Optimization and Evaluation of a Vehicular Exhaust Heat Recovery System

Meng Zhao and Mingshan Wei

(1.School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China; 2.Beijing Institute of Space Launch Technology, Beijing 100076, China)

Abstract: Exhaust waste heat recovery system based on organic Rankine cycle (ORC) has been considered as an effective method to achieve energy conservation and emissions reduction of engine. The performance of adiesel engine with an on-board ORC exhaust heat recovery system was evaluated through simulations in this study. The combined system was optimized through controlling the exhaust gas mass flow rate entering the ORC system. The models of the engine with ORC system were developed in GT-suite and Simulink environment. The validation results showed high accuracy of the models. The performance of the system recovering heat from different exhaust gas mass flow rates was evaluated. The comparative analysis of the performance between the optimized and un-optimized system was also presented. The results indicated that the exhaust gas mass flow rate had significant effects on the system performance. Integration with the on-board ORC system could effectively improve the engine power performance.The power output of the engine-ORC combined system with optimization had further improvement, and the maximum improvement could reach up to 1.16 kW.

Key words: diesel engine; exhaust gas; organic Rankine cycle(ORC); optimization

Fuel efficiency enhancement and emissions reduction of engines are becoming increasingly important. The technology of ORC system for recovering exhaust waste heat from engines has been considered as an effective method to reach energy conversion and emissions reduction.

The researches of the performance investigation of ORC systems for recovering waste heat from engines have been presented.Yang et al.[1]analyzed the thermodynamic and economic performance of an ORC system for exhaust gas waste heat recovery from a marine diesel engine. Shu et al.[2]analyzed the performance of different ORC systems for recovering exhaust heat from a large gaseous fuel engine. Song et al.[3]used an ORC system with an internal heat exchanger to recover exhaust heat from a compressed natural gas engine. Chen et al.[4]developed a confluent cascade expansion ORC system which has a simpler cycle than the conventional dual-loop ORC systems. The performance of the ORC system was evaluated under different truck engine working conditions. The results indicated that the ORC system can improve the engine thermal efficiency by 4.2%, and can increase 8% of power output than the traditional ORC system.Zhang et al.[5]analyzed the unsteady performance of an ORC exhaust heat recovery system based on the Monte Carlo simulation method. The effects of the engine power output on the ORC system performance were discussed. Michos et al.[6]analyzed the increased exhaust backpressure caused by an ORC system on performance of a diesel engine. Furthermore, Zhao et al.[7-8]evaluated the performance of a truck diesel engine with ORC system, and discussed the effects of the ORC system on the heavy-duty truck running performance.

The optimization of the ORC waste heat recovery system has been presented in many literatures.Yang et al.[9]investigated the effects of the exhaust gas temperature at the evaporator outlet including evaporation pressure, superheat degree and condensation pressure on the ORC system performance. Shi et al.[10]presented a novel design for an ORC system which was applied to recover unsteady exhaust waste heat from automotive engines. A transient control approach was developed by adjusting the exhaust gas mass flow rate and the ORC system evaporation pressure. The simulation results showed that the optimized system has higher efficiency. Amicabile et al.[11]introduced the comprehensive ORC optimization for different types of cycle layout. The results indicated that a regenerative subcritical cycle with Ethanol has the best performance. Shu et al.[12]developed a thermal oil storage/ORC system for waste heat recovery from exhaust gas, and the performance of the system was analyzed experimentally. The results showed that the thermal oil has inertia effects on performance of the waste heat recovery system under different engine conditions. Dimitrova et al.[13]presented the performance and economic optimization of an ORC system for recovering waste heat from cooling water and exhaust gas for a gasoline hybrid pneumatic powertrain. Zhang et al.[14]comparatively analyzed the thermos-economic performance of an ORC system with different heat exchangers. The system was optimized for obtaining the minimum electricity production cost.

In this study, the on-board ORC exhaust heat recovery system was optimized by adjusting the mass flow rate of exhaust gas entering the ORC system to obtain larger power output. The performance of the vehicular ORC system was evaluated at different engine working conditions through simulations. The models of the engine and the ORC system were built in GT-suite, and the combined system model was developed in the Simulink environment. The performance of the system was evaluated under different engine operating conditions. The performance of the on-board ORC system with and without optimization at the rated engine speed was comparatively analyzed in detail.

