Performance analysis of exhaust waste heat recovery system for stationary CNG engine based on organic Rankine cycle

• The characteristics of exhaust energy for the stationary CNG engine are investigated.
• The ORC system with IHE is designed to recover the exhaust energy from engine.
• Zeotropic mixture R416A is used as the working fluid of the ORC system.
• The electric efficiency of combined system is defined and investigated.
• BSFC of combined system is studied under various operating conditions of engine.

In order to improve the electric efficiency of a stationary compressed natural gas (CNG) engine, a set of organic Rankine cycle (ORC) system with internal heat exchanger (IHE) is designed to recover exhaust energy that is used to generate electricity. R416A is selected as the working fluid for the waste heat recovery system. According to the first and second laws of thermodynamics, the performances of the ORC system for waste heat recovery are discussed based on the analysis of engine exhaust waste heat characteristics. Subsequently, the stationary CNG engine-ORC with IHE combined system is presented. The electric efficiency and the brake specific fuel consumption (BSFC) are introduced to evaluate the operating performances of the combined system. The results show that, when the evaporation pressure is 3.5 MPa and the engine is operating at the rated condition, the net power output and the thermal efficiency of the ORC system with IHE can reach up to 62.7 kW and 12.5%, respectively. Compared with the stationary CNG engine, the electric efficiency of the combined system can be increased by a maximum 6.0%, while the BSFC can be reduced by a maximum 5.0%.