Performance Of Solar Receiver Research Articles

Convective heat loss from a modified solar cavity receiver with vertical plate fins: An experimental assessment

AbstractExperimental investigations have been conducted to study the convective heat transfer from a cylindrical solar cavity receiver with vertical fins. The experiments were performed under varying surface heat flux levels and at seven inclination angles, ranging from −90° (upward facing) to +90° (downward facing) at 30° intervals. The impact of fins on the heat transfer process was studied by conducting experiments in two scenarios, namely, finned and unfinned cavities. The findings of the study showed that with an increase in cavity inclination, the magnitude of convective heat loss decreased in both finned and unfinned cavities, while the cavity surface temperature increased. At +90° inclination, the convective heat loss and Nusselt number were observed to have the lowest value, while the surface temperature had the highest value. For a downward‐facing cavity, the fins reduced convective heat loss, leading to an increase in cavity surface temperature. The finned cavity performed better for a vertically downward‐facing inclination (+90°) as it had a contribution of only 11% for convection heat loss compared with the unfinned cavity, which had a contribution of 21% for the same. Furthermore, an empirical model was developed based on the experimental results for the Nusselt number, which correlates experimental data with an error margin of ±15%. This model can be used to predict the Nusselt number for different inclination angles and surface heat flux levels. The presence of vertical fins in the cavity was found to be effective in reducing convective heat loss, especially for downward‐facing cavities. Understanding the influence of fin and cavity inclination on convective heat transfer can lead to enhanced efficiency and performance of solar receivers, thereby increasing the overall energy output of the system.

An experimental study of solar receiver under cloudy weather conditions

The performance of the central solar receiver should be properly examined for an effective design of a solar power or solar heating system. The performance of solar receiver is affected by three parameters. These are the amount of solar radiations, the state of the heating medium and the temperature of the surrounding air. Also, while evaluating the performance of a solar receiver, the ambient weather conditions should be taken into account. As a result, when a large amount of direct light strikes a solar receiver, the power generated increases as well. In contrast, as the amount of direct light falling on the solar receiver diminishes, so does the amount of electricity generated. As a result, it is generally understood that on sunny days, the performance of the central solar receiver will be higher, while on overcast days, the performance will be reduced. The solar receiver collects heat from diffused or reflecting light (when the sun is obscured by clouds) and absorbs reflective light from glossy or light-colored surfaces. The experimental results reveal that solar radiation collection performance is dependent on climatic circumstances; thus, when installing a solar receiver, it should be suitable for the specific region depending on weather conditions.

A novel approach for volumetric solar receiver performance assessments

A novel design and an efficient operation of solar volumetric receivers remain critical for successfully integrating such receivers into solar energy harvesting devices. The performance of the solar volumetric receiver is highly dependent on the geometric connectivity of the actual pore-sites in the SiC open-cell foams. Although the performance of SiC foam receivers has been investigated previously considering the homogeneous foam structure, further studies are needed to explore the influence of the actual path of the pores and their connectivity on the flow and thermal performance of the receiver. Hence, the present study considers the novel design of a solar volumetric absorber incorporating the actual SiC foam receiver geometric features. Accordingly, the thermal and flow behavior of the working fluid within the receiver is examined considering the 3D connectivity of the pore sites. Moreover, the performance of the solar volumetric receiver is evaluated for different Reynolds numbers, solar concentrations, and foam porosities. A 3D-dimensional numerical investigation is carried out incorporating the interactions between convection, conduction, and radiation heat transfer for which the discrete ordinate radiation model is used. Porous SiC foam is placed in a channel having an optically transparent wall at the top and air is used as the working fluid extracting heat from foam. The overall heat transfer coefficient, normalized Nusselt number, and thermal effectiveness are evaluated for various operating conditions. Findings revealed that the receiver thermal performance improves at high Reynolds numbers. The convection heat transfer is found to be the apparent heat transfer mode within the porous SiC foam. In addition, the pumping power to overcome the flow resistance in the channel is considerably less than that of the rate of heat transfer gain.

