Closed Sorption Seasonal Thermal Energy Storage with Aqueous Sodium Hydroxide

Abstract

In this paper, a 1 kW closed sorption heat storage prototype is presented. The two main processes (charging and discharging) occur under reduced pressure. During the charging process, the thermal energy produced by the solar collectors in summer is used to partially vaporize the water contained in a diluted sodium hydroxide solution (NaOH). Then, the concentrated aqueous sodium hydroxide solution and the water are stored at room temperature in separate tanks until the discharging process. During discharging, ground heat is used as a heat source to evaporate the stored water under sub-atmospheric pressure in the evaporator. In a separate chamber, heat is produced during the exothermic process of water vapor absorption into the concentrated NaOH solution. The heat is then transferred to a working fluid that can be used, for example, in floor heating applications. Sodium hydroxide has a high water affinity; nevertheless, the fluid wetting on the heat and mass exchanger influences the absorption process efficiency. Different experimental methods to improve the surface wetting like surface modifications (structuring, use of ceramic foams) and fluid properties tuning (use of surfactants) are presented. The development of the structured surfaces and ceramic foams enhances the heat transfer and thus reduces the size of absorber unit.