Thermal management scheme of hydrogen fuel cell

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Proton exchange membrane fuel cell (PEMFC) has many advantages, such as high efficiency, clean, zero emission and so on. In practical application, 40% ~ 60% of the chemical energy of PEMFC fuel is converted into electrical energy, and most of the remaining energy is converted into heat energy. If the heat can not be discharged from the battery in time, the system temperature will continue to rise, resulting in local single cell or local area overheating phenomenon in the battery, which seriously affects the normal operation of the fuel cell.

The importance of thermal management

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

At present, most PEMFC use Nafion series membrane, the working temperature is not high, usually at 75 ~ 80 ℃. When the temperature is above 80 ℃, the thermal stability and proton conductivity of proton exchange membrane will decrease, and the membrane dehydration will occur when the temperature is serious, resulting in a sharp decrease in conductivity. At the same time, if the working temperature is too high, the attenuation of catalyst will be accelerated. When the temperature is higher than 130 ℃, the membrane will be irreversibly damaged, and local hot spots will lead to membrane perforation, which will eventually affect the safety of PEMFC stack operation. Therefore, in order to keep the fuel cell working in the normal temperature range, the heat must be discharged from the fuel cell.

Cooling scheme of fuel cell

There are three ways of heat dissipation in fuel cells: evaporation of water generated in fuel cells, radiation of stack, and heat carried away by circulating cooling medium. Circulating cooling medium takes away heat is the main heat dissipation way of fuel cell. For PEMFC, the cooling methods are divided into two categories: single phase cooling and phase change cooling.

Single phase cooling

The single-phase cooling method uses the sensible heat of the cooling medium to take away the heat generated in the working process of the fuel cell, which mainly includes air cooling and liquid cooling. It is the most widely used cooling technology at present.

(1) Air cooling

Air cooling is the simplest cooling method. Air is transferred through the cooling plate or cathode to take away the waste heat generated by the fuel cell. The structure of the cooling system is relatively simple. This kind of heat dissipation method is mostly used in small power (≤ 5kW) PEMFC system with few parts, low cost and high system efficiency, such as UAV power system and portable power supply.

2) Liquid cooling

Liquid cooling is to design an independent coolant channel between the cathode and anode plates of the fuel cell, which relies on the forced convection heat transfer of the coolant to take away the heat generated in the working process of the fuel cell.

The coolant can be deionized water or a mixture of water and glycol. Compared with air cooling, liquid cooling has the advantages of high heat transfer capacity and low flow rate. Using liquid cooling method, the temperature distribution of fuel cell is more uniform, but there are many parts, complex structure, and the power consumption of accessories used for heat dissipation is large, which generally accounts for about 10% of the effective output power. Liquid cooling is the most common cooling method for high-power fuel cells, such as vehicle fuel cells.

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

In the coolant circulation part, the coolant enters the stack through the water tank through the cooling water pump, and then flows out of the stack into the thermostat, which automatically adjusts the flow into the large cycle and small cycle. The large cycle passes through the radiator, and the higher heat is taken away by the radiator. The cooling liquid with lower temperature enters the stack through the radiator outlet, and then returns to the cooling water pump inlet after removing the internal reaction waste heat of the stack. The small cycle does not pass through the radiator, and the cooling liquid directly enters the stack through the thermostat outlet, and then returns to the cooling water pump inlet again.

Phase change cooling

Phase change cooling is to cool the heat source by absorbing a lot of heat when the object changes phase. The common phase change cooling methods of fuel cells are evaporative cooling and heat pipe cooling.

(1) Evaporative cooling

The evaporative cooling of fuel cell is that the coolant and air enter the system from the cathode side together, and the selected coolant is deionized water. The coolant can humidify the air, improve the water content of the proton exchange membrane, and improve the performance of the fuel cell; at the same time, most of the coolant will be carried into the core area of the reaction heat source by the air and evaporated, taking away the heat generated by the reaction. Evaporative cooling fuel cell system does not need humidifier, evaporation and condensation heat transfer is more efficient than single-phase convection heat transfer, greatly reducing the load of cooling water pump and radiator.

(2) Heat pipe heat dissipation

Heat pipe heat dissipation is to embed the heat pipe into the bipolar plate, in the case of no external power, the heat pipe will transfer a large amount of heat through the cross-sectional area for long-distance heat dissipation. The material of the heat pipe is usually copper or aluminum alloy, which can ensure that the temperature of the heat source surface is well distributed. The application research of heat pipe cooling technology in the field of fuel cell application has just started, and needs further research and development.

Summary and Prospect

Thermal management is very important to the performance of fuel cells, which affects the efficiency, life and safety of fuel cells. At present, the most widely used technology in the field of fuel cell is single-phase cooling. Phase change cooling technology has the characteristics of uniformity and high efficiency, which is very worthy of research.

At the same time, the effective thermal management control strategy is also the key to ensure the normal operation of the fuel cell. For example, when the temperature of the fuel cell rises, the thermal management system can not provide enough heat dissipation, and the control strategy of the power system control platform should consider measures such as limiting the output power of the fuel cell, so as to improve the life, safety and durability of the fuel cell.

In order to improve the heat dissipation ability of fuel cell thermal management system, it is necessary to improve the working temperature of fuel cell and the temperature characteristics of fuel cell materials. For example, if the working temperature of the fuel cell is raised to 95 ℃, the heat dissipation power of the thermal management system will be increased by more than 50%. According to the performance roadmap of fuel cell stack released by NEDO of Japan, the target maximum operating temperature of fuel cell stack is 120 ℃ by 2040. Increasing the working temperature of fuel cell is the fundamental way to solve the technical bottleneck of fuel cell thermal management system.