Energy storage plants; pumped thermal electricity storage plants; pumped thermal electrical energy storage plants; long duration energy storage plants; energy storage systems comprised of supercritical co2 heat pumps, supercritical co2 heat engines, hot reservoirs, cold reservoirs, and thermal batteries; pumped thermal electrical energy storage systems comprised of supercritical co2 heat pumps, supercritical co2 heat engines, hot reservoirs, cold reservoirs, and thermal batteries; long durati...