Thermal Performance Of Receiver Research Articles

The Effect of Uneven Metal Foam Distribution on Solar Compound Parabolic Trough Collector Receiver Thermal Performance

Solar energy is a key player among other renewable energies that reduce greenhouse gases and replace conventional fuel, i.e., to solve global warming and fossil fuel descending issues. However, the thermal solar systems’ performance should be enhanced to cope with intermittent solar radiation. Metal foam can be used as an enhancer in solar collectors’ receivers. However, metal foams are associated with pressure drop. To benefit from the metal foam as a thermal enhancer and overcome the pressure drop issue, the present study numerically and experimentally investigates a novel Compound Parabolic Solar Collector (CPC) receiver with innovative uneven metal foam inserts of varying thickness. Two pores per inch (PPI) Cu-foam inserts, PPI10 and PPI20, were tested. These inserts were strategically placed at three different positions along the receiver, with thicknesses of 3 cm, 2 cm, and 1 cm starting from the inlet side. In the experimental part, three tubular receivers were tested, empty, i.e., without metal foam, inserted with Cu-foam of PPI10, and inserted with Cu-foam of PPI20. The experiments were conducted from 09:00 AM to 04:00 PM. The investigation involved water volume flow rates from 0.2 to 0.6 l/min. The numerical part included solving the governing equations, i.e., mass, momentum, and energy conservation, simulating conditions similar to the experiments. The Brinckman model described the fluid flow through the metal foam. The thermal performance of the CPC system was evaluated using the Nusselt number (Nu), thermal efficiency, water bulk temperature, and water outlet-inlet temperature difference. Inserting Cu-foam of PPI20 resulted in the hourly maximum Nu and thermal efficiency compared to the empty and PPI10 cases. Experimentally, the hourly maximum Nu was 8.9, 8.4, and 7.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. The average thermal efficiency was 88.3, 85, and 81.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. As for the outlet-inlet water temperature difference, the highest values were at 0.2 l/min. Again, PPI20 recorded the best results, i.e., 23.3 K, 21.5 K, and 20.2 K for PPI20, PPI10, and empty cases, respectively.

A comparative study on optical and thermal performance of parabolic trough concentrator using a bundle of absorber tubes

Reorganizing the larger diameter receiver tube into a bundle of smaller diameter receiver tubes in regions of high solar flux density can enhance the thermal performance of the solar parabolic trough collector. The data that really is presently available in the literatures are studied. The parabolic trough concentrator's optical performance and the solar parabolic trough receiver thermal performance is 66.4% and 74.8% respectively. It is concluded from the comparative study that the use of the original receiver bundle (copper tube selective coating), has been ascertained, improving convective heat transfer coefficient, bringing down receiver tube surface temperature and increasing thermal performance, but the expense of raising friction coefficient and pumping power.

Numerical investigation to optimize the modified cavity receiver for enhancement of thermal performance of solar parabolic dish collector system

The concentrated parabolic dish collector (PDC) systems are widely utilized for the purpose of generating high-temperature heat by receiving and converting solar energy efficiently into thermal energy. Cavity receivers including spiral coils have a common use in PDC system for optimal thermal performance. Therefore, to enhance the thermal performance of a PDC system, the present study investigates different cavity receiver configurations, such as cylinder and conical shaped receivers with flat and spherical absorber surface as well as the effect of spiral coil arrangement. To identify the most optimal spirally coiled cavity receiver, the comparison is conducted based on outlet temperature, heat exchange rate, thermal efficiency, effectiveness, and thermal performance capability (TPC). The investigations were conducted using the ANSYS Fluent CFD simulation software package. The SolTrace software package was utilized to compute the maximum heat flux incident on the absorber surface of the receiver, and peak heat flux data was used as input parameter at the absorber surface of the receiver. Furthermore, to select the suitable working fluid in PDC system, different heat transfer fluids, such as Syltherm 800, Xceltherm 600, ParathermHR™ and water, respectively were studied. From the thermal analysis of various proposed receiver geometries, the receiver with conical shape and spherical absorber surface is more efficient over other configurations. It provides 50 % higher heat transfer rate thancylindrical receiver with flat absorber surface. Moreover, receiver's thermal performance could modestly be improved by modifying the spiral coil pitch (variable pitch). In terms of pumping power consumption, readily availability, low viscosity, and high specific heat, water is suggested as working fluid for receiver of PDC system.

