Balancing comfort and climate: the role of residential air-source heat pumps in future UK cities
As the UK advances toward Net Zero by 2050, the adoption of low-carbon heating solutions has gained momentum, with air-source heat pumps (ASHPs) emerging as a key alternative to traditional gas boilers. ASHPs transfer heat between indoors and outdoors, enhancing energy efficiency and reducing carbon emissions. However, their broader impact, particularly on outdoor temperatures, remains underexplored. Could their large-scale adoption exacerbate summer overheating or contribute to winter overcooling in urban areas?
Our study investigates these questions using advanced urban climate and building simulation models to analyse how ASHPs interact with the UK’s 2050s projected climate. By assessing heat emissions, cooling and heating demand, and urban microclimates, we explore how ASHPs might reshape the future of energy-efficient, climate-resilient cities.
How do ASHPs interact with urban climates?
Unlike gas boilers, which generate heat directly, ASHPs redistribute heat, influencing both indoor and outdoor temperatures. This means their effects extend beyond building-level efficiency to neighbourhood-scale climate conditions, with potential impacts on urban heat islands, cooling/heating demand, and energy efficiency.
To quantify these effects, we developed a multi-scale modelling approach that integrates:
- Surface Urban Energy and Water Balance Scheme (SUEWS, https://suews.readthedocs.io/en/latest/) –A local-scale urban land surface model that simulates air temperature, wind speed, humidity, and radiation exchanges in a neighbourhood setting.
- EnergyPlus – A widely used high-resolution building energy simulation tool that models indoor-outdoor heat flows, including ASHP heat emissions and their influence on local microclimates .
By coupling these two models, our framework captures the complex feedback loop between building energy use and urban climate, providing an innovative and computationally efficient way to assess ASHP-driven temperature changes.
The climate impact of ASHPs
Our modelling focused on two common residential building types: two-storey detached houses and four-storey low-rise flats, both evaluated under 2050s climate projections.
Summer overheating: Do ASHPs contribute to urban warming?
- ASHPs increase anthropogenic heat emissions by up to 19.3 W m-2 in dense neighbourhoods.
- This results in an outdoor temperature rise of up to 0.12 °C, particularly in London and Cardiff, where higher cooling demand and population density amplify urban warming.
- While 0.12 °C may seem small, in densely populated areas, cumulative effects could amplify urban heat islands, increasing cooling demand and straining local energy grids.
Winter overcooling: Could ASHPs make cities colder?
- ASHPs extract heat from the outdoor air for indoor heating, reducing neighbourhood-scale heat emissions by up to 11.1 W m-2.
- This leads to a local temperature decrease of up to 0.16 °C, with colder cities such as Aberdeen experiencing the most significant effects.
- While minor, this cooling effect could increase heating demand, particularly in low-income communities that rely on passive heat gains from surrounding buildings.
What does this mean for urban design, policy and research?
Our findings suggest that ASHP adoption should be carefully managed to ensure climate resilience while maximising energy efficiency.
For policymakers:
- Recognise that ASHPs influence urban heat dynamics, requiring policies that go beyond energy efficiency alone.
- Promote green infrastructure and passive cooling strategies to counteract overheating effects.
For architects & engineers:
- Incorporate passive cooling and ventilation solutions to reduce reliance on ASHP cooling.
- Use high-performance insulation and thermal mass to limit winter overcooling and maintain thermal stability year-round.
For researchers & modellers:
- Advance multi-scale simulation tools like SUEWS-EnergyPlus to better capture ASHP-induced urban microclimate changes.
- Improve model accuracy and computational efficiency to enhance real-world applicability.
- Develop open-source frameworks to enable more precise assessments of energy-climate feedback loops, guiding data-driven urban planning.
Final thoughts: The future of ASHPs in UK cities
ASHPs are vital for decarbonisation, but their broader impact on outdoor temperatures must be carefully managed to avoid unintended urban overheating or overcooling. By aligning ASHP deployment with urban planning, cities can strike a balance between sustainability and climate resilience. As urban areas face rising temperatures and evolving energy policies, ASHPs will play a pivotal role in shaping the future built environment, making it crucial to integrate climate-conscious strategies for their adoption.
Want to learn more?
This research is published in Sustainable Cities and Society: https://doi.org/10.1016/j.scs.2024.105811 (Xie et al., 2024).