Solar air conditioning
Solar air conditioning, or "solar-powered air conditioning", refers to any air conditioning system that uses solar power.
This can be done through passive solar design, solar thermal energy conversion, and photovoltaic conversion. The U.S. Energy Independence and Security Act of 2007 created 2008 through 2012 funding for a new solar air conditioning research and development program, which should develop and demonstrate multiple new technology innovations and mass production economies of scale.
History
In the late 19th century, the most common fluid for absorption cooling was a solution of ammonia and water. Today, the combination of lithium bromide and water is also in common use. One end of the system of expansion/condensation pipes is heated, and the other end gets cold enough to make ice. Originally, natural gas was used as a heat source in the late 19th century. Today, propane is used in recreational vehicle absorption chiller refrigerators. Hot water solar thermal energy collectors can also be used as the modern "free energy" heat source.A National Aeronautics and Space Administration sponsored report in 1976 surveyed solar energy system applications of air conditioning. Techniques discussed included both solar powered and solar related along with an extensive bibliography of related literature.
Photovoltaic (PV) solar cooling
can provide either indirect solar air conditioning power or, now, directly power to air conditioners. Indirect photovoltaic power for air conditioners consists of whole-house or whole-building solar which, traditionally for most users, has also meant net metering to the grid. Solar in this case is inverted to alternating current to run the appliances in the house or building, including the air conditioner. The advantage of this is the air conditioners don’t need any special electronics to accommodate solar, so it’s a simple implementation. The disadvantage is that these air conditioners usually have a SEER value of 14 or less, and the supplied solar has some loss from the power conversion of DC solar to AC even before it reaches the air conditioners. Another disadvantage is that these air conditioners cannot run when the grid is down, since, in effect, the net-meterOff-grid solar arrays instead use batteries to supply whole-house or whole-building solar. Such systems employ a voltage controller to manage battery charging, and then the battery power is inverted to provide alternating current for the home or building. Since they’re not grid tied or net metered, they can operate after a storm or other event brings down grid power. However, the power, once again, must be converted from DC from the solar panels and batteries to AC by inversion to run power remotely to the appliances.
More recently, true solar-powered photovoltaic air conditioners heat pumps have been developed. Such units run using DC power, and, as such, they can and do make use of the inherent DC power generated by photovoltaic solar panels. One mini split version of this units employs a 48v DC power bus and a 48v battery array, usually 4 x 12v batteries in series. Unlike the whole-house battery system, though, these batteries only run the air conditioner. The advantage of these systems is that, with enough solar and battery capacity, they can run at night or when it’s cloudy. Another mini split version allows the solar panels to be plugged directly to the outside part of the unit, uses a 310v DC power bus, and offers optional 120v plug-in backup grid power to be leveraged to fill in any lack of solar power available. The advantage of these inverter DC air conditioners is the lower cost, while the disadvantage is that they have no way to run without solar unless they're plugged in. Both of these systems make use variable refrigerant flow technology, with high-efficiency variable-speed DC motors and compressors to require very little run power, and both also offer heat in addition to air conditioning. A third type of unit is available for larger, usually commercial, buildings and offers both grid and battery backup as well as optional net metering. Like the two smaller units, these units are VRF, but unlike them, there’s an option to run heating in one part of the building and air conditioning in another part, making use of one outside/condensing unit and multiple inside/evaporative units located in different areas of the building to condition that areas based on specific user needs.
Photovoltaic can be combined with geothermal technology, too. An efficient geothermal air conditioning system would require a smaller, less-expensive photovoltaic system. A high-quality geothermal heat pump installation can have a SEER in the range of 20. A 29 kW SEER 20 air conditioner would require less than 5 kW while operating.
There are also new non-compressor-based electrical air conditioning systems with a SEER above 20 coming on the market. New versions of phase-change indirect evaporative coolers use nothing but a fan and a supply of water to cool buildings without adding extra interior humidity. In dry arid climates with relative humidity below 45% indirect evaporative coolers can achieve a SEER above 20, and up to SEER 40. A 29 kW indirect evaporative cooler would only need enough photovoltaic power for the circulation fan.
