## 3 Working Principles and _Cop_ Limit of Adsorption Systems  
Adsorption systems employ sorption phenomena between the solid surfaces and gas molecules [12; 13; 14; 15]. Unsaturated solid surfaces can attract gas molecules releasing some energy when these molecules are trapped by van der Waals forces. Being weak binding forces, the molecules can easily be released by supplying energy in the form of a low-temperature heat source. Thus, the desorption process is an endothermic process while the adsorption process is exothermic. When employed in a cycle, these adsorption and desorption processes can be applied to perform heat pumping. Adsorption process utilises the chemical potential stored in the adsorbent and can trigger latent heat removal from the evaporator due to the vapour migration [16]. On the other hand, desorption process releases previously-adsorbed gas molecules to the condenser while restoring the chemical potential. From the thermodynamic point of view, adsorption systems can be considered a \begin{table}
\begin{tabular}{c l l l} \hline \hline
**Parameter** & **Heat Recovery from the Engine Coolant** & \multicolumn{2}{c}{**Heat Recovery from the Exhaust**} \\ \hline Temperature level & \textless{}90 \({}^{\circ}\)C & From 400 \({}^{\circ}\)C to as high as 800 \({}^{\circ}\)C [10; 11] \\ \hline \multirow{6}{*}{Advantages} & \(\bullet\) & Temperature level suitable for the & \multirow{2}{*}{\(\succ\)} & High-quality energy \\  & & regeneration temperature of the & \multirow{2}{*}{\(\succ\)} & Design can be compact \\  & & low-temperature adsorption systems & & Reduced or no capacity limitation by \\  & \(\bullet\) & Design simplicity & & the heat source \\  & \(\bullet\) & No back pressure effect to the & & The possibility of both direct and \\  & \(\bullet\) & combustion process & \multirow{2}{*}{\(\succ\)} & Direct recovery, i.e., the direct heating & \\  & & of the adsorbent is possible & & indirect recoveries \\ \hline \multirow{6}{*}{Disadvantages} & \(\bullet\) & Capacity limitation due to sensible & \multirow{2}{*}{\(\succ\)} & \multirow{2}{*}{\(\succ\)} & \multirow{2}{*}{\(\succ\)} \\  & & heat recovery only & & \\ \cline{1-1}  & \(\bullet\) & Higher risk of engine overheating & & \\ \cline{1-1}  & \(\bullet\) & Possible temperature fluctuation due & & \\ \cline{1-1}  & & to the cyclic nature of the & & \\ \cline{1-1}  & & adsorption system & & \\ \cline{1-1}  & \(\bullet\) & Possible coolant flows mismatch & & \\ \cline{1-1}  & & between the engine and the & & \\ \cline{1-1}  & & adsorption cycle if the direct heat & & \\ \cline{1-1}  & & recovery is applied. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Temperature level and the advantages and disadvantages of heat recovery methods from internal combustion (IC) engine of an automobile. thermodynamic cycle that consists of a heat engine and a heat pump. The heat engine cycle operates between the high temperature (heat source for regeneration) and medium temperature (heat sink for condensation) reservoirs. The work produced by this heat engine is employed to pump heat from the evaporator (low-temperature reservoir) to the intermediate temperature reservoir via the adsorption process. Figure 2 shows the Clapeyron diagram of an adsorption heat pump. The cycle consists of two isobars and two isosters [17; 18; 19]. Adsorption and desorption processes perform under isobaric conditions while the preheating and precooling of the adsorber beds are achieved at constant uptakes. The saturated adsorber bed is preheated at constant uptake, i.e., \(q_{\text{max}}\) until it reaches the condenser pressure (1-2) while the corresponding temperature is \(T_{2}\). The adsorber bed is then regenerated isobarically (2-3) by supplying thermal energy, i.e., \(Q_{d}\). The dry or unsaturated bed is then cooled down to the evaporator pressure (3-4). The adsorption process then commences (4-1) and the cycle completes.