**4. Challenges**

High *ZT* pursued by TE community is not the only case for real automotive application. A high *ZT* with a limited temperature range is not favoured since varying heat source temperature results in low efficiency in most of time. In addition, conversion efficiency may not sit in the centre of the concern, high power density is the main factor in automotive application same as request for traction battery. Here we organised three main factors for consideration of TE selections including the limited space under chassis, the low power density and the real reliability under an all-weather condition.

#### **4.1. Limited space**

Although TEG technology is favoured in the lightweight vehicle, it is still challenge to spare a space for it. Changing the exhaust piping system is usually subject to the chassis design, which involves weight balancing, powertrain system structure and brake force distribution, etc. Unfortunately, it is not the option for most of car manufacturers. Moreover, the TEG heat recovery system should be ideally designed in a cylindrical shape as much same as the exhaust pipe.

In order to minimising the extra coolant loop or heat sink, extra coolant pumping power should be considered when routing the loop in/out of the TEG system. A proportional electric gas valve is usually integrated with TEG system in order to adjust the bypass flow of the exhaust. In some cases, a DC-DC converter or a DC-AC converter has to be positioned close to the TEG system with extra electric cabling and connection work.

Inevitably, all the added up functional components mentioned here occupy significant space. To squeeze such TEG system into a highly-packed and harsh environment is not always an easy task for the system designers.
