PB has developed a sustainable energy solution for Midlothian Council¹ in order to meet the needs of the new town of Shawfair, where some 4,000 homes are planned for construction. The town will grow up on the site of the old Monktonhall colliery (a coal mine and its facilities), which ceased production in the early 1990s. The mine is continuously dewatered to prevent groundwater rebound and the water is discharged to a local watercourse.

Midlothian Council intended that Shawfair be a model of sustainable development, and supplying the town's energy from a sustainable source was a key element in delivering this objective. The Council proposed to use the low-grade geothermal energy resource available in the minewater as an energy source for a district heating network supplying the new town.

The proliferation of abandoned mine workings in the central belt of Scotland and the associated minewater disposal issues are of environmental concern due to the pollution risks. Minewater is generally warmer than normal ground temperatures, however, so if it were possible to use it as a heat source for community energy projects, an environmental liability could be turned into a sustainable energy source.

Assessing the Opportunities

Midlothian Council appointed PB to develop the energy strategy and outline design for the new town. We considered the technical, environmental, economic and legal issues pertaining to a minewater energy solution for Shawfair, and compared it to more conventional energy supply approaches and other more innovative solutions, including combined heat and power (CHP). The energy provision options considered were:

1. Individual (decentralised) gas fired boilers in the homes and packaged boilers in the offices, schools and retail areas.
2. Electrically driven minewater heat pumps with centralised gas boilers feeding a community heating network
3. Various combinations of interlinking CHP with minewater heat pumps to feed a community heating network
4. CHP and centralised boilers feeding a community heating network with no minewater usage.

Technical Issues. The minewater options depended on the technical viability of the geothermal energy source, which were evaluated based upon on satisfying the following three criteria:

• The water source had to be hot enough to provide useful energy.
• There had to be sufficient water available for the life of the project.
• It had to be possible to abstract the water at a rate sufficient to provide the net power required by the project.

PB showed that these three criteria could be satisfied. The minewater would not be used directly in the district heating network but as a heat source for large heat pumps that would be capable of producing hot water at 60° C (140° F). Boiler plant and/or a CHP engine could then be used to raise the temperature to 80° C (176° F) for distribution in the district heating. CHP units would also generate electricity for supply to the town via a private wire electrical network. Technically, the cooled minewater could be discharged at the surface to the existing water treatment facility (reed beds) or discharged back down into the coal workings at return points away from the abstraction shaft.

Cooling for the office buildings in the centre of the town would be provided by conventional means. Free cooling from the discharge of the heat pump is still under consideration.

The community energy solutions considered included PB’s best practise designs of using large temperature differences across the flow and return pipework to reduce the network costs. Selecting a large temperature difference means that PB could suggest innovative design solutions to combine the characteristics of the heat pumps and gas engine CHP systems as shown in Figure 1.

Figure 1. Sustainable Energy Solution Using Minewater

Defining and Selecting the “Best” Solution

We also evaluated the anticipated environmental and economic performances of the energy provision options.

Environmental Issues. Interestingly the greatest emission saving option depended on the carbon value attributed to electricity in the UK. There are currently two schools of thought in the UK:

• For each kWh of electricity consumed in the UK, 430 g of CO₂ are produced. This view is based upon an overall average of emissions factors from all the grid generators in the UK, known as grid mix.
• Distributed generation, like that discussed in this instance, would be operating during times of peak demand and, therefore, would prevent the marginal plant from being called on line to meet that demand. In the UK, marginal plant is considered to be predominantly coal-fired generation and, therefore, has a much higher carbon emissions factor of 664 g CO₂/kWh.

Option 2 consumes electricity whereas the CHP options generate electricity, so Option 2 was ruled out from further consideration. The greatest emissions savings option for Shawfair Town (4,000t CO₂ per annum by year 15) was Option 3 if the emissions factor attributed to electricity is 430g CO₂/kWh. Based on an emissions factor attributed to electricity of 664 g CO₂/kWh, however, Option 4 yielded the greatest emissions savings (8,000t CO₂ per annum by year 15).

Economic Issues. An important aspect of the appraisal was to investigate the availability of and quantify grant support for the different options. The sources considered were INTERREG IIIB from the European Union (EU) structural fund, and various UK-based funding programmes. A prerequisite for INTERREG funding was project innovation, something Option 3 met but Option 4 did not.

The economic analysis was conducted using a whole life costing methodology as is prescribed for public spending in the UK by the Treasury guidance, Appraisal and Evaluation in Central Government. The enhanced funding opportunities open to the minewater options meant that, economically, Option 4 did not compare favourably.

The inclusion of a private wire network for direct electricity generation sales to the community and capital grant funding meant that Option 3 would deliver the least net, present-cost solution over 25 years. In addition, if the minewater were not used, the perceived social benefit would be lost. Therefore, Option 3, the minewater heat pump with CHP solution, was selected as the preferred option on a balance of economic, environmental and social considerations.

Leading the Way Forward

We developed business plans for the project and submitted all grant funding applications on Midlothian Council’s behalf under budget and within all project deadlines, and eagerly anticipate the next phase of the project. Midlothian is awaiting notification of grant funding support and hopes to be in a position to continue forward soon. We are particularly satisfied that this approach will be taken into account for a client’s new build project, which historically have tended to adopt solutions with the lowest capital cost.

The development work for the Shawfair project has been complemented by a national minewater energy potential study for Scotland, conducted by PB and funded by the Scottish Executive. This study was based on existing geothermal surveys and identified the availability of minewater across Scotland in correlation to the density of heat demand. The purpose of the study was to highlight areas where geothermal district heating should be considered in more detail as part of local sustainable energy plans. The study showed that minewater projects could meet up to three percent of Scotland’s heat load.