Concentrating Solar Power (CSP) plants have the potential to provide dispatchable renewable power generation to support the baseload need currently supplied primarily by coal and nuclear plants and peaking power capability to reduce the use of natural gas for load following. However, these plants have had difficulty achieving widespread use due to the low cost of combined photovoltaic and battery systems capable of providing similar services to the electricity grid. A new generation of CSP technologies must be developed to reduce the levelized cost of electricity (LCOE) to 6 cents/kWh by leveraging heat transfer fluids (HTF) capable of operation at higher temperatures and coupling with higher efficiency power conversion cycles. Three promising pathways for Generation 3 CSP (Gen3CSP) technology development have been funded by the U.S. Department of Energy (DOE) leveraging solid, liquid, and gaseous HTFs to transfer heat to a supercritical carbon dioxide (sCO2) Brayton cycle. The primary heat exchangers (PHX) necessary to couple these high-temperature HTFs to sCO 2 are an essential new technology that must be demonstrated at a scale relevant to commercial CSP to validate design expectations for performance, lifetime, and operability. The demonstration of these PHXs need a reliable 1 MWth-scale sCO 2 test system that can provide sCO 2 coolant to the PHX in a compact package suitable for installation near any Gen3CSP thermal storage system. This paper outlines the final design of such a system including the expected operating range and off-design capabilities. The system uses a dense-phase high pressure canned motor pump as the sCO 2 circulator and ambient air as the ultimate heat sink operating at pressures up to 250 bar and temperatures up to 715 °C with capability to supply up to 5.3 kg/s of sCO 2 flow to the primary heat exchanger. Key component requirements for this system have been frozen and procurement is underway. The expected completion date for heated acceptance testing of this system is September of 2020. This system is also capable of being upgraded through the addition of a turbo-compressor and turbo-generator to operate as a complete sCO 2 Brayton cycle with power generation in order to demonstrate an integrated solar to sCO 2 power pilot plant and understand transient interactions between the thermal storage system, sCO 2 turbomachinery, and ambient air temperature. In addition, this upgrade would provide experience with plant operating considerations including balancing charging the thermal storage system with generating and dispatching power to the electrical grid. A roadmap for this upgrade will be discussed including limitations and requirements for the necessary turbomachinery.

Nitrate molten salt concentrating solar power (CSP) systems are currently deployed globally and are considered state-of the art heat transfer fluids (HTFs) for present day high-temperature operation. Although slightly higher limits may be possible with molten salt, to fully realize SunShot efficiency goals of $15/kWh th HTFs and an LCOE of 6¢/kWh, HTF technologies working at higher temperatures (e.g., 650 °C to 750 °C) will require an alternative to molten salts, such as with alkali metal systems. This investigation explores the development of a 2.0 MW th sodium receiver system that employs a sodium receiver as the HTF, as well as with a ternary chloride (20%NaCl/40%MgCl/40%KCl by mol wt.%) salt as a thermal energy storage (TES) medium to facilitate a 6-hr. storage duration. A sodium-to-salt heat exchanger model as well as a salt-to-sCO 2 primary heat exchanger model are employed and evaluated in this investigation. A thermodynamic system design model was developed using Engineering Equation Solver (EES) where state properties were calculated at inlets and outlets along both hot and cold legs of the pilot-scale plant. This investigation assesses receiver performance as well as system efficiency studies for the pump and system operational ranges. Results found that high efficiency sodium receivers were found to have higher heat transfer coefficients and required far less spreading of incident flux. The system performance model results suggest that for a pump speed of 2400 RPM, respective hot and cold pump TDH values were determined to be 260.1–307 ft. and 260.1–307 ft for pump flow rates of 90–120 GPM.

A focus in the development of the next generation of concentrating solar power (CSP) plants is the integration of high temperature particle receivers with improved efficiency supercritical carbon dioxide (sCO 2 ) power cycles. The feasibility of this type of system depends on the design of a particle-to-sCO 2 heat exchanger. This work presents a finite element analysis (FEA) model to analyze the thermal performance of a particle-to-sCO 2 heat exchanger for potential use in a CSP plant. The heat exchanger design utilizes a moving packed bed of particles in crossflow with sCO 2 which flows in a serpentine pattern through banks of microchannel plates. The model contains a thermal analysis to determine the heat exchanger’s performance in transferring thermal energy from the particle bed to the sCO 2 . Test data from a prototype heat exchanger was used to verify the performance predictions of the model. The verification of the model required a multitude of sensitivity tests to identify where fidelity needed to be added to reach agreement between the experimental and simulated results. For each sensitivity test in the model, the effect on the performance is discussed. The model was shown to be in good agreement on the overall heat transfer coefficient of the heat exchanger with the experimental results for a low temperature set of conditions with a combination of added sensitives. A set of key factors with a major impact on the performance of the heat exchanger are discussed.