1 System Configuration and Modeling

1.1 System configuration

The engine is a six-cylinder four-stroke turbocharged diesel engine in a heavy-duty truck. The main specifications of the engine are presented in Tab.1.The mass flow rate and temperature of the exhaust gas at different engine speed and torque were measured experimentally and presented in Fig.1. The exhaust mass flow rate increases with both the engine speed and torque, and the maximum mass flow rate can be up to 0.417 kg/s. The highest exhaust temperature is 754 K at the engine speed of 1 600 r/min and the torque of 1 422 N·m.

Tab.1 Main specifications of the engine

Fig.1 Experiment results of the diesel engine

By calculation and analysis of the exhaust gas energy and exergy, it is known that the maximum exhaust energy is 372 kW at the rated engine condition and the exhaust exergy can reach up to 74 kW at anambient temperature of 298 K and anambient pressure of 99.5 kPa. The exhaust gas contained huge energy and exergy, which suggests a high potential for the heat recovery. Therefore, recovering and utilizing the exhaust heat based on ORC system can effectively improve the engine fuel economy.

Energy of the exhaust gas is calculated with

(1)

Exergy of the exhaust gas is defined as

(2)

An ORC system was developed to recover exhaust waste heat from a heavy duty engine as shown in Fig.2. The engine was integrated with the ORC system through a shell and tube evaporator. Proportional exhaust bypass valves A and B were used to adjust the mass flow rate of the exhaust gas entering the evaporator. The ORC system was cooled by an added water cooling system which is isolated from the engine cooling system. R245fa was used as the working fluid due to the high critical temperature and good environmental protection and 50% (v/v) ethylene glycol was used as the cooling liquid. A scroll expander and a diaphragm metering pump were used in the ORC system. The added water cooling system consisted a finned tube radiator, a centrifugal water pump and an electrical fan. The power of the pumps and the fan was supplied externally.

Fig.2 Schematic diagram of the engine with ORC combined system

The liquid working fluid was pumped into the evaporator and heated by the exhaust gas to produce vapor. The high temperature working fluid vapor enteredthe expander which converted heat into power. Then, the working fluid was cooled into liquid in theplate condenser and flowed into the reservoir. The liquid working fluid was pressurized by the pump again to form a cycle.

1.2 System modeling

The models of the engine/truck and the ORC/cooling system were developed in GT-suite, respectively. The models of the engine/truck have been described[15].Simple displacement models of pump and expander were selected in GT-suite libraries to simulate the working fluid pump, the water pump and the expander. The volumetric flow rate and the specific enthalpy change of fluid through the pumps and the expander were calculated with a constant displacement, isentropic efficiency and volumetric efficiency.The exhaust gas temperature and mass flow rate were calculated by the engine model and transmitted to the ORC system model. The exhaust gas pressure at the point “6” (as shown in Fig.2) was transmitted to the engine model as the backpressure. The face velocity of the radiator in this cooling system was equal to the truck speed. The ORC system operating condition was adjusted through the pump speed and the expander speed. The cooling system condition was controlled with the fan speed and the water pump speed. The data was passed between the GT-suite models by every 0.01 s in the Simulink environment to improve the calculation accuracy.

The engine model was validated experimentally, and the comparison results of exhaust gas temperature and mass flow rate between simulation and experiment are presented in Fig.3a. The model of a plate condenser in a medium temperature ORC system which has been described in Ref. [16] was validated. The condenser model was also validated. The results of the working fluid temperature at the condenser outlet under different operating conditions were compared between simulation and experiment as shown in Fig.3b. The models were confirmed with small errors and can be used to evaluate the performance of the vehicular exhaust heat recovery system.

Fig.3 Validation results of the GT-suite models

2 Performance Evaluation and Optimization of the Combined System

The performance of the engine with ORC combined system was evaluated through simulations. The comparison analysis of the performance of the combined system with and without optimization was presented.The engine was operated at different engine speeds and torque, and the power output of the ORC system was used to drive the engine. The operating conditions of the ORC system and the cooling system were constant. The net power output change, BSFC reduction, backpressure change of the engine, and the ORC system thermal efficiency were analyzed.

Backpressure change of the engine with ORC system is

ΔP=Pew-Pewo

(3)

Power output reduction of the engine in the combined system is

(4)

Power output of the ORC system is

(5)

Net power output change of the combined system is calculated with

(6)

BSFC reduction of the engine is defined as

(7)

Thermal efficiency of the ORC system is

(8)

3 Results and Discussion

3.1 Performance evaluation of the combined system with different exhaust pipe opening

The performance of the combined system was evaluated through controlling the openings of the valves to change the mass flow rate of the exhaust gas. The opening range of valve A was 5%-100%, and valve B was fully opened.The engine was operated at 2 100 r/min under different torque conditions, and the truck speed was 95 km/h.

