Energy storage helps in waste management, environmental protection, saving of fossil fuels, cost effectiveness, and sustainable growth. Phase change material (PCM) is a substance which undergoes simultaneous melting and solidification at certain temperature and pressure and can thereby absorb and release thermal energy. Phase change materials are also called thermal batteries which have the ability to store large amount of heat at fixed temperature. Effective integration of the latent heat thermal energy storage system with solar thermal collectors depends on heat storage materials and heat exchangers. The practical limitation of the latent heat thermal energy system for successful implementation in various applications is mainly from its low thermal conductivity. Low thermal conductivity leads to low heat transfer coefficient, and thereby, the phase change process is prolonged which signifies the requirement of heat transfer enhancement techniques. Typically, for salt hydrates and organic PCMs, the thermal conductivity range varies between 0.4–0.7 W/m K and 0.15–0.3 W/m K which increases the thermal resistance within phase change materials during operation, seriously affecting efficiency and thermal response. This paper reviews the different geometry of commercial heat exchangers that can be used to address the problem of low thermal conductivity, like use of fins, additives with high thermal conductivity materials like metal strips, microencapsulated PCM, composite PCM, porous metals, porous metal foam matrix, carbon nanofibers and nanotubes, etc. Finally, different solar thermal applications and potential PCMs for low-temperature thermal energy storage were also discussed.

Compared to solar photovoltaics, concentrated solar power (CSP) can store excessive solar thermal energy, extend the power generation, and levelize the mismatch between the demand and supply. Thermal energy storage (TES) system filled with phase change material (PCM) is a key to make CSP competitive, and it is also a promising indirect energy storage technique. It is of great interests to the solar thermal engineering community to apply the latent heat thermal energy storage (LHTES) system for large-scale CSP application, because PCMs can store more energy due to the latent heat during the melting/freezing process. Therefore, a comprehensive parametric analysis of LHTES system is necessary in order to identify the most sensitive ranges of various parameters to design the LHTES system with better systematic performances. In this study, unlike the existing parametric study based on dimensional parameters, we aimed to provide a more general analysis using dimensionless parameters; therefore, an 11-dimensionless-parameter space of LHTES system was developed, by considering the technical constraints (material properties and operation parameters), without economic constraints. The parametric study and sensitivity analysis were then performed based on a 1D enthalpy-based transient model, and the energy storage efficiency was used as the objective function to minimize the number of variables in the parameter space. It was found that Stanton number (St), dimensionless PCM radius (r/D), and void fraction ( ε ) are the three most important dimensionless parameters. It is expected that the discovery of this study can bring more discussions in the solar thermal engineering community about the implementation of LHTES system in CSP plant, to further explore the significances of these three dimensionless parameters to the operation of the LHTES system.

Phase change materials (PCMs) used in the building walls constitute an attractive way to reduce the energy consumption and to increase the occupant's thermal comfort. However, there are some challenges to be faced among which the critical one is the PCM layer location allowing the greater heat flux reduction. In this work, the potential of PCM wallboards is evaluated experimentally using a heated reduced scale cavity including walls with or without PCM in a laboratory conditions. The cavity at reduced scale provides the flexibility to test most kinds of wall constructions in real time and allows faster installation and dismantling of the test walls. Three different PCM layer locations inside the walls are examined in terms of heat flux reduction and outside surface temperatures. The results confirm that the PCM layer reduces the peak heat flux compared to a reference wall (wall without PCM). Indeed, the PCM layer hugely affects the peak heat flux when it is placed on the inner face of the walls, near to the heat source. At this location, the peak heat flux reduction, compared to the reference wall, is 32.9%. Furthermore, for numerical validation purpose, the outside overall heat coefficient of the cavity outside walls is determined based on the experimental data.

Solar dryer with thermal energy storage device is an essential topic for food drying applications in industries. In this work, a two-dimensional (2D) numerical model is developed for the application of solar drying of agricultural products in an indirect type solar dryer. The phase-change material (PCM) used in this work is paraffin wax. The study has been performed on a single set of concentric tube which consists of a finned inner copper tube for air flow and an outer plastic tube for PCM material. The practical domain is modeled using ANSYS, and computer simulations were performed using ANSYS fluent 2015. The air velocity and temperature chosen for this study are based on the observation of indirect type solar dryer experimental setup. From this numerical analysis, the temperature distribution, melting, and solidification fraction of PCM are estimated at different air flow velocities, time, and inlet temperature of air. It is concluded that the drying operation can be performed up to 10.00 p.m. as the PCM transfers heat to inlet air up to 10.00 p.m. and before it got charged up to 3.00 p.m. because of solar radiation. The maximum outlet temperature is 341.62 K (68.62 °C) which is suitable for food drying applications. Higher air flow velocity enhances quick melting of PCM during charging time and quick cooling during recharging of inlet air; therefore, higher air flow velocity is not preferred for food drying during cooling of PCM.

