A electric storage units device refers to a device that converts electrical energy into other forms of energy, stores the energy, and then releases the same into electricity to supply where needed. In the main view, it aims to balance power supply and demand, balance power supply reliability, and promote renewable energy consumption.

Types of Electric Storage Units Commonly Applied:

1. Mechanical Energy Storage

Pumped storage: During the low load, use the surplus power of the grid to pump the water from the low water level to the high water level, converting electric energy into the gravitational potential energy of water and storing it. During peak power consumption, putting down the high water level to drive the turbine for electricity generation and thereby achieving the release of electric energy. A great amount of energy is stored by pumping storage power stations, with the time of release ranging from several hours to several days. The main usages include peak filling, frequency modulation, phase modulation, emergency backup, and so on.

Compressed air energy storage: The excess power at periods of low grid load is utilized in compressing the air, which is stored in a high-pressure sealed facility, and it is released to drive the gas turbine to generate electricity during the peak of electricity consumption. In comparison with the pumped storage power stations, the construction and power generation cost invested in the method is less, yet it has very low energy density and is bounded by some limitations in the terrain condition, such as rock formations.

Flywheel Energy Storage: It consists of a cylindrical rotating mass block and magnetic levitation bearings that support a mechanism. During the valley load, the power frequency grid provides the required electric energy to drive the flywheel for high-speed rotation by storing energy in kinetic form. During peak load conditions, it is the high-speed rotating flywheel that pulls the motor for generating electricity. Some of the other characteristics of flywheel energy storage are that it has no friction loss, small wind resistance, long life, no impact on the environment, almost no maintenance, etc. Mainly used for uninterruptible power supply/emergency power supply, peak regulation of power grid, and frequency control.

2. Electromagnetic energy storage

The superconducting magnetic energy storage could store magnetic field energy with coils made of superconducting material. The characteristics of fast electromagnetic response and high efficiency in energy storage meet the grid voltage support, power compensation, frequency regulation, improvement of system stability, and enhancement of power transmission capability. However, the cost is too high at present. In addition to the superconductivity itself, low-temperature maintenance is costly.

Supercapacitor energy storage: electric field is used to achieve charge storage, porous dielectric is used between positive and negative plates, and electrical energy is stored and released by charge adsorption and desorption. Compared with the traditional capacitor, supercapacitor has higher dielectric constant, larger surface area or higher voltage resistance, and is more expensive, mainly used for smoothing of short-time high-power load and peak power quality.

3. Energy storage of chemical type

Lithium-ion battery energy storage: The application of lithium ion embedding and deembedding between positive and negative plates for charge storage and release has advantages such as high energy density, long life, and no memory effect; it is also widely used at present, from electric vehicles, mobile phones to energy storage of the power grid, which has extensive applications.

Sodium sulfur battery energy storage: the positive pole is liquid sodium, the negative pole is porous ceramic material, and in the middle, there is solid electrolyte isolation. During charging, the sodium flows to the negative electrode for storage, while during discharging, the sodium ion migrates from the negative electrode toward the positive electrode through the solid electrolyte to realize energy release, which is generally used in energy storage power grid and other such areas.

Flow battery energy storage: Using liquid as a charge carrier includes two liquid storage tanks, charging between the liquid storage tanks through the electrolyte charge transfer and charge storage in the liquid. Discharge again through the electrolyte charge transfer to release electric energy, with large capacity, long cycle life, and other characteristics.

The energy storage of lead-acid batteries is mature and inexpensive, with its energy density relatively low, the cycle life is limited, and the application is usually made in those cost-sensitive scenarios or areas where energy density is not high. It includes a small-scale distributed power generation system or backup power supply.

Nickel-metal hydride battery energy storage: with high energy density, the charge/discharge performance is good, and the cycle life is relatively long. For some specific energy storage application scenarios, such as energy storage systems of hybrid electric vehicles, nickel-metal hydride batteries can be applied.

What are the factors that determine the choice of electric storage units?

Purpose of energy storage

Power compensation and improvement in power quality: If utilized for power fluctuation compensation within a very short time, or for the improvement of power quality, such as the elimination of voltage dips, flickers, and other problems, a high-power-density power reserve device must be selected. For example, supercapacitor energy storage and superconducting magnetic energy storage equipment could release or absorb a mass of power in a very short time-milliseconds. They are excellent in coping with the instantaneous power shock of the power system. For instance, in industrial production, when a large motor starts to produce a momentary voltage drop, these devices can quickly release energy to stabilize the voltage.

Energy management and load transfer: in energy management, for the purpose of peak cutting and valley filling, that is, to store electricity during off-peak hours and utilize it during peak hours, the device requires consideration of energy density. For example, lithium-ion battery energy storage and pumped storage devices can store a large amount of electricity, which can be shifted in a number of hours or even days. It is suitable for balancing day and night or seasonal power load differences.

Backup and emergency power: In order to serve the purpose of backup power and emergency power, a device needs to come into operation instantly in such situations with operational reliability during failure to the main supply. Owing to mature technologies, low cost, and stable power output within a short time, the lead-acid battery energy storage is usually used as a UPS in data centers, communication base stations, and other places to avoid data loss and communication interruption caused by a power cut.

Energy Storage Scale

Small distributed energy storage (kilowatt-megawatt scale) : In distributed power generation systems (such as home solar power systems, small wind power systems) or small commercial users, the scale of energy storage is generally small. At this time, lithium-ion battery energy storage, lead-acid battery energy storage and other chemical energy storage methods are more suitable due to their flexible configuration and relatively small footprint. A residential photovoltaic installation could have a few kilowatt-hours to tens of kilowatt-hours of lithium-ion battery energy storage so as to store excess electricity generated during the day for use at night.

