These giant batteries store energy, but not as electricity

Hidden deep under a sprawling Toronto block of condo towers, offices, shops and six levels of underground parking is a giant battery — but not the kind you might be thinking of.

There's no lead or lithium inside it. Instead, it's full of water — enough to fill three Olympic-sized pools — and all the heat that water holds. 

It's called "The Well," and it supports the buildings' low-carbon heating and cooling system. 

Wind and solar generate cheap, clean power. But it's not always sunny or windy at the times we need it most, such as the hottest days of summer or the coldest nights of winter. That makes storing energy an important part of a low-carbon grid — and storing it as heat can be cheaper, safer and more convenient than storing it in traditional batteries.

Here's a closer look at "thermal batteries."

Most of us are familiar with electrochemical energy storage in batteries. Energy can also be stored behind hydroelectric dams (mechanical storage) or as chemicals such as ethanol or hydrogen.

But it can also be stored as heat.

Gabe Murtaugh, director of markets and technology at the Long Duration Energy Storage Council, said the concept is simple: 

The heat can be used to generate steam and turn turbines to generate electricity.

It can also be used directly for space or water heating. That's what happens to the heat stored in The Well system — it's part of a district energy system run by Enwave Energy Corp. that provides heating and cooling to more than 100 buildings in Toronto.

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Stored heat can also be used for manufacturing. 

For example, PepsiCo is replacing one of its natural gas boilers at a factory in Broek op Langedijk, Netherlands, by generating heat from cheap wind energy at off-peak times, and storing it at 800 C in thermal batteries made by a German startup called Kraftblock. The heat can be transferred as needed to the oil-filled fryers that make Cheetos and Lay's chips.

Murtagh said this "dual use" for either heat or electricity makes thermal batteries "really versatile resources."