The dynamic performance of a thermal energy storage tank containing phase change material (PCM) cylinders is investigated computationally. Water flowing along the length of the cylinders is used as the heat transfer fluid. A numerical model based on the enthalpy-porosity method is developed and validated against experimental data from the literature. The performance of this hybrid PCM/water system was assessed based on the gain in energy storage capacity compared to a sensible only system. Gains can reach as high as 179% by using 50% packing ratio and 10 °C operating temperature range in water tanks. Gains are highly affected by the choice of PCM module diameter; they are almost halved as diameter increases four times. They are also affected by the mass flow rate nonlinearly. A nondimensional analysis of the energy storage capacity gains as a function of the key nondimensional parameters (Stefan, Fourier, and Reynolds numbers) as well as PCM melting temperature was performed. The simulations covered ranges of 0.1 < Ste ̃ < 0.4, 0 < Fo < 600, 20 < Re < 4000, 0.2 < ( ρ C P ) * < 0.8 , and 0.2 < θ m < 0.8 .

Thermal storage plays a major role in a wide variety of industrial, commercial, and residential applications when there is a mismatch between the offer and the claim of energy. In this paper, we study numerically the contribution of phase change materials (PCMs) for solar thermal energy storage (TES) in buildings. The studied configuration is a plane solar collector incorporating a PCM layer and coupled to a concrete slab (a roof of a building). The study is conducted for Casablanca (Morocco) meteorological conditions. Several simulations were performed to optimize the melting temperature and the PCM layer thickness. The results show that PCM imposes, on the roof, a temperature close to its melting temperature. The choice of a melting temperature T melt = 22 °C (the local indoor temperature T c is fixed as T c = 22 °C) limits the losses through the concrete slab, considerably. This last seems to be, nearly, adiabatic, in this case. Also, the energy released by PCM solidification, overnight, increases the outlet temperature of the coolant fluid to 35 °C and the useful flux to 80 W/m 2 , increasing the efficiency of the solar collector by night. The PCM functioned both as an energy storage material for the stabilization of the coolant fluid temperature and as an insulating material for the building.

This study describes the development and characterization of novel high-temperature thermal storage media, based on inclusion of transition metal chlorides in the potassium–sodium chloride eutectic system, (K–Na)Cl (melting temperature of 657 °C, latent heat of 278 J/g). At the melting temperature of (K–Na)Cl, infrared (IR) radiation can play a major role in the overall heat transfer process—90% of spectral blackbody radiation falls in the range of 2–13 μ m. The authors propose inclusion of small amounts (less than 0.2 wt.%) of IR-active transition metal chlorides to increase radiative absorption and thereby enhance heat transfer rates. A new IR-reflectance apparatus was developed to allow for determination of the spectral absorption coefficient of the newly formulated phase-change materials (PCMs) in the molten state. The apparatus consisted of an alumina crucible coated at the bottom with a reflective (platinum) or absorptive (graphite) surface, a heated ceramic crucible-holder, and a combination of zinc sulfide (ZnS) and zinc selenide (ZnSe) windows for containment of the salt and allowance of inert purge gas flow. Using this apparatus, IR spectra were obtained for various transition metal chloride additives in (K–Na)Cl and improved IR activity, and radiative transfer properties were quantified. Further, thermophysical properties relevant to thermal energy storage (i.e., melting temperature and latent heat) are measured for the pure and additive-enhanced thermal storage media.

The heating process of melting margarine requires a vast amount of thermal energy due to its high melting point and the size of the reservoir it is contained in. Existing methods to heat margarine have a high hourly cost of production and use fossil fuels which have been shown to have a negative impact on the environment. Thus, we perform an analytical feasibility study of using solar thermal power as an alternative energy source for the margarine melting process. In this study, the efficiency and cost effectiveness of a parabolic trough collector (PTC) solar field are compared with that of a steam boiler. Different working fluids (water vapor and Therminol-VP1 heat transfer oil (HTO)) through the solar field are also investigated. The results reveal the total hourly cost ($/h) by the conventional configuration is much greater than the solar applications regardless of the type of working fluid. Moreover, the conventional configuration causes a negative impact to the environment by increasing the amount of CO2, CO, and NO2 by 117.4 kg/day, 184 kg/day, and 74.7 kg/day, respectively. Optimized period of melt and tank volume parameters at temperature differences not exceeding 25 °C are found to be 8–10 h and 100 m3, respectively. The solar PTC operated with water and steam as the working fluid is recommended as a vital alternative for the margarine melting heating process.

