How you can take potential energy,

like the energy in the chemicals in this battery,

and you can then just convert it to electrical energy.

And then convert that to whatever you can find a use for.

A battery is just a store of potential energy.

And that potential energy could be chemical,

or it could be gravitational.

This lake in this rainy bit of Wales is part of the largest battery in Britain.

And it's rechargeable.

Welcome to Dinorwig Power Station,

otherwise known as Electric Mountain.

- What we have is a large reservoir at the top of the mountain,

a large reservoir at the bottom,

and between the two, a power station,

which converts that potential energy into electricity.

The power station itself is located inside that mountain in deep caverns.

We release that potential energy through the station.

It drives a turbine that spins round at synchronous speeds, connected to the grid.

It drives a generator, converting that into electricity.

We then export that onto the National Grid.

And then at nighttime, we just reverse the whole process.

The turbine turns into a pump. The generator becomes a motor.

And we push that water back up the hill into the reservoir at Marchlyn Mawr,

at the top of the mountain.

The operating principle here is that the generation

that the station is able to do during the daytime

helps the nation meet those peaks of electricity demand.

And overnight, it's able to pump the water back when prices are lower.

In electrical terms, we have around nine gigawatt-hours

of storage at the top of the mountain.

And in fact over the course of a day,

the water passing through the station is broadly equivalent

to the total household consumption of water in the UK.

At full load, we can be meeting the demand

of around about two million homes.

- This sounds like a really simple idea,

but when there's this much energy involved,

the design challenges are ridiculous.

If you've ever lived in a house where you turn off a water tap

and the the pipes go clunk,

then you know about water hammer.

Water isn't compressible, so closing a tap

means that all the water that is under pressure

and rushing down the pipe suddenly has nowhere to go.

So if the pipes in your house aren't secured properly,

the crash and rebound of all that water

can make the plumbing move and make noise.

If that can happen with just a tap in a house,

imagine what could happen when one of the valves here closes.

At full capacity, there is somewhere around

a swimming pool every second

rushing through the turbines down there.

When that valve closes, all that water heading down here

suddenly has nowhere to go.

The pressure would do enormous amounts of damage.

It would break everything apart, if it wasn't for one extra bit of plumbing.

A shaft hundreds of metres deep that acts as a pressure release.

We can't see anything, on the surface or from down here, sadly,

it all happens deep within the mountain.

But every time those valves move,

[valve clanking]

that surge pipe is what stops the whole system

from tearing itself apart.

This station is not some quirky old curiosity.

This is serious engineering

that keeps the lights on around the country.

- Dinorwig Power Station is one of the largest

power stations of its type in the world,

and it's also one of the fastest-acting power stations.

So as well as being able to take advantage

of the peaks and troughs in the electricity market,

it was essentially built, primarily,

with the intent to provide very fast response onto the system

in order to help the National Grid balance supply and demand instantaneously

on a second-by-second basis.

So it has the capacity to be able to go from zero up to its full capacity,

maybe 2,000 megawatts, within 10 to 30 seconds.

When people talk about pumped storage,

they tend to associate it a lot with TV pickups,

and National Grid, in analysing that risk,

study the TV schedules and plan ahead and put Dinorwig on standby, if you like,

to be able to meet those pickups.

Going back to the 80s and the 90s,

everyone would be sitting around the same TV channels

with a limited choice, then.

All the ingredients you need for a sort-of collective putting-on of the kettle

and opening of the fridge. Pickups could be pretty significant.

But since then, people are streaming on YouTube, on Netflix,

all the different plethora of channels that we have, so that has changed

that sort of coincidence of viewing on one channel.

The current environment that we're in,

we have significant amount of renewable technology on the system, which provides

a level of unpredictability day to day.

And actually, the importance of storage now

is that we can use it to help develop the amount of renewables we have

and allow us to fill in those gaps

when the wind's not blowing and the sun is not shining.

So having storage is such an important component of the modern grid system.

- Thank you very much to all the team at Engie

who made this visit possible.

Pull down the description for more about them,

and more about Electric Mountain.

A battery is just... a store!... of potential energy.