Researchers continue to make progress in large-scale storage of solar energy for use when the sun's not shining. The latest comes out of the Universidad Politecnica de Madrid (UPM), where scientists have developed a thermal-based system that uses an abundant natural material, molten silicon, to store energy generated by the sun.

The system developed by researchers from the university's Solar Energy Institute can store up to 10 times more than existing similar solutions, according to researchers, and it could one day be coming to a city near you. That's because the team aims to develop a series of next-generation, low-cost solar thermal stations based on the system-which currently has patent-pending status in the United States-to store electricity in urban areas.

Solar thermal energy storage solutions store sunlight as heat molten salt, then convert the energy into electricity on demand using a thermal generator. The key difference in the solution developed by the UPM team from others is its use of molten silicon, the most abundant element in the Earth's crust, researchers said. This makes it a plentiful, natural, low-cost, and safe resource for use in storing solar energy, they said.

The solution developed by the team takes solar energy and stores it in the form of heat in molten silicon at a very high temperature, around 1400°C (2552°F), said Alejandro Datas, the research promoter of the project.

"At such high temperatures, silicon intensely shines in the same way that the sun does, thus photovoltaic cells-thermophotovoltaic cells in this case-can be used to convert this incandescent radiation into electricity," he explained. "The use of thermophotovoltaic cells is key in this system, since any other type of generator would hardly work at extreme temperatures."

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Indeed, silicon has properties that allow for storage of more than 1 mW per hour of energy in a cubic meter-10 times more than storage systems that uses salt, Datas said. The UPM system thermally isolates molten silicon from its environment until the energy is demanded and, when this occurs, the stored heat is converted into electricity.

The UPM team is manufacturing the first lab-scale prototype of the system and expect that the first application will be in the solar thermal energy sector. For future applications the system also is well suited to storing electricity in the housing sector and to provide and manage electricity and heating in urban areas.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.