SaFEGround - Sustainable, Flexible and Efficient Ground-source heating and cooling systems

Lead Research Organisation: Imperial College London
Department Name: Civil & Environmental Engineering

Abstract

Through the 2008 Climate Change act, the UK committed to reduce by 80% its carbon emissions. While great progress has been made so far, data suggests that reductions in emissions have been achieved through switching electricity production to greener, more environmentally friendly sources, such as offshore wind. Clearly, it is inevitable that, to achieve further reductions in carbon emissions, we need to look for improvements elsewhere, such as heating and cooling of buildings, which accounts for 25% of all UK final energy consumption and 15% of carbon emissions.

Project SaFEGround aims to provide a template for reducing emissions associated to heating and cooling through the deployment of heat pumps. These are efficient devices capable of extracting heat from a storage medium, e.g. air for air-source heat pumps or the ground for ground-source heat pumps, and this is done with high efficiency, since for each unit of electricity consumed by the system, it is usual to get 3-4 units of heat. Clearly, these are more environmentally-friendly than boilers as they require only electricity, which, as mentioned above, is increasingly being generated from renewable and low-carbon sources.

Therefore, SaFEGround will investigate how ground-source heat pumps can be coupled with civil engineering structures to deliver low-carbon heating and cooling in a sustainable, safe and efficient manner. To achieve this, SaFEGround will combine research on material science, heat pump technology, energy geotechnics, building energy systems modelling, whole-system modelling and finance, to demonstrate that ground source energy systems can play an important role in the UK's future low-carbon energy mix in a cost-effective manner.
 
Description 1. Role of ground-source energy systems in the net-zero transition using whole energy system models

Ground-source heat pumps (GSHPs) combined with thermal batteries and optional backup Joule heaters are high-performance yet high-cost low-carbon technologies and attractive candidates for domestic heating/cooling. The role of GSHPs in the transition to a net-zero energy future has been explored using both the household-level ground-source energy system design and operational optimisation framework developed within SaFEGround and a national whole-energy system optimisation model. After optimising the design (specific cost minimisation) and year-round operation (operating cost minimisation) of more than 1,000 GSHP systems for 5 different households representative of the UK landscape, optimised ground-source energy system candidates have been added to the technology portfolio of the whole-energy optimisation model to quantify both the impact of heat electrification on the wider energy system and the value of GSHPs at the national scale. At the household level, it was found that large heat pumps equipped with large thermal stores are preferred as they allow shifting operation to off-peak low electricity cost hours (resulting in lower levelised cost of heat), while smaller-scale lower-cost systems are favoured from the whole-system perspective. At the national scale, the uptake of GSHPs and their ability to compete with low-cost gas boilers strongly depends on subsidies and natural gas prices. In the UK, with the current Boiler Upgrade Scheme providing a subsidy of 7,500 £, gas price above 50 £/MWh would be required for ground-source energy systems to become the dominant domestic heating technology, while with subsidies above 10,000 £, GSHPS become competitive for gas prices of 20 to 30 £/MWh, which corresponds to the historical lower bound.

2. Economic viability of thermo-active foundations

Heating in the UK today is primarily based on natural gas boilers and is responsible for about 37 % of the total UK carbon dioxide emissions. Ground-source heat pumps (GSHPs) are an attractive technology to decarbonise domestic heating, as they are highly efficient and emission-free at the point-of-use. According to the International Energy Agency (IEA), GSHPs are also the most cost-efficient solution, with a levelised cost of heat (LCOH) significantly smaller than low-carbon alternatives such as air-source heat pumps, hydrogen boilers, or solar-thermal heating systems (based on 2019 electricity/gas prices). For the UK, the IEA estimates that the LCOH of GSHPS lies between 51 and 83 £/MWh, while that of their air-source counterparts lies between 77 and 115 £/MWh. Their deployment is however hindered by large upfront capital costs and often by restricted access to the ground, especially in cities. In this project, we have explored the techno-economic potential of thermo-active foundations (or thermo-active piles, TAPs), proposed to ease/allow installation of GSHPs in urban environments while reducing the cost of installation per household. However, a key question was: would a TAP-based ground-source energy system (GSES) retain its economic attractiveness despite necessary operational constraints to avoid concrete piles to freeze? We developed a GSES design and operational optimisation framework using: (i) bespoke building models to provide year-round heating and cooling demand profiles; (ii) high-fidelity three-dimensional thermal models of the underground heat exchangers to predict the ground thermal response; and (iii) comprehensive first-law based models to provide a comprehensive link between cost and off-design/part-load performance of GSHP units. By imposing a constraint on the operation schedule of the GSHP unit to keep the concrete pile edge temperature strictly above 0 °C, we found that GSES using thermo-active foundations are able to provide the year-round heat demand of a 6-storey office building in London with a LCOH of 60 £/MWh, which is using a single-acting heating-only heat pump. With using a reversible heating/cooling GSHP, the LCOH for the same building drops down to less than 40 £/MWh.

