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Low-concentration and low-profile compound parabolic concentrators (CPCs) in the quasi-stationary state were designed and investigated for potential use in rooftop and building-integrated concentrating photovoltaic–thermal applications. All the designs were investigated with a geometric concentration ratio of two. The first design was the lowest-position-truncated CPC (LEMR), and the second was the highest-position-truncated CPC (HEMR). The performances of both concentrators were compared with a full CPC (without truncation). At the outlet aperture, the optical efficiency within the half acceptance angle for the LEMR and HEMR CPCs averaged nearly 97%, whereas for the full CPC, it was 95%. At the best concentration plane (BCP), the optical efficiency within the half acceptance angle was the highest for the LEMR CPC 95%. A distributive method was used to estimate accurately the nonuniformity in the solar flux at both the outlet and the BCP. The nonuniformity analysis revealed that the LEMR and HEMR designs exhibited a uniform distribution of solar flux at the BCP. The incident-angle modifiers for all three cases were investigated, and the annual accumulated solar energy was estimated for Xi'an, China. The material consumption of the LEMR CPC was approximately 48% that of the full CPC and 78% that of the HEMR CPC. The LEMR also exhibited the highest annual-energy efficiency per unit material consumption (ηm = 1.30). Finally, the results indicate that the LEMR CPC possesses higher solar flux uniformity and represents a better trade-off between energy collection and cost reduction, and thus, the novel design can be used as a quasi-stationary concentrator. [ABSTRACT FROM AUTHOR]

Highlights • Thermal performance of an external receiver is numerically evaluated. • Different wind directions and wind velocities are experimented to evaluate the losses. • A novel design using wind blocking walls is tested to successfully reduce the losses. • Receiver and wind characteristics dependent correlation is derived to evaluate losses. Abstract The efficiency of a CSP plant heavily depends on the performance of the receiver. The receiver is responsible for light-heat conversion in a power generation system. With the objective of numerical evaluation of the thermal losses of an external receiver, a method was developed in the present study. Solar flux was obtained on a north-facing receiver using the "SolarPilot" software, through Monte-Carlo ray tracing technique. Apposite boiling heat transfer coefficients were selected and then applied to obtain the temperature at the surface of the boiling tubes. Numerical simulations were performed to quantify the heat losses in the receiver at different wind velocities. Maximum heat flux and temperature were obtained at the center of the receiver. The results indicated that wind direction and wind velocity have a significant impact on the extent of heat losses. Maximum heat losses occur in the side-on wind direction (i.e., west bound) while losses are minimum in the head-on wind direction (i.e., north bound). Thermal losses through the receiver contribute a sizeable portion to overall energy losses in power tower systems, hence, to attenuate the heat losses a new, yet simple, design is proposed with the addition of wind-blocking walls at the receiver's periphery. The walls achieve noticeable heat retention in all wind directions, but their role is more prominent in the head-on wind direction. In best case scenario, attaching 1 m wide wind blocking walls resulted in a diminution of about 33 percent of losses in the head-on wind direction at 9 m/s wind velocity. Results obtained are then employed to derive a simplified correlation model to evaluate convective heat losses as a function of wind direction, wind velocity, receiver's surface area and wind blocking wall width. [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]

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 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]

• Innovative combination of optical concentrator and solar steam generation module. • Better yield of the system with water flux of 3.5 kg∙m−2∙h−1 under outdoor conditions. • Economizing of installation area and a lower water production cost of 7.03 $/m3. Solar steam generation (SSG) for water treatment has attracted great interest for its convenience, accessibility and eco-friendliness. However, water production rate of traditional floating structure SSG system is relatively low. To further increase the water production rate, we develop a solar thermal-latent heat cascade utilization SSG system coupled with compound parabolic concentrator for desalination. A 2 × compound parabolic concentrator (CPC) was used in the outdoor experiment to improve the incident solar flux. Solar absorber of the SSG module has a high solar absorbance of 95 % which can effectively harvest solar energy and convert it into heat. A thin air gap above the solar absorber can restrain thermal loss through convection and radiation. Within the SSG module, seawater was supplied for the evaporation process driven by the capillary force in the capillary wick. Through the multi-stage design, solar thermal conversion and latent heat cascade recovery were achieved. As a result, the SSG module can achieve a gain to output ratio (GOR) of 185.09 % with a water flux of 2.82 kg∙m−2∙h−1 under one sun. Concentrating solar flux reached 1300 W∙m−2 in an early autumn day and a corresponding water flux of 3.5 kg∙m−2∙h−1 was achieved. Theoretical analysis showed that higher solar flux can increase the GOR. Optical concentration of 2 × can increase annual fresh water by 1.72 times while maintaining economic benefit. An insulation air gap above the solar absorber can reduce thermal loss from the solar absorber. Increasing stages can increase number of times the latent heat being reused and improve GOR, but economic benefit decreased with the increase of stage number. Considering the economic benefit and floor area, optical stage number of the system should be 4. Annual water production evaluation showed that a 1 m2 area system can annually generate 6321.29 kg fresh water with a water production cost of 7.03 $/m3. [ABSTRACT FROM AUTHOR]