In-depth research report on energy storage systems

1. Brief introduction of energy storage systems

① Energy storage technology routes and application fields

According to the different forms of energy storage systems, it is divided into mechanical energy storage, chemical energy storage, electromagnetic energy storage, thermal energy storage and hydrogen energy storage. Among them, chemical energy storage systems (electrochemical energy storage) refers to various secondary battery energy storage, mainly including lithium-ion batteries, lead batteries and sodium-sulfur batteries.

Electrochemical energy storage systems are currently the most widely used power energy storage technology with the greatest development potential. Compared with pumped hydro storage, electrochemical energy storage systems are less affected by geographical conditions and has a short construction period, and can be flexibly used in various links of the power system and other scenarios. As costs continue to drop and commercial applications become more mature, the advantages of electrochemical energy storage technology become more and more obvious, and it has gradually become the mainstream of newly installed energy storage capacity.

In the future, with the further emergence of the scale effect of the lithium battery industry, the cost of electrochemical energy storage systems still have a lot of room for decline, and the development prospect is broad. From the perspective of the entire power system, the application scenarios of energy storage systems can be divided into three scenarios: energy storage on the generation side, energy storage on the transmission and distribution side, and energy storage on the consumption side. 

Among them, there are many types of demand scenarios for energy storage systems on the power generation side, including power peak regulation, auxiliary dynamic operation, system frequency regulation, and grid connection of renewable energy. Besides, off-grid battery systems are also available on the market. 

The development and utilization of renewable energy has continued to increase, and the proportion of access to the grid and the proportion of final energy consumption has continued to increase. According to research, in order to meet the demand for new energy consumption, it is predicted that the United States, Europe, China and India will need to increase the grid-connected power storage capacity by 310GW by 2050, and at least 380 billion US dollars will be invested for this purpose. Taking the power system as an example, energy storage can provide functions such as frequency adjustment, load tracking, peak shaving and valley filling, and backup power.

② Composition of electrochemical energy storage systems

The electrochemical energy storage industry chain can be roughly divided into upstream raw material and equipment suppliers, midstream integrators and downstream applications. The upstream of the energy storage industry chain mainly includes battery raw material and production equipment suppliers; the midstream is mainly suppliers of batteries, battery management systems, energy management systems and energy storage converters; the downstream is mainly energy storage systems integrators, installers and terminals users, etc.

A complete electrochemical energy storage systems are mainly composed of battery pack, battery management system (BMS), energy management system (EMS), energy storage converter (PCS) and other electrical equipment. The battery pack is the most important part of the energy storage systems; the battery management system is mainly responsible for the monitoring, evaluation, protection and balance of the battery. The energy management system is responsible for data acquisition, network monitoring and energy scheduling.

The energy storage converter can control the storage battery. It can perform the conversion of AC and DC during the charging and discharging process of the battery pack. Electrochemical energy storage systems has the characteristics of flexibility and great potential for future development. Since lithium batteries are currently the mainstream technology, the rest of this article will use lithium batteries as an example.

Energy storage power stations mostly exist in the form of centralized boxes. With the rapid development of smart grids, a new type of prefabricated box production mode has emerged based on the concepts of standardized design, factory processing, and modular construction. The container or prefabricated cabin contains lithium battery system, battery management system, energy storage converter, energy management system, thermal management system, fire protection system, etc.

③ Electrochemical energy storage systems

Thermal management is the process of adjusting and controlling the temperature or temperature difference by means of heating or cooling according to the requirements of specific objects (such as battery systems, base stations, IDCs, new energy vehicles). Taking lithium batteries as an example, only lithium-ion batteries in a temperature environment of 15-20 °C can achieve higher working efficiency. If the battery operating temperature exceeds 50°C, the battery life will decline rapidly, and may even cause safety accidents.

Reasonable thermal management (or temperature control) design is required in the design of the energy storage systems to ensure two thermal management indicators: one is to ensure that the battery surface temperature is within a certain range, and the other is to keep the temperature difference between the batteries small. The thermal performance of lithium batteries is directly related to the discharge rate, and the thermal performance also determines the operating efficiency, safety and reliability of the battery itself. When the discharge rate of the battery increases, the capacity that the battery can discharge decreases, and the discharge plateau decreases, making it stable and causing the battery temperature to rise.

