Reducing reliance on fossil fuels – which are easy to store – creates the need for reliable, efficient energy storage technologies to manage intermittent supply from renewables. The challenge is not simply a matter of balancing daily shifts in supply and demand – our future energy landscape will need to feature high-capacity energy storage suited to seasonal variation.
It is easy to take for granted the convenience of fossil fuels, which can be stored without difficulty and burned on demand. But, as weather-based renewables such as solar and wind grow in the primary energy mix, energy storage capacity must expand with them.
Maintaining security and flexibility means rolling out technologies which provide short-term and long-term storage. The former allows grids to adjust to changing weather and demand through the day, while the latter is essential for balancing supply and demand through the year (e.g.: lower solar generation during winter).
Pumped hydroelectric is the commonest form of energy storage; in the US it accounts for 95 percent of grid storage. Pumped hydro works by using energy supply when it's abundant to store water in a raised reservoir behind a dam. The water is then released when power is needed to turn a turbine. Pumped hydro has by far the largest capacity for grid energy storage, excellent round-trip efficiency of around 70-85 percent, and storage cost once built. However, the infrastructure is not only expensive; it also requires certain geographic topologies and consumes vast amounts of water. Just 10 countries have built pumped hydro infrastructure.
The other established form of energy storage is battery storage, which converts stored chemical energy into electrical energy when demand arises. Efficient and versatile lithium-ion batteries are well suited for short-term grid energy storage and will play an important part in future energy systems (although alternatives such as flow batteries have also attracted interest and R&D funding). The cost of battery technology is falling and there are plans for large-scale facilities in Australia, Chile, Germany, Japan, and the UK. In January 2021, a record-breaking 300MWh battery became operational at the Moss Landing Energy Storage Facility in California.
However, lithium-ion batteries are poorly suited for long-term energy storage, becoming uneconomic when used over several hours – pumped hydro is similarly limited. At present, there is no obvious equivalent able to meet the need for long-term storage (weeks or months). In the coming years, innovation will be essential for filling this gap.
Tackling the challenge of seasonal storage
Pumped hydro is just one example of utilizing forces like gravity to store energy – other examples include flywheels and air compression. These conceptually simple systems incorporate sophisticated materials and control systems to boost efficiency and responsiveness.
Start-up Energy Vault has developed a system which uses a six-armed crane for stacking concrete blocks – these store far more energy than water of equal volume and height in a pumped hydro facility, on account of their density. Although this approach remains experimental, it has attracted attention thanks to its potential for long-term storage and high efficiency compared with Li-ion battery storage – and Energy Vault has attracted $110m investment from SoftBank.
Hydrogen storage – another mostly experimental approach – involves using excess energy to power electrolysis of water, separating hydrogen for storage and re-electrification on demand. While efficiency is comparatively low, hydrogen storage attracts considerable interest due to its potential to hold vast capacity for months. Storing 500,000 cubic-meters of hydrogen in an underground salt cavern at 2,900psi, for instance, could hold 100GWh of supply - that's enough to power the 2.1 million electric cars that were sold globally in 2019.
A third emerging approach, thermal energy storage, encompasses a very wide range of technologies. Optimizing heating systems is crucial for meeting Paris Agreement obligations. Key to this is connecting communities with district heating networks and incorporating thermal energy stores. Heat can be stored in various media, including insulated ice tanks, molten salts, and bedrock, with the possibility of storing thermal energy on a seasonal basis. The International Renewable Energy Agency predicts the market for thermal energy storage could triple from 2019 to 2030, to reach more than 800GWh capacity.
The flexibility and security provided by energy storage permits far greater reliance on renewable energy sources such as solar and wind power. As energy systems shift away from the convenience of fossil fuels, innovation and growth in a range of energy storage technologies – especially long-term seasonal storage – will be critical to supporting these systems.