Experimental and theoretical study of thermoelectric generator waste heat recovery model for an ultra-low temperature PEM fuel cell powered vehicle
Introduction
Maximizing energy utilization has become a major concern for global sustainability. Managing the usage of energy effectively at each step of a process would lead to significant savings. The approach of waste heat recovery positively contributes to the sustainability and cost reduction through greater fuel utilization. Extensive applications and research in waste heat recovery technology for energy utilization improvement have emerged in recent years. Technologies such as the Steam Rankine cycle, Organic Rankine cycle and Kalina cycle are already established but its use is suited for high energy grade systems with temperatures in excess of 232 °C [1]. Systems with ultra-low grade waste heat (less than 60 °C) such as low temperature hydrogen fuel cells faces a bigger challenge in recovering its waste heat. Practical methods to recover and utilize low grade waste heat is by directly reusing it for pre-heating purpose or by converting it directly into electrical energy using a thermoelectric generator (TEG) [2].
A hydrogen fuel cell is an efficient energy conversion device with very low greenhouse emissions and is able to be integrated with other types of energy technologies. It is considered as one of the solution for clean power generation especially in the transportation sector. A hydrogen fuel cell works by using oxygen and hydrogen as reactants that goes through steps of electrochemical processes and produces electrical energy as the output. Due to irreversibilities and exothermic reactions, heat is generated. The generated heat is normally transported out of the system via active cooling approach. Studies on waste heat recovery for hydrogen fuel cells is an emerging area in the thermal management of fuel cells that will contribute to increase the overall fuel utilization [3].
Among the types of hydrogen fuel cells, the Polymer Electrolyte Membrane (PEM) fuel cell is a highly promising technology due to its high power density, rapid start and numerous advantages for sub-100 kW power output applications. Open-cathode PEM fuel cell stacks have an operating temperature of 50 °C–90 °C [4] and produces waste heat stream approximately similar to the stack temperature, which will be too low for thermodynamic heat recovery cycles to work effectively and economically.
PEM fuel cells offer high conversion efficiencies in the range of 45%–60% [5] and is suitable for use as a power plant for future vehicles. Electric car prototypes using PEM fuel cells are rapidly maturing into commercial designs where pilot public trials have been conducted for advanced logistic analysis and consumer feedbacks. At the ground level, mini fuel cell vehicles usually apply small open-cathode PEM fuel cell stacks in the range of 1 kW–2 kW power, widely used in the design of single-seated cars for the Shell Eco Marathon Challenge and technology demonstrations due to its operating simplicity and system size [1]. The open-cathode type allows the air to act as the reactant and coolant at the same time when drawn into the fuel cell. Unfortunately, this reduces its efficiency rapidly at higher electrical current as more heat will be generated causing the electrolyte membrane to dry and increase the resistance to charge flow [6].
Integration of waste heat recovery technologies with fuel cell systems are mainly focused on Solid Oxide Fuel Cells (SOFC) that operates between a temperature range of 800 °C–1000 °C [4]. In the area of energy recovery from hybrid SOFC and gas turbine system, Granovskii et al. [7] obtained a 20% reduction in natural gas consumption while Chinda and Brault [8] obtained a 45% improvement to its cycle efficiency. A similar study by Sreeramulu and Deepak [9] related to the combined effect of a gas turbine with SOFC found that the total thermal efficiency using gasoline and diesel increased by 71.36% and 70.72% respectively. A power generation system combining SOFC, micro-gas turbine and organic Rankine cycle was studied by Ebrahimi and Moradpoor [10] and showed an increase of 65% in the overall efficiency and 45% increase in fuel saving. A study by Zhao et al. [11] was made exclusively on Direct Carbon Fuel Cells (DCFC) combined with thermoelectric generators. This hybrid system generated a 50% higher maximum power density and increased the overall efficiency by 19.6%.
In the mobility sector, Nissan and Ceres Power is seriously attempting using SOFCs on electric vehicles acting as an on-board power generator for the batteries of the power train [12]. The capacity for waste heat recovery from a 200 bar liquid methane SOFC electric vehicle was comprehensively modelled and optimized by Dimitrova and Marechal [13] by using gas turbines as an integrated energy converter. The overall conversion efficiency was reported to be 70% and it is concluded to be optimally environomic as a range extender at designs with an autonomy of 200 km.
