How to heat a city…and decarbonise it using heat pumps! David Boon and Gareth Farr

With only 11 years to go until the first UK emissions target deadline, the race is now on for the UK to reduce its greenhouse gas emissions by 57% by 2030 of 1990 levels. BGS geoscientists, David Boon and Gareth Farr ask ‘How on earth will we do it?’.

Infographic illustrating the concept of using shallow urban aquifers and
heat pumps in district heat networks.
Credits: City of Cardiff Council/BGS/WDS Green Energy Ltd
In our first blog titled 'How to Heat a City', we announced our intention to install a pilot open loop Groundwater Source Heat Pump (GWHP), as part of an InnovateUK funded feasibility study to better understand how UK shallow aquifers can supply low carbon heating in urban areas. The recently established Cardiff Urban Geo Observatory hosts the GWHP pilot, which is conveniently surrounded by a world class groundwater monitoring network with over 100 temperature sensors in 60 boreholes, providing high resolution baseline data. Groundwater in urban areas can be slightly warmer than rural areas, due to the ‘subsurface Urban Heat Island effect’, and this human footprint actually makes heat pumps run more efficiently.

The pilot GWHP scheme is a collaboration between WDS Green Energy, City of Cardiff Council and the British Geological Survey. The physical infrastructure comprises two shallow boreholes (~20 m deep) that abstract and simultaneously re-inject shallow groundwater from an ice-age sand and gravel aquifer which underlies much of the city.

The heat pump system works by passing groundwater through a heat exchanger where 2 Kelvin degrees of its thermal energy is transferred to a heat pump which uses a gas phase-change to raise the water temperature to a useable 46oC. The heat pump keeps the inside of the school at a comfortable 22oC using a renewable energy resource that has low carbon emissions and is cheaper to run than the old gas boilers. 

As part of the InnovateUK project we fully instrumented the heat pump system with sensors above and below ground to allow us to follow its long-term environmental impact and energy performance. The project proved this technology could be scaled-up across Cardiff and other UK cities with similar shallow aquifers, where the geology allows.

The GSHP pilot plant room and associated monitoring and data telemetry. (Credit: BGS-UKRI)
Shallow aquifers can be a super-efficient way to run a heat pump as the borehole pumps do not require much energy to lift the water compared with deeper schemes. The energy efficiency of our pilot GSHP system is around 450 %, over four times more efficient than a condensing gas boiler, and is actually saving the council money on its energy bills!  And what about that all important CO2 target? …well after 3 years' the pilot GWHP had reduced the cost of heating with a 35 % reduction in CO2 emissions from the school overall.

The project has also had impact on energy policy in Welsh Government and was featured as a case study in a recent National Assembly for Wales Low Carbon Heat Research Briefing. The Cardiff Urban Geo-Observatory, which includes the heat pump monitoring pilot, has also been selected along with the Glasgow UKGEOS site as a pilot area for a new 3-year EU GeoERA project called MUSE (Managing Urban Shallow Geothermal Energy), and as a European Plate Observing System (EPOS) site.

CO2 emissions reductions resulting from switching from a gas boiler to a
shallow groundwater source heat pump.
Data courtesy of Cardiff City Council.
We will continue to monitor the environmental impact of the scheme on the aquifer source, and the BGS Geothermal Team is keen to support other UK cities in their journey to explore their range of geothermal resources. The project has also attracted lots of interest from industry keeping the project’s Principal Investigator, David Boon, busy giving presentations to stakeholders such as the IEA Heat Pump Technologies Annex 52 meeting in London in September 2018, seminars for Construction Excellence Wales and APSE Energy. The research findings will feature in a new CIBSE Code Of Practice (CP3) and in peer-reviewed papers.

Although open-loop ground source heat exchangers are not suitable in all geological environments, we have been working to understand the wider 3D geological and hydrogeological settings at a city-scale, with the release of a 3D superficial geology model of Cardiff. This evidence will allow better ‘above’ and ‘below-ground’ planning and regulation, and will (we hope) stimulate market growth in renewable energy systems and supply chains.  More demonstration projects like this are needed to improve the image and public perception of renewables. Local authorities can invest in renewable energy such as heat pumps using interest-free Government-backed finance schemes like the SALIX finance and the Renewable Heat Incentive (RHI) scheme. Case studies such as ours can give society and business the confidence to invest in shallow geothermal technologies to accelerate the energy transition.

David Boon and Gareth Farr have jointly managed and overseen the creation of the Cardiff Urban Geo Observatory (2014 – 2018).  The project has evolved naturally out of City Region Geoscience project (under former Chief Geologist Wales, Dave Schofield), the 2015 InnovateUK feasibility project (led by David Boon), and by listening to local stakeholders in Wales. Massive thanks go out to the project team: Ashley Patton, David Schofield, Alan Holden, Rhian Kendall, Laura James, Steve Thorpe, Corinna Abesser, Johanna Scheidegger, Jon Busby, Susanne Self, BGS Dando Drilling Facility, and others. Key partners are Cardiff Harbour Authority, City of Cardiff Council, WDS Green Energy, David Tucker (Nu Vision Energy (Wales)), and Innovate UK/ BEIS. @BGSWales