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March 31, 2023

Is heat reuse the key to making data centres truly sustainable?

Heat reuse is touted as a way to make data centres sustainable and drivers of decarbonization. But it’s harder than it looks.

By Greg Noone

Mark Bjornsgaard is getting used to the attention. “Ten days ago, I wasn’t,” says the founder of Deep Green, referring to the unprecedented publicity his firm achieved by installing one of its digital boilers to heat a swimming pool in Exmouth, Devon. Something about using the waste heat generated from servers – a resource that apparently few people knew existed – seemed to capture the imagination of the British public. Since then, Bjornsgaard has been inundated by requests from other swimming pools for more information about its digital boilers, as well as local government leaders eager to learn how these pint-sized data centres might be used in their own public heating projects. 

“From a business point of view, it’s been absolutely amazing, because we are now walking into conversations which we didn’t think we’d have for a couple of years,” says Bjornsgaard. It’s also a sign of the times. Amid unprecedented fluctuations in energy insecurity thanks, in large part, to Russia’s invasion of Ukraine, anxiety about rising electricity prices is at an all-time high across Europe. As well as sparking a conversation about national reliance on natural gas, attention has drifted toward how energy might be saved or put to better use – not least in a cloud computing infrastructure that generates more carbon emissions than the aviation industry, thanks in part to its elaborate and power-hungry cooling systems. 

All that electricity and heat energy could, critics argue, be put to much better use outside the data centre and for the benefit of society at large. Going by the law of physics that energy is never lost but merely changes form, it is conceivable that the many gigawatts of electricity used to power data centres could, if left uncooled, be converted into heat to warm thousands of homes and businesses. That potential has seen Norway and Germany pass laws mandating data centre operators have a plan – any plan – for heat reuse on their premises, while similar projects have seen the cloud warm greenhouses, fish farms, homes and offices. “And that’s good,” says Bjornsgaard, who professes a great belief in practical green energy solutions. When push comes to shove, he argues, “we can’t muck about for the next ten years with space lasers and cracking hydrogen and trying all these elaborate [projects] that we love to engineer.”

data centre sustainable heat reuse
Cooling apparatus on a data centre roof. Critics argue that the cloud sector needs to do more to recycle the enormous amount of heat generated by its server racks. (Image by Shutterstock)

The mechanics of heat reuse

Servers get very hot, very quickly – so much so that your common variety rack would probably catch fire were it not artificially cooled. This inevitably compromises the overall electrical efficiency of the data centre, calculated using a ratio called Power Usage Effectiveness. The more power that can be used for computation over cooling, the better. Even with the most efficient cooling systems, however, the average PUE score for most data centres remained at an uncomfortably high 1.57 throughout 2021 – meaning as much as 40% of power running through these facilities was used for cooling. 

Reusing waste heat would, in theory, drag those numbers down, make those data centres sustainable and contribute heat and warmth to local communities in the bargain. Such has been the case in Dublin, where AWS is participating in a community heating scheme, and in Stockholm, where one data centre operator has set the goal of using its waste heat energy to heat 10% of the city by 2035. The scale of these schemes, however, is also a reminder that many of these projects usually only come about through massive capital investment – not least because most of them reuse heat from air-cooled data centres using expensive heat pumps. An alternative lies in immersion cooling for edge high-performance compute units, wherein the server is surrounded by a liquid – usually water, or oil – and then wrapped in copper piping to build a heat exchanger. This also describes Deep Green’s first, crude prototype digital boiler, which consisted of little more than a “homebrew bucket of massage oil with a computer dangled in it, an Argos radiator and a fish pond pump,” says Bjornsgaard, who recalls feeling relieved after “we didn’t kill ourselves when we put the computer in the oil and the radiator got hot.”

As well as allowing data centre operators to pack more computers into their premises, immersion cooling can easily be adapted for edge computing applications. German company Cloud&Heat Technologies, for example, demonstrated in 2017 how to use immersion-cooled servers to heat a Frankfurt skyscraper, while the French startup Stimergy utilised waste heat to warm a swimming pool in Paris (the facility was originally heated using a geothermal borehole.)

Exmouth Leisure Centre is only the latest application of the technology, says Bjornsgaard, and Deep Green’s first official foray into immersion-cooled edge computing. In time, he hopes that the digital boiler will eventually generate up to 80% of the pool’s heat. The relatively small size of the units also means that they can, in theory, be installed more or less anywhere where a boiler would usually go. “This is a land grab for the plant rooms of the world,” says Bjornsgaard. “They are the future high-value locations that we will need for edge computing to become a reality.”

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But don’t expect this to immediately lead to the mass fragmentation of server farms into innumerable storage closets and maintenance rooms – Deep Green’s digital boilers will, for the most part, only handle high compute tasks. But embracing this model of heat reuse means that the carbon contribution of HPC applications needn’t grow in proportion to our growing reliance on these services. “If you’re folding a protein, rendering a movie or modelling a weather pattern, or doing anything that the robots can do for us, those are not latency sensitive tasks,” says Bjornsgaard. “You can schedule those jobs, you can do them on our estate – and you can capture all that heat from them.”

heat reuse data centre sustainable
A cold aisle containment system. While heat reuse has its benefits, it’s also an expensive proposition for air-cooled facilities. (Image by Shutterstock)

Making data centres sustainable

If the practical benefits are clear then, why hasn’t the idea taken off already? One reason is, very obviously, the historical moment: patently, we weren’t in as anxiety-inducing a time for Western energy security as we’re now in. There’s also the up-front expense and, in the case of edge heat reuse solutions like Deep Green, the fact that the technology remains resolutely novel. Bjornsgaard likes to imagine how unusual the pitch for a digital boiler might sound to your average CIO. “These random people, who you’ve never heard of, are saying you can just rip out all your cloud compute and dump it in your local social housing project, and the head of IT goes, ‘No thanks!’” he says. “No-one gets fired for buying IBM.”

But imaginative thinking will be required if we’re going to get to grips with reducing the cloud’s ever-expanding carbon debt, adds the founder. It would help, he adds, if clear guidelines were set and informed by central government. “All of that high-performance compute, the heat should be recaptured,” says Bjornsgaard. “It should be the law that we should not have any high-performance compute in a data centre.”

The prospects of that happening seem far off. While Norway and Germany have rules mandating any kind of action from operators make data centres sustainable through heat reuse, EU-wide guidelines have been criticised as milquetoast, and there’s little sign that the UK or US governments have shown much interest in pushing cloud providers to manage their waste heat more effectively. Then there’s the hyperscalers. “What we need is the large players to start taking this seriously,” says Bjorsgaard. It’s all well and good talking about how renewable energy is powering your cloud estate, he argues, but little attention is being paid to the indirect, Scope 3 emissions that constitute the hidden bulk of the sector’s carbon contribution. 

For now, Bjornsgaard is focusing on delivering additional digital boilers to customers up and down the UK. Is he worried that enthusiasm for the model might fade as wholesale energy prices fall? The entrepreneur thinks not. Ultimately, reducing European reliance on natural gas imports will continue to see market prices fluctuate. Besides, argues Bjornsgaard, the value proposition for recycling waste heat is clear. “Even if you were so hard-nosed that you didn’t care about the polar bears and climate change,” he says, “it’s just economics.” 

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