IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
5 Solid desiccant cooling systems
Paul Bourdoukan, Etienne Wurtz, (LOCIE-INES/CNRS)
5.1 Desiccant cooling principle
Using solar energy appears to be an interesting option for cooling applications, as the
building load almost matches the availability of the source. In a solar desiccant cooling cycle,
solar energy is used to regenerate a desiccant that dehumidifies moist air; the resulting dry
air is cooled in a sensible heat regenerator and then in an evaporative cooler. By associating
different elementary changes in moist air (dehumidification, sensible cooling and evaporative
cooling) the technique uses water as a refrigerant and solar energy as a driving heat; while
electricity is only used in the auxiliaries, so the technique is environmentally friendly. It is a
thermally driven open cooling cycle based on evaporative cooling and adsorption.
With reference to figure 31, a desiccant cycle operates as follows: outside air (1) is
dehumidified in a desiccant wheel (2); it is then cooled in the sensible heat regenerator (3) by
the return cooled air before being further cooled in an evaporative process (4), finally, it is
introduced into the building. The operating sequence for the return air (5) is as follows: it is
cooled to its saturation temperature by evaporative cooling (6) and then it cools the fresh air
in the rotary heat exchanger (7). It is then heated in the regeneration heat exchanger by solar
energy (8) and finally regenerates the desiccant wheel (9) by removing the humidity before
exiting the installation.
The desiccant wheel is used to remove moisture from the outside air. Its function is first, to
reduce the humidity of the outside air in order to match indoor air standards and second, to
provide extra dehumidification to increase the cooling potential of the supply humidifier.
Depending on load conditions this system can operate under different modes. Simple
ventilation (fans only), indirect evaporative cooling for low cooling loads (fans, return
evaporator cooler and the sensible heat regenerator) and desiccant mode (all components
operational).
For moderate summer conditions, in the morning when the outside temperature is low, the
indirect evaporative cooling mode is able to keep the room in a comfortable range; there is
thus no need for regeneration and the solar heated water can be stored in the tank. During
the day with the outside temperature rising and increasing solar gains the indirect
evaporative cooling cannot provide the cooling and so the desiccant mode is then required,
while solar energy is needed for the regeneration of the desiccant wheel. The minimum
temperature required for regeneration depends on the nature of the desiccant material. It
varies from 50°C for lithium chloride to 60°C for silica gel. We chose silica gel for its higher
dehumidification performances in spite of the higher temperature needed for its regeneration.
The desiccant cooling process is well-suited to the requirements of non-residential buildings
with high occupancy needing high air exchange rates, e.g. seminar rooms and banks. In
these buildings the rooms are usually occupied during the day, so air-conditioning loads
match solar energy availability; so coupling desiccant cooling with solar collectors would
seem a very interesting option.
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