Jokingly speaking, in the field of energy storage temperature control, the first generation was air cooling, the second generation and currently the dominant one was cold plate liquid cooling, and immersion liquid cooling was still striving to become the third generation. Suddenly, direct cooling emerged and entered the market in a high-profile manner, competing for the position of the third-generation successor.

China's energy storage industry has entered a stage of rapid development, and continuous technological innovation and the synchronization of multiple technological routes are one of the important manifestations of this period.

In particular, as energy storage cells evolve towards larger capacity, system integration develops towards larger scale and higher energy density, and application scenarios become more complex and diverse, all of which put forward higher requirements on the life, safety, cost and other factors of energy storage systems. From system integration to core components including cells, 3S, temperature control and fire protection, technology iteration is continuing.

As a key link in the energy storage system, the temperature control system plays a vital role in the safety, efficiency, and life of energy storage. Especially with the increasing demand for applications such as long-term energy storage and high-rate energy storage, the overall performance indicators for temperature control components have been raised.

From the first generation of air cooling, to the current mainstream cold plate liquid cooling, to the immersion liquid cooling that is receiving widespread attention, the temperature control technology has been multi-pronged in recent years to continuously optimize issues such as the battery's susceptibility to heat and uneven temperature distribution.

At the beginning of the month, another big news came: CRRC Zhuzhou Institute, together with 14 industry chain companies including Invic, Hisense Network Energy, Tongfei Co., Ltd., and Midea, released a future-oriented 6.9MWh system, in which the temperature control link used a 12kW energy storage direct cooling unit for the first time. As soon as this news came out, it attracted the attention of the industry.

The direct cooling technology, which was originally used in the field of new energy vehicles, has entered the energy storage industry with great fanfare. There are voices of high-profile support, as well as voices of objection.

In the past two years, the global installed capacity of renewable energy has grown rapidly. According to the annual market report "Renewable Energy 2023" released by the International Energy Agency, in 2023, the global installed capacity of renewable energy will increase by 50% compared with 2022, and the growth rate of installed capacity has exceeded that of the past 30 years. Against this background, the development of the energy storage industry has ushered in an increasingly broad market space.

At the same time, Chinese energy storage companies are caught in a vortex of internal circulation. To break out, technology is the most core competitiveness, while high safety, low cost and high efficiency are the most important thresholds for upgrading energy storage technology.

Especially with the trend of large-scale battery cells and increasing integrated power density of energy storage systems, battery efficiency and the risk of thermal runaway have become the focus of the industry. Among them, the temperature control system plays an important role.

Looking closely at the advancement of energy storage temperature control technology, the first-generation air cooling system was simple, low in manufacturing cost, and easy to install; the second-generation cold plate liquid cooling began to use liquid as the heat exchange medium, with large heat carrying capacity and high heat exchange efficiency; and immersion liquid cooling, which is still in its early stages of development, has the advantages of effectively preventing thermal runaway and extreme temperature uniformity, but is trapped by the problem of high cost and has not yet been settled.

At a time when the industry is developing rapidly and technology is rapidly iterating, direct cooling suddenly ended in a high profile. It is reported that the above-mentioned 12kW energy storage direct cooling unit adopts refrigerant direct cooling technology, which reduces heat exchange losses, makes the system more energy efficient, and reduces costs; at the same time, it adopts a design that does not require water circulation, and the risk of leakage is "zero". The unit is smaller in size and has lower noise, and can provide a larger cooling capacity in a limited space, which is in line with the development trend of increasing energy density of energy storage systems and decreasing available space.

Some supply chain companies said that direct cooling temperature control technology will provide more options and directions for the development of the energy storage industry, and is expected to become the main development trend in the field of energy storage thermal management in the future.

Some companies have bluntly stated that because the heat generated by the battery cells is not concentrated enough and the heat generated per unit area is not very large, there is no need for high-intensity heat transfer refrigeration technology such as direct cooling to solve the problem.

What exactly is direct cooling? According to public information, direct cooling is a minimalist cooling design that does not require water circulation, allowing the refrigerant to directly cool the battery cell through the fluorine cold plate, and quickly remove the generated heat through heat exchange.