A two-dimensional numerical model is developed to simulate the transient response of a heat pipe-assisted latent heat thermal energy storage (LHTES) unit integrated with dish-Stirling solar power generation systems. The unit consists of a container which houses a phase change material (PCM) and two sets of interlaced input and output heat pipes (HPs) embedded in the PCM. The LHTES unit is exposed to time-varying concentrated solar irradiance. A three-stage operating scenario is investigated that includes: (i) charging only, (ii) simultaneous charging and discharging, and (iii) discharging only. In general, it was found that the PCM damps the temporal variations of the input solar irradiance, and provides relatively smooth thermal power to the engine over a time period that can extend to after-sunset hours. Heat pipe spacing was identified as a key parameter to control the dynamic response of the unit. The system with the greatest (smallest) heat pipe spacing was found to have the greatest (smallest) temperature drops across the LHTES, as well as the maximum (minimum) amount of PCM melting and solidification. Exergy analyses were also performed, and it was found that the exergy efficiencies of all the systems considered were greater than 97%, with the maximum exergy efficiency associated with the system having the minimum heat pipe spacing.

Research on phase change materials (PCM) as a potential technology to reduce peak loads and heating, ventilation and air conditioning (HVAC) energy use in buildings has been conducted for several decades, resulting in a great deal of literature on PCM properties, temperature, and peak reduction potential. However, there are few building energy simulation programs that include PCM modeling features, and very few of these have been validated. Additionally, there is no previous research that indicates the level of accuracy when modeling PCMs from a building energy simulation perspective. This study analyzes the effects a nonlinear enthalpy profile has on thermal performance and expected energy benefits for PCM-enhanced insulation. The impact of accurately modeling realistic, nonlinear enthalpy profiles for PCMs versus simpler profiles is analyzed based on peak load reduction and energy savings using the conduction finite difference (CondFD) algorithm in EnergyPlus. The PCM and CondFD models used in this study have been previously validated after intensive verification and validation done at the National Renewable Energy Laboratory. Overall, the results of this study show annual energy savings are not very sensitive to the linearization of enthalpy curve. However, hourly analysis shows that if simpler linear profiles are used, users should try to specify a melting range covering roughly 80% of the latent heat; otherwise, hourly results can differ by up to 20%.

Concentrating solar power plants (CSPs) are being explored as the leading source of renewable energy for future power generation. Storing sun's energy in the form of latent thermal energy of a phase change material (PCM) is desirable for use on demand including times when solar energy is unavailable. Considering a latent thermal energy storage (LTES) system incorporating heat pipes to enhance heat transfer between the heat transfer fluid (HTF) and the PCM, this paper explores the dynamic response of the LTES system subjected to repeated cycles of charging and discharging. A transient computational analysis of a shell-and-tube LTES embedded with two horizontal heat pipes is performed for repeated charging and discharging of the PCM to analyze the dynamic performance of the LTES, and the augmentation in the cyclic performance of the LTES embedded with heat pipes is investigated. A model low temperature phase change material system is considered in the present study, with the physical results being scalable to high temperature systems used in CSP plants.

Molten salts are currently the only thermal energy storage media operating with multiple hours of energy capacity in commercial concentrated solar power (CSP) plants. Thermal energy is stored by sensible heat in the liquid phase. A lower melting point in the range of 60–120 °C and a decomposition temperature above 500 °C are desired because such a fluid would enhance the overall efficiency of the plants by utilizing less energy to keep the salt in the liquid state and by producing superheated steam at higher temperatures in the Rankine cycle. One promising candidate is a multicomponent NaNO3—KNO3—Ca(NO3)2 molten salt. Different compositions have been reported in literature as the best formulation for CSP plants based on melting temperature. In this paper, the National Renewable Energy Laboratory (NREL) presents the handling, preparation, thermal properties, and characterization of different compositions for this ternary nitrate salt, and comparisons are drawn accordingly. This system has a high tendency to form supercooled liquids with high viscosity that undergo glass formation during cooling. When the proportion of Ca(NO3)2 decreases, the formulations become more thermally stable, the viscosity goes down, and the system increases its degree of crystalline solidification. Differential scanning calorimetry (DSC) tests showed the presence of a ternary eutectoid solid–solid invariant reaction at around 100 °C. The eutectic invariant reaction was resolved between 120 and 133 °C as reported in the literature. Based on DSC and viscosity results, the best composition would seem to be 36 wt. % Ca(NO3)2—16 wt. % NaNO3—48 wt. % KNO3, which showed a low solidification point.

