Current state-of-the-art development of concentrated solar power (CSP) applications targets cost-effective and highly efficient processes in order to establish commercialization of these technologies. The design of solar receivers/reactors and their respective flow configuration have a direct impact on the operational performance of the solar thermochemical processes. Thermal efficiencies, reaction kinetics, and other key output metrics are the intrinsic result of the chosen configuration. Therefore, reactor design optimization plays a crucial role in the development of solar thermochemical applications. In this study, a computational fluid dynamics (CFD) model of a directly-irradiated cavity receiver has been developed. The CFD-domain is coupled with incoming radiation that is obtained by using Monte Carlo ray tracing (MCRT). Experimental campaigns of the cavity receiver were carried out using a 7-kW high flux solar simulator (HFSS) as a radiative source. Temperature readings were obtained at different locations inside the cavity receiver for both wall and gas temperatures. In order to mimic naturally changing insolation conditions, the HFSS was run at different power levels. Heat flux at the aperture of the solar receiver was experimentally characterized. The acquired heat flux maps validated the intermediate results obtained with the MCRT method. The coupled computational model was validated against the measured temperatures at different locations inside the receiver. Computed temperature contours inside the receiver confirmed the experimentally observed non-uniformity of the axial temperature distribution. The validated analysis presented in this paper was then used as a baseline case for a parametric study. Design optimization efforts were undertaken toward obtaining temperature uniformity and achieving efficient heat transfer within the fluid domain. Enhanced flow circulation was achieved which yielded temperature uniformity of the receiver at steady-state conditions. The outcome of this parametric analysis provided valuable insights into the development of thermal efficient solar cavity receivers. Hence, findings of this study will serve as a starting point for the future solar reactor design. For example, it was found that reversing flow direction has an adverse effect on the temperature uniformity inside the receiver. Similarly, increasing the inlet angle does not positively affect the temperature distribution and hence should be chosen carefully when designing a solar reactor.

This paper presents the modeling theory and results of an innovative thermal energy storage (TES) facility, ideated, realized, and tested by ENEA (Italy). This prototype enabled the thermocline storage with molten salts in a novel geometry ideated for small-medium scale decentralized solutions, which includes two vertical channels to force the circulation through two heat exchangers, respectively, and realized for charging and discharging phases (in a single tank). A thermophysical model was built and tested properly for this particular geometry in order to analyze the temperature distribution along the radius. The numerical results well reproduced the experimental values. Furthermore, the analytical solution provided a short-cut methodology able to evaluate the thermocline distribution (along the vertical axis) depending on both the time and the radius values. Hence, the influence of the radial position (r) on the thermocline degradation was studied finding that, at the edges (r → 1), the thermocline remains unchanged for longer (around ten times more) than at the center of the tank (r → 0). The obtained numerical modeling and the analytical correlation can be useful for the process analysis to scale-up the thermal storage system and to evaluate the system reliability for industrial plants.

Conjugate laminar natural convection heat transfer and air flow with radiation of tube solar receiver with glass window were numerically investigated. The discrete ordinate method was used to solve the radiative transfer equation. And the three-dimensional steady-state continuity, Navier–Stokes, and energy equations were solved. The temperature difference based on environment and high temperature surface of receiver is varied from 100 K to 1000 K. The influence of the surface emissivity, heating temperature, convective coefficient, and convective temperature of environment on the heat transfer from the receiver with glass window has also been investigated. The numerical results indicated that the highest temperature of glass window increases and the high temperature area becomes wide, with the temperature of heating wall and surface emissivity increasing. Adopting higher convective coefficient of glass window can reduce the peak magnitude of temperature distribution on glass window of tube receiver up to 45%.

