Reading: Matching Features: Question 5

As electricity grids incorporate growing shares of solar and wind power, engineers have turned to several very different technologies to store surplus electricity for use when the sun is not shining or the wind is not blowing.

The oldest and still the most widely deployed method is pumped-hydro storage, in which surplus electricity is used to pump water uphill from a lower reservoir to a higher one; when electricity is needed, the water is released back downhill through turbines to generate power on demand. Because it requires two large reservoirs connected by a significant change in elevation, pumped-hydro storage is highly site-specific and can rarely be built close to the cities that most need the power, but a single large installation can store enough energy to supply a city for several hours and, once built, can operate for many decades with only modest maintenance.

Lithium-ion battery arrays, by contrast, can be installed almost anywhere, including directly beside the substations they serve, and can begin releasing stored power within milliseconds of being called upon, making them particularly well suited to smoothing sudden, short-lived fluctuations in electricity supply or demand. Their capacity gradually degrades with every charge-and-discharge cycle, however, so utilities typically plan to replace battery arrays after a decade or so of heavy daily use, far sooner than a pumped-hydro plant would need replacing.

Compressed-air energy storage takes a different approach again: surplus electricity drives compressors that force air into large sealed underground caverns, often ones left behind by mining or gas extraction, and the compressed air is later released through a turbine to generate electricity when demand rises. Like pumped-hydro, this method depends on finding a suitable underground cavern in the first place, but it can store energy for considerably longer stretches, sometimes across an entire season, without significant loss.

Flywheel storage instead keeps a heavy rotating disc spinning at very high speed inside a low-friction, near-vacuum housing; surplus electricity accelerates the disc further, and drawing electricity back out simply slows it down. Flywheels release stored energy quickly enough to help stabilise a grid's frequency from moment to moment, but they can typically only hold useful amounts of energy for a matter of minutes rather than hours, which limits their role mainly to short-term frequency regulation rather than storing energy for later use.

Hydrogen-based power-to-gas storage, which converts surplus electricity into hydrogen gas for later reuse, remains a comparatively small part of today's grid-storage mix and was not examined in detail in this comparison.

List of Storage Systems A. Pumped-hydro storage B. Lithium-ion battery arrays C. Compressed-air energy storage D. Flywheel storage E. Hydrogen power-to-gas storage

The following statements describe features of grid-scale energy storage technologies. Match each statement with the correct option, A–E. NB You may use any option more than once.

1. Depends on finding two large reservoirs connected by a substantial change in elevation.
2. Can operate for many decades once built, requiring only modest maintenance.
3. Can begin releasing stored power within milliseconds and is well suited to smoothing short-lived fluctuations in supply or demand.
4. Gradually loses capacity with each charge-and-discharge cycle and is typically replaced after about a decade of heavy use.
5. Uses surplus electricity to force air into sealed underground caverns, often left behind by mining or gas extraction.
6. Can store energy for an entire season with little loss, once a suitable underground site is found.
7. Keeps a heavy disc spinning at high speed inside a low-friction, near-vacuum housing.
8. Can typically hold a useful amount of energy for only a matter of minutes, limiting its role mainly to short-term frequency regulation.