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.
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.
Absorption Cooling in Commercial Buildings
Absorption cooling is most frequently used to air condition large commercial buildings. Absorption chillers can be teamed with electric chillers in "hybrid" central plants to provide cooling at the lowest energy costs. In this case, the absorption chillers are used during the summer to avoid high electric demand charges, and the electric chillers are used during the winter when they are more economical. Because absorption chillers can make use of waste heat, they can essentially provide free cooling in certain facilities.
Absorption cooling systems can most easily be incorporated into new construction, though they can also be used as replacements for conventional electric chillers. A good time to consider absorption cooling is when an old electric chiller is due for replacement.
Absorption chillers can be direct-fired or indirect-fired, and they can be single-effect or double-effect (explanation of these differences is beyond the scope of this discussion). Double-effect absorption cycles capture some internal heat to provide part of the energy required in the generator or "desorber" to create the high-pressure refrigerant vapor. Using the heat of absorption reduces the steam or natural gas requirements and boosts system efficiency.