تجزیه و تحلیل عملکرد از بازیافت حرارتی هدررفت با یک سیستم حلقه دوگانه آلی سیکل رانکین (ORC) برای موتور های دیزلی تحت شرایط عملیاتی مختلف

To take full advantage of the waste heat from a diesel engine, a set of dual loop organic Rankine cycle (ORC) system is designed to recover exhaust energy, waste heat from the coolant system, and released heat from turbocharged air in the intercooler of a six-cylinder diesel engine. The dual loop ORC system consists of a high temperature loop ORC system and a low temperature loop ORC system. R245fa is selected as the working fluid for both loops. Through the engine test, based on the first and second laws of thermodynamics, the performance of the dual loop ORC system for waste heat recovery is discussed based on the analysis of its waste heat characteristics under engine various operating conditions. Subsequently, the diesel engine-dual loop ORC combined system is presented, and the effective thermal efficiency and the brake specific fuel consumption (BSFC) are chosen to evaluate the operating performances of the diesel engine-dual loop ORC combined system. The results show that, the maximum waste heat recovery efficiency (WHRE) of the dual loop ORC system can reach 5.4% under engine various operating conditions. At the engine rated condition, the dual loop ORC system achieves the largest net power output at 27.85 kW. Compared with the diesel engine, the thermal efficiency of the combined system can be increased by 13%. When the diesel engine is operating at the high load region, the BSFC can be reduced by a maximum 4%.

With the rapid development of the auto industry, the automobile population has skyrocketed, and the energy sources consumed by automobiles have been growing rapidly. Meanwhile, given the low utilization rate, the effective power output of the total fuel combustion energy is less than 40% in internal combustion (IC) engines, and the remaining heat is released into the atmosphere. This mechanism not only harms the environment; it also results in energy dissipation. Currently, numerous research institutes and scholars are devoted to developing new energy vehicles and alternative fuel technologies. Therein, biodiesel is a kind of clean biofuel that holds much promise. However, the application of biodiesel remains limited by technology and cost, such that this technology cannot displace traditional IC engines in the near future [1], [2] and [3]. Therefore, recovering the waste heat from IC engines is an effective method for the improvement of thermal efficiency, reducing pollutant emission, and saving fuel. Reasonable measures should be employed to recover and utilize the low-grade waste heat of IC engines. Numerous scholars have recently investigated the use of organic Rankine cycle (ORC) system to recover low-grade waste heat. Research has focused on the selection of working fluids, optimization parameters, system configuration improvements, and so on. Mago et al. employed a basic ORC system and a regenerative ORC system to convert waste energy into power from low-grade heat sources. The results showed that the regenerative ORC system achieved a higher efficiency compared with the basic ORC system [4]. Wang et al. presented a method for selecting the working fluid and parametric optimization using a multi-objective optimization model by simulated annealing algorithm. They concluded that different working fluids should be selected to match the different heat source temperatures [5]. Roy et al. designed a set of ORC system, using R-12, R-123 and R134a as working fluids to recover the waste heat from flue gas. The analysis showed that R-123 had the maximum work output and efficiencies among all the selected fluids [6]. Wei et al. studied the performance and optimization of an ORC system using HFC-245fa (1,1,1,3,3-penta-fluoropropane) as a working fluid driven by exhaust heat. The results revealed that utilizing the exhaust heat as much as possible was a good method for improving the system output net power and efficiency [7]. Liu et al. presented a two stage Rankine cycle for power generation, which was composed of a water steam Rankine cycle and an Organic Rankine bottoming cycle. Optimal points were found at different cold source temperatures and steam turbine outlet pressures for each cycle [8]. Wang et al. conducted a multi-objective optimization of the ORC system to achieve the system optimization design from both thermodynamic and economic aspects using an evolutionary algorithm [9]. Gao et al. assessed 18 organic working fluids according to their physical and chemical properties for a supercritical ORC driven by exhaust heat. The effects of these working fluids on the performance of the supercritical ORC system were discussed. The results showed that R152a and R143a are recommended as the working fluids for the supercritical ORC system [10]. Hung proposed an ORC system to recover waste heat from low enthalpy heat sources using dry fluids. The study indicated that R113 and R123 performed better in recovering low temperature waste heat [11]. Quoilin et al. proposed a dynamic ORC model and three different control strategies. Their simulation results showed that a model predictive control strategy based on the steady-state optimization of the cycle under various conditions was the one showed the best results [12]. Dai et al. described an ORC for recovering low-grade waste heat using different working fluids. The effects of thermodynamic parameters on ORC performance were examined, and the thermodynamic parameters of the ORC for each working fluid were optimized with exergy efficiency as an objective function. The results indicated that the cycle with R236EA had the highest exergy efficiency [13]. Guo et al. proposed an innovative cogeneration system powered by low temperature geothermal sources, and the system consisted of a low temperature geothermal-powered ORC subsystem, an intermediate heat exchanger, and a commercial R134a-based heat pump subsystem. The suitable working fluids were screened and the performances of the novel cogeneration system under disturbance conditions were studied [14]. Hung et al. investigated the effects of dry, wet, and isentropic organic fluids as working fluids on the performance of ORC system. The results revealed that wet organic fluids with very steep saturated vapor curves in T–s diagram had a better overall performance in terms of energy conversion efficiencies than that of dry organic fluids. Furthermore, an appropriate combination of solar energy and an ORC system with a higher turbine inlet temperature and a lower condenser temperature would provide an economically feasible energy conversion system [15]. Wang et al. conducted an experimental study to investigate the performance of a low temperature solar Rankine cycle system. The results showed that the highest heat collecting efficiency of the collector is about 50% [16]. The ORC system has been widely applied to the recovery of waste heat from IC engines. Gao et al. proposed a waste heat recovery system where a high-speed turbocharged diesel engine acts as the topper of a combined cycle system and exhaust gases were used for a bottoming Rankine cycle. The conclusion was that introducing a heat recovery system could increase the engine power output by 12% [17]. Srinivasan et al. designed an ORC system to examine the exhaust waste heat recovery potential of a high-efficiency, low-emissions dual fuel low temperature combustion engine [18]. Vaja et al. considered three different ORC setups for recovering the waste heat of exhaust gases and engine cooling water. The analysis demonstrated that a 12% increase in the overall efficiency could be achieved with respect to the engine with no bottoming [19]. Yu et al. built an ORC system to recover waste heat both from engine exhaust gas and jacket water using R245fa as working fluid. The results indicated that the ORC system performs well under the rated engine condition with a recovery efficiency of up to 9.2% and exergy efficiency of up to 21.7% [20]. Tian et al. proposed an ORC system used in the IC engine exhaust heat recovery and techno-economical performances of the ORC system based on various working fluids. The results showed that R141b, R123, and R245fa had the highest thermal efficiency and the lowest electricity production cost [21]. Katsanos et al. investigated the potential improvement in the overall efficiency of a heavy-duty truck diesel engine equipped with a Rankine bottoming cycle for recovering heat from exhaust gas. They discovered that the specific fuel consumption improvement ranged from 10.2% (at 25% engine load) to 8.5% (at 100% engine load) for R245ca [22]. Wang et al. analyzed the characteristics of a novel system combining a gasoline engine with a dual loop ORC system that recovered the waste heat from both the exhaust and coolant system [23]. Peng et al. developed an Exhaust Energy Recovery (EER) system based on Rankine cycle to convert exhaust thermal energy to mechanical or electric energy [24]. Wang et al. investigated the improvement on a light-duty gasoline engine with experimental work and a numerical simulation based on a steam Rankine cycle EER system [25]. From the aforementioned analysis, ORC system is efficient for recovering the waste heat from IC engines. Currently, however, most research takes only IC engine exhaust energy into account. Few scholars have considered recovering the waste heat from the coolant system, and the released heat from turbocharged air in the intercooler of IC engine. In this paper, a set of dual loop ORC system is designed to recover exhaust energy, waste heat from the coolant system, and released heat from turbocharged air in the intercooler of a six-cylinder diesel engine. The dual loop ORC system consists of a high temperature (HT) loop ORC system and a low temperature (LT) loop ORC system. R245fa was selected as the working fluid for both loops. The HT loop ORC system is used to recover the exhaust energy, whereas the LT loop ORC system is used to recover the waste heat from the coolant system, the released heat from turbocharged air in the intercooler, and the released heat from the condensation process of the HT loop ORC system. First, the distribution characteristics of the waste heat from the diesel engine under various operating conditions are analyzed via a diesel engine performance test. Thereafter, the operating performances of the dual loop ORC system and the diesel engine-dual loop ORC combined system are evaluated

