Thermoelectric conversion of waste heat from IC engine‐driven vehicles: A review of its application,issues, and solutions

Thermoelectric generator (TEG) is a promising thermoelectric (TE) conversion technology to effectively recover and convert waste heat from vehicle exhaust into useful energy, ie, electricity. Exhaust TEG (ETEG) is a system that is incorporated into the exhaust manifold of a vehicle. Exhaust TEG comprises of a heat exchanger, TEG modules, heat sink, and power conditioning unit. The present work reviews different vehicular ETEGs based on engine type, engine‐rated power, type and number of TEG module, efficiency of ETEG and TEG, exhaust and coolant temperature, and power output of ETEG . In addition to these, the technical issues faced in these ETEGs are addressed under 2 categories, viz., primary (TEG with low ZT TE material and inefficient heat exchanger and heat sink) and secondary issues (low operating temperature TEG modules and installation position of ETEG). In addition to it, effects of vibration and thermal cycling of exhaust system on TEG modules that may arise in ETEG are also discussed. A review of preventive solutions to the issues is also presented. Finally, the economic aspects of an ETEG are also discussed. The review highlights the need of commercialization of TE materials with ZT > 2, high‐temperature operating range, and segmented TEG modules in large volumes so that their practice can be extended in vehicular applications. Heat exchanger modeling using computational fluid dynamics and interfacing with heat transfer theory is essential to maintain temperature uniformity across the TEG modules. Installation of ETEG in the exhaust pipe should be such that it does not affect the performance of the engine. It is also realized that sturdy TEG modules should be developed for long‐term operation to prevent degradation due to mechanical vibration and thermal cycling of the vehicle. Further, ETEG is economically beneficial in vehicles such as trucks owing to availability of high thermal energy in their exhaust stream.     

Powering microbial electrolysis cells by electricity generation from simulated waste heat of anaerobic digesters using thermoelectric generators

Waste heat from anaerobic digesters can be converted to electricity by using thermoelectric generators (TEG). Herein, such energy was employed to power a microbial electrolysis cell (MEC) for producing hydrogen gas. Four TEG units could deliver a voltage of ~0.5 V, sufficient to drive the MEC that achieved a hydrogen production rate of 0.48 ± 0.13 m³ m⁻³ d⁻¹. This rate was further improved to 0.75 ± 0.05 m³ m⁻³ d⁻¹ when the temperature difference for TEG was increased from 18 to 28 °C. There was no significant difference between the TEG-powered MEC and power supply-supported MEC (at 0.6 V), in terms of current generation, hydrogen production, and organic removal. Ambient air was also studied as a cold-side source for TEG, although some challenges were encountered to maintain a large temperature difference. Those results will encourage further exploration of using TEG as a feasible power supply for sustainable MEC operation.     

An experimental investigation on electrical performance and characterization of thermoelectric generator

Thermoelectric generator (TEG) is an attractive renewable energy source that utilizes waste heat energy from various sources to produce electricity. In this paper, a novel method has been proposed to investigate and characterize the TEG module by determining the exact maximum power point (MPP) and estimate the TEG module dynamic parameters. A DC-DC boost converter with a simple control method is proposed to obtain I-V and I-P characteristics and extract the exact MPP of the TEG module. The electrical performance and dynamic parameters such as Seebeck coefficient and internal resistance of the TEG module have been estimated at the MPP. MATLAB software package is used to model and simulate the complete system. Then, by using a low-cost Arduino microcontroller and a standard TEG module (HZ-14HV), a test rig is built to examine the electrical performance of the TEG. A comparison among simulation, experimental, and manufacturing data sheet is done to validate the accuracy of the proposed system. The proposed system tests and characterizes a TEG module at five values of temperature differences in the range from 377 K to 457 K and analyses the MPP generated in the range from 5 to 13.8 W . The obtained results confirm that there is good agreement among the simulation, experimental, and data sheet. To the best of our knowledge, it is the first time to use the boost converter to estimate the electrical characteristics and the dynamic parameters of the TEG. It can be said that the proposed DC-DC boost converter with the suggested control method is helpful for testing commercial TEG modules before implementing in a TEG system.     

