Review of Selected Heat Transfer Topics for Solar Thermal Energy Utilization and Storage Open Access

This review article, developed by the K6 Committee—Heat Transfer in Energy Systems, a part of the Heat Transfer Division (HTD) of the American Society of Mechanical Engineers (ASME), summarizes advancements in heat transfer technologies for solar thermal energy utilization and storage, focusing on concentrated solar power (CSP), solar-driven cooling, sensible and latent thermal energy storage (TES), and novel heat exchanger designs. Key topics include heat transfer enhancement strategies such as additive manufacturing, phase change materials (PCMs), and triply periodic minimal surface (TPMS) structures for improving efficiency. The advances in solar-driven cooling and multigeneration systems are analyzed, emphasizing thermodynamic optimization through exergy and entropy generation minimization. Additionally, the study examines emerging methodologies, including constructal theory and second-law analysis, to enhance the performance of solar thermal applications. The article highlights overlaps in TES strategies, heat exchanger innovations, and system optimization approaches, offering a comprehensive perspective on sustainable energy solutions. Future research directions include scaling advanced TES materials, optimizing hybrid cooling technologies, and improving structural integrity in high-temperature heat exchangers.

Energy systems are integral to modern society, encompassing processes such as energy conversion, transport, and storage. Within these systems, heat transfer plays a pivotal role in influencing efficiency, reliability, and overall performance. Given the critical importance of heat transfer, there is a compelling need to continuously examine and highlight recent advancements in this field. This review article, developed by the K6 Committee—Heat Transfer in Energy Systems, a part of the Heat Transfer Division of the American Society of Mechanical Engineers (ASME), aims to address this need by focusing on several selected topics within the expansive domain of energy systems. It is our aspiration that a series of review articles following this one will be published regularly, thereby enhancing the visibility of the topics considered and assisting the research community in this field. The selected topics represent key advancements in harnessing solar thermal energy efficiently for various applications. Concentrated solar power (CSP) plays a crucial role in large-scale energy generation, while solar-driven cooling addresses the growing demand for sustainable cooling solutions. Sensible and latent thermal energy storage (TES) is essential for overcoming the intermittent nature of solar energy, ensuring reliability and extended usability. Additionally, novel heat exchanger (HE) designs enhance heat transfer efficiency, improving overall system performance and feasibility. Together, these advancements contribute to optimizing solar thermal utilization for a more sustainable energy future.

The urgency for this review is underscored by the mounting global concerns about climate change and environmental sustainability. Energy systems are significant contributors to carbon emissions, necessitating innovations that enhance sustainability while maintaining high performance. Despite the abundance of scholarly publications, there remains a gap in comprehensive reviews that synthesize recent developments and identify future research directions. This article seeks to fill that gap and provide a consolidated resource that highlights key advancements and emerging issues in heat transfer within energy systems.

Given the vast and intricate nature of energy systems, this review does not attempt to cover every facet comprehensively. Instead, it narrows its focus to seven critical areas related to solar thermal energy that represent a balance between components and systems.

Capable of large capacity of solar thermal storage for extended power supply after sunset, concentrating solar power (CSP) technology has received significant development in the recent decade. Over 100 concentrating solar power plants have been built globally with a total operational capacity of 6.6 gigawatts (GW) and 1.5 GW under construction as of 2023. It is known that heat transfer and thermal storage technology plays a critical role in the high performance of a CSP system. The objective of this section is to examine the state-of-the-art heat transfer enhancement technologies for the application in modern CSP and further to survey the more recent research and innovation for the improvement of the performance of CSP technologies and application of concentrated solar energy for industrial process heating in the future.

CSP technology has experienced great development in the recent decade [1]. Especially, with the advancement of technology of large-capacity thermal energy storage [2], daytime stored thermal energy can make the extended nighttime power production for as long as 17.5 h,² which renders the 24 h solar power production possible. It is important that the 24 h nonstopping daily operation [3] not only reduces the levelized cost of electricity but also compensates for the fluctuations of power supply and makes a positive contribution to grid stability and security of supply on the consumer side [4,5]. There is no doubt that with cost-effective long duration thermal energy storage, concentrating solar power technologies will receive even more development in the renewable energy industry. Additionally, thermal energy storage even becomes an acceptable approach for wind energy storage to provide customers with either thermal energy or electrical energy [6,7]. When wind turbines generate more electricity than is needed, the surplus is used to heat a storage medium like molten salt or particles [8]; when demand is high, the stored heat is converted back into electricity through a heat engine, or thermal energy is directly fed to customers [9].

Due to the development of high-concentration ratio solar concentrators and receivers [10–13], high-temperature solar thermal energy (⁠$≥1000∘C$⁠) has obtained a significant development for the application of thermal chemical processes to produce hydrogen [14–17], capture $CO2$ [18,19], or provide high-temperature heat for cement manufacturing or firing ceramic ware [20,21].

Effective and innovative heat transfer technologies are pivotal to concentrating solar thermal systems including heat collection and thermal storage. Heat-receiving pipes/tubes have internal heat transfer to transmit solar energy to a fluid, and their external surfaces absorb concentrated solar energy and at the same time have natural convection and heat emission to the surroundings. Very high absorption of sunlight at the external surface and heat transfer enhancement for the internal flow are needed, but the external radiative loss and free convection heat transfer loss have to be inhibited. The objective of technology innovation and research and development in solar thermal society is to better solve these problems with reliable and cost-effective measures.

This section will present a survey of the state-of-the-art CSP technologies already well applied in industrial operations for electricity generation, followed by a literature review of the recent research and development showing a high potential for concentrated solar energy being applied for the advanced solar thermal energy industry in the future.

A CSP system must include a suitable power cycle with a proper working substance, a heat transfer fluid (HTF) that takes heat from the solar receiver, solar absorbing coatings at the external surface of solar receivers, heat transfer enhancement of internal flow under nonuniform heat flux, and thermal storage which is the indispensable advantage of solar thermal power.

The CSP stations currently in commission around the world have more than 74% based on parabolic trough solar collectors, 1% based on linear Fresnel reflector solar collectors, and the rest 25% are based on solar power towers for solar collection [1]. To fully exercise the advantage of CSP technology, almost all the current CSP stations have thermal storage system and the extended operation time after sunset can range from 2 h to 17.5 h. In fact, with 17.5 h of extended power station operation through stored energy in the nighttime, the power station can operate 24 h, which is very attractive to industrial electrical power suppliers.

The power cycle adopted in the current industrial CSP is mostly the Rankine cycle using water as the working substance. Solar thermal energy is mostly taken by synthetic oil (e.g., $SylthermTM$ and Therminol^®) in parabolic trough solar receivers and by molten nitrate salts in solar power towers [22], except one example of using air as heat transfer fluid for solar receiver [23]. Heat exchangers are needed for the heat transfer fluid from the solar receiver to deliver heat to the water in the Rankine cycle. Because of the temperature limit of the synthetic oil and molten nitrate salt, the steam temperature in the Rankine cycles in the CSP plant cannot reach the level of $560∘C$ as that of a coal-fired power plant [24]; instead, it is around $270∘C$ in a linear Fresnel reflector CSP system (using synthetic oil), $370∘C$ in a parabolic trough CSP system (using synthetic oil), and $542∘C$ [23] in a solar power tower CSP system (using nitrate molten salt). The temperature limit of synthetic oil is $400∘C$ and that for molten nitrate salts mixture is $560∘C$ [24]. Obviously, more elevated working temperatures at solar receivers and higher upper-bound temperatures at thermal power cycles are demanded in order to further advance CSP technologies in the future.

Effective heat transfer is very important to the energy efficiency and the levelized cost of thermal energy or electricity of a CSP station. The objectives of efficient heat transfer for solar receivers include enhanced convective heat transfer inside a solar absorber tube and also the reduction of heat loss at the outside of the solar absorber tubes. Currently, internal flow heat transfer enhancement is often limited by the difficulties of fabricating internal fins of required structures based on conventional manufacturing technologies. Although a lot of computational studies [25–31] reported simulation results of enhanced heat transfer using internal fins, there is not much information about manufacturers making internally finned tubes for parabolic trough solar collectors (of inner diameters greater than 700 mm) right now. As the parabolic trough absorber tubes are very long in a solar collection field, the increase of friction loss must be minimized for finned tubes. Nonetheless, spiral low fins at a fin height of less than 2.0 mm are manufacturable for tubes of inner diameter less than 1.0 in. (25.4 mm) but not seen for tubes with pipe diameters greater than 70 mm that can be used in parabolic troughs or linear Fresnel reflector solar collectors. Although spiral tape insertion has been used for heat transfer enhancement for large diameter tubes, the issue of vibration of the tape in the flow field and the significant pressure loss is often a concern and not well adopted in solar receiving tubes. Therefore, for parabolic trough and Fresnel reflector solar collection pipes, there is a good potential for internal heat transfer enhancement, if internal fins can be manufactured for large diameter pipes with cost-effective fabrication technologies and the friction loss is controlled. For example, additive manufacturing (AM) to make internal fins for metal pipes becomes very promising. Haddad et al. [32] used a 3D-print method and made heat transfer enhancement tubes using Inconel-718 material, as shown in Fig. 1. The tubes have 7-head spiral fins on the internal surface, and multiple sections were welded together to obtain a long heat transfer-enhanced tube. The heat transfer coefficient of the 7-head helical finned tube could achieve 1.6–2.4 times that of a smooth tube with the expenses of 4.5–8 times of pressure loss compared to that of a smooth tube. It is promising that additive manufacturing can be applied to fabricate various structures of internal fins for high heat transfer coefficient and low friction loss in solar absorber tubes. For solar power towers, optimal tube diameters of solar absorbing tubes were found to be around 1 in (25.4 mm) for molten salt fluid [33]. Therefore, conventional manufacturing techniques may still be cost-effective to make internal fins in tubes [34] so that many of them can be welded side by side to form panels of tubes for solar absorption at a solar power tower.

It is worth noting that the nonuniform circumferential solar heating flux on solar absorber tubes can result in large thermal stress which is one of the major reasons causing failure of the solar receiver. The spiral internal fins in the solar absorber tubes can introduce spiral flow that can level the circumferential temperature difference and thus help reduce thermal stress [35].

While the enhancement of convective heat transfer inside solar absorber tubes helps energy being transmitted under relatively low surface temperature at the solar receiver, the reduction of radiation heat loss at the external surface of the tubes is also important to reduce energy loss to the surroundings. Therefore, coatings of high absorptivity to sunlight but low emissivity of longwave radiation from surfaces at temperatures $200−1000∘C$ have been widely used for solar receivers. Pyromark 2500^® is one of the most common solar absorber black paints. Lanthanum strontium manganite oxide (LSM) and cobalt oxide (⁠$Co3O4$⁠) are also used as coatings for solar receivers. Reference [36] showed that Pyromark has the highest initial absorptivity of 0.955, and that of $Co3O4$ is 0.948, and for LSM it is 0.941. LSM has the lowest emittance at 0.782, and that of $Co3O4$ is 0.785, and for Pyromark 2500^® it is 0.879. Nonetheless, Pyromark 2500^® possesses the best overall cost-effectiveness among the three types of coatings. Because of its better performance and cost-effectiveness, Pyromark 2500^® has received extensive detailed studies [37–40].

Glass cover on solar receiving tubes is used to reduce heat loss, particularly for parabolic trough and Fresnel reflector solar collectors. Cavity receivers for solar power towers also have glass windows for heat loss reduction. The property of high transmittance to visible light and low transmittance to long-wave infrared radiation makes glass the perfect material to cover solar absorber surfaces [41]. Typically, the glass used for the protection of solar absorber tubes is coated with antireflective material to ensure high transmittance and high abrasion resistance.

To greatly advance CSP technologies, a large amount of research and development efforts have been put into two aspects in the past ten years. One is the improvement of the upper bound temperature of the heat from solar receivers so that high thermal efficiency of power cycles can be achieved according to the second law of thermodynamics. The other effort is to develop a reliable material and technology so that large capacity and cost-effective thermal storage is possible for extended power generation in the night to achieve the goal of 24 h power production using solar thermal energy. In fact, the two aspects of the studies are correlated because a heat transfer medium and thermal storage material may be the same material, which is beneficial for no heat exchange needed between HTF and the thermal storage material.

To improve the upper bound temperature of the thermal energy at a solar receiver and make the thermal power cycles have a high temperature at the level of 700–$800∘C$⁠, molten chloride salts high-temperature heat transfer fluid [42] and falling particle solar receiver [43] have been extensively studied in the past ten years for heat transfer fluid and thermal storage media. On the thermal power generation side, the supercritical $CO2$ (s$CO2$⁠) Brayton cycle has been proposed for the CSP system so that the high temperature of the power cycle can reach a level of $700∘C$ [44] and thermal efficiency can attain a point of 50% [45].

Power cycles at higher temperatures beyond $600∘C$ may include Brayton cycle and Rankine cycle combined system and supercritical $CO2$ Brayton cycles.

The two fundamental advantages of $sCO2$ cycles are elevated efficiency and reduced sizes of components such as $sCO2$ compressors [46–50] and turbines [51] due to the fluid density effect. Although there are a great number of studies in the academic society showing the advantages of $sCO2$ power cycle, limitations and challenges associated with the industrialization of the technology are still enormous.

