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ELI5 Yahaaa

Rain, stored on a hillside

How Does a Dam Make Electricity?

A hydroelectric dam is a battery filled by the weather, and the water gives back exactly what gravity put in.

1 Rain fillsa valleybehind a2 The wateris heldhigh up3 Fallingwater spinsa turbine4 A generatormakeselectricity
The weather does the lifting.

  1. Step 1 of 5

    Height is stored energy.

    The sun evaporates seawater It falls asrain on highground It is held behind a wall It runs backdown throughthe turbine

    Lifting water takes work, and water sitting high up holds that work until it is let go. The sun and the weather did the lifting, evaporating it from the sea and dropping it on the hills.

  2. Step 2 of 5

    Both height and quantity matter.

    A mountain dam A little water, a longdropA river dam Vast flow, a short drop

    A little water falling a long way and a lot of water falling a short way can give the same power. A mountain dam is tall and narrow; a river dam is low and enormously wide.

  3. Step 3 of 5

    The wall is thickest at the bottom.

    Near the surfacegentle push,thin wallHalfway downharder push,thickerAt the basehardest push,thickest of all

    Water pushes harder the deeper you go, so a dam is a wedge — thin at the top where the push is gentle and immensely thick at the base where it is not.

  4. Step 4 of 5

    It can be turned on in seconds.

    A coal station Hours to warm upA dam Full power in under aminute

    A coal station takes hours to warm up. A dam only has to open a valve, so it is used to cover sudden surges in demand — the whole country switching kettles on at the same moment.

  5. Step 5 of 5

    Some dams pump the water back up.

    Cheap spare electricity at night Water ispumped backup The reservoir refills Releasedagain whendemand peaks

    When electricity is cheap and spare, the same machines run backwards and push water up into the reservoir. It is not free — some is lost — but it stores energy nothing else can.

    A dam floods a valley, blocks the fish and traps the silt that used to fertilise the farmland downstream. The electricity is clean; the river is changed forever.

The short version

A dam stores rain high up and lets it fall through a turbine, turning the height it loses into electricity.

Try it yourself

Fill a bottle and pour it from ankle height, then from above your head, onto a spoon. The high pour spins it far harder.

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