Thermal Energy Storage

Thermal energy storage stores energy as heat for later use, a major branch of Grid Energy Storage. It uses three mechanisms: sensible heat (raising the temperature of a material such as water or rock), latent heat (a phase-change material that absorbs energy as it melts and releases it as it freezes, at near-constant temperature), and thermochemical storage (reversible chemical reactions, with the highest potential capacity). The best-known grid example is molten salt in concentrated solar power. A salt that melts around 131 degrees Celsius is heated to about 566 degrees in solar collectors and held in insulated tanks; the Solana plant stores about six hours of generating capacity, and Spain's Gemasolar once ran 24 hours a day for 36 straight days. So-called sand battery systems heat sand or crushed rock: Polar Night Energy in Finland stores around 100 MWh in sand at 600 degrees, and a Siemens-Gamesa system near Hamburg heats basalt to 750 degrees. Stored heat is recovered through a conventional steam turbine or used directly for heating. When a system stores electricity by converting it to heat and then converts the heat back to electricity, it is called a Carnot battery; its round-trip efficiency is bounded by thermodynamic limits and is typically in the 70 to 80 percent range, comparable to Pumped-Storage Hydroelectricity. Heat-only storage before any electricity conversion can be very efficient, with molten-salt stores estimated near 99 percent annual heat retention. Thermal storage sits alongside the electrochemical and mechanical families and is attractive because the storage medium (sand, rock, salt) is cheap and abundant.