Cooling scheme for thermal management of proton exchange membrane fuel cell (PEMFC)

Proton Exchange Membrane Fuel Cells (PEMFC) boast advantages such as high efficiency, cleanliness, and zero emissions, making them promising for widespread application. In practical applications, 40% to 60% of the chemical energy from the fuel is converted into electrical energy, while the remaining energy is mostly converted into thermal energy. If heat cannot be promptly dissipated from the cell, the system temperature will continue to rise, leading to localized overheating of individual cells or specific areas within the cell, severely impacting the normal operation of the fuel cell.

I. Importance of thermal management

The main heat sources in the process of fuel cell operation are ohmic resistance heating, reaction entropy heat, irreversible electrochemical reaction heat, water vapor condensation heat release, compressed air heat and environmental radiation heat, the latter two can be ignored.

  II. Cooling scheme for fuel cells

The main heat dissipation pathways for fuel cells are threefold: water vaporization from within the cell, radiative cooling of the stack, and heat removal by circulating cooling media. The latter is the primary method of heat dissipation for fuel cells. For PEMFCs, cooling methods can be broadly categorized into two types: single-phase cooling and phase change cooling.

1. Single-phase cooling

Single-phase cooling method is to use the sensible heat of cooling medium to take away the heat generated in the working process of fuel cell. There are two types: air cooling and liquid cooling, which are the most widely used cooling technology at present.

(1) Air cooling

Air cooling is the simplest method of cooling, where air passes through cooling plates or cathodes to carry away waste heat generated by fuel cells. The structure of the cooling system is also relatively simple. This type of heat dissipation is commonly used in low-power (≤5kW) PEMFC systems that have fewer components, lower costs, and higher system efficiency, such as in drone power systems and portable power sources.

Fuel cell system with air cooling

(2) Liquid cooling

Liquid cooling is designed to separate the coolant flow path between the cathode and anode plates of the fuel cell, and relies on forced convection heat transfer of the coolant to remove the heat generated during the operation of the fuel cell.

The coolant can be deionized water or a mixture of water and ethylene glycol. The specific heat capacity of liquids is greater than that of air, making liquid cooling more efficient in terms of heat transfer and lower flow rates compared to air cooling. Using liquid cooling, the temperature distribution in fuel cells becomes more uniform; however, it involves many components and complex structures, with significant power consumption for accessories used in heat dissipation, typically around 10% of the effective output power. For high-power (over 5 kW) fuel cells, such as those used in vehicles, liquid cooling is the most commonly employed method.

Take the vehicle fuel cell as an example, its thermal management system mainly includes coolant pump, heat exchanger, water tank, fan, pressure sensor and other components.

  III. Phase change cooling

Phase change cooling is to cool the heat source by using the characteristic of absorbing a large amount of heat when the object changes phase. The commonly used phase change cooling methods in fuel cells are evaporation cooling and heat pipe heat dissipation.

(1) Evaporative cooling

The evaporative cooling of fuel cells involves the coolant and air entering the system from the cathode side together. The coolant typically used is deionized water. The coolant can humidify the air, increasing the moisture content in the proton exchange membrane, thereby enhancing the performance of the fuel cell. At the same time, most of the coolant is carried into the core area of the reaction heat source by the air and evaporates, carrying away the heat generated during the reaction. An evaporative cooling fuel cell system does not require a humidifier, as evaporation and condensation heat exchange are more efficient than single-phase convection heat exchange, significantly reducing the load on the cooling water pump and radiator.

(2) Heat pipe heat dissipation

Heat pipe cooling involves embedding the heat pipe into a bipolar plate. In the absence of external power, the heat pipe transfers a large amount of heat over long distances through its cross-sectional area for cooling. The material of the heat pipe is typically copper or aluminum alloy, ensuring that the temperature at the heat source remains well-distributed. Research on the application of heat pipe cooling technology in fuel cell applications is still in its early stages and requires further development.

Thermal management is crucial for the performance of fuel cells, affecting their efficiency, lifespan, and safety. Currently, the most widely used technology in the fuel cell field is single-phase cooling. Phase change cooling technology, with its uniformity and high efficiency, is a highly promising research direction. At the same time, effective thermal management control strategies are key to ensuring the proper operation of fuel cells. For instance, when the temperature of the fuel cell rises and the thermal management system cannot provide sufficient heat dissipation, control strategies on the power system platform should consider measures such as limiting the output power of the fuel cell to enhance its lifespan, safety, and durability. To improve the heat dissipation capability of the fuel cell thermal management system, efforts must also be made to increase the operating temperature of the fuel cell and improve the temperature characteristics of the fuel cell materials. For example, if the operating temperature of the fuel cell is increased to 95℃, the heat dissipation capacity of the thermal management system can be improved by more than 50%.