Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 113

5.1 Absorption Cooling
Absorption cooling is a mature technology with the first machine being developed in
1859 by Ferdinand Carre. For the closed-cycle process, a binary working fluid that
consists of the refrigerant and an absorbent is necessary. Carre used as the work-
ing fluid ammonia/water (NH 3 /H 2 O). Today the working pair of lithium bromide
as the absorbent and water as the refrigerant is most commonly used for building
climatization (H 2 O/LiBr). In contrast to the ammonia/water system with its pressure
levels above ambient pressure, the water/lithium bromide absorption cooling machine
(ACM) works in a vacuum because of the low vapour pressure of the refrigerant water.
In 1945 the American company Carrier Corp. developed and introduced the first
large-scale commercial, single effect ACM using water/lithium bromide with a cool-
ing power of 523 kW. In 1964 the company Kawasaki Heavy Industry Co. from
Japan produced the first double effect water/lithium bromide ACM (Hartmann, 1992).
The double effect (DE) ACM is equipped with a second generator and condenser to
increase the overall COP by reusing the high-temperature input heat also for the lower
temperature generator.
Absorption chillers today are available in the range of 5 to 20 000 kW. In the last
few years some new developments have been made in the medium-scale cooling range
of 10 to 50 kW for water/lithium bromide and ammonia/water absorption chillers
(Storkenmaier et al ., 2003; Safarik and Weidner, 2004).
While absorption cooling has been common for decades, heat pump applications
have only become relevant in recent years, due to the improvement in recent years
in the performance figures; small gas-driven absorption heat pumps achieve COPs of
approximately 1.5, that is, by using 1 kWh of the primary energy of gas, 1.5 kWh of
heat can be produced using environmental energy, which is better than the condensing
boilers presently available on the market with maximum COPs of about 1.0. Differ-
ent manufacturers are producing absorption heat pumps of 10-40 kW output which
achieve COPs of about 1.3 at heating temperatures of 50 ◦ C. However, absorption
chillers today are mainly used as cooling machines rather than heat pumps.
5.1.1 Absorption Cycles
Absorption cooling machines are categorized either by the number of effects or by
the number of lifts: effects refer to the number of times high-temperature input heat is
used by the absorption machine. In general, increasing the number of effects is meant
to increase the COP using higher driving temperature levels. Lifts refer to the number
of generator/absorber pairs to increase successively the refrigerant concentration in
the solution and thus to reduce the required heat input temperature level.
The most important restrictions of single effect absorption cooling machines
(Figure 5.2) are the limitation of the temperature lifting through the solution field,