3. New design methodologies for thermo-active piles

A key barrier to the adoption of ground source energy systems is the lack of readily available methodologies for estimating the thermal performance of thermo-active piles. SaFEGround has originated a simple method that can be implemented using commonly available software and determines (a) the maximum heat flux that can be reliably extracted using thermo-active piles and (b) the temperature changes in the pile based on heat fluxes applied by the attached heat pump. This method has been validated and subsequently extended to account for thermal interference in thermo-active pile groups as in large residential and commercial buildings, pile foundations are typically deployed in groups, rather than in isolation, with values of spacing which are much smaller than those adopted for other heat exchangers, such as boreholes. This leads to a significant degradation in performance that needs to be accounted for in design, as it determines the overall cost of the system.
Exploitation Route The outcomes of this funding can be taken forward when developing software that can produce streamlined solutions for thermo-active pile design in different group arrangements. Moreover, project outcomes will serve as scientific evidence of the impact of ground source energy systems as regional and national level, potentially influencing policy.
Sectors Construction

Energy

Government

Democracy and Justice
 
Description Concrete Mix Design for Improved Thermal Conductivity in Thermal Energy Piles
Amount £25,000 (GBP)
Organisation Institution of Civil Engineers 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2023 
End 03/2024
 
Description Transition to Zero Pollution UROP Scholarship
Amount £2,800 (GBP)
Organisation Imperial College London 
Sector Academic/University
Country United Kingdom
Start 07/2022 
End 09/2022
 
Title Ground-source energy system (GSES) design and operational optimisation framework 
Description The GSES design and operational optimisation framework is an open-source library of comprehensive first-law models written in Python. Based on polynomial AHRI (Air-Conditioning, Heating, and Refrigeration Institute) compressor performance maps provided by manufacturers for various operating conditions (frequency, superheat, condensing and evaporating temperatures) and one-dimensional heat exchanger steady-state models, ground-source heat pumps (GSHPs) are designed to minimise the specific investment cost while maximising the coefficient of performance in standard conditions (e.g., B0W35 for brine at 0 °C and a heating water temperature of 35 °C). The operation of the GSHP coupled to underground heat exchangers (e.g., a borehole or thermo-active piles, the operation of which is estimated using G-functions) and cold/hot thermal batteries (e.g., water tank or phase change material) is then optimised to supply a given heating/cooling demand throughout the year at the lowest cost, i.e., by minimising the total cost of electricity consumed. This highly non-linear mixed integer optimisation problem is solved using the Pyomo optimisation tools and is able to take advantage of fluctuating electricity prices. This design and operational optimisation framework is currently stored on a private Github repository (please contact Paul Sapin, p.sapin@imperial.ac.uk for further details), which will be soon made publically available. 
Type Of Material Computer model/algorithm 
Year Produced 2023 
Provided To Others? No  
Impact This application-specific techno-economic optimisation framework based on a large database of manufacturer performance maps provides a clear and comprehensive link between the cost of a ground-source energy system and its off-design and part-load performance. It provides not only guidance to manufacturers, e.g., by identifying the optimal configuration (size and design of the heat pump unit and of the thermal battery) to minimise the levelised cost of heat over the lifetime of the system with adjustable interest rates, cost of electricity, incentives or subsidies. The framework and its capacities have been presented at a session keynote presentation at the 14th IEA Heat Pump Conference held on 15-18 May 2023 in Chicago, Illinois. The model has recently been combined with a national whole-energy system optimisation model to perform a holistic assessment of the role and value of ground-source energy systems in the UK net-zero energy system transition - results will be presented at the upcoming 37th International conference on efficiency, cost, optimization, simulation and environmental impact of energy systems (ECOS) to be held from 30 June to 4 July 2024 in Rhodes, Greeece. A journal publication is currently in preparation. 
 
Description Pavement solar collectors 
Organisation University of Antwerp
Country Belgium 
Sector Academic/University 
PI Contribution Provided expertise in the modelling of pavement solar collectors which share many commonalities with the ground source energy systems studied in SaFEGround.
Collaborator Contribution Provided computational models of pavement solar collectors and experimental data for their validation.
Impact Thermal and structural response of a pavement solar collector prototype (SEG 2023 conference paper and presentation), Thermal performance optimisation of Pavement Solar Collectors using response surface methodology (Renewable Energy, Volume 210, 2023).
Start Year 2022
 
Description Presentation to the EPSRC Network+ for Decarbonisation of Heating and Cooling 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact Discussion between all academic and industry partners involved in the EPSRC Network+ for Decarbonisation of Heating and Cooling. Resulted in multiple contacts being established with people working in this field and provided opportunity to share progress on SaFEGround.
Year(s) Of Engagement Activity 2023
URL https://www.durham.ac.uk/research/institutes-and-centres/durham-energy-institute/about-us/events/net...
 
Description Towards Net Zero (TNZ) seminar series - The role of ground-source heat pumps in the transition to net-zero 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact This 1-hour seminar entitled "The role of ground-source heat pumps in the transition to net-zero" was given by Paul Sapin at the Towards Net Zero (TNZ) seminar series on 16 February 2024. This in-person and remote event organised by the Sargent Centre for Process Systems Engineering of Imperial College London sparked many questions from the audience, mainly from established Professors and led to fruitful discussions on the potential of ground-source energy systems (GSESs) in a low-carbon energy future. In particular, we quantified the cost reductions, economies of scale, incentives/subsidies that would be required for a wide uptake of GSESs as a function of the highly uncertain future gas/electricity prices. Paul also engaged with Professor Jackson of the Earth Science & Engineering Department at Imperial College London, with whom we are currently discussing future projects/funding applications.
Year(s) Of Engagement Activity 2024
 
Description UCD Workshop - Geothermal Energy, Thermal Energy Storage & District Heating 
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 This was a one-day workshop organised by University College Dublin to discuss current research on geothermal energy, thermal energy storage and district heating. The audience was mostly engineering practitioners who engaged with SaFEGround's research on thermo-active piles.
Year(s) Of Engagement Activity 2022
URL https://alertgeomaterials.eu/2022/12/one-day-ucd-webinar-geothermal-energy-thermal-energy-storage-an...