However, when discharging at a large rate (ie, above 1.5 C), the operating temperature of the lithium battery will exceed the ideal operating temperature. Lithium battery thermal runaway refers to the fact that the battery generates a large amount of heat in a short period of time due to an internal short circuit or an external short circuit, causing the positive and negative active materials and the electrolyte to react and decompose, generating a large amount of heat and flammable gas, causing the battery to catch fire or explode.

Different battery materials have different thermal stability, and thermal runaway is the most serious safety accident of lithium batteries. In the process of thermal runaway, from low temperature to high temperature, lithium batteries will sequentially experience:

1. High temperature capacity decay; 
2. SEI film decomposition; 
3. Anode-electrolyte reaction; 
4. Separator melting; 
5. Cathode decomposition → electrolyte solution decomposition → anode and binder Reaction; 
6. Electrolyte combustion and other processes.

2. Brief introduction of energy storage thermal management industry

① Current technology of thermal management technology

Thermal management technologies are mainly air cooling, liquid cooling, heat pipe cooling and phase change cooling technologies. Different thermal management techniques can be used for application scenarios with different heat production rates and ambient temperatures.

1.  Air cooling: The thermal management technologies are mainly air cooling, liquid cooling, heat pipe cooling and phase change cooling technology. Different thermal management techniques can be used for application scenarios with different heat production rates and ambient temperatures. The specific heat capacity of air is low, and the thermal conductivity is also very low, which is difficult to meet the heat dissipation of energy storage systems with large capacitance, and the temperature difference between the inlet and outlet battery packs is too large, that is, the battery heat dissipation is uneven. It is relatively suitable for household low-power energy storage cabinets.
2. Liquid cooling: Using liquid as the cooling medium, the heat generated by the battery is taken away by convection heat exchange. The structure of liquid-cooled equipment is more complicated than that of air-cooled equipment, and the requirements for product quality and sealing design are quite strict. When the battery density is high, liquid cooling is suitable.
3. Phase change cooling: a cooling method that uses the phase change of the phase change material to absorb heat. The biggest influence on the heat dissipation effect of the battery is the selection of the phase change material. When the selected phase change material has a larger specific heat capacity and a higher heat transfer coefficient, the cooling effect will be better under the same conditions, and vice versa.
4. Heat pipe cooling: relying on the phase change of the cooling medium in the tube to achieve heat exchange, the phase change process can absorb or release a large amount of heat. Heat pipe cooling technology is suitable for lithium battery systems that often work in high-rate conditions, such as fast-charging battery systems, frequency modulation energy storage systems, etc.

② Thermal management technology principle and system composition

The key to air cooling is the air duct design. Generally, the air from the top of the air conditioner is used to send the cold (hot) air evenly to each battery cabinet through the air duct through the wind wall formed by the battery and the bulkhead. At the same time, the battery cabinet is designed with heat conduction holes to ensure that the cold (hot) energy can be smoothly To every battery pack, every module. At the same time, the air conditioner is provided with an upper air return port, and the air outlet and return air form an air circulation in the battery compartment.

Meanwhile, the thermal simulation software is used for correction and verification to ensure that the temperature difference between the batteries can also be controlled within 5 °C. The liquid cooling system mainly includes the battery liquid cooling plate, the water distribution line and the refrigeration/liquid supply system.

Equipment such as condensers and compressors force the cooling of the cooling liquid. After the low-temperature cooling liquid flows through the interior of the battery system and exchanges heat with the battery cells, it flows back to the heat exchanger for heat exchange with the low-temperature refrigerant, thereby transferring the heat generated by the battery to the battery. out the battery system.

There are two contact modes between the liquid and the battery: one is direct contact, the battery cell or module is immersed in the liquid, and the liquid is directly cooled by the liquid; the other is to set up a cooling channel or cold plate between the batteries to cool the liquid indirectly battery. The former generally uses organic substances such as silicone-based oil and mineral oil, and the latter generally uses water, ethylene glycol, a mixture of ethylene glycol and water, and the like.

At present, the liquid cooling system has been more popular in the field of new energy vehicles, but there are few applications in the energy storage power station. Now major manufacturers have begun to increase the research and development and application of the energy storage liquid cooling system.

③ Air cooling is the core of thermal management of energy storage system

In terms of lithium ion battery pack temperature, under the same inlet temperature and limit wind speed and flow rate, the temperature of the liquid-cooled battery pack is 30-40°C, while the temperature of the air-cooled battery pack is 37-45°C. The temperature of liquid cooling is better. In terms of operating energy consumption, in order to achieve the same average battery temperature, air cooling requires 2-3 times higher energy consumption than liquid cooling.

