Unlike lithium-ion batteries, vanadium flow batteries store energy in a non-flammable, liquid electrolyte and do not degrade with cycling. They hold the promise of more than 10-hour duration storage, high recyclability, and 25 years or more lifespan. However, these batteries suffer from comparatively low energy densities as well as complex thermal issues, which are important to understand in order to achieve their wide application and facilitate the design of next-generation VRFBs with high-power density.
To help understand how to operate VRFB systems efficiently and stably at high temperatures, scientists at the Hong Kong University of Science and Technology in China have composed a review in which they summarized the thermal issues of VRFBs reported in the literature to date and existing thermal management methods.
Complex thermal issues caused by excessive heat generation during high-rate operations and various heat transfer behaviors in diverse climates dramatically affect the efficiency and stability of VRFBs.
Therefore, according to the review, the operating temperature should be maintained in the range of 10 C to 40 C to ensure VRFBs with high efficiency, weak side reactions, high electrolyte stability, and low crossover.
“The thermal management system of VRFB can maintain the electrolyte temperature in the range of 10 C to 40 C in most climates,” researcher Ren Jiayou told pv magazine. “However, in extreme low temperature climates below -30 C, the thermal management system cannot operate stably.”
To ensure the electrolyte stability, it is important to predict the operating temperature of a VRFB system by cost-effective and efficient thermal models. These include two-dimensional, three-dimensional, and lumped models, as summarized in the research paper, noting that in the future, a combination of these approaches could enhance the simulation accuracy and calculation speed simultaneously.
Furthermore, existing thermal management methods are analyzed. Here, employing titanium heat exchangers with anti-corrosive properties to adjust the temperature of electrolytes is recommended as an efficient way to maintain the operating temperature of VRFB systems.
The paper takes the example of a 15 MW/60 MWh VRFB system applied in the Minami-Hayakita substation, Japan. In this project, the heat transfer through pipes plays the main role.
Specifically, when the temperature of electrolytes is higher than the upper limit of the safety threshold, the intake dapper and outlets of a VRFB stack will be opened, allowing the forced air flowing through the heat exchanger to carry the heat to the outside. When the electrolyte temperature is lower than the upper limit of the safety temperature threshold, the intake dampers and outlets will be closed.
In addition to that, with the range of ambient temperature at -10 C to 25 C, the room temperature could be maintained through a passive method, using the heat generation of VRFBs. Based on these two temperature control modes, the electrolyte temperature is maintained in the range of 30 C to 40 C.
Finally, the review highlights the remaining challenges to enhance the efficiency and stability of VRFBs under harsh thermal conditions.
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