Realizing cost-effective, dispatchable, renewable energy production using concentrated solar power (CSP) relies on reaching high process temperatures to increase the thermal-to-electrical efficiency. Ceramic based particles used as both the energy storage medium and heat transfer fluid is a promising approach to increasing the operating temperature of next generation CSP plants. The particle-to-supercritical CO 2 (sCO 2 ) heat exchanger is a critical component in the development of this technology for transferring thermal energy from the heated ceramic particles to the sCO 2 working fluid of the power cycle. The leading design for the particle-to-sCO 2 heat exchanger is a shell-and-plate configuration. Currently, design work is focused on optimizing the performance of the heat exchanger through reducing the plate spacing. However, the particle channel geometry is limited by uniformity and reliability of particle flow in narrow vertical channels. Results of high temperature experimental particle flow testing are presented in this paper.

In solar thermochemical redox cycles for H 2 O/CO 2 -splitting, a large portion of the overall energy demand of the system is associated with heating the redox material from the oxidation temperature to the reduction temperature. Hence, an important measure to improve the efficiency is recuperation of sensible heat stored in the redox material. A solid-solid heat exchanger can be subject to undesirable oxygen crossover, which decreases the oxygen uptake capacity of the redox material and consequently the system efficiency. We investigate the extent of this crossover in ceria based cycles, to identify, under which conditions a heat exchanger that allows oxygen crossover can improve the system efficiency. In a thermodynamic analysis we calculate the amount of transferred oxygen as a function of the heat exchanger efficiency and show the system efficiency of such a concept. A second law analysis is applied to the model to check the feasibility of calculated points of operation. For the investigated parameter set the heat exchanger design improves the system efficiency by a factor of up to 2.1.

The use of solid particles as a heat-transfer fluid and thermal storage media for concentrating solar power is a promising candidate for meeting levelized cost of electricity (LCOE) targets for next-generation CSP concepts. Meeting these cost targets for a given system concept will require optimization of the particle heat-transfer fluid with simultaneous consideration of all system components and operating conditions. This paper explores the trade-offs in system operating conditions and particle thermophysical properties on the levelized cost of electricity through parametric analysis. A steady-state modeling methodology for design point simulations dispatched against typical meteorological year (TMY) data is presented, which includes computationally efficient submodels of a falling particle receiver, moving packed-bed heat exchanger, storage bin, particle lift, and recompression supercritical CO 2 (sCO 2 ) cycle. The components selected for the baseline system configuration presents the most near-term realization of a particle-based CSP system that has been developed to date. However, the methodology could be extended to consider alternative particle receiver and heat exchanger concepts. The detailed system-level model coupled to component cost models is capable of propagating component design and performance information directly into the plant performance and economics. The system-level model is used to investigate how the levelized cost of electricity varies with changes in particle absorptivity, hot storage bin temperature, heat exchanger approach temperature, and sCO 2 cycle operating parameters. Trade-offs in system capital cost and solar-to-electric efficiency due to changes in the size of the heliostat field, storage bins, primary heat exchanger, and receiver efficiency are observed. Optimal system operating conditions are reported, which approach levelized costs of electricity of $0.06 kW e −1 hr −1 .

Heat pumps based on the vapor compression cycle account for a significant portion of energy use around the world. However, growing demands for energy efficient and environmentally friendly technologies have created a need for new space conditioning approaches. Novel systems which use elastocaloric material have shown potential to replace traditional vapor compression due to high energy efficiency and use of environmentally friendly, solid-state refrigerants. The solid-state refrigerants exhibit the elastocaloric effect, a phenomenon that occurs when metal alloys experience stress-induced reversible phase transformations resulting in latent heat release or absorption. Prototypes built in the Center for Environmental Energy Engineering have utilized the active elastocaloric regeneration (AER) operating method to develop high temperature gradients between the ends of a regenerative heat exchanger made of tubular elastocaloric material. Though this schema significantly increases the temperature span developed by elastocaloric cooling devices, the current heat pump design leads to temperature degradation as a result of conduction along the length of the tubes in the regenerator. The novel regenerator concept presented in this work mitigates that issue by using short, thermally insulated tubes layers which also enables fluid flow over external surface areas of the material.