In this study, the CO 2 -based photovoltaic–thermal hybrid system has been investigated with an objective to increase the power generation efficiency in photovoltaic solar panel and to improve the performance of supercritical CO 2 solar Rankine cycle system (SRCS). From a previous study, an improvement of 2% of power generation efficiency was confirmed via experimental investigation. In this study, the temperature distribution on the CO 2 -based photovoltaic–thermal hybrid system has been numerically and experimentally investigated and confirmed with referenced experimental results. Particularly, in this study, the one-dimensional (1D) calculation of CO 2 flow in the cooling tube and three-dimensional (3D) calculation of temperature distribution on the surface of the photovoltaic solar panel are conducted. The typical summer and winter weather conditions are used as the calculation references to investigate the effect of temperature distribution of the photovoltaic solar panel. The results show that the trend of temperature distribution from calculation was confirmed with the experimental data both in summer and winter conditions. Furthermore, in summer condition, the CO 2 temperature was increased to a maximum of 28 °C.

Performance of photovoltaic (PV) module decreases significantly with increasing cell temperature due to its overheating. Photovoltaic thermal (PVT) is an optimized technology that facilitates effective removal and utilization of this excess heat leading to enhanced electrical performance. In this article, a 3D numerical model has been developed and analyzed to investigate the PVT performance with a new pancake-shaped flow channel design. This flow channel is attached directly to the backside of PV module by using thermal paste. The governing equations are solved numerically by using Galerkin's weighted residual finite-element method (FEM), which has been developed using COMSOL Multiphysics ® software. The numerical results show that the cell temperature reduces on an average 42 °C, and the electrical efficiency and output power increase by 2% and 20 W, respectively, for both aluminum and copper channels with an increase in inlet velocity from 0.0009 to 0.05 m/s. On the other hand, overall efficiency of the PVT system drops about 13% in both cases as the inlet temperature increases from 20 °C to 40 °C. Cell temperature is found to increase approximately by 5.4 °C and 9.2 °C for every 100 W/m 2 increase in irradiation level of the PV module with and without cooling system, respectively. Regarding flow channel material, it has been observed that use of either copper or aluminum produces almost similar performance of the PVT module.

Volume - filling ratio of the working fluid has a predominant effect on the system performance of a closed two-phase solar water-heating (SWH) system. To study this effect, a prototype of a loop thermosyphon SWH system, which uses remolded flat-plate solar collector as the evaporator and the coil pipe in the water tank as the condenser, was set up. A set of long-term outdoor experiments under 10%, 20%, 30%, 50%, and 70% volume-filling ratios were conducted in this paper. R600a was used as working fluid. Loop thermosyphon solar collector thermal performance and system thermal performance under different volume-filling ratios, including the temperature distribution of loop thermosyphon evaporator, were presented. It is shown that the loop thermosyphon solar collector and the system had a better thermal performance than the conventional ones under 30% and 50% volume-filling ratio, and the loop thermosyphon evaporator had an even temperature distribution when the volume-filling ratio was higher than 30%. The optimum volume filing ratio lies in between 30% and 50% of the whole system volume.

A high-temperature pressurized air-based receiver is considered as a module for power generation via solar-driven gas turbines. A set of silicon carbide cavity-receivers attached to a compound parabolic concentrator (CPC) are tested on a solar tower at stagnation conditions for 35 kW solar radiative power input under mean solar concentration ratios of 2000 suns and nominal temperatures up to 1600 K. A heat transfer model coupling radiation, conduction, and convection is formulated by Monte Carlo ray-tracing, finite volume, and finite element techniques, and validated in terms of experimentally measured temperatures. The model is applied to elucidate the effect of material properties, geometry, and reflective coatings on the cavity’s thermal and structural performances.

This study focuses on thermochemical cavity-type reactor, with a reactive material directly irradiated by concentrated solar energy. General tendencies of reactor performance are analyzed as a function of the reactor geometry. The objective is to define a simplified model that can be adapted easily to different reactor designs or different operating conditions. For this reason, the chemical reaction is not precisely fixed but rather characterized by a reaction temperature and a useful power consumed by the endothermic reaction, inside the reactive material. In order to increase the efficiency, two new reactor designs are proposed. These designs allow obtaining a nonuniform distribution of the useful power consumed by the reaction with the depth in a circular cylindrical cavity (z-axis). This is done in two ways: by varying the reactive material thickness along the axis or by varying its density at a constant thickness. The results show that these reactor concepts lead to a more uniform temperature distribution along the z-axis and a diminution of the heat losses. Thus, the reactor efficiency can increase significantly. The results of the simplified model can be used as a system predesign. A more accurate CFD model could be used afterward to refine the optimal shape.

