Energy Storage Methods

Pumped hydro stores energy by lifting water

Use electricity to pump water uphill, then let it flow downhill through a turbine and generator when electricity is needed. The stored energy is gravitational potential energy.

The cycle returns less electrical energy than it consumed because pumping, flow and generation all have losses. It needs suitable elevations, reservoirs and water management; it cannot be placed wherever a battery can.

For the energy and efficiency relationships, see Work and Energy and Power and Efficiency.

Thermal storage saves heat for later

Thermal storage holds energy in a hot material, such as a fluid or solid. A concentrated solar plant can collect heat while sunlight is available, store it, and later use it to produce steam and electricity. Stored heat can also serve heating loads directly.

Insulation slows heat loss; it does not eliminate it. Returning heat to electricity adds conversion losses. Choose the storage route for the energy form the load needs.

Rechargeable batteries store energy chemically

A battery supplies DC through electrochemical reactions. A primary battery is intended for a single discharge life and replacement. A secondary battery is designed to be recharged with a suitable charger, restoring chemical energy for another discharge.

Primary cells are useful portable energy supplies, but they are not reusable storage for electricity from a generator. Do not try to recharge a primary cell. Rechargeable does not mean unlimited life: cycling and aging reduce usable capacity.

See Battery Chemistries for the families and Battery Capacity and Packs for the arithmetic.

Choose storage for the load and the site

A small portable instrument needs a compact energy supply, so a battery is a practical fit. A site with suitable upper and lower reservoirs can consider pumped hydro for large stationary storage. A solar thermal plant needing heat after sunset can store heat without first converting it to electricity.

Compare the required power, the energy needed over time, the storage duration, available space and terrain, losses, and service life. These examples identify a good fit; they do not establish that one technology is always cheapest.