Performance of Ground Energy Systems Installed in Foundations

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

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.

Planned Impact

The benefits of this research will be far reaching, providing greater understanding of the combined thermal behaviour of the ground and concrete in contact with it that will ultimately lead to guidance for designers and contractors within the civil engineering industry. It will facilitate greater uptake of closed loop energy foundations, bringing clean renewable energy to public and private buildings. The work will therefore contribute to the carbon dioxide reduction targets for the UK and the European Union. The project will be run by the University of Southampton in collaboration with a number of industrial partners who will provide practical input, function as a steering group and be amongst the initial beneficiaries of the research. The project partners are leading engineering practitioners who bring a range of complementary skills to ensure the research is relevant to industry and assist with knowledge transfer to reach the widest range of potential beneficiaries. The partners, of whom full details are provided in the impact plan, are: i) Mott MacDonald, who will provide guidance from the perspective of an Engineering Designer, ii) Cementation Skanska, who will provide access to sites and practical assistance. They have also recently worked on the instrumentation of an energy pile in south London, iii) Golder Associates, who will provide guidance from a design perspective. In addition, WJ Groundwater will assist with the installation of instrumentation and in situ testing and will provide reduced rate drilling, installation costs and thermal response testing. Provision of research findings to beneficiaries will take place throughout the project. Dissemination to the project partners will occur rapidly and frequently through quarterly meetings of the steering group and via the University web based teaching tool Blackboard . This means that results from the research can be passed on to industrial practitioners as they become available. Towards the end of the project, the results will be published in quality journals (such as Proceedings of the ICE, Geotechnique, Proceedings of the ASCE, Canadian Geotechnical Journal) to make them available to the wider civil engineering community. Shorter articles for publications such as New Civil Engineer and Ground Engineering, which have a very large readership, will also be produced. Presentation at prestigious European and International conferences will allow dissemination to a wider audience. Ultimately the results of the research are expected to be incorporated into industry standard guidance and design documents that will facilitate safe and efficient uptake of ground energy systems throughout the UK and beyond. Using educational establishments (schools and colleges) to provide study sites also offers a unique opportunity to engage teachers and pupils with the project and with the topics of both renewable energy and the role of civil engineers in providing for a sustainable future.

Publications

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Amis T (2014) Identifying best practice, installation, laboratory testing and field testing in DFI Journal - The Journal of the Deep Foundations Institute

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Bourne-Webb P (2014) Design tools for thermoactive geotechnical systems in DFI Journal - The Journal of the Deep Foundations Institute

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Katzenbach R (2014) New technologies and applications: materials and equipment in near surface geothermal systems in DFI Journal - The Journal of the Deep Foundations Institute

 
Description The key findings from this grant are:-
1. Piled foundations used as heat exchangers exhibit different thermal behaviour compared with better understood borehole ground heat exchangers. This is because piled foundations are shorter and wider than boreholes and reach a thermal steady state with the overlying ground surface more quickly.
2. Because piled foundations are larger in diameter than boreholes, the short term transfer and storage of heat within the concrete of the foundation is very important. If this thermal capacity is neglected during analysis then the energy performance of the associated ground source heat pump system will be underestimated.
3. New semi-analytical design approaches are proposed for use with piled foundations used as heat exchangers. These allow determination of the amount of energy obtainable for a given foundation arrangement.
4. Initial recommendations have been made about limited the use of thermal response testing for piles to small diameters only. Thermal response tests are a routine in situ test for use with boreholes to determine the thermal design parameters of the ground and heat exchanger. However, the method and standard interpretation approaches are found to be inappropriate for larger diameter piles.
5. The importance of heat losses in steady state thermal testing of soils to determine their thermal conductivity was confirmed. This lead to a change in currently recommended practices. Additionally, new interpretation approaches for transient laboratory tests for thermal conductivity were proposed.
6. The importance of thermal conductivity as a design parameter for ground source heat pump systems was confirmed for simple heating only applications. However, the reduced sensitivity of such systems to thermal conductivity when the heating load is more complex was also shown.
Exploitation Route Improved analysis methods for pile heat exchangers have been published, showing the benefit of carrying out transient calculations for large diameter piles.
Guidance for practice have also been included within the Ground Source Heat Pump Association thermal pile standard.
Recommendations for laboratory testing and in situ thermal response testing have also been made.
The investigators continue to be involved with a related COST Action which will lead to European level guidance in due course.
Sectors Construction,Energy,Environment

