Waste heat recovery from a heavy-duty natural gas engine by Organic Rankine Cycle

Waste heat recovery can be a key solution for improving the efficiency of energy conversion systems. Organic Rankine Cycles (ORC) are a consolidated technology for achieving such target, ensuring good efficiencies and flexibility. ORC systems have been mainly adopted for stationary applications, where the limitations of layout, size and weight are not stringent. In road transportation propulsion systems, the integration between the powertrain and the ORC system is difficult but still possible. The authors investigated an ORC system bottoming a spark ignited internal combustion engine (ICE) powering a public transport bus. The bus, fuelled by natural gas, was tested in real driving conditions. Exhaust gas mass flow rate and temperature have been measured for calculating the thermal power to be recovered in the ORC plant. The waste heat was then used as energy input in a model simulating the performance of an ORC system. The heat transfer between the exhaust gases and the ORC fluid is crucial for the ORC performance. For this reason, attention was paid to considering the interaction between hot fluid temperature and ORC maximum pressure. ORC performance in terms of real cycle efficiency and power produced were calculated considering n-Pentane as working fluid. The fuel consumption was reduced from 271.5 g/km to 261.4 g/km over the driving cycle, corresponding to 3.7% reduction.