Absorption chillers use heat energy to generate chilled water that can be used for air conditioning or process cooling applications. They are often used in conjunction with combined heat and power (CHP) making use of the surplus heat from CHP engines in summer to provide cooling.
An absorption chiller consists of a generator, a condenser, an evaporator, and an absorber. The basic absorption cycle employs two fluids, a refrigerant (typically water) and an absorbent (typically lithium bromide). As the absorption cycle proceeds, these two fluids are separated and combined, as follows:
In the generator a dilute lithium bromide solution is heated, causing water to evaporate off – resulting in water vapour and a concentrated lithium bromide solution. The water vapour is transported to the condenser and the lithium bromide is transported to the absorber. In the condenser, the water vapour from the generator condenses under high pressure.
This condensed water is then transferred to a lower pressure evaporator, where it evaporates and absorbs heat from water in adjacent cooling coils. This generates chilled water that is then distributed to the cooling system. From the evaporator, the low-pressure water vapour passes to the absorber where it re-combines and dilutes the concentrated lithium bromide solution, so that the cycle can be repeated.
Absorption chillers typically require a hot water temperature of 70-95 degC to produce chilled water at around 7 degC.