A very recent review article by Molire et al. [52] in 2024 reveals the current status of significant discrepancy in the results from academically studied systems versus those from industrialization and actual operational systems. The wide industrial application of using $sCO2$ power cycles for CSP generation is not yet fully ready. Nevertheless, some recent progress in system development for $sCO2$ power generation is still very impressive. In May 2024, a demo pilot plant funded under the program of Supercritical Transformational Electric Power (STEP) program initiated by the US Department of Energy has generated electricity for the first time using $sCO2$ power cycles. The 10-megawatt (MW) $sCO2$ facility at Southwest Research Institute (SwRI) in San Antonio has demonstrated the next-generation power production technology in a project led by GTI Energy in collaboration with SwRI, GE Vernova, Department of Energy National Energy Technology Laboratory (DOE/NETL), and several industry participants. The project team has successfully completed its phase-I testing of the $sCO2$ power cycle in a simple recuperating configuration, which could achieve a full operational turbine speed of 27,000 rpm at $500∘C$⁠, meeting test objectives and targets. The system could generate 4.0 MW of grid-synchronized power which is enough to supply electricity to 4000 homes [53]. In the final phase of the project, STEP Demo will operate the $sCO2$ power cycle at $715∘C$ and demonstrate a recompression closed Brayton cycle (RCBC) configuration. Thermodynamics analysis [52] has confirmed that RCBC is the most promising architecture for an $sCO2$ power cycle which can have a thermal efficiency of 48% at a turbine inlet temperature of $500∘C$⁠, and further at the top temperature of $715∘C$⁠, a thermal efficiency of 50% may be achievable in the future.

Although great achievement as mentioned above has been accomplished for $sCO2$ power systems, there are a number of challenges yet to be overcome [52], which need better and innovative techniques in the future. These challenges include (1) the rotational stability of the turbine shaft, (2) to manage and stabilize the strong pressure variations during the operation, (3) to bring the level of $CO2$ leaks at turbine and compressor bearings to a very low level despite the very high pressures at 25–30 MPa, (4) to improve the isentropic efficiencies of the compressors and the turbines through better design of the turbomachinery.

To have less operational pressure below that of $sCO2$ cycles, higher temperature CSP has been considered. Li et al. [54] proposed and analyzed a high-temperature combined cycle system for CSP, which includes the helium gas Brayton cycle as the top cycle and the steam Rankin cycle as the bottom cycle. The upper bound temperature for the helium Brayton cycle is at the level of $800$–$850∘C$ and the highest gas pressure is 8.0 MPa in their proposed system. Helium gas serves as the heat transfer fluid for the solar receiver and also the working substance of the Brayton cycle. The analysis indicated a system thermal efficiency of nearly 45%. Although the energy efficiency is not as high as that of $sCO2$ cycles, the convenience of using matured helium gas compressors and turbines as well as steam turbines is beneficial for low cost. Because of the high thermal conductivity of helium, 10–20 times that of air, solar receivers using helium gas at temperatures from $700∘C$ to $900∘C$ can be an interesting direction for future CSP applications.

Accompanying the development and adoption of supercritical $CO2$ power cycles, high-temperature heat transfer fluid and thermal storage medium are direly needed. This is also the driving force for the development of molten chloride salts heat transfer fluid and falling particle solar receivers. In 2012, the US Department of Energy first funded studies for molten chloride salts being used as heat transfer and thermal storage fluid being able to operate at temperatures up to $800∘C$⁠. Although the thermophysical properties and heat transfer performance of molten chloride salts, $KCl−MgCl2$⁠, $NaCl−KCl−MgCl2$⁠, and $NaCl−KCl−ZnCl2$ are acceptable [42,55–60] for serving as an HTF, there are some uncertainties on the corrosivity to metals (nickel-based high-temperature alloys) caused by the salts at high temperatures, which is affected by the impurities, especially water, hydroxide, and oxygen [61,62] in the salts. Alternatively, falling particles (such as silica and silicon carbide (SiC) sands) have demonstrated excellent absorptance to concentrated sunlight to reach a temperature above $800∘C$⁠, which can also serve as thermal storage materials at a very low cost [43,63]. Starting from 2012, a great amount of study worldwide has also focused on particle thermophysical properties [64], heat transfer between falling sand particles and metal surfaces [65], and fluidized bed heat transfer of sand particles with metal pipes [66]. The heat transfer coefficient of a fluidized bed of particles with metal pipes could reach $3.3kW/(m2K)$ [67,68]. A pilot particle solar receiver has been in testing at Sandia National Lab since Sep. 2024. The features of particle solar receiver and thermal storage designs and the CSP plant operation are available in the literature [69].

Solar concentration ratios above 1500 times allow the creation of high temperatures at the level of $1000−1500∘C$ that can facilitate solar thermochemical reactions at higher temperatures for the manufacture of solar fuels such as solar hydrogen and solar jet fuel [70]. High solar concentration ratios at above 1500 times and the created temperature of $2500∘C$ or above will also have a great potential for applications in industrial manufacturing in the future.

Numerous solar thermochemical water-splitting and $CO2$-splitting cycles have been investigated for hydrogen and jet fuel [71]. Ceria (⁠$CeO2$⁠) is mostly considered a state-of-the-art redox material for its rapid redox kinetics and long-term stability. When ceria is heated to a temperature of $1500∘C$⁠, part of its oxygen is released so that $CeO2$ becomes $CeO2−δ$⁠. At $900∘C$⁠, $CeO2−δ$ can react with $H2O$ to release $H2$ and get $CeO2$ recovered or react with $CO2$ to release CO and get $CeO2$ recovered. The $CeO2$ reduction reaction at $1500∘C$ and oxidation reaction at $900∘C$ need to occur alternatively and heating and temperature need to be well managed [72]. Better heat transfer design and management of the temperature cycles between $1500∘C$ and $900∘C$ for the $CeO2$ residing reactor is a very important topic in the future for better thermal efficiency.

Using concentrated solar energy to catch $CO2$ from the atmosphere for sequestration is another direction of utilizing concentrated solar energy for the benefit of the environment [73]. The technology is called calcium looping, where calcium carbonate (⁠$CaCO3$⁠) or limestone is heated by concentrated solar to split into CaO and $CO2$⁠. While $CO2$ is sequestrated for storage, the CaO can be exposed to air or flue gas from the power plant to absorb $CO2$ and turn CaO back to calcium carbonate. Cyclic or looping operation of these two processes, calcination (split $CaCO3$ into CaO and $CO2$⁠) and carbonation (CaO and $CO2$ from air or flue gas), will allow $CO2$ to be captured using solar energy and at the same time the heat recovered during the carbonation can be used for power generation or thermal energy utilization [74,75]. Interestingly, the temperature demand for the calcination is at around $950∘C$ which has less requirement for solar concentration ratios for the solar receivers.

Several researchers have used concentrated solar energy to provide heat for kilns that can fire ceramic ware [20,76] at temperatures from $1500to2500∘C$⁠. To provide relatively uniform heating, the ceramic ware is typically put on a rotatable platform. However, the research work seen so far is only made possible for single-piece ware due to the need for very high solar concentration ratios (1500–2000 times). Systems of increased scale of production for multiple pieces of ceramic ware with required uniformity of heating might be the direction of research and development for very high temperature (up to $2500∘C$⁠) concentrated solar kilns in the future. As ceramic ware production consumes a great amount of energy, the cumulative $CO2$ emissions attributable to the production of ceramic tiles and wares have reached approximately 100 million tons [77]. It will be greatly beneficial to use concentrated solar energy for ceramic ware and tile production.

The building sector consumes nearly 30% of global final energy consumption and produces about 26% of global energy-related carbon dioxide (⁠$CO2$⁠) emissions. Cooling systems are responsible for about 20% of global electricity use by buildings [78]. In addition to large energy consumption and the associated carbon footprint, the majority of cooling systems rely on mechanical vapor-compression refrigeration cycles which require the use of refrigerants. Many refrigerants that are currently in use continue to have considerable global warming potential which will further contribute to climate change [79]. According to the most recent Intergovernmental Panel on Climate Change assessment report, it is projected that from 1990 to 2100, the land surface air temperature will increase by nearly twice as much as the global average temperature, with an estimated rise of approximately $1.5∘C$ [80]. Considering the continuous rise in global temperature, the demand for air conditioning systems has been increasing over the past several decades. The International Energy Agency anticipates there will be 5.6 billion air conditioners by 2050, nearly three times as many as there are today [23]. As the demand for cooling continues to rise, particularly in warmer climates and urban areas, exploring methods for reducing building cooling demand [81,82] as well as alternative cooling technologies to support the global shift toward a more sustainable future has become essential [83–85].

Solar cooling technologies have become increasingly important over the past several years and have emerged as promising alternatives to traditional technologies for certain applications [85]. Solar cooling presents innovative solutions by using solar energy to drive cooling systems, resulting in a decrease in the carbon footprint and declining the burden on the electricity grid while promoting the use of renewable sources of energy. Additionally, integrating solar cooling systems with energy storage allows energy to be stored during peak sunlight hours and used later, improving system reliability and resilience [86].

Solar energy has emerged as a promising renewable source for various applications, including cooling. Solar cooling technologies can be categorized based on their driving mechanisms as solar-electrical, solar-thermal, or hybrid systems [87]. In this section, the basic concept and most recent advancements regarding each technology category are briefly discussed. A summary of the list of technologies covered in this article is shown in Fig. 2 in this section for an example.

Solar-electric cooling systems utilize solar energy to generate electricity, typically through photovoltaic (PV) cells. This electricity is then used to power cooling systems, which could include mechanical vapor-compression cycles, thermoelectric cooling systems, Stirling cycle, or other technologies [88].

The use of PV electricity to drive vapor-compression refrigeration cycles has been studied extensively over the past few decades [89]. The technology has become more cost-effective in recent years as the efficiency of PV cells significantly improved and the price of PV systems has reduced considerably [90,91]. Numerous case studies have been performed to better understand the performance of PV-powered air conditioning (PVAC) systems and compare it against conventional systems.

PVAC systems can be driven using AC or DC power. While AC-driven systems require an inverter, DC-driven systems can be directly supplied by PV panels and therefore offer enhanced efficiency. These systems have recently received more attention [89]. Recent studies are more focused on the experimental evaluation of PVACs to better understand challenges pertaining to this technology. Recently, Rebelo et al. [92] performed an experimental investigation of a PVAC system and compared with conventional unit for Piaui, Brazil. They reported that the PVAC system was able to accomplish competitive performance while offering a short payback period of 3.7 years. Ayadi et al. [93] investigated a PVAC system for a $36m2$ lab at The University of Jordan. The system, with 2.67 kWp PV panels, was tested in both on-grid and off-grid modes on clear and cloudy days. Results showed a 70% operational time on average, with off-grid operation successful on clear days but limited by charging power on cloudy days. Economic analysis found payback periods of 6.91 years (on-grid) and 6.4 years (off-grid).

Solar photovoltaic-thermoelectric (PV-TE) systems have also received increasing attention over the past few years. TE modules offer highly desirable characteristics including scalability, exceptional controllability, no need for major moving parts or refrigerant, and a DC-driven system [94]. PV-TE systems can be used in the form of radiative cooling systems by integrating TE modules in building envelope or modules can be incorporated in a separate cooling unit [95]. Seyednezhad and Najafi [83] performed a parametric study on a solar-powered PV-TE system integrated into the ceiling of a small office building. They presented a novel approach for sizing the system to address the cooling load of a building that was used for a case study. Seyednezhad et al. [84] assessed the possibility of incorporating phase change materials in integrated PV-TE systems. They discussed that maintaining a competitive coefficient of performance (COP) requires effective thermal management on the hot side of the TE module. Ahmed et al. [96] investigated the impact of modules number of thermoelectric cooler coupled with PV panels and phase change material on building cooling. They developed a model and evaluated the performance of the system for five wall positions (east, west, south, north, roof). The roof system generates the highest PV output, and the west system achieves the best cooling capacity. Optimal performance is reached with different numbers of thermo-electric elements for each position. A recent review of the TE systems used for building applications is presented by He et al. [95]. The COP of TE cooling systems is reported between 0.2 and 4.8 and remained on the lower side in most cases that are reported. This is partly due to the relatively low efficiency of the thermoelectric materials and also the challenges involved, and the energy required for the thermal management of these systems.

Solar thermal cooling systems generally operate by using solar collectors to absorb solar radiation as thermal energy. The thermal energy can be used for cooling purposes using various techniques such as absorption, adsorption, ejector, and desiccant systems [97]. The state of the art as well as challenges and prospects related to these technologies will be further discussed in this section.

In this section, we will focus on solar thermal cooling technologies, particularly thermal sorption systems. Sorption cycles are thermodynamic processes used in cooling and heating systems, where a refrigerant is absorbed or adsorbed by a solid or liquid sorbent material. In these cycles, thermal energy is used to drive the process, typically replacing electricity-driven compression. Sorption cycles are commonly used in solar thermal cooling, and refrigeration systems, offering energy efficient and environmentally friendly alternatives to conventional vapor-compression technologies.

Absorption Cooling. The absorption refrigeration cycle is a thermally driven cooling technology that operates similarly to the vapor-compression cycle but with a key difference: Instead of using a mechanical compressor to increase refrigerant pressure, the refrigerant leaving the evaporator is absorbed by a liquid solution. This solution is then pumped to a higher pressure, and heated to release the refrigerant as vapor (desorption), which then condenses and expands as it returns to the evaporator and cools the desired space. By relying on heat rather than mechanical compression, absorption refrigeration offers an efficient alternative to conventional mechanical vapor-compression systems, particularly when an abundant source of low-grade thermal energy, such as solar energy, is available [98].

