Proton Exchange Membrane Fuel Cells (PEMFCs) present a promising alternative to the conventional internal combustion engine for automotive applications because of zero harmful exhaust emissions, fast refuelling times and possibility to be powered by hydrogen generated through renewable energy. However, several issues need to be addressed before the widespread adoption of PEMFCs, one such problem is the removal of waste heat from the fuel cell electrochemical reaction at high ambient temperatures. Automotive scale fuel cells are most commonly liquid cooled, evaporative cooling is an alternative cooling method where liquid water is added directly into the fuel cell flow channels. The liquid water evaporates within the flow channel, both cooling and humidifying the cell. The evaporated water, along with some of the product water, is then condensed from the fuel cell exhaust, stored, and re-used in cooling the fuel cell.
This work produces a system level model of an evaporatively cooled fuel cell vehicle suitable for the study of water balance and heat exchanger requirements across steady state operation and transient drive cycles. Modelling results demonstrate the ability of evaporatively cooled fuel cells to self regulate temperature within a narrow region (±2°C) across a wide operating range, provided humidity is maintained within the flow channels through sufficient liquid water addition. The heat exchanger requirements to maintain a self sufficient water supply are investigated, demonstrating that overall heat exchange area can be reduced up to 40% compared to a liquid cooled system due to the presence of phase change within the vehicle radiator improving heat transfer coefficients. For evaporative cooling to remain beneficial in terms of heat exchange area, over 90% of the condensed liquid water needs to be extracted from the exhaust stream.
Experimental tests are conducted to investigate the condensation of water vapour from a saturated air stream in a compact plate heat exchanger with chevron flow enhancements. Thermocouples placed within the condensing flow allow the local heat transfer coefficient to be determined and an empirical correlation obtained. The corresponding correlation is used to produce a heat exchanger model and study the influence different heat exchanger layouts have on the overall required heat transfer area for an evaporatively cooled fuel cell vehicle.
A one-dimensional, non-isothermal model is also developed to study the distribution of species, current density and temperature along the flow channel of an evaporatively cooled fuel cell using different methods of liquid water addition. Results show that good performance can be achieved with cathode inlet humidities as low as 20%, although some anode liquid water addition may be required at high current densities due to increased electro-osmotic drag. It is also demonstrated that both good membrane hydration and temperature regulation can be managed by uniform addition of liquid water across the cell to maintain a target exhaust relative humidity.
Porous metal foams have been used as alternative flow-fields in proton exchange membrane fuel cells (PEMFCs), exhibiting improved performance compared to conventional 'land and channel' designs. In the current work, the mechanical behaviour of PEMFCs using metal foam flow-fields is investigated across different length scales using a combination of electrochemical testing, X-ray computed tomography (CT), compression tests, and finite element analysis (FEA) numerical modelling. Fuel cell peak power was seen to improve by 42% when foam compression was increased from 20% to 70% due to a reduction in the interfacial contact resistance between the foam and GDL. X-ray CT scans at varying compression levels reveal high levels of interaction between the metal foam and gas diffusion layer (GDL), with foam ligaments penetrating over 50% of the GDL thickness under 25% cell compression. The interfacial contact area between the foam and GDL were seen to be 10 times higher than between the foam and a stainless-steel plate. Modelling results demonstrate highly uniform contact pressure distribution across the cell due to plastic deformation of the foam. The effect of stack over-tightening and operating conditions are investigated, demonstrating only small changes in load distribution when paired with a suitable sealing gasket material.
Maintaining proton exchange membrane fuel cell (PEMFC) stack operating temperature across transient current profiles presents a significant challenge for fuel cell vehicles. Liquid cooled systems require active control of coolant temperature and flow rate to match heat rejection to heat generation. Evaporative cooling is an alternative to conventional liquid cooling in automotive sized PEMFC stacks. In an evaporatively cooled system, liquid water is injected directly into the cathode flow channels where it evaporates, both cooling and humidifying the stack. This paper uses a validated simulation to explore the inherent temperature regulation abilities of an evaporatively cooled PEMFC stack across a range of current profiles and drive cycles. Results show that throughout the normal operating current range, stack temperature varies by less than ± 2.0 °C, this is comparable to liquid cooling but without the need for active temperature control. The introduction of variable operating pressure and cathode stoichiometry using proportional integral control, can further reduce temperature variation to ± 1.0 °C and ±1.2 °C respectively for step increases in current demand. Variable operating pressure is also shown to improve warm up time and reduce heat loss at low operating loads.
Abstract The distribution of current/voltage can be further regulated by optimising the electrical connection topology, considering a particular battery thermal management systems. This study numerically investigates a 4P6S battery module with two connection topologies: 1) a straight connection topology, where the sub-modules consist of parallel-connected cells that are serial connected in a linear configuration, and 2) a parallelogram connection topology, where the sub-modules are serial connected in a parallelogram configuration. We find that the straight topology is more advantageous, as it allows the temperature gradient to be distributed among the parallel-connected cells in the sub-modules, mitigating over(dis)charging. Consequently, it achieves a 0.8% higher effective capacity than the parallelogram topology at 1C discharge, along with a higher state of health at 80.15% compared to 80% for the parallelogram topology. Notably, the straight topology results in a maximum current maldistribution of 0.24C at 1C discharge, which is considered an acceptable trade-off.
