## 7 Future Aspects and Tasks on Adsorption Systems for Automobile Air Conditioning  
The amount of waste heat available from the automobile engine and the basic characteristics of adsorption systems, i.e., the ability to operate using the waste heat, portray enormous potential for improved fuel economy and environmental friendliness. However, despite such obvious benefits and prospects, the widespread usage of adsorption systems has not been realised thus far. It is another obvious fact that adsorption systems/chillers have been around for several decades (in fact earlier than the mechanical vapour compression (MVC) systems) and they are unable to replace the MVC systems. The application of adsorption systems in automobile air conditioning enjoys the readily available waste heat which is going to be severely affected by the emergence of electric vehicles. However,  
Figure 13: Engine speed and the cabin temperature variation under NEDC-2 ECE mode [81]. IC engine-driven vehicles will still be on the road for the foreseeable future, and the successful emergence of adsorption cooling systems for automobile air conditioning might revive the industry with improved fuel economy and environmental friendliness. On the other hand, it is crucial to treat vehicle adsorption systems in a different domain separated from the so-called adsorption chillers which are mostly stationary or immobile and sometimes free from space and weight limitations Some key challenges to automobile adsorption systems are listed as follows:  
1. Energy density (W/kg or W/m\({}^{3}\)): The energy density of adsorption systems used to be extremely poor. The industry has adopted some salient developments such as (1) the adaptation of compact vapour duct and valve designs, (2) compact adsorber heat exchanger, (3) the usage of expansion valves replacing the U-bend, and (4) the introduction of composite adsorbent. The reported energy density values are 102 W/kg (total mass of the adsorption unit) [77], 33-50 W/kg [78], and 55-75 W/kg (based on the results from Figure 5). Note, however, that the energy density of a typical MVC system is \(\sim\)526 W/kg [77]. When it comes to the volumetric energy density of adsorption systems, the reported values are 11.8 to 17.6 W/m\({}^{3}\)[78] and 19.82 to 26.49 W/m\({}^{3}\) (based on the results from Figures 3 and 4). Thus, the energy density of adsorption systems must be improved. Composite adsorbents have been developed for improved thermal conductivity yet most of the reported works focus on elementary isotherm or kinetic studies. [PERSON] et al. reported an experimental unit on a resorption cycle for vehicles using the composite adsorbent with a MnCl\({}_{2}\)-CaCl\({}_{2}\) working pair. The unit has a cooling capacity of 3 kW, and the packing density of the composite was estimated to be 300 kg/m\({}^{3}\). Note, however, that this figure is not based on the overall volume of the refrigeration system [82]. Here, it should be stressed that the conventional terminologies, such as the specific cooling energy (_SCE_) or specific cooling power (_SCP_) based on the dry mass, are not helpful or, sometimes, give the wrong impression. Instead, the specific energy density based on the total mass or the volume of the system should be applied, and most importantly, apple-to-apple comparison with the conventional MVC system should be adopted when appropriate. 2. Reliability evaluation: Adsorption systems are reported to be operational for several years, for example, the adsorption prototype in the Air Conditioning Laboratory of the National University of Singapore has been operating since 2002 [83; 84; 85]. [PERSON] et al. studied ageing and its effect on the desiccant cooling performance for 50,000 cycles [86]. Note, however, that these systems are not for automobile air conditioning. The operational environment of the adsorption systems on an automobile can be subjected to vibration, sudden impacts, and high temperature surroundings. The vibrating condition might damage the adsorber heat exchanger.