Abstract
Proton exchange membrane (PEM) fuel cells are a promising technology with many features, including having a high energy density, efficiency, lightness, and producing energy directly instead of storing it. PEM fuel cells are currently used in mobile vehicles, military applications, portable systems, stationary systems, and air-independent propulsion systems for sea and space. Elimination of the cooling problems associated with PEM fuel cells in space applications is of great importance to produce more efficient systems. With this motivation in mind, this study examined PEM fuel cell elements and cooling flow channel design for space applications. The effect of the designed flow channel on the PEM fuel cell temperature distribution was investigated. Five different PEM fuel cell cooling channels were designed and modeled computationally, and the most suitable cooling channel design was selected based on the thermal analysis. While modeling the PEM fuel cell, space environment operating conditions, pure reactant supply (oxygen and hydrogen), electrochemical reaction in the membrane, physical properties of liquid coolant, physical properties of fuel cell elements, and heat transfer in the fuel cell were considered. The temperature distribution obtained as a result of the thermal analysis was examined, and it was seen that the PEM fuel cell cooling channel design was successful in maintaining the operating temperature of the PEM fuel cell. According to the data obtained, the cooler inlet temperature is approximately 40°C and the cooler outlet temperature is approximately 75°C in flat serpentine design. These values were obtained as a result of the analysis made with a mixture of 50% ethylene glycol and 50% water selected as the liquid coolant. As a result, a fuel cell can be formed by producing a fit bipolar plate with the analyzed PEM fuel cell cooling flow channel design, and current density and homogeneity can be determined by applying single-cell performance tests to the fuel cell.
1. Introduction
Fuel cell is an electrochemical device that converts chemical energy directly into electrical power by combining a fuel and an oxidizer [1]. They are used in many fields such as mobile vehicles, military applications, portable systems, fixed systems, and air-independent propulsion systems (space applications, submarine applications, etc.) [2]. In the use of fuel cells in space applications, fuel cells are defined as primary (nonrechargeable) or secondary (rechargeable). Fuel and oxidizer tanks for primary fuel cells in space applications are discharged gradually [3]. Secondary fuel cells, also known as regenerative fuel cells, produce water and electrical power using hydrogen and oxygen. The amount of energy stored in the fuel and oxidizer per unit mass is much larger than the amount of energy stored in a typical battery [4]. Unlike batteries, fuel cells do not store their fuel and oxidizers in the cell stack, and the fuel and oxidizer are stored outside of the stack. Due to this feature, the energy capacity of a fuel cell power system is determined by the size of the fuel tanks [3].
The National Aeronautics and Space Administration (NASA) has used proton exchange membrane fuel cells (PEMFC) or alkaline fuel cell (AFC) technology in space missions as the primary energy source in many manned space missions since the 1960s. Manned missions often require higher power levels and primary energy storage than unmanned missions due to their long discharge times. Considering that weight is an important criterion in space applications, the fact that fuel cells are much lighter than batteries is an important reason for choosing fuel cells [5]. The PEM fuel cell-powered spacecraft, developed by General Electric for use in the Gemini V spacecraft in 1962, was launched in 1965, and the Gemini V mission was the first space mission to use a fuel cell. The use of fuel cells continued to be used on Gemini 7, 8, 9, 10, 11, and 12 missions [6]. The water produced by the fuel cells on the Gemini missions was used as drinking water by the astronauts. In 1963, Pratt and Whitney selected alkaline fuel cells to power the Apollo Command and Service Module (CSM). Fuel cells provided the main power for the CSM. These fuel cells have been used on the Apollo, Apollo/Soyuz, and Skylab missions. In 1981, the electrical power required for NASA’s Space Shuttle Orbiter was provided by alkaline fuel cell power plants. These fuel cells are designed, developed, and manufactured by UTC fuel cells. In the Space Shuttle Orbiter, three 12 kW fuel cells generate all onboard electrical power. Therefore, there is no need for backup batteries and a single fuel cell is sufficient to ensure safe vehicle return and additionally water produced by the electrochemical reaction was used for crew drinking and spacecraft cooling [3–6]. Irregular temperature distribution, or improper cooling, is one of the most important problems in fuel cell applications. Another equally important and challenging issue is excess water formation on the cathode side, or inadequate humidity, which prevents the electrochemical reaction.