Commonly used solution pairs that are used in absorption systems are ammonia (⁠$NH3$⁠) as a refrigerant and water as a sorbent, or water as the refrigerant and lithium bromide (Li–Br) as a sorbent. With the $NH3−H2O$ pairs, evaporator temperatures below $0∘C$⁠, can be accomplished and condenser temperature could be as high as $50∘C$⁠. For the $H2O- LiBr$ pairs, the evaporator temperature has to remain above $4∘C$ and the condenser temperature cannot exceed $35∘C$ [97]. Solar absorption cycles come in different configurations. Single-effect absorption cycle is the simplest form. In a single-effect absorption cycle, a low-grade heat source powers the desorption of the refrigerant from an absorbent. The COP of single-effect cycles is about 0.5–0.75 and they operate at lower temperatures with a low-grade heat source at around $80−95∘C$ [97]. Double-effect absorption cycles use two stages for the desorption process, with high-grade heat driving the first stage and waste heat from the first stage running the second stage. Double-effect systems require a higher temperature heat source (⁠$130−160∘C$⁠) such as solar collectors as evacuated tubes, parabolic troughs, or concentrating collectors, and offer an enhanced COP of around 0.8–1.2, resulting in higher energy efficiency [99]. Cycles with more than two stages have been also explored in the literature. Triple-effect absorption cycles add a third stage of desorption and absorption to further enhance the energy efficiency of the cycle and reach COP as high as 1.5–1.7. Such an approach requires a higher temperature heat source (over $200∘C$⁠) and therefore is often more appropriate for industrial applications with very high heat availability. Given the transient nature of solar radiation, the efficiency and operation time of solar absorption cooling systems may vary. Therefore, variable effect absorption refrigeration cycles that can adjust COP based on the temperature has been also proposed and successfully tested with promising results [100].

Solar absorption systems have been studied extensively from various perspectives and numerous articles have discussed aspects such as absorbent materials, optimal cycle design and configurations, control, and parametric analysis [99]. Performance improvement of the absorption refrigeration cycle using nanofluids is investigated by Kumar et al. [101]. Advances in solar absorption technologies have been reviewed by Choudhury et al. [98]. A comprehensive review of different methods for improving energy performance of absorption cycles is presented by Nikbakhti et al. [99]. They highlighted the advantages of absorption systems as the utilization of low-grade heat sources and environment-friendly working fluid pairs and also discussed the two major disadvantages as the relatively large size of the cooling unit and the low COP. The article is focused on cycle design improvement, heat recovery method, development of new working fluid pairs, adding subcomponents, and improvement of operating conditions.

Adsorption Cooling. The solar adsorption cycle uses solar energy to drive an adsorption cooling process. A typical adsorption cycle includes an evaporator, an adsorber, a condenser, a throttle, a heater, and a cooler. The adsorber is packed with solid adsorbent material and the evaporator is charged with the adsorbate material (refrigerant). In this system, an adsorbent material captures refrigerant vapor at low pressure. Solar thermal energy is then used to release the refrigerant, which condenses and evaporates to provide cooling, making it an efficient, and environmentally friendly option for thermally driven cooling [98]. Adsorption cooling features major advantages such as eliminating large moving parts, durability, environmentally friendly working fluid, and flexibility to be used as cooling or heating systems by simply switching the direction of the refrigerant flow [102]. They also do not need a solution pump and rectifier, unlike the absorption cycles. Conventional adsorption systems have disadvantages including complex design, intermittent operation, and low COP and energy densities compared to vapor-compression cycles.

There are several possible adsorbent–adsorbate materials that can be used as working pairs. The selection must be made based on the temperature of the heat source, the desired performance of the refrigeration system, cost, availability, and environmental impact [103]. The most commonly used working pairs for adsorption cycles are zeolite–water, activated carbon–methanol, silica gel–water, and activated carbon–ammonia [89]. The regeneration temperature may vary between 60 and $120∘C$ and the associated COP ranges between 0.3 and 0.6. The COP of adsorption cooling systems may vary based on the type of adsorbent-refrigerant pair, regeneration temperature, and system configuration.

Silica gel–water has been widely used as a working pair for solar adsorption refrigeration cycles for the past few decades. However, these systems are typically expensive, offer limited capacity, and deliver low COP. To address these issues, modular adsorption beds are proposed and tested. The modular system features a compact adsorption bed formed by fin-tube units. The resulting design offers scalability and lower cost [104]. Pahinkar et al. [102] proposed a microchannel-based adsorption heat pump design to enhance specific cooling capacities (SCCs) and COPs. The design comprised alternating rows of adsorbent-coated microchannels and heat transfer fluid channels, with a thin adsorbent layer coating the inner walls. The results showed significant improvement in terms of SCC and COP compared to traditional adsorption systems.

A comprehensive review of adsorption cooling systems and their regeneration methods using solar, ultrasound, and microwave energy is presented by Sidhareddy et al. [105]. Several aspects of the adsorption refrigeration cycles have been investigated over the past decade and major improvements to the system have been introduced and assessed.

Desiccant and Hybrid Cooling. A solar desiccant cooling system consists of a desiccant wheel which dehumidifies the incoming air and an evaporative cooler to cool the dehumidified air. A solar collector is used to absorb the solar radiation and regenerate the desiccant wheel to ensure continuous operation [106,107]. Chun et al. [108] conducted a study and evaluated the thermodynamic performance of a solar-driven desiccant evaporative air cooling system (SDEAC) for outdoor environments. They explored the impact of various parameters such as rotational speed of the desiccant wheel and air flowrate on the performance of the system. Previous SDEAC systems have reported COPs ranging between 0.4 and 1.4 [108]. Several studies explored the possibility of hybridizing desiccant cooling systems with other solar cooling technologies.

Aboelala et al. [109] presented a preliminary investigation of a solar-powered absorption-desiccant-radiant cooling system for thermally active buildings. They assessed both energy and thermal comfort performance and the simulations showed promising results. Yang et al. [110] proposed a solar-assisted regenerative desiccant air conditioning with indirect evaporative cooling for humid climate regions. Bozorgi et al. [106] studied solar-driven desiccant cooling with integrated phase change material and adsorption chiller system. Their results showed improvement in COP and a reduction in carbon emissions while maintaining proper thermal comfort.

Although recent studies on hybrid systems have shown promising results, further experimental evaluations are needed to better understand the implementation challenges these systems may face. Moreover, conducting detailed cost analyses is crucial for informing decision-making and determining the optimal cooling system for specific applications.

Global warming and the rising demand for cooling and air conditioning systems have made the need for alternative technologies more urgent than ever. Solar cooling technologies can play an important role as we strive toward decarbonization of the building sector and a more sustainable future. A variety of technologies have been developed to harness solar energy for cooling, and despite significant progress over the past few decades, major challenges remain that require further research.

• Solar-electric cooling technologies have made significant improvements, thanks to the fast growth of PV cell efficiencies and decreasing in their price. Recent studies demonstrated competitive performance for PVAC systems at reasonable payback periods. Both AC-driven and DC-driven PVAC systems have been studied and the latter particularly requires further development to efficiently link between the PV and air conditioning systems. Hybridizing PVAC with other cooling technologies and efficient integration and control for these systems also demand more research.

• PV-TE systems have received increasing attention for the past few years. The scalable, solid-state cooling technology with no moving parts have been both used as integrated in building envelope and nonintegrated systems. The relatively low efficiency of TE modules demands more research and development on thermoelectric materials. More energy-efficient thermal management systems to maintain competitive operational efficiency are also essential. Addressing challenges ahead of implementation such as control and large-scale installation must be also explored.

• Solar absorption cooling systems continue to have a relatively low efficiency. However, new developments in the technology, such as variable effect absorption cooling system, integration of energy storage, and using nanofluid for enhanced heat transfer, have led to significant improvements. Exploring more efficient working pairs, innovative hybrid systems, incorporating energy storage along with control and optimization require more research and will lead to improve the system performance. There has been significant progress on solar adsorption systems which has resulted in improvement of their efficiency and reduction of their cost. Modular systems with heat and mass recovery, and microchannel-based adsorption heat pumps are examples of such innovations. More research and development are needed to accomplish efficient and cost-competitive solar adsorption chillers.

• Several innovative hybrid systems have been proposed and analyzed to accomplish an overall enhanced performance for cooling technologies. Majority of these works offer promising performances, many of them are based on simulations and rigorous experimental studies are needed to better understand the limits and challenges ahead regarding each of these designs.

As carbon emissions from the consumption of fossil fuels drive climate change to unprecedented levels, society must find ways to meet growing energy demands renewably and sustainably [111]. Thermal loads in buildings, including hot water, heat and air conditioning, represent nearly a quarter of the total energy use of the United States [112]. Electrification of these loads through the use of heat pumps, electric water heaters, etc. can reduce emissions if coupled with a low- or zero-carbon electric grid, but that approach is not ideal in all settings. Moreover, there are challenges associated with the strategy of “greening” the grid while electrifying a broad range of energy services. Increasing the demand for electricity makes the challenge of transitioning to renewable electricity harder, increases the demand for electricity storage, and exacerbates potential environmental problems of solar, wind, and batteries [113,114]. Where possible, it makes sense to advance options that reduce the overall use of electricity and fossil fuels in buildings [115]. Solar thermal systems capture heat from the sun and store it, generally in hot water tanks, to be used to meet building demands for domestic hot water and sometimes space heating or even dehumidification and/or air cooling [116–118]. Solar air conditioning is discussed in detail in Sec. 3. By capturing the diffuse energy of sunlight and using it to meet the relatively low-quality energy demands of buildings, solar thermal systems offer an elegant solution to reducing carbon emissions from building thermal loads.

Globally, the most common solar thermal systems are used to provide hot water in buildings. Due to the relatively low cost of natural gas, the United States has been slow to adopt these solar thermal systems. However, there is huge potential for reducing US fossil fuel and electric use in hot water loads through the use of solar thermal hot water. In fact, a basic thermal system with two solar collectors and a 60-gal storage tank would have an average solar fraction of over 50% everywhere in the US except some of the most northern edges of the country, while the southern half of the country would have average solar fractions of 75% or more [119]. In short, solar thermal systems can be used almost anywhere, and in many places, simple systems could dramatically reduce the electricity or fuel used to meet thermal loads.

There are a range of options for storing solar thermal energy, most notably sensible hot water storage, latent energy storage, and chemical energy storage. The focus of this review is on the use of baffles to passively improve heat transfer to immersed heat exchangers in sensible hot water storage tanks. A brief discussion of the range of solar thermal energy storage options will provide context for that body of work. Several reviews of the literature on solar thermal storage options highlight the work of scholars in improving storage for solar thermal systems [120–122]. Some researchers have investigated converting thermal energy into chemical energy through thermochemical sorption storage, allowing for seasonal storage of thermal energy [123,124]. Latent energy storage through the use of phase change materials can also lengthen the time frame and stability of the thermal energy storage relative to sensible energy storage [124–126]. Latent energy storage is discussed in detail in Sec. 5. Although sensible energy storage cannot be stored for more than a few days due to heat loss from the storage tank, it remains the only commercially viable storage option due to its relatively low cost, simplicity, and convenience. As such, research on improving sensible solar thermal storage continues [120].

A major focus of work on improving the performance of sensible storage tanks for solar thermal systems focuses on increasing thermal stratification in the tank, often with a forced flow of water through the tank for charging (adding energy) and/or discharging (extracting energy) [120,127–130]. Stratification can both increase the solar gain from the solar thermal panel by delivering the coldest possible fluid to the panel and decrease the need for auxiliary heating by having the hottest possible water at the top of the tank available for meeting building loads. In both experimental and numerical investigations of tanks with storage fluid flowing into and out of the tank, a range of strategies have successfully improved thermal stratification in the tank, including, for example, tank shape [131], horizontal flat plates that obstruct the tank flow [128,129], inlet devices [130], fabric pipes [132,133], and porous manifolds [127,134].

Stratification can also improve heat transfer to or from heat exchangers immersed in thermal storage tanks by increasing the temperature difference between the storage fluid around the heat exchanger and the temperature of the heat exchanger working fluid [135]. Immersed heat exchangers can be used both for discharging or charging tank energy and can be used in combination with or instead of forced flows in and out of the tank. Immersed heat exchangers offer some design benefits relative to direct draws for charging/discharging the tank. They can reduce the need for pumps, allow for the use of unpressurized tanks, and can be used to create useful thermal zones in the tank [136]. Additionally, immersed heat exchangers can also add options in more complicated systems designed to meet more than one building load [137–139].

Improving heat transfer to immersed heat exchangers through low cost and passive means can have a significant impact on system performance. Indeed, simple, passive improvements to heat transfer can increase the solar fraction of domestic hot water or heating load by as much as 5% [140]. When the heat exchangers are made of highly thermally conductive materials such as copper pipes, the dominant thermal resistance to heat transfer between the heat exchanger working fluid and the tank storage fluid is the natural or mixed convection heat transfer between the heat exchanger outer wall and the surrounding storage fluid. Thus, attempts to passively improve heat transfer to the immersed heat exchanger must focus on improving the storage side convective heat transfer, which can be achieved by either (1) increasing the driving temperature difference between the heat exchanger wall and surrounding fluid (e.g., increasing tank stratification) or (2) by increasing the velocity of the storage fluid flowing over the heat exchanger. The tank flow field can be controlled with strategically placed barriers in the storage tank—baffles—with the goal of improving heat transfer to immersed heat exchangers by one or both of those mechanisms. However, many baffles investigated have not successfully improved heat transfer, generally because designs chosen to improve stratification had the unintended consequence of dramatically reducing fluid velocity [141–143]. However, some simple baffles do improve heat transfer to the immersed heat exchanger. This section highlights research on baffles that create an annular region with the tank wall within which a copper coil heat exchanger is situated.

