Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 127

exhaust air is brought to the necessary regeneration temperature (8), takes up the water
adsorbed on the supply air side in the sorption wheel, and is expelled as warm, humid
exhaust air (9). If room air is recirculated, the desiccant wheel is used to dry the room
exhaust air, which is then precooled using the rotating heat exchanger and humidified
to provide the cooling effect. Regeneration of the desiccant wheel and precooling of
the dried recirculation air is done by ambient air, which is first humidified, then passes
the rotating heat exchanger, is heated to the necessary regeneration temperature and
finally used to regenerate the desiccant wheel.
The concept of desiccant cooling was developed in the 1930s and early attempts to
commercialize the system were unsuccessful. Pennington patented the first desiccant
cooling cycle (Pennington, 1955), which was then improved by Munters in the 1960s
(Munters, 1960). Good technology overviews are given by Mei et al ., (1992) Lavan
et al ., (1982) or Davanagere et al ., (1999). The most widely used desiccants are silica
gel, lithium chloride or molecular sieves, for example zeolites. Solid desiccants such
as silica gel adsorb water in their highly porous structure. Lithium chloride solution is
used to impregnate for example a cellulose matrix or simpler cloth-based constructions
and can then be used to absorb water vapour from the air stream (Hamed et al .,
2005).
The thermal COP is defined by the cold produced divided by the regeneration
heat required. For the hygienically needed fresh air supply the enthalpy difference
between ambient air and room supply air can be considered as useful cooling energy.
If the building has higher cooling loads than can be covered by the required fresh air
supply, then the useful cooling energy has to be calculated from the enthalpy difference
between room exhaust and supply air, which is mostly lower. The thermal COP is
obtained from the ratio of enthalpy differences (state points are given in brackets):

q cool
q heat =

h amb (1) − h supply (4)

COP thermal = (5.1)

h waste (9) − h reg (8)

Related to ambient air, COPs can be near to 1.0 if regeneration temperatures are kept
low, and reduce to 0.5 if the ambient air has to be significantly dehumidified. COPs
obtained from room exhaust to supply air are lower, between 0.35 and 0.55 (Eicker,
2003). The maximum COP of any heat-driven cooling cycle is given for a process in
which the heat is transferred to a Carnot engine and the work output from the Carnot
engine is supplied to a Carnot refrigerator. For driving temperatures of 70 ◦ C, ambient
air temperatures of 32 ◦ C and room temperatures of 26 ◦ C, the Carnot COP is 5.5:

1

T ambient
T heat

T room
T ambient − T room

COP Carnot = − (5.2)

However, as the desiccant cycle is an open cycle with mass transfer of air and
water, several authors have suggested the use of a reversible COP as the upper limit