• Energy Research

Abstract Redox thermochemical reactor model integrated concentrating solar power technology is finding increased interest due to the enormous advantages of storing renewable energy into high-temperature heat flux and clean transportable fuel. From the perspective of designing a scalable direct solar irradiated receiver, the reactor optimum design and solar thermochemical energy storage performance have been investigated. Numerical models combined with experiments were developed for the analysis of engineering design parameters affecting the reactor efficiency. The shape of the cavity receiver including the size of the glass-covered target radiant received surface and reactor volume, especially the axial length of the heat-storing medium can be considered as important design issues for improving thermochemical energy storage efficiency. The reactor efficiency of storing sunlight with safer operating conditions is reported to 85.27% ± 0.85% during thermal charging up to 1787.725 K ± 30.58 K and 76.9% ± 0.11% during thermal discharging step at 1315. 16 K ± 7.53 K. Increasing heat losses via receiver insulation layer significantly affects the reactor thermal performance. This study demonstrated that the solar-driven thermochemical process has the potential of achieving high-temperature storable heat and solar fuel production. Appropriate geometric parameters were provided for the scalability from the perspective of industrial implementation.

Abstract Redox thermochemical reactor model integrated concentrating solar power technology is finding increased interest due to the enormous advantages of storing renewable energy into high-temperature heat flux and clean transportable fuel. From the perspective of designing a scalable direct solar irradiated receiver, the reactor optimum design and solar thermochemical energy storage performance have been investigated. Numerical models combined with experiments were developed for the analysis of engineering design parameters affecting the reactor efficiency. The shape of the cavity receiver including the size of the glass-covered target radiant received surface and reactor volume, especially the axial length of the heat-storing medium can be considered as important design issues for improving thermochemical energy storage efficiency. The reactor efficiency of storing sunlight with safer operating conditions is reported to 85.27% ± 0.85% during thermal charging up to 1787.725 K ± 30.58 K and 76.9% ± 0.11% during thermal discharging step at 1315. 16 K ± 7.53 K. Increasing heat losses via receiver insulation layer significantly affects the reactor thermal performance. This study demonstrated that the solar-driven thermochemical process has the potential of achieving high-temperature storable heat and solar fuel production. Appropriate geometric parameters were provided for the scalability from the perspective of industrial implementation.

Abstract In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray–thermal–structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses.

Abstract In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray–thermal–structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses.

This paper aims at predicting radiation characteristics of the solar collector system by the Monte Carlo method with respect to the corresponding optical properties. Several probability models were introduced to analyze the effects of sunshape and surface roughness. Directional characteristics of radiative flux in the focal region and flux distribution of the cavity receiver were considered. An equivalent radiation flux method is presented for designing the shape of the cavity receiver. Based on the relative numerical simulation results, a new shape cavity receiver called “upside-down tear drop” is proposed to meet an almost uniform radiation flux field. Radiation effects due to multiple reflections and thermal emission in the cavity are parametrized by using the radiative exchange factor. The calculation results can be a valuable reference for the design and assemblage of the dish solar collector system.

This paper aims at predicting radiation characteristics of the solar collector system by the Monte Carlo method with respect to the corresponding optical properties. Several probability models were introduced to analyze the effects of sunshape and surface roughness. Directional characteristics of radiative flux in the focal region and flux distribution of the cavity receiver were considered. An equivalent radiation flux method is presented for designing the shape of the cavity receiver. Based on the relative numerical simulation results, a new shape cavity receiver called “upside-down tear drop” is proposed to meet an almost uniform radiation flux field. Radiation effects due to multiple reflections and thermal emission in the cavity are parametrized by using the radiative exchange factor. The calculation results can be a valuable reference for the design and assemblage of the dish solar collector system.