In this paper, in order to realize the cascade utilization of diesel engine waste heat, a set of dual loop ORC system is designed, with R245fa is chosen as the working fluid, to recover exhaust energy, waste heat from the coolant system, and released heat from turbocharged air in the intercooler. Based on an analysis of the distribution characteristics of the diesel engine waste heat and the operating performance of the diesel engine-dual loop ORC combined system, conclusions can be made as follows: (1) By employing the dual loop ORC system, the fuel economy of the diesel engine can be notably improved. At the engine rated condition, the effective thermal efficiency of the diesel engine-dual loop ORC combined system is 0.35, which increases by 13% compared with the diesel engine itself. When the engine is operating in medium–high load region, the combined system has better fuel economy, the BSFC can be decreasd to 186 g/(kW h) and can be improved by a maximum of 4% compared with the diesel engine itself. (2) For most operating conditions of the diesel engine, the net power output of the LT loop ORC system is greater than that of the HT loop ORC system. At the engine rated condition, the net power output of the LT loop ORC system is 17.85 kW, the net power output of the HT loop ORC system is 10 kW, the overall net power output of the dual loop ORC system is 27.85 kW. Over the whole operating range, the WHRE of the combined system can reach a maximum of 5.4%. (3) For the dual loop ORC system, over the whole operating range, the overall exergy destruction rate of the dual loop ORC system increases with the engine speed and engine load, such that the exergy destruction rate of the LT loop ORC system is higher than that of the HT loop ORC system. At the engine rated condition, the exergy destruction rate of the LT loop ORC system is 10.42 kW, whereas exergy destruction rate of the HT loop ORC system is 8.29 kW.