## 1 Introduction  
Emission from the vast numbers of vehicles significantly contributes to the adverse urban environmental issues. Subsequently, urban heat island effect and serious air pollution are becoming existential threats to city dwellers. Approximately 55% of the world's population resides in urban areas while this number is projected to be as high as 68% in 2050 [1]. As the urban population increases, the number of cars running in cities is expected to increase proportionally. This situation might become worse if public transportation is poor or underdeveloped. Meanwhile, total vehicle production increased by 2.4%, while the production for 2017 was 95 million units [2]. According to the 2018 report by the Japan Automobile Manufacturers Association (JAMA), Motor Industry of Japan 2018, the total number of passenger cars produced in 2017 was approximately 8.3 million which is a 5.8% increment [3]. The thermal efficiency of most internal combustion engines is well under 40% while most cars reject approximately 60-65% of the thermal energy from the combustion of fuel via the engine coolant and the exhaust [4]. Japan's National Greenhouse Gas Emissions reported that ~7% of the CO\({}_{2}\) emission in 2016 came from transportation and automobiles [5]. Thus, urban cities are subjected to the thermal energy rejection from vehicles, and the effective recovery of this rejection will immediately reduce the rise in the ambient temperature. Due to the urban heat island effect, the usage of an onboard air conditioner to achieve cabin thermal comfort has increased. However, conventional mechanical vapour compression (MVC) systems consume part of the engine power to operate the compressor and hence worsen fuel economy. Figure 1 depicts the breakdown of energy utilisation by a typical automobile. Approximately 65% of the thermal energy from the combustion of fuel is rejected through the exhaust, the engine coolant and the cabin heating during winter. This energy, for the cabin heating, can be considered as the useful effect in winter while in summer it can be considered as waste heat. Of the remaining 35% of thermal energy, friction losses and electricity generation account for 4% each. On average, the onboard air-conditioner consumes \(\sim\)5% of the engine torque. Depending on the cabin size, the capacity of the installed air conditioner varies. Following [PERSON] and [PERSON], Table 1 lists the influence of operating the air conditioner on the performance of different types of vehicles, i.e., mini, small, and standard as per the classification under the Road Vehicles Act of Japan [3; 6]. It is noted that the mechanical air conditioner used up a significant portion of the power production, especially during idling. Hence, the implementation of cabin air-conditioning systems that utilise or recover waste heat from the engine will provide a two-pronged benefit: (i) improved fuel economy and (ii) reduced heat rejection at a lower temperature. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Unit**} & \multicolumn{3}{c}{**Type**} \\ \cline{3-5}  & & **Mini** & **Small** & **Standard** \\ \hline Number of cylinders & - & 3 & 4 & 6 \\ \hline Engine displacement range & cc & \textless{}660 & \textgreater{}660 and \textless{}2000 & \textgreater{}2000 \\ \hline Engine displacement & L & 0.66 & 1.5 & 2.5 \\ \hline Height & m & \textless{}2 & \textless{}2 & \textgreater{}2 \\ \hline Width & m & \textless{}1.48 & \textgreater{}1.48 and \textless{}1.7 & \textgreater{}1.7 \\ \hline Length & m & \textless{}3.4 & \textgreater{}3.4 and \textless{}4.7 & \textgreater{}4.7 \\ \hline Heat of combustion & kJ/1 & 33,000 & 33,000 & 33,000 \\ \hline Air conditioner capacity & kW & 5 & 6 & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Influence of operating the air conditioner on the performance of different types of vehicles [3; 6]. Figure 1: Energy flow diagram of an automobile engine.