The waste heat generated as a result of the electrochemical reaction in the fuel cell is transferred to the cooler through the cooling channels in the bipolar plate [7]. The heated coolant is pumped to a heat exchanger and discharged or used for alternative purposes such as heating. Maintaining the operating temperature of the fuel cell and ensuring the homogeneity of the temperature distribution are vital for the performance of the fuel cell [8]. In the literature, different cooling channel geometries and flow field designs are compared through numerical simulations. Although there are many studies on hydrogen and air-cooling fuel cell stacks operating under low pressure in the literature, there are few studies on space applications. Burke [3] performed the analysis of the power density and energy density of fuel cells and compared the predicted performance with the current performance. Hoberecht and Reaves [9] discussed the advantages of PEM fuel cell technology and its potential for future spaceflight compared to alkaline fuel cells. They emphasized that PEM are currently under development and that attention should be paid to the removal of water from the PEM fuel cell and should be designed according to the effect of gravity. Guo et al. [10] showed that the flow channel orientation is affected by the gravity level by throwing the fuel cell from a drop tower in order to provide a microgravity environment in their study. According to the experimental results, it was stated that the vertical flow channel orientation is more suitable for the microgravity environment in terms of water removal and cell performance. Guo et al. [11] carried out their research on gas/water and waste thermal management for PEM-based electrochemical cell systems applied to future space exploration. Giacoppo et al. [12] carried out the design, production, and testing of a 2 kW PEM fuel cell suitable for the lunar exploration mission. The study demonstrated the usability of a low-temperature proton exchange membrane fuel cell stack in a regenerative fuel cell system for lunar surface missions in both stationary and mobile conditions.
When the PEM full cell studies on the liquid cooling flow field channel geometry are examined, Wang et al. [13] investigated the effects of cathode flow channel configuration on local transport and cell performance for parallel and integrated flow fields in proton exchange membrane (PEM) fuel cells. Nam et al. [14] observed an increase in cooling performance in general, especially in areas such as product concentration, temperature distribution, and water saturation, since convection and transport between adjacent flow channels are provided when single and parallel multipass serpentine flow fields (MPSFF) are used in small PEM fuel cells. Firat et al. [15] showed that the high thermal conductivity of the material and the complex geometry of these plates directly affect the cooling performance. Sasmito et al. [16] numerically evaluated the simultaneous performance of various gas and cooling channel designs and performed parallel, coil, oblique, helical, parallel coil, and a new hybrid parallel-coil-oblique channel design. The results were compared regarding thermal, water, and gas management. Alizadeh et al. [17] numerically designed nine different cooling flow field designs for PEMFC stacks and optimized them to eliminate heat in terms of homogeneous temperature distribution and minimum pressure loss. Afshari et al. [18] simulated the cooling flow and heat transfer in the cooling plates with parallel channels in the polymer membrane fuel cell, and they realized the effect of the dimensions of the cooling channels on the fuel cell thermal performance based on three indices: maximum surface temperature cooling, surface temperature homogeneity, and pressure drop. Kazemi and Shateri [19] used a periodic zigzag channel with rectangular cross-section to cool the polymer electrolyte fuel cell and to obtain a high-efficiency system. Rahimi-Esbo et al. [20] carried out a numerical design of a fuel cell with metallic bipolar plates in which the liquid passes through the cooling flow area by using a spacer plate in the cooling flow area. Rahgoshay et al. [21] performed isothermal analysis of a fuel cell with parallel and serpentine flow field geometries. According to the results, it has been observed that the flow area with serpentine geometry is more effective in terms of heat transfer rate. Miri Joibary et al. [22] performed 6 different cooling flow field designs and analyzed the water and temperature distribution of the fuel cell. It has been observed that the 4-section serpentine flow field design gives the best results according to the relevant parameters. In the main results of all these studies, fuel cells have many advantages in space applications and manned missions due to their being highly efficient, lightweight, and stationery.
There are many studies on the space applications of fuel cells mainly focused on lightweight systems, power management, and geometric configurations. However, there is limited work in literature regarding the effect of geometric properties of fuel cells on thermal management of space systems. In this study, the effect of geometric properties of fuel cell cooling channel and flow parameters on thermal management was investigated by considering the space environment. A steady-state analysis was performed instead of a transient simulation. Having this motivation in mind, issues and solutions associated with thermal management of the PEM fuel cell are examined in this study.
