View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NERC Open Research Archive MINING FOR HEAT Image: Yerko Espinoza/shutterstock.com Many UK towns were eep coal mining in the UK has coal deposits, coal is now regarded as a left a legacy of flooded former “dirty” fuel because of the associated CO2 built on coal. mines. Water within these and other pollutant emissions, and is mines can provide a source of being phased out of our energy mix. UK Charlotte Adams, D heat energy. Durham government demonstrated its University is researching the potential of commitment to reducing coal use by Alison Monaghan this resource and the British Geological joining 27 other national governments that Survey (BGS), commissioned by the have signed up to the “powering past coal and Jon Gluyas Natural Environment Research Council alliance” and there are now days when (NERC), are constructing and operating a coal is not being burned to produce highlight the vast research site in Glasgow to further electricity in the UK. potential to repurpose understanding of mine energy systems. The UK has made good progress in decarbonising its electricity supplies over our abandoned When coal was king the past decade, with around half of UK Over the past century, vast quantities of electricity demand supplied from low- mining infrastructure coal were mined from the UK subsurface carbon sources (DBEIS, 2018). However, as it fuelled our industrial and economic electricity provision is only part of the as a low-carbon heat growth. The 15 billion tonnes mined are story and half of the UK’s total energy equivalent to a 5-cm-deep layer of coal demand is used to produce heat, most of source spread over the entire UK land surface. which is consumed by the domestic sector. Despite the wealth generated by the UK’s Heat is predominantly produced from 10 | MAY 2019 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | MAY 2019 | 11 Fig 1: Schematic Diagram of a mine water heat pump system (Drawn by Charlotte Adams) Energy Centre or Heat User Heat Pump Warm Water Cooled Water Pumped Returned to Mine from Mine Water Level ç√ Mine Shaft Coal Seams ç√ Flooded Abandoned Mine burning natural gas. In the meantime, the but heat pumps can boost temperatures to Mine-water heat systems are generally UK has been a net importer of natural gas provide hot water and space heating. Heat operated open-loop, which offers better for over a decade and is reliant on other pumps require an energy input, but thermal efficiency than standard closed- nations to meet any shortfalls in demand. because each kW of electrical energy input loop ground-source heat systems. Finally, could be expected to deliver a heat output mine-water systems offer economies of Developing a legacy of 3-4 kW, heat is provided in an energy- scale, meaning that clusters of hundreds of Our abandoned mining infrastructure efficient way. properties could be served from a single now lies largely derelict and many have Mine water is accessed by drilling mine and a few boreholes. To deliver this forgotten its existence. Yet, people in boreholes into flooded workings through vision requires changes to planning and former mining towns and villages remain which water is abstracted, heat is removed building control policies that support the strongly connected to their mining and the temperature boosted with a heat future development of low-temperature heritage, even though its decline brought pump, before the water is returned to the energy systems. hardship and, in some areas, subsurface (Fig. 1). Using mine water as Durham Energy Institute at Durham environmental pollution. an energy source compared with University is undertaking national, Our mining legacy and the associated individual closed-loop, ground-source regional and local assessments of mine infrastructure could be repurposed and heat pumps offers some advantages for energy potential for a range of has potential as a future energy source. domestic properties. Decreased garden applications, under the auspices of The mine shafts and galleries that are now size of newer housing stock leads to BritGeothermal, which is a national flooded contain copious volumes of water reduced space for horizontal ground research partnership for deep geothermal at 12-20℃. Clearly you would not want to arrays, meaning that boreholes would be energy. This work includes assessing take a bath in water of this temperature, required, which are more capital intensive. domestic residences, industrial 10 | MAY 2019 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | MAY 2019 | 11 Fig 2: Stoop-and- room (Scottish terminology for pillar-and-room) mine workings subsequently exposed by opencast coal workings at Blindwells, Scotland. Photo number P001520 BGS©UKRI. The image illustrates the connected void space (‘rooms’) that could be targeted for mine water geothermal projects developments and municipal ochre precipitation is key for mine- that involves driving tunnels to the buildings for mine energy