Thermochemically regenerative flow batteries for solar electricity generation and storage

External thermochemical regeneration (charging) at high temperature for a flow battery by concentrated sunlight is proposed, and electricity is produced by conventional electrochemical discharge of the battery. Analysis of the conversion efficiency and plant sizing is presented for three material systems, sodium-sulfur, zinc-air, and lithium-air systems. Addition of carbon to the metal-air systems to reduce reaction temperature is also analyzed. The thermodynamic upper limit (ideal system) for solar to electricity conversion efficiency is above 60% for all systems, but the required reactor temperatures are very high. Hybrid carbothermic reduction reaches the same theoretical efficiency at lower temperatures. The practical efficiency with accounting for realistic losses can reach up to 35%, which is very high compared with existing solar electricity technologies. A smaller plant size, smaller and simpler storage tanks, and flexibility in electrochemical generation from storage without thermal machinery offer advantages that make this approach very promising.