Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 156

The author's research team has therefore developed a thermally driven diffusion-
absorption cooling machine (DACM) with ammonia/water and pressure-compensated
auxiliary gas. For a residential building in Germany the annual primary energy-saving
potential by using a solar thermal supported absorption cooling/heat pump system is
over 50% compared with an electrical heat pump/chiller, at estimated operating hours
of 700 h a − 1 and 1800 h a − 1 for cooling and heating operation, respectively.
Also, a new desiccant cooling cycle to be integrated in residential mechanical ven-
tilation systems has been developed by the author. The process shifts the air treatment
completely to the return air side, so that the supply air can be sensibly cooled by
a heat exchanger. Purely sensible cooling is an essential requirement for residential
buildings with no maintenance guarantee for supply air humidifiers. As the cooling
power is generated on the exhaust air side, the dehumidification process needs to be
highly efficient to provide low supply air temperatures.
5.3.1 Development of a Diffusion-Absorption Chiller
In 1899 H. Geppert was the first to propose the introduction of an auxiliary inert
gas (non-condensable gas) into the Carre absorption cooling cycle resulting in a third
cycle, the so-called auxiliary gas cycle. By heat input to the generator, the gas is first
displaced out of the generator and then out of the condenser. It collects inside the
evaporator and the absorber, where its pressure compensates for the refrigerant partial
pressure difference between the generator and absorber and between the condenser
and evaporator. This special kind of absorption cooling machine is called a diffusion-
absorption coolingmachine (DACM). The usual throttle for the pressure compensation
is removed and the solution pump can be a simple bubble pump because the whole
unit now has the same total pressure p total . Geppert suggested using air as the inert
gas, but the COP that he reached was too low for commercial use. This was due to two
reasons; the first one is that the diffusion of ammonia into air is very slow so that only
a very small cooling capacity can be achieved. The second and most important reason
is that the gas mixture of air and ammonia is less dense than air alone and because
of this it was not possible to get a thermosyphon circulation of the gas mixture. For
these reasons, Geppert had to use a ventilator for the circulation of the gas cycle.
In 1922 the two Swedish engineers von Platen and Munters developed the idea
of a DACM with pressure-compensated auxiliary gas and no mechanically moving
parts independently of Geppert's proposal. They suggested hydrogen as the auxiliary
gas and at first they also obtained low COPs. Hydrogen alone is lighter than the gas
mixture of ammonia and hydrogen, and with this mixture a thermosyphon circulation
is possible. With the use of a gas heat exchanger between the evaporator and absorber,
von Platen and Munters were able to reduce the cooling losses in the auxiliary gas
cycle and thus reached higher COP values. Consequently, this diffusion-absorption
cooling technology became interesting for commercial use for example, for absorption
refrigerators. Later, helium was also used as the auxiliary gas.