Parametric and exergetic analysis of waste heat recovery system based on thermoelectric generator and organic rankine cycle utilizing R123
Highlights
► Development of a TEG-ORC system using R123 as working fluid for WHR of engines. ► Performance of the developed cycle was investigated theoretically. ► Optimization of configurations and parameters can be obtained. ► Irreversibility in the evaporator, turbine, IHE, condenser, pump and TEG is revealed. ► Optimal net power and indicated efficiency is 27 kW and 45.7%, respectively.
Introduction
With the rapid development of population and vehicle industry in the world during the past 20th century, the demand on passenger vehicles has increased sharply. Recent studies [1], [2] indicate that, only 41% of a class-8 truck diesel engine's fuel combustion energy is converted into useful work to drive a vehicle and its accessory loads. The remainder is engine waste heat dissipated by engine exhaust system (20%), coolant system (18%), EGR (exhaust gas recirculation) cooler (11%), CAC (charge air cooler) (9%), and convection as well as radiation from engine block. This increases fuel consumption, which rises almost exponentially due to low engine efficiency and increasing demand on vehicles, brings serious energy crisis and has environmental effects. If this waste heat of engines could be recaptured efficiently, engine output power will be significantly enhanced without additional fuel consumption. Thus, large amount of fossil fuel can be saved [3], [4] and much less harmful exhaust gases are dissipated to ambient environment when the same output power is generated [5], [6]. Furthermore, global warming will be relieved. However, we should notice that the potential energy savings from improved energy efficiency are estimated using basic physics principles and engineering models. The actual energy savings from such improvements generally falls short of such estimates due to the rebound effect [7]. A possible explanation for this phenomenon could be that such improvements encourage the consumption of energy services where part or all of the gain would be offset by the increase in consumption [8]. Since the price of fossil fuel rises greatly due to serious energy crisis, renewable clean energy will play an important role in the future. However, with plenty of current technical obstacles, renewable technology cannot be widely applied to vehicle industry within short term. In other words, it is a considerable solution to improve engine efficiency via waste heat recovery system [3]. In particular, it has considerable potential environmental and economic benefits. Thus, many researches and projects focus on enhancing engine performance and thermal efficiency, aiming at lowering fuel requirement and exhaust that are harmful to human body. Converting exhaust heat to electricity by ORC (organic rankine cycle) is an interesting and actual avenue among the ways of recovering the waste heat [5], [9].
Explosive development of ORC technology has been achieved in the area of WHR (waste heat recovery) of low temperature/grade heat sources, such as geothermal sources [10], [11], [12], solar energy [13], [14], [15], [16], bio-fuel electricity production plants [17], [18], [19], [20] and vehicle exhaust gases [1], [3], [5], [21] during the last one hundred years. When it comes to engines, some obstacles should be cleared away before ORC technology can be applied to vehicle engines. Compared with the other two sources of low temperature, the temperature of IC engine exhaust gas is much higher, even than the decomposition temperature of most organic fluids, which generally are kept below 623 K. Thus, when organic rankine cycle technology is applied to IC engines, temperature difference between exhaust gas and organic working fluid becomes much greater. Working fluid may resolve due to high exhaust temperature, and then prevent WHR system from working fluently and safely. Previous studies [22], [23] indicate that using water only as working medium for steam rankine cycle is one of the potential solutions of overcoming high exhaust temperature. Since decomposition temperature of water is up to 2273 K, this could avoid working fluid resolving but lead to a lower thermal efficiency and large system size and weight. In the 1970s, a steam rankine cycle system is applied to a 288-horsepower truck engine to recapture energy from exhaust gases by the Mack Trucks company. The actual bench test indicates that an increase of substantial fuel efficiency has been achieved [22]. However, it produces much lower power output and the devices are huge and heavy, so it is of less practical value. Thus, steam rankine cycle is not suitable to gain higher thermal efficiency, although it provides security for vehicle engines. Another solution is to lower exhaust gas temperature to guarantee working fluid work properly with high temperature exhaust gas, and this would waste part of total energy to make it adaptable to ORC system when exchanging heat with high temperature gases. Karellas et al. [10] proposed the case that exhaust exchanged heat firstly with thermal oil, and then the heated oil conducted heat to organic fluids to provide heat supply for ORC system. They also simulated and analyzed of ORC system with oil circulation to select the best refrigerant to drive thermodynamic cycle. They focused on system parameters optimization and redistributed the energy in the system. However, the studies show that these cases mentioned above cannot take full advantage of the high-grade waste heat of exhaust gas. Since the former case has low efficiency but needs large devices, it cannot be widely used in actual vehicle applications; and exhaust temperature of the latter case is much lower and a large portion of the exhaust energy is dissipated to ambient environment. It makes output power generated in bottoming cycle be greatly reduced and thermal efficiency get much lower, while oil circulation needs a secondary heat exchanger, which increases irreversible loss.
