Parametric analysis of a dual loop Organic Rankine Cycle (ORC) system for engine waste heat recovery
Introduction
A large amount of the primary energy is consumed by the diesel engines, whereas nearly 2/3 of the fuel energy is wasted through the exhaust gas and the engine coolant [1]. It is therefore apparent that there is still a large fraction of the energy that is untapped, and the potential gain from efficiently utilizing the waste heat is appreciable, thereby improving the fuel economy and reducing the emission of the engine. Among all of the existing technologies, Organic Rankine Cycle (ORC) has been proven to be one of the most effective solutions for engine waste heat recovery [2], [3], [4], [5]. Various ORC systems have been designed and presented in previous publications, including simple ORC systems [4], [6], regenerative ORC systems [4], [7] and preheated ORC systems [8], [9].
Recently, dual loop ORC systems have been actively researched in the fields of engine waste heat recovery, which can simultaneously recover the waste heat of the exhaust gas and the engine coolant. A dual loop ORC system consists of a high temperature (HT) loop and a low temperature (LT) loop. The two loops are usually coupled via a shared heat exchanger, through which the residual heat of the HT loop is utilized by the LT loop. A large number of research studies have been conducted on the analysis and optimization of dual loop ORC systems. Yang et al. [10] designed a dual loop ORC system to recover the exhaust energy, the waste heat from the coolant system and the released heat from the turbocharged air. The system performance was discussed under various engine operating conditions. Zhang et al. [11] analyzed the characteristic of a novel system combining a vehicular light-duty diesel engine with a dual loop ORC. The relative power output could be improved from 14% to 16% in the peak effective thermal efficiency region of the engine and from 38% to 43% in the small load region. Wang et al. [12] evaluated the effects of three parameters, the expander isentropic efficiency, the evaporation pressure of the HT loop and the condensation temperature of the LT loop, on the performance of the dual loop ORC system. Wang et al. [13] evaluated the performance of a combined engine-ORC system across the engine’s entire operating region. The results showed that the relative output power improves by from 14% to 16% in the peak effective thermal efficiency region to 50% in the small load region, and the absolute effective thermal efficiency increases by 3–6% throughout the entire region. Yao et al. [14] evaluated the performance of a DORC–CNGE combined system. Results show that the maximum net power output and the maximum thermal efficiency of the DORC system are 29.37 kW and 10.81%, respectively, under the rated power output condition of the engine. Shu et al. proposed a novel dual-loop ORC system for engine waste heat recovery and carried out different research work [15], [16], [17]. The HT loop recovered the high-temperature engine exhaust and the LT loop recovered the waste heat of the engine coolant, the residual heat of the HT loop and the low-temperature exhaust in series. In Ref. [15], energetic and exergetic analysis of the dual loop system were conducted to evaluate the influence of the evaporation temperatures of both loops on the system performance. In Ref. [16], regenerative cycles were exploited in the system and different cycle modes (transcritical cycle and subcritical cycle) of the HT loop were investigated. In Ref. [17], subcritical cycle and transcritical cycle were adopted in the LT loop. The maximum net power output reached 39.91 kW, with R143a in the transcritical cycle. Choi and Kim [18] applied a dual loop waste heat recovery power generation system to an internal combustion engine, in which the HT loop (an upper trilateral cycle with water) recovered the waste heat of the engine exhaust gas and the LT loop (a lower organic Rankine cycle) recovered the low-temperature exhaust and the residual heat of the HT loop. The thermodynamic results confirmed that the dual loop system exhibited a maximum net power output of 2069.8 kW, and a maximum thermal efficiency of 10.93% according to the first law of thermodynamics and a maximum exergy efficiency of 58.77% according to the second law of thermodynamics.
For most presented dual loop ORC systems for engine waste heat recovery, the HT loop recovers the high temperature engine exhaust, while the LT loop utilizes other low temperature waste heat in addition to the residual heat of the HT loop. It is evident that the condensation parameters of the HT loop, i.e. the condensation temperature and the residual heat load, varies under different operating conditions. These operating parameters will definitely have an influence on the LT loop. However, few researches have concentrated on this influence. In this paper, this issue has been investigated in detail. A dual loop ORC system is designed to recover the waste heat of a diesel engine manufactured by Hudong Heavy Machinery Co., Ltd, which consists of a HT loop and a LT loop. The HT loop recovers the waste heat of the engine exhaust gas, and the LT loop recovers the waste heat of the jacket cooling water and the residual heat of the HT loop in series. The two loops of the system are coupled via a shared heat exchanger, which means that the condenser of the HT loop is the evaporator of the LT loop as well. Cyclohexane, benzene and toluene are selected as the working fluids of the HT loop. Different condensation temperatures are set to maintain the condensation pressure of the working fluids slightly higher than the atmosphere pressure. R123, R236fa and R245fa are chosen for the LT loop. Parametric analysis is conducted for the dual loop ORC system with different working fluids and under different operating conditions. The influence of the HT loop condensation parameters on the LT loop is evaluated using the pinch point method.
Section snippets
The diesel engine
The selected diesel engine for waste heat recovery in this paper is an inline six-cylinder turbocharged engine manufactured by Hudong Heavy Machinery Co., Ltd. The main parameters of the diesel engine under the design condition are shown in Table 1. The composition of the engine exhaust gas is measured and listed in Table 2. According to the calculation result by REFPROP 9.1, the average specific heat capacity of the engine exhaust gas is approximately 1.1 kJ/kg K. The temperature of the cooled
Thermodynamic model
The T–s diagram of the dual loop ORC system for engine waste heat recovery is shown in Fig. 2. The thermodynamic model of the system is developed, which can be described as follows.
Analysis of the HT loop
The heat load absorbed by the HT loop varies with the outlet temperature of the exhaust gas, . For each specified , a proper evaporation temperature of the HT loop can be calculated by the computer program, which is mainly related to the ratio of the latent heat to the sensible heat of the working fluid. Simulation is conducted for the HT loop with cyclohexane, benzene and toluene as the working fluids, with the results shown in Fig. 6, which demonstrates variations of the
Conclusions
This paper investigates the waste heat recovery of a diesel engine using a dual loop ORC system. The high temperature (HT) loop recovers the waste heat of the engine exhaust gas, while the low temperature (LT) loop recovers the waste heat of the jacket cooling water and the residual heat of the HT loop in series. The two loops are coupled via a shared heat exchanger, which means the condenser of the HT loop is the evaporator of the LT loop as well.
Cyclohexane, benzene and toluene are selected
Acknowledgement
This research study was supported by the cooperative scientific research project of energy conversion and emission reduction among China-Europe enterprises (No. SQ2013ZOC200005).
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