A direct absorption receiver using ceramic particles (CentRec) has been successfully developed by DLR and tested under solar conditions at the Juelich Solar Power Tower, demonstrating receiver outlet temperatures of more than 900 °C. The next step towards commercial application of the technology is to demonstrate a cost-effective, high temperature heat extraction and transfer to a process medium. Besides e.g. steam for electricity generation in a steam turbine, hot air can be used to supply heat to industrial processes with energy demand at high temperature level. A great potential for higher efficiencies and lower costs has been identified for a moving bed heat exchanger. Several concepts of direct contact heat exchangers have been analyzed and evaluated. The selected concept is a combination of several crossflow-sections that are arranged in series with fluid-mixing-chambers between each crossflow-section. Based on the selected design a heat exchanger prototype with 10 kW thermal power and a design air outlet temperature of 750 °C has been built and integrated into a test setup. The test setup provides particles at 900 °C that are heated up electrically inside a hopper on top of the heat exchanger. Hot particles are then moving downwards (moving bed) from the hopper through the direct contact heat exchanger driven by gravity. Cold air supplied by a compressor flows through the particle bed in cross-flow and is heated up. The hot air flow leaves the heat exchanger with a temperature of 750 °C. The particle mass flow is controlled by an oscillating mass flow controller, positioned under the heat exchanger. The cold particles are collected in a container on the bottom. The particle cycle is closed by transporting them back to the hopper. A measurement and control system is implemented to carry out the tests. The test setup has undergone successful commissioning in October and an extensive testing phase started in January 2019. This paper presents the development and manufacturing as well as the successful commissioning of the heat exchanger prototype.

The aim of this paper is to study the charging/discharging process in a Latent heat exchanger for the purpose of space heating by using a composite metal foam/PCM. The composite PCM-air system is modelled in a 3-D CFD approach for the purpose of 8h charging during the night and 16h discharging during the daytime using the non-equilibrium thermal model to simulate the presence of a porous medium in the domain. For the charging process, rod Positive Temperature Coefficient (PTC) heating elements with constant temperature are selected to heat the PCM based on the maximum operating temperature of the PCM. For the discharging process, a blower is assumed to pass the air from the middle of the PCM container and so the air can gain heat and its temperature rises which is used then for space heating. RT70HC is also selected as the PCM material due to the high capacity of latent heat and suitable melting point for domestic usage. The system is studied according to the average liquid fraction and temperature of the PCM during both charging and discharging as well as the outlet temperature of the air during discharging. The results show that by using two rod heating elements with the diameter of 1cm, length of 25cm and temperature of 95°C, the melting process is performed in less than 8h. Furthermore, a uniform output temperature of almost 29.5°C is also achieved in the next 16h during the discharging process with the air mass flow rate of 0.04 kg/s.

Particle receiver systems require durable, reliable, and cost-effective particle transport equipment. These lifts are critical pieces of equipment to transport the particles from the heat exchanger back into the receiver. There are challenges that must be overcome with any particle lift device including high temperatures (800°C), particle load and friction, and erosion from particle contact. There are several options commercially available for particle systems including a screw-type vertical elevator, bucket lift vertical elevator, and skip-hoist-style bulk vertical lifts. Two of the elevator types (screw and bucket) have been tested at the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories (SNL) in Albuquerque, NM. The two elevators are currently in operation on the 1 MW th falling particle receiver at the Solar Tower. The screw-type elevator consists of a stationary internal screw with an outer casing that rotates about the screw. The frictional forces from the casing rotation drives the particles upward along the flights of the screw. The casing rotational velocity is variable which allows for mass flow rate control. Identified issues with the screw-type elevator include particle attrition, uneven loading at the inlet causes casing deflection, bearing deformation due to casing deformation, and motor stalling due to increased resistance on the casing. The SNL bucket elevator is rated for temperatures up to 600 °C and consists of steel buckets and a steel drive chain capable of lifting particles at a rate of 8 kg/s. Identified issues with the bucket type elevator include discrete (non-continuous) discharge of the particles and a non-adjustable flow rate. A skip hoist type elevator has been studied previously and seems like the most viable option on a large scale (50–100MW th power plant) with a non-continuous particle discharge. Different control scenarios were explored with the variable frequency drive of the screw-type elevator to use it as a particle-flow control device. The objective was to maintain the feed hopper inventory at a constant value for steady flow of particles through the receiver. The mass flow rate was controlled based on feedback from measurements of particle level (mass) inside the top hopper.

