Assessing the Carbon Intensity of Low-Enthalpy Deep Geothermal Heat

Decarbonising heating presents a significant societal challenge. Deep geothermal energy is widely recognised as a source of low carbon heat. However, to date there has been no assessment of the carbon intensity of heat from low-enthalpy deep geothermal as previous studies have focussed on geothermal power or higher enthalpy heat. Further, there is currently no established method for assessing the CO₂ emissions reduction from implementing a deep geothermal heating scheme.

To address these gaps, we performed a life cycle assessment of greenhouse gas emissions relating to a typical deep geothermal heat system to (i) calculate the carbon intensity of geothermal heat (ii) identify the factors that most affect these values (iii) consider the carbon abated if geothermal heat substitutes conventional heating sources and (iv) set a benchmark methodology that future projects can adapt and apply to assess and enhance the carbon emissions reduction offered by geothermal heat development in the UK and internationally.

In the absence of an established deep geothermal heat system in the UK, to inform our work we adopted parameters from a feasibility study for a potential geothermal heat system in Banchory, Scotland. The Banchory project aimed to deliver heat to a network sourced from 2-3 km deep in a radiothermal granite where temperatures were predicted to be 70-90 °C. We assumed a 30 year project lifetime and that the heat system operation was powered by the UK electricity grid which was decarbonising over this period.

Our analysis found that the carbon intensity of deep geothermal heat is 9.7 - 14.0 kg(CO₂e)/MWh_th. This is ~5% of the value for natural gas heating. The carbon intensity is sensitive to several factors, and so the carbon intensity of deep geothermal heat could be reduced further by: replacing diesel fuelled drilling apparatus with natural gas or electricity powered hardware; decarbonise the power grid more rapidly than forecast; or substitute mains power with local renewable electricity to power pumps – or decarbonising the electricity grid faster or deeper; source lower carbon steel and cement; design projects to minimise land use change emissions.

Overall, our study provides quantitative evidence that deep geothermal systems can produce long term very low carbon heat that is compatible with net-zero, even for low enthalpy geothermal resources.