HVAC SYSTEMS - HFT Stuttgart

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CHAPTER 01
lower. The thermal COP is obtained from the enthalpy ratio. Related to ambient
air, the thermal COP can be near to 1.0, if regeneration temperatures are kept
low, and reduces to 0.5, if the ambient air has to be significantly dehumidified.
Thermal COP’s obtained from room exhaust to supply air are lower between
0.35 and 0.55 (Eicker, 2003).
Under conditions of 35°C ambient air temperature and 40% relative humidity,
defined by the American Air conditioning and Refrigeration Institute (so called
ARI conditions), reversible thermal COP´s of 2.6 and 3.0 were calculated for
ventilation and recirculation mode (Kanoglu, 2007). The real process, however,
has highly irreversible features such as adiabatic humidification. Furthermore,
the specific heat capacity of the desiccant rotor increases the heat input
required.
Simple models have been used to estimate the working range of desiccant
cooling systems, for example to provide room conditions not just for one set
point, but for a range of acceptable comfort conditions (Panaras, et al., 2007).
The performance of the desiccant rotor itself can be evaluated by complex heat
and mass transfer models based on Navier Stokes equations (Maclaine-Cross,
1988; Gao, et al., 2005). This allows the evaluation of the influence of flow
channel geometry, sorption material thickness, heat capacity, rotational speed,
fluid velocity etc. They are mostly too time consuming to be used in full system
simulations including solar thermal collectors, where mostly simpler models are
available, based either on empirical fits to measured data (Beccali, et al., 2003)
or on models of dehumidification efficiency.
Different control strategies have been compared by Ginestet, et al. (2003) to
study the influence of air volume flow and regeneration temperature. As the
increase of regeneration temperature does not linearly lower the supply air
temperature, the study concluded that increased air flow rates are preferable to
increased thermal input by auxiliary heaters, if the cooling demand is high.