Abstract
Organic Rankine Cycle (ORC) power plants are characterized by high efficiency and flexibility, as a result of a high degree of maturity. These systems are particularly suited for recovering energy from low temperature heat sources, such as exhaust heat from other plants. Despite ORCs having been assumed to be appropriate for stationary power plants, since their layout, size and weight constraints are less stringent, they represent a possible solution for improving the efficiency of propulsion systems for road transportation. The present paper investigates an ORC system recovering heat from the exhaust gases of an internal combustion engine. A passenger car with a Diesel engine was tested over a Real Driving Emission (RDE) cycle. During the test exhaust gas mass flow rate and temperature have been measured, thus calculating the enthalpy stream content available as heat addition to ORC plant in actual driving conditions. Engine operating conditions during the test were discretized with a 10-point grid in the engine torque–speed plane. The ten discretized conditions were employed to evaluate the ORC power and the consequent engine efficiency increase in real driving conditions for the actual Rankine cycle. N-pentane (R601) was identified as the working fluid for ORC and R134a was employed as reference fluid for comparison purposes. The achievable power from the ORC system was calculated to be between 0.2 and 1.3 kW, with 13% system efficiency. The engine efficiency increment ranged from 2.0% to 7.5%, with an average efficiency increment of 4.6% over the RDE test.
Highlights
Fossil fuel availability and the environmental impact of their usage in energy conversion systems are big concerns for the scientific community, which is focusing on the development of renewable energy sources
The maximum Organic Rankine Cycle (ORC) pressure is different for each fluid since the saturation pressures in the condenser at T1 = 314 K are different, Table 4
A passenger car powered by a compression ignition engine was tested on an Real Driving Emission (RDE) cycle and the thermal power available in the exhaust gases exploited into an ORC system in order to promote an engine efficiency increase
Summary
Fossil fuel availability and the environmental impact of their usage in energy conversion systems are big concerns for the scientific community, which is focusing on the development of renewable energy sources. While for trucks, off-road engines, power generation engines, as well as marine engines, it is convenient to consider solutions to exploit the lower temperature heat sources to preheat the working fluid before entering the evaporator [16], simple configurations are usually more appropriate for passenger cars [7]. The engine backpressure can be counter-balanced implementing appropriate turbocharging strategies, choosing for example a Variable Geometry Turbine (VGT) Such a solution would help in limiting the negative effect of the increased back pressure caused by the exhaust gas heat exchanger. It has been demonstrated by means of numerical simulations that, adopting a VGT, the engine brake specific fuel consumption is only weakly affected by the ORC system if the backpressure increase is below 100 mbar [17]. N-pentane (R601) was chosen as working fluid for the ORC while R134a has been considered only for comparison purposes
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