Solar air conditioning units have been unreliable in the past because of the size of the units needed to provide adequate cooling. Better refrigerants and more efficient hybrid designs are being studied by government agencies in the U.S., Australia and Europe as summertime cooling needs continue to grow. Several conditioning models are on the market, but the cost is the most daunting feature they all have in common.
Photovoltaic
Using the electricity produced from solar photovoltaic panels is one way to provide the energy needed by small vapor compression system such as those found on an average household refrigerator. Voltage needs of the compressor will need to be matched with the output of the PV array. However, this form of solar cooling is susceptible to the variations of sunlight available and will most likely need a battery backup system in order to maintain cooling operation.
Solar Mechanical
Solar mechanical is a vapor compression system that works by heating, vaporizing, compressing and expanding refrigerants such as lithium bromide. Coolants such as antifreeze are heated in the solar collectors or solar panels, and are then transferred to a heat exchanger to transfer heat to the refrigerant. The now heated and expanded refrigerant flows to the mechanical expander, causing vaporization and continued pressurization. Next it flows to the condenser, where it will return to a liquid state. As the pressure is reduced, it causes a drastic drop in temperature.
Solar Absorption
Unlike PV and vapor compression, absorption systems require little to no mechanical power for the compression cycle. However, absorption operates the same as vapor compression except for the compression. As the ammonia vapor, refrigerant, is introduced to an ammonia-water mixture in the heat exchanger, it acts the same as a condenser, releasing heated molecules in exchange for cool air molecules. Small amounts of electricity will be required to pump the coolant to the solar collectors, but the actual refrigeration process requires no moving parts.
Desiccant Cooling
Using dehumidifying materials such as silica gel to remove moisture from the air is called desiccant cooling. The process is used to reduce humidity in the air that is introduced by an evaporative "swamp" cooler or a passive cooling tower. Air is passed by the desiccant material by way of convection, and solar thermal heat is used to remove the moisture from the material. The cycle is self-completing as the moisture is reintroduced in the cooling phase. Only a small pump is needed to circulate the desiccant liquid.
Considerations
Vapor compression refrigeration methods require additional electrical energy to drive the compression pumps and the forced-air components of the system. PV power or outside electricity are needed to maintain an efficiency margin in line with operating the system versus the cost of operating a conventional cooling system. The coefficient of performance for PV electrical supply for some systems can be as high as 80 percent. The size of the system is also important. Absorption and solar desiccant systems need to be large enough to operate efficiently while mechanical and PV can be scaled to meet residential needs.
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