Earth
The earth is very hot within.
Every 100 metres it gets 3 degrees hotter.
You can use this heat.
You pump cold water down.
You let it heat up and then you pump it up again.

You can also save water heated in the summer.
You store it underground until it is winter.
Then you take it out again.

People, animals and plants live on the earth.
You can also use their waste.
You can use the manure to make biogas.
You can even burn fast-growing plant.
You can heat a house with that heat.
You can also generate electricity with it.
That is what happens in a power station.