Potential for deep geothermal energy in Scotland: study volume 2

Appendix 1 The Scottish National Minewater Potential Study and the Shawfair Minewater Project

The Scottish study was done by PB Power (Energy Services Division) for Midlothian Council ( PB Power, 2004). The report states that the study was based on existing geothermal surveys, although it doesn't reference these. Although the title of the report is the Shawfair Minewater Project, the subtitle (Scottish National Minewater Potential Study) and the report content make it clear that this study considered the potential for minewater heating at Shawfair in the context of the potential across the Midland Valley of Scotland.

Both the specific potential at Shawfair and the wider potential are summarised here.

The Shawfair minewater project

Shawfair is the name of a planned new town to be built largely on the site of the former Monktonhall colliery in Midlothian, south-east of Edinburgh and currently on hold due to economic conditions. It overlies substantial former coal mine workings. Early in the development plans for Shawfair there was considerable interest in the use of minewater and heat pump technology to supply a community heating scheme ( e.g. Banks et al., 2003; PB Power, 2004; Malolepszy et al., 2005; McLoughlin, 2006). The PB Power feasibility study on the resource potential, energy demand and technological options for the site concluded that there was potential for such a scheme, and a successful grant application to the EC was made in 2003 with £3.5 million being awarded. However, further funding applications by Midlothian Council from UK and Scottish government sources (McLoughlin, 2006) failed when the business case for the mine water heating scheme could not be successfully concluded, in part due to issues of the ownership of the resource, and the plans for using mine water at Shawfair have been abandoned (Minewater Project, 2009).

The Coal Authority has an ongoing commitment to pumping mine water from Monktonhall to control mine water levels (Coal Authority pers.comm.). Monktonhall Colliery originally had an 11 km ² footprint accessed by two 1,000 m deep shafts. The temperature at 1,000 m depth is some 25°C and the feasibility studies suggested that sustainable pumping of mine water at 16°C would be possible (Banks et al., 2003). A heat pump could raise this temperature to between 40 and 60°C. Waste mine water exiting the heat pumps at around 8°C could be re-injected to the mine periphery and/or at a level below the abstraction point in a nearby shaft, or could be discharged at surface to reed bed water treatment facilities, or used for other purposes such as cooling or grey water (Banks et al., 2003; PB Power, 2004). Other options could have been the separate abstraction of shallower, cooler water for use in cooling installations, leading potentially to seasonal manipulation of the mine water and thermal energy storage in the flooded mine workings (Banks et al., 2003). The studies raised concerns about mine water quality, specifically the high iron content, and its potential corrosive and encrustation effects on heat exchangers (Banks et al., 2003).

The Scottish National Minewater Potential Study

The Scottish National Minewater Potential Study ( PB Power, 2003) reviewed rebounding water level monitoring data, provided by the Coal Authority and their consultants, IMC Consulting Engineers Ltd, for closed coal mines in Scotland in order to estimate the potential thermal resource available from the old mine workings. The monitoring data were originally collected to ascertain the rates and influence of rebounding water levels in the mines, not for the purpose of estimating available thermal energy ( PB Power, 2004). Most of this study concentrated on the heat demand and economic cost of mine water heating schemes: only approximately 16 out of 153 pages deal with the technical potential of mine water for heating.

The report identifies more than 60 underground coal mine locations in Scotland and categorises them as follows ( PB Power, 2004):

1. Sites with a pumping system already installed to manage groundwater levels and discharge.
2. Sites with a known discharge by gravity and natural flows.
3. All other sites where drilling and deep pump installations would be needed to create a mine water circulation system.

The report states that the deepest mine workings have virgin rock temperatures of about 37 to 40 °C, and that the water in the deepest parts of the mines may well be close to the maximum rock temperatures, but that near to shafts, natural convection systems will destroy the thermal convection ( PB Power, 2004). It is not clear if this agrees with other reports that the water temperature at approximately 1 km depth ( e.g. in Monktonhall) is approximately 25°C (Banks et al., 2003), or with other data collected recently by BGS. However, the remainder of the report assumes that the mixed water temperature delivered to heat pumps is not more than 13°C, which is typically equivalent to water abstracted through boreholes or shafts ( i.e. categories 1 and 3, above) from 100 m depth ( PB Power, 2004). This is acknowledged as probably a conservative estimate. No thermal data are available for Category 2 (gravity) discharges, but it is assumed that temperatures will be lower than pumped discharges, possibly around 10°C ( PB Power, 2004). This is supported by a single temperature measurement of gravity mine water drainage taken during a separate BGS project into groundwater chemistry (Baseline Scotland), which was 9.8°C.

Estimates of the potential flow rate of mine water from the mines were made based on records of the numbers of men known to be employed underground during mine operation. Additional information was obtained on mine size and interconnectivity, although the report doesn't make clear how detailed this information was compared to what may potentially be available, nor how it was used in the estimate of mine water flow rate.

Estimates of heat resource were then made from estimates of the flow rate of mine water. The conversion factor for the Monktonhall site was 16.75 litres/second (l/s) mine water flow per 1000 kWth (kilowatt thermal) ( PB Power, 2004). It isn't clear if the same factor was used for all the examined mines in the study.

Issues related to mine water chemistry, solids content and corrosion / precipitation potential were not considered in detail, but most potential problems of this nature were assumed to be treatable given available technology ( PB Power, 2004).

The report provides maps of identified collieries, pumping sites and/or gravity discharges in Scottish coalfields, and information on the pumping and/or discharge flow rates from each. Most of the larger sites and flows are in the east side of central Scotland, but smaller schemes are present throughout former coal mining areas ( PB Power, 2004). The report recommends that category 1 and 2 sites should be developed first, because the mine water resources have already been proved and the risks are therefore lower.

The study then combines these assessments / estimates of mine water flow with a detailed assessment of the heat demand and economic costs of developing heating schemes at each of the identified abandoned mine sites. Based on this, it provides a list of the ten preferred sites for developing mine water heating schemes in Scotland ( Table 2), with the thermal resource interpreted from the estimated flow rates. The total estimated heat capacity of the ten sites is 83 MW. It is noted that the economic ranking of sites varies depending on the technology option selected and the discount rate applied; that sites where there is little or no current heat demand are not ranked highly ( e.g. Monktonhall); and that more detailed evaluation may show that the two smallest sites in this list would prove to be less economically viable than some of the larger ones ( PB Power, 2004). Most of these too are in the east of the Midland Valley; only two (including one of the smallest) are in the general Glasgow area (in Motherwell).

Table A2 The ten preferred abandoned Scottish coal mines for mine water heating schemes (from PB Power, 2004)