Modeling of a thermal energy storage system based on coupled metal hydrides (magnesium iron - sodium alanate) for concentrating solar power plants

- A detailed transport model developed to simulate coupled metal hydride systems.
- A low cost metal hydride couple (magnesium iron-sodium alanate) identified.
- A bench scale thermal energy storage system modeled.
- Technical feasibility of the system demonstrated with proper hydrogen transfer.
- Achieved temperatures of 450-500 °C and energy densities 5 times the DOE target.

Concentrating solar power plants represent low cost and efficient solutions for renewable electricity production only if adequate thermal energy storage systems are included. Metal hydride thermal energy storage systems have demonstrated the potential to achieve very high volumetric energy densities, high exergetic efficiencies, and low costs. The current work analyzes the technical feasibility and the performance of a storage system based on the high temperature Mg2FeH6 hydride coupled with the low temperature Na3AlH6 hydride. To accomplish this, a detailed transport model has been set up and the coupled metal hydride system has been simulated based on a laboratory scale experimental configuration. Proper kinetics expressions have been developed and included in the model to replicate the absorption and desorption process in the high temperature and low temperature hydride materials. The system showed adequate hydrogen transfer between the two metal hydrides, with almost complete charging and discharging, during both thermal energy storage and thermal energy release. The system operating temperatures varied from 450 °C to 500 °C, with hydrogen pressures between 30 bar and 70 bar. This makes the thermal energy storage system a suitable candidate for pairing with a solar driven steam power plant. The model results, obtained for the selected experimental configuration, showed an actual thermal energy storage system volumetric energy density of about 132 kWh/m3, which is more than 5 times the U.S. Department of Energy SunShot target (25 kWh/m3).