Application about Heat Pipes in Thermal Management of Fuel Cells

Proton exchange membrane fuel cell (PEMFC) adopts heat management technologies such as air cooling and liquid cooling, which can effectively transfer excess heat from the battery. However, auxiliary work is required to drive the fluid flow, which undoubtedly reduces the overall power of the battery. The pulsating heat pipe (PHP), as a novel heat dissipation device, can provide effective thermal management in PEMFC due to its compactness, fast heat transfer, and no auxiliary working support.

Proton exchange membrane fuel cells (PEMFC) are considered to be the most promising candidates for next-generation transportation, stationary, auxiliary, and portable applications due to their advantages such as low operating temperature, high power density, fast start-up, transient capability, and low emissions. Despite extensive research and progress on fuel cells, there are still several technical obstacles to their commercialization, especially in terms of their durability and cost. Due to electrochemical reactions and electrical resistance, a large amount of heat is generated in the fuel cell stack, which is almost equivalent to the electrical power output, so effective thermal management should be carried out to avoid overheating of components and to guarantee the favorable operating temperature range of current PEMFC (typically in the 60~80℃). Improper thermal management and uneven temperature distribution within the fuel cell stack can lead to electrolyte drying (global or local) or electrode flooding, both of which degrade fuel cell performance. On the other hand, the temperature difference between PEMFC and ambient temperature is very small compared to internal combustion engines, so proper thermal management of PEMFC battery packs is very challenging, especially when high power output and high power density are required. stack automotive applications.

Commercial PEMFC cooling is usually performed by forced convection of air or water, however, the adopted air cooling method consumes a significant portion of the battery power and reduces the overall range of electric vehicles. Toyota, for example, employs fan-forced convection cooling for thermal management, utilizing about 40 percent of the battery's energy. In high-power PEMFC stacks, the liquid cooling method is most widely used due to the high heat transfer coefficient, and so far, a lot of work has been done by designing the coolant flow field parameters, cooling channel geometry, developing alternative coolants and cooling systems, This is achieved with minimal additional energy loss and uniform temperature distribution throughout the battery. With the advancement of technology, PEMFC is gradually miniaturized and centralized. At the same time, higher power densities can be achieved. Therefore, the traditional cooling method can no longer meet the demand. Developing and designing a more efficient cooling method has become a research hotspot in the academic field at home and abroad.

Due to the high thermal conductivity and no additional power input, heat pipes can transfer a large amount of heat over a considerable distance even with a small cross-sectional area. Heat pipes allow efficient and timely transfer of heat generated within the battery pack to the environment, or use of waste heat generated by fuel cells. The pulsating heat pipe (PHP) has the advantages of small size, light weight, good effective thermal conductivity, and low thermal gradient. The use of PHP allows for a more uniform temperature distribution in the PEMFC and ultimately improves its performance by avoiding the disadvantages of local temperatures.

The structure of PEMFC is shown in Figure 1, which consists of bipolar plates and membrane electrodes. Bipolar plates are assembled on both sides of the membrane electrodes to provide gaseous reactants for electrochemical reactions. Membrane electrodes consist of two gas diffusion layers, two catalytic layers and a proton exchange membrane.

The working principle of PEMFC is that hydrogen and oxygen (air) are delivered to the anode and cathode respectively. At the anode electrode (CL), hydrogen is oxidized and split into protons and electrons. Protons pass through the PEM to the cathode, while electrons are conducted from the anode to the cathode through an external circuit. On the cathode electrode, oxygen diffuses from the flow field to the electrode through the GDL/MPL. Where electrons and protons combine with dissolved oxidant (oxygen) to produce water and heat. The basic electrochemical reaction of a fuel cell is as follows.

The formula is for the hydrogen oxidation reaction that occurs in the anode electrode.

The formula is the hydrogen oxidation reaction that takes place in the cathode electrode.

The formula represents the entire reaction process.

The energy conversion efficiency of fuel cells is about 50%. This suggests that nearly half of the energy will be released as heat during operation. The main heat sources of fuel cells are entropic heat of reaction (30%), irreversible heat of electrochemical reaction (60%), Joule heat of ohmic resistance (10%) and latent heat of phase change of water. The energy flow in PEMFC is shown in Figure 2.

