Science & Technology in Action

14th Edition

Storing Renewable Energy


Pumped hydroelectric energy storage is a proven technology which today accounts for about 99% of global energy storage. It combines high power with large capacity. At present there is only one such facility in Ireland. The overall efficiency of pumped hydroelectric energy storage is around 75%.

Available downloads

The full lessons along with a supporting toolkit are available in three different formats, A4, A3 and as a Powerpoint deck.

Download Lesson Kit

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

As renewable energy grows and fossil fuels decline there will be increasing need to store renewable energy so that peak demands can be met.


Between 1990 and 2017 the world population increased from 5.3 billion to 7.6 billion, i.e. a 43% increase. During that same period demand for electricity more than doubled worldwide while in the EU it grew by 26%.

Until 2005 over 99% of global electricity was generated from fossil fuels (burning coal, peat, oil or gas), hydro and nuclear energy. Since then the contribution to electricity generation from other energy sources has increased from about 1% to 6.7% globally and from 2.6% to 15% in the EU. These contributions are expected to increase significantly.

Meeting demand

On a large scale the demand for electricity varies during the course of the day and from season to season in fairly predictable ways. When demand is low some power stations might be taken off the national grid to be brought on stream again when higher demand is expected. Hydro power is particularly useful for meeting short term peaks in demand.

Like hydro power, wind and solar power are renewable forms of energy. However, unlike hydro power, wind and solar power cannot be turned on just when required. They vary with the weather and time of day in sometimes unpredictable ways. Wind turbines typically require a minimum wind speed of around 10 km/h (3–4 m/s) and solar power cannot operate at night.

Download the lesson to find out more.

True or False?

  1. Since 1990 the population has increased by 43% but the electricity demand has increased by more than 100%. true
  2. Hydro, wind and solar power are renewable energy sources that can supply all the energy needs of the country. false
  3. Because wind and solar power are not available all the time the energy must be stored in some way until required. true
  4. The national grid uses alternating current. true
  5. The output from wind and solar generators can be linked directly to the national grid. false
  6. Pumped hydroelectric energy storage (PHES) is the most widely used large scale energy storage method. true
  7. Batteries store chemical energy. true
  8. Flywheels store electrical energy. false
  9. CAES is more efficient than PHES. false
  10. Energy storage systems are widely used to supply electricity when the demand fluctuates rapidly. true