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Nov 29, 2023

The enormous heat pumps warming cities

It's another cold snap and the fields of Cornwall, in south-west England, are

It's another cold snap and the fields of Cornwall, in south-west England, are blanketed in snow. But down a windy lane, Ceri Simmons' home is toasty warm. Her living room is a jungle of hanging plants and, through the kitchen, glimpses of a wood-lined studio reveal Simmons' job as an aerial-yoga teacher. "It's not just lovely for me to have a warm house, it's also important for my clients," she says.

The remote village of Stithians, close to the most south-westerly tip of the UK mainland, where the Simmons family live has become an unlikely frontier in the race to decarbonise heating. It is piloting a new approach to low-carbon heating which could be key to the rapid scale-up needed worldwide.

The project zooms out from the obstacles facing individual homeowners and designs a heat pump system that can be delivered at scale across streets, towns and cities. In doing so, it could provide a model for urban spaces across the world pondering how to decarbonise their heat systems quickly and effectively.

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In the UK today, 74% of people heat their homes using gas boilers, with mostly electric heaters and oil comprising the rest. This leads the heating sector to account for a third of the UK's greenhouse gas emissions – comparable to the emissions of all its petrol and diesel cars. Similar values are seen in the US, where around half of heating comes from gas.

To limit global warming, this needs to change drastically, and in many places, that means installing many more heat pumps. By 2030, around a quarter of UK buildings should be heated using them, according to the UK government's climate advisory body, rising to 52% by 2050. Electrifying heating will also be key to decarbonising buildings in the US, says Melissa Lott, director of research at the Centre on Global Energy Policy at Columbia University. One study in San Francisco referred to heat pumps as the "single most impactful lever" to reducing emissions.

Rather than burning a fuel, heat pumps concentrate heat energy already present in air, ground or water and pump it through a building's pipes and radiators.

They do this with incredible efficiency, converting 1 kilowatt (kW) of electricity into 3-5kW of heat, as opposed to 1kW for a direct electrical heater and 0.9kW for a gas boiler. This means they provide practically "free heat", says Lott. However, as with all heating systems, efficiency depends on how well the building is insulated to minimise heat loss, she notes.

If the source of electricity is renewable, heat pumps themselves emit no carbon. In the UK, almost half of the electricity provided to the national grid comes from renewable sources, compared to 20% in the US. Both countries aim for sharp increases in these percentages.

Ceri Simmons' heat pump, seen here outside her house, now supplies all her heat and hot water from a network of pipes under the street (Credit: Evie Townend)

The Heat the Streets project in Stithians provides a whole new template for how ground source heat pumps can work.

Ground source heat pumps are more efficient than their air source counterparts. This is due to the ground having a consistent temperature. Most ground source heat pumps have a vertical piping which requires drilling of a deep, costly borehole 60-200m (200-650ft) into the ground. Alternatively, they can use a horizontal loop that is far shallower in the ground but requires a large surface area that most people don't have, especially in cities.

What's more, installing heat pumps tends to be the responsibility of individual homeowners. Despite incentives such as the UK's Boiler Upgrade Scheme and US federal tax credits under Biden's Inflation Reduction Act, there remain significant barriers to widespread rollout. There is often a lack of understanding and awareness of the technology, which, combined with large upfront costs and few trained installers, can prevent homeowners from making the change. Architecture can also be a barrier: houses also simply need enough outdoor space to install the heat pumps, something obviously lacking in flats and dense urban settings.

Rather than each home drilling a single borehole for a single heat pump, however, Heat the Streets uses over 200 boreholes drilled 100m (330ft) beneath the street linked to a huge communal network of horizontal, underground pipes just below street level, known as a heatmain.

Glycerol – an odourless, non-toxic, viscous liquid – is passed vertically through the boreholes to absorb heat and then circulate it in these horizontal pipes, which in turn supply heat pumps in individual properties along the whole street and, eventually, the whole neighbourhood.

The heat pumps – no larger than a typical gas boiler – are fitted either inside or outside individual homes, depending on the property's size, suitability and owner preference.

Just a few metres below the surface of Cornwall, the ground has a constant temperature of around 11C (52F) from absorbing sunlight for millennia, says Max Bridger, project operations manager of Heat the Streets, and it's this heat that is harvested by the heatmain.

Heat pumps run more efficiently when buildings are insulated to minimise heat loss (Credit: sturti/Getty Images)

The heat pumps then perform another series of exchange, compression and evaporation that brings the temperature to around 50C (122F). Finally, this heat is transferred to water, which is pumped through a house's specially upgraded pipes and radiators.

Kensa Utilities, the company in charge of installing the network in Stithians, will remain the owner of the infrastructure. For residents, connecting to the heatmain works like it does with other utilities, such as broadband or water. Residents own their individual heat pumps and pay a connection fee to join whenever they're ready.

"When a [gas] boiler breaks, there'll now be an alternative to simply replacing it. But this system also means people don't have to finance the large upfront infrastructure costs or connect all at once," says Bridger.

The ground source heat pumps provide all the heating and hot water needed by the house, and cut the greenhouse gas emissions released by them by 70%. Residents keep full control of their heating and can switch energy suppliers whenever they want.

Simmons' home was fitted with a heat pump that shares a heatmain with several neighbours. It took around a week to do the interior and exterior work and "really wasn't too disruptive or noisy", she says. The water cylinder, tucked away inside an old linen cupboard, is almost exactly the same size as an average boiler.

She ultimately plans to use her solar panels, currently set up to sell the electricity to the National Grid, to directly power the heat pump – which she says would make her home almost entirely self-sufficient.

