The Heat Beneath Our Feet: Modelling Mine Energy Systems

Overview

Over half of UK energy demand is used to produce heat, most of this comes from burning gas and most is consumed by the domestic sector. The UK has been a net importer of gas for over a decade meaning that we now rely on gas supplies from other nations. This brings our long-term energy security into question. In addition to improving our energy security, we also need to decarbonise our energy supplies to meet carbon reduction targets and combat climate change. Over the past decade, around half of our electricity demand has been decarbonised using renewables and nuclear, yet around three quarters of our heat demand is still derived from fossil fuels and only one percent of our transport demand is met from low carbon sources.
A political decision during the 1980s to rapidly abandon UK collieries and switch off water pumps led to high levels of unemployment and social deprivation in mining areas. These communities were further affected by rising water levels underground which threatened surface and groundwater. Over the past couple of decades, extensive measures have been taken to maintain pumping in strategic locations to control rising water levels and intercept and treat any potentially problematic discharges from abandoned mines which has a cost of around £4bn per annum to the UK taxpayer. There is a huge opportunity to re-use the infrastructure that was so hard won by miners and tap into the water within flooded abandoned mines to provide a source of geothermal heat for the future. This could also deliver economic opportunities to former mining areas.

Over 15 billion tonnes of coal were extracted from the UK subsurface over the past century. Spread over the entire UK land surface, this would create a layer of coal 5cm deep. Allowing for some post abandonment subsidence, it is estimated that there are around 2 billion cubic metres of water within flooded mines that contain around 2.2 million GWh of heat. There is also good overlap between coalfield areas and areas of heat demand. This is not surprising as many of our towns and cities were developed as a direct result of their coal reserves. Around one quarter of UK homes are located in coalfield areas meaning there is good potential to access this energy source.
The water within the mines is tepid, at temperatures of 12-20 °C. Clearly this is not hot enough for taking a bath in or to heat a room but by using a heat pump, temperatures can be increased to a more comfortable 40-50 °C. Although the heat pump requires an electrical input, it is an energy efficient device because for every 1 kW of electrical input a heat output of 3-4 kW would be expected from the heat pump. This research has shown that we don’t need access to volcanoes to develop geothermal energy and that our abandoned mines can augment the UK’s geothermal resources which could in their entirety meet our heat demands for a century or more.

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Methodology

Although geothermal mine energy is successfully used at some places within the UK and outside (notably the city of Heerlen, The Netherlands), the associated research is still very much in development. Developing a new mine energy site requires drilling boreholes, abstracting water from and returning it to the subsurface. Anticipated uptake in the development of mine energy systems means that there is the potential for interference between adjacent systems which may impact upon energy recovery and system longevity. Therefore, numerical modelling of mine flow and associated heat exchange is an essential first stage for the successful deployment of these geothermal mine systems.

This project envisions significant interaction and collaboration with local private and public stakeholders (such as the Coal Authority) and local county councils. Such interaction will ensure that the significant local expertise skills will be embedded in the project. This interaction is also vital for access of regional flow and mine plan data to constrain models and testing of model results and calibration against real systems.

The heat exchange between mine water and the regional subsurface will be investigated using numerical modelling techniques. Energy recovery relies upon removal of heat from abstracted mine water and re-injection of cooled mine water without short-circuiting occurring between these wells. This research will investigate the hypothesis that temperature exchange between host rock and groundwater influences potential energy recovery from the mine water system. The mined subsurface can be thought of as an anthropogenically enhanced aquifer hence the development of bespoke models is required to better understand the thermal and hydrogeological behaviour of mine energy systems. Modelling of such systems will be done using a combination of existing software (such as the groundwater software package Modflow) and user-written software that will target fluid flow through abandoned mines and associated geothermal energy exchange.

The newly set-up UK NERC Geoenergy Observatory in Glasgow will be used as a test site and calibration of the numerical models against real data. Once calibrated, the models will be applied to potential future mine geothermal sites in the region such as the Louisa Leisure Centre Stanley where test drilling is underway and a planned large heat network in County Durham. Where suitable data from ongoing Coal Authority pumping operations will also be used for model development and testing.

Project Timeline

Year 1

Literature study, data and knowledge exchange with regional institutes, county councils and the Coal Authority, training in numerical modelling, and setting up and testing of first models; training in project specific and transferable skills.

Year 2

Further code development and application to targeted test sites; writing of first academic publication; industrial secondment to enhance skill set and future employability.

Year 3

Collect scientific results that will be written up in the form of several scientific publications; these will be combined with further chapters to integrate into a first draft of the PhD thesis.

Year 3.5

completion of thesis and final submission of scientific manuscripts.

Training
& Skills

The student will become part of a vibrant research culture in the department of Earth Sciences, in which ~70 PhD students work on a wide range of Earth Science research projects. In particular, Geoenergy, Environmental Science, and Computational Geoscience form major research clusters in the department of Earth Sciences, and have dedicated group meetings and seminars. The PhD student will become an active member of these research communities. The student may also be eligible to join the Centre for Doctoral Training in Energy which would facilitate access to a range of additional training activities such as lectures and site visits. We would also encourage the student to engage with our research institutes, specifically Durham Energy Institute (DEI). DEI also provides the opportunity to engage in networking with other energy researchers and energy professionals involved with industry, policy and governance and opportunities for the student to engage with outreach events, competitions and public lectures.

At the start of each academic year, the student and supervisor will decide on a list of required skills and training, both topic-specific, and more generic, transferable skills, and a plan will be formulated or revised to acquire those. Throughout the project, training will be provided in geodynamical modelling (programming, code development, model setup, and usage) as well as data management of high-performance computing systems. The project is an opportunity for the student to become proficient in computer programming and large dataset analysis. In addition, the student will receive training in general and transferable skills.

The student is expected to attend national and international conferences to disseminate research results and to spend time away from Durham to integrate all project partners at the partner institutes.

An industrial secondment will provide valuable experience to do research in a commercial environment. The skills acquired through academic training and research can be applied in a different environment, while this secondment will also provide a direct link into industry, as an essential network component.

The student will also become part of the IAPETUS DTP which offers a multidisciplinary package of training focused around meeting the specific needs and requirements of each of our students who benefit from the combined strengths and expertise that is available across our partner organisations.

References & further reading

Bailey et al. (2016). Heat recovery potential of mine water treatment systems in Great Britain,
International Journal of Coal Geology, Volume 164, 77-84, https://doi.org/10.1016/j.coal.2016.03.007.

Loredo et al., 2016. Modelling flow and heat transfer in flooded mines for geothermal energy use: A review, Int. J. Coal Geol., http://dx.doi.org/10.1016/j.coal.2016.04.013

Rodriguez & Diaz, 2009. Analysis of the utilization of mine galleries as geothermal heat exchangers by means a semi-empirical prediction method. Renew. Energy 34, 1716-1725. http://dx.doi.org/10.1016/j.renene.2008.12.036.

Further Information

For any information on the various aspect of the PhD project, the department of Earth Sciences or, more generally, matters related to doing a PhD in Durham, please feel free to contact Charlotte Adams (c.a.adams@durham.ac.uk) or Jeroen van Hunen (jeroen.van-hunen@durham.ac.uk).

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