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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. 

A man stands in what looks like a mall.
Carson Gemmill, vice-president of solutions and innovations at Enwave, stands on a walkway at The Well, a sprawling Toronto development featuring shops, condo and office towers, and six levels of parking atop a huge water-filled thermal battery. (Paul Smith/CBC News)

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.”

What is a thermal battery? 

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: 

A view of the Enwave Well, under construction.
A view of the Enwave Well under construction. It’s about 46 metres (or about 12 storeys) deep and 15 metres in diameter. (Enwave Energy)

What can the heat be used for?

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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A new project in the Ottawa-Gatineau region is reducing emissions by capturing the wastewater heat from a local paper plant and using it to heat residential buildings.

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.”

Why would you want to store heat this way?

It improves energy efficiency and lowers costs in a number of ways.

Energy use typically peaks at certain times — say, the evenings on the coldest days of winter and the late afternoons on the hottest days of summer. Those peaks are expected to become bigger as more heating systems electrify

Giant glassy office tower
This is the Well complex in Toronto as viewed from the outside. It’s heated and cooled by Enwave’s district energy system, which gained 10 million square feet of capacity with the addition of thermal storage under the complex. (Paul Smith/CBC News)

But that demand may not align with peak power production, especially with a growing share of renewable electricity sources such as wind and solar.

Many utilities try to smooth out demand by charging more for power at peak times and far less when there is excess supply, such as overnight.

Thermal batteries allow utilities or customers to move energy from one time of day — or even one time of year — to another, said Murtaugh.

Inside a huge thermal storage tank
Journalists tour the inside of Vattenfall’s vast thermal tank to store hot water in Berlin, Germany. Thermal batteries work well with district heating, which is widespread in Europe. (Michael Sohn/The Associated Press)

The International Renewable Energy Agency (IRENA) said that enables the use of more renewable energy and reduces the need for costly grid upgrades.

Heat storage also lets buildings and manufacturers buy power only when it’s cheapest. 

The Energy Innovation report found thermal batteries could make industrial heating costs using electricity competitive with natural gas, while displacing 75 per cent of fossil fuels burned for heat by U.S. industry.

For space heating, Enwave collects waste heat from customers, such as data centres, and burns natural gas. Thermal storage reduces its reliance on natural gas (and hence, its carbon emissions).

But thermal storage is also used for cooling — Enwave normally uses cold water from Lake Ontario to provide air conditioning to its customers in the summer through its deep lake cooling system. But when its pipe runs out of capacity, the company turns on huge, electrically powered chillers to make up the difference, said Carson Gemmill, the company’s VP of solutions and innovations.

With pre-chilled water stored in The Well, the deep lake cooling system can serve an additional 10 million square feet before switching on the chillers.

How long can heat be stored like this?

Heat can be stored and discharged over a few hours. Examples designed for this include Icebear system tested by the Toronto Zoo and the heat pump with built-in energy storage made by Fredericton-based Stash Energy.

But many systems can store it for months — in fact, they’re often used for seasonal storage. 

For example, the Drake Landing Solar Community built in 2006 in Okotoks, Alta., was designed to collect solar energy in the summer, store it underground and then release it for heating in the winter, supplying 90 per cent of the community’s winter space heating. (The equipment is now reaching the end of its life.)

An illustration of how the solar energy community in Okotoks, Alta. operates. It shows solar energy being collected from garage roof-top solar panels and the energy being stored in a number of underground bore holes.
An illustration, courtesy of High Performance Buildings, of how the solar district heat community in Okotoks, Alta., captures solar energy during the summer and stores it for use in the winter. (High Performing Buildings)

Enwave charges The Well with hot water in the fall to help heat buildings on its district heating system through the winter. In the spring, it charges The Well with cold water, and uses the stored “cold” to help provide air conditioning.

Finland-based Vantaan Energia is planning the world’s largest seasonal thermal storage facility. It will stash away surplus heat from renewable energy and waste-to-energy during the summer to heat an entire medium-sized city in the winter.

If it saves money and energy, why haven’t I heard of it before?

Partly, there isn’t a lot of it. Murtaugh said, in general, for long duration storage, compared to what the world needs for full decarbonization, “We’re at a drop in the bucket.”

A 2020 report from IRENA expected the global market for thermal energy storage to triple by 2030, to 800 gigawatt hours (about enough to power 800,000 average Canadian homes for a month).

On the manufacturing side, Murtaugh said thermal batteries make sense for industries needing heat below 500 C and that are subject to volatile gas prices. The technology is not quite there yet for higher temperatures.

Costs are also coming down. CSIRO, Australia’s national science agency, estimates that thermal energy storage will be roughly a third cheaper than both lithium-ion batteries and pumped hydro for storage longer than four hours by 2050.

A bunch of big pipes and machines with a series of blue lights running down the middle of the room.
This is the chiller room at The Well. On the right are heat exchangers that transfer heat between storage in The Well and Enwave’s district energy system. On the left are chillers that pre-cool the water in the spring to charge The Well. (Emily Chung/CBC News)

On the space heating side, projects like The Well make more sense in district energy systems that serve many buildings, said Carlyle Coutinho, CEO of Enwave Corp. and chair of the International District Energy Association. 

The Well wasn’t easy to build, he added, but the system is working as planned, and Enwave is now looking for other thermal storage opportunities, especially for larger-scale developments.

“I think it’s really, really important, as we think about energy and we think about electrification … to look at all the tools that we have in our tool case to be able to do this as efficiently and effectively as possible,” Coutinho said.

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