What is Energy Storage?
If we have access to more energy than we need at a given time, it is often beneficial to store the extra energy for future use. This process is called energy storage. In most cases, electricity is converted to another form of energy (such as potential energy, chemical energy, etc.), stored for a period of time (ranging from seconds to months), and then converted back into electricity when needed. The process of storing the energy is called charge, while the process of retrieving the stored energy is called discharge.
There are several methods of converting between electricity and stored energy, leading to an assortment of energy storage technologies with various advantages and disadvantages. The best technology for a given energy storage project will depend on a number of factors 1:
- Capacity: How much energy can it store?
- Power: How quickly can it be charged and discharged?
- Price: How much does it cost to build and operate?
- Life: How long does it last in operation before it needs replacement?
- Efficiency: How much energy can be discharged after a charge? This can range from less than 50% to more than 99% depending on the technology.
- Safety: Does it pose any risks or hazards to human/environmental health?
For storing large amounts of energy on the electricity grids, four technologies are most common today: pumped hydro (lifting water), battery (chemical reactions), thermal (heat storage), and flywheel (spinning a disk) 2. Check out the Dive Deeper section to read more about different energy storage technologies.
For an electricity system to work, the electricity being generated by power stations must be closely matched to the electricity being consumed by users. Energy storage offers numerous benefits for maintaining this delicate balance of electricity supply and demand 3.
Power stations must be sized to match the maximum expected electricity demand (usually occurring on the days with the highest heating or cooling requirements), even if the average demand is much lower. With energy storage however, energy can be stored overnight (when demand is low) and then used during the high demand period of the following day. This use of energy storage is called peak shaving, which reduces the need to build and operate expensive power stations and transmission lines. The result is less waste and lower electricity prices.
Since wind and sunshine depend on daily and seasonal weather, an electricity grid cannot rely solely on these renewable energy sources if the supply-demand balance is to be maintained. To avoid reliance on fossil-fuel power stations, energy storage technologies can be charged when there is excess wind or sunshine, and later discharged when there is insufficient wind or sunshine. This use of energy storage is called renewable energy integration, which will be critical for the clean energy transition.
Where we have previously seen a small number of large fossil-fuel power stations in a given region, we are now starting to see larger numbers of smaller renewable energy farms backed by energy storage. This is called distributed energy resources, which will allow for increased energy independence at the community level. Remote communities relying on diesel generator microgrids are also starting to reduce or eliminate their dependence on expensive, air-polluting fuels in favor of emission-free renewables and energy storage.
There are also a number of additional functions that energy storage technologies can perform to increase electricity grid reliability. These functions are called ancillary services, which will become increasingly important as more wind and solar energy farms are connected to the grid.
The state of South Australia has been a global leader in adopting renewable energy, but large amounts of wind and solar power without energy storage made the electricity grid more vulnerable to an extreme wind event in 2016, which led to a state-wide blackout 4. The state government responded by building the world’s largest lithium-ion battery in 2017, called the Hornsdale Power Reserve. This giant battery is charged from a nearby wind farm and maintains grid reliability with greater performance and lower cost than fossil-fuel power stations. In its first four months of operation, the 90 million AUD battery saved Australians 35 million AUD in electricity costs and successfully protected the grid when a coal power station unexpectedly went offline 5. Due to its initial success, plans were released in 2019 to increase the size of this battery by 50% 6.
As countries around the world take steps to develop more renewable energy and improve electricity reliability, energy storage is increasingly necessary and valuable. The main drawbacks of today’s energy storage technologies are high economic cost and reliance on mining operations that are not geared towards the clean energy transition. are not always environmentally or socially acceptable 7. The world needs skilled engineers and scientists to “lead the charge” in developing energy storage solutions that are both affordable and sustainable. We also need policy-makers and community members to pay attention to how new technologies are manufactured, and advocate for responsible and ethical sourcing of the required raw materials.
Dive Deeper: Types of Energy Storage 2:
Elevation (gravitational potential energy):
- Electricity can be used to pump water from a low elevation to a high elevation. Electricity can then be generated later by allowing the water to flow back down through a turbine. This is called pumped hydro energy storage, which is the oldest and most-used form of large-scale energy storage.
- Electricity can also be used to temporarily force massive objects uphill or straight into the air, which is generally called gravity energy storage. Several new start-up companies are trying to make these ideas commercially viable for large-scale energy storage. Advanced Rail Energy Storage uses heavy train cars on a slope, while Energy Vault uses a reversible crane-lift system with large concrete blocks.
Compression (elastic potential energy)
- Electricity can be used to compress air into a container at high pressure. Electricity can then be generated later by allowing the air to flow back out through a turbine. This is called compressed air energy storage, which has only a handful of existing large-scale projects but is now seeing increasing research and development. Some projects store the air in tanks, while others store the air in underground caverns. There is even a demonstration project where air is stored in giant balloons at the bottom of Lake Ontario.
- Springs are another form of energy storage through compression, but this technology has not been used for storing electricity at a large scale.
Rotation (kinetic energy)
- Electricity can be used to spin a massive wheel. Electricity can then be generated later by allowing the spinning wheel to slow down as it drives a generator. This is called flywheel energy storage, which is becoming popular for electricity grid applications with short storage periods (seconds to minutes).
Chemistry (chemical energy)
- Electricity can be used to change the chemical bonds in a material. Electricity can then be generated later if this chemical process can be reversed. This is called battery energy storage, which is the most popular technology for new large-scale energy storage projects today due to the wide range of suitable applications. There are many different types of batteries within this category. Lithium-ion batteries and lead-acid batteries are the most common, but other types such as sodium-based batteries and vanadium-redox flow batteries are being increasingly used as well.
- Electricity can also be used to manufacture gases such as hydrogen. Electricity can then be generated later by burning the gas or running it through a special engine called a fuel cell. This is called power-to-gas (P2G), which has potential for long-term storage of excess renewable energy.
Capacitors (electrostatic energy)
- Capacitors are electrical devices that store electricity in an electric field rather than converting it into another form of energy. Advanced capacitor technology (called supercapacitor or ultracapacitors) combines the designs of capacitors and batteries to allow for much more energy storage than a regular capacitor. This technology can be used for electricity grid applications with short storage periods (seconds to minutes).
Magnetism (electromagnetic energy)
- When electricity flows through a coil it generates a magnetic field. Almost all the energy stored in this magnetic field can be retrieved as electricity when the coil is cooled to very low temperatures. This is called superconducting magnetic energy storage, which can be used for electricity grid applications with short storage periods (seconds to minutes).
Heat (thermal energy)
- Electricity can be used to raise the temperature of water or other specialized materials. Electricity can then be generated later by using the high temperature to produce steam and drive a turbine. The stored heat can also be used directly for heating purposes instead of converting it back into electricity. This is called thermal energy storage, which is one of the more common energy storage types in use today.
Electricity can also be used to lower the temperature of a gas until it can be stored as a liquid. Electricity can then be generated later by allowing the cold liquid to expand into a gas and drive a turbine. The stored cold liquid can also be used directly for cooling purposes instead of converting it back into electricity. This is called liquified air energy storage or cryogenic energy storage, which is a new technology still under development.