Thermal performance and thermal stress analysis of a supercritical CO2 solar conical receiver under different flow directions

To ensure the safe and efficient operation of the receiver in the concentrated solar power system, it is necessary to study its performance characteristics. Different from considering the thermal performance alone, this study comprehensively analyzed the thermal and mechanical performance of the solar receiver by using a coupled thermal-mechanical model. The critical factors including geometric parameters (aspect ratio H/D and angle ratio φ/φmax), operation parameters (mass flow rate, inlet temperature and direct normal irradiance (DNI)) and flow directions (up-flow and down-flow) were investigated. The results demonstrated that the optimum thermal efficiency achieved 85.67% when H/D = 1.5 and φ/φmax = 0.25, and the maximal thermal stress was 87.61 MPa. Increasing the flow rate from 0.01 kg/s to 0.06 kg/s improved the thermal performance, but increased the thermal stress. An opposite trend was observed when the inlet temperature increased from 573 K to 773 K. As the DNI increased from 200 W/m2 to 1000 W/m2, the maximal thermal stress increased by approximately 3 times, when the DNI >800 W/m2, the thermal performance would be deteriorated. Although the thermal performance under up-flow was better, the thermal stress was greater, the receiver should adopt an up-flow flow direction.

A comparison between lumped parameter method and computational fluid dynamics method for steady and transient optical-thermal characteristics of the molten salt receiver in solar power tower

Lumped parameter method (LPM), which neglects the change of the temperature within the cross-section of the molten salt, is widely used to predict solar receiver's steady and transient optical-thermal characteristics. To consider the influences of the temperature variation inside molten salts on the performance of solar receivers, the receiver's steady and transient thermal performance are comparatively studied by computational fluid dynamics method (CFD) and LPM. The results show that the rise time of the outlet temperature predicted by the CFD model under the start process, the change of mass flow rate, and the change of direct normal irradiance are more than two times those of the LPM model. Take the start process on spring equinox as an example, the rise time predicted by the CFD and LPM model are 52s and 22s, respectively. Therefore, the temperature distribution within the molten salt should be considered for predicting a more precise transient thermal performance of the receiver. The predicted outlet temperatures in the steady state by the two models are nearly the same, with an absolute difference less than 2.0 °C, which indicates that whether considering the temperature distribution within the molten salt does not influence receiver's steady thermal characteristics.

A spectral self-regulating parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating

A significant decrease in the solar-thermal conversion efficiency of parabolic trough collectors occurs at high operating temperatures, mainly due to the massive radiant heat loss from the parabolic trough solar receiver incurred in this case. In this paper, a novel parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating is proposed to effectively reduce the radiant heat loss and improve the overall performance of solar receivers. The thermochromic layer, deposited on the glass envelope’s inner surface in the negative thermal-flux region, has a reversible transition from a monoclinic (M) phase to a rutile (R) phase at the critical temperature of 68 °C. The occurrence of the phase transition in the thermochromic coating induces beneficial self-regulation of the spectrum-selectivity characteristics of the glass envelope, i.e., transmittance, reflectance, and absorptance (emittance) of glass envelope to solar and infrared radiation, for enhancing the thermal performance of solar receivers. Comprehensive heat transfer models based on the finite volume and spectral radiant heat transfer methods are established in this study. It is validated the models can predict the thermal performance of the solar receivers with high accuracy. In this framework, the total heat loss, thermal efficiency, and exergy efficiency of parabolic trough collector systems integrated with the proposed novel solar receivers are investigated and analyzed. Besides, the effects of transmittance of thermochromic coating (M phase) and emittance of thermochromic coating (R phase), two key parameters, on the overall performance of proposed solar receivers are also studied. The results show that no matter under the M phase or R phase, the thermochromic coating can play unique advantages to improve the thermal performance of the proposed solar receivers. The heat loss and thermal efficiency of the proposed solar receivers are effectively reduced and enhanced by 18.2% and 4.8% at the absorber temperature of 600 °C and inlet fluid temperature of 580 °C, respectively.