systems. water treatment, in a heat-exchange farthest extent of the mine, then For these assessments, we need to system this can cause severe clogging removing coal from the seam laterally understand the nature of the and fouling of system components. whilst retreating from the workings. subsurface infrastructure and Ochre precipitation is normally Many areas formerly mined by the appreciate how water quality can avoided by keeping systems under room-and-pillar method were latterly evolve, both during mine positive pressure, limiting dissolution reworked using longwall extraction. abandonment and following periods of oxygen. As longwall mining proceeds the of abstraction. Research shows that To assess the potential of this overburden above the seam subsides following long periods of resource, it is necessary to obtain from producing “goaf” (collapsed waste). abandonment, water within mines the UK Coal Authority the Consequently, an area mined by can become highly stratified, with abandonment plans for seams within room-and-pillar methods can be better-quality water generally lying the collieries worked, and then use assumed to have around 50% of the above poorer-quality water (Nuttall & these to calculate the worked areas original void space remaining and, for Younger, J. Contam. Hydrol. 2004). and seam thickness. The large-scale longwall mining, around 20% of the Pumping and abstracting water from extraction of coal from the subsurface original void space remains (Younger mines induces turbulence and mixing leaves voids that remain long after & Adams, Tech. Report 1999). These within the mine. This means that the mine abandonment. Yet, these voids voids, in effect, have created an eventual quality might be poorer than will not remain exactly as they were anthropogenically-enhanced aquifer expected and it is important to take at abandonment. Shafts were often in which heat can be extracted from samples of the entire water column if filled with rubble from the or reinjected into the large water possible. Water quality may change demolished topside colliery volumes existing within the mine with pumping rate and is linked to infrastructure before being capped, workings. the areas of workings being drawn rendering many of no value for future The potential for the flooded from. Quality may improve once a few water pumping. The floor of galleries abandoned collieries of the UK to shaft volumes have been pumped. may heave and roof material may provide a source of heating, cooling Many mine waters contain iron, collapse, leading to tunnels with and energy storage is huge. The UK which is a remnant of the interaction partial blockages along their length. Coal Authority estimates that between the oxygenated mine-water The amount of remnant void space abandoned flooded mines contain and pyrite within the coal depends upon the method of deep around 2.2 million GWh of heat, (particularly high-sulphur coals) and mining employed. Early ‘room-and- with an even greater potential for is the nemesis of mine-water pillar’ mining involves working a heat storage. Furthermore, because management. As long as air is grid, leaving pillars of coal intact for coal spawned the development of excluded from a mine-water heat roof support and mining the areas many of our towns and cities, mine pump system, iron remains in between (Fig. 2). Room-and-pillar energy resource aligns well with solution, but introducing air even mining was later replaced by centres of heat demand. The Coal when there are low concentrations of longwall mining (first developed in Authority estimates that around one iron present (1mg/l) leads to the Shropshire in the 17th Century), a quarter of UK housing stock overlies deposition of ochre. Though iron more efficient means of removing coal areas of abandoned mine workings. WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | MAY 2019 | 13 GEOSCIENTIST FEATURE There are a few examples of mine to flow adequate quantities of water for relatively cheap gas supplies provide energy projects that serve smaller and energy extraction. There is only one some answers. Complexities associated individual developments in the UK. operational deep geothermal heat scheme with retrofit of heat exchangers and heat Bridgend Council in Wales is working to in the UK, at Southampton. It was pumps, economic risks linked to deliver a larger district heating system developed in response to a national audit difficulties in securing long-term that will initially supply heat to around of the UK’s geothermal resources in the contracts for heat supply and system 150 homes. At Heerlen in the 1980s, prompted by the oil crisis. maintenance, as well as concerns over Netherlands, abandoned mines are used Although abandoned mines are water quality and subsidence may effectively to deliver space heating and shallower and the temperature of water present barriers to widespread cooling to around 200,000 m2 of mixed within them is cooler, copious quantities development.