2. System Description of Fuel Cell
2.1. PEM Fuel Cell
Proton exchange membrane fuel cells (PEMFC) are based on a solid polymer electrolyte and operate at a low temperature (∼60-80°C) and high temperature (~80-120°C) [23–25]. In PEM fuel cells, hydrogen is fed from the anode section and oxygen is fed into the cell from the cathode section. Hydrogen enters the anode section of the fuel cell, passes through the gas diffusion layer, and then reaches the platinum catalyst. In the catalyst layer, it dissociates into protons and electrons, and the protons pass through the polymer electrolyte membrane, and the electrons pass through the external electric circuit, producing an electric current [26, 27]. By combining the oxygen entering the cathode section and the protons passing through the membrane, water is released as a product. In addition, since the electrochemical reaction is an exothermic reaction, heat is released [28]. Fast start-up times, low-temperature operation, and high-power densities make PEM fuel cells an easy-to-use technology for transport applications [29]. Carbon monoxide (CO) poisons the catalyst, and therefore, the hydrogen fuel must be very pure [30]. Water management is one of the critical aspects of successfully operating a PEMFC, as the polymer membrane must be kept moist for good proton conduction. It is also important to keep the temperature of the cell within a certain range due to the electrochemical reaction taking place in the fuel cell.
2.2. Heat Generation and Thermal Management
The most difficult problems often encountered in a fuel cell are the thermal management and the water management of the fuel cells [31]. Factors such as homogeneous temperature distribution, pressure drop, maximum temperature, and heat transfer, which are parameters related to the cooling performance of the fuel cell, should be considered during designing process. Considering the increase in the kinetic rate in the reaction zone, the fuel cell stack is much more efficient at high temperatures, but also, overheating of the membrane should be avoided. Overheating of the membrane and catalyst can damage them and thus reduce the performance of the fuel cell [32]. This can be avoided by a cooling cycle flowing through channels through the bipolar layer. In addition to optimizing the temperature value as high as possible and not damaging the membrane, it is also quite important to ensure a homogeneous temperature distribution. Heat generation in PEM fuel cells occurs for four reasons: reversible heat of electrochemical reactions (also entropic heat), irreversibility of reactions, ohmic resistance, and heat from water vapor condensation [33]. Considering performance, liquid cooling can be used to solve thermal and water management issues in PEMFCs [7].
Convectional heat transfer with liquid coolant is the most common method of removing waste heat from high-power PEM fuel cell stacks of more than 5 kW [34]. Liquid cooling has a high rate of heat removal due to its high thermal conductivity and heat capacity. The liquid cooling system has a complex design as it contains additional tools such as coolant loop, heat exchanger, flow regulating valve, and deionizing filter, but it is the most efficient cooling method to be used in high-power fuel cell integrations such as space applications. The circulating liquid coolant is pumped by flowing through both a sealed cooling channel between two adjacent bipolar plates and an external heat exchanger. To the external heat exchanger, the waste heat generated by the electrochemical reaction is continuously carried by the coolant from the spacecraft cooling system. High thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low abrasion, low toxin ratio, and thermal stability are among the parameters to be considered when determining a heat transfer fluid [35]. In addition, the relevant chemical properties of the preferred coolant such as viscosity, electrical conductivity, specific heat capacity, and thermal conductivity need to be evaluated. In the determination of the cooling liquid for the space environment of the fuel cell design, the water-based ethylene glycol liquid solution, which has been used in space missions previously, has been determined. It was decided to use a mixture of 50% water and 50% ethylene glycol as the coolant in the fuel cell. The specific heat, viscosity, and specific gravity of a water and ethylene glycol solution vary significantly with the percentage of ethylene glycol and the temperature of the liquid. Ethylene glycol has desirable thermal properties, including high boiling point, low freezing point, stability over a wide temperature range, and high specific heat and thermal conductivity. It also has a low viscosity and therefore reduces pumping requirements. As the glycol concentration in the solution increases, the thermal performance of the heat transfer fluid decreases. PEMFCs convert approximately 55% of fuel chemical energy into waste heat [7]. Due to the low (60–70°C) PEMFC operating temperature, it becomes a challenge to remove this heat [36]. When the cooling system temperature is around 40°C, the small temperature difference between the radiator and the environment creates a very small temperature gradient to dissipate the heat. The small temperature gradient requires a large heat transfer area, but there are very strict weight restrictions in aircraft and space applications, so care must be taken to achieve the lightest cooling option [37]. The properties of the ethylene glycol water mixture selected as the liquid coolant are given in Table 1.
