Solar Power on Demand: Making Energy Available 24/7

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Project Manager, the DLR Institute for Solar Research, Markus Eck elucidates on water having a number of advantages in this context: “If we supply water under a pressure of up to 120 bar, we can produce superheated steam at a temperature in excess of 500°C.” This would significantly increase the process temperature of the power plant and thus boost efficiency levels. However, direct solar steam generation calls for specially adapted storage technologies.

Salt as a Medium

Molten salt, for instance, is a suitable medium to store and transfer such heat. “Solar salt—currently the salt mixture of choice—consists of potassium and sodium nitrate,” avers Product Manager, Bertrams Heatec, Switzerland, Markus Weikl. Both solar salt components must be extremely pure as contaminants such as chloride could result in pipeline corrosion. State of the art solar thermal power plants working with thermal oil use this solar salt to store sensible heat. “Sensible refers to the fact that energy accumulates as the temperature of the storage medium increases,” voices Weikl.

Two large tanks hold the molten salt mixtures at different temperature levels. Collectively, the tanks store more than 30,000 tons of salt. During the day, the molten salt is pumped from the ‘cold’ tank (290°C) to a heat exchanger, where it absorbs energy from the thermal oil that has been heated by the sun to reach a temperature of around 380°C. During the night, the salts are fed back to the colder tank. Heat exchangers extract the stored energy and feed it to the power plant to generate electricity.

The potential heat capacity of molten salt is even higher—with maximum temperatures of up to 565°C. This would be an ideal temperature for the superheated steam, and an ideal way to maximize plant efficiency. Currently, tests are underway to explore the benefits of feeding molten salts instead of thermal oil through the solar field. Not only would this enable higher process temperatures, but the direct use of salts would also provide an easy-to-use sensible heat store. “But molten salts bring their own challenges. If the temperature drops below 240°C, the salts freeze. As the receiver tubes are very thin, the salts can quickly freeze overnight and this would bring the power plant to a standstill,” continues Weikl.

“Once salt has hardened, it is extremely difficult to melt again,” discloses Weikl. “To prevent this happening, the receiver tubes must be kept warm during the night. This eats up around six per cent of the total volume of solar power generated during the course of an average year.”

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