Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 155

5.2.7 Summary of Desiccant Cooling Plant Performance
Two large desiccant cooling systems with solar air collector energy supply were
conceptually designed and monitored by the University of Applied Sciences in
Stuttgart. Both systems work well with average coefficients of performance around
1.0, when all operating modes are included. If only full desiccant operation is con-
sidered, COPs are between 0.5 and 0.6. A detailed measurement analysis in the
Althengstett system showed that COPs under full regeneration were around 0.37 if
the air temperature measurement was taken directly at the DEC machine entry rather
than on the roof, especially as the air channels are long and the entry air is precooled
in the building. This reduces the total system efficiency and care should be taken to
insulate the air channels well.
Using the full system simulation with validated component models, it can be shown
that the cooling energy demand could be covered with very little auxiliary energy
consumption. The main problem in real machine operation was that the regeneration
air temperature was set to a fixed value. As the deviation of room temperature from the
setpoint was often high, all components including full auxiliary heating were switched
on, although pure solar operation with lower regeneration temperatures would have
given the same supply air temperatures. Adjustable regeneration temperatures are
absolutely necessary to reduce auxiliary energy consumption. The solar thermal air
collectors have a relatively low energy yield during summer, although the efficiency
is good at near 50% during full regeneration mode. The reason for the low thermal
yield is that the number of regeneration hours is quite low (e.g. only 20% of the total
operating hours or 83 hours during July in Germany), from which the collector can
supply energy for 64% of the time. During the remaining hours solar irradiance is
too low for a significant collector contribution. Increasing the collector surface area
obviously increases the solar fraction during those days with high irradiance, but is
a very costly measure for just a few hours of regeneration mode. To achieve higher
solar fractions, the control strategy of the desiccant system must be adapted more to
the solar air collector system, allowing full regeneration operation, whenever solar
irradiance is available and using the building's heat and humidity storage capacity to
dry and cool it down more than required by static setpoints.
5.3 New Developments in Low-Power Chillers
Whereas thermal cooling technologies are available on the market in the medium-
and high-power range, there are hardly any systems below 10 kW of cooling power.
However, these small air-conditioning units dominate today's market, which is almost
exclusively covered by electrical compression systems. In order to supply this market
with sustainable technologies the development of cost-effective solar or waste-heat-
driven cooling systems is necessary.