Groundwater heat pump feasibility in shallow urban aquifers: Experience from Cardiff, UK

Ground source heat pumps have the potential to decarbonise heating and cooling in many urban areas. The impact of using shallow groundwater from unconsolidated sedimentary aquifers for heating in urban areas is often modelled, but rarely validated from field measurements. This study presents findings from the 'Cardiff Urban Geo-Observatory' project. This study focuses on an experimental open loop ground source heat pump scheme retrofitted to a school building. Field monitoring for three years between 2015 and 2018 provided data on the environmental impact of the scheme on aquifer conditions. Average aquifer thermal degradation in the first three years was kept below 2 °C, with a maximum change of 4 °C measured during the heating season. The numerically modelled predictions of thermal degradation around the production and injection wells are compared with long-term field monitoring data, providing new insights into both aquifer, and user, behaviour. The Seasonal Performance Factor (SPF_H4) of the pilot installation was 4.5 (W13/W50) in the monitoring period. An initial thermal resource estimation of the wider aquifer volume suggests that lowering the temperature of the aquifer by 8 °C could generate equivalent to 26% of the city's 2020 heating demand, but achievable heat extraction would in reality, be less. The study concludes that large parts of the aquifer can sustain shallow open loop ground source heat pump systems, as long as the local ground conditions support the required groundwater abstraction and re-injection rates. Future schemes can be de-risked and better managed by introduction of a registration of all GSHP schemes, with open sharing of investigation, design and performance monitoring data, and by managing thermal interference between systems using spatial planning tools.