A review and selection of engine waste heat recovery technologies using analytic hierarchy process and grey relational analysis

In this paper, a brief review and comparison of the engine waste heat recovery technologies have been made. These five technologies are electric turbocompounding systems (ETC), thermodynamic organic Rankine cycle (ORC), thermoelectric generators (TEG), hydrogen generation by using exhaust gas heat energy, and hybrid pneumatic power systems (HPPS). According to comparison results, the HPPS system can achieve the highest fuel economy improvement among the five technologies. Though there are their own benefits by utilizing these different technologies, their disadvantages prevent the application of these advanced technologies to different extent. Besides, a combined evaluation method consisting of grey relational analysis and analytic hierarchy process has been applied to assess the five new engine waste heat recovery technologies from the perspective of technical, economic, and environmental aspect. Based on the final results of the new evaluation method, the HPPS was found to be the most promising WHR technology for vehicle engines. But because of the emphasis on economic benefit, TEG was found to be more favorable for working conditions, like power plant and marine engine. What is more, as is shown in the sensibility analysis, the weighing of the environment relevant factors can prominently influence the comparison results between ETC and HPPS. Copyright © 2014 John Wiley & Sons, Ltd.     

Liquid metal based thermoelectric generation system for waste heat recovery

A new type of thermoelectric generator (TEG) system based on liquid metal which serves to harvest and transport waste heat, is proposed in this paper. To demonstrate the feasibility of the new TEG system, an experimental prototype which combined commercially available thermoelectric (TE) modules with the electromagnetic pump was set up. Output voltage from TE modules and temperature changes of the main parts (waste heat source, liquid metal heating plate, water-cooling plates I and II) of the liquid metal based TEG system were experimentally measured, as well as the flow rate of cooling water and the load resistance. It was shown that the maximum open-circuit voltage of 34.7 V was obtained when the temperature of the waste heat source was 195.9 °C and the temperature gap between liquid metal heating plate and cooling-water plates was nearly 100 °C. These experimental results obviously verify that using liquid metal based TEG system for waste heat recovery is highly feasible. In addition, the TEG system performance is discussed and a calculated efficiency of 2% in the whole TEG system is obtained. Possible suggestions to further improve this type of generator in the future are given.     

Application of biochar derived from used cigarette filters in direct carbon solid oxide fuel cell

As a typical waste, used cigarette filters (UCFs) are difficult to biodegrade and harmful to the environment. The direct carbon solid oxide fuel cell (DC-SOFC) is an energy conversion device that can utilize carbon directly, including biochar, as fuel. We report a superior DC-SOFC powered by Fe-loaded UCF biochar in this paper. The microstructure and composition are characterized, indicating that the UCF biochar is micron-sized and contains metal elements such as K and Ca that are beneficial to the performance of DC-SOFC. The peak power density of the cell fueled by pure UCF biochar is 308 mW cm^?2 and increases to 341 mW cm^?2 after loading Fe as the catalyst, which is comparable to that of the cell with Fe-loaded activated carbon (368 mW cm^?2). It proves the feasibility of the UCF biochar as fuel for DC-SOFCs, providing a theoretical basis and technical demonstration for the disposal and transformation of solid waste.     

Integration of molten carbonate fuel cell and looped multi-stage thermoacoustically-driven cryocooler for electricity and cooling cogeneration