Thermal performance analysis of a porous solar cavity receiver

A solar cavity receiver filled with a porous layer now is popularly found in solar energy industry. As its thermal performance will influence the efficiency of solar energy utilization systems considerably, deep understanding on it is urgent for the development of solar energy industry. In the present work, the influences of thickness, effective thermal conductivity and permeability of a porous layer on thermal performance of a porous solar cavity receiver are investigated comprehensively for the first time. The analysis is carried out not only from the viewpoint of the first law of thermodynamics but also from the second law of thermodynamics. Three new findings are obtained: (1) a critical value of thickness of the porous layer, (2) a critical value of effective thermal conductivity of the porous layer and (3) a critical value of permeability of the porous layer, across which the thermal performance of the solar receiver will alter significantly. In addition, some comparison is conducted between the observations in this research and those in previous publications to check the effects of the configuration and flow driven mechanism of a porous solar cavity receiver on its thermal performance for the first time.

Comprehensive experimental testing and analysis on parabolic trough solar receiver integrated with radiation shield

Parabolic trough collectors (PTCs) are the most mature way to harvest high-temperature heat source and widely applied in solar thermal utilizations. Parabolic trough solar receivers as the heat-collecting elements (HCEs) are the key parts of PTC, but face with a knotty problem that is exploding radiative heat loss under high operating temperature, which exerts a significantly negative role on the overall performance of the PTC system. For effectively reducing the heat loss and improving the thermal performance of solar receiver, a structurally optimized HCE with an inner radiation shield was proposed, designed, and manufactured. Furthermore, the indoor heat loss and outdoor thermal efficiency testing were carried out in the Institute of Electrical Engineering, Chinese Academy of Sciences (IEECAS) to validate comprehensive thermal performance of the proposed HCEs. The results show that the radiation shield plays an effective role in reducing the heat loss and improving the thermal efficiency. The heat loss of the proposed HCE is significantly reduced by 28.0% compared to the conventional HCE at the absorber temperature of 550 °C. And the proposed HCE possesses superior performance at high operating temperature and low solar irradiance. In the case of inlet temperature of 350 °C and solar irradiance of 600 W/m2, the thermal efficiencies of proposed HCE and conventional HCE are 49.4 and 51.8% respectively, and the thermal efficiency of the proposed HCE is effectively enhanced by 4.9%.

Comparison of control strategies for a solar heating system with underground pit seasonal storage in the non-heating season

Abstract The solar heating system coupled with seasonal thermal storage (SHSSTS) is a promising solution to solve the seasonal mismatch between the solar energy supply and heating demand. The performance of SHSSTS in the non-heating season has a vital impact on the discharging process during the heating season, not only the quantity of energy, but also the level of energy. The aim of this work is to analyze the impact of different control strategies on the performance of the system during the non-heating season by means of both experiments and simulation methods. A pilot solar heating system integrating with a 3000 m3 underground water pit seasonal storage (UWPS) was built in Hebei, China. A calibrated system model was established in TRNSYS. Good agreement between measurement and simulation was obtained. The solar collection efficiency, stratification of the UWPS, energy storage efficiency and exergy storage efficiency were demonstrated for evaluating the system performance. Results showed that the control strategies were significant for improving the heat collection performance of solar receiver and the exergy efficiency of the UWPS. The stratification of the seasonal storage has an impact on the collection efficiency of the receiver, especially at the end of the non-heating season. In addition, at the end of the non-heating season in typical year, the solar collection efficiency could be increased by 10% in variable flow control compared to the temperature difference control.

