Technical details

The process of producing power from the sun’s radiation involves several steps. Firstly, the concentrating mirrors focus the solar radiation onto the receiver to heat up a heat transfer fluid to high temperatures. In the case of tower technology, molten salts are typically used as the heat transfer fluid. The same molten salts, stored in large tanks, are also used as the thermal energy storage medium.

In order to produce power, the hot salts are withdrawn from the storage tank and are made to circulate through a series of heat exchangers where they generate steam by heating up water. The pressurised steam is what drives the steam turbine and this rotational motion is then converted into electrical power in the generator. The cool steam from the turbine is then condensed into water in an air-cooled condenser, recompressed and pumped back into the salt/water heat exchangers.

For a tower power plant, the mirrors consist of an array of heliostats (large mirrors tracking the sun along two axes) that concentrate the sunlight onto a fixed receiver mounted at the top of a tower. The high level of concentration produces high temperatures and therefore high efficiency energy conversion at a single large receiver point. High temperatures also make the thermal storage cheaper as the same amount of heat can be stored in a smaller amount of thermal medium.

Thermal storage is what enables a CSP plant to produce 24‑hour solar power. The system consists of two large insulated tanks (one hot tank and one cold tank) that store a mixture of potassium and sodium nitrate salts, at a sufficiently high temperature to maintain them in a molten state. These salts are cheap, safe and never need replacement (they already have a large market as fertilisers).

During the day, the molten salts are moved from the ‘cold tank’ and heated up in the receiver (close to 600ºC), then they are transferred to the ‘hot tank’ by the use of pumps. At night, they follow the opposite path and the energy contained in the salts is used to power the steam cycle.

The salts can retain their heat for weeks due to the effective insulation of the tanks. In situations when heat cannot be provided from the solar field (eg programmed maintenance), electric heaters are used to keep the salts in a molten state. In the case of forced curtailment to nearby wind and solar PV farms, the same heaters could be used to store the surplus energy in the salts to be dispatched at later times when there is a need for power. This effectively allows the plant to pursue additional revenue from energy arbitrage.