Thermal management is essential for high-temperature molten carbonate fuel cell (MCFC) because the accumulated waste heat may degrade the durability. In this paper, looped multi-stage thermoacoustically-driven cryocooler (LMTC) is proposed to reuse the waste heat from MCFC for cooling production, which not only can tackle with the thermal management issue but also can provide additional usages. Accounting various irreversible dissipation, the models of MCFC, LMTC and MCFC-LMTC hybrid system are analytically formulated. Performance features of MCFC-LMTC hybrid system are revealed and the advantages are expounded via calculation examples. Calculations indicate that the maximum power density and corresponding efficiency of the hybrid system are 1688.9 W m^?2 and 39.7%, which are 11.4% and 1.3% bigger than that of the sole MCFC system, respectively. By comparing with other available systems, the superiority of using LMTC to recover MCFC waste heat for refrigeration is clearly demonstrated. Considerable parametric studies show that the heat-transfer coefficient of hot heat exchange for LMTC is not suggested to be greater than 2.5 × 10^?3 W m^?2 K^?1. In addition, an increase in the working temperature, working pressure of MCFC, reactant concentration or engine stage number of LMTC positively benefits the hybrid system performance, while an increase in the thermodynamic loss coefficient worsens the hybrid system performance. The obtained results may offer new insights into improving the performance of MCFCs through thermal management approaches.     

Novel hybrid system of pulsed HHO generator/TEG waste heat recovery for CO reduction of a gasoline engine

Environmental crisis requires using cleaner energy sources for different sectors including the transportation. Hydrogen can support the transition of the automotive industry from petrol and diesel into a sustainable fuel. It could be the main source of energy or the auxiliary fuel in vehicles. As an auxiliary fuel, it has recently been considered in hydroxyl (HHO) form for reducing the emissions from transportation fleet. In this study, an HHO generator with the optimum power consumption was utilised for HHO injection into the intake manifold of a petrol engine as the case study. High concentration of CO is expected to be produced during idling, so the experiments were designed to inject ultra-low HHO for reducing CO emissions. The results were very promising, and it was shown that the CO emission could be reduced by about 98%. Furthermore, a novel design was developed based on the concept of waste heat recovery (WHR) for powering the HHO unit. Engine was simulated in AVL software to design a thermo-electric generators (TEG) for running the HHO unit. Based on the results, TEG can provide the energy required for HHO unit as the energy output of the TEG was between 91 kJ and 169 kJ for the case study while the energy consumption of the proposed HHO generator was just about 22.5 kJ. The results of this study are recommending a practical solution for bringing HHO injection from laboratory research into the real practice.     

Design and energy efficiency analysis of a pure fuel cell vehicle for Shell eco racer

The target of Shell Eco‐marathon competition of vehicle is to drive a fixed distance with the lowest quantity of fuel. To win the competition, the fuel cell‐powered propulsion system needs to be ultra efficient since the fuel cell system and transmission system are the key effects on the performance of the fuel cell‐powered propulsion system. In this study, a high‐efficiency fuel cell propulsion system has been designed and integrated in a prototype vehicle to participate the Shell Eco‐marathon Asia 2018 race. To achieve that, the vehicle dynamic is modeled to make the selection of the key components, and some experiments have been conducted to obtain the properly vehicle driving strategy. Based on the results of vehicle dynamic analysis, a high specific power proton‐exchange membrane fuel cell (PEMFC) stack with 1000 W and a high‐performance direct current (DC) brushless motor (1000 W) are selected to build the propulsion system of the Shell Eco‐marathon vehicle. Based on the experimental result, the racing time (1300‐1440 seconds) and varied range of racing speed (23‐27 km/h) are selected as the driving strategy. Finally, the efficiency of the fuel cell‐powered vehicle is analyzed. In the race at the year of 2018, the designed vehicle won the first place.     

An efficient high-temperature PEMFC/membrane distillation hybrid system for simultaneous production of electricity and freshwater