Attempts to recover waste heat from PEM fuel cell systems have been mainly studied using Combined Heat and Power (CHP) methods due to the much lower waste heat temperature. Gigliucci et al. [14] assessed the suitability of CHP system based on PEM fuel cells power generation for Italian residential customers with promising results. Shabani and Andrews [15] investigated the CHP effect of a 500 W PEM fuel cell with solar-hydrogen for remote area power supply system and obtained an increase on the overall fuel efficiency to 70%. Hwang et al. [16] conducted a study on the implementation of a heat recovery unit in a 5 kW PEM fuel cell cogeneration system, resulting in a maximum overall system efficiency of 82% based on the lower heating value of hydrogen. A CHP and thermoelectric cooler combined system was proposed by Ebrahimi and Derakhshan [17] using a PEM fuel cell with low quality waste heat of 80 °C and successfully generated 2.79 kW of electricity at an overall efficiency of 53.86%.
A thermoelectric module is a solid-state device that converts thermal energy directly into electrical current, or vice versa, depending on the type of thermoelectric modules used. There are two types of thermoelectric modules; thermoelectric generators (TEG) that operate based on the Seebeck effect and thermoelectric coolers (TEC) based on the Peltier effect. A TEG offers electrical generation when the applied heat causes a temperature gradient across the TEG where the magnitude of electrical power generated is directly related to the temperature difference between the TEG surfaces.
Thermoelectric modules consist of no moving parts, no electromagnetic interferences and can withstand extreme harsh environments. Thermoelectric material technology now reaches maturity where there are increasing number and variety of thermoelectric products available for high temperature thermoelectric and converter technology; thus, the application of thermoelectric module is becoming widespread especially for waste heat recovery purposes. Current modules are simple, compact, environmentally friendly, with wide range of scalability, and most significantly, it has the capability to convert low grade waste heat to electricity [16]. These characteristics make TEG an attractive option for waste heat recovery compared to mechanically driven technologies such as the Steam Rankine cycle, the Organic Rankine cycle, the Kalina cycle and the Piezoelectric Generator. Table 1 lists a comparison on the characteristics of these energy recovery technologies.
Research in applied use of TEG to improve the energy efficiency are spread across diverse thermal energy systems such as on fuel cells [17], gasoline engine exhaust [18], natural gas-fired furnace [19], gas turbines [20], hybrid photovoltaic system [21], photovoltaic refrigerator [22], photovoltaic trough collector [23] and many others. The mentioned studies have successfully proven that the integration of TEG leads to improvements in energy efficiency. Different setups showed unique results depending mainly on the source energy quality, the TEG configuration and the cooling method. The main parameters towards obtaining an optimal TEG power output are the fluid-surface flow mechanics, high TEG temperature gradient and the resistance value between the electronic load and the TEG [24].
In order to obtain greater energy recovery efficiency, the application of thermoelectric modules is normally coupled with heat pipes and finned heat sinks as it provides larger heat transfer rates across the TEG cell [25]. A heat pipe is a passive heat exchanger that can be applied to improve the heat dissipation from the cold side of a thermoelectric module or to transfer heat effectively from a heated space to a cooler space [26]. Heat pipe designs have been improved over the years such as the recent design of the gravity heat pipe heat exchanger [27] and heat pipes heat exchangers for automotive heating [28].
The integration of heat pipes into the design of TEG systems have been performed with success for different applications. The use of heat pipe and heat sinks improved the TEG performance by 12.2% based on a study by Makki et al. [29] on the heat recovery of a hybrid photovoltaic system. Remeli et al. [30] experimentally explored a novel design of a sandwiched heat pipe and heat sink assisted TEG system for industrial waste heat, obtaining an overall conversion efficiency increase of approximately 1%. An increase of 0.8% on the overall energy efficiency was reported by Ding et al. [31] for heat extraction from a solar pond. Tundee et al. [32] investigated the effect of 16 cells of thermosyphon assisted TEG heat pipes to recover waste heat from an 80 °C solar heat collected in a solar pond. Theoretical and experimental work by Date et al. [33] studied the combined effects of a heat pipe assisted thermoelectric power generation system and solar water heating where it managed to generate 0.685 kWh of power output at a temperature difference of 150 °C. Nuwayhid et al. [34] applied a TEG-heat sink system for a domestic woodstove, generating 4.2 W of electrical power output at a temperature difference of 88 °C. A thermoelectric generator system with metal pin-fin array heat sink developed by Tzeng et al. [35] generated 200 W electrical power using 100 TEG cells at 252 °C temperature difference.
