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Highlights • Evaluation of a novel double-windowed pressurized volumetric receiver (DW-PVR). • A numerical model combining MCRTM and FVM to predict optical and thermal performance. • A lower temperature field occurring at the outer window of DW-PVR. • DW-PVR showing advantages with greater windows distance and smaller cavity size. Abstract 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. [ABSTRACT FROM AUTHOR]

A vector based freeform approach is proposed for the geometrical reverse design of reflecting solar concentrators. The effect of solar cone angle of incident vectors is taken into consideration in the method based on three-dimensional freeform optics. The method developed is a closed-loop program and did not need any other commercial software. According to the transmission pattern of solar concentrators, the source-target mapping should be initially established prior to constructing an initial geometry formed by discrete points using the Geometric Construction Method. The freeform surface is generated by connecting several triangular flat elements defined by these points and then the Monte-Carlo ray tracing was conducted. The freeform surface can then be further optimized through feedback modifications. Finally, the optimal shape is obtained when the design examples are the off-axis freeform reflectors with target circular/rectangular receivers for the use of Concentrated Photovoltaic. This method provides a practical approach for improving the optical performance of solar concentrators. [ABSTRACT FROM AUTHOR]

The conventional imaging solar concentrators normally obtain a Gaussian distribution on the receiving surface. The aperture size of a thermal receiver (TR) is always limited primarily to the high concentration area, discarding the edge part to be lost. The current study proposes a novel free-form Cassegrain concentrator (FFCC) for PV/T combining utilization. The remaining heat flux by the edge of focal spot is directed by the secondary mirror for electric generation. The profile of secondary reflector is constructed by the geometric construction method (GCM), aiming at achieving high concentration for the TR and uniform flux distribution for the PV receiver (PVR). The energy transmission characteristics of FFCC, including the incident vectors and heat flux distribution on the TR/PVR are both investigated. The effects of changing the receiving locations, PV/T dividing ratio, etc. are analyzed. The sun size/solar conic angular is also considered for practical use. The GCM method and simulation results in this article can provide a reference for the PV/T solar energy system and free form surface design. [ABSTRACT FROM AUTHOR]

• A multi-physical coupled model is established to analyze the thermophotovoltaic performance. • The energy conversion process is explained in detail by an integrated energy flow and multi-scale analysis. • Different strategies are designed and their effect is evaluated from different energy perspectives. Thermophotovoltaic (TPV) emerges as a novel high-efficiency generation technology, showcasing enormous potential in various sustainable energy solutions. Aiming to understand the complex conversion process between radiative, electrical and thermal energies within TPVs, a multi-physics coupled model is established. From the heat generation, temperature and carrier perspectives, the output performance is analyzed in detail. Under 1200 K black-body radiation, the energy loss distribution in the GaSb cell is analyzed meticulously, where Joule heat (40.4 %), surface recombination heat (23.1 %) and thermalization heat (20.8 %) play predominant roles. As it stands, the cell temperature rises to a maximum of 317.0 K with a cooling intensity of 300 W/(m2·K). The minimum of the average cell temperature (approximately 312.3 K) occurs at the optimized working voltage of 0.3 V. Subsequently, the electrical characteristics are illustrated. The results indicate a maximum output power underestimation of 4.6 % compared to the uncoupled isothermal case. Based on the coupled model, a quantitative analysis of the anti-reflection coating (ARC), the structural adjustment (SA) and the selective emitter (SE) is conducted. The results reveal that the ARC case exhibits the most enhancement in the output power (55.3 %) followed by the SA case, which effectively improves it by 12.2 %. Although the SE case partially decreases the output power (−9.8 %), it shows promise in promoting conversion efficiency. Integrating the above strategies, the TPV system achieves an impressive conversion efficiency of 29.38 %, representing a 76.46 % improvement compared to the prototype with the cell only. [ABSTRACT FROM AUTHOR]

• 2D coupled heat transfer model with MCRT for thermal analysis of windowed VSR. • Effects of porosity and cell size of porous on thermal efficiency of windowed VSR. • Models based on P 1 approximation were compared with MCRT. • High thermal efficiency results from at a large cell size or a low porosity. • P 1 underestimates the thermal efficiency and overestimates the window temperature. A two-dimensional coupled heat transfer model with Monte-Carlo ray tracing (MCRT) method is developed to analyze the heat transfer in windowed volumetric solar receiver. The local thermal non-equilibrium (LTNE) effects in ceramic foam absorber and spectral selectivity of quartz window are considered. With the solar absorption term calculated by the MCRT, the P 1 -approximation for thermal radiative exchange in the foam absorber is examined by comparing with the MCRT. In the model based on P 1 -approximation, net radiation method (NRM) and MCRT are applied to calculate the thermal radiative exchange between the window and absorber. The effects of porosity and cell size of the absorber foam on temperature distribution and thermal efficiency are investigated. The results show that for heat transfer in foam absorber at a highest temperature below 1727 K, the P 1 -approximation underestimates the absorber temperature by about 27 K and overestimates the window temperature by 106–133 K. And the thermal efficiency is underestimated by about 2.1% using the P 1 -approximation. There is small difference between the temperature in the absorber predicted by different models, but the NRM cannot predict the axial temperature gradient in the window. Under a constant air mass flow rate, within the absorber foam with the cell size range of 2–4 mm and porosity range of 0.7–0.9, a higher thermal efficiency can be reached at a larger cell size or a lower porosity. [ABSTRACT FROM AUTHOR]

• Start-up duration of a tubular receiver filled with nickel foam is predicted. • Preheating performances are compared by operating synergy electric heater and solar simulator. • Transient temperature and thermal stress distribution of solar receiver is obtained. • The maximum difference of duration is 11 min between cold and hot start-up strategy. A cylindrical tubular receiver filled with nickel foam (CTRNF) appears to be a promising receiver concept due to its capability to provide temperatures over 900 °C at a high pressure. Nevertheless, CTRNF is challenged by the unsteadiness and high intensity of radiant flux may cause inaccurate start-up time predictions, providing unrealistic and unstable outlet temperatures. In this article, these matters are addressed by a numerical model coupling the Monte Carlo ray tracing (MCRT) method and discrete coordinate (DO) radiative transfer model. The model considers the cold, semi-cold, semi-hot, and hot start-up strategies on the CTRNF using N 2 and Ar as the working medium. Three preheating methods and their influence on the thermal characteristic of the CTRNF are proposed. Temperature and equivalent stress distribution of internal heat absorber is simulated and analyzed. The results indicate that the duration for four start-up strategies takes 1711 s, 2189 s, 1212 s and 1032 s as the outlet temperature reaches to 1200 K, respectively, when Ar is used as the working medium (10 g/s) and radiative power of 10.5 kW incidents on the solar receiver. Compared with the 68.44 kg Ar consumption of cold start-up strategy, the Ar working medium consumption decrease by 24.44 kg, 51.68 kg, and 63.16 kg for semi-cold start-up, semi-hot start-up, and hot start-up strategy. However, the maximum equivalent stress of the CTRNF under the hot start-up strategy is 100 MPa higher than under the cold start-up strategy, which requires accurate monitoring of the cavity temperature in solar receiver. [ABSTRACT FROM AUTHOR]