When most of us think of sunlight being used to generate power, we likely picture photovoltaic cells. Concentrated solar power plants however, use lenses or mirrors to heat fluid – such as synthetic oil – which in turn is used to generate high-pressure steam to drive a conventional turbine. A new experimental solar steam generation power plant that opened last week in southern Spain is aiming to improve on the efficiency of existing systems by using water as the direct working fluid and incorporating novel methods for storing the energy, so it can be dispensed even on cloudy days or at night.
The water in the receiver tubes is kept at a pressure of up to 120 bar (1740 psi), which results in the creation of superheated 500C (932F) steam. That higher temperature allows the whole process to work more efficiently, bringing down the cost of solar thermal power generation and making it a more viable option.
Heat gathered by the plant can be stored in both sensible and latent forms, for use in creating energy when the sun isn't brightly shining. In the sensible heat system, heat from the steam is absorbed by a concrete storage system, so it can later be released to create more steam and drive the turbine. In the latent heat system, the steam is used to heat salt, which absorbs energy when it reaches a temperature of 305C (581F) and changes from a solid to a liquid state. When the salt cools back down below 305 degrees and reverts to its solid form, that energy is released.
The Carboneras plant now boasts the world's largest high-temperature latent heat storage facility.
"The advantage of such a system is its capacity to store large amounts of energy in a small volume and with a minimal temperature change," said Doerte Laing, Thermal Energy Storage Research Area Manager at the DLR Institute of Technical Thermodynamics. "The energy in the system can be transferred and absorbed very efficiently by phase transition at a constant temperature."
Researchers plan to keep the plant in use until the end of the year. They will be validating the direct solar steam generation process itself, the storage methods, and the flexible pipe connections that are necessary for the mirror and receiver tube assembly to pivot as it tracks the Sun.
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