Proposal of a thermocline molten salt storage tank for district heating and cooling

•Development of a low-melting temperature molten salt based on Sr(NO3)2/NaNO3/KNO3/LiNO3.•Application of a new low melting point quaternary salt for energy storage in district heating and cooling for the WEDISTRICT project.•Sizing and engineering description of a thermocline molten salt operating between 150 and 235 °C.•Steady state nominal design and dynamic model to evaluate the expected performance of the molten salt storage. Thermal Energy storage is one of the critical components in district heating and cooling (DHC) facilities when based on intermittent energy sources (as solar). Currently, DHC energy storage is based on water tanks. Nevertheless, water storage cannot be used with medium temperature processes as double effect absorption chillers or other services requiring temperatures well above 100 °C. Current thermal storage at higher temperature may be based on a variety of technologies. Among then, molten nitrate salts are widely used for heat storage in concentrated solar plants (CSP), limited by the melting temperature of available commercial solar salts and their solidification risk. Such commercially available salts are not applicable to district heating and cooling as they are solid in the expected operational temperature range. Other storage media may be solid materials, either by the use of packed-bed materials, concrete or phase change materials. In this paper, an innovative molten salt mixture with a melting temperature below 150 °C is used. The availability of a storage medium in a temperature range from 150 up to 250 °C might increase the solar share in DHC installations, integrating concentrated solar technologies. Such storage allows to apply solar technologies to supply heat to services for DHC that now can be only provided by boilers. In this communication, a molten salt storage tank operating in thermocline mode will be proposed in the framework of the WEDISTRICT project [1]. The project seeks to deliver the highest possible share of renewable sources for the energy needs of a district demand, maximizing performance by the application of advanced absorption chillers, that requires temperature feeds well above 100 °C.