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

Activated carbon is a form of carbon that has a large specific area available for adsorption approximately between 800 and 1500 m<sup>2</sup> /g for most used carbon. Initially, raw materials such 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 of ARSs.

#### **a.** Activated carbon-ammonia

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 pair are the toxicity and pungent smell of ammonia.

**b.** Activated carbon-methanol

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.

#### **2.2. Chemical adsorbents-adsorbate pair**

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 adsorption capacity, and they involve calcium chloride (CaCl<sup>2</sup> ), strontium chloride (SrCl<sup>2</sup> ), magnesium chloride (MgCl<sup>2</sup> ), barium chloride (BaCl<sup>2</sup> ), manganese chloride (MnCl<sup>2</sup> ) and cobalt chloride (CoCl<sup>2</sup> ), among others. For example, in CaCl<sup>2</sup> /ammonia pair, 1 mole calcium chloride can adsorb 8 moles ammonia [15].

Generally, chemical adsorbents have very large uptakes with specific adsorptions approaching 1 kg/kgads in some cases, and desorption temperatures varying from 40 to 80°C which are very promising. However, chemical adsorption systems stability is lower than that for physical adsorption systems due to agglomeration and swelling phenomena, which are common in chemical adsorbent beds. This instability reduces heat and mass transfer which limits the cooling capacities of chemical adsorbents. Consequently, heat-driven chillers utilizing these adsorbents have been less common than those using physical adsorbents. To overcome this problem, the porous heat transfer matrixes were put forward for the improvement of mass transfer as well as the heat transfer by using composite adsorbents.
