Lithium Bromide-based absorption chillers (an air-conditioner that “chills” water which then cools the air) have been around since the 1950’s and used as a simple cooling system when there was free or inexpensive waste-heat or steam available.

The basic principle of a heat-driven absorption chiller and a conventional air-conditioner are the same: the cooling effect is achieved by evaporation of a refrigerant at a lower pressure; the heat is rejected by condensing the refrigerant at a higher pressure. A conventional air-conditioner accomplishes this by electric energy while an absorption chiller accomplishes this by heat energy and the chemical affinity of water for a salt (lithium bromide). With a solar fired absorption chiller, the heat is supplied by the evacuated-tube solar collectors.

The process goes something like this:
• The solar collectors heat the water in the storage tank and as soon as the temperature reaches about 158oF, the solution pump comes on, and a diluted lithium bromide solution is pumped into the generator component of the chiller which is under a low pressure

• Heat (in our instance solar heated hot water) is introduced evaporating the water out of a dilute solution of lithium bromide. A now concentrated lithium bromide solution flows back to the absorber.

• The water vapor flows to the condenser. Here under a slightly higher pressure, the water- refrigerant vapor condenses on the surface of the cooling coil and latent heat, removed by the cooling water, is rejected to a cooling tower. The condensed water flows to the evaporator.

• In the evaporator, the water is exposed to a substantially lower pressure than in the condenser due to the influence of the absorber. As the water (refrigerant) flows over the evaporator coil, it evaporates removing heat from the chilled water circuit. The lower vacuum in the evaporator is maintained by the fact that the water molecules can’t resist being attracted to concentrated lithium bromide solution in the absorber compo- nent of the machine.

• As the lithium bromide solution becomes diluted, it looses its capacity to absorb the water vapor. It thus needs to be re-concentrated and is next pumped back to the generator where the process is repeated.