At present, the more common temperature control technologies are mainly air cooling and cold plate liquid cooling, and immersion liquid cooling is still in the early stages of development. Among the four temperature control technologies shown in the table above, except for air cooling, which uses air as the cooling medium, cold plate liquid cooling, immersion liquid cooling and direct cooling all use liquid.

Among the three liquid cooling technologies, only immersion cooling uses direct contact by immersing the battery cells directly in the immersion liquid without any heat transfer link in between. Cold plate liquid cooling and direct cooling both use indirect contact.

From the structural point of view, direct cooling and cold plate liquid cooling are quite similar. Industry insiders said that the traditional cold plate liquid cooling technology dissipates heat to the bottom of the battery by introducing cold water into the liquid cooling plate, while direct cooling replaces the water in the cold plate liquid cooling with a refrigerant, which is then used to cool the battery cell through a fluorine cold plate.

However, although the forms are similar, the heat exchange principles of these two technologies are not exactly the same.

In direct cooling, on the one hand, temperature difference heat exchange is utilized. Because the refrigerant temperature is relatively low and the refrigerant itself has a specific heat capacity much greater than that of water, higher heat exchange efficiency can be achieved. On the other hand, direct cooling also utilizes the principle of evaporation heat absorption, absorbing the surrounding heat by transforming the refrigerant from liquid to gas.

In this regard, some industry insiders explained that "the high coupling of the battery cooling system with the air-conditioning system is equivalent to putting the evaporator in the air-conditioning system directly into the battery pack."

It can be seen that the amount of heat that can be removed by direct cooling in this dual heat exchange method is far greater than that of cold plate liquid cooling that simply relies on temperature difference heat exchange. The excellent heat exchange capacity and overall efficiency of the machine do make direct cooling seem to have considerable market space in the field of energy storage.

In fact, the idea of applying direct cooling temperature control technology to the field of energy storage has been proposed for a long time, but related products and applications are relatively rare, even in new research applications. The reason is that direct cooling technology still has many problems that have not been broken through.

In the promotion of direct cooling temperature control products, safety is often placed in a very prominent position. It is reported that once a leak occurs, the refrigerant will automatically evaporate into gas, making the risk of leakage zero, and can effectively avoid electrical short circuits and thermal runaway caused by leakage of conventional cooling media.

It is worth noting that the direct cooling system faces greater pressure intensity. On the one hand, the pressure of fluorine is much greater than that of water. The water pressure is only a few kilograms, but the fluorine pressure is dozens of kilograms higher than that; on the other hand, the evaporation pressure of the refrigerant generally reaches 3-4 atmospheres, while the working pressure of the liquid cooling plate is generally within 1.3 atmospheres.

Therefore, direct cooling will greatly increase the pressure-bearing strength requirements of the cold plate, joints, and pipelines. For example, conventional nylon pipes cannot withstand such pressure at all. The pressure resistance level of the direct cooling plate must be at least 4 times the evaporation pressure.

In addition, direct cooling has much higher requirements for the sealing of the cold plate than traditional liquid cooling.

All these factors will make it very difficult for supply chain companies to iterate their technology, and the cost of parts will also increase accordingly. In terms of system control, direct cooling is also more complicated because it is necessary to consider the flow distribution between different PACKs, the control of evaporation temperature, and the design of the cold plate flow channel, etc.

Taking the design of the refrigerant flow direction in the direct cooling plate as an example, the battery pack must not only ensure that the battery cells operate at a reasonable temperature, but also control the temperature difference between different modules. Generally, the temperature difference of the battery cells is required to be no more than 5°C. Therefore, it is particularly important to ensure the uniform temperature of the battery cold plate itself. Therefore, optimizing the flow direction of the refrigerant in the direct cooling plate and improving the temperature uniformity of the energy storage battery are the difficulties that the direct cooling system needs to overcome.

It can be seen that there are still many problems for direct cooling technology to be truly applied in the field of energy storage, and it will take a long time to achieve large-scale application.