Solar-desiccant air-conditioning systems - Solar Energy Sciences and Engineering Applications - page 513

progressing well. As sorption processes are natural and occur on the surface of the
desiccant, the desiccant has another advantage: it can treat the biological and chemical
contents of air owing to direct contact of the air with the desiccant. Airborne micro-
organisms can be treated by desiccants (Goswami et al., 1997). Wang et al. (2011)
show that desiccants can make airborne fungi inactivate. Thus, serious problems in air
quality can also be resolved by the open-cycle desiccant-based air-conditioning system.
Nevertheless, it was shown by Goswami et al. (1997) that titanium dioxide (TiO 2 ) des-
iccant material can be used to control air micro-organisms through a photocatalytic
process.
16.2.3 Desiccant materials
The adsorption process is a surface phenomenon occurring at the interface of two
phases in which cohesive forces, including Van der Waals forces and hydrogen bond-
ing, act between the molecules of all substances irrespective of their state of aggregation
(Srivastava and Eames, 1998). This process is called physisorption. Absorption is a
chemical process caused by valency forces called chemisorption (Low, 1960). The pro-
cess of attracting moisture from the air is done either by adsorption or by absorption:
the adsorption process is a physical process in which the property of the desiccant mate-
rial remains the same; while in the absorption process, upon attracting moisture, the
physical characteristic of the material changes. The desiccant materials can be either
solid or liquid: the solid desiccant and hydrophilic desiccants are silica gel, activated
alumina, and zeolites, while calcium chloride is an absorbent desiccant. Commercial
hydrophobic solid desiccants are activated carbons, metal oxides, specially developed
porous metal hydrides and composite adsorbents (Srivastava and Eames, 1998).
Some desiccant materials combine absorbent and adsorbent desiccants to form
composites which enhance their physical properties and sorption capacity (Tokarev
et al., 2002). The basic mechanism in the sorption of moisture between air moisture
and the desiccant material is the difference in the water vapour pressure on the sur-
face of the desiccant and of the material. The uptake of moisture from the air to the
desiccant occurs when vapour pressure in the air is high; the removal of vapour from
the desiccant material is done when the vapour pressure in the air is lower than on the
desiccant material. When the vapour pressure is the same both in the air and on the
desiccant material, an equilibrium is reached and the sorption process stops. The only
means to make the sorption process proceed is to use outside forces such as increasing
the air pressure, decreasing the temperature or by artificial electromotive force (Low,
1960). The same procedure, but in reverse, is applied for the removal of moisture from
the desiccant material.
The most common absorbents are lithium bromide, lithium chloride, calcium chlo-
ride and triethelene glycol. Other possible candidates as absorbents are salt-based
solutions or related materials which attract water molecules. Examples of alternative
absorbents are potassium chloride and sodium chloride. Other candidates are a mix-
ture of the commonly used absorbents mentioned above. Table 16.2.1 summarizes
the common absorbents and their properties and compares them for their thermo-
chemical, environmental, human toxicity and cost properties. Lithium chloride (LiCl)
has a low vapour pressure at a given temperature but the material cost is high (Mei
and Dai, 2008); Figure 16.2.3 shows LiCl in the psychometric chart. According to