3. Materials and Methods
3.1. Mathematical Modeling and Design Parameters
In this study, an altitude of kilometers has been determined for the design of the fuel cell-operated system for space application. This altitude, which is referred to as the upper mesosphere layer, is the altitude just before the thermosphere; the temperature decreases as the altitude increases. In the thermosphere, the temperature increases with increasing altitude. This is because most of the X-ray and UV radiation from the sun is absorbed in the thermosphere. When the sun is very active and emits higher energy radiation, the thermosphere warms and expands.
Depending on the chosen altitude, gravitational acceleration, temperature, pressure, and density are calculated. Gravitational acceleration () due to altitude is calculated 9.5525 m/s2 using Equation (1). Here, is the earth’s radius, is the altitude, and is the standard gravitational acceleration.
The relationship between geopotential altitude () and geometric altitude () is given in Equation (2). The geopotential height () is calculated as 82904.5 m.
The altitude-dependent temperature calculation is as stated in Equation (3) [39].
Here, is the geopotential altitude, is the molecular temperature at the geopotential altitude, is the layer spacing number, is the base geopotential altitude above the mean sea level, is the base temperature, and is the increase rate of the base temperature per kilometer at the geopotential altitude. The molecular temperature at geopotential altitude was calculated as 190.841 K.
The altitude-dependent formula of the kinetic temperature is up to 86 km, where is the kinetic temperature (K), which is expressed in Equation (4). Molecular weight ratio depends on geopotential and geometric height [40].
Temperatures are in Kelvin scale, being the linear function of geopotential altitude, kinetic temperature, molecular mass of air at sea level, and molecular mass of air at altitude. The kinetic temperature was calculated as 190.800 K. Equation (5) is used to determine the pressure at various altitude ranges from 0 to 86 km [40].
Here, is the base temperature of the layer, is the base temperature increase rate of the layer, is the universal gas constant which is N∙m/kmol∙K, is the gravitational acceleration, and is the molar mass of the air on earth. base static pressure was calculated as 0.58 Pa. The air is considered to be dry up to an altitude of 86 km. It is also assumed that the atmosphere mixes homogeneously, resulting in a constant molecular weight. In this case, the ideal gas equation expressed in Equation (6) can be applied.
If the density is written from Equation (6), Equation (7) is obtained.
The altitude-dependent density () is calculated as 0.0000105 kg/m3.
3.2. Governing Equations
If the continuity equation for the conservation of mass is written, it is expressed in Equation (8) as fluid density and fluid velocity.
Equation (9) for constant density is expressed as follows, with () velocity components.
The convection diffusion equation is expressed by
Incompressible flows with being the kinematic viscosity and the conservation of linear momentum where the viscosity is constant can be written as in Equation (11).
3.3. Thermal Management and Heat Transfer
The heat flux towards a single cooling plate due to the heat generation resulting from the electrochemical reaction in the fuel cell is expressed in the following equation (12).
is the vertical temperature gradient of the plate channel surface. Surface boundaries in contact with the environment are considered insulated [41]. The convective heat transfer between the solid region and the fluid channel is expressed in the following equation:
is the vertical temperature gradient of the plate channel surface and the flow channel surface. Surface boundaries in contact with the environment are considered insulated. To analyze thermal management, a proper calculation of the heat produced by a fuel cell is required. Since all energy that is not converted into electrical energy is dissipated as heat, the heat produced by a fuel cell can be calculated as in the following equation: where is the waste heat () produced in the fuel cell, is represents the consumption rate of the water produced.
Substituting Equation (15), Equation (16), and Equation (17) in Equation (14), Equation (18) is obtained and refers to the amount of heat produced by the fuel cell and must be absorbed by the coolant.
Typically, the actual fuel cell output current is approximately equal to the theoretical fuel cell output current. Assuming that the two quantities are equal, it is expressed in Equation (18) [42].
