How can a solar power station keep generating after sunset?

A concentrating solar-thermal station can continue generating electricity after the Sun sets because it stores thermal energy collected earlier. In a two-tank molten-salt design, the storage medium is a hot liquid salt mixture, not a battery holding electrical charge. Mirrors direct sunlight toward a receiver. Salt pumped through the receiver absorbs energy and leaves at a higher temperature. The hot salt enters an insulated tank. Another tank holds cooler salt awaiting reheating. Both tanks can contain liquid salt; the words hot and cold describe their relative operating temperatures, not whether one is frozen.

When electricity is required, hot salt flows through a heat exchanger. Energy crosses metal surfaces into water, producing steam for a turbine-generator cycle. The salt cools as it transfers heat and returns to the cooler tank. The water-steam circuit remains separate from the salt circuit. After the steam expands through the turbine, a condenser and pumps prepare it for another pass.

The amount of sensible heat stored is approximately the salt mass multiplied by its heat capacity and usable temperature change. In symbols, Q is approximately m c ΔT when heat capacity can be treated as nearly constant. Twice as much salt over the same temperature range holds roughly twice as much usable thermal energy. This relationship concerns thermal storage, not the final electrical output.

Electrical output is smaller than the released thermal energy because the power cycle rejects heat and equipment has losses. Pumps, tracking mirrors and other components use electricity, while insulation slows but does not stop losses from the tanks. For a store containing 100 units of usable heat, a stipulated net conversion of 40 percent yields about 40 electrical units, before any losses excluded from that efficiency are accounted for. Energy capacity describes how much can be delivered; power describes how fast. The available duration depends on both, together with conversion efficiency and operating limits.

The salt must stay within a suitable temperature range. Excessive cooling can let it freeze in equipment; excessive heating can damage materials or degrade the fluid. Mixtures, insulation, trace heating and controls help maintain operation. A cloudy week can still exhaust the reserve. The achievement is separating collection time from generation time: sunlight heats the salt earlier, and controlled heat transfer makes that stored energy useful after sunset.