Environmental Engineering Reference
In-Depth Information
of the desiccant cycle, which is calculated from Carnot temperatures given by the
enthalpy differences divided by entropy differences (Lavan et al ., 1982). This allows
the calculation of equivalent temperatures for the heat source (before (7) and after the
regenerator heater (8)), evaporator (room exhaust (6) minus supply (4)) and condenser
(waste air (9) minus ambient(1))
m in h in
m out h out
T equivalent =
(5.3)
m in s in
m out s out
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 COPs of 2.6 and 3.0 were calculated for the ventilation and
recirculation modes (Kanoglus et al ., 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.
Crucial for the process is an effective heat exchange between the dried fresh air
(state 2) and the humidified exhaust air (state 6), 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 conditions
of 32 C and 40% relative humidity to below 16 C, which is already slightly below
the comfort limit (see Figure 5.12).
Simple models have been used to estimate the working range of desiccant cooling
systems, for example to provide room conditions not just for one setpoint, 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
air heater
T = 56.6 °C
= 18%
x = 19.4 g kg -1
T = 70°C
= 7%
x = 14 g kg -1
T = 40.4 °C
= 29.5%
x = 14 g kg -1
T = 20 °C
= 95%
x = 14 g kg -1
T = 26 °C
= 55%
x = 11.6 g kg -1
ϕ
ϕ
ϕ
ϕ
ϕ
waste air
exhaust air
sorption
wheel
heat
exchanger
humidifier
outside air
inlet air
T = 32 °C
= 40%
x =12 g kg -1
T = 45.5 °C
= 10.7%
x = 6.6 g kg -1
T = 24 °C
x = 6.6 g kg -1
T = 15.4 °C
x = 10.4 g kg -1
ϕ
ϕ
ϕ
= 32%
ϕ
= 95%
Figure 5.12 Temperature and humidity levels in a high-performance desiccant cooling process
 
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