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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
Figure 3: Principle of the periodical absorption refrigeration process.
a: first period, cooling of A and B: evaporation of the refrigerant in A, absorption of the
refrigerant by an absorptive substance in D; b: second period, generation of refrigerant in A:
boiling of a mixture refrigerant and absorptive substance in D and condensation in A.
The adsorptive substance (silica gel, zeolite) adsorbs the liquid refrigerant water with a
physical bond onto its extremely large surface. It is worth mentioning that the surface of 1
gram of silica gel lies in the range of 300 to 400 m 2 . So in the case of adsorption, which is
normally an exothermal process, bonding energy has to be rejected out of the system.
2.2.3 Continuous (ab)sorption refrigeration processes
2.2.3.1 Components and definitions
These sorption processes use liquid absorbents and have one common characteristic
feature, the continuously working thermal compressor. It replaces the mechanical
compressor of a vapour compression cooling system. See the important components of the
thermal compressor in Figure 4.
- Generator G: gaseous refrigerant is generated by boiling the working fluid
- Absorber A: cool, gaseous refrigerant coming out of the evaporator is absorbed
- Working fluid heat exchanger (WFHE): heat recovery from the hot and weak to the
cold and strong working fluid
- Working fluid pump: in the case of ammonia/water, the pressure difference is in the
range of 8 to 11 bar
- Working fluid control valve (WFCV): controls the level of the working fluid in the
generator
The single stage, continuous absorption refrigeration machine needs, in addition to the
thermal compressor, a condenser C, an evaporator EV, a refrigeration control valve RCV and
a working fluid control valve WFCV. This process is the basic process of all systems in group
(b) of Figure 2.
Description of the thermal compressor: The working fluid, mainly ammonia and water (or
another working fluid pair like water/lithium bromide) is boiled in the generator by supplying
heat at a suitable temperature, e.g. at 70 …100°C. Mainly ammonia leaves the generator as
vapour and is condensed at the cool condenser heat exchanger, e.g. at 25 … 35°C. The
ammonia which leaves the generator leads to a decrease of the ammonia concentration in
the generator; therefore the boiling working fluid in the generator has to be renewed
continuously. This is managed by the working fluid pump, which delivers the strong working
fluid with a concentration of, e.g. 40% ammonia, from the absorber via the working fluid heat
exchanger into the generator. The working fluid heat exchanger heats up the strong WF from
the absorber temperature, e.g. 30 … 35°C, by heat recovery to about 65°C. So, the WF
enters the generator at 65°C and starts boiling after additional heating. After a certain time of
boiling, the concentration of the WF is decreased from, e.g. 40% to 35%, and is led back as
weak WF via the WFHE to the absorber. The weak WF leaves the generator with an outlet
temperature of about 80°C and enters the WFHE at the same temperature. The weak WF is
cooled down in the WFHE by the cold, strong WF, which comes from the absorber with a
temperature of about 35°C. The WFCV is controlled by the WF level in the generator and
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