In this work, we investigate the effects of the width of an annular baffle region on natural convection heat transfer to an immersed, coiled heat exchanger in an otherwise quiescent sensible hot water storage tank. In experiments, the coiled heat exchanger sits in an annular region created by the tank wall and a straight, cylindrical baffle. The width of this baffle region is 1.5, 2, 3, or 4 times the heat exchanger diameter, These experiments are compared to each other and to corresponding control experiments with no baffle. In general, all baffles create considerable benefits over their respective control experiments, consistent with past studies. The considered metrics of heat transfer rate, fraction of energy discharged from the tank, and heat exchanger outlet temperature show that heat transfer is improved slightly by narrowing the baffle region. For example, relative to their respective controls, the energy extracted from the tank after 30 min of discharge in the 1.5 D , 2 D , 3 D and 4 D experiments is 23.2%, 20.8%, 18.1%, and 14.7% higher, respectively. This improvement in natural convection heat transfer as the baffle region narrows is attributed to the increasing thermal stratification observed in experiments with increasingly narrow baffle regions.

Particle-based power tower systems are a promising technology that can allow operation of concentrating solar power (CSP) systems at temperatures higher than what today’s commercial molten salt systems can achieve, making them suitable for use in a variety of applications, including supercritical CO 2 cycles, air Brayton cycles, and high-temperature process heat. In this concept, particles, instead of molten salt, are heated by the concentrated sunlight. In 2015, this concept was successfully tested at Sandia National Laboratories. In the mean time, an integrated system incorporating a particle heating receiver, a particle-to-air heat exchanger and a 100-kWe microturbine was designed, built, and tested at King Saud University in Riyadh, Saudi Arabia. The integrated system was run in 2018, and results from that test campaign were very promising, with temperatures of the particles leaving the receiver exceeding 600°C despite a number of challenges. The utility sponsoring the project is now planning to move forward with building a 1-MWe plant using the same concept, thereby moving closer to large-scale deployment, and making this facility the world’s first commercial concentrating solar power plant that uses the particle heating receiver concept. Moving from a 100-kWe scale to a 1-MWe scale requires modifications to the design of some components. The most likely plant location is the city of Duba in northwestern Saudi Arabia where the average daily total DNI is 7,170 Wh/m 2 and an integrated solar combined cycle power plant exists on the premises. This paper discusses the design features of the main components of the new plant. Those features include a north field design, a 7.22-m 2 single-sheet heliostat design, a cavity receiver to improve receiver efficiency by reducing radiative and convective losses, temperature-based particle flow regulation within the receiver, six hours of full-load thermal energy storage, with the tanks integrated into the tower structure and made of cost-effective masonry material, a shell-and-tube particle-to-air heat exchanger, a 45% efficiency recuperated intercooled gas turbine, and a high-temperature bucket elevator. The heliostat field was optimized using SolarPILOT. Results show that 1,302 heliostats are needed. The aperture area was found to be approximately 5.7 m 2 , while the total illuminated receiver surface area is about 16.8 m 2 . This design was found to be capable of achieving the particle temperature rise of 416°C, which is necessary to allow the turbine to rely entirely on the solar field to bring the temperature of air to the firing temperature of the turbine, thereby eliminating the need for fuel consumption except for back-up and for assistance at off-design conditions.

