Thermo-Electro-Chemical Storage (TECS) is a novel concept for thermal energy conversion and storage in electro-chemical form, where a thermo-chemical reaction is used to charge a battery directly without prior conversion to electricity. The battery discharges electricity in a conventional fashion. This work describes a specific case of TECS based on the sodium-sulfur (Na-S) battery. During discharge, sodium ions cross the inorganic solid electrolyte (β-Al2O3 membrane) from the anode compartment to the sulfur cathode, and react to produce sodium polysulfides. During charging, sulfur and sodium polysulfides are pumped from the cathode to a reactor, where the polysulfides are thermally decomposed to produce sulfur and sodium vapors. Quenching of the products yields liquid sodium with gaseous sulfur, and allows separation of the two components. The sulfur and sodium are stored in separate tanks after cooling to the battery operating temperature and return to the battery when needed. This thermo-chemical charging process is equivalent to electrical charging the battery, but uses external heat instead of electricity to decompose the sodium polysulfides before returning the sodium to the anode compartment. The cycle is both a power generator and an energy storage device, without the need for a heat engine or a high-temperature heat storage unit. This work analyzes the thermodynamics of the TECS cycle for different temperatures and pressures, and for different process configurations that include heat recuperation and waste heat utilization. Results will be presented showing that proper internal and external utilization of waste heat leads to very high conversion efficiency.
