Waste heat recovery to improve fuel economy on a hybrid bus

You are here: Homepage > Search

Waste heat recovery to improve fuel economy on a hybrid bus

Nolan, Cathal / Douglas, R. / O'Shaughnessy, R. / Lopes, J.D. / Rouaud, C. / Hanna, A.

An experimental WHR ORC (Waste Heat Recovery - Organic Rankine Cycle) system has been designed and installed in a hybrid bus to improve its fuel efficiency. The system was first modelled to determine the most suitable working fluid and to estimate the power such a system would generate. Having installed the system it was then tested to determine the amount of power recoverable. The predicted benefit to fuel economy ranges from 3.4% to 9.1% depending on the driving scenario. As expected the testing phase has shown that at a given condition the exhaust system generates more power than the coolant system. However the exhaust system requires an additional condenser while the coolant system can eliminate the requirement for a radiator on the vehicle, instead using the coolant condenser to cool the engine. Expander sizing is critical to the power generation of the system and the authors note that more power could have been recovered had a custom sized expander been available. Also using a more efficient generator/inverter configuration and pump would increase the efficiency of the system. If a generator/inverter configuration with an efficiency of 90% was used the net power of the system would be increased to 2.2 kW. Running the system at higher pressures would also increase the power generation. The expander used for the testing had a working pressure of 13 bar and therefore the testing stopped once this pressure had been reached. The overall system (ORC on exhaust line and cooling circuit) is being tested on the hybrid bus in order to improve overall efficiency and maximize the power recovery in order to reach a fuel consumption benefit of 10% over a typical bus drive cycle.