Modelled annual thermal performance of a 50MWth refractory-lined particle-laden solar receiver operating above 1000°C

The paper reports on the thermal performance of the sub-system for a solar thermal particle technology used to generate high temperature air, including refractory-lined particle-laden receiver, particle separator, particle storage and particle feeder. These assessments are made with a transient mathematical model developed to calculate the heat and mass transfer within the cavity of the receiver together with the thermal losses to the surroundings, incorporating the influence of solar transients during start-up, turndown or shutdown periods. New insights are provided of the influences of the variables of refractory configuration and of the potential operating controller parameters to manage the influence of solar variability on the annual thermal performance of the system, considering the useful thermal gain of hot air. The model is further used to advance the understanding of the sensitivity of the thermal performance to the mass flow rate of inlet air and mass loading of particles in the receiver on the sensible energy harnessed. The influence of the returned air temperature on the receiver thermal performance is also assessed, to provide insights on the suitability of the present configuration to re-heat already hot air in a CST system. Further to this, the thermal outputs are compared with available CFD data for this configuration, and with that reported for a cavity reactor, to provide information on the model validation.

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.

Numerical study on the transient thermal performance of a lab-scale molten salt receiver

The non-uniformity of temperature distribution and thermal stress distribution in the on-site high-temperature molten salt receiver is frequently reflected in the receiver's unstable operating circumstances. As a result, the focus of this research is on the transient thermal performance of the receiver in non-steady-state situations. An in-house software was used to estimate the transient temperature distribution of a lab-scale receiver using a three-dimensional transient model built for the receiver's thermal performance calculation. The transient thermal performance of a lab-scale receiver was studied under some variable operating conditions, such as the startup process, varying mass flow rate, varying radiation flux, and varying ambient wind speed, using a combination of numerical prediction and variable-condition experiment on a lab-scale receiver. When the working environment changed, the temperature distribution of the receiver took around 12 s to return to a stable condition. In addition, when the working environment changed, the transient temperature fluctuations of the receiver were given and evaluated in depth in this study.

Coupled optical and thermal simulation of the thermal performance of a 50 MWe external cylindrical solar receiver

The performance of the central receiver is directly related to the efficiency and power generation capacity of the solar tower power plant. In this paper, by combining the analytical approximation method and the finite volume method, the coupled optical and thermal performance of a 50 MWe external cylindrical receiver was numerically evaluated. The thermal performance of the receiver under design parameters was first simulated, and the model was verified by the comparison with the method in Zhou et al. [Renewable Energy 164, 331–345 (2021)]. The effects of key parameter and uniformity of incident flux distribution on the thermal performance were then extensively investigated. Finally, the thermal performance changes in one day were explored. The results showed that direct normal radiation (DNI), salt inlet mass flow, salt inlet temperature, and coating absorption rate strongly affected the receiver's thermal performance. Compared with the non-uniform incident flux distribution, the accurate salt outlet temperature could also be obtained under the uniform incident flux distribution, but the thermal efficiency was underestimated by 0.5%, and the wall temperature on the front side of the tube was generally underestimated, reaching 5.6% at the location of the maximum wall temperature. Evolution trend of the salt outlet temperature and maximum wall temperature on vernal equinox was similar to that of the DNI distribution. From 14:00 to 18:00, they decreased by 18.3% and 12.3%, respectively. The maximum wall temperature from 13:00 to 16:00 exceeded the design value of 700 °C, which was detrimental to the safe operation of the receiver and might cause decomposition of molten salt in the tube.

