## 4 Adsorbent Materials  
Adsorption heat pumps utilise the differential chemical potential of the porous solids. The sorption potential of the regenerated adsorbent is utilised in the adsorption process while the desorption process restores this chemical potential. Thus, the adsorbent material can be considered as the bottleneck of the adsorption heat pumps. Generally, porous materials with a huge specific surface area (typically \(>500\) m\({}^{2}\)/g) are employed as adsorbents. Other important characteristics of the adsorbent materials are thermophysical properties such as porosity, pore volume (micro, meso, and macro), pore width, and pore size distribution, as well as the polarity, surface heterogeneity, hydrophobicity, and hydrophilicity. Meanwhile, adsorption heat pumps utilise chemisorption and physisorption processes. Moreover, the performance of a particular adsorbent may depend on the type of adsorbate (the refrigerant). Regardless of the nature of adsorbate, i.e., the refrigerant, the high surface area is always preferable in adsorption heat pumps. The temperature of the recoverable heat in automobiles varies in a wide range (between 50 and 800 \({}^{\circ}\)C), as such, most adsorbent materials can generally be applied. Nevertheless, various adsorbents can be readily grouped as (i) carbon-based, (ii) silica-based, (iii) zeolite and molecular sieve-based, and (iv) metal organic frameworks (MOFs). Table 3 lists commonly-used adsorbents along with their porous characteristics that are widely studied in the research field of adsorption heat pumps with potential for automobile applications.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & \multicolumn{2}{c}{**Surface Area**} & \multicolumn{2}{c}{**Pore Volume**} & \multirow{2}{*}{**Pore Width (Å)**} & \multirow{2}{*}{**Reference**} \\  & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & & & \\ \hline \multicolumn{5}{c}{**(i) carbon-based**} \\ \hline Activated carbon fibre, ACF (A-20) & 1900.0 & 1.028 & 21.6 & [33; 34] \\ \hline Activated carbon fibre, ACF (A-15) & 1400.0 & 0.765 & 21.75 & [33; 34] \\ \hline Activated carbon fibre, ACF (FX-400) & 1458.0 & 0.683 & 23.50 & [35] \\ \hline Activated carbon fibre, ACF (KF-1000) & 1227.0 & 0.648 & 22.34 & [35] \\ \hline Activated carbon fibre, ACF (A10) & 1200.0 & 0.530 & & [36] \\ \hline Activated carbon, BPL & 1150.0 & 0.430 & & [36] \\ \hline Cocnut shell-based activated carbon (CSAC) & 804.0 & 0.430 & 17.60 & [37] \\ \hline Chemviron & 945.0 & 0.510 & & [38] \\ \hline Norit R 1 Extra & 1450.0 & 0.470 & & [36] \\ \hline Norit RB3 & 978.0 & 0.510 & 21.00 & [39] \\ \hline Norit Darco (100 mesh) & 876.45 & 0.730 & 33.30 & [39] \\ \hline Norit Darco (12 \(\times\) 20 US mesh size) & 462.67 & 0.500 & 42.80 & [39] \\ \hline Maxsorb III & 3250.0–3311.0 & 1.740–1.790 & 22.00 & [36; 40] \\ \hline Activated carbon from waste palm trunk (WPT-AC) & 2927.0 & 2.510 & 16.80 & [41] \\ \hline Activated carbon from mangrove (M-AC) & 2924.0 & 2.180 & 14.70 & [41] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Commonly-used adsorbents along with their porous characteristics.  
(8) Simple tube, and (9) Plate, were described together with the performance results in terms of the _COP_ and _SCP\({}_{d}\)_. Hairpin type heat exchangers were reported to exhibit the highest _COP_ while the plate type offered the best _SCP\({}_{d}\)_. On the other hand, annulus tube, fin tube, and plate tube configurations showed a high adsorbent mass to heat exchanger (bed) mass ratio. [PERSON] et al. reported the development of a novel compact adsorber regenerator achieving a packing density of 435 kg/m\({}^{3}\) with the adsorbent mass of 1 kg, while the total weight of the bed is reported to be 9 kg [75].