What are the new energy storage technologies?

New energy storage technologies include energy storage body and support technology.

Energy storage body technology includes five significant types: electrochemical energy storage technologies such as lithium-ion batteries, sodium-ion batteries, flow batteries, and solid-state batteries; mechanical energy storage technologies such as compressed air and flywheels; electromagnetic energy storage technologies such as supercapacitors and superconducting magnetic energy storage; thermal energy storage (cooling) technologies; and hydrogen energy storage technologies.

Energy storage support technologies include energy electronics, energy storage system integration, and whole-process safety control and management, among others. New energy storage construction features flexible site selection, short construction periods, rapid and flexible response, and diverse functional characteristics. It can complement pumped-storage energy storage in terms of development sequence, construction layout, and response characteristics, and together, they can support the construction of new power systems. Various new energy storage technologies differ in power density, storage duration, cycle life, and construction cost. Depending on the power system's need for energy storage, the application scenarios for energy storage technologies involve the power supply side (centralized renewable energy grid connection, auxiliary frequency regulation for thermal power plants), the grid side (grid transmission and distribution and auxiliary services), and the user side (distributed household photovoltaics, industrial parks, and smart microgrids). Different application scenarios correspond to energy storage technologies with varying performance requirements. The diversity of energy storage application scenarios determines that technologies must develop in a diversified direction, and no “super” energy storage technology can do everything.

1. Divided by duration

Energy storage duration is when an energy storage system can continuously operate (discharge) at rated power. Energy storage technologies can be divided into short-term high-frequency energy storage (<30 minutes), short-to-medium-term energy storage (30 minutes to 4 hours), and long-term energy storage (>4 hours) according to energy storage duration.

2. Divided by scale

According to the construction method of energy storage projects, they can be divided into energy storage power stations and decentralized energy storage devices. Energy storage power stations can be divided into large-scale, medium-scale and small-scale energy storage according to their power and capacity.

Key indicators of new energy storage

Although there are many types of new energy storage and their technical principles are different, all of them involve storing and releasing energy. Taking electrochemical energy storage power stations as an example, the essential characteristics of energy storage technology can generally be evaluated using the following indicators.

1. Installed power (MW or GW): Installed power is one of the critical indicators for measuring the scale of new energy storage. The installed power of new energy storage refers to the maximum power the energy storage system/power station can output within the power regulation range.

2. Installed capacity (MWh or GWh): Installed capacity is also one of the critical indicators for measuring the scale of new energy storage. The installed capacity of new energy storage is the guaranteed value of the power storage system's maximum available energy storage capacity under normal operating conditions. Generally, the installed capacity of new energy storage is described by the two indicators of installed power/capacity. For example, an energy storage system with an installed power of 100MW and an energy storage time of 2 hours has an installed capacity of 200MWh, described as 100MW/200MWh.

3. Energy density (Wh/kg or Wh/L): This refers to the amount of energy stored in the energy storage system per unit weight/volume. 1 Wh is equal to 3600 joules (J) of energy. The material properties of the energy storage system determine the energy density. For example, the energy density of a lithium iron phosphate battery is about 160 Wh/kg.

4. Power density (W/kg or W/L): refers to the rate at which energy is discharged from an energy storage system per unit weight/volume. The characteristics of the material also determine power density, and there is no direct relationship between power density and energy density. It does not mean that the higher the energy density, the higher the power density. Power density describes the rate performance, i.e., how much current the energy storage system can discharge.

5. Charge-discharge rate (C): Charge-discharge rate = charge-discharge current/rated capacity, expressed in “C”, which indicates the charge-discharge capacity rate of the battery. 1C indicates the current strength when the battery is fully discharged in 1 hour.

6. Energy conversion efficiency (%): This refers to the ratio of the total discharge capacity of the energy storage unit to the total charge capacity during the evaluation period of the energy storage system. It is generally used to evaluate the economic efficiency of the energy storage system.

7. Cycle life (times): The energy storage system's complete charge and discharge process is considered a cycle. The maximum number of cycles that the energy storage system can achieve during its life cycle is the cycle life.

8. Self-discharge rate (%): The self-discharge rate, also known as the charge retention capacity, refers to the rate at which the power of an energy storage system spontaneously decreases when it is not in use. Specifically, the self-discharge rate is the ability of an energy storage system to retain the stored power under certain conditions when it is in an open circuit state. Self-discharge is measured as a percentage of total capacity over a certain period. For example, the self-discharge rate of a lithium-ion battery is generally less than 1% per month.

In addition, there are two other important economic indicators for new energy storage construction projects: initial investment cost and life-cycle cost of electricity. The initial investment cost of new energy storage is the sum of the energy cost, PCS system cost, BMS system cost, EMS system cost, construction cost, and other costs. The energy cost is the cost of the energy storage technology itself. Taking a lithium iron phosphate battery energy storage power station as an example, the energy cost is the battery system cost. The levelized cost of energy storage over the entire life cycle of a new energy storage system, also known as the levelized cost of energy storage, takes into account the investment cost, operation and maintenance cost, and life cycle of the energy storage system, and divides them equally to get the average cost per unit of energy stored. The formula is the ratio of the total initial investment cost, charging cost, operation and maintenance cost, etc., to the cumulative transmitted power.