The topics discussed in this section consider the design of a cylindrical baffle and its impact on heat transfer to an immersed copper coil heat exchanger. In all of these studies, a coiled heat exchanger constructed from a 10 m long copper tube with an outer diameter, $D$⁠, of 9.5 mm was situated at the top of a 300 L, well-insulated tank. In most studies presented, the tank was initially isothermal and $61∘C$⁠. The baffle was a cylinder constructed from a 3 mm thick polycarbonate sheet. It created an annular region with the tank wall within which the heat exchanger was situated. These studies investigated the effect of the baffle on heat transfer to the immersed heat exchanger while optimizing the heat exchanger and baffle design. The placement of the heat exchanger at the top of the tank is ideal for energy discharge, and as such cool, $20∘C$ water runs through the heat exchanger with a mass flowrate of $0.1kg/s$⁠. Cool plumes that form from the surface of the heat exchanger descend into the baffle region and create a flow field in the tank. In that flow field, storage water enters the baffle region from the top of the tank, flows over the heat exchanger, and exits the baffle region at the bottom of the tank. System performance is consistently evaluated with and without the cylindrical baffle. The system and design parameters explored in the articles discussed here are illustrated in Fig. 3. In general, design parameters such as the width of the baffle region and the pitch of the heat exchanger are given in terms of the diameter of the heat exchanger tube, $D$⁠.

The first pair of articles on this type of system demonstrated both experimentally [147] and numerically [143] that the cylindrical baffle improved heat transfer to the immersed heat exchanger, primarily by increasing the velocity of storage fluid flowing over the heat exchanger. The numerical work [143] investigated heat transfer to a heat exchanger using an axisymmetric model in which the heat exchanger was represented as a porous medium located along the wall of the top of the tank. They modeled heat transfer with no baffle, with the simple annular baffle, and with the simple baffle combined with hydraulic resistance elements in the center portion of the tank. The simulation found that the simple annular baffle improved heat transfer relative to the tank without the baffle by increasing the velocity over the heat exchanger. In contrast, the hydraulic resistance elements created significant thermal stratification at the cost of velocity and resulted in decreased heat transfer. The benefit of heat transfer of the simple straight baffle was confirmed experimentally [147]. In this work, the cylindrical baffle created an annular region with the tank wall with a constant width of 4 times the heat exchanger diameter, or 4$D$⁠. The baffle improved heat transfer relative to the tank with no baffle by 6.6% over the first hour of discharge. The experimental work also found that while the baffle did not generate significant thermal stratification (⁠$1∘C$⁠) in an initially fully charged isothermal tank, it did help maintain stratification in an initially stratified tank.

Given the demonstrated benefit of the baffle on heat transfer to an immersed heat exchanger, subsequent work sought to optimize the design of the system. The shape of the annular baffle was explored experimentally [144] and subsequently numerically [148]. The experimental study investigated three baffle shapes: (1) a straight cylindrical baffle that created a constant-width annular gap with the tank wall, where the width was twice the diameter of the heat exchanger, or 2$D$⁠; (2) a complex baffle-shroud in which the annular gap remained 2$D$ around the heat exchanger coil and narrowed to 0.75$D$ below the bottom loop of the coil; and (3) a hybrid of the two designs in which only the baffle side narrowed, creating an annular gap below the bottom of the heat exchanger coil of 1.38$D$⁠. The straight and complex baffle-shrouds are illustrated in Fig. 3. These designs were inspired by a series of numerical investigations that used a 2D simulation of a single cold cylinder in a rectangular hot water store to optimize baffle and shroud configurations [149–151]. In these studies, the shroud was composed of arcs that partially surrounded the heat exchanger and the baffle was a narrower region that extended below the heat exchanger. The experimental design in Ref. [144] attempted to apply the results of that series of numerical baffle-shroud investigations to the more complex geometry of a coiled heat exchanger in a large cylindrical storage tank. All baffle-shroud configurations improved heat transfer relative to control experiments without the baffle by at least 10% over the first 30 minutes, but the straight cylindrical baffle resulted in the best performance of the three, with 17% more energy extracted after 30 min than in the control. Of the three designs, the straight baffle generated the highest storage fluid velocities through the baffle region, explaining the observed improvement in heat transfer.

The subsequent numerical work [148] used a 2D axisymmetric model to explore the fluid dynamics of the system with the straight cylindrical baffle and the complex baffle-shroud design described above. The two designs were considered in a domain with the heat exchanger represented by a single cold loop and by two cold loops. The results of the simulations were consistent with the experimental results, especially when two loops were modeled. The simulations provided insights into the interactions of the cool plumes that form on the surface of the heat exchanger and interact with the heat exchanger loops below, as well as the fluid dynamics that resulted in lower velocities around the heat exchanger in the simulations with the complex baffle shroud. The wider baffle region in the straight cylindrical baffle decreased the viscous resistance to flow compared to the narrow baffle in the complex baffle-shroud. The resulting higher fluid velocity in the straight baffle improved heat transfer to the cylinder.

Having established the benefits of the straight baffle design, the next study experimentally investigated the effect of the width of the annular baffle region on heat transfer to the immersed heat exchanger [145]. Baffle region widths of 1.5$D$⁠, 2$D$⁠, 3$D$⁠, and 4$D$ were investigated and compared to experiments without the baffle. The heat exchanger coil diameter was adjusted so that it consistently sat in the center of the annular baffle region. The baffle continued to improve heat transfer relative to control experiments without the baffle for all widths. With the baffles in place, the heat exchanger extracted 14.6–23.3% more energy from the tank than in the control experiments without the baffle. Heat transfer increased as the baffle region narrowed. However, the effects of the baffle region width were modest—heat transfer over the first 30 min of discharge was 4.2% higher in the 1.5$D$ baffle case than in the 4$D$ baffle case. In this investigation, velocity was largely constant across the widths studied, but the baffle with the narrowest annular region did generate slightly more thermal stratification than that with the widest annular region (⁠$2.1∘C$ versus $0.9∘C$⁠), resulting in somewhat higher water temperatures around the heat exchanger. Thus, the increased heat transfer was attributed to the increased thermal stratification in experiments with narrower baffle widths.

Using the straight baffle that created a 1.5$D$ region with the tank wall, the effect of the pitch, or distance between coils, of the copper coil heat exchanger on heat transfer was the next parameter investigated [146]. Heat transfer to heat exchangers with pitches of 2$D$⁠, 3$D$⁠, 4$D$⁠, 6$D$ and 12$D$ were investigated with and without the cylindrical baffle. Without the baffle, a larger heat exchanger pitch resulted in higher rates of heat transfer to the heat exchanger because those larger pitches created greater thermal stratification in the tank. In contrast, when the baffle was in the tank, heat transfer was very similar for the heat exchangers with pitches of 2$D$⁠, 3$D$⁠, 4$D$ and 6$D$⁠, with 3$D$ slightly outperforming the others. With more spacing between the heat exchanger coils, there was more water between each coil to mix with the cool plumes from the surface of the heat exchanger. This mixing both slowed the velocity of the fluid approaching the coil below and resulted in warmer water driving heat transfer between the storage tank and that coil. This trade-off between temperature difference and velocity meant that with a baffle in place, heat exchangers with pitches between 2$D$ and 6$D$ would all perform similarly well. As in all prior work, the presence of the baffle resulted in significantly higher heat transfer rates for all heat exchanger pitches relative to all experiments without the baffle.

The baffle and heat exchanger optimization studies were all carried out in initially fully charged isothermal tanks with initial temperatures of $61∘C$⁠. However, as discussed in the introduction, thermally stratified tanks are highly desirable in solar thermal systems. Thus, the impact of the baffle in a stratified tank and the role of stratification in a tank with and without the baffle were investigated in the optimized system [135]. Three stratification cases were explored and compared to initial isothermal tanks with the same initial energy. In stratified tanks without the baffle in place, the heat exchanger cooled the upper, hotter zone of the tank uniformly until it reached the temperature of the bottom zone, after which the whole tank cooled uniformly. In initially stratified tanks with a baffle, the baffle maintained the high initial temperature of the upper zone of the stratified tank for 10–16 min, while the boundary between the hot and cool zones moved upward in the tank. During discharge in tanks with the baffle, the cool plumes that form on the heat exchanger were confined to the annular baffle region until they exited at the bottom of the tank, moving water from the top of the tank to the bottom without mixing. As such, the water temperature around the heat exchanger remained elevated and relatively constant for the first portion of the experiment.

In the initially stratified tanks and the isothermal tanks with equal initial energy, the baffle improved heat transfer to the immersed heat exchanger. This result in the isothermal tanks is consistent with prior work. In stratified tanks with the baffle, energy extracted by the heat exchanger during the first 30 min of discharge was up to 16% higher than in stratified tanks without the baffle. Initially, the baffle improved heat transfer in stratified tanks because of the higher water temperatures around the heat exchanger. Later, after all the water from the hot zone had entered and flowed through the baffle, the tank was basically isothermal. At that point, the velocity increased as the fluid temperature near the heat exchanger dropped, maintaining rates of heat transfer higher than that in the tank without the baffle. Stratification improved heat transfer in tanks without a baffle because, by design, the driving temperature difference between the heat exchanger wall and the surrounding fluid was considerably higher. However, in tanks with the baffle, stratification only results in a slight improvement in heat transfer to the immersed heat exchanger. The study demonstrates that in both stratified and isothermal conditions, the baffle always improves heat transfer to the heat exchanger: either by increasing the storage fluid velocity when the tank is isothermal or by maintaining the temperature difference between the storage fluid and heat exchanger wall when the tank is thermally stratified.

The body of work described in this review demonstrates that the cylindrical baffle improves heat transfer to the immersed copper coil heat exchanger in initially isothermal and initially stratified tanks. Although the studies described here investigated a heat exchanger placed at the top of the tank to discharge energy, the fluid dynamics would be the same for a charging heat exchanger placed at the bottom of the annular baffle region. In that case, the warm plumes that would form on the surface of the charging heat exchanger would rise through the baffle region, increasing fluid velocity and/or maintaining thermal stratification. Thus, the conclusions about the benefit of the cylindrical baffle can be extended to a copper coil heat exchanger used for tank charging.

However, in solar thermal storage systems, charging and discharging tank energy both occur, often simultaneously. Thus, future work should investigate the simultaneous charging and discharging of the tank to determine whether the positive effect of the baffle on heat transfer to the heat exchanger will persist when the tank is also being charged. Charging options are many and varied. The tank can be charged with an immersed heat exchanger placed at the bottom of the tank, through which hot water would run. The heat exchanger would need to be placed in the center portion of the tank—within the wall of the annular baffle—but the design of the heat exchanger would be an interesting parameter to explore. For example, the charging heat exchanger could be a coil with a vertical axis, a horizontal axis, or it could be a spiral. Using an immersed heat exchanger to charge would add a complicating element to the fluid dynamics in the tank, but as with the previous work, tank flow would be entirely buoyancy-driven. It is likely that a charging heat exchanger generating a buoyant flow in the center portion of the tank combined with the discharging heat exchanger creating a negatively buoyant flow in the annular region of the tank would increase the velocities in the tank and improve heat transfer.

In many solar thermal systems, water is directly drawn from and deposited into the tank, creating a forced flow in the tank. Many studies have investigated the success of different inlet devices in generating and maintaining thermal stratification. Thus, adding a charging loop with a variety of inlet devices is another interesting avenue for future research. A simple straight inlet tube would provide a baseline for exploring a range of other inlet devices, such as diffusers [127,130], fabric tubes [132,133], and porous manifolds [127,134]. It is important to determine whether the baffle continues to result in higher heat transfer to the immersed heat exchanger with the introduction of a forced flow in the tank. Furthermore, the fact that the baffle can maintain existing thermal stratification [146] leads to the question of whether it can enhance stratification when paired with various inlet devices. Preliminary results of ongoing experiments indicate that the baffle does continue to enhance heat transfer to the immersed heat exchanger when used in combination with an inlet pipe for charging.

The straight cylindrical baffle paired with an immersed heat exchanger shows promise as a very low cost, passive way to improve the performance of solar thermal systems. Solar thermal systems can be a valuable element in decarbonizing building thermal loads, and low cost ways to improve their performance can help make them more commercially desirable and expand their role in reducing greenhouse gas emissions.

Aligning with the global efforts to battle climate change through sustainability, improving upon the current state-of-the-art TES systems used in building applications has become a priority within the research community. Currently, most buildings employ outdated solid-to-liquid latent heat TES technologies typically consisting of large ice-water tanks and polyethylene tubes. Since the 1980s, these ice storage tanks have been used to shift peak daytime electrical loads to nighttime [152]. However, these systems require a large footprint and possess inherent operational difficulties, limiting their use to large commercial-scale buildings and reducing their overall efficiency. Both of these factors directly affect the power density of such systems, which are typically around $3kW/m3$⁠. While previous efforts mainly focused on enhancing material properties, such as increasing thermal conductivity and maximizing latent heat of fusion, these improvements have often been achieved at the expense of each other. Introducing additive manufacturing techniques can help improve system power density and overall efficiency by reducing the length scale and increasing the surface-area-to-volume ratio. The objective of this review is to examine the advancements in HE design through the utilization of advanced manufacturing techniques for the development of next-generation latent heat TES systems.

For the plane wall equation, $kw$ is the material thermal conductivity, $t$ is the wall thickness, and $A$ is the area normal to the direction through which heat transfer is occurring. Similarly in the annulus case, $L$ is the length of the pipe, and $ro$ and $ri$ are the outer and inner radii, respectively. As can be seen in Eq. (1), decreasing the thermal resistance can be accomplished by producing complex geometries with thin-walled features (shorter length scale) and high-surface-area-to-volume ratios that are favorable for heat transfer [154]. Such geometrical features are not practically achieved using traditional manufacturing techniques such as casting and injection molding [155,156]. These increase the power density of the system, requiring a smaller footprint and making latent heat TES technologies accessible to a wider range of applications such as residential and smaller scale commercial buildings. The additive manufacturing techniques that are discussed in this section allow for novel designs that cater toward efficiency and introduce the potential for modular, scalable systems for varying storage requirements and temperature ranges.

