In the case of ammonia absorption cooling (Fig. 3.8) the operation relies on the fact that ammonia is soluble in cold water but not in hot water. When a solution of water and ammonia is heated, ammonia vapour is driven off at high pressure.
If this gas is then cooled in a condenser to ambient temperature it becomes a liquid. This pure liquid ammonia is brought into contact with a small amount of hydrogen gas in an evaporator. This causes the ammonia to revert again to a gas, absorbing heat from its surroundings in the process. This is the cooling phase of the process.
The ammonia-hydrogen mix is then brought into contact with water at the absorber at ambient temperature. At this temperature the ammonia is able to dissolve into the water, leaving the pure hydrogen to be returned to the evaporator for the process to begin again.
Condenser at 35°C
Condenser at 35°C
There is no mechanical support for the circulation process which is driven by a 'bubble pump'. This comprises a narrow vertical heated vessel where bubbles form, lifting the liquid in the process. The most common application of bubble pump technology is the espresso coffee machine. This pump is cheap and maintenance-free.
The technology has some inherent problems such as the fact that if the boiler does not reach a critical temperature it does not drive off all the ammonia as a vapour which, in turn, undermines the effectiveness of the purification of the hydrogen. As the evaporation temperature of the ammonia in the evaporator depends on the vapour pressure of the ammonia in the hydrogen, an inadequate boiler temperature would significantly reduce the cooling efficiency of the system.
The basic system requires the boiler which heats the ammonia-water mix to reach a temperature of 150°C. In solar energy terms this can only be achieved by solar concentrators. Researchers in the Technical University of Vienna redesigned the cycle, inserting an extra loop called a 'bypass' which makes it possible to extract much more ammonia from the ammonia-water solution at a relatively lower temperature than was previously possible. As a result, a boiler temperature of 75-80°C brings about the necessary evaporation of ammonia. This brings the technology within the range of normal evacuated tube solar collectors for providing the heat. This is the ideal heat source since it is at its most abundant during the summer when cooling is in greatest demand.
To provide space cooling a fan drives ambient air over the evaporator to be ducted from there throughout the building.
The heat source could also be spare heat from a CHP system driven by an internal combustion engine, a microturbine or a Stirling engine (see Fig. 3.8).
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