Modelagem transiente do armazenamento térmico em sistemas CSP

Abstract

Among the renewable energies, concentrated solar power is capable of store energy in its thermal form, which is cheaper and has energy density higher if compared to the batteries of photovoltaic systems. This technology allows solar thermal plants to dispatch energy even when climatic conditions are unfavorable, such as in periods of cloudiness and after sunset. This work seeks, in a general way, to verify the solar potential of the brazilian Northeast region through a transient study of the heat losses of a sensitive heat storage system by two active-direct or indirect tanks in relation to environmental variables such as solar irradiation, ambient temperature and wind speed. In addition, the work also aims to verify the sensitivity of the system to its loading level and operating temperatures. The solar potential of the brazilian Northeast region was compared to the main world producers of heliothermic energy. The main storage thermal energy systems in operation in concentrated solar power plants were identified, besides the storage and insulation materials of the tanks. The transient analysis was performed by solving the diffusive terms of the differential equations resulting from energy balance and heat exchanges using the FDM in the implicit formulation for discretization of the system and the TDMA method for solving the equations. The sensitivity study of the system to the climatic conditions was conducted for two regions with distinct characteristics, South and Northeast. Experimental data of the Solar Two thermal storage system were used for validation of the model proposed by this work. The heat loss rates for the simulated validation model presented values of 97.36 kW for the fully charged hot tank and 46.12 kW for the fully charged cold tank, these values presented differences, related to experimental models, of 4.55% and 4.82%, respectively. The heat losses of the hot tank were higher for the region of São Martinho da Serra - RS, however the difference in storage efficiency between these two regions was less than 1.0 %. The heat losses were relatively sensitive to the tank loading level, with a difference of 37.35% between the fully charged and fully discharged tank. Operating temperatures were also quite influential on the thermal storage efficiency, being 10.78% for the tank at 300 C and 89.42% for the tank at 600 C. The model developed for this work presented results consistent with experimental measurements and with the physical phenomenon of storage. Finally, the tank showed little sensitivity to changes in climatic conditions, however it showed real sensitivity to the loading level and operating temperatures. The heat losses of a thermal storage system is function of several variables and understanding the influence of each parameter on the system is primordial for the advancement of the technology.