A Novel design of the solar central receiver to improve the performance of the central solar power tower plant

A novel design for a central receiver prototype was manufactured with dimensions of (50*50) cm staggered configuration using copper pipes of 9.5 mm diameter was tested to produce thermal energy from solar energy, had been analyzed numerically and experimentally. central solar tower receiver prototype was manufactured and mounted at Al-Nahrain University in Baghdad - Iraq, were high solar radiation is available. Heliostats with automated dual axes tracking systems were attained to capture the solar rays. Heliostats used to redirect the sun rays to a central receiver which is constructed using coated copper tubes to convert solar to heat and transfer it to the receiver working thermo-fluid.According to a precise design consideration presented procedure in this study, a numerical and experimental investigation was performed. Also, an automated tracking system was installed on the heliostat to govern the title and azimuth movements.The working fluid used is water. The supply tank was manufactured from plastic equipped with a digital flow meter to control flow rate to the receiver. In this study, a detailed layout, investigation for each part is shown. It’s found that optimum tower height is 4.5m, optimum heliostat horizontal distance from the tower is 7m and the offset angle is 30°. Receiver thermal performance enhanced by 6.17% from the one-row receiver and about 12.98% for the evacuated tube receiver.

Enhancing the overall thermal performance of a large aperture parabolic trough solar collector using wire coil inserts

With the use of large apertures (higher concentration ratios) in parabolic trough solar collectors, increased temperature gradients, increased heat losses and increased heat transfer irreversibilities become inevitable. As such, means of reducing the magnitude of these operating parameters to enhance the overall thermal and thermodynamic performances become crucial. In this study, the use of wire coil inserts in the receiver’s absorber tube to improve the parabolic trough solar collector’s performance and to lessen the associated temperature gradients is presented. The parabolic trough solar collector having an aperture width of 9 m and a rim angle of 80° was modeled. Using Monte-Carlo ray tracing, the realistic heat flux profile on the receiver’s absorber tube was obtained. The resulting non-uniform heat flux profile was later coupled to a finite volume based computational fluid dynamics model. The working fluid properties were considered to be temperature-dependent. The wire coil inserts with a pitch of 0.076, 0.114 and 0.152 m and widths of 0.03, 0.033 and 0.036 m were examined in this study. The wire coil has a triangular cross-section of 0.0076 m in size. Results show significant improvements in receiver thermal performance with the use of wire coil inserts owing to the improved fluid mixing, disruption of the thermal boundary layer and reduction in the absorber tube temperatures. The heat transfer performance is increased up to 183% whereas the thermal efficiency improves between 0.4 and 1.4% when the flow rate is below 13 m3/h.

Effects of the superheater layout on the thermal performance of a central cavity receiver

For a water/steam solar cavity receiver, the superheater usually works at a very high temperature. It is necessary to consider the installation location of the superheater inside the cavity for its safe operation. In the present work, the effects of the superheater layouts on the thermal performance of the receiver were numerically investigated. A computational model was proposed to solve both the heat transfer and the thermal stress. With this model, the heat losses of the receiver and the temperature distributions were obtained under different superheater locations. Besides, the thermal elasticity theory was employed to quantify the thermal stress in the superheater. The results showed that the heat flux of the superheater apparently drops when it is moved from the center to the upper zone of the cavity. Consequently, the maximum wall temperature of the superheater is remarkably reduced from 740℃ to 470℃, resulting in the reduction of thermal stress by 50% approximately. In addition, due to the cavity effect, the thermal efficiency of the receiver changes slightly as the superheater location varies. Moreover, with the incident power rising, the boiler and superheater panels can absorb more solar radiation simultaneously, since they are installed together inside the cavity. It will lead to the evaporation rate and flowrate of superheated steam increasing. Therefore, the outlet temperature of superheated steam has noobviouschange even though the incident power varies significantly.