Effects of double windows on optical and thermal performance of solar receivers under concentrated irradiation

To fulfill the working condition at high pressure and temperature, a double-windowed pressurized volumetric receiver (DW-PVR) was proposed in this work. A computation model combining Monte-Carlo Ray Tracing method (MCRTM) and Finite Volume method (FVM) was established to evaluate the temperature field, energy losses, and efficiencies of the receiver. The model also includes a gray-band approximation to consider the spectral properties of windows. The performance of receiver with two windows and with single one (SW-PVR) was compared with each other. Based on the model, effects of five influential factors were fully studied. It is found that the total efficiency is improved by increasing cavity size, inner window thickness, emissivity of inner surfaces and distance between windows. Compared with SW-PVR, the DW-PVR gets a higher thermal efficiency at the expense of a lower optical efficiency. With a relatively large distance between windows and relatively small cavity size, the total efficiency of DW-PVR can be higher than that for SW-PVR. The peak temperature of the outer window in DW-PVR is 38.6–82.6 K lower than that in SW-PVR. The present work can provide a deeper understanding of characteristics of DW-PVRs.

Analysis of Solar Receiver Performance for Chemical-Looping Integration With a Concentrating Solar Thermal System

This paper introduces a chemical-looping configuration integrated with a concentrating solar thermal (CST) system. The CST system uses an array of mirrors to focus sunlight, and the concentrated solar flux is applied to a solar receiver to collect and convert solar energy into thermal energy. The thermal energy then drives a thermal power cycle for electricity generation or provides an energy source to chemical processes for material or fuel production. Considerable interest in CST energy systems has been driven by power generation, with its capability to store thermal energy for continuous electricity supply or peak shaving. However, CST systems have other potential to convert solar energy into fuel or to support thermochemical processes. Thus, we introduce the concept of a chemical-looping configuration integrated with the CST system that has potential applications for thermochemical energy storage or solar thermochemical hydrogen production. The chemical-looping configuration integrated with a CST system consists of the following: a solar-receiver reactor for solar-energy collection and conversion, thermochemical energy storage, a reverse reactor for energy release, and system circulation. We describe a high-temperature reactor receiver that is a key component in the chemical-looping system. We also show the solar-receiver design and its performance analyzed by solar-tracing and thermal-modeling methods for integration within a CST system.

Experimental and numerical performance analyses of Dish-Stirling cavity receivers: Radiative property study and design

The solar receiver performance has a direct impact on the CSP power plant performance and, thereby, its levelized cost of electricity. Improved receiver designs supported by new advanced numerical tools and experimental validation campaigns directly help to make CSP technology more competitive. This paper presents an experimental and numerical investigation of the influence of the cavity receiver radiative properties and the thermal power input on the Dish-Stirling performance. Three cavity coatings are experimentally investigated: the original cavity material (Fiberfrax 140), Pyromark 2500 and Pyro-paint 634-ZO. Moreover, simulations validated with the experimental measurements are utilized to define a higher performance cavity receiver for the Eurodish system. The results indicate that the absorptivity of the cavity should be as low as possible to increase the receiver efficiency whereas the optimum emissivity depends on the operating temperatures. If the cavity temperature is lower than the absorber temperature, low emissivities are recommended and vice-versa. All material/coatings analyzed for the cavity provide similar receiver efficiencies, being Fiberfrax 140 slightly more efficient. Finally, a total receiver efficiency of 91.5% is reached by the proposed Eurodish cavity receiver when operating under the most favorable external conditions.

