Conception and design of a thermal energy, storage system

In order to provide the capability of a thermal storage to produce superheated steam with a temperature of about 330 degrees C (pressure 70 bar), a new phase change material based on a Tin-Zinc alloy was explored as the heat storage medium. It was shown that 70 wt% Zn in the alloy would be optimal to attain a thermal storage temperature of 360-370 degrees C, using solar superheated steam 400 degrees C as the heat charging fluid. At the stage of heat discharge, high quality steam is produced on the base of utilizing mainly the phase change heat of zinc solidification and partly sensible heat of the liquid phase. A heat transfer fluid (e.g. DOWTHERM-A) is used to enhance heat transfer between the storage medium and built-in heat exchangers by the effect of reflux evaporation-condensation. Since the storage process is essentially non-isothermal, a secondary, relatively small thermal buffer following the main storage is required to stabilize the steam outlet temperature. The results of calculations for a pilot of 5MWh thermal storage show that steam sent to a turbine after the buffer may have a temperature near 330 degrees C during the entire charge-discharge cycle comprised of 6 and 1 hr periods respectively.