Thermal and mechanical analysis of a sodium-cooled solar receiver operating under a novel heliostat aiming point strategy

The nature in which a solar receiver in a concentrated solar power plant interacts with an accompanying heliostat field plays a significant role in plant performance and economics. An appropriate heat flux distribution should help deliver maximum receiver thermal performance, while minimising mechanical damage – thereby maximising power production and reducing costs. The current work presents an investigation into the thermal performance and mechanical reliability of a sodium-cooled solar receiver operating under heat flux profiles generated by a novel heliostat aiming strategy. A modification of the HFLCAL model is used to generate heat flux profiles for individual heliostats in a representative plant, and simulated annealing optimisation techniques are used to produce a novel heliostat aiming strategy. The importance of giving consideration to receiver limitations under non-uniform thermal boundary conditions in the development of a heliostat aiming strategy is demonstrated in this study, with mathematical optical, thermal, and mechanical models used to complete the analysis. An investigation has been conducted for a point-in-time resulting in maximum thermal loading conditions, with theoretical modelling techniques used to calculate receiver tube temperatures for aiming strategy yielded heat flux profiles, thereby allowing for the determination of heat losses and mechanical reliability through creep-fatigue damage. Results show that the simulated annealing algorithm can significantly improve heat flux homogeneity on the receiver, potentially reducing peak heat flux to less than 10% that of a single aiming point strategy, given an appropriate spillage allowance and aiming point grid size. A satisfactory configuration of spillage allowance and aiming grid size exists so as to supply maximum power to the receiver, while uniformly distributing the incident heat flux in order to meet mechanical reliability requirements. Based on the receiver design and conditions simulated in the analysis, a grid constructed of more than 81 aiming points (receiver area coverage of 32.7%), and an additional spillage allowance of 10% allows the receiver to deliver maximum power output while retaining mechanical durability through a 30 year plant life cycle.

Effects of surface optical and radiative properties on the thermal performance of a solar cavity receiver

Solar cavity receiver is one of the most crucial components in a solar power tower system, where the solar-thermal conversion process is achieved. Optimizing the surface properties (viz. absorptivity and emissivity) of a designed cavity receiver is a viable option to improve its thermal efficiency. In the present paper, an integrated simulation approach was employed to quantify the influence of surface optical and radiative properties on the thermal performance of a typical water/steam solar cavity receiver. Firstly, the different candidate surface materials, solar selective and non-coated, were compared. The results showed that the thermal efficiency of the receiver with the ideal coating and Pyromark 2500 is respectively enhanced by 12.5% and 7.8% compared with that of the non-coated receiver. And then, the effects of surface spectral selectivity on the thermal performance of the receiver were carefully analyzed. It can be found that the solar absorptivity is the most critical parameter for improving the receiver efficiency, which is enhanced by about 12.6% as the solar absorptivity rises from 0.8 to 1.0. However, the receiver efficiency shows different variation tendencies with the thermal emissivity of active surfaces, which are closely related to the infrared heat transfer direction between the passive surfaces and the active surfaces. Furthermore, with the increase of reflectivity of passive surfaces over the full spectrum, the receiver efficiency is determined by a trade-off between the increasing reflective heat loss and the decreasing radiative and convective heat losses. For the present receiver, the thermal efficiency is improved by about 4.8% with the reflectivity increasing from 0 to 1.0 due to its great cavity effect. Therefore, the passive surfaces should be highly reflective throughout the spectrum inside the present receiver.

Cavity receiver thermal performance analysis based on total heat loss coefficient and efficiency factor

A cavity structure effectively decreases the heat loss of a receiver. This paper applies the widely used collector analysis method to derive the total heat loss coefficient and efficiency factor for a cavity receiver. Based on the derived factors, this paper investigates the effect of different parameters, such as absorber surface temperature and emissivity, on thermal performance. The absorber surface emissivity significantly affects the total heat loss coefficient at high temperatures. The effect of the absorber surface temperature on the total heat loss coefficient is significant, while the effect of the ambient temperature on the total heat loss coefficient can be ignored. The present results can help to determine the testing conditions for the cavity receiver by using molten salt as the heat transfer fluid.