Experimental evaluation of a novel solar receiver for a micro gas-turbine based solar dish system in the KTH high-flux solar simulator

This work presents the experimental evaluation of a novel pressurized high-temperature solar air receiver for the integration into a micro gas-turbine solar dish system reaching an air outlet temperature of 800 °C. The experiments are conducted in the controlled environment of the KTH high-flux solar simulator with well-defined radiative boundary conditions. Special focus is placed on providing detailed information to enable the validation of numerical models. The solar receiver performance is evaluated for a range of operating points and monitored using multiple point measurements. The porous absorber front surface temperature is measured continuously as it is one of the most critical components for the receiver performance and model validation. Additionally, pyrometer line measurements of the absorber and glass window are taken for each operating point. The experiments highlight the feasibility of volumetric solar receivers for micro gas-turbine based solar dish systems and no major hurdles were found. A receiver efficiency of 84.8% was reached for an air outlet temperature of 749 °C. When using a lower mass flow, an air outlet temperature of 800 °C is achieved with a receiver efficiency of 69.3%. At the same time, all material temperatures remain below permissible limits and no deterioration of the porous absorber is found.

Optical efficiency measurement of solar receiver tubes: A testbed and case studies

Knowledge of the parabolic-trough collector (PTC) solar receiver performance is essential in solar thermal power plants using this type of collector technology because it is the component responsible for collecting the solar radiation and transferring it in the form of thermal energy to the heat transfer fluid. Its performance depends on two parameters, heat loss and the optical efficiency of the solar receiver tubes. Therefore, the receiver optical efficiency must be known for correct sizing of a PTC solar field. This article presents an outdoor testbed developed to measure the optical efficiency of solar receiver tubes under real solar radiation conditions, including a detailed description of the system developed, working principle, test procedure, and performance results for two sample receiver tubes tested showing proper system functioning.

Numerical investigation and experimental validation of the impacts of an inner radiation shield on parabolic trough solar receivers

Conventional parabolic trough solar receivers are widely used to harvest heat energy at temperatures ranging from 400 °C to 550 °C. However, high temperatures cause excessive heat loss in solar receivers. Two types of novel solar receivers with an inner metal radiation shield (RS), one with solar selective absorbing coating on the outer surface and one without, were proposed and constructed to improve the thermal performance of solar receivers. Experiments were conducted in an enthalpy difference lab, and mathematical models with spectral radiant distributions were established to predict the thermal performance of the solar receivers. A comparison between the simulated and experimental results showed satisfactory consistencies. Predictions were obtained using the models with the root mean square deviation of less than 6%. The novel solar receiver without solar selective absorbing coating on the outer surface of the RS showed superior performance at absorber temperatures exceeding 550 °C. At the absorber temperature of 600 °C, the percentage of heat loss reduction of the receiver with solar selective absorbing coating and of that without reached 23.4% and 24.2%, respectively.

Numerical investigation of pitch value on thermal performance of solar receiver for solar powered Brayton cycle application

Small scale open solar thermal Brayton cycle with Concentrated Solar Power (CSP) system can provide an efficient, clean, sustainable and low cost energy conversion system to produce different forms of energy such as electricity. The current paper investigates various configurations of open cavity solar receivers including cylindrical, conical and spherical with an aperture of 0.02835 m2 and average irradiance values of 2.5 kW/m2. Advanced ray tracing (OptisWorks®12), as this is the latest available version) and computational fluid dynamic (CFD, ANSYS® 15) simulations are used to reduce optical and thermal losses and maximize the exit temperature of the working fluid. The received irradiance on the external surface of the helical coils inside these receiver geometries was used to predict the heat transfer fluid temperature through CFD analysis. Investigations were carried out to discover the effects of coil pitch and tube diameter on the working fluid's exit temperature. The results showed that for a pitch value of 3 mm and a tube diameter of 10 mm, the exit temperature is 401.3 K, 405.7 K and 409.4 K for each of the spherical, cylindrical and conical geometries respectively; indicating that the conical shaped receiver is more efficient than the other geometries. Moreover, the best pitch value, when the higher outlet temperature was achieved, depends on both the tube diameter and the cavity configuration. The effect of some other factors such as the ambient temperature and the pressure losses on the receiver's performance have also been investigated in this study.