Climate change and the major threat it poses to the environment and human lives is the major challenge the world faces today. To overcome this challenge, it is recommended that future automobiles have zero carbon exhaust emissions. Even though battery electric vehicles reduce carbon emissions relative to combustion engines, a carbon footprint still remains in the overall ecosystem unless the battery is powered by renewable energy sources. The proton exchange membrane fuel cell (PEMFC) is an alternate source for automotive mobility which, similar to battery electric vehicles, has zero carbon emissions from its exhaust pipe. Moreover, the typical system level efficiency of a PEMFC is higher than an equivalent internal combustion powertrain. This review article covers the background history, working principles, challenges and applications of PEMFCs for automotive transportation and power generation in industries. Since the performance of a PEMFC is greatly influenced by the design of the anode and cathode flow channels, an in-depth review has been carried out on different types of flow channel designs. This review reveals the importance of flow channel design with respect to uniform gas (reactant) distribution, membrane proton conductivity, water flooding and thermal management. An exhaustive study has been carried out on different types of flow channels, such as parallel, serpentine, interdigitated and bio-inspired, with respect to their performance and applications.
Proton exchange membrane fuel cells (PEMFCs) using porous metallic foam flow-field plates have been demonstrated as an alternative to conventional rib and channel designs, showing high performance at high currents. However, the transport of liquid product water through metal foam flow-field plates in PEMFC conditions is not well understood, especially at the individual pore level. In this work, ex-situ experiments are conducted to visualise liquid water movement within a metal foam flow-field plate, considering hydrophobicity, foam pore size and air flow rate. A two-phase numerical model is then developed to further investigate the fundamental water transport behaviour in porous metal foam flow-field plates. Both the experimental and numerical work demonstrate that unlike conventional PEMFC channels, air flow rate does not have a strong influence on water removal due to the high surface tensions between the water and foam pore ligaments. A hydrophobic foam was seen to transport liquid water away from the initial injection point faster than a hydrophilic foam. In ex-situ tests, liquid water forms and maintains a random preferential pathway until the flow-field edge is reached. These results suggest that controlled foam hydrophobicity and pore size is the best way of managing water distribution in PEMFCs with porous flow-field plates.
Despite having efficiencies higher than internal combustion engines, heat rejection from fuel cells remains challenging due to lower operating temperatures and reduced exhaust heat flow. This work details a full system simulation which is then used to compare a conventional liquid cooled fuel cell system to two types of evaporatively cooled fuel cell systems. Both steady state and transient operation are considered. Results show the radiator frontal area required to achieve thermal and water balance for an evaporatively cooled system with an aluminium condensing radiator is 27% less than a conventional liquid cooled system at 1.25 A/cm2 steady state operation. The primary reason for the reduction is higher heat transfer coefficients in the condensing radiator due to phase change. It is also shown that the liquid water separation efficiency has a significant influence on the required radiator frontal area of the evaporatively cooled system.
Flat plate Loop Heat Pipes (LHPs) have emerged as an attractive battery thermal management system (BTMS) as they allow long-distance heat transfer at no power cost due to their unique porous structure. Additionally, the flat contacting surface is suitable for end cooling in cylindrical cells due to the higher axial thermal conductivity of the jelly roll structure. Previous studies on LHP BTMS have mainly focused on the cooling performance of prismatic or pouch cells, without considering cell ageing. This study instead numerically investigates the cooling performance, the resulting current distribution, and the ageing of a 6S4P 18650 battery module equipped with LHPs BTMS, comparing the performance against a natural air convection BTMS. This study has taken various thicknesses of the heat collector plate (i.e. 0.5 mm, 1 mm, 2 mm and 3 mm) and the number of LHPs (i.e. 1 or 2) into consideration. The results indicate that utilising a single LHP in conjunction with a 2 mm thick heat collector plate emerges as the optimal configuration. This design achieves a maximum average temperature of 29.7 °C and a minimal temperature gradient of 0.02 °C under a 1C discharge condition. The proposed design also significantly enhances battery lifespan, reaching up to 310 cycles with a maximum ageing gradient of 0.005, compared to the 146 cycles and 0.04 ageing gradient observed for the module with natural air convection. This study presents the potential of LHP BTMS for future electric vehicles, demonstrating its superior effectiveness in enhancing battery performance and longevity.
With the increasing severity of environmental problems and energy scarcity, fuel cell electric vehicles (FCEVs), as a sustainable and efficient means of transportation, are attracting more attention. The ageing of fuel cells (FCs) has become an urgent problem with the development of FCEV. In order to prolong the lifetime of FCs, this paper builds a model of a vehicle driven by two power sources, FC and lithium battery (Lib) using AVL Cruise. A rule-based energy management strategy (EMS) is developed in Simulink to explore the optimal control strategy for the vehicle in terms of the durability of the FC. An FC ageing model is used to quantify the degradation voltage of different duty cycles. The results show that the FC engagement levels, OCV operations, and start/stop operations can affect the lifetime of the FC significantly. By optimising the EMS, the lifetime of the FC is improved by 9.47%.
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