The amount of heat generated in a fuel cell stack can be determined by comparing the operating voltage to the thermal neutral voltage or the thermal voltage of individual cells. This process is represented by the following formula.

In the formula, Q is heating rate. Eth is the thermoneutral voltage of the fuel cell, which represents the maximum voltage of a single cell under the assumption that the transmission efficiency of the fuel cell reaches 100%. Vcell is operating voltage. i is current density. Acell is active area of a single cell.

Traditional heat pipes are made of sealed tubes with a wick structure. After evacuation, the heat pipe is injected with working medium. Under the action of heat input at the evaporation end, the medium will be heated and evaporated. Due to the small pressure difference, the vapor reaches the condensing end and condenses into a liquid state at the condensing end. The capillary force provided by the liquid-absorbing core structure returns the condensed fluid to the evaporating end to realize the circulating flow of the working fluid. The two-phase circulating flow in the heat pipe provides it with good thermal conductivity.

Different from traditional heat pipes, as a new type of heat pipe, the pulsating heat pipe (PHP or OHP for short) is a high thermal conductivity device that relies on the gas-liquid phase change of the internal working fluid to achieve large heat flux transmission. It was first proposed by Japanese scholar Akachi in the 1990s Suggested.

The working mechanism of the pulsating heat pipe is mainly to utilize the vapor-liquid plug formed by the working fluid in the pipe. The internal pressure changes due to the phase change of heat input, which in turn causes the working medium in the tube to oscillate irregularly to realize heat transfer. Figure 3 describes the working principle of the closed loop pulsating heat pipe in detail. The pulsating heat pipe is bent by a capillary to form a serpentine loop structure. According to different thermal boundaries, it is divided into three parts: evaporation end, condensation end and adiabatic end. The pulsating heat pipe, which is in a vacuum state inside, is filled with working fluid through the liquid filling port. According to the capillary principle, the friction and surface tension of the liquid working medium and the wall are balanced with its gravity. Therefore, the vapor-liquid plugs are distributed in phases inside the pipeline. With the heat input from the evaporating end of the pulsating heat pipe, the working medium in the tube absorbs heat and evaporates to generate bubbles, which gradually grow to form a vapor plug under the action of internal pressure. As the pressure of the vapor plug further increases and reaches a certain value, it can overcome the gravity and frictional resistance of the liquid plug and push the adjacent liquid plug to move to the condensation end, thereby realizing heat transfer. When the vapor-liquid plug reaches the condensation end, the vapor plug condenses and releases a large amount of latent heat. At the same time, the temperature of the liquid plug working medium is also reduced due to heat release. Finally, the working fluid realizes the pulsating circulation flow under the joint action of thermal driving force, surface tension, capillary resistance and gravity.

At present, most research institutions have conducted more experiments and simulations on working fluids such as water, methanol, ethanol, acetone, and boiling immiscible working fluids. In general, alcohols have relatively low boiling points, relatively high latent heats of vaporization, and ratios of saturation pressure gradients to temperatures. It works with PHP's quickstart feature. Under high heating power conditions, the PHP with mixture can start quickly and achieve a stable unidirectional pulsating cycle. Nanofluids have excellent physical properties. It is of great significance to overcome capillary return and promote fluid circulation and oscillation.

The configuration of the PEMFC stack cooled by PHP is as follows. There are 5 battery cells and PHP in the stack. Each battery cell is sandwiched between two pulsating heat pipes (individual cells are represented by C1-C5), as shown in Figure 4. A complete battery cell consists of an anode flow plate (AFP), a cathode flow plate (CFP) and a membrane electrode assembly (MEA). MEA includes membrane, anode catalyst layer (ACL), anode gas diffusion layer (AGDL), cathode catalyst layer (CCL), cathode gas diffusion layer (CGDL). It is one of the viable combinations of PEMFC and FPHP.

In summary, PHP has the advantages of small size, light weight, good effective thermal conductivity, and low thermal gradient. By avoiding the disadvantages of local temperatures, the use of PHP allows for a more uniform temperature distribution in PEMFC and ultimately improves their performance. The pulsating heat pipes containing Nanofluids have potential value as a new heat dissipation device to improve the thermal management of PEM-FC.