Characteristics and analysis of an Open-loop Ground Source Heating System

Thesis

Open-loop ground source heating systems (GSHS) provide cooling and heating to a space with several positive sustainable and economical factors over fossil-fuel-based alternatives. The worldwide number of GSHS is increasing but they remain relatively rare compared to alternatives. Therefore the overall GSHS industry might still be considered young in its GSHS-specific experience of testing, design and modelling practises. Characterisation and analysis of an operational system, in light of past design assumptions, modelling and testing practices, remains a novel opportunity to provide key insights and learnings. Several studies have been conducted into the testing and operational performance of an open-loop GSHS. Historical operational data showed thermal fluctuations at an abstraction well in advance of predicted thermal breakthrough times. An explanation of these occurrences was sought by evaluating prior testing and modelling assumptions. Models were constructed to evaluate pump and tracer testing data, calibrating hydraulic and solute-transport properties of the host aquifer. Calibration of the thermal transport properties, not previously conducted, was pursued through isolating an early thermal event and several years of operational data. Work identified differing hydraulic and transport properties to those previously assumed in design. The significance and sensitivity of free parameters in each model variant was evaluated. Re-calibrated properties yielded inconclusive results; attributing thermal fluctuations to either regional groundwater temperature fluctuations (reinforcing earlier predictions that thermal breakthrough was not yet expected) and/or earlier than expected thermal breakthrough. Calibration efforts highlighted the importance of comprehensive testing and instrumentation to support design and modelling efforts --- namely, careful consideration and increased density of sensor placement, along with geophysical investigation of down-hole well properties. Modelling of open-loop GSHS operational strategies (unidirectional and seasonally reversed) was conducted to provide quantitative support to the naive intuitive benefits of the latter. A seasonally reversed strategy was found to outperform unidirectional modes considering varying injection temperature, peak energy loads and seasonal imbalances in operation. Furthermore, the superior performance in both energy efficiency and predicted design life was observed without permitting the system to be responsive to detected thermal stores. A responsive seasonally reversed system is expected to further outperform its unidirectional counterpart.

Authors: Delport, T

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: seasonally reversed; well tests / pump tests; ground source heating system; unidirectional; open-loop; well doublet; thermal tracer test; geothermal; tracer test; transport in porous media
• Subjects: Geothermal resources; Heat as a groundwater tracer; Ground source heat pump systems; Tracers in hydrogeology; Geothermal space heating; Hydraulic models; Geothermal wells