Search

The receiver serves as a critical component in tower-type concentrated solar power plants. Responsible for light-heat conversion, the efficiency of the receiver significantly affects the overall performance of the power plant. In the current study, the thermal performance of external receivers was investigated. An experiment was set up similarly using the solar simulator to experimentally investigate the heat losses of a billboard receiver. A billboard-type external receiver was designed, fabricated, and experimented with. A solar simulator having seven xenon lamps characteristics similar to the sunlight spectrum was used to obtain heat flux at the surface of the receiver. Convection losses in the head-on wind direction were evaluated, along with the radiation losses. The thermal efficiency of the billboard receiver calculated experimentally was around 83.9%. Numerical simulations were also carried out to compare the results against the experimental data. A variation of ±5% observed between both results validate the model proposed in the current study. [ABSTRACT FROM AUTHOR]

A simulation method consisting of Monte Carlo Ray-Trace and Finite volume coupled model is proposed in the present work for simulation of complex photo-thermal conversion in the particle entrapped cavity receiver. The solar heat flux distribution in the cavity receiver was simulated by using the Monte Carlo ray tracing, while the convection-radiation-conduction process taking place in the cavity receiver was simulated by the Finite Volume Method. Solar heat flux was introduced as a source term in the energy equation for the Finite Volume Method part, and correlations for flow boiling heat transfer were chosen to calculate the surface temperature of the boiling tubes. Based on the simulation model, the thermal performance of the receiver, the temperature distribution and the mean square deviation of temperature at the outer surface of the boiling tubes were analyzed at varying albedo ω , diameter D and time (8:00–16:00). It was revealed that the temperature distribution on the outer surface of the boiling tubes can be uniformed by entrapping carbon particle sin the receiver. The results indicate that the collision between photon and carbon particles resulted in scattering and absorbing, with a tendency to decrease the maximum heat flux and temperature gradient at the surface of tubes. Although the thermal efficiency of the receiver decreases by 4.5%, the maximum temperature was reduced from 665 K to 630 K and the mean square deviation of temperature on the outer surface of backpanel was reduced from about 43 K to 26 K at noon. The results are useful for choosing suitable parameters for entrapped particles in cavity receivers to improve efficiency and operational safety. • MCRT and FVM methods were applied to evaluate the thermal efficiency of a cavity receiver. • Thermal efficiency of the cavity receiver at varying sun positions along the passage of day was evaluated. • Effects of particle's radiation properties on thermal performance of the receiver were investigated. • Carbon aerosol was used to achieve uniform heat flux distribution and overcome the localized heating problem. [ABSTRACT FROM AUTHOR]

Highlights • An integrated method based on a two-band semi-gray surface approximation is employed. • The solar absorptivity is a critical parameter relative to other surface properties. • The receiver efficiency shows different variations with the thermal emissivity. • The infrared heat transfer direction between passive and active surfaces is crucial. • The passive surfaces should be highly reflective due to the great cavity effect. Abstract 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. [ABSTRACT FROM AUTHOR]

The performance of cavity receiver plays a fatal role in light-heat conversion process in a concentrating solar power (CSP) plant. In our present work, an experimental platform is designed and built, in which a solar simulator consisting of seven mutually independent xenon lamp units is used for supplying the solar energy needed. The cold start-up and steady state performance of the cavity receiver were studied under 0.5MPa. The results show that for a certain cavity structure, temperatures on boiling panels and thus the heat loss can be affected not merely by the heat flux, other parameters can influence the temperature of absorbing surfaces, such as the heat transfer in the boiling tubes, fluid flow states and the wind effect, so the temperature distribution is highly non-uniform on boiling panels. However, the temperatures of passive surfaces depend greatly on the input solar energy, thus the heat losses of the passive surfaces will vary with the input energy. Overheat occurs if a high heat flux doesn’t have a sufficient heat transfer inside boiling tubes internal. The stagnation zone does exist in the cavity and its boundary varies with a different input energy, thus the convective heat loss can be affected. The cavity receiver thermal efficiency is 72.92% under 0.5MPa, may be the reason behind this low efficiency is the strong cooling wind of the solar simulator. [ABSTRACT FROM AUTHOR]

