Solar air conditioning

Solar air conditioning refers to any air conditioning (cooling) system that uses solar power. This can be done through passive solar, photovoltaic conversion (sun to electricity), or solar thermal energy conversion. The U.S. Energy Independence and Security Act of 2007^[1] 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.

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Solar A/C Using Desiccants

A packed column air-liquid contactor has been studied in application to air dehumidification and regeneration in solar air conditioning with liquid desiccants. A theoretical model has been developed to predict the performance of the device under various operating conditions. Computer simulations based on the model are presented which indicate the practical range of air to liquid flux ratios and associated changes in air humidity and desiccant concentration. An experimental apparatus has been constructed and experiments performed with Monoethylene Glycol (MEG) and Lithium Bromide as desiccants. MEG experiments have yielded inaccurate results and have pointed out some practical problems associated with the use of Glycols. LiBr experiments show very good agreement with the theoretical model. 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.^[2].

Passive Solar Cooling

Main articles: Passive cooling and Passive Solar

Passive solar building design is used to (1) slow the rate of heat transfer into a building in the summer, and (2) remove unwanted heat from a building. The principles of Physics are holistically integrated into the exterior envelope. This is much easier to do in new construction.

It involves a good understanding or the mechanisms of heat transfer: heat conduction, convective heat transfer, and thermal radiation (primarily from the sun).

For example: One sign of poor thermal design is an attic that gets hotter than the peak outside summer air temperature. This can be significantly reduced or eliminated with a cool roof or a green roof, which can reduce the roof surface temperature by 70 degrees F (21 degrees C) in the summer. Below the roof there should be a radiant barrier and an air gap, which blocks 97% of downward radiation from the sun.

There are hundreds of design specifics involved in Passive Solar Cooling. It is a primary element of a Zero energy building in a hot climate.

Photovoltaic Solar Cooling

Main article: Photovoltaic electricity

Photovoltaics can provide the power for any type of electrical air conditioner. At the current time, this is NOT cost effective for use on a building with a conventional compressor-based cooling load (without a large system subsidy).

For example, a 100,000 BTU U.S. Energy Star rated air conditioner with a high seasonal energy efficiency ratio (SEER) of 14 would require over a 7 kWh solar photovoltaic electricity generation system (with morning-to-evening, and seasonal solar tracker capability to handle the 47-degree summer-to-winter difference in solar altitude). The photovoltaics would only work during the sunny part of clear days.

A solar-tracking 7 kWh photovoltaic system would probably have an installed price well over $60,000 USD (with PV equipment prices currently falling at roughly 17% per year). If you need your air conditioner to work when skies are cloudy, or in the evening, you would require more than 7 kWh of PV power, plus expensive electrical batteries that have to be replaced about every 6 years. Or, you would need to purchase electricity from your local power company when photovoltaic power is not available.

A more-efficient air conditioning system would require a smaller, less-expensive photovoltaic system. A high-quality geothermal heat pump installation can have an SEER in the range of 20 (+/-). A 100,000 BTU SEER 20 air conditioner would require less than 5 kWh while operating.

There are new non-compressor-based electrical air conditioning systems with 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 buildngs without adding extra interior humidity (such as at McCarran Airport Las Vegas Nevada). In dry arid climates with relative humidity below 45% (about 40% of the contenental U.S.) indirect evaporative coolers can achieve SEER above 20, up to SEER 40. A 100,000 BTU indirect evaporative cooler would only need enough photovoltaic power for the circulation fan (plus a water supply).

A less-expensive partial-power photovoltaic system can reduce (but not eliminate) the monthly amount of electricity purchased from the power grid for air conditioning (and other uses). With American state government subsidies of $2.50 to $5.00 USD 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 back to the power grid in many locations, which can reduce (or eliminate) annual net electricity purchase requirement.

The key to solar air conditioning cost effectiveness is in lowering the cooling requirement for the building. Superior energy efficiency can be designed into new construction (or retrofitted to existing buildings). 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 proctices are still producing inefficient new buildings that need weatherization when they are first occupied.

It is fairly simple to reduce the heating-and-cooling requirement for new construction by one half. This can often be done at no additional net cost, since there are cost savings for smaller air conditioning systems and other benefits.

Since Jimmy Carter created the Solar Energy Tax Incentives in 1978, hundreds of thousands of passive solar and zero energy buildings have demonstrated 70% to 90% heating-and-cooling load reductions (and even 100% reduction in some climates). In contrast, well over 25 million new conventional U.S. buildings have ignored well-documented energy efficiency techniques since 1978. As a result, U.S. buildings waste more energy (39%) than transportation or industry. If their architects and builders had listened to the U.S. Department Of Energy presentations at the National Energy Expositions three decades ago, American buildings would be using $200 billion USD less energy per year today.

When cooling requirement is reduced far below conventional construction (70% to 90% reduction), THEN solar air conditioning can be very cost effective.^[3]

Solar Thermal Cooling

Main article: absorption chiller

There are multiple alternatives to compressor-based chillers that can reduce energy consumption by 80%, with less noise and vibration.

Solar thermal energy can be used to efficiently cool in the summer, and also heat domestic hot water, and the building in the winter. The Audubon Environmental Center in Los Angeles is one example among many).^[4]

Single, double or triple iterative absorption cooling cycles are used in different solar-thermal-cooling system designs. The more cycles, the more efficient they are.

In the late 1800's, the most common phase change refrigerant material for absorption cooling was ammonia + water. Today, lithium + bromide 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 1800's. Today, propane is used in recreational vehicle absorption chiller refrigerators. Innovative hot water solar thermal energy collectors can also be used as the modern "free energy" heat source.

Efficient absorption chillers require water of at least 190 degrees F (88 degrees C). Common, inexpensive flat-plate solar thermal collectors only produce about 160 degree F (71 degree C) water. More-expensive solar-energy-concentrating collectors are required for solar-powered absorption chiller systems.

Flat plate solar collectors work partially in overcast skies, but concentrating solar collectors only work well with dry clear skies, since diffused light cannot be focused. Concentrating solar collectors required for absorption chillers are less effective in hot humid, cloudy environments, especially where the overnight low temperature and relative humidity are uncomfortably high.

Where water can be heated well above 190 degrees F (88+ degrees C), it can be stored and used when the sun is not shining.

For 150 years, absorption chillers have been used to make ice (before the electric light bulb was invented).^[5]

This ice can be stored and used for cooling when the sun is not shining, as it was in the 1995 Hotel New Otani in Tokyo Japan.^[6]

Geothermal cooling

Earth sheltering 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 build up 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 6 to 10 gallons per minute). 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 that an outside air heat pump, (which is useless when outside air temperature is below 40 degrees F).

The same type of geothermal well can be used without a heat pump. Ambient Earth temperature water is pumped through a shrouded radiator (like an automobile radiator). Air is blown across the radiator, which cools and dehumidifies it 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. (Not the tropics)

See also

• Air Conditioning
• HVAC
• Passive solar
• Passive cooling
• Passive solar building design
• Passive house
• Absorption chiller
• Photovoltaics
• Solar power
• Solar thermal energy
• Geothermal heat pump
• Geothermal exchange heat pump
• Earth sheltering
• Earth cooling tubes
• Zero energy building

References