Experimental analysis of heat transfer in passive latent heat thermal energy storage systems for CSP plants

Thermal energy storage is a key factor for efficiency, dispatchability and economic sustainability of concentrated solar plants. The latent heat storage systems could ensure a significant reduction in construction. costs and environmental impact, because of its high. storage energy density. In LHTES, the heat transfer between the heat transfer fluid and the storage system is strongly limited by the reduced thermal conductivity of the storage media.. For operating temperatures between 200 and 600 degrees C, the most used storage media are salts. In order to evaluate solutions which promote the thermal conductivity, by increasing the exchange surface and/or the addition of nanoparticles to the storage media, Enea set up a. small facility to test some storage concepts. In this facility, a. diathermic oil flows through three elementary "shell-and-tube" storage systems, connected in series, reaching a. maximum temperature of about 280 degrees C. The elementary storage systems are filled with a mixture of sodium and potassium nitrates salts, which melt at about 225 degrees C. Moreover a small percentage of alumina and silica nanoparticles were added to this mixture. The results of the experiments show an increase of the thermal diffusivity of the medium not only for the presence of tins on the heat transfer tubes but also because of convective flows within the melted traction were established. These phenomena strongly reduce the charging times of the system (by about 30%). Instead, the presence of nanoparticles increases the thermal capacity and the thermal conductivity of the storage system but seems not to have a relevant effect on the thermal diffusivity of the mixture. This behavior depends on the type of used nanoparticles, which can significantly change over time some characteristics of the storage medium, in which they are dispersed, leaving other characteristics unchanged, according to mechanisms which. are still to be well understood. (C) 2015 The Authors. Published by Elsevier Ltd.