Workers operate a drill rig for a borehole in Enfield, London, where they are installing ground source heat pumps in high-rise flats (Credit: Kensa Utilities)

This concept of a heatmain buried beneath a communal street is by no means restricted to single-storey properties like Simmons'. Another Kensa project in Enfield, London, is installing ground source heat pumps in high-rise flats.

Heatmains have been developed beneath each of eight tower blocks, totalling 400 flats, using their car parks to drill the required boreholes.

A vertical shaft that is used to transport other utilities throughout the building, known as a service riser, allows the technology used in Stithians to work for multi-storey buildings, says Bridger. With smaller spaces, retrofitting each flat with a "shoebox" heat pump can supply enough heat while minimising the space taken up inside, he adds.

The future of heat pumps in high-rise buildings is receiving growing attention across the world. In New York City, it is air-source heat pumps that have become the focus of efforts to decarbonise the city's 6,000 high-rise buildings.

Currently, most of these have a single, large gas boiler that controls the whole building's heating. Each individual flat also uses an air conditioning unit attached to an outdoor window that works separately from the heating system.

In 2021, the New York City Housing Association launched the Clean Heat for All Challenge, a competition to encourage industry innovation in line with an upcoming local law that will limit the greenhouse gas emissions of buildings from 2024. The design that emerged as the winner is an air-source heat pump which hangs "like horse saddlebags" from the base of apartment windows, not blocking any light or taking up space inside apartments, Lott says.

"The beauty of heat pumps is that they can both heat and cool spaces, using the same process in reverse," Lott adds. "It's one modular unit performing two functions."

The design would also give apartments control of their own heating and cooling. "It's more efficient because you have the ability to customise your apartment to your own comfort so there'll be no more windows flung open from overheated apartments in winter," says Lott.

The new heat pumps are being rolled out this year for a trial at Woodside Houses, a complex of 20 brick buildings in Queens where residents went without heating and hot water last winter following Hurricane Ida.

While it is not expected that the local grid infrastructure will need to be upgraded immediately, an increased electricity demand is an important consideration in the increased use of heat pumps generally.

Jan Rosenow, director of European programmes at the Regulatory Assistance Project (RAP), says that the UK could see pressure on the grid double or even triple in the next 20 to 30 years. "There will need to be ways to store electricity other than as large batteries in people's homes," he says. There is a range of promising technologies out there, he adds, such as flow batteries and green hydrogen. (Read more about whether gravity batteries can solve our energy storage problems).

Stockholm's district heating network supplies electricity to 800,000 homes using industrial-scale heat pumps and other sources of heat (Credit: Christine Olsson/Getty Images)

In a sense, Heat the Streets taps into another major solution for heating homes in a low-carbon world: district heating.

In essence, district heat networks are "just whacking empty vessels that connect properties together", says David Barns, a heat decarbonisation expert at the University of Leeds. "How you get heat energy into that heat network is a related but separate question."

District heating boomed in Nordic countries during the 1970s' oil crisis, but the systems were mostly powered by burning fossil fuels in large combined heat and energy plants. Still, this design of a single centralised system can make it far easier to switch a significant number of homes to low-carbon heating – without thousands of boilers being "ripped out", says Barns.

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For example, Stockholm's district heating network has 3,000km (1,860 miles) of pipes and now supplies electricity to 800,000 homes using industrial-scale heat pumps which capture heat from domestic wastewater, data centres and seawater, alongside incinerated non-recyclable waste and forest biofuels.

The Heat the Streets model in Stithians uses "fifth-generation district heating" – a localised network combined with heat pump technology. The advantage of this kind of street-by-street project is being able to focus on whatever energy resources there are nearby to make the best match, says Caroline Haglund Stignor, a researcher in energy technologies at Rise Research Institutes of Sweden. "You start small and then you build on little by little," she says.

This can allow for more innovative methods of sourcing heat, such as a network in Islington harnessing heat from the hot currents from London Underground tunnels, or flood-water in old mines.

In the UK, however, there is a lack of awareness and regulation around district heating, which is delaying its expansion, says Barns.

The UK government has committed to designating heat network zones no later than 2025 that will situate heat networks in the best places and mandating that people connect to them. This will help to make the business model for building heat networks more financially viable, Barns explains, because private investors will have more certainty that people are going to use them.

This type of large-scale work can be coordinated with other groundworks, such as cables for electric vehicle charging points, to reduce disruption, says Rosenow. But these rollouts at the local level requires "radical change" in planning that is not yet reflected in policies or frameworks, he adds.

It's far more common to see air-source heat pumps on individual houses than apartment blocks (Credit: Mark Morgan/Alamy)

Still, district heating is not necessarily the right answer everywhere. It is most suitable for densely populated areas due to the significant heat loss that comes from transporting water at high temperatures over long distances.

District heating already exists in some US cities, such as Milwaukee and Baltimore, but the country doesn't have the same density of population as the UK and most of Western Europe, bar a few exceptions, says Lott. "For most of the US, we're talking about heating individual buildings and so district heat networks don't make sense. [Individual] heat pumps, both air or ground source, are a much better option for us."

The road to decarbonised heating is not simple but heat pumps are one technology more or less ready and waiting, so long as the cost hurdles can be breached. But even as question marks hang over energy security, fuel prices and climate-related weather extremes, the yoga studio in the Simmons' family home feels like an inviting place to weather the uncertainty.

This article was updated on 2/2/23 to clarify a quote about long term storage solutions from Jan Rosenow.

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