**2. Working adsorbent/refrigerant pairs**

The overall performance as well as the design and operating parameters for an ARS are greatly affected by the employed working adsorbent/refrigerant pairs. In general, good adsorbents should have wider range of adsorption capacity with temperature variation, higher heat and mass transfer properties, along with thermal stability and low susceptibility to contamination. In addition, distinctive properties of a refrigerant should be examined, and that include heat of vaporization, thermal conductivity, boiling point and working pressures, reactivity and stability, toxicity, environmental impact and freezing point. The adsorption capacity of an adsorbent-refrigerant pair is commonly determined from plots known as adsorption isotherms as shown in **Figure 4** [5]. These isotherms give the amount of adsorbed mass taken up by the adsorbent, after reaching the thermodynamic equilibrium, as a function of pressure at constant temperatures. Accordingly, adsorbent-adsorbate pairs and their developments can be compared based on their isotherms. However, when the adsorbent domain undergoes transient operating conditions, a kinetic model is required to define the mass transfer kinetics and gives the instantaneous amount of adsorbate through a relation with the equilibrium uptake that is given by the isotherms. Mass transfer kinetics is a catch-all term related to intraparticle mass transfer resistance. The increase in the adsorption capacity increases capability of an ARS to have a large cooling capacity, where it sets up the total amount of refrigerant that can be adsorbed in a cycle. However, faster mass transfer kinetics is required to insure higher cooling capacity as it controls the duration of the adsorption cycle.

**2.1. Physical adsorbent-adsorbate pair**

Silica gel is an amorphous silicon dioxide, SiO<sup>2</sup>

applications:

*2.1.1. Silica gel-water systems*

specific surface area is 400–700 m<sup>2</sup>

carbon (100–1000 m2

large chilled water flow rates.

*2.1.2. Zeolite-water systems*

The physical adsorbents that are used in ARSs rely on van der Waal's forces to contain adsorbate. Three adsorbent-adsorbate pairs are generally considered to be the best available in

has a granular, vitreous and highly porous form. The high-density silica is the common type of silica gel used in adsorption systems such as Fuji Davison types 'A' and 'RD' silica gel,

sizes can be used as a host material in composite adsorbents. The thermodynamics characteristics of silica gel-water working pair were investigated experimentally by several researchers as in [5, 7], and the empirically determined parameters for the isotherm equations had been calculated from the experimental data. The performance of the two-bed silica gel-water was

In general, the main advantage of silica gel over other adsorbents is that the regeneration temperature is typically 85°C which makes such system to be suitable for solar energy use and low temperature waste heat sources. Moreover, it could be as low as 50°C when multi-stage configuration system is applied [11]. In such case, for non-regenerative cycle, the dynamic losses due to the heat capacities of the adsorber components will be reduced which lead to higher COPs since the adsorbent itself and the container vessel do not need to be heated to high temperatures. However, desorption temperature must not be too high. If it is higher than 120°C, silica gel will be destroyed. The adsorption heat is relatively higher than activated carbon pair between 2500–2800 kJ/kg. Also, silica gel porosity level is lower than activated

0.35 and 0.4 kg/kg silica gel, while the net change in the instantaneous amount of adsorbate may not exceed 0.1 kg water/kg silica gel under typical operating conditions which is low. Another drawback is the limitation of evaporating temperature due to the freezing point of water and the uptake also is effected badly under a very low vacuum, that make silica gelwater refrigeration system be better to be applied in the air conditioning applications with

Zeolites are microporous, alumina silicate crystals composed of alkali or alkali soil. The zeolite-water working pair has a wide range of desorption temperature (70–250°C). Due to its stable performance at high temperatures, the adsorber can be directly heated by the exhaust gases from engines. Therefore, the zeolite-water system is simpler than that one driven by the hot water. However, the adsorption heat of zeolite-water is higher than that of silica gelwater, between about 3300 and 4200 kJ/kg [12], which will lead to low COPs, in addition to the drawbacks associated with using the water as a refrigerant. Several studies had been

/g). The maximum adsorption capacity at equilibrium could be between

which have pore diameter in the range of 2.0–3.5 nm, the pore volume is 0.3–0.4 cm3

evaluated experimentally and analytically by several researchers [8–10].

, made synthetically from sodium silicate, and

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167

/g [6]. The other types of silica gel with relatively high pore

/g and the

77

The most commonly used adsorbent/refrigerant pairs are silica gel/water, zeolite/water, activated carbon/methanol, activated carbon/ammonia, calcium chloride/ammonia and composite adsorbent/ammonia. In general, according to the nature of the forces involved in the adsorption process, they are classified into three categories such as physical, chemical and composite adsorbent/adsorbate pair.

**Figure 4.** Isotherms for type RD silica gel-water pair [5].
