Active Solar Energy Systems

Active solar energy systems are employed in residential and commercial/industrial buildings for the provision of space conditioning (heating and/or cooling) and/or hot water. Another common application is the heating of swimming pool water.

The basic building block of an active solar energy system is the collector. The collector contains a receiver or absorber that converts the incident solar radiation into heat. The heat collected by the absorber is transferred to a working fluid, such as a water-glycol solution or air, for transport directly to the load or to storage for later use. Transport is usually accomplished with pipes, pumps, and valves for liquid systems and with ducts, dampers, and fans for air systems. For liquid systems, insulated tanks are generally used to store the working fluid until there is a demand for the heat. For air systems, potential storage media include rocks or phase-change materials. Normally, storage capacity is limited to that required to meet the portion of the daily load that occurs when the sun is not shining. To meet these loads during prolonged periods without sunshine, most active solar heating systems are supplemented by a backup conventional system.

The combination of solar cooling with solar heating can provide for year-round utilization of collected solar heat and thereby significantly increase the cost-effectiveness and energy contribution of solar installations. Currently, solar desiccant and solar-driven absorption systems are the active cooling technology options that appear to have the greatest potential. Solar desiccant systems use a desiccant or drying agent to adsorb water vapor in recirculated or ventilated air to reduce humidity levels. The warm dry air is subsequently cooled evaporatively to the required temperature. The solar heat from the collectors is used to dry or regenerate the desiccant so it can regain its moisture-trapping or sorption capacity. A solar absorption system uses the thermal energy from the solar collector to separate a binary mixture of an absorbent and a refrigerant fluid. The refrigerant is condensed, throttled, and evaporated to yield a cooling effect, after which it is reabsorbed to continue the cycle.

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