Generally, in the automotive application, there have been successful attempts to recover the waste heat energy from internal combustion engines. A 96 cell of heat pipe assisted TEG system developed by Goncalves et al. [36] obtained an increase in the overall efficiency of an internal combustion engine system by 1.7% at a temperature difference of 165 °C. Kim et al. [37] used 72 cells of heat pipe-heat sink assisted TEG installed in an air-cooled engine and achieved a 0.3% overall system efficiency increase. In another work, Kim et al. [38] applied 224 cells of heat pipe assisted TEG to the exhaust of a hybrid vehicle, resulting in 350 W of maximum power output at a temperature difference of about 140 °C. Orr et al. [39] applied thermoelectric modules and heat pipes to the exhaust of an internal combustion engine and managed to generate 38 W of maximum power output at an overall efficiency of 2.46%.
The main issue in fuel cells is the source of hydrogen. A common method to produce hydrogen is through electrolysis process that requires costly electrical energy. An alternative electric supply solution for electrolyzers is to utilize TEG by harnessing waste heat to generate electricity. Gholamian et al. [40] proposed two Organic Rankine Cycle (ORC) systems that were modelled and compared with a basic ORC system. The first ORC system employs TEG to generate extra electricity from the waste heat of an ORC turbine. The second system is similar to the first system except that electricity generated by TEG is used in a PEM electrolyzer to produce hydrogen. Another feature in the second system is that the heated cooling water leaving the TEG was reused for the electrolysis process. Systems 1 and 2 have higher exergy efficiencies by 21.9% and 12.7% respectively than the basic ORC system. In addition, System 2 has the lowest specific product cost among the other systems, even though the total configuration cost is higher.
Another study by Habibollahzade et al. [41] employed a hybrid solar thermal system consisting of parabolic trough solar collectors (PTSC) field, a thermoelectric generator (TEG), a Rankine cycle and a proton exchange membrane (PEM). The exhaust steam from the turbine enters the TEG module and produces secondary electricity. The proposed system was evaluated based on energy, exergy and exergoeconomic assessment with multi-objective optimization. The simulation model shows that the exergy efficiency is 12.76% and the total cost is $61.69 per GJ when the system runs at optimal working condition.
Demir and Dincer [42] proposed a similar hybrid solar thermal system with phase change materials (PCM) as a heat storage medium. The PCM functions to provide heat after sunset and to avoid excessive temperature differences during the day due to solar irradiance fluctuation. The TEG module taps the energy from the PCM to generate electricity for the PEM electrolyzer. Thermodynamics analysis shows that the overall energy and exergy efficiencies of the proposed system are 42.5% and 40.5% respectively, while the total electrical energy and hydrogen output are 50.6 GJ/day and 129.9 kg/day.
A study on a proposed integrated multigeneration system utilizing solar photovoltaic (PV) and parabolic trough solar thermal collector to produce electricity, hydrogen, heat, hot water, thermal energy storage and cooling was conducted by Islam et al. [43]. The study demonstrated that the exergy and energy efficiencies of the solar PV alone (without TEGs) are 5.6% and 5.9% respectively, while an increase of 10.1% and 10.7% in exergy and energy efficiencies respectively were obtained for the solar PV with TEG system. The increase in efficiency also led to an increase in hydrogen production.
From the sustainability point of view, research in recovering thermal energy using TEG should be intensified especially for systems that consumes fuel such as vehicles running on hydrogen fuel cells due to the energy-intense production of hydrogen. However, there are very few studies in the application of TEG in the unique domain of hydrogen fuel cell vehicles. Kim et al. [44] investigated the performance of a hexagonal shaped TEG applied in a hybrid fuel cell and internal combustion engine electric vehicle. Using 18 TEG units and 8 different engine operating conditions, a TEG conversion efficiency range between 1.3 and 2.6% were obtained. In another work, Nasri et al. [45] proposed a thermoelectric energy storage that was able to store large amounts of recovered energy using metal hydrides. The recovered energy in the storage can be used for preheating during start up and improves the travelling distance of the electric vehicle. Moreover, it increases the overall efficiency of the vehicle by 17%.