A new hybrid system model mainly consisting of a high-temperature proton exchange membrane fuel cell and a direct contact membrane distillation is proposed. According to thermodynamics and electrochemistry theories, key performance indicators of the proposed system are formulated, from which feasibilities and effectiveness as well as performance features of the proposed hybrid system are verified. Numerical calculation results indicate that the hybrid system's maximum power density, the according energetic efficiency and exergetic efficiency are, respectively, 5580.3 W m^?2, 49.4% and 23.3%, which are, respectively, 51.8%, 110.9% and 112.0% greater than that of a single fuel cell system. Simultaneously, the corresponding exergy destruction rate density is decreased by 28.1%. Direct contact membrane distillation can be regarded as an efficient waste heat recovery technology. Furthermore, extensive parametric studies show that feed water temperature, flow velocities of both feed water and permeate water, convective heat transfer coefficients of both feed side and permeate side as well as porosity of hydrophobic membrane have positive effects on the hybrid system performance, while hydrophobic membrane thickness and permeate water temperature have negative effects on the hybrid system performance. The results obtained may be beneficial to design and run such a real hybrid system.     

Road testing of a three-wheeler driven by a 5 kW PEM fuel cell in the absence and presence of batteries

The road testing and demonstration of a three-wheeler vehicle driven by a 5 kW proton exchange membrane fuel cell (PEMFC) was carried out in the absence and presence of lead acid batteries. Prior to integrating the PEMFC module and batteries in the three-wheeler, they were tested and demonstrated separately. The PEMFC module had a very fast response as the load was manually or, especially, automatically changed and it could supply a continuous power when the reactant was supplied continuously. In contrast, the 5 kW lead acid batteries alone could supply power for no longer than 300 s. In the presence of both the PEMFC module and batteries, when the drawing power was in the range of the PEMFC module capacity the propulsion motor gained its energy from the PEMFC module only, whilst the stack power output at all conditions was greater than the setting power of approximately 400 W. After integrating the PEMFC module and batteries into the three-wheeler, both energy sources were found to power the vehicle effectively. The motor power as well as the stack power changed as a linear proportion to the throttle. The motor consumed more power in case of high speed driving, take off or hill climbing, while it used only 0.354 kW in the absence of throttle. The hybrid system can achieve a maximum speed in this three-wheeler of around 24.9 km/h with a hydrogen consumption of 11 g H₂/km (71 g H₂/kWh) and an operating cost of 1.99 USD/km. The thermodynamic efficiency of the vehicle was 42.9%.     

Tracking maximum efficiency of hydrogen genset used as electric vehicle range extender

This paper investigates a hydrogen-based genset maximum efficiency tracking problem in the context of electric vehicle range extension. This genset is cheaper than fuel cells and has the desirable property of being greenhouse gas emission free in addition to being less pollutant than the conventional gasoline based gensets. Using Taylor's series, a parametric efficiency model is built iteratively. This model is used by a nonlinear optimization method which searches for the optimal operating conditions for a maximum achievable efficiency. The root-mean-square-error between experimental data and the model is less than 5 × 10⁻⁴. The hardware-in-the-loop simulation demonstrated that the proposed tracking approach is effective. In addition, it can improve the hydrogen-based genset efficiency up to 7.15% compared to the commonly used industrial method based on a constant speed drive approach.     

A theoretical study of molten carbonate fuel cell combined with a solar power plant and Cu–Cl thermochemical cycle based on techno-economic analysis

A novel power and hydrogen coproduction system is designed and analyzed from energetic and economic point of view. Power is simultaneously produced from parabolic trough collector power plant and molten carbonate fuel cell whereas hydrogen is generated in a three-steps Cu–Cl thermochemical cycle. The key component of the system is the molten carbonate fuel cell that provides heat to others (Cu–Cl thermochemical cycle and steam accumulator). A mathematic model is developed for energetic and economic analyses. A parametric study is performed to assess the impact of some parameters on the system performance. From calculations, it is deduced that electric energy from fuel cell, solar plant and output hydrogen mass are respectively 578 GWh, 25 GWh and 306 tons. The overall energy efficiency of the proposed plants is 46.80 % and its LCOE is 7.64 c€/kWh. The use of MCFC waste heat allows increasing the solar power plant efficiency by 2.15 % and reducing the annual hydrogen consumption by 3 %. Parametric analysis shows that the amount of heat recovery impacts the energy efficiency of fuel cell and Cu–Cl cycle. Also, current density is a key parameter that influences the system efficiency.     

