Electrical Energy Storage (EES) technologies - an Overview & Types | SEA Energy

Developing countries have been taking advantage of the potential of renewable energy to curb their power demand. The growing number of such countries is leading to an increasing need for reliable and cost-effective storage applications.

With growths such as distributed generation and smart grid, there is a need to store electricity where it is needed. Due to recent evolution in storage technologies and developments, electricity can be stored in a megawatt-scale. This electricity energy storage (EES) application is increasingly becoming possible around the world. In this article, we look at which type of technology is available to store Electrical Energy.

Electrical Energy Storage (EES) technologies

The potential of energy storage is growing at a tremendous rate, and it’s expected to grow exponentially in the coming years. Here are some technologies that can help you store energy more efficiently. Energy storage technologies are broadly classified as mentioned below:

1. Mechanical Energy Storage

A. Pumped Storage

Hydro-power Pumped storage hydro-power is an efficient method of storing electricity for use at a later time. In pumped storage hydroelectricity, water is used to pump excess electricity from one reservoir to another, and vice versa. The electricity can then be used for industrial purposes, or it can be stored in a second reservoir, where it can be released during periods of high demand. Water is allowed to flow back from the upper reservoir to run a turbine and generate electricity when required. The long lifetimes and stability make them the ideal storage systems.

B. Compressed air energy storage (CAES)

Compressed air energy storage (CAES) is a technology that involves capturing, compressing, and storing energy in the form of compressed air. This technology is based on the conventional gas turbines and stores energy by compressing air in an underground storage. Electricity is used to compress air and when needed the compressed air is mixed with natural gas, combusted and expanded in a modified gas turbine. The turbine produces the same amount of output power as conventional gas turbines but uses only 40% of the gas. The advantage of CAES is its large capacity; disadvantages are low round-trip efficiency and geographic limitation of locations where it can be installed.

C. Flywheel energy storage (FES)

Kinetic energy is stored in a large rotational cylinder where the energy is maintained by keeping its speed constant. A transmission device is used to accelerate or decelerate the flywheel by supplying and extracting electricity. When the speed is increased higher amount of energy is stored. Flywheels are extensively used for space applications and latest generation flywheels are reported to be suitable for grid applications. The long life of this technology with relatively less maintenance requirements makes it another ideal storage solution. However, high levels of self-discharge due to air resistance and bearing losses may make it less efficient.

2. Thermal Energy Storage (TES)

Thermal energy storage (TES) is a solution that harnesses the thermal energy of the surrounding environment. It enables electricity to be generated by converting heat into mechanical energy and storing it for later use. The most common method of Thermal energy storage involves using an insulated tank or container filled with molten salts. The temperature of the molten salts can be adjusted so that it matches the temperature of your desired application and then the heat is used to create electricity. These systems use chilled water, ice storage, hot water, molten salt as storage medium. The efficiencies vary with the material. These storage systems are becoming relevant for integrating large scale renewable energy such as concentrated solar thermal technology which can be used as a reliable and despicable source of energy to balance the supply and demand.Thermal energy systems are divided in three types:

• Sensible heat

• Latent heat

• Thermochemical

Sensible heat thermal energy storage is considered to be the most viable option to reduce energy consumption and reduce CO2 emissions. They use water or rock for storing and releasing heat energy. This type of thermal energy storage is most applicable for residential buildings.

Latent heat storage systems store energy without the medium changing in temperature but rather depends on the changing state of a medium. So called ‘phase change materials’ have been developed, which can store heat in their mass as latent heat. These materials are commonly used in solar applications and building materials, where they absorb and store excess building heat.

Thermochemical heat storage systems on the other hand, are based on chemical reactions.

3. Chemical Energy Storage

Hydrogen Energy Storage Hydrogen energy storage is a promising future-proof technology that could help power the 21st century with renewable energy. Hydrogen is an important part of our society, powering transportation and electricity production, but it can also be used to store energy in the form of hydrogen gas or as a liquid. Excess electricity from renewable energy can be used to produce hydrogen by electrolysis and therefore can be considered as zero-carbon fuel. Hydrogen can be stored as gas under pressure or liquid at low temperatures. It can then be used to create electricity in conventional reciprocating engines, gas turbines or in fuel cells, transport etc. A further area of application is that the hydrogen can be injected into existing natural-gas networks.

4. Electrochemical Energy Storage

Battery storage is a solution to the intermittency of renewable energy sources such as solar and wind. As battery costs continue to drop, battery storage will become an increasingly attractive option for storing electricity from such renewables.

Lead acid batteries are the world's most widely used battery type. Valve-regulated lead-acid (VRLA) batteries absorbed glass mat (AGM) designs have increased performance and total energy output making them a good choice for renewable energy off-grid applications at a lower cost than other batteries. However, their lifespan tend to be relatively short because of lower depth of discharge.

VRLA batteries with added nanocarbon are more resistant to sulfation which can reduce the life of batteries faster. The carbon slows sulfation and allows the battery to charge faster and cycle more than traditional lead acid.

Lithium-ion batteries are most popular as it powers the lives of millions of people each day ranging from laptops and cell phones to hybrids and electric cars. Lithium provides the highest energy density per weight--far lighter and more efficient than the popular lead acid battery.

Lithium-ion batteries have a significantly higher cycle life than lead acid batteries in deep discharge applications. This means that lithium-ion battery can support a higher number of complete charge/discharge cycles before its capacity falls under 80%. Lithium ion’s high energy density and long cycle life has made it dominant in electric vehicle applications. Electric vehicles could also have an impact on energy storage through vehicle-togrid technologies, in which their batteries can be connected to the grid and discharge power for others to use.

Sodium Sulfur (NAS) Battery is a type of molten-salt battery constructed from liquid sodium (Na) and sulfur (S). This type of battery has a high energy density, high efficiency of charge/discharge and long cycle life. The operating temperatures of 300 to 350 °C and high corrosive nature of sodium makes it suitable only for large-scale grid storage applications. These batteries are ideally suited for supporting peak demand and stabilizing the grid.

Redox (reduction–oxidation) flow battery is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by an Ion exchange membrane accompanied by flow of electric current through the membrane. The major advantage over other rechargeable batteries is a long lifespan.

5. Electrical Energy Storage

Super Capacitor is device which store current as static energy, rather than traditional storage of energy which uses a chemical reaction Super capacitors have a very high energy density (energy per unit volume or mass) than normal capacitors. Super-capacitors use two layers of the dielectric material separated by a very thin insulator surface as the dielectric medium, whereas normal capacitors use only a single layer of dielectric material Unlike battery, a super capacitor can be re-charged indefinitely and do not have issues such as battery life, over-charging, and maintenance. The super capacitors can withstand much higher numbers of charge/discharge cycles and their response time is fast. They are ideally suited for very short-term power applications. However, the cost per unit of energy storage capacity is higher than for batteries.