Modeling of heat leak effect in round trip efficiency for Brayton pumped heat energy storage with liquid media, by cooling and heating of the reservoirs tanks

Abstract

For a pumped heat energy storage technology with commercial solar salt and a cold fluid such as methanol, the performance in long times of the Brayton like model associated to the losses of the four tanks (two high temperature units and two low temperature units) was studied. A round trip efficiency evaluation model linked with the heat leak coefficient was proposed for the sensible storage tanks, considering the losses for each face exposed to the environment. The different energy leaks and temperatures in the tanks were calculated for hourly climatic fluctuations during a reference year for a reference city. A zero-dimensional model was developed where radiation (direct and diffuse), convection and conduction effects were considered. The obtained results showed that for long periods, time and insulation thickness have a significant influence on the performance of the technology for long periods. For Brayton pumped heat energy storage technology with solar salts the round trip efficiency, which can be reduced daily by 0.4% even for a considerable insulation thickness of around 10% of the tank diameter. Winter–summer climatic conditions do not show a significant difference for tanks at higher temperatures, but as the temperature decreases, the effect becomes more visible but not decisive for the performance of the technology. The relatively high crystallization point of solar salt represents a significant solidification risk that limits the operation of the PHES technology during waiting periods for a surplus power to charge the system.