3.4. Conceptual System Design
A fuel cell for space application needs special system requirements, operating conditions and designs due to its isolated low gravity environment, which is significantly different from the environment compared to terrestrial applications [43]. In spacecraft, reactive gases are stored as liquids in cryogenic tanks. Liquid hydrogen and oxygen are heated and fed into the fuel cell in gaseous form before being fed into the fuel cell. As a result of the reaction in the fuel cell, water is produced. The exiting water is separated by passing through the condenser and the dryer. The fuel cell requires dehydration (drying) of water to balance the complex moisture conditions inside. Remaining water in the transmission line should be removed as it may block the reactive gas flow. In addition, the resulting water must be separated for use as drinking water in manned space missions. It is very important to prepare a system as simple as possible for the use of fuel cells in space.
In space applications, pure hydrogen and oxygen are used as reagents. Reactive gases must not be humidified before being supplied. The fuel cell should consume as much reagent as possible, and the amount of waste gas should be minimal. The water produced from the fuel cell must be collected [43]. The use of pure oxygen instead of air has a significant effect on the power density and efficiency of polymer electrolyte fuel cell systems. The main difference is the greatly reduced parasitic power in the balance between the oxygen plant and the air system. Using pure oxygen in fuel cell level has higher power density, lower platinum requirement, higher efficiency, and no possible poisoning due to poor ambient air quality. At system level, other advantages are higher system efficiency, easier humidity management, decreased noise, and high dynamics of the fuel cell system. With those advantages, vehicles have higher fuel efficiency, higher range, and global weight reduction [44]. The fuel cell system design diagram for space applications, prepared by considering all these factors, is shown in Figure 1.
The fuel cell thermal control system is a pump loop system that circulates coolant through the fuel cell to a radiator, which collects waste heat from the fuel cell stack cold plates, returning the waste heat to the environment. The sizing of the system depends on the heat load to be dissipated and the heat transfer from the radiator to the surroundings. Heat transfer from the radiator takes place by both radiation and convection. Despite the sparse atmosphere, convection still plays a role in removing heat from the radiator and therefore, the radiator must be modeled correctly in order to be sized appropriately [45].
3.5. Simulation Model of PEM Fuel Cell
When it is desired to provide heat management and homogeneous heat distribution in PEMFC, it becomes very important to design a suitable cooling flow area to prevent heat generation [21]. The elements modeled in the PEM fuel cell are anode current collector, anode gas diffusion layer, anode catalyst layer, membrane (electrolyte), cathode catalyst layer, cathode gas diffusion layer, and cathode current collector, respectively. The location of the elements in the fuel cell is shown in Figure 2 (from left to right: anode current collector, anode gas diffusion layer, anode catalyst layer, membrane (electrolyte), cathode catalyst layer, cathode gas diffusion layer, and cathode current collector).
Anode and cathode current collector bipolar plates have cooling channels passing through them. Hydrogen flow from the anode and oxygen flow from the cathode takes place in serpentine-type flow channels. The cooling channels passing through the inner sides of the bipolar plates are modeled in 5 different types, and the focus of this study is on the effect of these cooling channel geometries on the PEM fuel cell temperature distribution. Figure 3 shows the geometry of the cooling channel passing through the bipolar plate modeled as a single piece. In order to clearly observe the cooling channel passing through the anode and cathode plates, which are a single plate, it is actually the only plate that looks like 2 separate plates shown in the left picture of the Figure 3. The plate is referred to throughout the text as the anode or cathode plate is a single plate with a cooling flow area within it.
The inlet of the cooling channel is modeled to coincide with the inlet of the hydrogen channel in the anode bipolar plate and the inlet of the oxygen channel in the cathode bipolar plate. Hydrogen and oxygen fed flow channels in the bipolar plates of the fuel cell are modeled with serpentine geometry. Serpentine gas flow channel is used with all cooling channels. For the hydrogen and oxygen flow channel geometry, a classical and simple model in the literature was preferred, and the analysis was carried out quickly, and the focus is on the cooling channel geometry. The cooling channels are modeled in five different ways as three parallel and two serpentine types and are shown in Figure 4.
(a)
(b)
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While modeling the cooling channels, it is important to keep the vertical parts of the channels long along the -axis, since it is designed for space applications. In some of the geometries, pointed or curved transitions were created in order to ensure that the flow fills the channels properly, it can be seen whether the flow creates a vortex while passing through the channels, and the effects of the changes in the velocity of the flow during the transition can be seen in accordance with the geometry. By changing the frequency of the vertical cooling channels and the angle between them, it is desired to observe the effect on the analysis results. Modeling of the cooling channel geometries was carried out in such a way that all cooling channel models have the same active area. The geometric properties of the PEM fuel cell model are given in Table 2.
