Solar thermal energy is energy produced through heat from the sun which is then converted into heat that is captured for energy use. Thermal collectors come in low, medium, and high forms, and are also able to store the energy for later use.
Thermal energy is stored in a thermal reservoir. Solar thermal energy can be used immediately in full, or it can be used in part balanced between day time and night time and in addition to utility energy. The thermal reservoir can come in varied forms and may be maintained at a temperature either hotter or cooler than the ambient environment.
Solar thermal energy storage generally stores heat, most often in solar collectors which are considered to be insulated repositories. This energy can later be used for space heating, water heating, or even to generate electricity. Most often thermal energy is stored for a few hours or days, but seasonal thermal energy storage is becoming more and more popular.
There are two general types of thermal energy storage, water-based technology and molten salt technology. Water-based technology has been used for decades, originally beginning with the transportation of ice from city to city as a coolant. It was discovered at this time that a small storage facility can hold enough ice to cool a large building anywhere from a day to a week. This same concept was taken and then applied to air conditioning.
Air conditioning is combined with the use of thermal energy storage to minimize the capital investment. The partial storage system paired with the air conditioner produces ice for storage, which chills water in the air conditioning system. The water circulating through the melting ice can work for six hours a day. The storage system minimizes 50% of costs by shutting off the chillers during peak hours. Water-based technology is efficiently offset by the amount of heat loss involved in the use.
While water-based technology may not always be the most efficient, it still has its benefits. The fuel used to run these water coolers is local and minimal, and the emissions are proven to be low. The plants that run these coolant systems often run off low-emission base load facilities in the evening to insure lower emissions.
The condensing form of water-based technology has recently been added to the forum. Refrigerants have become popular over the last hundred years, using pumped coolers, and ice to chill the liquids. Compressors are being removed from the design, using water chill technology instead.
Molten salt technology has recently become part of thermal energy storage. The salt can be used as a heat store to retain heat collected by solar energies. The efficiency is thought to be around 99%, although this is not conclusive at this point. Molten salt technology is non-flammable and nontoxic and has been used in heat-transport fluid. The salt melts and is kept liquid in an insulated cold storage tank. The salt is pumped through solar panels, heated through the sun and then sent to a hot storage tank. This thermal energy can then be used for up to a week.
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