Solar Power on Demand: Making Energy Available 24/7

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TURCK GmbH

EDL Anlagenbau Gesellschaft mbH

Pörner Ingenieurgesellschaft mbH

Linde GmbH, Head Office Munich

Linde AG, Linde Gases Division

Challenges to be Overcome

And so researchers are on the lookout for salt mixtures that only freeze at lower temperatures. As part of the DSG Store project, Linde and DLR are exploring a different avenue. They are looking to generate steam directly in the solar field and combine this with a latent heat storage system. This approach combines the advantages of high steam temperature, and thus process efficiency, with the benefits of an easy-to-use heat transfer fluid that would not require costly antifreeze protection.

“But even direct steam generation calls for specially adapted storage technologies in order to efficiently store the huge volume of energy contained in the steam,” states Eck. Latent heat is ‘hidden’ energy, which means that energy is accumulated as the storage medium changes phase—from solid to liquid for instance. In other words, it is invisible to a thermometer. Special nitrate salts can act as latent heat stores—which is why they are at the heart of the Linde–DLR cooperation. The storage systems consist of a salt tank fitted with long tubes through which the steam can flow. The exterior of the tubes feature fine aluminum fins.

“The fins allow the steam to transfer its thermal energy most effectively to the salt store,” elaborates Eck. This design increases the effectiveness of the heat transfer process by several orders of magnitude. “The molten salt has to freeze in order to release the energy so it can be used,” clarifies Eck.

The system works in a similar way to reusable hand-warmers based on crystallization. In the case of solar thermal power plants, the sun heats up the crystallized salts by means of steam. This melts the salt mass. If the energy is subsequently withdrawn, the salt recrystallizes and can be powered up with heat from the sun again. The advantage of latent heat storage systems is that they can capture the hidden energy contained in the steam itself—also known as the enthalpy of vaporization.

According to Hübner: “Over 60 per cent of the energy absorbed from the sun is concentrated in the phase change from boiling water to saturated steam. The only way to efficiently capture this energy is with latent salt storage systems.”

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