In this study, a phase change material (PCM) sandwich panel was developed and tested to evaluate the resulting decrease in heating and cooling loads of a test cabin in Adana, Turkey, where Mediterranean climate prevails. The panel was formed by a macropackage of microencapsulated PCM layer together with an insulation panel. Two different PCMs, with melting points 26 ° C and 23 ° C , were used in the panel. Temperature distribution in the cabin was measured for four different cases. In summer, the maximum average temperature reduction achieved in the cabin was 2.5 ° C when only the PCM was used. This corresponded to a summer cooling load reduction of 7%. In winter, the maximum average temperature increase achieved in the cabin was 2.2 ° C with the PCM sandwich panel. The winter heating load was decreased by 17%. Energies conserved in cooling and heating were calculated as 186 kWh/year and 206 kWh/year, respectively.

Thermal mixing and stratification are explored numerically and experimentally in a cylindrical tank, which simulates a storage of water heated by a solar collector. The tank is 70 cm in height and 24 cm in diameter. The inlet and outlet are vertical and located off the centerline of the tank. The study is conducted in a transient mode, namely, the tank is filled with hot water, and as the hot water is being withdrawn, the tap water replaces it in a stratified way or by mixing. The flowrates of 2 l ∕ min , 3 l ∕ min , 5 l ∕ min and 7 l ∕ min , which correspond to superficial velocities of 4.35 cm ∕ min , 6.52 cm ∕ min , 10.87 cm ∕ min , and 15.2 cm ∕ min , are explored. Temperature of hot water ranges within 40 – 50 ° C , while the tap water is about 25 – 27 ° C . Installation of one and two horizontal baffles above the inlet is examined. Simultaneous experimental and numerical investigations are performed. In the experiment, both flow visualization and temperature measurements are used. Three-dimensional transient numerical simulations are done using the FLUENT 6 software. Validation of the numerical model is achieved by comparison with the experimental results. Then, the numerical model is applied to a study of various possible changes in the system. The results show that at low flowrates, up to a superficial velocity of about 11 cm ∕ min through the tank, the baffles have no effect on tap water mixing with the stored hot water. At higher flowrates, a single horizontal baffle prevents the mixing and preserves the desired stratified temperature distribution in the storage tank.

In order to optimize the solar field output of parabolic trough collectors (PTCs), it is essential to study the influence of collector and absorber geometry on the optical performance. The optical ray-tracing model of PTC conceived for this purpose uses photogrammetrically measured concentrator geometry in commercial Monte Carlo ray-tracing software. The model has been verified with measurements of a scanning flux measurement system, measuring the solar flux density distribution close to the focal line of the PTC. The tool uses fiber optics and a charged coupled device camera to scan the focal area of a PTC module. Since it is able to quantitatively detect spilled light with good spatial resolution, it provides an evaluation of the optical efficiency of the PTC. For comparison of ray-tracing predictions with measurements, both flux maps and collector geometry have been measured under identical conditions on the Eurotrough prototype collector at the Plataforma Solar de Almería. The verification of the model is provided by three methods: the comparison of measured intercept factors with corresponding simulations, comparison of measured flux density distributions with corresponding ray-tracing predictions, and comparison of thermographically measured temperature distribution on the absorber surface with flux density distribution predicted for this surface. Examples of sensitivity studies performed with the validated model are shown.

A critical step of sulfur based thermochemical cycles for hydrogen production from water is the endothermic decomposition of sulfuric acid. The necessary heat can be provided by concentrated solar radiation. A solar receiver-reactor has been developed and built to investigate this process. Experiments with the test reactor were carried out in the DLR solar furnace in Cologne and confirmed the technical feasibility of a receiver-reactor containing porous ceramics for the decomposition of sulfuric acid. The receiver-reactor and strategy of operation were iteratively optimized with respect to chemical conversion and reactor efficiency. Parametric studies were conducted with varying partial pressure of S O 3 , residence time, absorber temperature, the presence of catalyst, and the performance of different catalysts to quantify their influence on chemical conversion and reactor efficiency. The absorber temperature distribution was found to be the most crucial process parameter. Conversions close to the equilibrium—in some cases exceeding 90%—were achieved with a platinum catalyst. Thermal efficiencies of up to 35% for the foam vaporizer and 30% for the overall reactor were achieved. Enclosing the absorber in a cavity and using separate chambers for the evaporation and the S O 3 -decomposition were identified as potential measures to improve the reactor.

