Absorption coolers use heat rather than electricity as their energy source. Because natural gas is the most common heat source for absorption cooling, it is also referred to as gas-fired cooling. Other potential heat sources include propane, solar-heated water, or geothermal-heated water. Although mainly used in industrial or commercial settings, absorption coolers are commercially available for large residential homes.
The absorption process is thermochemical in nature, as opposed to mechanical. Also, absorption chillers circulate water as the refrigerant instead of chlorofluorocarbons or hydro chlorofluorocarbons (CFCs or HCFCs, also known as Freon). The standard absorption chiller system uses water, as a refrigerant, and lithium bromide, as an absorbent, in its cycle. The lithium bromide has a high affinity for water. The process takes place in a vacuum, allowing the refrigerant (water) to boil at a lower temperature and pressure than it normally would, helping to transfer heat from one place to another. Small residential-sized units use ammonia as the refrigerant, and water as the absorbent.
HOW ABSORPTION COOLING WORKS
An absorption cooling cycle relies on three basic principles:
- When a liquid is heated it boils (vaporizes) and when a gas is cooled it condenses
- Lowering the pressure above a liquid reduces its boiling point
- Heat flows from warmer to cooler surfaces.
Absorption cooling relies on a thermochemical “compressor.” Two different fluids are used: a refrigerant and an absorbent. The fluids have high “affinity” for each other, which means one dissolves easily in the other. The refrigerant—usually water—can change phase easily between liquid and vapor and circulates through the system.
Heat from natural gas combustion or a waste-heat source drives the process. The high affinity of the refrigerant for the absorbent (usually lithium bromide or ammonia) causes the refrigerant to boil at a lower temperature and pressure than it normally would and transfers heat from one place to another.
In addition to being direct fired by natural gas, absorption chillers can run off of hot water, steam, or waste heat, making them an integral part of cogeneration systems or anywhere that waste heat in any form is available. Absorption chillers are generally used where noise and vibration levels are an issue, particularly in hospitals, schools, and office buildings. The primary advantage of absorption chillers is lower electricity costs. Costs can be even further decreased if natural gas is available at a low price or if we can utilize a source of low grade heat that is otherwise wasted in a plant. Comparing with electric chiller, absorption chiller has many advantages as above. However, it need more place to set the equipment. What is more, its efficiency is low.