Thermal management of flow batteries

Liquid flow batteries (RFBs) generate a lot of heat during operation. If the heat cannot be dissipated in a timely and effective manner, the battery temperature will rise, thus affecting the battery performance and safety. The electrochemical reaction conditions, ion conductivity, the rate at which ions move across the membrane, and the viscosity of the electrolyte are all closely related to the temperature during operation. Specifically, increasing the temperature can increase the reaction rate constant and promote the reaction kinetics in the electrochemical reaction. At the same time, high temperature will also reduce the viscosity of the electrolyte, thereby increasing the transmission efficiency of vanadium ions from the main body to the electrode surface and reducing the concentration polarization potential. However, when the temperature exceeds a certain range, it will have a fatal effect.

Taking the vanadium redox flow battery (VRFB) as an example, its normal operating temperature range is 0~40°C. As the temperature increases, the hydrogen evolution reaction on the negative electrode will be significantly enhanced, resulting in a decrease in Coulombic efficiency. At the same time, the diffusion ability of vanadium ions through the ion membrane is enhanced, which intensifies the capacity fading. In addition, the vanadium active ions in the electrolyte are unstable and prone to precipitation when the temperature is abnormal. When the electrolyte of 2 mol/L VO+2+3 mol/L H2SO4 is placed at 40°C for 2 days, the VO+2 Converted into V2O5 precipitation; and after being placed at 15°C for 7 days, V2+ in the electrolyte will precipitate. This generated precipitate will block the flow channel, cover the carbon felt and ion membrane, lead to increased pump power loss and battery failure.

Sustained high temperature will also accelerate the aging of the internal electrodes, proton membrane and other materials of the battery, thus shortening the service life of the battery. Therefore, temperature thermal management is of great significance to maintaining the stable operation of flow batteries.

In order to ensure the stable and safe operation of flow batteries, it is necessary to establish a thermal model to predict and control the temperature of the electrolyte and further guide battery optimization control, which is also an important part of the thermal management system.

The factors that generate heat during the operation of all-vanadium liquid flow batteries include electrochemical reactions, overpotential, hydraulic friction, cross-reactions and shunts, among which electrochemical reactions and overpotential heat generation account for a larger proportion compared to the other three.

At present, the thermal management technology routes of electrochemical energy storage systems are mainly divided into four categories: air cooling, liquid cooling, heat pipe cooling and phase change cooling. The mainstream technology routes for thermal management of liquid flow battery energy storage in the market are air cooling and liquid cooling. The choice of these heat dissipation methods depends on the scale, design, operating conditions and cost-effectiveness of the battery.

1) Air cooling

Air cooling is wind cooling, which uses air as a medium to remove the heat inside the system by heat conduction and heat convection, thereby cooling the system. Air cooling is divided into natural air cooling and forced air cooling according to the driving mode. Natural air cooling uses natural conditions such as natural wind pressure, air temperature difference, and air density difference to achieve a cooling effect on the battery.

The convection heat transfer coefficient of natural air cooling is much lower than that of forced air cooling, so it is difficult to completely dissipate the heat generated by the battery. For low-rate charge and discharge of the battery, the system temperature can be controlled within a certain temperature range, but the increase in the system current density can easily cause the temperature to exceed the limit range. Therefore, although natural air cooling has the advantages of simplicity, lightness and low cost, its scope of application is extremely small and it is rarely studied now. Forced air cooling is to take away heat through forced airflow generated by a blower or fan. At this time, the heat transfer coefficient of the forced airflow is greatly improved. Compared with liquid cooling, air cooling has the advantages of simple structure, easy maintenance and low cost, but it requires a certain amount of electricity, and the heat dissipation efficiency, heat dissipation speed and temperature uniformity are poor. It is usually suitable for small or medium-sized battery systems.

2) Liquid Cooling

Liquid cooling (liquid cooling) uses coolant as the medium and utilizes higher specific heat and heat transfer coefficient to dissipate heat. Liquid cooling systems can provide higher heat dissipation efficiency and better temperature control effects, but the system complexity and cost are also relatively high, and are suitable for large battery systems. Commonly used coolants include water, ethylene glycol aqueous solution, pure ethylene glycol, air conditioning refrigerant, and silicone oil. Since the charge in the electrolyte of the flow battery easily flows along the coolant to the entire system, it is more dangerous, so the choice of cooling medium is also very important. However, the most common method for flow batteries is to use corrosion-resistant and non-conductive heat exchangers. The internal materials are generally the same as those of the electrolyte storage tanks, using PVC or PP, or using titanium metal tubular heat exchangers, and the inner surface is covered with a corrosion-resistant TiO2 layer to protect the heat exchanger from sulfuric acid corrosion.

As one of the most promising renewable energy storage technologies, the overheating problem of vanadium flow battery during operation greatly affects the efficiency and stability of the system. Therefore, various feasible methods are needed to provide a feasible solution for the VRFB thermal management system.