Monte Carlo radiative transfer analysis is applied to a cylindrical cavity-receiver containing an array of high-temperature tubular absorbers directly exposed to concentrated solar power entering through a spectrally selective window. The cavity walls are assumed either diffusely or specularly reflective. The relative dimensions, the number of tubes, and their position are optimized for maximum energy transfer efficiency or maximum absorber temperature. A single-tube absorber operating at 2000 K performs best when located at 60% relative distance to the cavity’s aperture. Higher absorber temperatures are attained for a specularly reflective cavity that serves as internal infrared mirror but at the expense of lower energy transfer efficiencies. In contrast, diffuse reflecting cavity walls promote a more uniform temperature distribution around the tubular absorber. Decreasing the window-to-cavity areas ratio further results in an increase of the absorber temperature, which peaks for an optimum absorber-to-cavity radii ratio. This optimum ratio shifts to lower values for multiple-tube absorbers. However, the average absorber temperature is not significantly affected by using multiple-tube absorbers of constant total cross sectional area.

Theoretical and experimental investigations of the flow and temperature distribution in a 12.53 m 2 solar collector panel with an absorber consisting of two vertical manifolds interconnected by 16 parallel horizontal fins have been carried out. The investigations are focused on overheating and boiling problems in the collector panel. Single-phase liquid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Differently designed collectors are investigated with different collector fluid volume flow rates. The effect of friction and the influence of the buoyancy effects are considered in the investigations. Further, experimental investigations of the solar collector panel are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the back of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good agreement between the measured and calculated temperatures. Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of collector fluid flow rate and inlet temperature on the flow and temperature distribution are shown. The flow distribution through the absorber tubes is uniform if a high flow rate of 10.0 l ∕ min is used. By decreased collector fluid flow rate and by increased collector fluid inlet temperature, the flow distribution gets less uniform due to the influence of buoyancy force. If the collector fluid flow rate is small and the collector fluid inlet temperature is high enough, severe nonuniform flow distribution may happen with a small flow rate or even zero or reverse flow in the upper horizontal strips, resulting in overheating or boiling problems in the strips. The CFD calculations elucidate the flow and temperature distribution in the collector panels of different designs. Based on the investigations, recommendations are given in order to avoid overheating or boiling problems in the solar collector panel.

The current trend in volumetric solar receiver technology is to build modular receivers cooled by air (Hitrec I and II, Solair 200 kW and 3 MW ) in order to facilitate the replacement of broken absorber modules (cups) and to simplify the upscaling of the receiver. In addition, the modular designs include an air return circuit to cool down the structure supporting the cups. Usually, the air outlet temperature from each module is characterized by measurements taken from a single thermocouple. However, the air temperature distribution behind the volumetric absorber module is not homogeneous, as it can be seen in some specific tests where several thermocouples were added behind different absorber modules. The radial distribution of outlet air temperatures shows very high temperature gradients. The goal of this work is to explain the inhomogeneous thermal maps behind the metallic absorbers by comparing some experimental results with numerical simulations performed using the computational fluid dynamics FLUENT code. The results show the wind influence over the air recirculation flow and its effects on the outlet air temperature radial distribution. Thus, the simulations suggest different ways to reduce the temperature gradients behind each cup.