A less-expensive partial-power photovoltaic system can reduce the monthly amount of electricity purchased from the power grid for air conditioning. With American state government subsidies of $2.50 to US$5.00 per photovoltaic watt, the amortized cost of PV-generated electricity can be below $0.15 per kWh. This is currently cost effective in some areas where power company electricity is now $0.15 or more. Excess PV power generated when air conditioning is not required can be sold to the power grid in many locations, which can reduce or eliminate annual net electricity purchase requirement.
Superior energy efficiency can be designed into new construction. Since the U.S. Department of Energy was created in 1977, their Weatherization Assistance Program has reduced heating-and-cooling load on 5.5 million low-income affordable homes an average of 31%. A hundred million American buildings still need improved weatherization. Careless conventional construction practices are still producing inefficient new buildings that need weatherization when they are first occupied.
Geothermal cooling
or earth cooling tubes can take advantage of the ambient temperature of the earth to reduce or eliminate conventional air conditioning requirements. In many climates where the majority of humans live, they can greatly reduce the buildup of undesirable summer heat, and also help remove heat from the interior of the building. They increase construction cost, but reduce or eliminate the cost of conventional air conditioning equipment.Earth cooling tubes are not cost effective in hot humid tropical environments where the ambient Earth temperature approaches human temperature comfort zone. A solar chimney or photovoltaic-powered fan can be used to exhaust undesired heat and draw in cooler, dehumidified air that has passed by ambient Earth temperature surfaces. Control of humidity and condensation are important design issues.
A geothermal heat pump uses ambient earth temperature to improve SEER for heat and cooling. A deep well recirculates water to extract ambient earth temperature, typically at of water per metric ton per minute. These "open loop" systems were the most common in early systems, however water quality could cause damage to the coils in the heat pump and shorten the life of the equipment. Another method is a closed loop system, in which a loop of tubing is run down a well or wells, or in trenches in the lawn, to cool an intermediate fluid. When wells are used, they are back-filled with bentonite grout or another grout material to ensure good thermal conductivity to the earth.
In the past the fluid of choice was a 50/50 mixture of propylene glycol because it is non-toxic unlike ethylene glycol. Propylene glycol is viscous, and would eventually gum up some parts in the loop, so it has fallen out of favor. Today, the most common transfer agent is a mixture of water and ethyl alcohol.
Ambient earth temperature is much lower than peak summer air temperature, and much higher than the lowest extreme winter air temperature. Water is 25 times more thermally conductive than air, so it is much more efficient than an outside air heat pump,.
The same type of geothermal well can be used without a heat pump but with greatly diminished results. Ambient earth temperature water is pumped through a shrouded radiator. Air is blown across the radiator, which cools without a compressor-based air conditioner. Photovoltaic solar electric panels produce electricity for the water pump and fan, eliminating conventional air-conditioning utility bills. This concept is cost-effective, as long as the location has ambient earth temperature below the human thermal comfort zone.
Solar open-loop air conditioning using desiccants
Air can be passed over common, solid desiccants or liquid desiccants to draw moisture from the air to allow an efficient mechanical or evaporative cooling cycle. The desiccant is then regenerated by using solar thermal energy to dehumidify, in a cost-effective, low-energy-consumption, continuously repeating cycle. A photovoltaic system can power a low-energy air circulation fan, and a motor to slowly rotate a large disk filled with desiccant.Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. Air is passed through an "enthalpy wheel" to reduce the cost of heating ventilated air in the winter by transferring heat from the warm inside air being exhausted to the fresh supply air. In the summer, the inside air cools the warmer incoming supply air to reduce ventilation cooling costs. This low-energy fan-and-motor ventilation system can be cost-effectively powered by photovoltaics, with enhanced natural convection exhaust up a solar chimney - the downward incoming air flow would be forced convection.
A desiccant like calcium chloride can be mixed with water to create a recirculating waterfall that dehumidifies a room using solar thermal energy to regenerate the liquid, and a PV-powered low-rate water pump to circulate liquid.
Active solar cooling wherein solar thermal collectors provide input energy for a desiccant cooling system. There are several commercially available systems that blow air through a desiccant impregnated medium for both the dehumidification and the regeneration cycle. The solar heat is one way that the regeneration cycle is powered. In theory packed towers can be used to form a counter-current flow of the air and the liquid desiccant but are not normally employed in commercially available machines. Preheating of the air is shown to greatly enhance desiccant regeneration. The packed column yields good results as a dehumidifier/regenerator, provided pressure drop can be reduced with the use of suitable packing.