Multi-parameter optimization design of parabolic trough solar receiver

The design parameters of parabolic trough solar receiver are interrelated and interact with one another, so the optimal performance of solar receiver cannot be obtained by the convectional single-parameter optimization. To overcome the shortcoming of single-parameter optimization, a multi-parameter optimization of parabolic trough solar receiver is employed based on genetic algorithm in the present work. When the thermal efficiency is taken as the objective function, the heat obtained by working fluid increases while the average temperature of working fluid and wall temperatures of solar receiver decrease. The average temperature of working fluid and the wall temperatures of solar receiver increase while the heat obtained by working fluid decreases generally by taking the exergy efficiency as an objective function. Assuming that the solar radiation intensity remains constant, the exergy obtained by working fluid increases by taking exergy efficiency as the objective function, which comes at the expense of heat losses of solar receiver.

Numerical Heat Transfer Analysis of a 50 kWth Pressurized-Air Solar Receiver

A high-temperature pressurized-air solar receiver, designed for driving a Brayton cycle, consists of a cylindrical SiC cavity and a concentric annular reticulated porous ceramic (RPC) foam enclosed by a steel pressure vessel. Concentrated solar energy is absorbed by the cavity and transferred to the pressurized air flowing across the RPC by combined conduction, convection, and radiation. The governing mass, momentum, and energy conservation equations are numerically solved by coupled Monte Carlo (MC) and finite volume (FV) techniques. Model validation was accomplished with experimental data obtained with a 50 kWth modular solar receiver prototype. The model is applied to elucidate the major heat loss mechanisms and to study the impact on the solar receiver performance caused by changes in process conditions, material properties, and geometry. For an outlet air temperature range 700–1000 °C and pressure range 4–15 bar, the thermal efficiency—defined as the ratio of the enthalpy change of the air flow divided by the solar radiative power input through the aperture—exceeds 63% and can be further improved via geometry optimization. Reradiation is the dominant heat loss.

Experimental investigation on the energy and exergy performance of a coiled tube solar receiver

In this article, an experimental investigation is carried out to examine the heat transfer characteristics of a coil type solar dish receiver under actual concentrate solar radiation conditions. During the test, the concentrated solar flux is approximately 1000kW/m2 at aperture. The solar irradiance is almost unchanged (650W/m2) for continuous two hours in the afternoon, which is used to analyze the energy and exergy performance of the solar receiver. Experimental results show that, the efficiency of the solar receiver is normally above 70% with the highest efficiency of 82%, whereas at steady state, the efficiency is maintained at around 80%. A very low value of the heat loss factor (0.02kW/K) could be achieved during the current steady state operating conditions. The highest value of the exergy rate is around 8.8kW, whereas the maximum energy rate can reach 21.3kW. In addition, the highest exergy efficiency is approximately 28%, and the highest energy efficiency is around 82%.

Simplified heat loss model for central tower solar receiver

Heat loss is an important factor in predicting the performance of solar receiver of concentrated solar power (CSP) systems. This study presents a numerical simulation calculating convection and radiation heat losses from four different receiver shapes including external and cavity type receivers with different opening ratios (ratio of cavity aperture area to receiver area). The simulation was carried out using Fluent CFD (computational fluid dynamics) software considering three different receiver temperatures (600, 750, and 900°C), three wind velocities (1, 5, and 10m/s), and two wind directions (head-on and side-on). The simulation results were then used for deriving a simplified correlation model which gives the fraction of convection heat loss by a function of opening ratio, receiver temperature, and wind velocity. The calculated fraction can be easily converted to convection heat loss, total heat loss, or receiver efficiency once the radiation heat loss is estimated by any applicable prediction model. Calculated heat loses by the proposed simple correlation model showed good agreements with the simulation results with 11.4% and 5.9% average absolute deviations for convection heat loss and total heat loss, respectively. Validation of the model with experimental data was also carried out using test results available from three central receiver systems (Martin Marietta, Solar One and Solar Two).