Performance of Ground Energy Systems Installed in Foundations

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The UK and the European Union have legally binding targets for reducing carbon dioxide emissions and for the increasing renewable energy generation. As about 25% to 33% of the UK's annual energy usage is expended on space heating, the provision of renewable heat energy is an area of critical importance if emissions and energy targets are to be achieved. Increased use of ground energy systems within foundations and other underground structures would be beneficial in both these respects, and will be eligible for financial support through the forthcoming government Renewable Heat Incentive. However, despite a recent increase in the use of ground energy systems, there remain key areas of uncertainty about their performance. This is especially important in the long term, where multiple installations will interact with each other and where unbalanced heating or cooling loads will lead to changes in the thermodynamic regime in the ground. This project aims to address some of the uncertainties surrounding ground energy systems installed in foundations by comprehensively instrumenting and monitoring two sites in contrasting ground conditions. This will allow the real response of the ground to known heating and cooling loads to be measured, and comparisons made with predictions based on analytical and numerical models. The use of contrasting geological regimes will allow investigation of the impact of groundwater on the performance of systems, something rarely considered and not well understood. The field monitoring will be accompanied by a programme of in situ and laboratory testing to assess differences in thermal behaviour at different scales and temperatures relevant to ground energy systems. The testing programme will address questions relating to degrees of uncertainty in determining key thermal properties and how this may compare with other uncertainties in the system design, such as heating/cooling loads. Numerical modelling, including back analysis of the in situ thermal response testing and operation of the ground energy systems, will allow assessment of the sensitivity of the systems to different input parameters. The modelling will also allow evaluation of the numerical and analytical techniques currently used for the design of ground energy systems and assessment of the importance of key factors (geological variation, groundwater, surface boundary conditions, geothermal gradient) not currently accounted for in existing methods. Taken together, the various strands to the project are expected to provide an important dataset which will add substantially to the understanding of the performance of ground energy systems. By addressing uncertainties surrounding design input parameters, geological conditions and design approaches, the project will also provide relevant lessons for direct application to the design and construction of ground energy systems installed in foundations, which it is expected will ultimately form part of improved guidance for industry.