Effect of the optical properties of the coating of a concentrated solar power central receiver on its thermal efficiency

Solar receivers are one of the critical components of tower central receiver concentrated solar power plants (CSP), as this is where the light-heat conversion occurs. Better receiver designs provide higher plant efficiencies, better coupling with thermal energy storage and cost reductions. Current solar receivers are usually coated with a high sunlight absorptivity layer applied over the bare surface of the absorber receiver's tubes, in order to enhance their absorptivity and light-to-heat conversion. Here, we study the effect of the optical properties (viz. absorptivity and emissivity) of these coatings on the thermal performance of the receiver. The thermal performance of different coating candidates, solar selective and non-selective, were numerically simulated and compared against the current standard coating Pyromark 2500. Variation tendencies and the variation of the common accepted Figure of Merit (FOM) curve with temperature in different optical scenarios were also studied. Our results show that the thermal efficiency increases with the absorptivity of the coating (up to 4%). On the other hand, we observed that the emissivity has a very minor effect on the thermal performance of Molten Salt external receiver at its nominal working temperature, as the efficiency increases only by 0.6% when the emissivity of the coating decreases from 0.9 to 0.5 in a molten salts external receiver. Variation tendencies are also carefully analyzed, as well as the variation of the FOM curve with temperature in different optical scenarios for the design conditions of each receiver. At current molten salts working temperatures, improving the absorptivity of a non-selective coating leads to higher thermal efficiency than when using a selective coating. Nevertheless, for superheated steam cavity receivers, the effect of using a selective coating can be noticed at temperatures greater than 500°C. These results can be used for selecting the optimum coating type for each type of receiver, incident flux and working temperature.

In-situ heat loss measurements of parabolic trough receivers based on transient infrared thermography

The design lifetime of parabolic trough receivers is at least 25years. However, depending on the specific operating conditions, this expected lifetime may be shortened. A periodic inspection of receivers mounted in a solar field is thus recommended. Several field methods with distinct depth of information and field throughput exist to estimate receiver thermal performance. This paper gives an overview of these field methods and presents the application of a transient infrared thermography method, which combines dynamic temperature measurements with a numerical heat transfer model in order to derive specific receiver heat losses and separate internal heat loss mechanisms. This method has been previously investigated under laboratory test conditions and is here applied under field test conditions for in-situ thermal measurements on mounted receivers.The transient thermography method allows a correct estimation of specific heat losses within a precision range below ±10% for all tested receivers under field test conditions. Relevant separation of heat loss mechanisms could be derived for evacuated PTRs with solar selective absorber coating, while partially biased diagnoses could be observed for partially or non-evacuated PTRs with highly emissive absorber coatings. Active thermography is considered as a relevant tool for receiver field inspection, in combination with a complementary field screening measurement technique.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

The Required Concentration Ratio for Two Stage Water/Steam Cavity Receiver

The pioneering water/steam cavity receiver CESA-1 is not very successful for that the evaporator and superheater are in the same cavity cause it is very difficult to control the heat flux distribution on the absorber surface and hence the flow stability inside the receiver.The two-stage water/steam receiver is a promising way for separating the evaporator and superheater in two different cavities or parts. Because the two different parts have different thermal performance, the required concentration ratio(CR) of per heliostat projected two different parts will be different. Based on the transient efficiency equation and existed DAHAN solar tower power plant, the paper research the influence principle of many factors such as receiver thermal performance,superheater operation capacity, heliostat size and numbers of heliostats projected to the absorber. The results show that the required CR of per heliostat for superheat part is decreasing with the superheater thermal performance improving. When the receiver operates in lower capacity, the required CR of per heliostat is lower. With more heliostats project to the superheater, the required CR of per heliostat becomes lower. The smaller heliostat size, the lower required CR of per heliostat. The results also give us some inspirations that: 1) using the method in the paper, one can evaluate whether the existing heliostat field is fit for the two-stage receiver. 2)two stage receiver capacity and heliostat size should be in harmony for different CR requirement for evaporator and superheater.3) the results about how the heliostat size influences the required CR of per heliostat for superheater give the reference to industrial choose of heliostat size. 4)from the required CR point, the smaller heliostat is better than bigger one. Of course, in the real commercialization process, the cost of the heliostat should also be considered.