Introducing an integrated SOFC,linear Fresnel solar field,Stirling engine and steam turbine combined cooling,heating and power process

A new integrated combined cooling, heating and power system which includes a solid oxide fuel cell, Stirling engine, steam turbine, linear Fresnel solar field and double effect absorption chiller is introduced and investigated from energy, exergy and thermodynamic viewpoints. In this process, produced electrical power by the fuel cell and steam turbines is 6971.8 kW. Stirling engine uses fuel cell waste heat and produces 656 kW power. In addition, absorption chiller is driven by waste heat of the Stirling engine and generates 2118.8 kW of cooling load. Linear Fresnel solar field produces 961.7 kW of thermal power as a heat exchanger. The results indicate that, electrical, energy and exergy efficiencies and total exergy destruction of the proposed system are 49.7%, 67.5%, 55.6% and 12560 kW, respectively. Finally, sensitivity analysis to investigate effect of the different parameters such as flow rate of inputs, outlet pressure of the components and temperature changes of the solar system on the hybrid system performance is also done.     

Design of a radial vanadium redox microfluidic fuel cell: A new way to break the size limitation

Microfluidic fuel cell (MFC) suffers from small single cell output power due to the inherent cell size limitation as microscale geometries are prerequisite to prevent reactant crossover between the anode and cathode. To meet the power demand of practical applications, previous works mainly focus on the creating of MFC stacks with multiple cells connected in series, parallel, or mixture of both series and parallel to increase the output power. Yet, low energy efficiency is observed because of the flow distribution nonuniformity and shunt current losses. In this work, a high performance radial vanadium redox MFC is presented to address the size limitation issue by adding a separate layer between the porous electrodes of the conventional plate‐frame MFC. Specific cell characteristics are detailed by mathematical modeling, and parametric studies are performed to evaluate the influences of the geometrical and operational parameters on the cell performance. The results show that this new radial MFC can provide a higher fuel utilization and meanwhile an improved cell performance under a fixed electrode size compared with the conventional plate‐frame MFC. Moreover, the electrode size limitation due to the reactant crossover between the anode and cathode is broken as the influences of the electrode size on the mixing region are greatly reduced. In the case with the electrode size equal to 18 mm × 18 mm , single cell output power of 0.35 mW with a fuel utilization of 53.33% is obtained under the reactant concentration of 2 mol L^?1 and flow rate of 300 μL min^?1 .     

Performance simulation of a low‐swirl burner for a Stirling engine

A new type of gas burner for Stirling engine that can recover adequate heat from exhaust gas was designed based on the plate heat exchanger and low‐swirl combustion technology, which consists of three components: a cyclone, a burner, and a circular plate heat exchanger. The circular plate heat exchanger tightly wound around the combustion chamber plays a high efficiency of heat recovery role. In consideration of the radial symmetry of the burner, a three‐dimensional numerical simulation was carried out by Ansys15. The velocity distribution, temperature distribution, and pressure distribution of the combustion gas were presented respectively. Strong backflow that came from the exhaust gas around the root of the flame in the combustion chamber and a vortex below the inlet of the exhaust gas channel were found, which were beneficial for the combustion and improving the uniformity of temperature distribution. Combustion behaviors of the burner under standard operating conditions were obtained, the highest temperature was about 2200 K in burner and the exhaust gas entered the plate heat exchanger at the temperature of 1375 K and exited at 464 K, with the waste heat recovery efficiency over 65.8%. And, the air‐fuel ratio and combustion power had negligible effect on the waste heat recovery efficiency.     

