How an absorption refrigeration system works

Most often, cooling is created with the aid of compression refrigeration systems, which use an electrically powered compressor to increase the pressure of the gaseous coolant.

In place of this, absorption refrigeration systems use a so-called thermal compressor. By contrast with the traditional compressor, the thermal compressor is powered by heat rather than electricity. The thermal compressor works on the basis of a mixed substance comprising coolant and a sorption medium. The sorption medium is able to absorb the coolant, hence the term 'sorption medium'.

At the start of the thermal compressor the coolant-sorption mixture is in the form of a fluid, sometimes referred to as a 'rich mix', because a comparatively large volume of coolant is dissolved in it. It is possible to increase the compression level of the fluid with an extremely small compression force from an electric pump, which means that the absorption refrigeration units do consume a small amount of electricity.

Because they boil at different temperatures, it is possible to separate the coolant from the sorption fluid by means of heating. This is done in the generator, otherwise known as the expulsion unit, through the introduction of external heat, for example the excess heat from the CHP unit. The boiled off coolant vapour is channelled to the condenser (liquefier), where it turns back into a liquid whilst emitting heat to the surrounding environment. The coolant is de-pressurised to the lower pressure level with the aid of a throttle and channelled to the evaporator. Due to the low pressure of the gas, the residual heat in the cooling circuit return flow piping is sufficient to evaporate the coolant. This removes heat from the cooling circuit which can then provide refrigeration. In a subsequent step, in the absorber the gaseous coolant is brought into contact with the low pressure and at this point coolant-depleted solution from the generator, whereby the coolant is absorbed into the solution and can then be pumped back to the generator.

The solution saturation difference between the coolant rich and coolant depleted solution is known as the degassing spectrum, and can be regarded as an efficiency measure for the thermal compressor, whereby the objective is to maximise the degassing spectrum. The back-cooling temperature is determined by the ambient temperature of the surrounding environment, and the target refrigeration temperature is dependent on the process conditions. The result of this is that the temperature of the heat source which is used to expel the coolant in the generator needs to be raised in order to increase the degassing spectrum. The greater the degassing spectrum, the greater the extent to which the heating medium can be refrigerated, thus enable greater transfer capacities.

If one only takes the capacities into account, one can use the thermal ratio, which describes the refrigeration capacity in relation to the thermal capacity used at the applicable system temperatures.

Additional internal heat exchangers are used to improve the processes inside the absorption refrigeration unit, by means of which it is possible to further increase the efficiency of the system.

Two systems have become established in the market, each of which uses a different media mixture: water with lithium bromide, and ammonia with water. The first is used when cooling temperatures above 0 °C are required. If deep freezing is required, then ammonia has to be used as an alternative coolant. Units of this type are considerably more complex and require rectification systems in order to produce a highly concentrated coolant vapour.

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