Powering vehicles from waste heat
Motor vehicles rely on combustion of fossil fuels for motion and an alternator (converts mechanical energy into electrical energy) for electrical supply to on-board components. EU-funded scientists are working on technology exploiting thermoelectric (TE) materials to harvest waste heat from exhaust gases and convert it into electricity. This should mitigate to some extent the effects of rising fuel prices and increasing electrical demands of cars. The project 'Reduced energy consumption by massive thermoelectric waste heat recovery in light-duty trucks' (HEATRECAR) is developing thermoelements to provide electricity, either to on-board components or the power train of hybrid electric vehicles. Reduced fuel consumption for these purposes translates to important emissions reductions. TE materials have been employed previously in automotive applications but have not achieved reasonable conversion efficiencies. Scientists tackled this issue in two ways. They selected bismuth telluride (Bi2Te3) suitable for lower operating temperatures in a diesel engine. They also optimised the geometry of heat transfer surfaces to maximise the temperature difference available to the TE modules. The technology was implemented in a prototype TE generator (TEG) for a diesel light-duty truck (LDT) in common use in the EU. Performance of TE materials was increased by over 20 % through ball milling and subsequent spark plasma sintering. Driving cycle tests demonstrated that the TEG system decreased fuel consumption by about 2.2 % on the New European Driving Cycle (NEDC) and by 3.9 % on the more heavily loaded Worldwide Harmonised Light Vehicles Test Procedures (WLTP) cycle. Increases in TEG electric output corresponded to the same decrease in alternator demand. The technical feasibility of a Bi2Te3-based TEG for application to a diesel LDT has been successfully demonstrated. In order to enhance marketability, further work should be focused on decreasing the cost together with several recommendations regarding engine type, driving conditions and materials properties. HEATRECAR technology has the potential to significantly decrease fuel consumption and associated carbon dioxide emissions for important socioeconomic impact.