Numerical simulation on the thermal performance of a solar molten salt cavity receiver

The receiver thermal performance directly affects the efficiency, reliability and safety of the entire solar tower power system. A model combining radiation, convection and conduction heat transfer modes was developed to model a 1 MW molten salt cavity receiver with determined basic design parameters. The coupled heat transfer model was used to predict the receiver's steady-state efficiency, temperature distribution and heat losses for various flow layouts and receiver aperture lip sizes.The results show that the fraction of reflective loss (18.78%) is the largest in all heat losses, followed by radiative and convective heat losses, with conduction heat losses as the smallest part which is usually negligible. The heat loss distribution is closely related to the external tube temperature field in the cavity receiver and the specific structure of the receiver. 10 cm lips on both the top and bottom of the receiver aperture reduced the reflective heat loss by 1%. The results for different flow layouts show that center-side flow layout has the best temperature homogeneity, with less possibility of overheating in the central area. This, however, comes at the expense of increased irreversibilities and decreased efficiency. The sides-center flow layout has the worst temperature homogeneity, but the receiver efficiency is 2.06% higher. These results provide a useful reference for designing and optimizing cavity receivers.

Numerical study on thermal performance of a solar cavity receiver with different depths

Solar cavity receiver usually has an aperture for concentrated sunlight coming in. However, there is apparent heat loss because of this aperture. And the asymmetric approach of energy supply causes highly non-uniform distribution of heat flux inside the cavity. As a result, it is necessary to investigate geometrical effects on thermal performance of the receiver. A saturated water/steam solar cavity receiver with different depths was numerically studied by adopting a combined computational model. The variation trends of thermal efficiency and heat loss with depths were obtained. The influential factors on the change of three kinds of heat loss were specifically analyzed. The net energy absorbed by the boiling panels, the mass flow rate of generated steam and the distributions of heat flux and wall temperature on the boiling panels were also figured out. The results showed that a peak thermal efficiency exists as the receiver is stretched. And the distribution of heat flux on the boiling panels appears more and more uniform with the stretched length increasing. A suitable cavity depth was finally found for the receiver studied in the present work.

Effects of Absorber Emissivity on Thermal Performance of a Solar Cavity Receiver

Solar cavity receiver is a key component to realize the light-heat conversion in tower-type solar power system. It usually has an aperture for concentrated sunlight coming in, and the heat loss is unavoidable because of this aperture. Generally, in order to improve the thermal efficiency, a layer of coating having high absorptivity for sunlight would be covered on the surface of the absorber tubes inside the cavity receiver. As a result, it is necessary to investigate the effects of the emissivity of absorber tubes on the thermal performance of the receiver. In the present work, the thermal performances of the receiver with different absorber emissivity were numerically simulated. The results showed that the thermal efficiency increases and the total heat loss decreases with increasing emissivity of absorber tubes. However, the thermal efficiency increases by only 1.6% when the emissivity of tubes varies from 0.2 to 0.8. Therefore, the change of absorber emissivity has slight effect on the thermal performance of the receiver. The reason for variation tendency of performance curves was also carefully analyzed. It was found that the temperature reduction of the cavity walls causes the decrease of the radiative heat loss and the convective heat loss.