There is growing
interest in a method of cooling buildings which uses gas as a fuel instead of
electricity. The technology is known as absorption cooling. The biggest differences
between this and vapour compression cooling are that the compressor is replaced
by a gas fired generator and the refrigerant is replaced by a refrigerant/ absorber
mixture. A diagram of an absorption chiller is shown in below figure. The generator
is filled with a mixture of refrigerant and absorber (solvent) which can be
either water/lithium bromide (>35kW capacity) or ammonia/water. (>11kW).
Note because water freezes the lithium bromide/water units can only cool down
to 5oC, ammonia/water units on the other hand can cool down to -10oC. The way
the system works can be illustrated using the ammonia/water pairing as an
example. In this case water is the absorber and ammonia is the refrigerant. The
water is called the absorber, giving the process its name, as it is so
chemically attracted to ammonia vapour that it absorbs it out of the
atmosphere.
A
concentrated solution of ammonia in water is heated in the generator (figure
5.4) using a gas burner. The ammonia component vaporizes first, as it has a
lower boiling point than water, and passes into the condenser. The water which
is left behind passes back to the absorber. The ammonia vapour condenses back to
liquid ammonia in the condenser giving out waste heat. This heat is removed
from the system by air which is blown over the condenser by a fan. The ammonia
now passes from the condenser into the evaporator via an expansion valve. In
doing so its pressure drops and so it can evaporate once more. It does this by
absorbing heat from the chilled water circuit. Chilling has therefore, been
achieved.
The
ammonia vapour now passes into the absorber where it is absorbed by the water
from the generator to create a concentrated ammonia solution. Heat is given out
when the two chemicals combine. This waste heat is also removed by the
condenser cooling air flow. The ammonia solution is pumped back to the generator
where the cycle continues once more.
The above
device is known as a single effect absorption chiller. Double effect units are
also available which use water and lithium bromide. This solution is pre heated
on its way back to the generator by passing it through a heat exchanger. This
improves the efficiency of the unit. Double effect units require a higher
temperature heat source (>140oC) derived from a direct gas fired burner or
pressurised hot water.
Absorption
chillers are less efficient than vapour compression chillers with a COP of
approximately 0.7-1.2. It follows that more gas energy will be required than an
equivalent electric chiller (COP = 3.0). However the cost and pollution
differentials will be reduced because electricity costs and pollutes
approximately four times more per unit of energy than gas because of wastage in
the power stations. Contract gas prices are lower still in summer when gas is
needed for cooling as less is needed for space heating.
As well
as direct gas firing some absorption chillers can be operated using waste heat.
One form of surplus heat is that generated by combined heat and power units. In
winter their heat output is used for space heating. In summer this heat is
surplus to requirements and so can be used to drive the absorption chiller.
This is known as trigeneration or combined cooling and power. When heat, which
would normally be wasted, is used absorption chillers emit much less CO2 into
the atmosphere than a vapour compression chiller for a given cooling effect. Research
is currently underway which is investigating the linking of absorption chillers
with solar panels as a source of generator heat. It is an advantage that the appearance
of large amounts of cost and pollution free solar energy coincides with the
need for cooling.
Other Benefits
The only
electrical elements in an absorption chiller are the pumps used to move the
ammonia/water solution from the absorber back to the generator. These pumps consume
much less power and produce less noise and vibration than a compressor. This
latter point is useful if the chiller is to be sited near to a noise sensitive
area.
The pump
along with the air cooled condenser fan and the gas burner fan are the only
moving parts. The rest of the device consists of sealed metal chambers. This configuration
means that maintenance costs are low.
External,
air cooled, modular packaged units mean that cooling capacity can be easily
expanded as the building is developed or as heat loads increase. Flexibility is
further enhanced as units are also available that provide heating in winter and
switch to cooling in summer.
Selection
Criteria
From the
above it can be seen that absorption chilling is particularly appropriate
where:
· You
have excess heat production from your CHP plant in summer or a production
process which can be used to drive the absorption chiller
· The
electrical supply to the site is not robust enough to supply the necessary
electricity required for vapour compression chilling and an expensive upgrade
would be necessary.
· You
wish to optimize the use of clean gas as a fuel throughout the year, not just
in winter.
· You
have a source of low cost or free heat energy available such as solar energy or
heat released from the combustion of landfill gas.
