N°30 I quarterly I july 2013 Focus | 23 w Our Stone Age ancestors developed a sure-fire technique for boiling water: they heated a stone in fire and dropped it into a skin filled with water, repeating the operation until boiling point. Heat storage had just been invented. The same principle applies to water heaters, or even a simple hot water bottle—a portable, relatively longlasting heat source that can be used as needed. Today’s interest in renewable energy, which is intermittent by nature, makes this principle more topical than ever. Concentrated solar power plants, by integrating heat storage, can produce electricity day and night. Sunlight is focused by a mirror array and used to heat a liquid to more than 400°C (usually molten nitrate salts, which can remain in a fluid state even at very high temperatures). This liquid is then routed to a steam generator that powers a turbine to produce electricity. At this stage, thermal storage plays a key role. Part of the fluid from the solar unit is used to heat an enormous reservoir that also contains molten salts. These salts cool down during the night, returning the heat accumulated during the day to the power plant, allowing it to function continuously. CAPT URING HEAT Although significantly perfected, this technology has one inherent drawback. “A 50-megawatt solar power plant requires some 28,000 tons of salts to store the heat,” says PROMES1 researcher Xavier Py. “To meet the objectives that have been set for 20502 in terms of concentrated solar power, we would need 10 times the total world output of natural nitrate salts every year!” To overcome this obstacle, the French researcher and his colleagues are developing a new material for heat storage in solid form: a ceramic made from hazard- 11 Concentrated solar power stations (like the Gemasolar plant in Spain, shown here) focus the sun’s heat on a spot at the top of a tower. 12 Potassium and sodium nitrates, visible here in solid form, are liquefied and stored in the Gemasolar power station. They can then be used to power the plant’s turbine for up to 15 hours without sunlight. 11 12 Running Hot and Cold © p hotos 11-12 : p . Psaïla
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