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Thermal storage is one of the main parts of a solar heating, cooling, and powergenerating system. Because for approximately half the year any location is in darkness, heat storage is necessary if the solar system must operate continuously. For some applications, such as swimming pool heating, daytime air heating, and irrigation pumping, intermittent operation is acceptable, but most other uses of solar energy require operating at night and when the sun is hidden behind clouds.

Usually the design and selection of the thermal storage equipment is one of the most neglected elements of solar energy systems. It should be realized, however, that the energy storage system has an enormous influence on overall system cost, performance, and reliability. Furthermore, the design of the storage system affects the other basic elements, such as the collector loop and the thermal distribution system.

A storage tank in a solar system has several functions, the most important of which are:

• Improvement of the utilization of collected solar energy by providing thermal capacitance to alleviate the solar availability and load mismatch and improve the system response to sudden peak loads or loss of solar input.

• Improvement of system efficiency by preventing the array heat transfer fluid from quickly reaching high temperatures, which lower the collector efficiency.

Generally, solar energy can be stored in liquids, solids, or phase-change materials (PCM). Water is the most frequently used storage medium for liquid systems, even though the collector loop may use water, oils, water-glycol mixtures, or any other heat transfer medium as the collector fluid. This is because water is inexpensive and non-toxic and it has a high storage capacity, based on both weight and volume. Additionally, as a liquid, it is easy to transport using conventional pumps and plumbing. For service water heating applications and most building space heating, water is normally contained in some type of tank, which is usually circular. Air systems typically store heat in rocks or pebbles, but sometimes the structural mass of the building is used.

An important consideration is that the temperature of the fluid delivered to the load should be appropriate for the intended application. The lower the temperature of the fluid supplied to the collectors, the higher is the efficiency of the collectors.

The location of the storage tank should also be given careful consideration. The best location is indoors, where thermal losses are minimal and weather deterioration will not be a factor. If the tank cannot be installed inside the building, then it should be located outside above the ground or on the roof. Such a storage tank should have a good insulation and good outside protection of the insulation. The storage tank should also be located as close as possible to the collector arrays to avoid long pipe runs.

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