Medium energy storage (megawatt-10 megawatts) : For medium-scale energy storage applications such as distributed energy stations in some industrial parks, or for medium-sized substation energy storage facilities, flow battery energy storage is suitable or small pumped storage (under given geographical conditions). Flow battery energy storage can be flexibly configured by adjusting the size of the electrolyte tank and reaches a scale of megawatt energy storage.

Large-scale power grid energy storage (10MW-gigawatt level) : At the large-scale power grid level, huge energy storage capacity is needed when it is used for peak regulation and frequency regulation, stable power grid and other purposes. Pumped storage power plants are currently one of the main options for large-scale energy storage (gigawatt-hour level) due to their ability to store a large amount of electricity. In addition, compressed air energy storage can be used for large-scale power grid energy storage in areas with proper geological conditions, and the energy stored is able to reach tens to hundreds of megawatts.

Storage duration

The energy storage at a minute level is mainly applied to those application scenarios requiring energy release or absorption in an extremely short time, such as seconds to minutes. For example, suppressing high-frequency oscillations in the power system can be best solved by supercapacitor energy storage and superconducting magnetic energy storage. Their response speed is super fast, and the charge and discharge cycle of energy can be done within a very short time. However, for energy storage time, it's comparatively short; if one wants to extend the time, it will notably raise the cost.

Hour-level energy storage: if you need an energy storage device capable of supplying power in the case of several hours, such methods as chemical energy storage methods, lithium-ion battery energy storage, sodium-sulfur battery energy storage, and flow battery energy storage will be more appropriate. The energy storage time of these batteries can be flexibly adjusted with different battery capacities and loads, and it usually reaches several to ten hours, which can satisfy daily peak and valley electricity price differences in energy storage and short-term power supply.

Day-week energy storage: In some special cases, like a prolonged period of bad weather that has resulted in insufficient renewable energy generation, or during grid failure maintenance, electric storage units become necessary to provide a power supply for several days or even weeks. The pumped storage power plants are ideal owing to their huge energy storage capacity and relatively long storage time up to several days. However, the construction period is long, the requirements for geographical environment are high, and it may not be implemented in some areas.

Technical performance and efficiency

Energy density and power density: Energy density determines how much electrical energy can be stored per unit volume or per unit mass of the device, and power density reflects the ability of the device to charge and discharge quickly. For example, lithium-ion batteries have high energy density, store more electricity in small spaces, suitable for energy storage applications with space limitations; Though the energy density of supercapacitors is low, the power density is extremely high, suitable for high-power application scenarios requiring fast response.

Charge and discharge efficiency: The electrical energy lost in the energy conversion process of devices with high charge and discharge efficiency is less. For example, lithium-ion batteries can normally achieve a charge and discharge efficiency of more than 90%, which means only less energy will be lost as heat during charging and discharging, and the energy stored effectively. However, some of the traditional lead-acid batteries have comparative low efficiency in charging and discharging, and there will be more energy loss in the frequent charging and discharging process.

Cycle Life: Cycle life refers to the number of complete charge/discharge cycles a device can handle before significant performance degradation occurs. For instance, lithium-ion batteries normally have a cycle life of several thousand to ten thousand times, while that of lead-acid batteries is usually only several hundred times. For energy storage application scenarios used frequently, such as the peak regulation and frequency regulation of power grids every day, long cycle life electric storage units shall be selected to reduce replacement costs.

Cost factor

The initial investment cost of various power storage devices is quite different. For example, the construction of a pumped storage power station needs huge capital investment in reservoir construction, water turbine and other equipment purchase, installation, but it has a long service life; the initial investment cost of lead-acid battery energy storage is relatively low, suitable for small-scale energy storage application scenarios where cost is highly sensitive.

Operation and maintenance cost: it includes equipment overhaul, replacement parts, energy consumption, etc. For example, superconducting magnetic energy storage must be operated in a low-temperature environment, and the cost of keeping a low-temperature environment is very high. The lead-acid battery energy storage regularly checks the electrolyte and plate, and the maintenance cost cannot be ignored. In selecting the storage device, it shall consider the whole life cost of it.

Geographical surroundings and the conditions for installation

Geographical and topographic requirements: pumped storage requires suitable topographic conditions, that is, two reservoirs with large height difference; Compressed air storage also needs to have suitable geological conditions, like underground caves or salt caverns for storing compressed air. Without these geographical conditions, neither of these energy storage methods can be carried out.

Installation space limitation: Some electric storage units, such as supercapacitor energy storage and lithium-ion battery energy storage, require a relatively flexible installation space indoors or outdoors in containers. Pumped storage and large-scale compressed air energy storage require more land for facility construction. If the resources of land are very tight in cities or areas, whether energy storage facilities can meet the installation space requirements should be taken into consideration.

Environmental adaptability: the different electric storage units have a different adaptability to ambient temperature, humidity, and so on. Take the lithium-ion battery, for example: it can show degraded performances or even safety risks while operating at high temperatures, while under low temperatures the discharge capacity of lead-acid batteries will be weakened sharply. Therefore, when selecting an energy storage device, it is necessary to consider the local environmental conditions, choose a device with strong adaptability to the environment, or take corresponding environmental regulation measures.