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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]

A non-uniform solar flux leads to a non-uniform temperature distribution on the walls of cavity receiver. There exists a large temperature gradient, which can cause great challenges for the safety and high-efficiency operation of concentrating solar power system (CSP). To overcome this problem, micron particles with strong properties of absorption, scatter and radiation are added into the cavity receiver to improve the non-uniform heat flux distribution on the wall. In the present research, a coupled simulation method based on Monte Carlo Ray Tracing (MCRT) method and Finite Volume Method (FVM) is established to simulate the complex photo-thermal conversion process of a two-dimension square solar cavity filled with air-carbon particle mixtures. In the model, first, the solar heat flux distribution on the surface of cavity receiver is simulated by MCRT and the photo-thermal conversion process in the receiver is modeled by FVM. Then both are coupled in the cavity receiver by transferring the absorbed solar energy computed by MCRT to FVM, and this energy is applied as a source term for the energy equation in the FVM part. Based on the coupling model, the thermal performance and the temperature distribution characteristics on the wall of the cavity receiver are studied at various particle densities. The results show that as carbon particle density increases, the heat flux and temperature distribution on the walls of the receiver becomes more uniform and the effects of different L / D and N on σ T of the receiver are further investigated. [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]

Solar cavity receiver is quite often utilized in the solar tower power system. It is a photo-thermal conversion component, which absorbs solar radiation energy and heats the working fluid. The safety and thermal efficiency of solar cavity receiver can directly affect the efficiency of the entire power system. In this paper, a modified combined method is proposed to simulate the thermal performance of a saturated water/steam solar cavity receiver. Compared with the combined method presented previously, this modified one can describe the whole steady process of the receiver and accurately estimate the steady-state efficiency and the mass flow rate of the generated steam in the boiling tubes. Under the given boundary condition of incident solar flux, the modified method is adopted to find an optimized distribution of absorptivity in the solar wavelength band for the absorber surfaces in order to improve the uniformity of surface heat flux distribution. The simulation results show that the uniformity of surface heat flux is improved effectively and the maximum mean square deviation of the external tube temperature drops from 32.4 °C to 23.6 °C after optimizing. Based on the relatively uniform heat flux distribution, the effect of thermal emissivity of the absorber surfaces on the thermal performance of the cavity receiver is further investigated. Being different from those of parabolic trough and external cylindrical receivers, both the radiative and the convective heat loss of the present cavity receiver decrease with increasing thermal emissivity, which improves the receiver’s efficiency. These results can provide a useful reference for the design and selection of surface coatings for the cavity receivers. [ABSTRACT FROM AUTHOR]