**4.2. Power density**

Power density is usually the main concerns that car manufacturers focus on. Every car manufacturer pursues highest power output in a certain weight. At present, the threshold of power output in automotive market is 1 kW per unit. In other way, the thermoelectric have to obtain at least 10 kW heat energy from the exhaust given that most of current system efficiency are less than 10%. It is a dilemma for TE material scientist and thermal system designer. To maintain such power density, system designer have to find ways to deliver sufficient heat to the TE material. Material scientist have to tune the TE into best power output capacity within a defined space.

#### **4.3. Reliability**

the TEG system can be measured, and a maximum power output of 944 W was obtained,

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

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.

which can fulfil the power for some accessories in automotive application (**Figure 14**).

**4. Challenges**

158 Bringing Thermoelectricity into Reality

**4.1. Limited space**

reliability under an all-weather condition.

**Figure 14.** Multi cross flow arrangements for TEG heat exchanger.

**Figure 13.** Fin arrangement studies for heat transfer enhancement in TEG heat exchanger.

The lifespan of a common passenger car can reach over 25 years or more. Although no moving parts in the TEG system, TEG system exposes to an extremely varying thermal cycling condition. Tiny cracks between soldering layers and metalized layer will decrease the efficiency and cause mismatched resistive load. In addition, the clamping is prone to failure under a shock or vibration. Unfortunately, there is little research in this area but it should be included in any road test in future.

Nonetheless, considering major services in certain intervals and payback time for the customer, it is crucial for TEG developer to define the business model for selling this technology in term of installation cost and repair cost.
