HeatReCar project demonstrates technical feasibility of thermoelectric generator for waste heat recovery; economic case more difficult
A recently completed European project coordinated by Centro Ricerche Fiat (CRF) demonstrated the technical feasibility of a Bi2Te3-based thermoelectric generator (TEG) for waste heat recovery for application to a diesel light-duty truck (LDT). The project “Reduced energy consumption by massive thermoelectric waste heat recovery in light-duty trucks” (HeatReCar) focused on thermoelectrics to provide electricity, either to on-board components or to the power train of hybrid electric vehicles. Reduced fuel consumption for these purposes translates to emissions reductions.
TE materials have been employed previously in automotive applications but have not achieved reasonable conversion efficiencies. The researchers tackled this issue in two ways. They selected bismuth telluride (Bi2Te3) suitable for lower operating temperatures in a diesel engine. They also optimized the geometry of heat transfer surfaces to maximize the temperature difference available to the TE modules. The technology was implemented in a prototype TE generator (TEG) for a diesel IVECO Daily light-duty truck (LDT) in common use in the EU.
Performance of TE materials was increased by more than 20% through ball milling and subsequent spark plasma sintering (SPS). Sintering is a standard method for consolidating powders without melting them. Compaction is achieved by sintering at high temperatures compared to the melting point of the processed materials. Spark plasma sintering uses an electric current to heat the sample and to activate sintering.
The initial HeatReCar base case was a TEG producing 1 KWe at 130 km/h. At this power level the potential for CO2 emissions reduction ranged from 50 g CO2/km in real traffic at 30 km/h (18.6 mph) average city speed to 10 g CO2/km on the motorway at 130 km/h (81 mph).
Test bench results showed that the TEG was able to deliver 500We at the design point, with inlet gas at 450 ˚C and a mass flow rate of 90 g/s. A thermoelectric by-pass was included also to limit the highest thermoelectric modules temperature at 270 ˚C to prevent Bi2Te3 material damage.
This strategy caused a penalty in the thermoelectric performance, as at medium-high engine load, where the thermoelectric potential is higher, the system was to be switched to by-pass in order to lower the temperature over the thermoelectric modules. With this limitation the thermoelectric was operated at full flow up to a 110 km/h (68 mph) vehicle speed, and run with partial by-pass above this speed. In these conditions the output did not exceed 170 We. After improvement, the by-pass system should allow to reach 250 We at constant speed.
Driving cycle tests results:
On the NEDC Cycle the thermoelectric system dropped the fuel consumption by about 2.2% (6.7 g CO2/km reduction) with a peak thermoelectric electric power of 150 We.
On the more heavily loaded WLTP Cycle the thermoelectric dropped the fuel consumption by about 3.9% (9.6 g CO2/km reduction) with a maximum thermoelectric electric power of 200 We.
Increases in TEG electric output corresponded to the same decrease in alternator demand.
To enhance marketability, the researchers concluded, further work should be focused on decreasing the cost together with several recommendations regarding engine type, driving conditions and materials properties.