Environmental Engineering Reference
In-Depth Information
5.3.2 Liquid Desiccant Systems
As buildings with low energy demand are often equipped with mechanical ventila-
tion systems, it is useful to consider air-based thermal cooling technologies, such as
open desiccant cooling systems (DCSs), for low-power applications. Small desiccant
rotors, heat exchangers and humidifiers are available on the market for volume flows
in the range of typical mechanical ventilation systems (around 300m 3 h 1 fresh air
supply for single family houses). Liquid sorption systems have also been patented for
such applications (Bachofen, 1999), but there is no system yet on the market and only
limited experimental results are available (Saman and Alizadeh, 2002). Both conven-
tional desiccant cooling units and liquid sorption systems dehumidify the outside air,
which is then precooled, humidified and injected into the rooms. The direct humidi-
fication of inlet air still causes concerns about hygiene, especially if maintenance is
not guaranteed, which is generally the case in residential buildings.
To avoid any hygienic problems for low-power cooling applications, a new system
configuration is proposed here, which shifts the whole air treatment to the exhaust
air side and uses only sensible cooling for the outside air stream. The room exhaust
air is dehumidified by a desiccant wheel or by liquid desiccant systems, precooled
in a heat exchanger using an additional humidified air stream and further cooled by
direct humidification. Finally the cooled return air is used to cool the supply air in an
efficient heat exchanger.
The disadvantage of the proposed system is the requirement of an additional heat
exchanger to transfer the generated cooling power and additional air volume flows
to provide the precooling after the sorption process. The additional heat exchanger
reduces the available cooling power and the additional volume flow increases the
pressure drop and fan power. In order to reach a cooling performance comparable
with the conventional system, highly efficient dehumidification combined with an
effective removal of the condensation/absorption enthalpy is therefore essential.
The driving force for mass transfer in the dehumidification process is the difference
in water vapour pressure between process air and liquid desiccant. The best perfor-
mance is achieved if the desiccant material maintains a low vapour pressure during the
whole absorption process. As the water vapour pressure of desiccants, here solid silica
gel or concentrated salt solutions, increases with temperature, it is essential to remove
the heat generated during the absorption process. Heat removal during the absorption
process can be achieved using finned cooling coils as the contact surface in liquid
desiccant systems (Lavemann et al ., 1993; Oberg and Goswami, 1998a; Park et al .,
1994a,1994b,1994c). However, an additional circuit for cooling water combined with
a cooling tower is necessary, which increases the costs of the system.
Considering these facts, a cross-flow type of plate heat andmass exchanger used as a
direct contactor for liquid desiccants seems to offer the best opportunities (Saman and
Alizadeh, 2001). The channels of the proposed heat and mass exchanger are sprayed
with liquid desiccants for dehumidification on the return air side and with water for
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