Waste heat recovery concept developed in PowerDriver project
The PowerDriver project is a European Union funded collaborative research initiative involving major UK based end-user organizations Jaguar Land Rover Ltd and Rolls-Royce PLC together with supply chain and research and development partners and universities. The project is focused on the conversion of energy from combustion engine exhaust gas into electricity utilizing thermo electric generator (TGEN) technology. Jaguar Land Rover Ltd is interested in technology capable of being applied to petrol engine passenger cars while Rolls-Royce PLC is interested in marine applications related to diesel engines.
In order to extract the energy from the exhaust gas flow the TGEN has to be mounted between two heat exchangers – a hot side heat exchanger and a cold side heat exchanger. This is necessary because the thermo electric materials produce energy when exposed to a large difference in temperature. In both cases the hot side heat exchanger forms part of the exhaust line and the cold side heat exchanger forms part of the engine cooling system.
“Thermoelectric generators are a very promising technology that enables the recapture of heat energy that would otherwise be lost,” commented Dr. Barri Stirrup, of European Thermodynamics Ltd. “The PowerDriver project is an important European research collaboration that aims to help bring this technology much closer to commercial realization. With the simulation work of the automotive TGEN system indicating a power output equating to a very significant fuel saving over the NEDC, the project will now progress the design of prototype systems aimed at providing cost-effective implementations of this technology.”
From a technological perspective the PowerDriver project has a number of challenging areas. For example, the thermoelectric materials under consideration for the automotive application – both n-type and p-type, to form a couple – are silicide based materials. These have a potentially low cost base but need further development to achieve the performance and thermal stability required for the application. This is not least due to the fact that the TGEN is located within the exhaust line and is subject to significant thermal cycling. In addition, the lead telluride based thermo electric materials being investigated for the marine application have a proven track record in similar applications but present financial and thermal stability issues which need to be overcome. Furthermore, the thermo electric generators require electronic controls which also need to be developed to maximize output efficiency. The joining of current conductors to the thermo electric material also presents issues which will need to be overcome.
Simulation work completed
Engineering simulation modelling work is necessary to design both the TGEN and the heat exchangers to obtain the optimum system performance (euro/watt) and thermal stability using Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) techniques, in addition to achieving volume and weight constraints for the target vehicle. Currently this work suggests that a prototype TGEN design can be achieved with a predicted performance output of 300W, based on the NEDC operating cycle, which would provide a potential fuel saving of up to 2.5% with a comparable reduction in carbon footprint. During the remainder of the project, activity will be directed at the production of a prototype for evaluation on a hot air test rig to confirm the projected power output performance; the cost/watt of power for a complete commercial system will also need to be established. This will validate the commercial potential of the system before more costly in-vehicle testing is undertaken.
The lead telluride thermo electric materials used for the marine application have proved challenging, however, the developed n-type material has achieved electrical performance suitable for taking forward into a TGEN design, although cost difficulties have resulted in a change to the selected p-type material. Remaining work will centre on the development and characterisation of the new material, including the confirmation of a viable manufacturing process. The final thermo electrical material couple may include functionally graded materials (FGM) which consist of a sandwich of materials in which the individual layers are tuned to the temperature profile being experienced.