Electricity is an essential part of modern life, but one of the key challenges with it is that it is difficult to store large amounts efficiently. This article explores whether viable ways exist to store electricity on a large scale.
Can electricity be stored directly?
No, electricity itself cannot be directly stored. This is because electricity refers to the flow of electric charges, typically through a wire. Once the flow stops, the electricity disappears. So in order to store electricity, it needs to be converted into another form of energy that can be stored.
What are the main ways to store electricity?
There are three main ways electricity can be stored:
- Chemical energy storage – The electricity is used to drive a reversible chemical reaction and the energy is recovered by reversing the reaction.
- Mechanical energy storage – The electricity is converted to potential energy by raising a weight. The weight falling releases the energy.
- Thermal energy storage – The electricity is used to heat or cool a storage material. The stored heat or cold can then provide energy.
Of these, chemical energy storage is most convenient for large scale grid applications. The most common technology used is rechargeable batteries.
Chemical Energy Storage
How do batteries store electricity?
Batteries are electrochemical devices that directly convert chemical energy to electrical energy and vice versa. They contain two electrodes (the anode and cathode) and an electrolyte solution.
When the battery is being charged, electricity drives reactions that store chemical potential energy in the electrodes. This reverses during discharge, converting the stored chemical energy back into electricity.
What battery technologies exist for large scale storage?
Several rechargeable battery technologies have potential for utility-scale electrical energy storage:
- Lithium-ion batteries – This is the most promising technology currently. They have high efficiency (90-95%), good energy density and rapid response. Tesla has built major lithium-ion battery facilities.
- Lead-acid batteries – The oldest rechargeable battery technology. Low cost but limited energy density. Used for small scale storage.
- Sodium-sulfur batteries – Operate at high temperatures. Good energy density. Used for grid storage in Japan.
- Flow batteries – The electrodes are in liquid form. Allows larger scale storage but lower energy density.
Of these, lithium-ion batteries have emerged as the preferred choice for large grid-scale storage systems.
What are the largest grid scale battery installations?
Some major grid-scale lithium-ion battery storage facilities worldwide include:
- Tesla Hornsdale Power Reserve, Australia – 150 megawatt capacity, provides stability to grid.
- Moss Landing system, California – 400 megawatt capacity, helps integrate renewable energy.
- Minhua Energy Storage Power Station, China – 200 megawatt capacity, evens output from wind and solar.
Many more major facilities are being built globally as interest in grid-scale batteries increases.
Mechanical Energy Storage
How can electricity be stored as mechanical potential energy?
Two main methods exist to convert electricity into mechanical potential energy:
- Pumped hydro – Water is pumped uphill into a reservoir using electricity. When power is needed, water is released to turn hydro turbines.
- Compressed air – Electricity is used to compress air in large underground caverns. The high pressure air can turn turbines to generate electricity when needed.
Pumped hydro is the most widespread technology currently, representing around 95% of grid-scale mechanical storage.
Where are the major pumped hydro plants?
Some major pumped hydro storage plants worldwide include:
- Bath County Pumped Storage Station, USA – Capacity of 3,003 MW, can power 750,000 homes.
- Guangdong Pumped Storage Power Station, China – Capacity of 2,400 MW.
- Okutataragi Pumped Storage Power Station, Japan – 1,932 MW capacity.
- Dinorwig Power Station, Wales – Capacity of 1,728 MW, stores 6 GWh.
Pumped hydro allows very large scale grid storage but sites are geographically limited by landscape.
What are the main compressed air projects?
Some major compressed air storage projects include:
- Huntorf plant, Germany – Built in 1978, 290 MW capacity.
- McIntosh plant, Alabama – Completed in 1991, 110 MW capacity.
- Advanced adiabatic compressed air plant, Ontario – Proposed, 150 MW capacity.
Compressed air storage could be expanded significantly but currently lags behind pumped hydro.
Thermal Energy Storage
How can electricity be stored as heat or cold?
Two main thermal energy storage technologies exist:
- Molten salt storage – Excess electricity is used to heat molten salt, stored in insulated tanks. The heat can generate steam to produce electricity later.
- Cryogenic energy storage – Electricity is used to liquify air. The liquid air is stored and expanded through a turbine when needed.
Molten salt storage allows major solar thermal plants to continue generating electricity overnight. Cryogenic storage is an emerging large scale technology.
Where are major molten salt facilities located?
Some major molten salt storage facilities include:
- Andasol solar power station, Spain – Capacity of 1,010 MW. The tanks store 28,500 tonnes of molten salt.
- Crescent Dunes solar facility, Nevada – 110 MW capacity. 10 hours of full load storage.
- Gemasolar Thermosolar Plant, Spain – 120 MW output. 15 hours full load storage.
Spain and the US southwest have suitable solar resources for these plants. Molten salt offers unique long duration storage.
Are any major cryogenic energy storage facilities in operation?
Cryogenic energy storage is still an emerging technology, with pilot plants being tested but no full-scale facilities yet. Some demonstration projects include:
- Pilot plant in Lyon, France – 350 kW capacity, tested in 2010.
- ETES pilot installation at University of Birmingham – 60 kW capacity, trialled in 2018.
- Highview Power storage system, UK – Demonstration project with plans to scale up to 50 MW.
Commercial viability is still being established but the technology shows promise for long duration storage.
Comparisons Between Technologies
| Technology | Maturity | Storage Duration | Discharge Time |
|---|---|---|---|
| Lithium-ion batteries | Mature | 2 – 8 hours | Milliseconds to seconds |
| Pumped hydro | Mature | Days to months | Minutes |
| Compressed air | Early stages | Days to months | Minutes |
| Molten salt | Demonstration stage | 5 – 15 hours | Minutes |
| Cryogenic | Developmental | Days to months | Minutes |
How do the characteristics compare?
As the table shows, no single electricity storage technology has ideal characteristics across the board. Lithium-ion batteries offer short duration, fast discharge well suited for smoothing grid fluctuations. Pumped hydro provides the lowest cost very long duration storage. Thermal storage can cover multi-hour gaps. An optimal electricity grid may use a mix of these storage methods.
Challenges With Grid Scale Storage
While grid scale electricity storage can provide major benefits, some key challenges remain:
- High capital costs – Storage facilities are still very capital intensive to build.
- Geographic constraints – Pumped hydro and compressed air require suitable terrain.
- Energy losses – No storage method is 100% efficient, some energy is lost.
- Long duration seasonal storage – Storing electricity from summer to winter remains difficult.
Continued technology development and more optimized grid integration will help overcome these.
Conclusion
There are proven viable technologies available to store electricity on a large scale, with lithium-ion batteries and pumped hydro storage leading the way. As storage technologies continue to advance and deployment expands, they will play an increasingly pivotal role in building resilient, renewable powered electricity grids. Combining multiple storage methods tailored to different grid needs will unlock the full benefits. While challenges remain, grid scale electricity storage is vital for a sustainable energy future.
