Simulation and Performance Analysis of Organic Rankine Systems for Stationary Compressed Natural Gas Engine

The organic Rankine cycle (ORC) can be used to recover the waste heat from a stationary compressed natural gas (CNG) engine. However, the exhaust energy rate varies with engine load, which can influence the operating performance of the ORC system, therefore, it is necessary to study the running state of the ORC system. In this paper, first, the numerical simulation model of the ORC system is built by using GT-Suite software, with R245fa selected as the working fluid of the ORC system. The boundary conditions of the numerical simulation model are specified according to the measured data obtained by the stationary CNG engine test. Subsequently, the power output and dynamic characteristics of expander are analyzed to determine the running state of the ORC system. Investigations indicate that the fluctuation of refrigerant mass flow rate in the expander is obvious in the engine’s low-load regions (from 20% engine load to 40% engine load). The performances of ORC system and stationary CNG engine-ORC combined system (combined system) are finally investigated, respectively. The results show that the thermal efficiency of the combined system can be increased by a maximum 5.0% (at the engine rated condition), while the brake specific fuel consumption (BSFC) can be reduced by a maximum 4.0% (at the engine rated condition).