Methanol decomposition fuel processor for portable power applications

A new fuel processor approach for portable fuel cell power sources significantly improves upon microreformers by overcoming the difficulties with heat deficiencies and contaminants in the product hydrogen. Instead of reforming, the processor uses methanol decomposition to enable the byproduct, carbon monoxide (CO), to be used as the heat source. A hydrogen permselective membrane segregates the CO for combustion in an integrated burner, maximizes the decomposition conversion, and provides pure hydrogen for a fuel cell. Discharging the CO-rich retentate through an ejector to draw combustion air into the burner greatly simplifies the system. High and stable hydrogen yields are attained with optimized catalysts and fuel compositions. The resultant simple, efficient, and self-heating processor produces 85% of the hydrogen content of the fuel. A 20 W autonomous power source based on this novel fuel processor demonstrates a fuel energy density >1.5 Wh g^?1_(electrical), nearly twice as high as microreformer power sources.     

Experiments and simulations on low-temperature waste heat harvesting system by thermoelectric power generators

In this case study, a system to recover waste heat comprised 24 thermoelectric generators (TEG) to convert heat from the exhaust pipe of an automobile to electrical energy has been constructed. Simulations and experiments for the thermoelectric module in this system are undertaken to assess the feasibility of these applications. A slopping block is designed on the basis of simulation results to uniform the interior thermal field that improves the performance of TEG modules. Besides simulations, the system is designed and assembled. Measurements followed the connection of the system to the middle of an exhaust pipe. Open circuit voltage and maximum power output of the system are characterized as a function of temperature difference. Through these simulations and experiments, the power generated with a commercial TEG module is presented. Overview this case study and our previous work, the results establish the fundamental development of low-temperature waste heat thermoelectric generator system that enhances the TEG efficiency for vehicles.     

Performance of a hybrid direct ethylene glycol fuel cell

In this work, a hybrid fuel cell is developed and tested, which is composed of an alkaline anode, an acid cathode, and a cation exchange membrane. In this fuel cell, ethylene glycol and hydrogen peroxide serve as fuel and oxidant, respectively. Theoretically, this fuel cell exhibits a theoretical voltage reaching 2.47 V, whereas it is experimentally demonstrated that the hybrid fuel cell delivers an open‐circuit voltage of 1.41 V at 60°C. More impressively, this fuel cell yields a peak power density of 80.9 mW cm^?2 (115.3 mW cm^?2 at 80°C). Comparing to an open‐circuit voltage of 0.86 V and a peak power density of 67 mW cm^?2 previously achieved by a direct ethylene glycol fuel cell operating with oxygen, this hybrid direct ethylene glycol fuel cell boosts the open‐circuit voltage by 62.1% and the peak power density by 20.8%. This significant improvement is mainly attributed not only to the high‐voltage output of this hybrid system design but also to the faster kinetics rendered by the reduction reaction of hydrogen peroxide.     

Feasibility study and techno-economic analysis of an SOFC/battery hybrid system for vehicle applications

A feasibility study and techno-economic analysis for a hybrid power system intended for vehicular traction applications has been performed. The hybrid consists of an intermediate temperature solid oxide fuel cell (IT-SOFC) operating at 500–800 °C and a sodium–nickel chloride (ZEBRA) battery operating at 300 °C. Such a hybrid system has the benefits of extended range and fuel flexibility (due to the IT-SOFC), high power output and rapid response time (due to the battery). The above hybrid has been compared to a fuel cell-only, a battery-only and an ICE vehicle. It is shown that the capital cost associated with a fuel cell-only vehicle is still much higher than that of any other power source option and that a battery-only option would potentially encounter weight and volume limitations, particularly for long drive times. It is concluded that increasing drive time per day decreases substantially the payback time in relation to an ICE vehicle running on gasoline and thus that the hybrid vehicle is an economically attractive option for commercial vehicles with long drive times. In the case where the battery has reached volume production prices at £70 kWh⁻¹ and current fuel duty values remain unchanged then a payback time <2 years is obtained. For a light delivery van operating with 6 h drive time per day, a fuel cell system model predicted a gasoline equivalent fuel economy of 25.1 km L⁻¹, almost twice that of a gasoline fuelled ICE vehicle of the same size, and CO₂ emissions of 71.6 g km⁻¹, well below any new technology target set so far. It is therefore recommended that a SOFC/ZEBRA demonstration be built to further explore its viability.