Any proposed particle to working fluid heat exchanger as part of a CSP Particle Heating Receiver system is challenging. A principal challenge is achieving adequate heat exchange (HX) from the high temperature particles to the working fluid such as sCO2 or air flowing in tubes or other passages. To reduce the required HX area, a high particle side heat transfer coefficient is needed, and counterflow is always the best overall arrangement. Consequently, a promising approach is implementing an open channel flow of fluidized particles actually flowing in a general counterflow with respect to the working fluid, which is contained in tubes or passages immersed in the channel. This arrangement provides (1) excellent particle side heat transfer, (2) convenient particle re-circulation, and (3) almost ideal counterflow with the working fluid. To advance the understanding and support the design and applications of such exchangers, this investigation has been conducted to study the possibility of local effects of the particle flow path on the fluidized heat transfer. To this end, a series of smaller fluidized bed heat exchangers were built utilizing an axially flowing open channel for the moving bed of fluidized particles. These designs featured a serpentine flow path representative the full scale HX design proposed by others. The proposed serpentine flow design is based on an existing particle cooling system; however, questions were raised about this design that had not yet been conclusively answered and promoted this investigation. The test bath supporting this investigation contains one bend around which the particulate flows prior to exiting the heat exchanger. The intent of this larger scale apparatus is to observe the variables affecting the stability or uniformity of the particle flow and provide insight into potential problems with the operational unit. The test rig consists of two stacked sections. The lower container is the fluidizing air plenum, which provides a uniformly distributed airflow through the bottom plane of the upper container. The interface comprises a structural perforated plate, stacked layers of filter paper to balance the pressure drop, and a fine stainless steel wire mesh to ensure that the particulate remains in the upper container. This upper container represents the particulate flow area. Clear conductive PETG polymer walls were used for the fluidized bath to reduce electrostatic buildup while still providing a transparent material through which the flow can be observed. The current design uses an air conveyor to recirculate the particulate from one end of the test bath back to the other closing the particle loop. The tests described investigate the effectiveness of fluidization in specific regions of the serpentine path. Measurements have been taken in these regions to determine the local heat transfer coefficient. This is accomplished by inserting a cartridge heater with a known power input and heated area, instrumented with a fine bead surface thermocouple to measure the heater surface temperature. In addition, two probes are immersed in the fluidized bed surrounding the cartridge heater to measure the free stream temperature in the bed. The air input for fluidization and air conveyor lift are also measured and recorded as test parameters along with approximate bed height in each region. In addition to the quantitative measurements of the flow, the test unit is used to observe the effect of fluidization, bed height, and outlet locations on the axial mass flow rate of the particulate. These results will be presented in the proposed paper. Going forward, this setup will allow for testing of various mass flow control schemes for the system. Currently this design, with the instrumented heater and free stream temperature probes, allows measurement of the local heat transfer properties anywhere in the particle flow path. The present tests provide a localized map of heat transfer coefficients in the fluidized bath design and a description of the flow behavior which will be reported and presented to support future open channel particle to sCO2 heat exchanger designs.

The global energy demand has increased at a very large rate, and in parallel, the Municipal Solid Waste (MSW) has also increased, both posing enormous technological challenges to world sustainable growth. Therefore, in order to contribute with concrete alternatives to face such quest for sustainability, this work presents an analysis of an integrated power plant fired by municipal solid waste that uses a biological filter for the combustion emissions fixation. The facility located in the Sustainable Energy Research & Development Center (NPDEAS) at Federal University of Parana is taken as a case study to analyze the process of technical and economic viability. For that, an exergoeconomic optimization model of the waste-to-energy power plant that generates electricity and produces microalgae biomass is utilized. An incineration furnace, which has a 50 kg/h capacity, heats the flue gas above 900°C and provides energy for a 15 kW water-vapor Rankine cycle. A set of heat exchangers preheats the intake air for combustion and provides warm utility water to other processes in the plant, which assures that the CO 2 rich flue gas can be airlifted to the microalgae cultivation photobioreactors (PBR) at a low temperature, using a 9 m high mass transfer emissions fixation column. Five 12 m 3 tubular photobioreactors are capable of supplying up to 30,000 kg/year of microalgae biomass with southern Brazil solar conditions of 1732 kWh/m 2 per year. The results show that considering the incineration services, the integrated power plant could have a payback period as short as 1.35 years. In conclusion, the system provides a viable way to obtain clean energy by thermally treating MSW, together with microalgae biomass production that could be transformed in a large variety of valuable bioproducts (e.g., nutraceuticals, pharmaceuticals, animal feed, and food supplements).

Cooling towers are widely used to remove heat in buildings with chilled water air conditioning systems. Moreira et al. [1] performed an experimental comparison between a cooling tower (CT) and a geothermal heat exchanger (GHE) in Guayaquil-Ecuador (hot/humid climate) and the results show an advantage of 39% of GHE systems regarding energy efficiency. This study compares the emissions of greenhouse gases (GHG), considering the results of the research mentioned above and comparing both systems. A life cycle assessment (LCA) approach was used to estimate the GHG emissions, assuming three scenarios for the electricity supply: the electricity generation mix in 2016, the planned electricity generation mix in 2025, and the profile for marginal electricity generation (peak demand). The estimated reduction of GHG emissions due to the use of GHE systems could be up to 50%. GHEs for building air conditioning applications is a technological option with potential to reduce energy consumption and GHG emissions. However, additional work is necessary to evaluate the complete environmental profile and its cost-effectiveness.

