Updated: Oct 6, 2020
Energy storage is simply storing excess energy which can then be fed back into the grid later when it is needed. There are many different ways in which we can store this surplus energy for later use. This article will take a look at some of the most common energy storage technologies that are enabling a cleaner, greener energy mix.
Why is Energy Storage Important?
In order to ensure we have energy when we need it, the National Grid need to balance energy supply to match the demand. The growing usage of renewable energies can pose an issue when it comes to this balance - we can't just make the wind blow when we need power. The solution? Store any excess energy that is produced when it is particularly windy or sunny and access it again when we need it.
One of the oldest forms of energy storage, Pumped Hydro has been around since 1907. In Pumped Hydro, energy is stored in the form of gravitational potential energy. Surplus energy is used to pump water from a reservoir at a lower elevation to another reservoir at a higher elevation. This water can then be stored there for extended periods of time and recovered during periods of high energy demand by allowing the water to flow back through the turbines, thus producing energy that can be used by the grid.
Pumped Hydro storage is one of the most efficient forms of energy storage, with 70-90% of energy recovered. It is also by far the most prevalent energy storage technology currently used, accounting for 95% of all stored energy worldwide (US Dept. of Energy).
Possibly the most familiar form of energy storage to the general public, Lithium-Ion Batteries store energy in chemical form. There are other types of battery, such as Lead-Acid Batteries and Sodium-Ion Batteries, however Lithium-Ion Batteries are by far the most prevalent in the current energy mix.
During charging, lithium ions move from the positive electrode through an electrolyte to the negative electrode, and back when discharging.
The largest Lithium-Ion Battery currently supplying grid is the Australian Hornsdale Power Reserve with an output of 100MW.
Also called Solid Mass Gravitational, this works on the same concept as Pumped Hydro - storing energy a gravitational potential energy. The difference is that instead of water being moved to a higher height, a solid object is moved to a higher height. This can be done using cranes, towers, rails or moving a weight up and down abandoned mine shafts. This form of energy storage is efficient, with efficiencies as high as 85%.
Flywheel Energy Storage works by accelerating a heavy rotor (or flywheel) to a very high speed. When energy is added the flywheel rotates faster, when energy is extracted the speed decreases. Energy is stored as rotational kinetic energy.
Flywheel Energy Storage, with a peak power of up to 20MW (enough for about 10,000 homes), can't store as much energy as technologies such as Pumped Hydro but offer an alternative to Lithium Batteries with the benefit of having a very long lifespan - flywheels built as part of James Watt steam engines have been working continuously for more than 200 years!
Compressed Air Energy Storage (CAES)
Surplus energy can be used to compress air, store it underground (often in salt caverns) and use it later to generate electricity. Energy is stored as confined kinetic energy. As heat is produced when air is compressed, this heat must also be stored and re-used when expanding the air in order to achieve the greatest efficiency. Heat can be stored using solutions such as molten salt (discussed later). When heat is captured and stored to be used during expansion of the air, efficiencies of CAES can be around 70%. If this heat is wasted the air must be heated using other means (often natural gas), meaning efficiencies are much lower - around 27%.
Often abbreviated to P2G, this uses electrical power to produce gas for fuel. When the electricity used is generated from wind power, the technology is sometimes called wind-to-gas or windgas. P2G uses electrolysis to produce hydrogen from water. By adding carbon (C), often in the form of CO2, the hydrogen (H) can then be converted into methane (CH4). This concept is also used to create methanol, syngas and LPG.
The gas can then be stored and transported, often using existing infrastructure, before being converted back into electricity using conventional generators and gas turbines. The efficiency of this technology can be up to 70%. Whilst there are still greenhouse gas emissions when generating electricity from gas, this technology offers the greatest potential storage capacity of current energy storage technologies. CO2 emissions are often offset by using Carbon Capture and Storage in the production of the Methane. The gas can also be used as fuel for transport; one of the main projects of this sort is the Audi e-gas project. Audi estimates that the CO2 emission saving is 80% when using e-gas (from a P2G scheme) when compared to cars powered by petrol.
Typical storage capacities and time scales of different network scale technologies.
As with Methane, Hydrogen is also a Power-to-Gas technology. Electrical power is used to generate hydrogen using electrolysis. Where this differs to the above technology is that the hydrogen is not then converted into another fuel, instead it is kept as hydrogen. One of the main reasons Hydrogen storage is not more common is because hydrogen is more difficult to handle than methane. However, when hydrogen is burned to create electricity, no CO2 is emitted making it a very clean energy. Hydrogen can also be used as a clean fuel for transportation. When hydrogen is converted back to electricity, efficiency is around 30-40%, when it is used for transportation efficiencies can rise as high as 80%. Whilst the efficiency when generating electricity is lower than Power-to-Methane, Hydrogen has the huge advantage of no CO2 emissions when burned. Hydrogen is considered to be one of the most exciting fuels of the future and is likely to become a crucial part of our energy mix.
Thermal storage can be done in several different ways. The basic concept behind thermal storage is to store energy as heat and recover it when needed either through generation of electricity, or without converting the heat (e.g. heating our homes). The same concept is used in air-conditioning buildings, in these cases liquid is cooled to provide cooling for buildings at a later stage. These thermal storage schemes can be stand-alone projects or developed in conjunction with other schemes.
Where air-conditioning is concerned, cooling occurs at night when energy demand is lower, energy is cheaper and ambient temperature is lower. The ice that is created can then be used throughout the day to cool the building. This technology is only used on a relatively small scale, e.g. providing air-conditioning for an office block.
An example of a larger scale thermal store is the Crescent Dunes Solar Energy Project in Nevada. This is a Concentrated Solar Power (CSP) plant which uses molten salt to store energy. Using mirrors, the sun's energy is focused on a tower which then uses that to heat up ~ 32,000,000 kg of salt to a temperature of over 550°C. The molten salt is then used to heat water and create steam which powers a turbine, generating electricity that can be fed into the grid. Whilst the Crescent Dunes Solar Energy Project experienced problems, it demonstrated that thermal energy storage could be used at a grid scale.
Although there has been significant progress in energy efficiency and the use of renewable energy, more must be done to meet international climate goals established under the Paris Agreement. One of the main parts of this will be the ever increasing roll out of renewable energy. As renewable energy passes 20% of the total grid demand, energy storage is a necessity in order to provide the grid with the resilience it requires. There are numerous different energy storage technologies at our disposal, each with its own pros and cons with no single solution. In the coming years we can expect our energy mix to be much more diverse than what we see today.
This article is part of a series explaining some of the key technologies that are enabling the energy transition. Click Here to see the full series.