When you hear about "the cloud", what do you imagine? Some sort of electronic gas distributing internet coverage far and wide? Maybe it's just that the name suggests something nebulous – but the cloud needs infrastructure, just like electricity, gas and other modern-day essentials.

The cloud is run by data centres. Lots of data centres. And these data centres come at a cost. As well as running the digital economy that we've come to rely on, they're also huge carbon emitters.

This is partly because they get hot. Step into a data centre and it's like stepping into a sauna. And when computers get hot, they fail. That's why your laptop has a fan and why data centres have costly, carbon-chomping cooling systems – to ensure uptime wherever possible.

In Arizona, to take one example, there are sprawling data centres that get through an Olympic-sized swimming pool's worth of water every year. And in an age where uninterrupted uptime is increasingly the norm, they're running 24/7/365.

So what's to be done about data centres' carbon footprints? While tech titans like Microsoft and Google claim to be greener than ever, there's still a long way to go.

That's one reason why some start-ups are looking for innovative ways of using excess heat to reduce their overall carbon footprint.

How to warm a swimming pool

In March 2023, Exmouth Leisure Centre announced it was using a data centre the size of a washing machine to heat its swimming pool.

Created by start-up Deep Green, and given to the leisure centre free of charge, the data centre can heat the pool to 30°C around 60% of the time.

This is good news for the swimming pool. In 2022, the BBC reported that 65 swimming pools had closed in the UK since 2019. The cost of heating the pools was a significant driver of these closures.

Deep Green's data centre heater could be a godsend for leisure centres. Not only will it keep them open, but it can also help them to reduce their carbon footprint. Win-win!

And in their quest to provide eco-friendly data solutions, Deep Green has said it will cover any electricity costs incurred by the leisure centre for running the gizmo.

How does it work?

The data centre is a small computer processing unit placed beneath the pool. The servers inside the box are surrounded by hot oil which captures the heat. This warms the water – and in return, the computers are cooled by transferring the heat.

This is clearly greener than your average data centre, which requires elaborate – and carbon-consumptive – cooling to stop the servers from overheating.

This "digital boiler" is one example of excess heat being used and not wasted. But in Denmark, among other places, they've taken things one step further.

Could data centres be the new central heating?

Odense is the third largest city in Denmark, with a population of 200,000 and 100,000 households. Ten per cent of these are kept warm by excess heat from a nearby hyperscale facility.

The facility on the outskirts of the city is one of several scattered across Scandinavia. It's part of the cloud infrastructure that makes sharing pictures and videos over social media and messaging apps a reality.

Like all hyperscale facilities, the one at Odense keeps its servers on around the clock, all year round. After all, that's what we've come to expect as digital consumers. And like its counterparts around the world, it produces a huge amount of heat.

But where most facilities let their excess heat warm up passing wildlife, the one in Odense traps the surplus behind servers and then gives it a second life as a substitute for central heating.

It works through something called "heat exchange". The heat from the servers rises to the roof, where it's cooled and sent through almost 200 heat exchanges. The cold water comes from the city's district heating – a type of community heating used in Denmark. In return, the data centre provides warm water to households in Odense.

It's the Exmouth Leisure Centre on a grander scale. Instead of wasting excess heat, the data centre puts it to good use elsewhere – and in the process, keeps itself cool and reduces the risks of overheating. This means reliable uptime for internet users and warm toes for the Danes.

In the households that take advantage of this heat exchange, there's no need for a boiler. Instead, each household has a pay-as-you-go meter.

Is this method being used elsewhere?

The idea has also taken off in Sweden – but it's in Finland where we're likely to see the biggest adoption.

In summer 2022, Microsoft and energy firm Fortum announced that they would capture the excess heat from a Microsoft data centre near Helsinki and transfer it to homes, services and businesses that, as in Odense, are plugged into the district heating system.

If all goes to plan, this will be the largest heat exchange of its kind. On top of this, the data centre itself will run on 100% emission-free electricity.

Other beneficiaries so far include a Norwegian lobster farm, an Austrian hospital and a German algae farm. 

It's clearly a step in the right direction – but is it enough?

Is heat exchange the solution?

Putting excess heat to good use elsewhere is an innovative solution – but it's also relatively niche. It's unlikely to scale up in the way necessary to keep data centres in line with global climate targets.

Whether tech leaders will adopt this new technology at scale remains to be seen. But in the meantime, it's a clever and effective way of stopping all that heat from going to waste.

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