The mass flow rate of the exhaust gas entering the evaporator decreases with the increasing ofthe valve opening, as shown in Fig.4a. The mass flow rate of the exhaust gas passing through the ORC system is 0.25 kg/s when the valve A was fully opened, and it can be further reduced through decreasing the opening of the valve B. Fig.4b presents the backpressure of the exhaust gas at different locations. The exhaust backpressure at the valve A decreases with the increasing of the opening. This is because the volume flow rate decreases gradually, although the exhaust gas mass flow rate increases. The exhaust backpressure at the valve B decreases with the decreasing of the exhaust gas mass flow rate. Therefore, the exhaust backpressure at the turbine outlet (point 6) decreases.

Fig.4 Variations of the exhaust mass flow rate and backpressure

The exhaust gas mass flow rate was controlled with an internal increment of 0.01 kg/s. The net power output improvement of the combined system at different engine torques was presented in Fig.5. The system obtained the maximum power output increment under the condition points which were filled with black as shown in Fig.5. The exhaust gas mass flow rate increases with the increasing of engine torque as shown in Fig.1a. The net power output increment increases as the exhaust gas mass flow rate grows for the engine torque lower than 992 N·m, which increases firstly and then decreases with the increasing of exhaust gas mass flow rate as the engine torque increases. This is due to the limited cooling capacity of the cooling system and the gradual increase of the exhaust heat, causing both the expansion ratio of the working fluid and the output power of the ORC system to decrease gradually. As shown in Fig.5, the suitable mass flow rate of exhaust gas for the on-board ORC system decreases with the increasing of the engine torque. When the engine was operated at 1 168 N·m, the optimization exhaust gas mass flow rate was 0.35 kg/s, and the rest exhaust gas should be released to the environment through the valve A. Therefore, the combined system should be optimized by controlling the mass flow rate of the exhaust gas entering the ORC system.

Fig.5 Net power output improvement at different engine torque conditions

3.2 Optimization analysis of the combined system

Fig.6 Performance of the combined system with and without optimization

The performance change between the optimized and un-optimized combined system at the engine speed of 2 100 r/min was discussed in detail. The comparison results are presented in Fig.6.

The backpressure changes of the optimized and un-optimized combined system are described in Fig.6a. In the un-optimized system, the mass flow rate and backpressure of the exhaust gas increase gradually as the engine torque increases.Therefore, the power output of the engine in the combined system decreases due to the higher backpressure,while the backpressure of the optimized system was significantly decreased at high load conditions, due to the decrease of mass flow rate of the exhaust gas entering the evaporator. Therefore, the power output reduction of the engine in the combined system was decreased according to Eq.(4). Fig.6b shows the changes of the ORC system output power. In the un-optimized system, the power output increases firstly and then decreases with the increasing of the engine torque according to Fig.5, while the output power of the ORC system has obvious increment in the optimized combined system. This is because the condensation pressure of the working fluid decreases as the exhaust gas mass flow rate decreases at the constant operating condition of the cooling system causing the expansion ratio and power output to increase. The power output of the un-optimized and optimized on-board ORC system are 4.09 kW and 5.24 kW, respectively. Therefore, the maximum power output of the ORC system can be further improved by 1.15 kW at the engine torque of 1 168 N·m.

The net power output improvement of the engine with un-optimized and optimized on-board ORC system under full engine operating conditions was further analyzed, and the comparison results are presented in Fig.7. The engine power output improvement increases with the increasing of engine speed and torque when the engine operated with low speeds. The improvement of engine output power has remarkable increase at high engine speed conditions for the optimized system. Therefore, integration with the optimized on-board ORC system can effectively improve the engine power performance.

Fig.7 Net power output improvement of the engine at different operating conditions

4 Conclusions

The engine with ORC combined system was optimized, and the performanceof the combined system at different operating conditions was evaluated through simulations in this paper.The main conclusions are listed as follows.

Integration with the on-board ORC system can effectively improve the engine power performance, especially if integrated with the optimized system which can further improve the engine power output under high engine load conditions.Compared with the previous system, the optimized combined system has lower exhaust backpressure change, higher net power output improvement and larger ORC system thermal efficiency. The maximum power output improvement of the un-optimized systemis 2.78 kW, and the optimized system has further improvement of 1.16 kW.