Additive manufacturing techniques range in different deposition methods and material mediums. The American Society of Testing and Materials categorizes such techniques into seven main groups of which the two most commonly applied in the manufacturing of low-temperature TES are vat-photopolymerization and material extrusion [157], both utilizing polymers as the medium. While polymers are less commonly used for HEs when compared to metals, they do exhibit beneficial behaviors such as their low cost, low weight, and their antifouling/anticorrosion properties [155,158]. Vat-photopolymerization encompasses methods such as digital light processing (DLP), liquid crystal display (LCD), and stereolithography (SLA) in which a photocurable resin is photopolymerized by a powerful light source in a layer-by-layer fashion. While the light medium varies across these methods, the resulting components exhibit qualities such as smooth surface finish, high precision, and resolution in relatively short manufacturing times [159]. Material extrusion methods such as the commonly used fused filament fabrication (FFF) rely on a layer-by-layer deposition in which a molten thermoplastic is extruded following the desired pattern [160]. FFF is typically inexpensive and operates within a wider range of materials when compared to other methods such as the aforementioned resin-based techniques, making it easily accessible for the fast prototyping of TES components. Both of these additive manufacturing categories exhibit unique advantages that can be tailored specifically toward heat-transfer-enhancing geometries crucial for the improvement of the current low-temperature TES systems.

FFF has been widely researched as a method for manufacturing TES systems due to their low cost, scalability, and wide material selection. Furthermore, significant effort has been dedicated to creating customized material blends that increase the effective latent heat of fusion of the components, directly improving the effective energy storage of the designed system [161–165]. A shape-stabilized composite material suitable for FFF 3D printing is achieved by introducing alternative phase change materials in their pure and/or encapsulated form and a host polymer, such as was introduced by the seminal work of Inaba and Tu [166]. The 3D printed functional composites have varying effective latent heats of fusion, mainly driven by the selected phase change material (PCM), the PCM wt%, and any substantial losses occurring along the manufacturing process. The polymer filaments that have been developed include high-density polyethylene (HDPE) [161,162,165–167], thermoplastic polyurethane (TPU) [164,168], nylon [163], polycaprolactone [169], and acrylonitrile-butadiene-styrene (ABS) [170,171]. Neat PCM blending, dispersion of microencapsulated PCMs, and copolymerization have all been explored as potential techniques for PCM incorporation into the host polymer [172]. With this material-focused approach, leakage, material stability during printing, and compatibility remain the biggest issues; however, the methods show great promise in the development of functional polymer-PCM composites for the additive manufacturing of TES systems using FFF.

While the precision of FFF and other extrusion-based methods is much lower than that of vat-photopolymerization, it still allows for the manufacturing of complex geometries that can improve overall system efficiency. A great example of these intricate designs is triply periodic minimal surface (TPMS) structures. These porous structures are a type of periodic implicit surface with zero mean curvature [173]. Additive manufacturing techniques, such as FFF, allow for the precise fabrication of these geometries which would otherwise be unachievable through traditional manufacturing methods. While FFF 3D printing of TPMS structures requires a large number of support structures that may affect the surface quality, it is still a suitable method as shown in the work of Singh et al. and Khan et al. [163,174]. With favorable material selection, such as ABS and dissolvable supports, FFF successfully produced the TPMS structures within reasonable quality. For the application of TES, their interconnected surfaces and controllable degree of porosity make these structures highly conducive to improved heat transfer. To evaluate the performance of TPMS structures in TES applications, several research groups have conducted numerical analyses of unique structures and their performance [175–177]. Due to the inherently high surface-area-to-volume ratio seen in TPMS structures, the numerical study done by Beer found that the presence of TPMS structures greatly increased the charging rate of their sodium acetate trihydrate-based TES unit [175]. Furthermore, the work done by Chen et al. focused on enhancing the heat transfer efficiency of a gyroid HE by implementing a gradient thickness design and found that HEs with gradient thicknesses heavily outperformed ordinary TPMS structures in heat transfer efficiency [177]. Additionally, less complex alterations that can enhance heat transfer, such as small pin fins, can be implemented in TES system designs [178]. In Mulholland’s work, 1.2 mm-diameter pin fins were easily implemented in an FFF print to enhance heat transfer in air-cooled HEs, showcasing the versatility of FFF in implementing design features that favor heat transfer in various magnitudes.

When cross-comparing additive manufacturing methods for TES applications, vat-photopolymerization techniques provide higher precision/resolution, an overall smoother surface finish, and reduced material waste [159,179]. While FFF is typically lower in cost, has a wide material selection, and is widely scalable, it lacks in print-quality. This shortcoming is seen in the poor resulting surface finish, weak interlayer bonding, and interlayer airgaps, all of which can present inconsistencies in performance and reduced system longevity. Because of these advantages, an increasing amount of research has been dedicated to the implementation of vat-photopolymerization techniques in TES. However, the photocurable resins used are extremely sensitive to optical and rheological modifications [159] resulting from introduced additives, making these methods more suitable for geometrical-based innovation rather than composite material development, such as what was achieved through FFF.

Because of the high precision associated with vat-photopolymerization methods, intricate geometrical features can be easily incorporated into HEs to enhance heat transfer performance in ways that would be difficult or impossible to fabricate through traditional methods [180]. Recent advances that result in greater accuracy in shorter printing times make these methods especially suitable for fast prototyping and design optimization of thermal systems [181]. Work done by Kirsch and Thole focused on several types of micro-cooling channels accurately replicated through SLA 3D printing [182]. The study found minimal deviation between the original design and the resulting 3D printed product, which also resulted in an agreement between the numerical and experimental results. Along the same lines, face-centered cubic lattice structures were manufactured through SLA and implemented in a rectangular channel by Liang et al. [183]. Three unit element geometries were fabricated and compared regarding heat transfer performance: circular, rectangular, and elliptical. The work highlights the ease of fabrication of such porous geometries that can be customized to decrease pressure drop and increase heat transfer in a wide number of applications. While FFF is suitable for complex geometries, such as the aforementioned TPMS structures that have recently gained popularity, vat-photopolymerization techniques such as DLP and SLA succeed in the areas where FFF falls short. Since the resolution of resin 3D printing is mainly driven by the light spot size, thin walls, microchannels, and high-surface-area designs are easily achievable and highly accurate. When fabricating TPMS structures with commercially available photocurable resin, Yu et al. found that the wall thickness, ranging between 0.50 and 1.50 mm, was 94% accurate across the geometry [184]. Furthermore, the predicted weight of the $50mm×50mm×50mm$ TPMS components deviated by a maximum of 2.89% across four samples of varying parameters. Similar results were achieved by Bagheri Saed et al. which employed DLP 3D printing for TPMS structures and optimized parameters for mechanical performance [185]. Both of these studies showcase the potential for further implementation of vat-photopolymerization in various engineering fields, including TES. Recently, investigators have been adding thermal energy storage capability to resin for vat photopolymerization (VPP) [186–188]

While additive manufacturing facilitates geometrical innovation, it also provides the opportunity to introduce additives and create customized composite materials tailored to the desired application. In the field of TES, most enhancements aim to increase the material’s effective thermal conductivity or effective latent heat of fusion, which directly correlates to the energy storage capacity of the material. Additionally, some additives can serve as structural reinforcement and improve mechanical performance. Ibrahim et al. successfully incorporated continuous carbon fibers in a nylon filament for the FFF 3D printing of polymer-based HEs [189]. In the study, directional dependency was explored and, in the case where heat transfer occurs in the direction of the fiber, thermal conductivity increased by nearly one order of magnitude. Combining these continuous fibers with additive manufacturing techniques also introduces the potential for controllable anisotropic conductivities [189]. Other nonmetallic fillers have been explored, such as boron nitride [190,191], aluminum nitride [192], and silicon carbide [193,194]. Such ceramic fillers are usually introduced when high thermal conductivity and electrical resistance are needed whereas carbon [189,195], graphene [196], and various metal particles [197–199] are used when electrical resistance is not required [194]. Su et al. prepared hydroxyl-functionalized boron nitride (OH-BN) particles aimed at increasing the thermal conductivity of the material by improving surface compatibility between the base TPU and OH-BN particles. The composites containing OH-BN particles exhibited a thermal conductivity of 2.5 times greater than nonfunctionalized boron nitride composites. Furthermore, the mechanical properties of the 3D-printed samples significantly improved and the boron nitride introduction did not affect the FFF process [190]. While the discussed works show that material additives can easily address some inherent material limitations, such as the low thermal conductivity of polymers, there are also some limiting factors that need to be considered when adapting this approach to TES. These include material compatibility, anisotropy, and filler loading capacities. Combining property-enhancing additives with additive manufacturing techniques enables the co-optimization of current TES systems, creating the next generation of latent heat TES systems.

Another area of application where thermal energy storage is of great importance and has the potential to significantly reduce greenhouse gas emissions is process heating. According to the Office of Industrial Technologies from the Department of Energy, process heating is the most significant source of energy use and greenhouse emissions in the industrial sector. Process heating refers to thermal energy used in the treatment or manufacturing of goods, involving processes such as drying, heat treating, metal forming, smelting, and steam generation, among others. Process heating consumes about 50% of all energy used by industry, where about 92% is obtained from fossil fuels [200]. Since approximately 50% of the most energy-intensive manufacturing processes in the US occur at temperatures of $300∘C$ or less [201], the adoption of solar industrial process heating, waste heat utilization, and thermal storage technologies that operate in this temperature range, provides an effective way of increasing decarbonization in the industrial sector. In particular, direct contact latent thermal storage systems are well suited to operate in this temperature range when integrated with medium-temperature solar thermal installations.

Due to the high energy storage density of latent heat storage, PCMs have been intensively studied, with inorganic salts and metallic alloys being used for applications that require storage temperatures above $300∘C$ [202]. A significant amount of research and development has been performed for low-temperature applications that require $TES<100∘C$⁠, such as energy conservation in buildings, as described in Sec. 5.2 and also, for TES temperatures higher than $200∘C$⁠, in applications related to power generation [203–206]. However, there is a large number of solar and waste heat recovery systems that operate in the moderate TES range between $100∘C$ and $150∘C$⁠, which require PCMs that match the operating temperature, have high heat of fusion and fast charge/discharge rates, are thermally stable and nontoxic [207]. In addition, conventional TES HE designs include shell-tube or fined tube configurations where a thermally conductive wall separates the PCM from the HTF, which leads to thermal resistance due to the HE wall and the conduction distance in the storage media. Alternatives to this design have been proposed where there is direct contact between the HTF and PCM, for instance in a packed-bed configuration.

Figure 4 shows the schematic of the PCM pellets (rectangular regions) in an HTF (other regions) and the corresponding implementation in a packed-bed configuration [208]. Representative values for pellets with a thickness of 4 mm result in heat transfer coefficients of the order of $500W/m2K$ compared to $75W/m2K$ for conventional packed-bed configurations, thus, resulting in a significant reduction of the convective resistance. A variety of polymeric materials such as HDPE have a melting temperature within a moderate range, featuring a large heat of fusion, with nontoxic and chemically stable characteristics, but the transition from solid to liquid in contact with the HTF means that an encapsulation material needs to be utilized that maintains the original geometry of the pellet. HDPE has been studied extensively as an encapsulation material [209,210], but Roy et al. utilized HDPE as the PCM with a glass fiber-reinforced epoxy coating to maintain the pellet’s shape in a packed-bed configuration [208]. Experimental results showed a thermal capacity of 160 kJ/g, with high discharging rates $>100W/kg$ and with exergetic efficiencies of approximately 79% for operating temperatures between $120∘C$ and $140∘C$⁠. Along the same line of research and with the intention of developing tubeless heat exchangers that operate at a similar temperature range (⁠$100−180∘C$⁠), Wang et al. utilized a SnBi58 alloy microencapsulated PCM/ceramic composite that achieved a $59.1MJ/m3$ heat storage density, a 30.4% increase compared to conventional ceramic-based heat exchangers [211]. A number of studies have numerically analyzed the effect of PCM particle size and flow distribution in packed-bed TES systems for low-temperature operating conditions (⁠$<100∘C$⁠). For process-heat temperatures, Junior et al. investigated the influence of nonuniform heat flow on different geometrical configurations of spheres melting in upstream and downstream external flows, utilizing Erythritol as PCM, which has a phase change temperature of $118∘C$⁠. Results showed that the temperature difference between the external flow and the phase change (⁠$ΔT=T∞−Tm$⁠) has a nonlinear effect on the melting rate, and the blockage ratio, i.e., the ratio between the distance between spheres and their diameter, has a minor effect on melting times [212]. Future directions in this area include taking advantage of the high energy density and swift charge/discharge capability of these energy storage systems to be implemented in large-scale solar thermal installations.

For the solar thermal energy utilization and storage to be applicable, subcomponents, such as the HEs are critical. Compact HEs emerge as critical thermal systems supporting such technologies. However, conventional techniques face challenges in meeting the current demands, including the necessity for increased heat transfer density, heightened effectiveness, minimal pressure drop, and reduced capital and operation and management costs. As a result, there is a need for innovative approaches to overcome the limitations inherent in the manufacturing processes of established heat exchangers.

Existing heat exchangers, such as printed circuited heat exchanger (PCHE), meet notable challenges. PCHE exhibits drawbacks like high-pressure drop [213], frequent fouling occurrences [214], maldistribution issues [215], and constraints in cross-sectional and channel shapes [216]. Similarly, plate-fin heat exchangers encounter limitations imposed by the current manufacturing processes, restricting the creation of intricate geometries. Moreover, their susceptibility to extreme conditions is restricted in brazing and welding processes [217]. While shell and tube heat exchangers remain popular [218]; the bonding process introduces potential issues, causing flow channel obstructions and creating localized stress concentrations at the weld of the manifold [219]. Addressing these challenges becomes imperative to enhance the efficiency and applicability of heat exchangers.

