Solar Energy Thermal and Electrical Storage
Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used. Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well designed systems can lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements. Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are non-flammable, nontoxic, low-cost, have a high specific heat capacity, and can deliver heat at temperatures compatible with conventional power systems. The Solar Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m³ storage tank with an annual storage efficiency of about 99%.[97] A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin, which has a high enthalpy of fusion. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water to 64 °C and can reduce conventional energy use by 50 to 70%.[98][99] Eutectic salts such as Glauber's salt can also be employed in thermal storage systems. Glauber's salt is inexpensive, readily available and can deliver heat at 64 °C. The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system in 1948.[100] Rechargeable batteries can be used to store excess electricity from a photovoltaic system. Lead acid batteries are the most common type of battery associated with photovoltaic systems because they are cheap and available. Batteries used in off-grid applications should be sized for three to five days of capacity.[101] Excess electricity can also be fed into the transmission grid. Net metering programs give photovoltaic system owners a credit for the electricity they deliver to the grid. This credit is used to offset electricity provided from the grid when the photovoltaic system cannot meet demand, effectively acting as a battery. For large scale use of renewable energy the most practical storage is hydro-storage, although vehicle-to-grid is also being developed, which will become viable when more plug-in hybrids and electric cars are in use. Notes
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