3.6. Analysis Method of PEM Fuel Cell
PEM fuel cell cooling channel flow analysis was performed using the ANSYS program PEMFC Add-on Module. The PEMFC module is provided as an add-on module to the standard ANSYS Fluent software. A fuel cell is an energy conversion device that converts the chemical energy of the fuel into electrical energy. With the PEMFC model, both the three-phase boundary (TPB), also known as the catalyst layer, and the ionic conductive electrolyte, also known as the membrane in the PEMFC terminology, are included in the calculation area. The PEMFC module was preferred in this study because it provides modeling of polymer electrolyte membrane fuel cells. The mesh structure of the PEM fuel cell model is properly constructed, and the model consists of 522197 nodes and 2635287 elements. The transmission characteristics of PEM fuel cell elements are given in Table 3, and the flow characteristics of the PEM fuel cell model are given in Table 4.
4. Results and Discussion
For the performance of the flow channels in the fuel cell to be high and the fuel cell temperature distribution to be realized homogeneously and to be kept at the operating temperature, the pressure difference should be as low as possible and the flow of the fluid in the channels should be smooth [46]. Flow channel orientation also plays a vital role in fuel cell performance [10]. Since fluid velocity and flow regime are two important parameters in fluid mechanics, the effect of velocity and flow regime on the distribution of temperature and pressure drop in the flow field of fuel cell is investigated [20, 21]. Selected five different cooling channel geometry designs were evaluated by comparing the pressure difference and the transition behavior of the flow in the channels. PEM fuel cell cooling channel flow geometry designs are specified as design 1, design 2, design 3, design 4, and design 5, respectively, and the analysis results are interpreted.
4.1. Pressure Difference between Cooling Channels
The pressure distribution of five different cooling channel designs is given in Figure 5. The pressure distribution of design 1 is considered to be a homogeneous distribution. Since the pressure difference between the inlet and the outlet is low, it is possible to observe a smooth flow profile. It is an expected result that design 1, which is modeled as similar to the parallel flow type, shows a low-pressure difference.
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(b)
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(e)
The pressure distribution of design 2 is also considered to be a homogeneous distribution. Since design 2, which is modeled as similar to the parallel flow type, has a small pressure difference, it is possible to observe a smooth flow profile, but it has a higher-pressure difference compared to design 1. The pressure distribution of design 3 is also considered to be a homogeneous distribution. The pressure difference of design 3, which is modeled similar to the parallel flow type, is small, and it is possible to observe a smooth flow profile. Although this design is similar to design 1 and design 2 in terms of being parallel type, it has a lower pressure difference compared to first two designs since the number of channels is higher. The pressure distribution of design 4 is not homogeneous. The difference between the inlet pressure and the outlet pressure is quite large, so it will be difficult to observe a smooth flow profile. Since this design is modeled similar to the serpentine type, unlike the first three designs of the parallel type, design 4 has a much higher-pressure difference. The pressure distribution of design 5 is also not homogeneous. The pressure drop is quite large. Since design 5 has a large pressure difference, it will be difficult to observe a smooth flow profile. Design 5 is modeled as having the same number of channels as design 4. The reason why design 5 has a lower pressure difference compared to design 4 is due to the smoothing of the pointed curved structure in the channel transitions. Pressure difference comparison of cooling channel designs is given in Table 5. In general, it can be said that flow fields with parallel geometry have much lower pressure drop than serpentine geometry [22].
4.2. Flow Passing through Cooling Channels
The flow in the cooling channel should continue along the channel and maintain its velocity as much as possible. As the flow passes through the channels, it must completely fill the channel [47]. There should be no gaps between the channels where the flow does not pass. In some parts of the channel geometry, layers that formed as a result of the acceleration of the fluid flow, called vortex, may occur due to the flow velocity and the geometry of the flow line. This prevents the flow from flowing smoothly along the channel and is an indication that the geometry is not suitable for the flow. The transition behavior of the flow through the cooling channels is given in Figure 6. When design 1 is examined, it is seen that the flow fills the channels and there are no channels that the flow cannot pass through.
