Absorbing absorption

At its simplest, an absorption refrigerator consists of four elements:: evaporator, absorber, generator and condenser.

Water acts as the primary refrigerant. Starting in the evaporator, the water is made to boil at as little as 0°C. This is possible due to low pressure conditions – achieved by keeping air out of the system.

As some of the water changes state from liquid to gas (steam), energy is drawn from the remaining water and its temperature falls. This creates the necessary cooling effect.

The steam then passes into the absorber, which contains a concentrated salt solution. This solution absorbs the heat of the steam and so becomes warmer.

To stop the salt solution becoming saturated with water – which would prevent the absorption of steam and heat – some of the salt solution is removed. This is circulated by pump through the generator.

When the solution enters the generator it is heated to about 75°C. It boils, forcing steam out into the condenser at high pressure (5 kPa absolute). The salt solution left in the generator is now concentrated, and is returned to the absorber through a control valve.

The high pressure steam that has entered the condenser changes state back to water, before being returned to the evaporator to start the cycle once more.

Lithium bromide salt is commonly mixed with the water for absorption refrigerators used in buildings. It is odourless and offers an advantage over hydrocarbons because it does not represent a fire hazard.

However, lithium bromide solutions may crystallise in pipes and heat exchangers, or cause corrosion. Further, although a water refrigerant has environmental advantages, it limits the minimum evaporator temperature to 0°C.

Fair cop?

The coefficient of performance (cop) of an absorption refrigerator measures the ratio of chilling capacity to heat input required at the generator. The cop of the simple system described above is typically 0·5 ie 200 kW at the generator for 100 kW of cooling.

Most research to improve cop has concentrated on reducing heat input at the generator – by using the waste heat it produces. Adding a solution heat exchanger between the hot and cold streams to and from the generator raises the cop to about 0·7.

As an extra measure to raise the cop, it is possible to use hot vapour from the generator. So-called 'double-effect' systems have been made for several years and produce cops of about 1·2. A 'triple-effect' unit could theoretically yield a cop as high as 1·52.

An experiment carried out by Nottingham University to raise cop uses an ejector to compress steam coming from the generator. The compressed steam is then reused to heat the generator, thereby lifting the cop³.

Thermal ice storage is also being linked to absorption refrigeration. The water refrigerant is being used to freeze a body of water into ice as an energy store. This kind of system is appropriate when waste heat that could be used for absorption refrigeration is available at different times to the demand for cooling.

In another project the University is testing a small-scale absorption refrigerator with no pump or moving parts. This may be suitable for domestic and light commercial buildings – using either hot water or solar power as the heat source².

Who knows, in the future we could see green absorption systems in the home.