HVAC SYSTEMS - HFT Stuttgart

Page - 5 -
CHAPTER 01
the implementation of such improved control strategies and control algorithms
new system controllers need to be developed, which are able to control the
whole solar cooling system.
Purely solar driven absorption cooling systems often suffer from a delayed
system startup in the morning caused by the relatively high inertia of the
collector field and of the hot water storage tank. To overcome this problem
actual research work focuses on direct solar driven absorption chillers without
hot water storage (Kühn, et al., 2008, Safarik, 2008). These systems operate
very well on cloudless days and if the cooling load of the building is constantly
nearly equal or higher than the cooling capacity of the absorption chiller. Other
conditions lead to several shutdowns and startups of the chiller and in
consequence result in low energy efficiencies. These problems of direct solar
driven absorption chillers can be reduced if a hot water storage with storage
bypass can be implemented in the systems. However, intelligent storage charge
and discharge strategies are required to ensure an early system startup and a
stable operation of the absorption chiller on partly cloudy days.
1.2 SOLAR DRIVEN OPEN DESICCANT EVAPORATIVE
COOLING SYSTEMS (DEC)
Desiccant cooling systems are an interesting technology for sustainable building
climatisation, as the main required energy is low temperature heat, which can
be supplied by solar thermal energy or waste heat. Desiccant processes in
ventilation mode use fresh air only, which is dehumidified, precooled and
humidified to provide inlet air at temperature levels between 16 and 19°C.
Crucial for the process is an effective heat exchange between the dried fresh air
and the humidified exhaust air, as the outside air is dried at best in an
isenthalpic process and is warmed up by the heat of adsorption. For a rather
high heat exchanger efficiency of 85%, high humidification efficiencies of 95%
and a dehumidification efficiency of 80%, the inlet air can be cooled from design
condition of 32°C and 40% relative humidity to below 16°C, which is already
slightly below the comfort limit.