Gas-molten salt direct contact heat exchange for porous phase change materials

Abstract

To accelerate the charging and discharging of molten salt thermal storage and to enhance heat transfer, the authors of this paper propose a novel idea of using direct contact heat exchange. Under this proposal, a gaseous fluid from the solar field is directly injected into the molten salt system without any heat exchanger. In direct contact heat exchange, efficient energy exchange takes place within a smaller volume, which contributes an order of magnitude increase in heat transfer. Relative to liquid metals, whose thermal conductivity varies from low 10 W/m.K to 180W/m.K, the thermal conductivity of molten nitrate salts is fairly low i.e. 0.55W/m.K to 0.654W/m.K The low thermal conductivity of molten salts in liquid, mushy and solid phase limits their charging, discharging efficiency and makes the heat transfer process an energy and engineering resource-intensive process. The proposed direct contact heat exchange will eliminate the use of electric blanket heating of molten salt tank and pipe and address the issue related to molten salt freezing in the tank, pipes, pumps or the solar field. The authors of this research paper are investigating an innovative concept in which a non-reactive gaseous heat transfer fluid from solar field is directly injected into the molten carbonate to melt and remelt and to enhance the heat transfer. During the cooling cycle, the liquid to solid phase change process will take place; the gaseous fluid cooling will enforce a porous solidification of the molten salts. This porous phase change phenomenon is of critical importance and is beneficial during the remelting of the molten salts. In the current research, the role of forced convection, liquid and vapour phase drops and the role of mass transfer due to evaporative losses are investigated. In addition, the issues related to the freezing and melting of phase change material by forced convection are also addressed. Modelling and analysis results of heat transfer due to forced convection are presented. The results of low and medium temperature testing, their analysis, some preliminary modelling and simulation work is presented in this research paper. The role of low vapour pressure, thermal stability, surface tension, the density of the molten salt and areas of critical importance are analyzed. Analysis of experimental results is presented to determine the energy exchange the exchange effectiveness and heat transfer coefficients.