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Although it is seen that the flow velocity in the vertical channels located on the sides is relatively lower than the other channels, it is possible to say that the flow proceeds at a homogeneous speed by looking at the overall design. Some vortex formations are observed in the angular and pointed parts of the vertical side channels. When design 2 is examined, it is seen that the same channel geometry is used as design 1, but the channels are created in a symmetrical way. It is aimed at reducing the gaps that may occur in the flow channels and at progressing the flow rate homogeneously by performing the analysis at the same flow rate with the channels in the symmetrical structure. Although better results were obtained than the first design in terms of no gaps, it is seen that there are still areas where the flow does not pass in the pointed and angular parts of the channels. When the results of the first two designs were examined, it was observed that a curved and smooth transition design should be made in the channel geometry. In design 3, the vortex formation caused by the pointed structure observed on the vertical side edges of the previous designs has been largely prevented by the curved channel structure. The number of vertical channels has been increased to ensure that the vertical flow channels are evenly penetrated with the membrane. It was observed that the fluid filled all the flow channels of design 3 and the formation of voids was very low. When design 4 is examined, vortex formation is observed in the turning regions at the ends due to the pointed and angular shape. Not all of the flow area has been used beneficially, and voids are seen in the channels during flow passage.
On the other hand, there are changes in the flow rate due to the sharp and angular parts. It is seen that there is no homogeneity in flow rate compared to the first three designs. The channel transition regions of design 4 are modeled as pointed, similar to designs 1 and 2. It is seen that the fluid cannot fill all the flow channels of design 4 and there is a void formation. Design 5 is created by making the pointed and angular parts of design 4 curved and has the same number of channels. In design 5, it is still possible to observe vortex formation, even though the vortex formation in the return regions of the flow channel is reduced compared to design 4. Although the flow rate is also made more homogeneous compared to design 4, there are still fluctuations in the flow rate compared to the first three designs. Comparison of cooling channel designs with each other is given in Table 6.
4.3. Temperature Distribution
The temperature distribution of the coolant flow of the selected as design 3 is shown in Figure 7. The cooler inlet temperature is approximately 40°C, and the cooler outlet temperature is approximately 75°C. It is seen that the temperature distribution of design 3 is homogeneous. Cooling channels of design 3 are located on the anode bipolar plate and the cathode bipolar plate. There are hydrogen and oxygen flow channels between the cooling channels and a membrane. The membrane is the area where the electrochemical reaction takes place. The positioning of the cooling channel geometry should be done in such a way that it coincides with the channels that carry hydrogen and oxygen and that it properly covers the membrane active area. Thus, the temperature distribution of the fuel cell will be distributed homogeneously. Considering that the reaction taking place in the membrane takes place at a certain temperature, the temperature must also be kept at a certain level [7].
The cooling channel geometry should follow the membrane with a smooth pattern so that the transition of the reactants to the membrane is properly done. The anode and cathode cooling channels and the anode and cathode gas flow channels positioned one above the other are given in Figure 8. Blue indicates the anode cooling channel, orange indicates the cathode cooling channel, green indicates the anode (hydrogen) gas channel, and yellow indicates the cathode (oxygen) gas channel.
After selecting design 3 as the coolant flow channel, the PEM fuel cell was modeled and thermally analyzed using the ANSYS PEM Add-on Module. The temperature distribution of the PEM fuel cell obtained as a result of the thermal analysis performed is given in Figure 9.
The maximum temperature observed in the fuel cell is approximately 80°C, while the minimum temperature is 26.85°C. According to the results of the analysis, the temperature distribution obtained is compatible with the operating temperature range of the PEM fuel cell. The regions where the temperature is maximum are the areas of the membrane where the electrochemical reaction takes place in the PEM fuel cell. When Figure 9 is examined, it is seen that the temperature distribution is in the PEM fuel cell operating temperature range and the thermal distribution is homogeneous. The coolant flow field design allowed to keep the PEM fuel cell temperature at operating temperature and had an improving effect on the fuel cell performance [48]. The coolant flow area design designed in this context has achieved a successful result in terms of cooling performance according to the design criteria. Design is successful in terms of keeping the stack within the safe range of operating temperatures of 60–90°C [49].