A scheme to streamline the electric power generation profile of concentrating solar power plants of the parabolic trough collector type is suggested. The scheme seeks to even out heat transfer rates from the solar field to the power block by splitting the typical heat transfer fluid loop into two loops using an extra vessel and an extra pump. In the first loop, cold heat transfer fluid is pumped by the cold pump from the cold vessel to the solar field to collect heat before accumulating in the newly introduced hot vessel. In the second loop, hot heat transfer fluid is pumped by the hot pump from the hot vessel to a heat exchanger train to supply the power block with its heat load before accumulating in the cold vessel. The new scheme moderately decouples heat supply from heat sink allowing for more control of heat delivery rates thereby evening out power generation.

This paper presents an evaluation of alternative particle heat-exchanger designs, including moving packed-bed and fluidized-bed designs, for high-temperature heating of a solar-driven supercritical CO 2 (sCO2) Brayton power cycle. The design requirements for high pressure (≥ 20 MPa) and high temperature (≥ 700 °C) operation associated with sCO2 posed several challenges requiring high-strength materials for piping and/or diffusion bonding for plates. Designs from several vendors for a 100 kW-thermal particle-to-sCO2 heat exchanger were evaluated as part of this project. Cost, heat-transfer coefficient, structural reliability, manufacturability, parasitics and heat losses, scalability, compatibility, erosion and corrosion, transient operation, and inspection ease were considered in the evaluation. An analytical hierarchy process was used to weight and compare the criteria for the different design options. The fluidized-bed design fared the best on heat transfer coefficient, structural reliability, scalability and inspection ease, while the moving packed-bed designs fared the best on cost, parasitics and heat losses, manufacturability, compatibility, erosion and corrosion, and transient operation. A 100 kW t shell-and-plate design was ultimately selected for construction and integration with Sandia’s falling particle receiver system.

Compressed air energy storage is an effective energy storage technology to solve the instability of wind power in distributed energy resources. In this paper, a multistage compressed air energy storage system optimization model is constructed based on the energy conservation equation. Then the system is optimized by differential evolution to improve the system efficiency. Optimal pressure ratios are proposed to distribute the pressures of compressors and expanders. The impact of pressure ratio distribution curve on the system energy efficiency suggests that the change curve of the characteristics vary in different heat exchanger performance. Results show that the change of thermal transfer reactor performance leads to the variety of optimal distribution pressure ratio and energy efficiency of the system. In addition, the differential ratio distribution factor can be effective on the pressure ratio of reasonable allocation. System efficiency optimization results increased by about 1% compared mean value.

Biomass torrefaction is a thermal pretreatment which takes place at a temperature between 200–300 °C in a non-oxidative environment. The process requires thermal energy for drying and torrefying the raw biomass. The amount of the required heat may vary depending on the biomass moisture content, operating temperature and residence time. The volatiles released during the torrefaction are usually burnt in a combustor to meet the heat requirement of the process. If the energy content of the volatiles is less than the thermal energy required for the process, the operation of the torrefaction unit is below the autothermal mode so an auxiliary fuel such as natural gas is burnt together with the volatiles. This paper investigates autothermal operation of a torrefaction unit which consists of a dryer, a torrefaction reactor, a combustor, and two heat exchangers. An experimentally validated process model is employed to identify a relation between the moisture content, torrefaction temperature, and residence time at autothermal operation. The model is capable of predicting the composition of volatiles and torrefied biomass, mass and energy yields, process heat requirement, and CO 2 emissions. The results are graphically presented allowing one to determine whether a torrefaction unit operates below or above the autothermal mode at given torrefaction temperature, residence time and moisture content. Furthermore, the effect of the main operating parameters on the carbon dioxide emissions of the torrefaction unit is discussed.

In geothermal heating and cooling, there exists an opportunity to improve the efficiency by utilizing non-uniform soil properties of a ground source heat exchanger during installation. This paper presents a gradient approach based upon finite element mathematics to determine an optimal distribution of heterogeneous soils with varying thermal conductivities. The numerically simulated case studies demonstrate the good performance of this algorithm to minimize the cross-talk of heat flux between pipes and maximize the overall efficiency.