About Solar Cooling

When you hear about solar power, you probably first think about heating a home, but what about cooling it?

Solar thermal energy or solar electricity can be used to power cooling appliances as well as to heat a home or business. Just as solar heating reduced energy bills, so does solar cooling. In addition, solar cooling also has the advantage of not requiring the need for storage, this is because the cool air rises and falls almost precisely with the available solar energy.

There are several types of solar cooling technologies available. The first is absorption cooling, which uses solar thermal energy to change the refrigerant into a vapor. The next type of solar cooling technology is called desiccant cooling, which uses solar thermal energy to dry out or regenerate the desiccant. The third type of solar cooling is known as vapor compression cooling, which uses solar thermal energy to power a Rankine-cycle heat engine. The fourth type of solar cooling technology that we will describe is known as evaporative cooling. This type of technology is used in heat pumps and air conditioners, which are powered by solar photovoltaic systems.

Absorption cooling technology is a form of heat pump technology. Absorption systems typically use ammonia, hydrogen gas, and water. At room temperature ammonia is normally a gas that has a boiling point of -33°C, however, the absorption cooling system is pressurized to the point that the ammonia is held in a liquid state at room temperature.

The evaporator part of the absorption cooling system contains the hydrogen, which lowers the partial pressure of the ammonia, so that not all of the pressure is being exerted by ammonia. The hydrogen is used to fill space created by pressure as the rest of the system, but is not ammonia. The boiling point of the ammonia is now lowered so that it will now boil below room temperature, as though if is wasn't under the pressure of the absorption system. When the ammonia boils, it removes some of the heat from the evaporator, thus producing the desired cool temperature.

The next process is known as the absorption phase and is the separation of the ammonia from the hydrogen then transforms the gas ammonia back into its liquid state. Separating the hydrogen is relatively simple as ammonia readily mixes with water whereas hydrogen does not. The gases flow into the absorber, which is a cascade of tubes where the mixture of gases flows while water drips from mixing with the gases and separating the hydrogen from the ammonia.

At this point, it's now necessary to separate the ammonia from the water. This is achieved by heating the ammonia water mixture until the ammonia evaporates out. This phase is known as the generator. The water is then circulated back through the absorption phase.

The next phase of the process is known as the condenser and is where a heat exchanger cools the ammonia gas to room temperature, reverting back into a liquid state because of the pressure and the absents of hydrogen. The condensed ammonia is now suitable as a refrigerant and the process starts over.

The key aspect of the absorption cooling system is that it cools by using heat energy, rather than mechanical energy.

The mechanisms of an absorption chiller have to be integrated more closely than those of a compression system. This results in all absorption systems being contained within a single compact unit. It is for this same reason that there are few variations between absorption cooling units.

The main variations between models seem to be in the heat source and also in the number of stages or phases. Originally, steam or high-temperature water was the energy source for absorption chillers. However today, this is being replaced with direct firing using an integral boiler because of its improved efficiency.

Desiccant cooling is both a new and clean technology, which can be used to cool the inside of a home or commercial building without using any harmful refrigerants. Such as those used in conventional air conditioning systems. Desiccant cooling systems are an open heat driven cycle that uses a desiccant wheel as well as a thermal wheel in order to both cool and dehumidify the air.

Desiccant materials absorb moisture from the air. These materials can be regenerated or dried out by using heat. The desiccant wheel in most systems turns at an extremely slow rotation, giving the desiccant time to absorb the humidity from incoming air, and then discharge it into the outdoors. Desiccant cooling can also be used in junction with a conventional air conditioning system in that the desiccant removes the humidity from the air as the AC unite cools the air.

Residential use of desiccant cooling is being explored in conjunction with energy recovery ventilators or ERV. During the winter, ERVs are designed to provide energy recovery in a mechanical ventilation system. Energy recovery ventilators work by recovering heat and humidity from indoor air to preheat and humidify incoming fresh air, whereas desiccant cooling systems are designed to cool and dehumidify incoming fresh air during the summer. When combined these two systems create a heating and cooling system to keep a home or building comfortable year-round.

In addition, mechanical ventilation replaces conditioned air from within a home or building with unconditioned air from the outside, which is hot during the summer and cold during the winter months, ERVs recapture some of this energy, thus increases the mechanical ventilation’s efficiency.
 

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