 
Description Findings from this project have been included in the Ground Source Heat Pump Association Thermal Pile Standard.
First Year Of Impact 2012
Sector Construction,Energy,Environment
Impact Types Economic

 
Description Ground Source Heat Pump Association Thermal Pile Standard
Geographic Reach National 
Policy Influence Type Participation in a advisory committee
URL http://www.gshp.org.uk/pdf/GSHPA_Thermal_Pile_Standard.pdf
 
Description 2014 Geotechnical Engineering Lecture. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact 2014 Geotechnical Engineering Lecture. Thermal Response Testing: Challenges and New Applications. Hosted by the British Geotechnical Association at the Institution of Civil Engineers, 12th November 2014. [And repeated to the North West Geotechnical Group on the 4th March 2015.]

International Conference Presentations
Year(s) Of Engagement Activity 2014
 
Description Energy piles: developments in Europe. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Energy piles: developments in Europe. Invited lecture to the Australian Geomechanics Association, Melbourne, Australia, 28th October 2015.
Year(s) Of Engagement Activity 2015
 
Description Geothermal Energy and Geotechnical Engineering: The Heat Beneath Your Feet 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact Talk to the South East Section of the Institution of Civil Engineers.

ICE Regional Evening Meeting
Year(s) Of Engagement Activity 2014
 
Description Group thermal response testing for energy piles. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Group thermal response testing for energy piles. Paper selected for presentation at the European conference for Soil Mechanics and Geotechnical Engineering, Edinburgh, UK, 13 - 17 September 2015.
Year(s) Of Engagement Activity 2015
 
Description Performance of Piled Foundations Used as Heat Exchangers 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Presentation to session of 18th ICSMGE Paris 2013.
Year(s) Of Engagement Activity 2013
 
Description Progress on Thermal Piles 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited presentation to the Piling and Deep Foundations Conference.
Year(s) Of Engagement Activity 2012
 
Description Task Force Position Paper 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Position paper, Task Force 3: Energy geo-structures and storage of thermal energy in the ground. Invited paper to be presented to the International Symposium on Energy Geotechnics, 2nd June 2015
Year(s) Of Engagement Activity 2015
 
Description The behaviour of foundation piles used as heat exchangers 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited presentation to the Ground Source Heat Pump Association Technical Seminar held at University of Cambridge on 16th November 2011.
Year(s) Of Engagement Activity 2011
URL http://www.gshp.org.uk/Cambridge_Seminar_2011.html
 
Description The importance of concrete thermal properties for thermal pile performance 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact presentation to ground source heat pump association technical seminar.
Year(s) Of Engagement Activity 2012
 
Description The thermal performance of piled foundations used as heat exchangers 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited presentation to the British Geotechnical Association Annual Conference.
Year(s) Of Engagement Activity 2012
 
Description Themo-active structure functioning. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Themo-active structure functioning. Invited contribution to the workshop "Thermo-active geostructures: experiences around Europe, to be held under the auspices of the Portuguese Platform for Shallow Geothermal Energy and the Portuguese Geotechnical Society in Lisobon on the 10th December 2015.
Year(s) Of Engagement Activity 2015
 
Description Thermal analysis: the limitations of existing design approaches for pile heat exchangers 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Contribution to research workshop organised by the National Science Foundation.

Workshop organised by NSF to bring together researchers in field
Year(s) Of Engagement Activity 2013
 
Description Thermal response testing for large diameter heat exchangers 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited presentation to the Ground Source Heat Pump Association.

One day technical seminar
Year(s) Of Engagement Activity 2013