The maximum temperature of the battery pack under the same power consumption, air cooling is higher than liquid cooling 3-5℃, the power consumption of liquid cooling is lower. In terms of battery thermal runaway risk, due to factors such as air specific heat capacity and convective heat transfer coefficient.

The battery air-cooled technology has low heat exchange efficiency and increased battery heat, which will lead to excessive battery temperature and thermal runaway risk; liquid cooling system ( The specific heat capacity of water is 4 times that of air, and the thermal conductivity of water at room temperature is dozens of times that of air), which can greatly reduce the risk of thermal runaway of the battery. New energy storage systems such as CATL and BYD are gradually adopting liquid cooling systems.

④ Introduction to the application of thermal management in other fields

The sales of new energy vehicles continue to increase, and the demand for power battery temperature control systems increases. Liquid cooling technology excels in thermal management of power batteries. The energy density and power density (or cruising range) of new energy power batteries are improved, and air cooling is difficult to meet the cooling requirements of new energy vehicles for battery systems, and a battery liquid cooling system needs to be used for heat management.

The thermal management technology of Tesla and other car companies has adopted liquid cooling technology, and liquid cooling has also become the main cooling method for power batteries. In addition, liquid cooling technology is a reliable solution for thermal management of high-power charging piles. The V3 super charging pile launched by Tesla in 2020 also uses liquid cooling technology.

3. Fire protection system and layout of energy storage power station

At present, the investment in fire protection in the overall cost of energy storage power stations in China accounts for less than 2%, while the proportion of power stations in other countries is 3%-5%. At present, the commonly used fire extinguishing agents mainly include carbon dioxide, heptafluoropropane, perfluorohexanone and fine water mist. Carbon dioxide and heptafluoropropane have poor fire-extinguishing and cooling effects, and re-ignition is easy to occur after the battery flame is extinguished.

The fire extinguishing effect of perfluorohexanone is good, but its cooling effect is not significant. The fine water mist is good for cooling, but the fire extinguishing effect is not as good as that of perfluorohexanone, and it can cause secondary damage to the battery system. Since the containerized lithium battery energy storage systems contains a large number of electrical devices and lithium battery systems, a single fire protection system cannot effectively control the fire of the containerized lithium battery energy storage systems. The spray water gun achieves the purpose of continuous fire extinguishing and cooling.

4. Driving factors and market size

① Large-scale lithium battery energy storage station scale

In order to achieve high rate, long life and high safety of large-capacity lithium-ion battery energy storage systems, the demand for temperature control continues to grow. Compared with the power battery system, the energy storage systems gather more batteries, and the battery capacity and power are also larger. A large number of batteries are closely arranged in a space, and the operating conditions are complex and changeable. This is likely to cause problems such as uneven heat generation, uneven temperature distribution, and large temperature difference between batteries.

If things go on like this, it will inevitably lead to a decline in the charge-discharge performance, capacity, and lifespan of some batteries, which will affect the performance of the entire system. Lithium-ion battery energy storage stations are generally above the MWh level. Energy storage applications pay more attention to the "low cost, long life and high safety" of batteries.

China's large-scale energy storage applications are mostly used on the grid side and power supply side. At present, many energy storage power stations are planned to be more than 100 MWh level, and the operating environment is more complex, which is expected to drive the demand for large-scale energy storage temperature control and fire protection systems to increase.

② The newly installed capacity of electrochemical energy storage continues to increase

Electrochemical energy storage systems have gradually become the mainstream of newly installed energy storage capacity. The installed scale of electrochemical energy storage systems continues to increase. In 2021, the installed power of the world energy storage market will be 205.3GW, of which the installed power of electrochemical energy storage systems will be 21.1GW, accounting for 10.05%.

By the end of 2021, the scale of electrochemical energy storage systems in China will reach 1.87GW/3.49GWh, and the planned scale of construction will exceed 20GW. Among the electrochemical energy storage projects, there are 120 lithium-ion battery energy storage projects. From the perspective of cumulative installed capacity, among electrochemical energy storage systems, lithium-ion batteries account for more than 90% of the installed power, while lead storage, liquid flow and other batteries account for a lower proportion.

The installed capacity of electrochemical energy storage systems continue to increase, and lithium-ion batteries will still be the mainstay in the next few years, driving the demand for temperature control and fire protection systems.

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