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Active Solar Cooling

Active solar cooling is somewhat harder to achieve than passive cooling or heating of any kind. Nevertheless, there are several techniques currently available as well as several under research.

Photovoltaic Panels and Standard Air Conditioning

Using solar panels or other solar form of generating electricity to power a traditional air conditioner is probably that most obvious and straightforward mechanism. It is widely known that air conditioners use a tremendous amount of energy and so power a traditional air conditioner with solar energy can require rather large installations, which means high cost. Standard air conditioning is one of the least energy efficient technologies used in the average home. Central air is less efficient than are window air conditioners.

Across most the Northern Hemisphere, the average BTU requirement ranges from 30 to 60 BTU/square foot. For a 2000 square foot house, that is anywhere from 60 to 120,000 BTU. A standard 100,000 BTU central air conditioner will need 7 kW of power on a summer day. This system alone would have a price tag in the $20,000 to $25,000 range. This system would supply air conditioning power only and also requires that the DC power from the PV panels be converted to standard AC before being used.

Photovoltaic Panels and DC Heat Pumps

Heat pumps are a viable alternative to standard air conditioning. These operate much like an air conditioner, but can also provide heat when run in reverse. They are DC, which means there is no need to convert the current from the solar panels. These units generally require 800 W of power, but only offer 5,000 to 18,000 BTU per hour of cooling. They can provide up to 20,500 BTU of heating. There are larger units available that require up to 5 kW of energy.

Photovoltaic and Non-Compressor Cooling

Most the energy to run an air conditioning system goes to the compressor, which reduces the volume of a refrigerant to release the heat it has collected. When the refrigerant expands, it collects heat (from indoor air) and is then compressed again to release that heat outdoors. This cycle is repeated multiple times per day by air conditioners and is the reason they require so much energy.

A non-compressor system uses desiccants (dying agents) to remove moisture from the air. This causes the air to cool as the warm moisture and the heat it contains are removed. The desiccant is then regenerated using solar thermal to provide heat to dry it. This cycle can be repeated many times and only requires enough solar energy to run a fan to circulate the air and a small motor that rotates a disk of the desiccant as it goes through its own cycle of being regenerated.

While non-compressor systems are energy efficient, their ability to cool air is also limited. Additionally, they are unsuitable for dry climates where water must be added to the air and then removed again to aid in cooling. This is usually impractical and reduces the efficacy of the system to near zero. They are best suited for mild cooling in humid climates.

Adsorption Cooling

Ammonia systems are discussed extensively in the forced air heating subsection under active solar heating. A heat pump uses a compression/expansion cycle of a gas to move heat from one location to another. By removing heat, an area is cooled. So, if the gas expands outdoors and is compressed indoors, then heat is transferred from outside to in. The opposite cycle, which transfers heat from indoors to out, provides cooling.

These systems do not require pumps, but instead rely on the adsorption of one liquid into another with subsequent boiling to separate them again. The heat for the boiling process can be derived from solar thermal.

A major benefit to these systems is their ability to provide both heat and cooling, greatly simplifying any total solar installing. They are also much cheaper than PV systems because they require only solar thermal collectors and not expensive PV cells.

The downside to these systems is that they require water temperatures of around 88 degrees Celsius (190 degrees Fahrenheit) to operate. In locations where the temperature is well above this, heat energy can be stored for overnight cooling. In locations where the temperature barely reaches this level, daily cooling can be difficult to achieve, let alone over night cooling. Cloudy, humid climates are often poor locations for this particular type of system. To overcome limited sun exposure, more solar collectors can be installed. This, of course, adds expense.
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