TEG (thermoelectric generator) has become another emphasis in the area of waste heat recovery in recent years [24], [25], [26]. But there hasn't been an abundance of literature and researches about using TEG to recover waste heat from automotive applications. That's because the conversion efficiency is low and the cost of thermoelectric material is high. But as the price of fossil fuel goes higher, and due to recent advancements in nanotechnology and semiconductor physics, more dedicated companies and government agencies contribute to improving and implementing the thermoelectric technology. As thermoelectric generator directly converts heat into electricity without moving parts and environmental side effects, and much less components are needed, thus less packaging and weight constraint compared with rankine cycle system, TEG method has become a renewed emphasis on vehicle applications in recent years with current nanotechnology applied in this area [27]. And it is interesting that Miller [28] tried to combine both TEG and ORC to recover heat from engines. Further researches [29], [30] indicate a bottoming system combining TEG and ORC has potential for the area of waste heat recovery from engine exhausts.
There are a lot of difficulties in waste heat recovery from passenger vehicle engines. The most important one is that, high exhaust temperature would lead to great difference between refrigerant and exhaust gases. However, the maximum temperature of organic working fluid is much lower and would resolve if the temperature goes higher than decomposition temperature. Thus it's difficult to combine traditional ORC systems with heavy-duty vehicle engines and make full use of exhaust heat. Therefore, a system utilizing both TEG and ORC technologies, which would be called TEG-ORC System in the later sections, has been adopted to recapture power from exhaust gases in this paper. The working fluid won't resolve and additional power could be recovered, thus it effectively broadens the range of applications for ORC systems. In the bottoming cycle, the hot side of TEG would exchange heat directly with the original high temperature exhaust gases coming out from supercharger, while TEG releases a large portion of heat absorbed from exhaust into the working fluid at the cold joint, so the temperature of the working fluid becomes higher. Then the exhaust with lower temperature supplies the working fluid in the evaporator with adequate heat during evaporation and superheat processes. Besides, the optimization of the ORC working parameters is concluded in this paper. The combined TEG-ORC system is established on a diesel engine, and the computational model of analysis is developed using MATLAB/SIMULINK. The performance of the system has been simulated and analyzed to offer data supply for further test researches and vehicle applications.
Section snippets
The topping ICE system
The diesel cycle of a commercial engine is considered as topping cycle [5]. The engine is an inline 6-cylinder 4-stroke supercharged diesel engine, and the main parameters of the engine are presented in Table 1. As the aim of the analysis in the study is to obtain parameters optimization for ORC use in exhaust heat recovery of ICE, we assume the engine to operate under rated conditions. It has been calculated that air fuel ratio is 28.59 and excess air coefficient is 2 under nominal conditions
Analyses of thermodynamic processes
Fig. 3a and b illustrates the thermodynamic processes under subcritical and supercritical conditions separately. To describe the thermodynamic processes, R123 is selected, while other candidate refrigerants show similar trends. The theoretical cycle consists of the following processes:
Under both subcritical and supercritical conditions (pevap = 3 MPa/5.5 MPa) 1–2: Expansion (expander) 2–3: Isobaric internal heat exchange (recuperator) 3–4: Isobaric heat rejection (condenser) 4–5: Compression
Thermal modeling of the comprehensive system
In the study, all the calculations and evaluations of the ideal cycles are based on optimized mass flow rate of working fluids where gains maximum net output power. To make the analyses simple and clear, some assumptions are made:
- (1)
Flow directions of working fluids in recuperator and evaporator are countercurrent, while in TEG are parallel current, and leakage of heat in recuperator and condenser is ignored;
- (2)
The internal resistances in heat exchangers are negligible and the condensation and
Results and analysis
Compared with solar energy and geothermal resource, the heat capacity of exhaust gases is limited, thus the net system output is much more important and attractive than the output per unit of mass flow rate of R123, and system weight and size must be controlled to reduce load and save space when WHR system is applied to vehicle engines. Thus the net output power of this system wnet and volumetric expansion ratio v2/v1, which determines system performance and the size of turbine expander, were
Summary/Conclusions
TEG-ORC system has many benefits compared with the ORC only system in recovering waste heat of engine exhaust gases. The limitation of fluid resolving due to high temperature exhaust is overcome. An energetic and exergetic calculation is conducted for the TEG-ORC system. The effects of relative TEG flow direction, TEG scale, the highest temperature, condensation temperature, evaporator pressure and efficiency of internal heat exchanger (IHE) on the system performance are investigated, and they
Acknowledgments
The authors acknowledge the system parameters, experimental results and financial support provided by the State Key Laboratory of Engines, Tianjin University, P.R. China. This work was supported by a grant from the National Basic Research Program of China (973 Program) (No. 2011CB707201)
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