Spacecraft use photovoltaics as power-generation systems. Although photovoltaics have advantages such as no requirement of fossil fuel or moving parts, a disadvantage is that generating power without sunlight is impossible. Spacecraft require new power-generation systems that do not depend on sunlight to make power generation possible in deep space where sunlight cannot reach. In this study, a thermoradiative (TR) system was investigated. The TR system generates power by thermal radiation from the TR cells to the surrounding environment. The possibility that the TR system would be most effective in spacecraft for which the surrounding temperature is 3 K was considered. Because the power generation of the TR system depends on the amount of radiation from the TR cells, the ideal state was simulated by assuming that the TR cell emits blackbody radiation to obtain the upper limit of the power generation. Furthermore, the effects of output voltage, cell bandgap, and TR cell temperature on the power generation were investigated numerically by using a HgCdTe photodiode, and the results were compared with those for blackbody radiation.
Direct simulation was performed for the solar porous receiver having cubic lattice as elementary structure with high porosity (0.80 - 0.98). The cell size was changed in four steps from 0.49 mm to 2.94 mm at three levels of the power over air mass (POM). Silicon carbide (SiC) was assumed as receiver material, and the absorptivity of the wall surface was assigned 0.9. The numerical data for the porous receiver was compared with the honeycomb receiver with low porosity. Such comparison proved the superiority of the highly porous receiver: the receiver-exit temperature and the receiver efficiency surpass those of honeycomb receiver at the same level of POM. Regarding the size effects, the performance of the porous receiver improves as the cell size reduces. When the cell size is 0.98 mm at POM =1000 kJ/kg, the receiver exit temperature increases beyond 1000 K and the receiver efficiency 0.8. When the cell size is 0.49 mm at POM = 2000 kJ/kg, the receiver exit temperature exceeds 1700 K and the efficiency 0.8. The numerical results thus demonstrated that the smaller cell size maintains the efficiency at the higher levels of POM. The mechanism for superiority of the small cell size was investigated through analysis of the heat transfer loss. This investigation revealed that the convective heat transfer enhancement in the case of small cell size decreases the temperature in the inlet region and attenuates the thermal radiation loss. Therefore, the high performance of the small-cell size receiver is attributed to the cooling effect of the air stream with high heat transfer coefficient.
Human demand for energy has reached unprecedented levels. A high-efficiency chemical heat pump (CHP) that uses solar thermal as heat source called SCHP system has been studied for decades in our laboratory. The solar chemical heat pump (SCHP) system using CaSO4∙1/2H2O/CaSO4 can store solar thermal energy in the form of chemical energy and release it as hot heat and cold heat. This study presents a simplified three-dimensional unsteady model of reactant bed using CaSO4∙1/2H2O/CaSO4 for heat releasing step of SCHP system. The simplified model is programmed by finite difference method and enabled operation by Microsoft Excel on a personal computer instead of other complex programming languages on workstations etc. Even the simplified model can simulate the heat/mass transfer in the reactant bed under different evaporator temperatures. The simulation results of temperature distribution and overall hydration conversion are in good agreement with the experimental results. Furthermore, the effects of fins inserted in the reactant bed and fins thickness on the enhancement of the hydration conversion and the hot/cold heat power were shown at different evaporator temperatures.
Solar thermochemical energy storage systems, utilize the entire spectrum of solar radiation to drive endothermic chemical reactions, have received great interest in concentrated solar power applications during the past years. Storing solar radiation as chemical energy during the day can be utilized at night times and cloudy days. In these solar thermochemical processes, chemically reactive and radiatively participating multiphase flows in various regimes are frequently encountered. Numerical modeling of multiphase flows assists to optimize the processes of solar thermochemical reactors by reducing the time-consuming experimental testing and cost. In this study, an Euler-Euler two phase model has been developed to investigate the fluidization behavior of 2:1 iron-manganese oxide redox and spinel particles for thermochemical and sensible heat storage systems respectively. In order to validate the model, a pseudo 2D experimental set up has been made. Experimental and numerical results have been compared for various conditions. The effect of gas flow rate on the fluidization behavior has been analyzed.
To perform solar thermochemical conversion, by utilizing high-temperature solar heat as an energy source and redox metal oxide particles as a chemical source, fluidized bed reactor has been developed to produce clean fuels. In this study, an Euler-Lagrange model has been developed for the simulation of particulate and gas flows in fluidized bed reactor for hydrogen production, by two-step water splitting cycles, using solar beam down concentrating system. The solid phase is modelled by Discrete Element Method (DEM) using soft-sphere approach and the gas phase is modelled as continuum by Navier-Stokes equations. The flow behavior of newly developed fine ceria particles has been analyzed for various conditions using the 30 kWth fluidized bed reactor prototype. The effect of particle size on the flow-dynamics at the spout, fountain periphery and annulus of the internally circulating fluidized bed has been examined. The results indicate that the particle size distribution should be minimized as much as possible to avoid the segregation behavior of different size particles.