Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

Renewable energy systems produce electrical power. One of the most frequent objections to the use of these systems is that they don’t always produce power when it is needed.

“Solar power arrays can’t produce power at night.” “Wind power only generates electricity when the wind is blowing.” — These, and other complaints, highlight what is one of the key benefits of fossil fuel, namely that it is stored energy.

A sail ship can move only when the wind is blowing, but a steam ship could move as long as it had coal to fire its boilers. Stored energy systems allowed the directed application of energy to whatever purpose it was needed for at the time and place of the user’s choosing.

Modern life has made us accustomed to this convenience. In the developed world, the supply of gas and electricity to our buildings are utilities, basic services so ubiquitous that they are provided to all.

But a number of systems are under development or are already in place to allow the clean energy generated from wind, solar, and other renewable sources to be effectively stored until needed.

One of the oldest and most widely used systems for storing power is pumped hydro. The first pumped hydro installations were built in the 1890s in the Swiss Alps and Italy, where narrow valleys could be easily dammed to create the higher reservoirs needed for a pumped hydro installation.

Pumped hydro uses flowing water to turn turbines that generate electricity. But where conventional hydropower uses a flowing watercourse like a river to run its turbines, pumped hydro uses a lake or other reservoir to collect and store the water until electricity is needed. Using simple physics, potential energy is stored by pumping water to a higher elevation and storing it until power is needed. Then the water is allowed to flow out through the turbines to produce the needed electricity.

The generating turbines at a pumped hydro facility are usually also used to pump water up into the storage pond, which helps lower capital equipment requirements.

Diagram of a pumped hydro system

Pumped storage facilities are fairly economical to build and to operate because they do not require hazardous chemicals or especially complex industrial systems for their operation. When natural geography provides the proper conditions, a pumped hydro facility can be built without a great deal of additional infrastructure. They do well when located next to natural bodies of water, which can serve as lower ponds.

Because pumped storage facilities require a large area of land (the storage pond at the Ludington, Mich., facility covers 1.3 square miles, or 3.4 square kilometers), they are not well suited for proximity to urban centers, so they require access to high capacity electrical transmission. However, this proximity also makes it possible for them to be used to store excess power from whatever source is producing excess electricity.

Some of the largest pumped hydro facilities are able to supply thousands of megawatts of power for several hours, if needed. The largest pumped storage facility in the world is the Bath County Pumped Storage Station in northern Virginia, which has a maximum capacity of 3 gigawatts (3,000 megawatts). Because they can begin generating additional power very quickly, pumped hydro facilities are a very good option instead of gas-fired peaker plants for responding to changing power demand.

Ocean shore pumped hydro systems are possible, particularly in locations with high cliffs which permit the storage pond to be higher above sea level (and thereby creating a greater potential difference for the stored water). A potential drawback to these systems is that salt water is more corrosive and typically requires more maintenance for the equipment than freshwater systems. However, seashore locations are particularly good for wind farms, and there is a good synergy to be had from the pairing of a coastal wind farm with a pumped storage facility. One proposed alternative energy system, called Searaser would use a fleet of offshore ocean buoys equipped with pistons to pump water up to an onshore storage pond

As with every mode of energy conversion, there are losses from using pumped hydro to store energy. Pumped hydro has an average system efficiency in the range of 70-80 percent. Pumped hydro can also suffer from evaporative losses during a period of drought (though it also gains “free” energy from rainfall and other runoff that feeds into the storage pond).

Image credits: Ludington MI Pumped Storage Consumers Power; diagram funjoker23/Wikimedia Commons