S ome renewable energy power plants store solar energy as heat, which is used to generate electricity after sunset. But as with many clean energy technologies, economics infl uences how far energy storage can spread through the emerging market for commercial-scale solar power. A class of materials called metal hydrides could be the next generation of heat storage materials to help reduce the cost of thermal energy storage at solar power plants, and thus further the development of renewable

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... y worldwide. The most effi cient use of sunlight to generate enough power for base-load consumption is called concentrating solar power (CSP). At CSP plants, mirrors focus sunlight onto a fl uid, like oil or molten salts. The hot fl uid then fl ows through a heat exchanger, producing steam to drive a turbine that generates electricity. There are 80 operational CSP plants around the world, mainly in Spain and the United States, with 1.9 GW of total capacity as of March 2012. Another 23 are under construction in India, China, Australia, and South Africa, among other places. Currently 40% of the commercial CSP plants store thermal energy to generate from 30 minutes to 15 hours of electricity on demand. One CSP plant with thermal storage is a state-of-the-art 110 MW plant in Nevada called the Crescent Dunes Solar Energy Plant, with construction to be fi nished by the end of 2013. At this plant, energy storage is integrated into the plant's power generation system, and electricity production can occur without intermittency. "Depending on what a utility company needs, we can develop this plant to run 24 hours a day, seven days a week," said Mary Grikas, Vice President of Communications at SolarReserve, the company developing the Crescent Dunes plant. "That makes this plant more like a coal or natural gas plant, as far as a utility company is concerned." At this plant, solar energy is collected and stored in a material called molten salt, a mixture of sodium nitrate and potassium nitrate, both ingredients commonly found in fertilizer. During the day, mirrors shine sunlight onto a receiver atop a central tower. Molten salt fl ows through the tower, where it is warmed to 565°C as it fl ows through the receiver. The hot salt is then pumped into an insulated storage tank. When electricity is needed during the day or night, the hot salt is moved from the tank through a heat exchanger, where it cools to about 290°C as the stored heat generates steam to power a standard turbine. The cooled salt then travels to a second storage tank until being pumped into the receiver again. The 10-hour heat storage capability at the Crescent Dunes plant requires about 32,000 metric tons of molten salts. That large amount of salt can increase costs to build CSP plants with storage. Using a storage material with a higher energy density than molten salt could help reduce some of the storage costs in a CSP project because less material would be required to store the same amount of energy. A well-researched class of materials called metal hydrides appears poised to be the next generation of storage at concentrated solar power plants. Made from inexpensive, readily available materials such as magnesium, calcium, and titanium, these materials are 10 to 30 times more energy dense than molten salt (see Table) , though some of that theoretical thermal storage capacity will be lost when engineering a storage system. Solar thermal storage using metal hydrides requires a smaller storage tank and less storage material than a system with molten salts, said Ewa Rönnebro, a senior research scientist at Pacifi c Northwest National Laboratory where she is leading a project on developing a thermal energy storage technology utilizing metal hydrides for high-temperature power generation. "Comparing the technical solutions, it's possible to see that metal hydrides will be cheaper than molten salt," she said. Metal hydrides like titanium-iron hydride were fi rst explored during the oil crisis of the 1970s as materials to store hydrogen for cars and to increase the energy density of batteries. These materials reversibly absorb and desorb hydrogen gas from their crystalline matrix. Energy is stored in the chemical bonds that change as a metal hydride releases hydrogen gas to become just a metal. That energy is released as heat when the metal absorbs hydrogen and reforms the metal hydride. Magnesium hydride, another material fi rst explored as a result of the oil crisis 40 years ago, is commonly studied for thermal energy storage because of its high hydrogen storage capacity: 7.7 wt%. The material reversibly absorbs hydrogen gas at around 400°C, the optimal temperature for many solar thermal plants Research on metal hydrides revived for next-generation solutions to renewable energy storage