Abstract In solar thermochemical systems, the utilization of porous ceramics plays an important role in the enhancement of heat transfer and optimization of reaction conditions, thereby effectively improving the energy conversion and storage efficiency. Compared with the common filling pattern of one-layer porous ceramic (1-LPC), novel changes in the thermal and chemical characteristics can be induced using multilayer porous ceramics (MPCs). To determine whether MPCs have advantages over 1-LPC in solar thermochemical applications, a numerical model was established in this study by combining computational fluid dynamics with dry reforming of methane reaction kinetics. The local thermal non-equilibrium model coupled with the P1 approximation was adopted to solve the solar radiation diffusion and convective heat transfer problems, while the non-Darcy flow effect was considered to predict the momentum dissipation resulting from the porous ceramics. Based on this, the effects of layer number, gap position, porosity, and cell size were investigated to find the optimal application strategies for MPCs. The simulation results indicate that a large temperature gradient in the first gap between two layers of MPCs can usually reduce the wall heat loss and improve the thermal efficiency, but has no universal effect on improving the solar-to-fuel efficiency. Under the current operational conditions, although improvement of the solar-to-fuel efficiency by approximately 0.03%–2.43% can be obtained using a 4-LPC in the cases of high porosities ( ϕ ⩾ 0.86 ) and large mean cell sizes ( d p ⩾ 7 mm ), 1-LPC remains the most reliable filling pattern with a wider range of applications and stable performance.

Abstract In solar thermochemical systems, the utilization of porous ceramics plays an important role in the enhancement of heat transfer and optimization of reaction conditions, thereby effectively improving the energy conversion and storage efficiency. Compared with the common filling pattern of one-layer porous ceramic (1-LPC), novel changes in the thermal and chemical characteristics can be induced using multilayer porous ceramics (MPCs). To determine whether MPCs have advantages over 1-LPC in solar thermochemical applications, a numerical model was established in this study by combining computational fluid dynamics with dry reforming of methane reaction kinetics. The local thermal non-equilibrium model coupled with the P1 approximation was adopted to solve the solar radiation diffusion and convective heat transfer problems, while the non-Darcy flow effect was considered to predict the momentum dissipation resulting from the porous ceramics. Based on this, the effects of layer number, gap position, porosity, and cell size were investigated to find the optimal application strategies for MPCs. The simulation results indicate that a large temperature gradient in the first gap between two layers of MPCs can usually reduce the wall heat loss and improve the thermal efficiency, but has no universal effect on improving the solar-to-fuel efficiency. Under the current operational conditions, although improvement of the solar-to-fuel efficiency by approximately 0.03%–2.43% can be obtained using a 4-LPC in the cases of high porosities ( ϕ ⩾ 0.86 ) and large mean cell sizes ( d p ⩾ 7 mm ), 1-LPC remains the most reliable filling pattern with a wider range of applications and stable performance.

Abstract Chloride salts are widely used as thermal energy storage (TES) media for high-temperature solar TES systems. Their thermal properties are crucial for the performance of TES systems. In this study, we prepared and characterized chloride salts/nanoparticles composite phase change materials (CPCMs) for high-temperature thermal energy storage. The ternary chloride salts (MgCl2:KCl:NaCl with 51:22:27 molar ratio) were used as base salt and Al2O3, CuO, and ZnO nanoparticles were dispersed into the base salt at 0.7 wt% to form various composite phase change materials (CPCMs). The thermal properties of the base salt and CPCMs were measured. The results showed that the melting temperature of the CPCMs was very close to that of the base salt. The phase change latent heat of the CPCMs was slightly lower than that of the base salt while the addition of dopant nanoparticles clearly enhanced the thermal diffusivity and thermal conductivity of the CPCMs. In particular, the thermal conductivity of the CPCM doped with Al2O3 nanoparticles showed the most obvious enhancement, which increased by more than 48%, compared to that of the base salt. Al2O3 nanoparticles could be considered as an optimal additive to improve the thermal conductivity of chloride salts. Moreover, the CPCM with Al2O3 also exhibited excellent thermal stability. These good thermal characteristics of CPCM with Al2O3 nanoparticles endow it promising applications for high-temperature TES system.