**2.3. Composite adsorbents-adsorbate pair**

presented experimentally and theoretically to investigate and improve the performance of

Activated carbon is a form of carbon that has a large specific area available for adsorption

as coal, lignite, wood, nut shells and synthetic polymers undergo number of special pyrolysis or chemical treatment at high temperatures (700–800°C) to produce activated carbons. They can be produced in many forms including powders, microporous, granulated, molecular sieves and carbon fibers. Activated carbon has advantages of that: a relatively low adsorption heat among the other types of physical adsorbent pairs (1800–2000 kJ/kg), low adsorption heat is beneficial to the system's COP because the majority of heat consumption in the regeneration phase is the adsorption heat [12], higher surface reactivity, suitable pore size [13] and large surface area. However, the thermal conductivity of activated carbon is poor and is near to the insulation material. For example, ACF-methanol system with a higher specific adsorption reaches up to 0.55 kg/kgads, and good mass transfer characteristics where void fraction of ACF layer is more than 0.90%, but the measured thermal conductivity is as low as 0.0893 W/(mK) [14]. The carbon physical characteristics could be optimized to obtain the best performance

While most of adsorbent-adsorbate pairs operate under high vacuum, an activated carbonammonia pair system has a high working pressure (about 1600 kPa when the condensing temperature is 40°C). So, permeability of sorbent is not critical and it can be easier and more applicable than sub-atmospheric systems. It is also more suitable than the activated carbon/methanol pair for heat sources of 200°C or higher. The drawbacks of this working

Large adsorption capacity of activated carbon-methanol pair has adsorption capacity of about 0.45 kg/kgads. Low regeneration temperature can be used to drive ARS employing activated carbon-methanol pair (about 100°C). On the other hand, it should not be used with regeneration temperature higher than 120°C, where activated carbon will catalyze methanol to decompose into dimethyl ether at a temperature more than 150°C, and operating pressure of the system will be sub-atmospheric and that requires assistant vacuum system.

Chemical adsorbents sorb the refrigerants differently than physical adsorbents where the strong chemical bond between the adsorbent and the refrigerant takes place in chemical adsorption. The uptake in the chemical adsorbents is not limited by the surface area of the material, which generally leads to higher mass transfer kinetics when compared to physical adsorbents. The metal chlorides are commonly used as chemical adsorbents due to their high

/g for most used carbon. Initially, raw materials such

zeolite-water adsorption system particularly for vehicle air conditioning.

*2.1.3. Activated carbon (ammonia/methanol) systems*

approximately between 800 and 1500 m<sup>2</sup>

78 Sustainable Air Conditioning Systems

of ARSs.

**a.** Activated carbon-ammonia

**b.** Activated carbon-methanol

**2.2. Chemical adsorbents-adsorbate pair**

pair are the toxicity and pungent smell of ammonia.

Composite adsorbents, also called "Salt in Porous Matrix (CSPM)" represent the promising solution of aforementioned drawbacks associated with pure physical and chemical adsorbents. Thus, many of these composites, which are typically made of porous media and chemical adsorbents, have been developed synthetically to be applied in adsorption refrigeration systems as in Refs. [16–18]. In such composites, porous media work on improving the heat and mass transfer properties of the chemical adsorbents along with limiting the swelling characteristics of the chemical adsorbents, while the chemical adsorbents increase the refrigerant uptake of the adsorbent pair. The common examples of these composites are combinations of metal chlorides and AC, ACF, expanded graphite, silica gel or zeolite. For example, silica gel and chlorides/water which are known as selective water sorbents (SWSs) which are tested and studied by Aristov et al. [6]. Composite adsorbents of silica gel and chloride are usually produced using the impregnation method. The silica gel is immersed in a chloride salt solution and is then dried to remove the water. There are also four types of porous media were used with chlorides to produce composite adsorbents/ammonia: activated carbon, activated carbon fiber, expanded graphite or vermiculite.