Current heat transfer fluids for concentrated solar power applications are limited by their high temperature stability. Other fluids that are capable of operating at high temperatures have very high melting points. The present work is aimed at characterizing potential solar heat transfer fluid candidates that are likely to be thermally stable (up to 500 °C) with a lower melting point (∼100 °C). Binary and ternary mixtures of nitrates have the potential for being such heat transfer fluids. To characterize such eutectic media, both experimental measurements and analytical methods resulting in phase diagrams and other properties of the fluids are essential. Solidus and liquidus data have been determined using a differential scanning calorimeter over the range the compositions for each salt system and mathematical models have been derived using Gibbs Energy minimization. The Gibbs models presented in this paper sufficiently fit the experimental results as well as providing accurate predictions of the eutectic compositions and temperatures for each system. The methods developed here are expected to have broader implications in the identification of optimizing new heat transfer fluids for a wide range of applications, including solar thermal power systems.

The melting temperature of acetanilide C8H9NO equals 116 °C which lies in the range of solar cooking (boiling cooking) of most kinds of food. Acetanilide was used in previous studies as a storage medium integrated within box type solar cookers. However, detailed studies of melting and/or solidification behavior of acetanilide were not reported. Therefore, the main aim of this work was to study the effect of fast thermal melting/solidification cycling of commercial grade acetanilide on its melting point and latent heat of fusion. One thousand thermal cycles had been performed and the thermophysical properties of acetanilide were measured using the differential scanning calorimetric technique. It was indicated that acetanilide solidifies with ∼15 °C of supercooling; consequently, it is recommended to overcome the supercooling problem before using acetanilide as a thermal storage material for solar energy applications.

The thermal stability of a molten LiNaK carbonate salt, potentially suitable for thermal energy storage, was studied up to a temperature of 1000 °C. The salt investigated was the eutectic Li2CO3–Na2CO3–K2CO3 in the proportions 32.1–33.4–34.5 wt. % and the study was done by simultaneous differential scanning calorimetry (DSC)/thermogravimetric–mass spectrometric (TG–MS) analysis in gas atmospheres of argon, air, and CO2. It was found that (i) under a blanket gas atmosphere of CO2 the LiNaK carbonate salt is stable up to at least 1000 °C. (ii) In an inert atmosphere of argon, the salt evolves gaseous CO2 soon after melting and begins to decompose at between 710 °C and 715 °C with acceleration in the CO2 evolution rate from the melt. An increase in the rate of weight loss is also observed after 707 °C. (iii) Under a blanket atmosphere of air, the gaseous CO2 evolution from the salt is observed to commence at 530 °C, the onset of decomposition detected by DSC analysis at 601 °C and the rapid rate of weight loss determined by TG analysis at 673 °C. The melting point of the LiNaK carbonate studied was between 400 °C and 405 °C. Thermodynamic modeling with Multi-Phase-Equilibrium (MPE) software developed in CSIRO Process Science and Engineering indicated that additives such as NaNO3, KCl, and NaOH lower the melting point of the LiNaK carbonate eutectic, and this was experimentally verified.

Molten salts are a promising medium for thermal energy transfer and storage. They have a very low vapor pressure and most are unreactive in air. Over the past 3 decades, Sandia National Laboratories has investigated a variety of molten salt mixtures of alkali nitrates and, most recently, quaternary mixtures of sodium, calcium, lithium, and potassium nitrate salts. This effort led to the discovery of mixtures with liquidus temperatures below 100 ° C . We have now extended this work to the mixed nitrate/nitrite anion system and found compositions with liquidus temperatures below 80 ° C . In this paper, we present experimental results exploring the lithium, sodium, and potassium compositional space with a 1:1 molar mixture of nitrate/nitrite. From our work, we have identified a five-component system with a liquidus temperature near 70 ° C . Physical properties of these salts, such as viscosity and density, are reported as well as thermal stability in air. Such a molten salt mixture, with a low liquidus temperature, has the potential to make parabolic trough collectors economically competitive with traditional power generation schemes.