AM emerges as a novel process, enabling the utilization of conventional materials suitable for solar thermal energy and storage conditions while offering a broader spectrum of geometries. A diversity of applications is already applying AM heat exchangers in the aerospace, chemical, process, food, and pharmaceutical industries, Fig. 5 shows different examples of commercial heat exchangers produced using AM. In this process, raw materials in powdered form are positioned on a platform, and an energy source melts or sinters the material, layer by layer. Although various AM techniques exist, each with its unique advantages and disadvantages [224], the powder bed fusion (PBF) technique stands out as the predominant choice for manufacturing heat exchangers under extreme conditions. This technique, known for its efficacy, plays an essential role in enhancing thermal-hydraulic performance and expanding the geometrical possibilities of these critical components.

The PBF technique offers distinct advantages, notably a high-resolution sample coupled with commendable structural performance [225]. For the selective laser melting (SLM) technique the resolutions can reach up to $150μm$ [226]. The PBF technique proves to be ideal for manufacturing intricate geometries, thereby enhancing thermal-hydraulic performance [216]. Moreover, its versatility extends beyond the HE core, incorporating the production of manifolds and headers [227,228].

Several studies highlight the effectiveness of AM in enhancing high-pressure high-temperature (HPHT) HEs. Ahmadi et al. [229] developed a ceramic heat exchanger using the SLA technique. This 3D-printed Silica heat exchanger, featuring a lung-inspired geometry, was compared with an Alumina counter-flow heat exchanger exposed to temperatures reaching $700∘C$ and pressures of up to 300 kPa. The lung-inspired design exhibited superior thermal-hydraulic performance compared to the millichannel heat exchanger. The normalized pressure drop had an enhancement from 63% to 83%. In addition, the volumetric power density experienced an improvement from 61% to 73%. Güler et al. [230] also studied a lung-inspired heat exchanger using direct metal laser sintering (DMLS), comparing with a brazed plate heat exchanger, it performed up to 75.2% better in terms of heat transfer, and up to 31.8% in the pressure drop.

In the CSP application, Du et al. [231] employed SiC to create a semi-elliptical cross-sectional channel within a counter-flow heat exchanger, simulating conditions with molten salt as the hot fluid and supercritical carbon dioxide (⁠$sCO2$⁠) as the cold fluid. Operating at a hot side temperature of $750∘C$ and a cold side temperature of $540∘C$⁠, with a pressure of 20 MPa, the heat exchanger showcased its adaptability to extreme conditions. Notably, the developed heat exchanger uses the benefits of AM by integrating the header with the heat exchanger. This design choice not only reduced the number of joints but also contributed to an overall enhancement in structural performance.

Considering the advantages of AM for heat exchanger designs, Fuchs et al. [217] conducted experimental evaluations on three distinct heat exchangers featuring different fin types, operating at temperatures up to $700∘C$⁠. Employing the SLM technique and utilizing stainless steel (SS) 316L powder material, the study compared a conventional fin with two optimized fins. The optimized fins aimed to achieve specific objectives: one prioritizing increased heat transfer, and the other focusing on minimizing pressure drop. Results indicated that the reference fin exhibited a friction factor 15–20% higher than the optimized fins. The optimized fins showcased superior thermal efficiency, demonstrating a 5–9% performance improvement over the conventional fin. Thus, the creation of complex structures made possible by AM can lead to enhancements in the overall thermo-hydraulic performance of HPHT HEs.

Another geometry that has been largely studying for HE is TPMS [232,233]. Reynolds et al. [234] characterized additively manufactured heat exchangers with TPMS geometries, showing that the Nusselt number can be improved by 13% when compared to straight tube HE for Reynolds numbers between 100 and 2500. Additionally, Yan et al. [235] also showed the improvement in the thermal performance using TPMS when compared to conventional geometries. Notably, TPMS structures demonstrated the potential to mitigate fouling issues attributed to fluid momentum within the core, aligning with the findings of Sreedhar et al. [236], who reported a 38% reduction in biofouling. This demonstrates the benefits of TPMS-inspired designs in enhancing HPHT heat exchanger efficiency while addressing fouling concerns in extreme environments.

In the scope of CSP applications, TPMS geometries have started in recent years with Kelly et al. [237], which employed $ZrB2−MoSi2$ ceramic in the production of a HE utilizing Schwarz-D TPMS for CSP applications. The design achieved a power density of $50MW/m3$⁠. However, the study faced constraints due to the size limitations of the printer. The maximum capacity for a one-piece heat exchanger that can be printed once was restricted to 3 MW, underscoring the technological boundaries. Additionally, when considering binder jet printing, the investigation highlighted that the commercial printers had a maximum size of $0.16m3$⁠. The size limitations of 3D printers are very well described by Careri et al. [238].

The $sCO2$ power cycle and solar thermal energy systems are closely connected. In solar thermal energy systems, heat is captured from concentrated sunlight and stored in thermal energy storage systems. This stored heat can be converted into electricity using high-efficiency power cycles like the $sCO2$ power cycle. The $sCO2$ cycle is particularly well suited for solar thermal applications due to its high thermal efficiency at elevated temperatures and compact system design, making it ideal for integrating with solar energy systems that operate in extreme conditions.

Li et al. [239] extended the application of TPMS structures, inspired by biological forms, to heat exchangers within the $sCO2$ cycle. In their numerical evaluation, the study was conducted under maximum temperature and pressure conditions of $280∘C$ and 8.28 MPa, respectively, two distinct TPMS geometries, Schwarz-D and gyroid, were compared against a zigzag PCHE. TPMS structures exhibited enhancements in heat transfer coefficients ranging from 16% to 120%. However, the hydraulic performance experienced a decline, increasing the friction loss by 50–100%. Despite this trade-off, the overall performance of TPMS-based heat exchangers surpassed that of traditional PCHEs by 15–100%.

Table 1 presents the summary of the main heat exchangers produced via additive manufacturing, showing their maximum pressure and temperature condition, AM technique, material, type of study, geometry, and applications.

The structural integrity of heat exchangers is important, particularly in extreme conditions. Zilio et al. [242] conducted hydrostatic tests, subjecting a heat exchanger to pressures up to 70 MPa at room temperature. Notably, the SS 316L-produced HE exhibited structural integrity with no leakage observed. However, the influence of temperature on structural behavior necessitates a comprehensive analysis under combined high-pressure and high-temperature conditions. Ma et al. [243] addressed this concern by simulating an AM-produced HE in a $sCO2$ power cycle environment. The simulation involved a low-pressure side at $1100∘C$ (at 8 MPa) and a high-pressure side at 24 MPa (at $300∘C$⁠). Employing the ASME Boiler & Pressure Vessel Code Section VIII, Div. 2. offers a recognized method for analyzing heat exchangers [244,248]. Despite this, there remains a critical need for more experimental tests specifically evaluating HPHT conditions for HEs manufactured via AM. Additionally, the assessment of thermal-mechanical fatigue becomes crucial due to varying process environments, start/stop cycles, and other related conditions. Liang et al. [249] highlighted the dynamic nature of HPHT conditions, illustrating rapid changes of $750∘C$ in temperature and 3.6 MPa in pressure within 3 s in a reactor. Maleki et al. [250] further investigated potential challenges, revealing that distinct AM techniques and surface roughness significantly influence mechanical fatigue life cycles. Their results emphasize the importance of such evaluations to uncover potential hidden issues within the AM technique under demanding operational conditions.

When considering materials applying AM for solar-applied heat exchangers, three predominant categories emerge: ceramic, stainless steel, and nickel superalloy. SS 316L, as employed by Refs. [217,242,245], demonstrates reliability under both high pressure (70 MPa; [242]) and high temperature (⁠$700∘C$⁠; [217]) conditions, aligning with the operational parameters for $sCO2$ applications. However, caution is warranted, as material properties may have degradation when exposed to temperatures surpassing $800∘C$ [251]. Nickel superalloys are widely employed for HPHT applications [227,228,240,252]. Battaglia et al. [227,252] studied Inconel 718 through the DMLS technique. These studies applied Inconel 718 to withstand high temperatures (maximum of $600∘C$⁠; [227]) and low pressures (maximum of 450 kPa; [228]). However, as indicated by Kim et al. [253], the maximum working temperature for Inconel 718 is $900∘C$⁠. Introducing a novel nickel superalloy material for AM, Rasouli et al. [240] utilized H282 at $720∘C$ and 20 MPa, specifically for CSP and $sCO2$ power cycle applications. Otto et al. [254] evaluated the performance of H282 and Inconel 718 in additive manufacturing for high-temperature applications.

Ceramic materials have emerged as a prominent choice for solar thermal energy conditions, because of their mechanical resistance, corrosion resilience, and resistance to abrasion and wear. Numerous HPHT HE studies have explored the advantages of ceramics as the primary material in additive manufacturing processes [229,231,237,243,255]. Except for Kelly et al. [237], which stands out with its distinct approach, other studies predominantly utilized SiC as the raw material. They studied thermal storage applications for concentrating solar power, employing $ZrB2−MoSi2$ in the ceramic formulation, showcasing the versatility of ceramics in diverse HPHT scenarios. Techniques such as binder jet and SLA were employed for the AM process. Ma et al. [243] incorporate zirconium diboride (⁠$ZrB2$⁠) into the ceramic composition. The study was conducted under conditions reaching a maximum temperature of $1100∘C$ and a pressure of 25 MPa. Furthermore, the utilization of SiC in AM has been extensively reviewed by Boretti and Castelletto [256]. The application of ceramic matrix composites to enhance resistance to fracture while maintaining high-temperature stability, mechanical strength, and chemical resistance, using the incorporation of fibers or nanoparticles within the ceramic matrix, has been widely studied during the recent years [257–259].

Despite the advantages of additive manufacturing, the technology faces challenges. The surface roughness is usually significantly high, which can range from 5 to $60μm$ [226]. The surface roughness is related to the specific AM technique employed. Studies have commonly explored post-treatment options to mitigate surface irregularities. A comprehensive study by Maleki et al. [250] delved into various post-treatment methods, offering insights into their effectiveness across diverse materials and AM techniques. Additionally, the significance of the printing direction should not be overlooked as it exerts an influence on critical factors such as channel diameters [260,261], surface roughness [262,263], wall thickness [264], concentricity [261], and circularity [261]. These factors impact key parameters, in particular Reynolds number, Nusselt number, friction factor, and overall pressure drop and heat transfer [265]. This interplay of variables underscores the importance of optimizing the AM process to improve heat exchanger performance, aligning with the broader exploration of advanced heat exchanger designs discussed previously.

While the novelty of AM holds promise for HE in solar applications, there are concerns regarding its economic viability for real-world implementations. Addressing these concerns, Rasouli et al. [240] have undertaken an assessment of manufacturing parameters and heat exchanger designs to evaluate the manufacturing cost. The investigation reduced the cost of an AM heat exchanger designed for CSP applications from $780/kW-th to $570/kW-th, with potential further reductions to $270/kW-th. Notably, the study employed Haynes 282, a nickel superalloy, as the raw material. The HE operated at a low pressure of 100 kPa with a temperature of $720∘C$ on one side and a high pressure of 20 MPa with a temperature of $500∘C$ on the other. Das et al. [246] also studied a Haynes 282 3D printed HE for CSP applications, reaching a cost of $19.2/kW. A recuperative HE for the $sCO2$ power plant was $60/kW, but the operating temperature is $343∘C$⁠.

Complementary, the research by Caccia et al. [241] investigated the cost evaluation of AM-produced heat exchangers, utilizing ZrC/W ceramic as the raw material specifically for CSP plants. The heat exchanger, operating at $750∘C$ K and 20 MPa, demonstrated a cost of $44/kW. This cost comparison revealed competitive pricing when compared with traditional stainless steel (SS 316L) and nickel superalloy PCHE, both with lower power density but similar costs, priced at $44/kW and $41/kW, respectively. These studies emphasize the economic advantages of AM-produced HPHT heat exchangers, positioning them as increasingly viable options for extreme applications.

Additive manufacturing has the potential to achieve high efficiency in heat exchangers by enabling complex geometries. Intricate designs such as lung-inspired configurations, Schwarz-D TPMS, and optimized fins aim to enhance thermal-hydraulic performance. Studies have demonstrated improvements in heat transfer coefficient and pressure drop using such designs. However, several concerns have arisen. Manufacturing variables significantly influence channel diameters, surface roughness, wall thickness, concentricity, and circularity, all of which directly affect the thermal-hydraulic performance of the heat exchanger. Additionally, how the AM can enhance the fouling resistance by using new concepts.

Although studies of heat exchangers applied for solar thermal energy utilization and storage have been emerging in the past decade, the field faces several challenges that necessitate further research. In terms of complex channels, innovative approaches using nature-inspired configurations could enhance the thermal-hydraulic behavior of heat exchangers. Additionally, channel entry designs could improve flow distribution by restricting the mass flowrate in central channels, which typically receive more fluid, and allow more flow in adjacent areas, as done for plate heat exchangers [266].

Additionally, the economic feasibility of implementing AM in heat exchangers for solar applications remains a concern. Nevertheless, studies have shown progress in this area, highlighting potential cost savings and efficiency gains. While cost considerations have been studied, there is a need for context-specific studies focused on mass production. A cost-effective comparison of different AM techniques and manufacturing parameters is crucial to identify potential paths toward real-world implementation.

