Abstract
The Organic Rankine Cycle (ORC) has been proved a promising technique to exploit waste heat from Internal Combustion Engines (ICEs). Waste heat recovery systems have usually been designed based on engine rated working conditions, while engines often operate under part load conditions. Hence, it is quite important to analyze the off-design performance of ORC systems under different engine loads. This paper presents an off-design Medium Cycle/Organic Rankine Cycle (MC/ORC) system model by interconnecting the component models, which allows the prediction of system off-design behavior. The sliding pressure control method is applied to balance the variation of system parameters and evaporating pressure is chosen as the operational variable. The effect of operational variable and engine load on system performance is analyzed from the aspects of energy and exergy. The results show that with the drop of engine load, the MC/ORC system can always effectively recover waste heat, whereas the maximum net power output, thermal efficiency and exergy efficiency decrease linearly. Considering the contributions of components to total exergy destruction, the proportions of the gas-oil exchanger and turbine increase, while the proportions of the evaporator and condenser decrease with the drop of engine load.
Highlights
At present, increasing global concerns over climate change and energy shortages have resulted in a strong interest in energy development
The medium cycle (MC)/Organic Rankine Cycle (ORC) system is designed based on the rated engine condition
The feasible operation points are subject to the following constraints: (a) the working fluid is completely vaporized at the outlet of evaporator to avoid blade liquid corrosion [41]. (b) the exhaust gas temperature at the evaporator outlet should be higher than the acid dew temperature to avoid exhaust gas acid corrosion
Summary
At present, increasing global concerns over climate change and energy shortages have resulted in a strong interest in energy development. In China, improving the proportion of natural gas (NG) in the primary energy mix is the main energy conservation and emission reduction strategy in the near future [1]. Most CNG engines in China are developed based on gasoline and diesel engines, the thermal efficiency of most CNG engine is about 30% lower than that of diesel engines due to the restraints of engine structure [2], that is to say, a large amount of fuel energy (about 35%) is wasted through the exhaust. The conversion of exhaust waste heat into useful output is a promising approach which will improve overall thermal efficiency and save fuel. Previous researchers have shown that among the various techniques for residual energy utilization, the Organic Rankine Cycle (ORC) is a promising method for engine waste heat recovery [3,4]
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