CPD Programme
IN
or water) for heat rejection, the greater the
Condensing water
Condenser
COP.
Condensing
• Enhanced absorption systems that use
water
Out
double and triple effect generators have
improved COPs of 1.2 and 1.7.
With regard to capital and running cost
Generator
comparison between absorption and vapour
Steam
compression, as a very general estimate,
figures for a 800 kW cooling plant showed
that absorption plant capital cost was 30
per cent higher than, but annual running
costs were 10-15 per cent lower than vapour
compression plant.
A potential selling point for absorption
Evaporator Out
chillers is that they do not use global warming
Chilled
fluids such as HCFC, or HFC refrigerant fluids
water
found in vapour compression systems. This is
Absorber
an important advantage of absorption units,
In
but it is clear that the environmental effects
of refrigerant leakage on ozone depletion and
global warming is minimal compared to the
effect on global warming of CO2 generation
from the energy production required to
operate the system. Absorption chillers are
also marketed as environmentally friendly
because their power input is not primarily
Heat
electricity, but a heat source. This would
exchanger
appear to produce lower CO2 emissions than
Solution pump Refrigerant pump vapour compression systems, but this will
Figure 3 Components of a two shell lithium bromide water chiller
depend on the energy source for generating
the electricity used in vapour compression
Operation and performance Tr(Ts – Ta)/Ts(Ta – Tr), where: systems. If the electricity generation for
In a typical absorption system producing Tr is the evaporator refrigerant vapour compression is from fossil fuel, then
chilled water, the evaporating temperature temperature; overall CO2 emissions may be lower from a
might be 5C, chilling water to about 7C, and a Ts is the generator temperature; gas powered absorption system. However,
condensing temperature of 40C. The absorber Ta is the absorber temperature. if greener electricity is produced, say from
will be at a temperature of about 25C. The From the above example the ideal COP is 2.8. hydropower plants, then vapour compression
solution is pumped to the generator, where Compare this with the Ideal Vapour systems will have lower CO2 emissions than
heat is supplied at a temperature ranging Compression COP, operating at the same gas fired absorption. The situation is fairly
from 80C to 140C, say 100C, driving the temperature difference, of 7.9. complex and each application would need to
refrigerant from the solution to the condenser. In practice a typical COP for an absorption be considered with all the relevant data.
The interesting feature of using water as the cycle in air conditioning would be about From an environmental position,
refrigerant is the low pressures within the 0.7, compared to about 3.5 for a vapour considering primary energy requirements
system. In the evaporator and absorber the compression system. It appears that only, today’s absorption systems can be
pressure will be 0.87kPa for an evaporating absorption systems require about five times effectively applied for use with integrated
temperature of 5C. In the condenser and more energy than vapour compression, but energy systems such as waste heat or
generator the pressure will be 7.38kPa of course, the energy for absorption is heat Combined Heat and Power(CHP).
corresponding to a condensing temperature energy, not work (electrical) energy. Heat © Terry Welch
of 40C. In other words the whole system energy is cheaper than electrical energy and in
operates well below atmospheric pressure. some applications this heat energy is free, or References
This means that any point of leakage in the is waste heat from another use, such as waste [1] Refrigeration & Air Conditioning, W F
components will result in air being drawn steam, hot water, gas, solar energy etc., which Stoecker/ J W Jones
into the absorption system, which will reduce makes it advantageous to use absorption. [2] DETR Good Practice Guide 256, An
cooling capacity or at worst stop the process Note the performance characteristics of introduction to absorption cooling, 1999
altogether. absorption systems: [3] ASHRAE Handbook, Fundamentals,
The Coefficient of Performance (COP) for • The higher the heat supply temperature to Chapter 1, 2005
an absorption system is defined as:- the generator, the greater the COP. [4] ASHRAE Handbook, Refrigeration,
COPc = Cooling Duty(kW)/Generator • The higher the refrigerant evaporating Chapter 41, 2006
Heating Duty(kW) temperature, the greater the COP. [5] Principles of Refrigeration, W B Gosney,
The ideal, theoretical Carnot COPc is: • The lower the ambient temperature (air Chapter 6, 1982
www.cibsejournal.com November 2009 CIBSE Journal 57
CIBSEnov09 pp55-58 cpd.indd 57 10/23/09 1:20:09 PM
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