Thermal and mechanical behaviors of absorbers of different material for its application in parabolic collectors during the direct steam generation, with low mass flows ( 90 kg ∕ h to 120 kg ∕ h ) and low pressures ( 9 × 10 2 kPa to 16 × 10 2 kPa ) were studied. The distribution of temperatures in the absorber periphery of copper, steel, and bimetallic copper-steel were analyzed using a theoretical model with the influence of a stratified two-phase flow in the internal wall of the absorber, as well as heat flux in a section of the external wall; the previous situation led to the construction of the copper-steel absorber and experimentally to a comparison with a steel absorber. In the experimental part, the thermal and mechanical analysis of the three absorbers is included, emphasizing the thickness of the wall and the thermal conductivity of each material, under the same thermohydraulic conditions. The theoretical and experimental work is analyzed for solar beam irradiance arriving from below the absorber for an east-west orientation.

This paper describes a combined system of stratificated air conditioning and natural ventilation for large enclosures, which uses stratificated air conditioning to cool the occupied part of a space and uses natural ventilation to cool the upper part to reduce heat penetration into the lower air-conditioned part. A zonal model is constructed to predict the vertical temperature profiles of large enclosures under such a combined system. This model incorporates airflow and heat transfer throughout the space into the mass and heat balance equations for each horizontally settled zone. It introduces some particular flow dynamics and thermal effects into the predictions of mean airflows and temperature distributions. Different from those pressure-based zonal models applied generally to the predictions for small building rooms, it is termed a temperature-based zonal model, which uses correlations based on temperature differences in combination with submodels for modeling of mass flow and heat transfer in the large enclosures. The present paper provides a calculation procedure for the model. Model performances are then discussed through analyzing the impacts of some influential factors on the space air temperature profiles.

Experimental results with a 50 kW solar reactor producing 10 - 15 g ∕ h single-wall carbon nanotube’s rich soot have shown that good quality product was obtained with helium and rather bad product with argon. This result is explained using a computational fluid dynamics model of the reactor accounting for fluid flow, heat transfer, and mass transfer. The bad results in argon were linked to the quenching of carbon vapor in the vaporization zone that resulted in the growth of carbon nanoparticles in spite of carbon-metal clusters (single-wall carbon nanotube precursor). By contrast, the vaporized materials (C, Ni, and Co species) at the target surface were well mixed in helium, and single-wall carbon nanotubes (SWNTs) might grow in the annealing and cooling zone at the backside of the reactor. The same numerical approach may be used to design modifications of the reactor in order to favor the growth of SWNTs.

Radiation heat transfer within a solar chemical reactor for the co-production of zinc and syngas is analyzed by the Monte Carlo ray-tracing method. The reactor is treated as a 3D nonisothermal cavity-receiver lined with ZnO particles that are directly exposed to concentrated solar irradiation and undergo endothermic reduction by CH 4 at above 1300 K. The analysis includes coupling to conduction/convection heat transfer and chemical kinetics. A calculation of the apparent absorptivity indicates the cavity’s approach to a blackbody absorber, for either diffuse or specular reflecting inner walls. Numerically calculated temperature distributions, zinc production rates, and thermal efficiencies are validated with experimental measurements in a solar furnace with a 5-kW prototype reactor. At 1600 K, the zinc production rate reached 0.12 mol/min and the reactor’s thermal efficiency exceeded 16%. Scaling up the reactor to power levels of up to 1 MW while keeping constant the relative geometrical dimensions and the solar power flux at 2000 suns results in thermal efficiencies of up to 54%.

Natural convection heat transfer coefficients for a rectangular array of eight tubes contained in a thin enclosure of aspect ratio 9.3:1 and inclined at 30 deg to the horizontal are measured for a range of transient operating modes typical of a load side heat exchanger in unpressurized integral collector-storage systems. Water is the working fluid, and thermal charging is accomplished via a constant heat flux on the upper boundary. All other boundaries are well insulated. Results for isothermal and stratified enclosures yield the following correlation for the overall Nusselt number: Nu D = 0.728 ± 0.002 Ra D 0.25 , 4.0 × 10 5 ⩽ Ra D ⩽ 1.4 × 10 7 . The flow field in the enclosure is inferred from measured temperature distributions. The temperature difference that drives natural convection is also determined. The results extend earlier work for the case of a single tube and provide limiting case heat transfer data for a tube bundle that occupies the upper portion of the collector storage.