Future Proofing Heat. For Good.
In recent years, the topic of decarbonising UK heating has intensified from a niche discussion to a full-blown national conversation. And with Russia’s invasion of Ukraine destabilising Europe’s gas supply and driving millions of British homes into fuel poverty, district heating has emerged as one of the most sustainable and secure alternatives. These underground piping systems, which are simple in nature and offer distinct economies of scale, can quickly decarbonise entire streets, neighbourhoods, or towns.
However, while various studies agree that heat networks are vital to our net-zero future, they can only fulfil their green potential by utilising a variety of sustainable, low-cost heat sources. This need is driving commercial interest in large-scale heat pumps and Large Thermal Energy Storage (LTES) systems, which can absorb excess renewable generation and supply heat networks with on-demand green heating.
The Incredible Potential of Large-Scale Heat Pumps
With a well-established national gas network and around 85% of homes relying on a gas boiler, the concept of ‘electrifying’ UK heating is often lost in translation. While it may conjure up images of expensive-to-run direct electric heaters, it actually refers to district heat produced by heat pumps and electric boilers utilising renewable energy.
With a growing number of UK councils investigating district heating, large-scale heat pumps are gaining traction as a sustainable alternative to fossil fuels. They can range in capacity from hundreds of kilowatts to multiple megawatts, utilising their size and scale to deliver remarkable heating performance. Heat pumps can extract thermal energy from various renewable heat sources (including industrial waste heat) and use refrigerants, compressors, and electricity to raise it to the desired temperature.
Types of Large-Scale Heat Pumps
There are two common types of large-scale heat pumps, each with unique benefits and considerations:
- Air-source heat pumps extract heat from ambient air and offer high efficiencies even in very cold climates. Their use of an abundant heat source means they can generate reliable heat all year round, although their performance reduces when air temperatures drop. A key practical consideration of large air-source units is their size and weight: they are far too large and heavy to be mounted on the roof of a building. When combined with LTES, these systems can take up the space of multiple tennis courts, stand 25+ meters tall, and weigh several thousand tonnes. As these heat pumps are outdoor units that can generate substantial operating noise, their location must also balance their proximity to the heat load with space and community considerations.
- Water-source heat pumps absorb heat energy from oceans, lakes, rivers, wastewater, and mine water. They offer higher performance than air-source models, as water temperatures are more stable during the year and, specifically, higher than air in winter. Water-source heat pumps can supply population centres near coasts or large bodies of water, as well as the 25% of UK homes and businesses located above disused coal mines. Water-source systems typically have higher capital costs than air-source models due to requiring a substation heat exchanger. When utilising ground source water such as mine water, there are also expenses for drilling test boreholes to assess site viability and potentially higher pumping costs. One advantage of water-source units is that they are typically indoor systems that eliminate visual and audible disturbances to nearby properties.
In addition to utilising air and water, both heat pumps can be used to capture industrial waste heat. Common sources of this heat include power stations, data centres, water treatment plants, commercial distilleries, supermarkets, and many others, which can be leveraged to ensure a diverse and resilient heating supply throughout the year.
Converting Renewable Electricity into Green Heat
Electric heat pumps can offer lower carbon emissions than polluting gas boilers, but they only produce truly ‘green’ heat when powered by renewables. However, while the UK has drastically increased its uptake of solar and wind in recent years, large shares of their production are currently going to waste. In particular, wind turbines are facing increasing levels of curtailment, which is when they’re deliberately shut down due to producing more electricity than our grid can use, transmit, or store.
If wasting this precious clean energy wasn’t bad enough, it’s also costing British taxpayers big money. From January 2021 to April 2023, the UK spent £1.5 billion curtailing over 6.5 TWh of wind power—enough to power around one million homes. And with Labour planning to double our onshore wind capacity and quadruple our offshore wind capacity by 2030, our curtailment woes will likely only get worse. Thankfully, large-scale heat pumps coupled with LTES offer a direct solution. With their ability to power up and down within mere minutes, they can be operated during periods of high clean energy production when wholesale power prices are low or even negative, the latter of which occurred for a record 214 hours in 2023.