Structural numerical analysis at temperatures up to $1100∘C$ and pressures up to 2.4 MPa has demonstrated the reliability of heat exchangers under static pressure conditions. The materials employed, such as ceramics, stainless steel, and nickel superalloys, play a crucial role in withstanding these extreme conditions. Overall, while additive manufacturing offers promising advancements for HPHT heat exchangers, further research and development are needed to address the remaining challenges and ensure widespread implementation.

More experimental tests regarding the structural aspects should be conducted, emphasizing mechanical and thermal fatigue, and the influence of HPHT conditions on surface roughness, anisometric features, and discontinuous geometries. The consistency of the powder used in AM is another significant issue, as it may cause production inconsistencies. Addressing these concerns through meticulous research and development will be essential for advancing AM heat exchangers and realizing their full potential in solar thermal energy utilization and storage applications.

Based on the first law of thermodynamics, the performance evaluation follows an energy-based criterion and the most common indices are the thermal efficiency (⁠$ηI$⁠) and the COP [267]. Thermodynamic systems receiving and delivering several forms of energy can be broadly divided into two main types: direct heat engines and reversed heat engines. Direct heat engines whose function is to convert a portion of the primary energy input into work, typically use thermal efficiency (⁠$ηI$⁠) as a performance index. On the other hand, reversed heat engines whose function is transferring heat from a low-temperature reservoir to a high-temperature reservoir by consuming energy, typically use the COP as a performance index [268].

The debate over the ability of assessing the performance of multigeneration systems is intrinsically resulting from combining different types of heat engines mentioned earlier (i.e., direct and reversed heat engines) into one system. Not only are the performance indices of each type of these engines different, but also the energy quality (or grade) of their products (e.g., power, heating, or cooling) is different. Both mechanical and electrical energies are considered higher-grade forms of energy that are produced by a direct heat engine. On the other hand, compared to electricity, cooling or heating are lower-grade forms of energy produced by reversed heat engines. Each type of these two heat engines utilizes a different performance index (i.e., the thermal efficiency and the COP), and both indices are inherently different and cannot be added together. Furthermore, the variation in the energy quality (grade) of energies produced by a multigeneration system prevents adding both quantities to each other without considering their respective qualities (grades) [270]. Thus, different researchers have discussed various approaches to overcome these issues. In this section, the available performance assessment methodologies and indices of multigeneration systems will be reviewed in order to determine their related limitations and issues. This will help guide us to the most accurate and meaningful performance index for assessing the performance of multigeneration systems.

The RAI can be thought of as an exergetic version of the primary $ζen$⁠. In this section, the exergy saving ratio (⁠$ζex$⁠) will refer to the RAI. As mentioned earlier, the energy and exergy saving ratios are considered reliable indicators of the goodness of multigeneration. However, they cannot be considered performance indices and cannot be used for comparing multigeneration systems with any single-generation system.

By comparing the results of these indices with the other results obtained from other common performance indices and methods, Alghamdi and Sherif [294] found the following:

• The trends of the exergetic and first-law efficiencies, obtained by Eqs. (6) and (7), respectively, conform with the trends of the energy and exergy saving ratios. Accordingly, the trends of the equivalent efficiencies, calculated as per Eqs. (16) and (18), respectively, conform with the trends of these efficiencies because $ηeqv$ and $ψP,eqv$ are functions of $ζen$ and $ζex$⁠, respectively.

• Since the saving ratios are an indication of the goodness of the combined systems, the exergetic efficiency (⁠$ψ$⁠), conforming with the saving ratios and accounting for the quality of the consumed and produced energies, can be considered as an indication of the goodness of the multigeneration system. Thus, the exergetic efficiency, that is simply and directly calculated as per Eq. (7), can be used for purposes of comparisons among similar multigeneration systems and for optimizing the performance of such systems.

• The performance indices using a weighting factor for the cooling production such as $ηI,eff$ and $ψeff$⁠, obtained by Eqs. (8) and (9), respectively, can be misleading when the cooling production, for example, is much more than the power production. However, when the production of the lower-grade product (e.g., cooling or heating) is low compared to the production of power, the technique of using a weighting factor can acceptably be used.

It can be concluded that the performance of multigeneration systems cannot be directly determined due to the variations among the types of subsystems comprising the multigeneration system along with the differences in the energy quality of the products that these subsystems produce. This issue is present whenever we attempt to assess the performance of multigeneration systems. Multiple methods and expressions have been proposed by different researchers to overcome this problem. Investigating the available performance indices yielded some main findings which are summarized below:

• The exergetic and first-law efficiencies calculated based on Eqs. (6) and (7) are thermodynamically correct and can be used for comparison among multigeneration systems that produce the same products. They can also be used for optimizing these systems. However, these two equations cannot be used for comparing multigeneration systems that produce different products.

• The relative energy and exergy saving ratios are reliable indices to examine the goodness of multigeneration systems.

• The performance indices rely on calculating an equivalent electricity for the thermal effect produced by a multigeneration system such as $ηI,eff$⁠, EEE, and $ψeff$ can be misleading in some cases.

• For comparison among multigeneration systems whose products are similar or different, the equivalent efficiencies can be used as long as the multigeneration systems being compared have at least one common product.

The increasing demand for modern energy services, together with the increasing concerns of climate change due to global warming, requires the deployment of carefully designed and optimized energy systems. The urgent need for efficient, renewable, and sustainable energy systems is underpinned by the impending catastrophic consequences of climate change if temperatures increase by more than $1.5∘C$ above pre-industrial levels [295]. From an engineering perspective, the performance of energy systems tends to be quantified by thermodynamics metrics (first and second-law efficiencies). In such a context, it is important to link the enabling heat and mass transfer processes to the overall energy system’s thermodynamic performance. This section will cover recent studies that rely on the theoretical frameworks of the second law of thermodynamics and the constructal theory to design and enhance heat and mass transfer in energy systems. The analytical and numerical techniques of incorporating the second law into the analysis of heat and mass transfer processes via the entropy generation minimization method to determine optimal configurations and operating conditions will be presented. Moreover, the review section will summarize recent (last 5 years) methodological and theoretical improvements, as well as highlight relevant applications.

In Eq. (24), $sgen‴$ is the entropy generation per unit volume. Similar to Eq. (23) this equation has two parts: the first part represents the irreversibilities that arise from heat transfer across a finite temperature difference, while the second one is the heat transfer due to fluid friction. Equation (24) has been extended by Kock and Herwig [300,301,309] to the analysis of turbulent flows in complex geometries. They also showed how the entropy generation can be implemented in CFD codes [309]. The detailed equations can be found in Ref. [301]. At the core of the entropy generation minimization method is the determination of the irreversibilities occurring in processes, devices, and systems and minimizing them. The irreversibilities arise from the entropy generation rates as given by Eqs. (23) and (24). In his various texts on thermodynamic optimization, various applications have been pointed out [296–298]. In heat transfer applications, the entropy generation minimization method has been used to optimize various heat transfer enhancement techniques. This requires determining the entropy generation rates for enhanced and nonenhanced configurations of the heat exchangers. Such that the enhancement entropy generation number, $Ns,a=Sgen,en/Sgen,o$⁠, which compares enhanced and nonenhanced configurations, is less than 1 for thermodynamically optimal configurations. In Secs. 8.3 and 8.4, the review of exergy analysis and entropy generation in solar thermal systems is presented.

The second law of thermodynamics has been widely used to optimize thermal energy systems. The exergy analysis approach to applying the second law uses the fundamentals of thermodynamics through Eq. (22) to determine the irreversibilities in thermal systems. This approach has been extensively applied to solar thermal systems, including low-temperature systems and complex high-temperature concentrating systems. Moreover, researchers have applied this approach to system components as well as entire solar thermal systems [310–312]. Regardless of the system or component considered, Eq. (22) or its variants are used. Researchers have applied the second law of thermodynamics using the exergy analysis approach to several low-temperature solar thermal collector configurations, including air collectors, flat plate collectors, evacuated tube collectors, and direct expansion solar collectors. Koholé et al. [313] developed a detailed exergy model of a flat-plate solar collector. They showed that increasing the number of tubes from one to sixteen increases energy and exergy efficiency by 34.61% and 92.95%, respectively. They showed that the irreversibilities were greater for the absorber plate than for the other components of the collector. Venkatesh et al. [312] investigated the thermal and exergy performance of a solar air heater enhanced with phase change material. In this study, they defined exergy efficiency as the ratio of the exergy out to the exergy entering the air heater to evaluate the performance of different models of air heaters. In a recent study, Alrashidi et al. [314] investigated the energy and exergy performance of an innovative finned plate solar air heater. They showed maximum energy and exergy efficiencies of 38.2% and 26.4% at noon for a tilt angle of $20$ deg. In another recent study, the energy, exergy, and enviro-economic performance of a solar air heater with porous wire mesh was investigated [315]. They used exergy efficiency to show the configuration that performs the best (minimum destruction of available work). The second-law analysis based on exergy analysis has also been used to determine the performance of high-temperature solar collectors. Barghi Jahromi et al. [316] investigated the energetic and exergetic performance of a solar parabolic dish collector. The maximum energy efficiency was 40.85%, while the maximum exergy efficiency was 47.3%. Issa and Thirunavukkarasu [317] determined the energy and exergy performance of a solar parabolic dish integrated with thermal energy storage. They showed peak energy and exergy efficiencies of 79.37% and 11.12%, respectively. Zaharil and Yang [318] conducted a fundamental investigation on the use of supercritical carbon dioxide in parabolic trough solar collectors. They considered the energy, exergy, and entropy behavior in their analysis. In addition to analyses for solar collectors in high-temperature systems, several researchers have investigated the overall exergetic performance of solar thermal systems for different applications, such as parabolic dishes in a steam power plant. Regardless of the type of the system, the starting equation for exergy analysis is (22) together with the relevant governing equations for the relevant component or system. Through most of the studies, the conditions for optimal energy efficiency do not always align with those of maximum exergy efficiency. There is still a need for studies at the fundamental level to develop a unified approach for the characterization and optimization of solar thermal systems for aligned optimal performance with both the first and second laws. Moreover, the exergetic efficiency is always shown to be lower than the energetic efficiencies, showing opportunities for further system enhancement to reduce irreversibilities and exergy losses in solar thermal systems.

Another related approach for the application of the second law in solar thermal systems is through the entropy generation minimization method. Although the goal of the two approaches is the same, the entropy generation minimization method can be applied directly to optimize systems with complex geometries and boundary conditions in combination with computation fluid dynamics. In this section, we present the use of the second law to determine and minimize the entropy generation rates in solar thermal systems. First, a review of studies on entropy generation in low-temperature solar thermal systems is presented, and then the works on entropy generation in high-temperature solar thermal systems are presented.

The application of the entropy generation minimization method to the study and optimization of high-temperature solar thermal systems becomes important owing to the high finite temperature difference. Typical high-temperature solar thermal systems involve the concentration of solar radiation on a smaller area, resulting in high temperatures and, thus, high heat transfer irreversibility. Moreover, owing to the very nature of concentrating solar thermal systems, the heat flux on the receivers is nonuniform, further leading to higher heat transfer irreversibilities [327]. The entropy generation minimization method has also been applied to the study of heat transfer enhancement in concentrating solar thermal systems [29,328,329]. Flesch et al. [330] considered the entropy generation analytically using the approach in Eq. (23) for a high-temperature solar thermal system using solar salt, sodium, and lead-bismuth eutectic as the heat transfer fluids with a constant receiver tube heat flux. However, for concentrating solar thermal systems, where the heat transfer is nonuniform, Eq. (23) and an assumption of constant heat flux can lead to under prediction of the entropy generation rates, as shown in Ref. [328]. Mwesigye et al. [328] determined the entropy generation rates in parabolic trough systems at different rim angles and concentration ratios. They showed that the irreversibilities due to heat transfer increase as the rim angle reduces and as the concentration ratio increases. As Fig. 6 shows, the higher rim angles give a more uniform heat flux and temperature distribution around the circumference of the receiver’s absorber tube.

The purpose of the second-law analysis is to identify where most losses occur in processes and systems and propose designs to reduce these losses and improve performance. Many studies are now available in the literature on the minimization of entropy generation rates in high-temperature solar systems with heat transfer enhancement. Unlike most studies on heat transfer enhancement, where the improvement in performance is shown by heat transfer and pressure drops, with the second law, the percent reduction in the irreversibilities, as well as conditions for minimum entropy generation rates, can be identified [330,331]. Moreover, owing to the nonuniform heat fluxes in concentrating systems, the approach of determining the entropy generation rates locally in Eq. (24) is used together with computational fluid dynamics tools. Using this approach, Mwesigye et al. [331] showed that the entropy generation rates in a parabolic trough receiver could be reduced by up to 58% using twisted table inserts placed separately from the receiver tube wall. Oketola and Mwesigye [332] also showed a reduction in entropy generation rates with the use of novel multi-twisted tape inserts. Liu et al. [333] investigated the entropy generation in a parabolic trough receiver equipped with conical strip inserts. They showed that with the heat transfer enhancement, the entropy generation rate is reduced by 74.2%. Pazarlıoğlu et al. [334] evaluated the exergetic, energetic, and entropy generation in a parabolic trough system with an elliptical dimpled receiver tube and a hybrid nanofluid. The enhancement showed a reduction in the entropy generation rate by 16.82%. The study used the analytical equation for the determination of entropy generation rates. Recently, Wang et al. [335] numerically investigated the entropy generation in the absorber tube of a parabolic trough solar collector equipped with eccentric structure inserts. Performance improvement was shown by the ability of the insert to reduce the Bejan number, which is defined as the ratio of the entropy generation due to heat transfer to the total entropy generation rate. In the analysis of entropy generation rates due to heat transfer and fluid flow in linear receivers of concentrating solar thermal systems, such as receivers in parabolic trough systems, it has been demonstrated that the fluid flow irreversibility (⁠$Sgen′)F$ and the heat transfer irreversibility (⁠$Sgen′)H$ are always conflicting, leading to an optimal condition that minimizes entropy generation. In general, the heat transfer irreversibility will reduce with increasing flowrates/Reynolds numbers as the finite temperature differences reduce, while the fluid flow irreversibility increases as the flowrate/Reynolds increase. This was demonstrated in Ref. [336] in a study of different liquid heat transfer fluids for high-concentration parabolic trough systems. The entropy generation and the optimal Reynolds number, in this case, as shown in Fig. 7, depend on the heat transfer fluid used. For less viscous heat transfer fluids, the fluid flow irreversibility is much lower and becomes more than the heat transfer irreversibility at much higher flowrates/Reynolds numbers.

