How do pumped-storage stations store energy?

1. At midday the sun is bright and the wind is strong. Solar panels and wind turbines make more electricity than the town is using. Where can the spare go?
2. The spare electricity runs a big pump deep inside the hill. It pushes water from the low lake up a long pipe to the high lake.
3. Now the high lake is full. Water held up high has stored energy, because gravity is ready to pull it down. It can wait for hours with almost nothing lost.
4. In the evening the sun sets and everyone switches on lights, ovens and screens. The gate opens, and the water rushes back down the pipe.
5. The rushing water spins the same wheel backward. Now it is a turbine, and the motor above it works as a generator. Electricity flows to the town.
6. By morning the high lake is low again. When the sun and wind return, the pump fills it back up. A little energy is lost each time, but most of it comes back.

Tap each part. Follow the water from the high lake down to the low lake.

• Upper reservoir. This lake sits high on the hill. When it is full of water, the station is fully charged, like a full battery.
• Intake gate. This strong gate holds the water back. When it opens, water pours into the pipe. A screen keeps branches and fish out.
• Penstock (the big pipe). The penstock is a huge pipe or tunnel through the rock, wide enough to drive a truck through. Water flows up it when pumping and down it when making electricity.
• Pump-turbine. This wheel with curved blades does two jobs. Spun by a motor, it pumps water uphill. Spun by falling water, it is a turbine.
• Motor-generator. This machine also does two jobs. Fed with electricity, it is a motor that spins the pump. Spun by the turbine, it is a generator that makes electricity.
• Lower reservoir. The water ends up in this low lake after making electricity. It stays here until there is spare electricity to pump it back up.
• Power lines. These wires work in both directions. They bring spare electricity to the pump, and later they carry electricity back to the town.

Pump the high lake full, then let all the water run back down. Compare the electricity you used with the electricity you got back.

Experiment: You pump the high lake full, then let it all run back down. How much of your electricity do you get back?

1. Press Pump up and wait until the high lake is full.
2. Press Let it flow and wait until the high lake is empty.
3. Compare the two numbers: used for pumping, and made by the turbine.

What we found: You get most of your electricity back, but not all. The pump, the turbine and the pipes all waste a little as heat and swirling water. Even so, getting back about three quarters makes this the biggest kind of battery in the world.

Turn on JavaScript to try it yourself.

Press Start the day. Pump while the sun and wind make spare electricity, then let the water flow when the town needs more in the evening.

Electricity has to be used at the very moment it is made. That is awkward, because the sun and the wind do not always match what people need. At midday there may be far too much, and on a still evening far too little. So power companies need a way to save electricity for later.

One of the best ways is surprisingly simple: lift something heavy. A pumped-storage station has two lakes, one high on a hill and one low. When there is spare electricity, a pump pushes water from the low lake up to the high one. The electricity is gone, but its energy is now stored in the lifted water.

When electricity is needed, a gate opens and the water rushes back down through a turbine. The turbine turns a generator, and electricity flows out again. The same machine usually does both jobs: it is a pump one way and a turbine the other. It can start in about a minute, so it is perfect for sudden rushes of demand.

The water battery is not a perfect battery. Every pump, turbine and pipe wastes a little energy as heat, so only about three quarters comes back. It also needs a hill, two lakes and a lot of water, so it cannot be built just anywhere. But once built, a station can work for many decades.

Key words

Reservoir

A lake used for storing water.

Potential energy

Energy something has because it is up high and could fall.

Head

How much higher the top lake is than the bottom lake.

Penstock

The big pipe or tunnel that carries water between the reservoirs and the turbine.

Turbine

A wheel with blades that spins when water rushes through it.

Efficiency

How much of the energy you put in comes back out as something useful.

Check your thinking

1. What does the station do when there is spare electricity?

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   It pumps water up to the high lake. Spare electricity runs the pump, and the lifted water stores the energy for later.

2. It is evening and the town needs more electricity. What should the station do?

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   Open the gate so water runs down through the turbine. Falling water spins the turbine and generator, which send electricity to the town.

3. Why do you get back a bit less electricity than you used for pumping?

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   The pump, turbine and pipes waste a little energy as heat. No machine is perfect. Rubbing and swirling turn a little of the energy into heat each way.

4. Two stations have the same size of lake. One hill is twice as high. Which stores more energy?

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   The one on the higher hill. Water lifted higher has farther to fall, so each bucketful can do more work on the way down.

5. About how much energy is stored by lifting 1,000 kilograms of water (one cubic meter) through 100 meters? Use g = 9.8 newtons per kilogram.

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   About 980,000 joules. E = m g h = 1,000 times 9.8 times 100 = 980,000 joules, which is about 0.27 kilowatt-hours.

6. Does a pumped-storage station add new energy to the grid over a full day?

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   No. It returns less than it takes, but at a more useful time. It is storage, not a source: it shifts energy from times of surplus to times of need, losing about a quarter on the way.