Studies in the application of thermoelectric modules for PEM fuel cells are relatively new and offer a diverse opportunity to model and analyze its performance for different operational settings. A thermal model consisting of a fuel cell and thermoelectric coolers was simulated and analyzed by Parise and Jones [46] to simulate the cooling behavior of thermoelectric coolers on the fuel cell membrane where the majority of the waste heat is generated. The model shows that thermal management of a fuel cell is improved with the presence of thermoelectric coolers by maintaining the fuel cell operating temperature between 45 °C and 60 °C. A thermoelectric and PEM fuel cells model was developed by Chen et al. [47] using computational fluid dynamics that attempted to optimize the system efficiency and performance. Then, Chen et al. [48] successfully developed a hybrid system model consisting of thermoelectric generators and PEM fuel cell to determine the optimal operating regions of the hybrid system.
A study by Gao et al. [49] simulated the potential use of thermoelectric devices in a high temperature PEM fuel cell to mitigate the dependence on Li-ion battery during system startup. The results indicate that proper heat flux regulation and minimal heat losses resulted in fuel cell system improvement. An investigation to study the effect of compact plate-fin heat exchanger towards thermoelectric generator and high temperature PEM fuel cell hybrid system using numerical model was also conducted [50]. The numerical model is based on finite-element approach which offers more accurate gas properties and heat transfer for recovering heat from the exhaust gas. Gao et al. [51] also performed an optimization of a thermoelectric generator subsystem which improves high temperature PEM fuel cell exhaust heat recovery.
Experimental work by Hasani and Rahbar [3] consisting of TEC, heat exchanger, heat pipe and a 5 kW PEM fuel cell was performed to study the effects of TEC towards the overall efficiency of the PEM fuel cell hybrid system. About 10% of waste heat was recovered from the fuel cell and 0.35% of overall efficiency of the system was successfully increased. Kwan et al. [52] performed an analysis on the optimization of thermoelectric system for thermal management of PEMFC based on genetic algorithm with multi-objective optimization of the TEG module parameters. An important outcome showed that optimal waste heat recovery can be achieved when the cooling convection coefficient are technically improved using heat sinks. In another work, they investigated the bidirectional operation of the thermoelectric device for active temperature control of fuel cells in which thermoelectric modules can operate as a TEG and TEC for energy harvesting involving a 500 W rated PEM fuel cell stack [53], and this research may lead to revolutionary changes in future stack designs.
Industries such as food manufacturing, district heating, cement, glass, chemical and ceramics produces low grade waste heat [54]. The amount of global waste heat from industries was estimated to be around 20–50% of the total energy input [55,56] with an average temperature of 88 °C [57]. Moreover, 50% of low grade waste heat is coming from geothermal and waste heat from low to mid temperature solar collectors [58]. The challenges in recovering low grade waste heat using thermodynamic cycles such as the Kalina cycle and Organic Rankine cycle is the high overhead and operating cost relative to its energy recovery efficiency which is limited due to the small temperature difference between the heat stream and the working fluid [59]. As such, these technologies are not suitable to recover ultra-low grade waste heat.
A thermoelectric module (TEM) has the capability to generate electricity or provide cooling depending on the electron movement direction through the semiconductor material. The major difference between a TEM and thermodynamic heat recovery technologies is the capability to recover ultra-low grade waste heat, which is defined as waste heat in the range of 50 °C to 100 °C. It is also available in numerous sizes depending on the type of waste heat recovery application. There are two types of TEM; the thermoelectric generator (TEG) and the thermoelectric cooler (TEC). A TEG generates electricity when heat is applied to one of the TEG surfaces and a temperature gradient is maintained between the surfaces. In contrast, a TEC provides cooling from one of its surfaces when electrical current is applied by absorbing heat from an instrument or space [60]. Both TEM consists of no moving parts, continuous operating hours without any maintenance required and can operate under all weather conditions [61]. The application of TEG has expanded towards numerous energy systems such as internal combustion vehicles, solar ponds and even fuel cells. For TEC, it is typically used as portable coolers and microprocessor cooling [62].
There are many advantages offered by TEM such as the capability to recover low grade waste heat in which most of existing thermodynamics systems are not capable of, availability in any size depending on type of application and it has no moving parts. However, the disadvantages of TEM is that it has very low energy conversion efficiency (3%–5%) and electrical output that requires the use of high numbers of TEM to be applied in a WHR system design [63]; thus, leading to innovative engineering solutions for practical application.