The addition of latent heat storage systems in solar thermal applications has several benefits including volume reduction in storage tanks and maintaining the temperature range of the thermal storage. A phase change material (PCM) provides high energy storage density at a constant temperature corresponding to its phase transition temperature. In this paper, a high temperature PCM (melting temperature of 80 ° C ) made of a composite of paraffin and graphite was tested to determine its thermal properties. Tests were conducted with a differential scanning calorimeter and allowed the determination of the melting and solidification characteristics, latent heat, specific heat at melting and solidification, and thermal conductivity of the composite. The results of the study showed an increase in thermal conductivity by a factor of 4 when the mass fraction of the graphite in the composite was increased to 16.5%. The specific heat of the composite PCM (CPCM) decreased as the thermal conductivity increased, while the latent heat remained the same as the PCM component. In addition, the phase transition temperature was not influenced by the addition of expanded graphite. To explore the feasibility of the CPCM for practical applications, a numerical solution of the phase change transition of a small cylinder was derived. Finally, a numerical simulation and the experimental results for a known volume of CPCM indicated a reduction in solidification time by a factor of 6. The numerical analysis was further explored to indicate the optimum operating Biot number for maximum efficiency of the composite PCM thermal energy storage.

A new method for characterization of shape-stabilized phase change materials (PCMs) based on one single sample and one single experimental device has been proposed. The simplicity of the experimental device is comparable to that of the T-history method: a cylinder of PCM, which is heated/cooled in a furnace following specific temperature patterns (steps, isotherms, and ramps). Instead of simple energy balances as in the T-history method, a numerical heat transfer model is used to retrieve the whole set of parameters/functions characterizing the PCM from temperature measurements at one single point within the PCM. A powerful inversion technique has been proposed for that. Its most striking feature is that it allows identification of enthalpy-temperature functions in an easy way. They are retrieved by solving a problem of time-dependent source estimation by inversion of a linear heat conduction model. It is shown that unknown sources are the output of a linear and invariant state model whose inputs are measured temperatures within the PCM. Enthalpy-temperature functions, as well as parameters derived from heat capacities, transition temperatures, and latent heat, are thus calculated in a simple way. An experimental test for characterization of graphite/salt composites has been carried out to illustrate the appropriateness of our developments.

The present work investigates, theoretically and experimentally, the thermal performance of a packed bed combined sensible and latent heat storage unit, integrated with the solar water heating system. A one-dimensional porous medium approach with the finite difference technique is used to develop the numerical model to obtain the temperature profiles of both the phase change material (PCM) and heat transfer fluid (HTF), and the molten mass fraction of the PCM at any axial location of the cylindrical storage tank during the charging process. The model also incorporates the effect of the varying fluid inlet temperature to accommodate the actual conditions that prevails in the solar collector. Experimental apparatus utilizing paraffin as PCM, which is filled in high-density polyethylene spherical capsules, is constructed and integrated with a solar flat plate collector to conduct the experiments. The water used as HTF to transfer heat from the solar collector to the storage tank also acts as a sensible heat storage (SHS) material. The results of the numerical model are compared into the experimental results of the temperature profile for various porosities and HTF flow rates. It is found that the results of the numerical model are in good agreement with the experimental results. The performance parameters, such as instantaneous heat stored, cumulative heat stored, and charging rate are also studied in detail.

The solidification and melting processes in a spherical geometry are investigated in this study. The capsules considered are filled with de-ionized water, so that a network of spheres can be thought of as being the storage medium for an encapsulated ice storage module. ANSYS GAMBIT and FLUENT 6.0 packages are used to employ the present model for heat transfer fluid (HTF) past a row of such capsules, while varying the HTF inlet temperature and flow rate, as well as the reference temperatures. The present model agrees well with experimental data taken from literature and was also put through rigorous time and grid independence tests. Sufficient flow parameters are studied so that the resulting solidification and melting times, exergy and energy efficiencies, and exergy destruction could be calculated. All energy efficiencies are found to be over 99%, though viscous dissipation was included. Using exergy analysis, the exergetic efficiencies are determined to be about 75% to over 92%, depending on the HTF scenario. When the HTF flow rate is increased, all efficiencies decrease, due mainly to increasing heat losses and exergy dissipation. The HTF temperatures, which stray farther from the solidification temperature of water, are found to be most optimal exergetically, but least optimal energetically. The main reason for this, as well as the main mode of loss exergetically, is due to entropy generation accompanying heat transfer, which is responsible for over 99.5% of exergy destroyed in all cases. The results indicate that viewing the heat transfer and fluid flow phenomena in a bed of encapsulated spheres, it is of utmost importance to assess the major modes of entropy generation; in this case from heat transfer accompanying phase change.