5. Conclusions
In this study, PEM fuel cell cooling flow channel design for space applications has been made and the suitability of the design has been demonstrated by creating a PEM fuel cell model and performing thermal analysis. PEM fuel cell elements were designed and modeled using the SOLIDWORKS. The designed PEM fuel cell model was transferred to ANSYS, and the assembly process was carried out. Flow and thermal analyses of the PEM fuel cell were performed using the ANSYS PEM Add-on Module. Five different PEM fuel cell cooling channels were designed, and according to the analysis results, the optimum cooling channel design was selected by comparing the criteria such as pressure difference, void formation, flow rate homogeneity, vortex formation, and the presence of channels that the flow cannot pass. While modeling the PEM fuel cell, space environment operating conditions, pure reagent supply (oxygen and hydrogen), electrochemical reaction in the membrane, physical properties of liquid cooler, physical properties of fuel cell elements, and heat transfer in the fuel cell are considered. The temperature distribution obtained as a result of the thermal analysis was examined, and it was seen that the PEM fuel cell cooling channel design was successful in maintaining the operating temperature of the PEM fuel cell. Evaluation of the numerical data obtained at the end of the study and evaluations for the development of similar studies to be carried out in the future are given below: (i)Parallel-type flow channels have much lower pressure difference than serpentine-type flow channels(ii)With the same surface area and the same flow rate, the flow proceeds more homogeneously compared to other geometries in the compositions with more channels in the flow geometry and it is also effective in lowering the pressure difference(iii)In two geometry designs with the same active area and number of channels, velocity variation and vortex formation are observed more in the flow channel design, where the regions at the channel transitions are modeled in a pointed structure, compared to the design in which the curves are modeled by softening(iv)When the same channel structure was remodeled symmetrically by reducing the number of channels, the channel occupancy could be increased and the void formation could be reduced compared to the first case with same flow rate, but it was observed that the pressure difference increased(v)It is a fact that a curved smooth transition design for all cooling type channels in general reduces vortex formation, but the parallel structure that proceeds in a straight and arc-free manner is avoided in the channel design. The reason for this is to increase the active area of the channels, in other words, the area in contact with the membrane as much as possible(vi)The positions of the designed cooling channels both relative to each other (anode and cathode side) and relative to the anode and cathode gas flow channels affect the fuel cell cooling performance. Positioning should overlap with the gas channels as much as possible and should proceed in a pattern. This is important in terms of homogeneity of the temperature distribution of the fuel cell and keeping the temperature of the fuel cell within the operating temperature range(vii)Cooler inlet temperature is approximately 40°C, and the cooler outlet temperature is approximately 75°C. This value was obtained as a result of the analysis made with a mixture of 50% ethylene glycol and 50% water selected as the liquid coolant. Various researches on different liquid coolers are available in the literature, and similar design studies related to fuel cells can be applied, and results can be improved(viii)A fuel cell can be created by producing an appropriate bipolar plate with the analyzed PEM fuel cell cooling flow channel design, and current density and homogeneity can be determined by applying single-cell performance tests to the fuel cell(ix)While modeling the PEM fuel cell, a flat serpentine-type design is preferred as the hydrogen and oxygen gas flow channel. In another study, the analysis can be repeated by performing the gas flow channels parallel, serpentine, integrated, radial, spiral, or another much more complex new design
As a result, the cooling flow channel of the PEM fuel cell was modeled together with the other elements of the fuel cell and analyzed for the space environment. In this context, in future studies, a fuel cell model can be produced and a parabolic flight or drop tower experiment can be performed by providing microgravity environmental conditions.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare no conflict of interest.
Authors’ Contributions
Conceptualization was performed by A.N., A.A., and N.Y. A.N. and A.A. were responsible for the methodology. A.N., A.A., and N.Y. were responsible for the software. Validation was performed by A.N. and A.A. Formal analysis was performed by A.N. and A.A. Investigation was performed by A.N. and A.A. A.N. was responsible for the resources. Data curation was performed by A.N., A.A., and N.Y. Writing original draft preparation was performed by A.N., A.A., and N.Y. Writing, review, and editing were performed by A.N., A.A., and N.Y. Visualization was performed by A.N. and A.A. Supervision was performed by A.A. Project administration was performed by A.N., A.A., and N.Y. A.A. was responsible for funding acquisition. All authors have read and agreed to the published version of the manuscript.