Two tanks hold the same molten-salt inventory at two different temperatures: a cold tank near 290 °C and a hot tank near 565 °C. Charging pumps salt from the cold tank through a heater (a solar receiver or excess grid/industrial heat) into the hot tank — the fill level shifts from cold to hot and the reservoir's usable heat rises. Discharging reverses the pump: hot salt flows through a heat exchanger that drives a power block (steam turbine or ORC), giving up its heat and returning to the cold tank. Because the storage medium is just hot vs. cold liquid changing tanks, round-trip efficiency is high (typically 95–99%) — the main losses are ambient heat leakage through the insulated tank walls, shown here as a slow drift back toward "cold" even while idle.
dE/dt = +P_charge · η (charging)
dE/dt = −P_discharge / η (discharging)
dE/dt = −loss_rate · E (always, idle heat leak)
SoC = E / capacity, clamped to [0, 1]
- Tank capacity — total thermal energy the pair of tanks can hold between fully-cold and fully-hot, in MWhth. A bigger capacity means longer duration for the same power rating.
- Charge rate — thermal power delivered into the hot tank while charging (e.g. solar-field or waste-heat input).
- Discharge rate — thermal power drawn out through the power block while discharging.
- Heat loss rate — passive leakage through tank insulation, expressed as % of stored energy lost per hour; it never stops, even in Idle.
Real-world relevance: this is the same architecture used at concentrated solar power (CSP) plants and increasingly at stand-alone "electric thermal energy storage" sites — cheap off-peak electricity or curtailed renewable power heats the salt, and the plant discharges heat (or dispatches electricity) for hours after the sun sets or demand peaks, decoupling when heat is collected from when it is used.