Is the answer to net zero just digging big holes and filling them with warm water?

We build large reservoirs feeding water treatment plants and a network of pipes to bring the resulting clean water to our houses. Water systems in developed countries are shining examples of the power of collective endeavour, which means when it rains in Wales in November, I don’t rush outside with a bucket in Surrey. That’s because someone else catches it, someone else treats it and, the following July, I turn on a tap and there it is.

My key point is that the technology that turns unusable rain into usable drinking water is located centrally, and the transformed product is distributed locally.

Now imagine we had designed water the same way we have designed heating.

Every house would have its own little water treatment plant for the winter rain, and in the summer, we would buy huge amounts of water from rainy countries, pipe it untreated to your house and, if your tiny water treatment plant broke down, then tough.

This sounds absurd, but that is almost exactly how we produce heat in this country.

We buy the raw fuel, gas, and distribute it in untreated form through a vast network of underground pipes before burning it in around 20 million individual heating systems to create the thing we actually want: heat.

This means millions of people solving the exact same engineering problem every winter with their own little gas-to-heat conversion plant, typically called a boiler. If  an engineer were designing Britain’s heating system from scratch today, there is almost no chance they would design it this way.

And every year we spend half of our time ignoring the enormous amount of heat that comes from the sky.

Part of my defence to HR is that it really was exceptionally hot last month. In June alone, roughly 3,000 TWh of solar energy fell on the UK.

That is a big, meaningless number until you put it into context.

The UK burns around 700 TWh of natural gas in an entire year. London alone uses around 66 TWh of heat every year.

This means that if you could capture all the heat that fell on the UK in June, you could theoretically heat London for around 45 years.

Now, of course, we cannot capture all of it. But you cannot catch all the rain either. Nobody built Kielder Reservoir expecting to collect every drop that fell on Northumberland. The point is not about capturing everything. The point is capturing enough to make a difference.

Suppose we managed to capture just 10% of June’s sunshine as usable heat. That would give us around 300 TWh, over 40% of our annual gas demand, from just one summer month, and enough heat to supply London for around four and a half years.

And this is not rocket science either.

You capture the heat with solar thermal panels, store it underground in natural aquifers or man-made reservoirs, and bring it back to the surface when you need hot water. Then you connect groups of users together through district heating networks, creating a single point of heat production instead of hundreds of thousands of little boilers.

It is a simple concept, and the Danes, Dutch and Swedes have been using variations of it for decades.

The world’s largest operational example of this is in Denmark, in a small town called Vojens with around 3,500 homes.

The Vojens plant is a snappily titled Pit Thermal Energy Storage (PTES) system. In other words, it stores around 200 million litres of hot water in a giant insulated hole in the ground beneath a floating lid and supplies heat to the town’s district heating network.

During the summer, it is charged using large solar thermal fields and surplus renewable electricity. During winter, the stored heat is released back into the network.

As we’ve already seen, this isn’t especially complicated.

The important number for our purposes is that the plant stores around 12 GWh of heat and supplies approximately 3,500 homes. That is impressive, but it is still tiny compared with UK demand.

If we stick with London as our example and modestly aim to cover just 10% of winter heat demand, we will need roughly 8.8 TWh of seasonal storage.

A Vojens-sized store (12 GWh) would cover only around 0.14% of that requirement.

So, supplying just 10% of London’s winter heat demand would require roughly 700 Vojens-scale stores.

That’s a huge number.

But then everything in net zero is a huge number.

The UK now has well over a thousand solar farms larger than 1 MW and around 700 biogas plants. Building hundreds of thermal storage facilities was not impossible, but historically we chose a different path from the Danes. We spent billions building renewable electricity while largely ignoring how we create, store and distribute heat.

The second problem is historical. More than a century ago, we decided individual heating systems were the best solution. Now almost every house in the UK has its own boiler tucked away like Harry Potter in a cupboard under the stairs. Every large building has its own heating plant in the basement. Very few, if any, share a heat network with their neighbours.

This is the fundamental difference between Denmark and the UK.

Denmark built the pipes and collective heat infrastructure first, then started building seasonal thermal storage.

The UK remains rooted in Victorian individualistic thinking about heat. Changing from an individualistic approach to a collective one in a city like London will be incredibly hard. We cannot suddenly capture London’s summer heat at scale because we would need to build hundreds of kilometres of heat networks and hundreds of thermal storage systems beneath one of the world’s oldest continuously occupied cities.

So, places like London are probably out of the question.

The good news is that new developments are different.

The UK builds around 200,000 new homes each year, many of them on large housing developments.

A 1,000-home development might have a peak winter heat demand of around 5–10 MW and an annual heat demand of roughly 10–20 GWh. A Vojens-sized store (12 GWh) could therefore theoretically store close to a year’s worth of space and hot water heating demand for a development of that size.

This is why new developments are a much easier starting point for district heating and seasonal storage than retrofitting London’s existing housing stock. The pipes, heat source and storage can all be designed in from day one. All it would really take is a relatively minor change to planning rules requiring these systems to be installed from the outset.

Which makes me wonder: why don’t we do more of this?

I think the answer is that the renewable industry in the UK has developed an innate preference for complexity over simplicity.

Look across the UK energy policy landscape, and you will find endless enthusiasm and conferences for esoteric things like artificial intelligence, blockchain, e-fuels, tidal power and other clever technologies. 

Things that are difficult and glamorous.

Nobody is hosting conferences called: “Is the future of net zero just digging big holes and filling them with warm water?”

But perhaps they should.

Because maybe it is.

Sometimes the answer is simply looking at an old problem differently. Water taught us this lesson hundreds of years ago. We do not each have our own reservoir and, more pertinently, we do not each treat our own water.

We built a network that relied on collective rather than individual transformation technology and, despite its current travails, the UK water and sewage system remains a remarkable piece of engineering.

We could do the same with heat.

In conclusion, I hope I don’t get sacked for answering a Teams video call topless.

In my mitigation, it was a really, really hot day and my colleague Nichlas sort of saw the funny side.

Every summer Britain receives an astonishing amount of free heat from the sun.

We admire it, complain it’s too hot and then let almost all of it disappear.

Come January we vaguely wonder where all that energy went and then go out and buy more gas and therefore more heat from the US and the Middle East.

We don’t do this with water because, on a rainy island, it would be ridiculous not to store it.

In an increasingly hot Britain, heat deserves exactly the same treatment.

The answer to decarbonising heat isn’t hidden in some futuristic technology. It doesn’t require another fashionable technology with a three-letter acronym.

Hopefully, for my sake, it doesn’t involve HR either.

The answer to heat has been sitting there all along. Quietly beneath a floating lid in a very big hole in the ground, next to an unremarkable little town in Denmark.