The geometry of a solar cavity receiver affects the absorption of incident solar energy and the temperature distribution on the absorber tube panels, and hence affects the operation stability and efficiency of the solar receiver. A modified combined computational method was proposed to investigate the thermal performance of the three dimensional (3-D) molten salt cavity models in steady state. This computational method was also used to calculate the thermal performance of the receiver in the Molten-Salt Subsystem/Component Test Experiment (MSS/CTE) in the United States, and the results agree well with published data. Molten salt cavity receivers with different heights were studied. The results show that a too-low or too-high height leads to low thermal efficiency of a receiver. Therefore, appropriate heights shall be chosen to enable receivers achieve relatively high efficiencies and reach low temperatures. The molten salt cavity receivers with different cross-sections in the chosen heights were then studied, to obtain a high-efficiency cavity receiver structure without increasing the inner surface area or the depth of the cavity, under the condition that the molten salt cavity receivers were in the same perimeter of the cross-section, aperture size, cavity height, tube panel and fluid flow layouts but different cavity depths. When only the lengths of the back walls next to the side walls had been increased, the thermal efficiency decreased first and then increased with the depth decreasing, which reached the lowest value when the depth was 2.71 m. When the depth of the modified receiver is small enough, its thermal efficiency can be higher than that of the MSEE receiver. When only the lengths of the two walls adjacent to the aperture had been increased, the thermal efficiency increased with the depth decreasing, and was always higher than that of the receiver in the Molten-salt Electric Experiment (MSEE) in the United States. The thermal-efficiency difference between the two arrangements of the side walls decreased with a decrease in the cavity depth. The cross-section of the MSEE receiver is similar to that of the receiver in the molten salt electric experiment by Sandia National Laboratories. Under the condition of side-on wind at a velocity of 8.3 m/s, the thermal efficiency of the modified cavity with the minimum depth is 2% higher than that of the MSEE cavity with the maximum depth. The convective heat loss is 3.1% lower, while the radiative and reflective heat losses changed slightly. Therefore, the modified cavity in the present study is capable of achieving a higher efficiency with a less depth. [ABSTRACT FROM AUTHOR]

• Supercritical CO 2 mixture Brayton cycle with floating critical points is proposed. • Energy consumptions of distillation based composition tuning systems are analyzed. • CO 2 -H 2 S cycle shows the most significant efficiency improvement. • Hourly efficiency of SBC-CTS is calculated based on typical daily temperatures. A novel concept of supercritical Brayton cycle with floating critical points is proposed to cope with the rapid deterioration of cycle performance at high heat sink temperatures. By coupling the power cycle with a distillation based composition tuning subsystem, the circulating mixture composition in the cycle can be altered dynamically to obtain the desired critical points at various ambient temperatures, thereby leading to a better match between the cold-end parameters of power cycle and the heat sink conditions. A thermodynamic model is established for the proposed novel system based on a recompression layout with single stage reheating. Four binary mixtures, including CO 2 –H 2 S, CO 2 –Butane, CO 2 –Isobutane, and CO 2 –Butene are employed. Simulation results indicate that an increasingly larger improvement in cycle efficiency compared to the pure CO 2 cycle can be obtained as the ambient temperature increases from 20 to 50 °C. When the composition tuning process is not considered, the CO 2 –H 2 S cycle shows the best performance among the four mixture cycles, with an improvement between 1.3% and 7.4% being achieved over the entire temperature range. Several feasible schemes for composition tuning subsystem are proposed and compared in terms of energy consumption. Finally, the hourly efficiencies of the integrated system are evaluated based on typical daily weather conditions, with a piecewise control strategy for critical points. It is found that although the use of CO 2 –H 2 S causes relatively high energy consumption for distillation, a significant improvement in the daily average thermal efficiency of the integrated system can still be achieved, with efficiency being increased by 3.5%-5.1% in comparison with pure CO 2 cycle on hotter days. [ABSTRACT FROM AUTHOR]