Environmental Engineering Reference
In-Depth Information
COP of above 5. It also shows that the coefficient of performance increases as the air
humidity increases. This means that the system is suitable in humid climates where air-
conditioning is important. The system is also more compact than other solid desiccant
air-conditioning systems.
16.4 LIQUID-BASED SYSTEM
16.4.1 Basic concept
The design of the liquid desiccant air-conditioning system uses the falling film type
in the membrane, with air passing to its surface (Ren et al., 2007). Some designs
apply the spray type to increase the surface area of air-desiccant contact. The design
of the air dehumidifier uses an isothermal process which passes cool air/water at the
back of the falling desiccant film (Yin et al., 2008). Since the regeneration of the
desiccant material is by means of heat, many designs of liquid desiccant regenerators
are done with solar energy. The cooling of air after the desiccant material is performed
in the same way as in solid-desiccant air-conditioning systems. The liquid-based system
utilizes liquid desiccant materials in removing air moisture content. The most widely
used liquid desiccant materials are lithium chloride, lithium bromide, calcium chloride
and glycol-based substances (Yin et al., 2009). The application of these materials
depends on cost, type of operations and the source of thermal energy. In addition,
some liquid desiccants are corrosive and require proper handling in their application.
However, the main advantage of liquid desiccant is its high moisture removal capacity
with lower regeneration temperature requirement.
Liquid desiccant air-conditioning systems rely on the liquid desiccant to control
air moisture content, which is achieved by means of an absorption process. One of
the main advantages of liquid desiccant air-conditioning systems is the lower regener-
ation temperature requirement along with higher thermal and chemical storage. The
advantage of the hybrid liquid desiccant air-conditioning system is the complete oper-
ation of the system using electric energy at higher performance. This means, for small
applications, that the hybrid liquid desiccant air-conditioning system will prevail over
the pure liquid desiccant air-conditioning system.
16.4.2 Typical systems
Dehumidifiers or regenerators based on spray, wetted wall (falling film) and packed bed
tower are the typical arrangements (Jain and Bansal, 2007). The wetted wall system
uses a falling film absorbent in the plate while the air is in contact with the absorbent.
This concept is practical for applications which do not need complex air dehumidi-
fiers, such as for low thermal capacity buildings. Mesquita et al. (2006) developed
the numerical models for simultaneous heat and mass transfer in parallel-plate dehu-
midifiers. Here, two polypropylene twin-wall plates form the channel, inside which
cooling water flows in a cross-flow configuration with respect to the absorbent and
air streams. Water mass flow rate is maintained high enough to keep the plates' walls
essentially isothermal, with water temperature gains throughout the plate of less than
0.4 C. The constant thickness and simplified model under-predict the dehumidifica-
tion, especially for low absorbent flow rates. The numerical model can be adapted
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