Theoretical analysis of engine waste heat recovery by the combined thermo-generator and organic Rankine cycle system

The combined thermo-generator and organic rankine cycle (TEG-ORC) used in exhaust heat recovery of internal combustion engine (ICE) is analyzed theoretically. Only about one third of the total energy released from fuel combustion is converted into useful work in engines, while the remaining energy goes into ambient environment, among which exhaust gas possesses high-grade thermal energy. Most of previous studies on energy recovery from engines have focused on exhaust heat recovery by ORC. However, if the heat is exchanged directly with high-temperature exhaust gas, organic working fluid would resolve with its lower decomposition temperature, and this is extremely harmful to ORC system. To avoid this phenomenon and utilize waste heat, preliminary thermoelectric modules are used to lower exhaust temperature and to generate electricity simultaneously. The heat rejected by TEG is used to preheat working fluid, and more energy is recovered to improve gross output power and thermal efficiency. A theoretical numerical model has been established in this paper to study the performances in both supercritical and subcritical combined TEG-ORC systems. The results suggest that, this model could efficiently identify the optimal performance parameters of both TEG and ORC systems. The utilization of TEG can extend the range of choosing working fluids if the temperature of waste heat source is high, so combined TEG-ORC system is suitable to recover waste heat from automotive vehicle engines, and thereby to improve the fuel economy of a passenger vehicle