In general, the determination of entropy generation rates shows the presence of optimal operating conditions at which the entropy generation rates are minimum [29,332,337]. However, this might not align with the system’s requirements for the given application. For example, in parabolic trough systems where the mass flowrate is a design parameter to ensure a specified outlet temperature based on the incident solar radiation, the optimal Reynolds number or flow may be larger than the Reynolds number at the design or operation mass flowrate. In either case, the flowrates in actual systems always lead to lower entropy generation rates from the fluid friction irreversibility, thus reducing the pumping power requirements.

Proposed by Bejan [338,339], the constructal law states that “For a finite-size system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed currents that flow through it,” along with constructal theories developed around the law, have gained an important presence in the energy conversion and heat transfer literature, including solar technologies. Initially, the constructal law was invoked to develop theories explaining the organization of structures used to connect area-to-point flows, like those of street networks that connect a city (area) to a central point (e.g., market) that facilitates the flow of inhabitants [338] and heat-conducting structures that facilitate the flow of heat from a two-dimensional domain occupied by heat-generating components and a point (sink) [339]. Since then, the use of the Constructal law has been expanding, initially in the fields of thermodynamics [340,341] and heat transfer [342,343] and now into multiple fields including economics [344], ecology, and architecture [345].

Some major groups of studies can be identified in the literature. The first and larger one is formed by studies that seek to improve the performance of engineering systems and link their design and optimization methods to the constructal law. Some relevant examples within solar energy conversion fall within this category, including for example Ref. [346], which optimized a flat plate collector with conventional and constructal-like configurations. The study found that the Pareto front indicated that the constructal designs, in general, led to higher efficiencies. In Ref. [347], a constructal approach is used to optimize the geometry of a solar chimney for maximum power production under fixed area and volume constraints. Among the results, the authors identify geometrical ranges for sveltness (Sv) beyond which the friction losses in the solar collector can be neglected. Morega et al. [348] formulated and documented the optimization results of arranging ensembles of honeycomb spherical photovoltaic cells, in a sequence of “constructs.” Starting with an optimized elemental cell, the higher-level constructs were created in a time arrow sequence, from lower to higher-order structures, employing a constructal law-guided building process. The study found that the approach led to all constructs exhibiting almost the same series resistance and structures of busbars that evolved naturally, following deterministic principles, to display appealing finger-like structures.

In power generation, Feng et al. [349] use a constructal design approach to minimize a cost function accounting for entropy generation and power consumption for a marine boiler. The study reveals a three-way minimization from variations of the outer diameters of the evaporator, superheater, and economizer. In the field of ocean energy conversion, [350] optimized the geometry of an overtopping wave energy converter. The degrees-of-freedom were the ratio between the height and length of the ramp and the distance between the bottom of the wave tank and the device and the objective function was the available power. The results indicate that the optimal shapes are dependent on the wave conditions. In the field of power and refrigeration, Ref. [351] used a construtal design approach to optimize the internal structure and operational parameters of a vapor-compression refrigeration system. Optimal area allocations in the HEs were identified.

A second group of studies has developed constructal theories around the Constructal Law. This includes efforts to explain global circulation and climate [352], the unification of scale effects in animal locomotion [353], a theory explaining shapes in tree canopies and root systems [354], and a theory of the hierarchy of bodies in space [355]. Some articles in this group focus on developing theoretical frameworks to explain the physics of evolution, as pointed out by [356].

Another group of studies has been focused on investigating and documenting the directional evolution of technology. The fundamental trade-off between efficiency gains due to size increase in the systems that enable the flight to compete with the penalty associated with carrying them on board was presented by Ordóñez and Bejan [357] in the context of aircraft and their energy systems. This trade-off is used in Ref. [358] to explain the evolution in size of commercial aircraft. In addition, Bejan et al. [358] develop arguments to support that the scaling rules observed in animal fliers are the same that are observed in aircraft. These groups are not disjoint and as the constructal law continues its journey to maturity by being exposed to further testing [359], its role in the thermal sciences will continue to evolve.

In this section, a review of the second law and constructal theory approaches to the analysis of solar thermal systems is presented. For low-temperature and high-temperature solar thermal systems, the review shows that using entropy generation minimization and exergy analysis is a better way to optimize their performance. The irreversibilities and location of exergy destruction in solar thermal systems can be identified and improvements made. Two approaches for the determination of entropy generation rates are common in the literature: one numerical and one analytical. Future studies should compare the deviation of the results of the two approaches and an accurate one recommended. This is mainly important because concentrating systems receive nonuniform heat fluxes, while the analytical approach uses Nusselt numbers based on uniform heat fluxes. In addition, integrating equations for the determination of the entropy generation rates in computation tools can increase the use of approaches that are accurate for systems with complex boundary conditions and complex geometries.

This article presents a holistic view of key innovations and ongoing research efforts aimed at enhancing solar thermal energy utilization and storage across seven topics. First, the recent heat transfer enhancements for modern CSP systems highlight novel approaches to elevate the receiver temperature, improve heat transfer inside absorber tubes, and reduce radiative losses with advanced coatings and selective surfaces. These developments enable higher-efficiency power cycles and expand CSP’s potential in industrial process heating. Second, solar-driven cooling technologies continue to make strides through PV- and thermal-based approaches, including absorption and adsorption cycles as well as hybrid desiccant systems, all of which reduce peak electricity demand and leverage solar energy to meet growing cooling requirements sustainably. Third, improvements in sensible thermal energy storage—particularly innovations in tank stratification and baffle designs–demonstrate that smart, passive configurations can substantially raise heat exchanger performance without sacrificing simplicity or cost-effectiveness. Fourth, in next-generation latent heat storage, additive manufacturing and tailored composite materials are ushering in high-power density solutions for compact ice- and PCM-based systems that can serve both building applications and industrial process heating needs. Fifth, the adoption of additively manufactured heat exchangers points toward intricate, customizable geometries (such as triply periodic minimal surfaces) and advanced materials (including ceramics and metal alloys) to deliver higher heat transfer density, structural stability, and reduced fouling under extreme conditions—critical factors for $sCO2$ cycles and other high-temperature processes. Sixth, the performance evaluation of multigeneration systems emphasizes that rigorous thermodynamic metrics (incorporating exergy, equivalent efficiencies, and primary energy-saving ratios) are essential to fairly assess integrated systems’ effectiveness when producing power, heating, cooling, or water. Finally, the application of the second law of thermodynamics and the constructal theory provides a powerful theoretical framework to minimize entropy generation. This fuels the design optimization of solar thermal components and drives the broader effort to link fundamental transport phenomena with overall system performance.

Taken together, the collective advances in hardware, materials, and rigorous thermodynamic analysis showcased throughout these seven focal areas underscore a central theme: solar thermal energy systems, aided by next-generation designs and manufacturing methods, are poised to play an increasingly significant role in meeting both heat and power demands sustainably. By refining heat transfer processes, storage strategies, and evaluation metrics, researchers and industry practitioners can unlock greater reliability and cost-effectiveness in solar thermal applications, which will enable cleaner, more flexible, and higher-efficiency energy systems for a decarbonized future.

The K-6 Committee sponsors topics and sessions at two annual ASME conferences: The International Mechanical Engineering Congress and Exposition (IMECE) and the Summer Heat Transfer (SHTC). Researchers from all fields are welcome to attend the conferences and learn/share more about new advances in other topics as well as those covered in this review article.

The authors attest that all data for this study are included in the article.

Dr. Li led Sec. 2. Dr. Najafi contributed to Sec. 3. Dr. Nicodemus completed Sec. 4. Dr. Boetcher along with her graduate students, Melendez and Morgan, and Dr. Diaz led the preparation of Sec. 5. Dr. Samadi and her graduate student, P. P. da Silva, led Sec. 6. Section 7 was prepared by Drs. Alghamdi and Sherif. Drs. Mwesigye and Ordonez led Sec. 8. Drs. Chen and Lee led the efforts in creating this review article and integrating all topics into one review article.

$h$ =

specific enthalpy (kJ/kg)

$k$ =

thermal conductivity $(W/(mK))$

$p$ =

pressure (kPa or MPa)

$r$ =

radius (m)

$s$ =

specific entropy $(kJ/(Kkg))$

$t$ =

wall thickness (m)

$m˙$ =

mass flowrate (kg/s)

$A$ =

area (m$2$⁠)

$L$ =

thickness or length (m)

$Q$ =

thermal energy (kJ)

$R$ =

thermal resistance (K/W)

$S$ =

entropy (kJ/K)

$T$ =

temperature (K)

$W$ =

shaft work energy (kJ)

$E˙$ =

energy rate (kW)

$Q˙$ =

heat transfer rate (kW)

$S˙$ =

entropy generation rate (kW/K)

$W˙$ =

work rate (kW)

$E˙x$ =

exergy rate (kW)

$Δ$ =

difference

$ζ$ =

saving ratio

$η$ =

efficiency

$μ$ =

viscosity

$ρ$ =

density

$ψ$ =

exergetic efficiency

$∘$ =

state at ambient

$b$ =

black body

$i$ =

$i$th layer or inner

$o$ =

outer

$s$ =

species

$w$ =

wall

$C$ =

cooling

$E$ =

Earth

$H$ =

heating

$P$ =

power

$S$ =

Sun

I =

first law of thermodynamics

II =

second law of thermodynamics

eff =

effective

en =

energetic

eqv =

equivalent

ex =

exergetic

gen =

generation

in =

input

out =

output

ref =

reference

CCHP =

combined cooling, heating, and power

CCP =

combined cooling and power

CHP =

combined heating and power

$′$ =

per unit length

$‴$ =

per unit volume

AM =

additive manufacturing

BPVC =

boiler and pressure vessel code

CCDP =

combined cooling, desalination, and power

CCHP =

combined cooling, heating, and power

CCP =

combined cooling and power

CFD =

computational fluid dynamics

CHP =

combined heating and power

COP =

coefficient of performance

DLP =

digital light processing

DMLS =

direct metal laser sintering

EEE =

equivalent electrical efficiency

EUF =

energy utilization factor

FFF =

fused filament fabrication

GOR =

gain output ratio

GW =

gigawatt

GWP =

global warming potential

HE =

heat exchanger

HPHT =

high-pressure high temperature

HTF =

heat transfer fluid

HVAC =

heating, ventilation, and air conditioning

IEA =

International Energy Agency

IPCC =

Intergovernmental Panel on Climate Change

LCD =

liquid crystal display

LDED =

laser-directed energy deposition

MMC =

manifold microchannel

MW =

megawatt

OSUC =

octet-shaped unit cells

PBF =

powder bed fusion

PCHE =

printed circuited heat exchanger

PESR =

primary energy saving ratio

PFHE =

plate fin heat exchanger

PURPA =

Public Utility Regulatory Policies Act

PV =

photovoltaic

PV-TE =

photovoltaic-thermoelectric

PVAC =

photovoltaic-powered air conditioning

RAI =

relative avoided irreversibility

RCBC =

recompression closed Brayton cycle

SCC =

specific cooling capacity

StCCSC =

straight circular cross-sectional channel

SDEAC =

solar-driven desiccant evaporative air cooling

SiRCSC =

sinusoidal rectangular cross-sectional channel

SLA =

stereolithography

SLM =

selective laser melting

SPVC =

spherical photovoltaic cells

SSECSC =

straight semi-elliptical cross-sectional channel

SCCSC =

straight circular cross-sectional channel

STEP =

supercritical transformational electric power

STHE =

shell tube heat exchanger

StRCSC =

straight rectangular cross-sectional channel

Sv =

sveltness

SwRI =

Southwest Research Institute

TES =

thermal energy storage

TPMS =

triply periodic minimal surface

VPP =

vat photopolymerization

$H2$ =

hydrogen

ABS =

acrylonitrile-butadiene-styrene

$CaCO3$ =

calcium carbonate

CaO =

calcium oxide

$CeO2$ =

ceria

CMC =

ceramic matrix composite

$Co3O4$ =

cobalt oxide

$CO2$ =

carbon dioxide

$H2O$ =

water

H282 =

HAYNE^® 282^® nickel alloy

HDPE =

high-density polyethylene

KCl =

potassium chloride

Li–Br =

lithium bromide

LSM =

lanthanum strontium manganite oxide

Mar M247 =

superalloy Mar M 247^TM

$MgCl2$ =

magnesium chloride eutectic salt

$MoSi2$ =

molybdenum silicide

NaCl =

sodium chloride

$NH3$ =

ammonia

OH-BN =

hydroxyl-functionalized boron nitride

PCL =

polycarbonate

PCM =

phase change material

$sCO2$ =

supercritical carbon dioxide

SiC =

silicon carbide

SS 316L =

stainless steel 316L

TPU =

thermoplastic polyurethane

$ZnCl2$ =

zinc chloride eutectic salt

$ZrB2$ =

zirconium diboride