When large-scale heat pumps are located in the same region as sizable renewable energy stocks—such as ocean-source units near wind farms—they can utilise green electricity that would otherwise be curtailed. This ‘renewable-led’ approach can enable large-scale heat pumps to produce large volumes of low-cost heat while providing extra network stability. Even without targeted operation, the government’s plan to achieve a zero-carbon power grid by 2035 means large-scale heat pumps (and the heat networks they supply) will become greener over time.
Decoupling Supply from Demand with LTES
A core challenge of powering heat pumps with renewables is that intermittent solar and wind production often doesn’t align with the demand for heating. To a degree, our existing gas grid mitigates this problem by functioning as a natural storage reservoir. However, our power grid has no inherent capacity to store electricity, and large-scale batteries are complex, highly expensive, and consume vast amounts of precious natural resources. To solve this problem, district heating operators need the ability to capture and store significant volumes of heat so that it’s available exactly when they need it.
The solution lies in coupling heat pumps with LTES systems. They can take the form of industrial water tanks or pit thermal energy storage (PTES), with capacities ranging from hundreds of thousands of litres to several million. These safe, environmentally friendly systems can store heat for days, weeks, or even entire seasons, completely decoupling supply from demand. They can store excess heat generated by large-scale heat pumps that utilise surplus renewables such as wind power in winter and solar power in summer. In recent years, LTES investment costs have plummeted to between 4€/kWh and less than 1€/kWh of capacity, and as their heat losses decrease as storage volumes increase, they offer lucrative economies of scale.
However, thermal storage operators can realise the full benefits of LTES by pairing large-scale heat pumps with electrical boilers. Compact in size and despite a coefficient of performance (COP) of 1:1, electrical boilers can provide cheap peak capacity for short periods and low-cost backup generation throughout the year. Combining heat pumps with electrical boilers helps LTES operators minimise system costs by leveraging low or even negative electricity prices in spot and balancing markets. Electrical boilers can also react extremely quickly and modulate their operation according to the grid, providing valuable frequency balancing services.
One practical challenge of LTES systems is that while they are economical to build and operate, urban areas typically lack the space to house large volumes of water. Thankfully, most cities have surrounding regions that are suitable, which heat highways equipped with heat pumps can collect and feed into district heat networks as required. As the electrification of UK heating gains momentum, LTES will become essential to decarbonising heat networks and achieving a 100% renewable-based energy system.
Denmark’s Green District Heating Success
The award-winning district heating company in Sønder Felding has deployed these power-to-heat technologies to showcase the future of heat networks in action. Its system combines a 3.5 MW air source heat pump with 10 MW electric boiler capacity and a 3,300 m3 thermal storage tank, which holds up to a week’s worth of district heating consumption. The system absorbs large volumes of green electricity (primarily wind power) and draws from thermal storage when the heating demand exceeds the available supply.
This innovative project now delivers green and affordable heating to around 740 local homes. In addition to reducing heat network costs and emissions, the system has been a lucrative investment for the district heating company. The role of its energy manager has evolved into that of an energy trader, who maximises profitability by purchasing electricity when it’s cheap, free, or available at negative pricing.
Large-Scale Heat Pumps and LTES: Future-Proofing UK Heating
With gas bills still rising, fuel poverty at crisis levels, and our net zero target drawing closer, the UK urgently needs to lay the foundations for a cleaner and more affordable national heating system. District heat networks offer a direct solution, with unique economies of scale and the ability to incorporate a diversity of low-carbon heat sources. These include large-scale heat pumps, which can extract heat from ambient air, water sources like oceans, lakes, and disused coal mines, and capture industrial waste heat in a broad range of applications.
When heat pumps are coupled with electrical boilers and LTES, they can become ‘green heat converters’ that soak up cheap renewables that may otherwise go to waste. And once connected to heat transmission highways, these power-to-heat systems can supply urban heat networks with on-demand clean heat. Deploying these technologies at scale is vital to the electrification of UK heating—reducing costs and emissions for millions of homes and creating a thriving, gas-free future.