It can be concluded that the TEG offers a viable solution in recovering ultra-low grade waste heat. Moreover, the sources of such low grade waste heat is ubiquitous and can be found mostly in generic and sector specific processes. Research opportunity to recover waste heat from a PEMFC system exists as there is extremely limited notable research in this area using TEG. By recovering the waste heat, the TEG would increase the overall energy efficiency of a fuel cell vehicle with a positive contribution towards sustainable mobility for the future.
Previous studies have successfully developed specific applications for TEG with reported improvements in overall energy efficiency that would lead to significant fuel and energy savings in long-term operation. There are two major limitations in this area of study:
- •
There are no evidence of existing research using an integrated thermoelectric generator and heat pipe system that relates to ultra-low grade waste heat recovery specifically from a fuel cell vehicle. Thus, this research takes the direction of experimentally investigating the characteristics of a passive heat pipe and TEG (HP-TEG) system using ultra-low temperature waste heat from a PEM fuel cell that powers a mini fuel cell vehicle.
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Analysis that accounts for variations in air cooling modes at the heat sink due to vehicle motion has not been studied elsewhere. Thus, another objective of the work was the modelling of the system using a one-dimensional analysis based on the thermal circuit heat transfer analogy. The model was validated with the experimental results where heat transfer characteristics under the influence of the variable parameters and system configurations were obtained. The main novelty of the work is the development of the validated model that can be applied as a base platform for further improvements in the HP-TEG system, with a specific target of contributing towards fuel cell vehicle technology advancement.
In the introduction, detail review of work in the areas of TEG and waste heat recovery for PEM fuel cells was given to prove that the study of TEG for PEM fuel cell vehicle would have a significant contribution to the body of knowledge in the sustainability agenda. Then, the design and modelling section reported the development and operating methodology of the test bench starting with the energy mapping of the PEM fuel cell stack and subsequently the TEG waste heat recovery system. The experimental method explained and rationalized the cases to be investigated such as using natural convection and forced convection cooling modes at the heat sink. The section on theoretical modelling describes the thermal resistance method to develop the analytical model of the test bench, especially on detailing the convection heat transfer coefficients on the TEG surface. The results and discussion section focuses on profiling the Maximum Power Point (MPP) for each experimental cases to obtain the most effective system configuration for actual application. The theoretical thermal resistance model was validated for selected cases. Finally, a section was provided to discuss the limitations of the results relative to practical FCV application.
Section snippets
Design and experimental
The system design process involves two separate stages:
- i.
PEM fuel cell thermo electrical characterization
- ii.
HP-TEG system development and experimental
Thermoelectric analysis
The thermoelectric generator operates with a temperature difference between the hot and cold sides of the TEG. Such condition can be achieved by applying heated air to the hot side surface of the TEG and then dissipated at the cold side.
Heat applied to the TEG hot side, and heat dissipated at the cold side, can be expressed as shown in Equations (1), (2)).where is the thermal conductivity, is the Seebeck coefficient, and
Results and discussion
The results are discussed following the chronology of these experimental objectives:
- i.
The effect of heat loads from the heat source towards TEG electrical generation,
- ii.
The effect of different convection modes (natural and forced convections) towards TEG behavior, and
- iii.
The effect of different TEG orientations (normal and parallel) towards flow of heat sources.
Model validation
In this section, the theoretical modelling was validated with the data from the experimental works. In Fig. 22, a comparison of the power output from the experimental and theoretical results at different heat loads were plotted, based on data from the forced convection experiments at normal orientation of the HP-TEG system setup.
The theoretical model and experimental results show similar exponential profiles for the TEG maximum power output. When the fuel cell power load increases, the TEG
Conclusion
A waste heat recovery system simulating a novel case of a PEM fuel cell on-board a mini vehicle was successfully developed and tested under limited operating conditions of stationary and 20 km per hour cruising motions. The main contributions of this work are the evaluation of fundamental thermal and electrical profiles of a single TEG unit coupled with a finned heat pipe under different heat loads, TEG orientations and convection mode configurations. The setup manages to obtain a maximum power
Acknowledgement
The authors would like to extend our gratitude for the funding of this work by the Ministry of Higher Education, Malaysia (600-RMI/FRGS 5/3 (150/2014)), and the Bestari research grant provided by Universiti Teknologi MARA (600-IRMI/MYRA 5/3/BESTARI (010/2017)).
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