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

Abstract

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 2015-2018 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 Journal Pre-proof J o u r n a l P r e-p r o o f 2 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.

A review of thermal energy storage technologies for seasonal loops

“…Although global analysis is useful in predicting total potential, local characterisation is also needed. Boon et al [35] used the process set out by Kessler et al [36] and data from over 3000 historical geotechnical and geological borehole records through third parties to generate a 3D geological model. This was used to define the extent and thickness of the sand and gravel aquifer units and confining layers, however, was made easier through the extensive data already available.…”

Bayesian parameter inference for shallow subsurface modeling using field data and impacts on geothermal planning

“…To assess the potential impact of parameter uncertainty on GSHP design, an existing open-loop GSHP system within the studied area of the city center of Cardiff is examined as an example case. This system has been installed at the Grangetown Nursery School, in the southwestern region of the modeled domain, and is monitored by the BGS (Boon et al, 2019). It includes two ground-source heat pumps that can provide up to 11 kW each, noting that, at the time of designing, few data from the site and aquifer were available and thus assumptions were made in the design.…”

Research on heat transfer characteristics and borehole field layout of ground heat exchangers to alleviate thermal accumulation with groundwater advection

“…is the soil temperature variation at distance r from the center of borehole, K; h is the length along the borehole depth, m; H is the borehole depth, m; erfc is the complementary error function. According to the superposition principle, the dynamic heat load in operation of boreholes is considered [27], and then the variable heat source theory [28] of boreholes is used to obtain the MFLS model of multi-boreholes, which can describe the temperature variation of any point at the soil temperature field around multi-boreholes. The expression can be written as,…”