Pre-design of waste heat recovery ORC systems for heavy-duty trucks by means of dynamic simulation

Internal combustion engines (ICE) of heavy-duty trucks show efficiencies larger than 40% in nominal operating conditions. Despite these rather good efficiencies in comparison with smaller ICEs, a large amount of heat is wasted to the ambient in the engine coolant circuit, in the exhaust gas and in the exhaust gas recirculation cooler. These different heat sources differ by their capacities and levels of temperatures. In order to meet the upcoming international CO2 regulations and in order to increase the competitiveness of transportation by trucks, truck industry is currently investigating the technical and economical feasibility of waste heat recovery by means of Organic Rankine Cycle heat engines. In this context, the present paper aims at comparing the energy and economical performance of different topologies of ORC on representative driving cycles. These topologies differ by the configuration of the different evaporators (EGR evaporator in parallel or in series with the exhaust gas evaporator), by the working fluids (R245fa, ethanol, acetone, cyclopentane and dimethylcarbonate) and by the expansion machines (volumetric expanders and radial inflow turbine). The choice of the investigated working fluids and expansion machines is justified by previous research projects whose results are summarized. Dynamic simulations are conducted with Amesim simulation tool. The control of the ORC is achieved by two different gain scheduling PIDs. Manipulated variables are the expander and pump rotational speeds and controlled variables are the expander supply pressure and temperature. The models of the components and the parameters identification process are described. Performance achieved with the different topologies are assessed in terms of fuel saving, power rejected at the condenser (which increases the cooling load of the truck) and payback time for the truck owner. Estimated fuel savings range from 2.3% to 3.2% and minimum payback time is 2 years.