This feasibility study explored the potential for a deep geothermal heat project at Hill of Banchory, Aberdeenshire. The geology of the Hill of Fare, to the north of Banchory, gives cause to believe it has good geothermal potential, while the Hill of Banchory heat network, situated on the northern side of the town, offers a ready-made heat customer.
The partners in the consortium consisted of academics and developers with relevant expertise in deep geothermal energy, heat networks, and financial analysis, together with representatives of local Government. They conducted geological fieldwork around the Hill of Fare, engaged with local residents to establish their attitudes to geothermal energy, and built business models to predict the conditions under which the heat network at Hill of Banchory would be commercial if it utilised heat from the proposed geothermal well. They also estimated the potential carbon emission reductions that could be achieved by using deep geothermal energy, both at Hill of Banchory and more widely.
A vision of how the geothermal heat system would work
The core element of a geothermal heat generation system at Banchory would comprise at least one pair of deep boreholes drilled into the Hill of Fare Granite. The medium for heat transport from the granite would be water: hot water from depth would be pumped to the surface via one of the boreholes, the heat transferred into Banchory via a heat exchanger and water pipes, and the now cooler water would then returned to depth by injection into the second borehole. The following schematic diagram indicates the system layout.
Figure ES.1: An indicative diagram of the proposed geothermal doublet at Hill of Fare
The equipment required at the wellhead would be minimal, and overall the environmental footprint of the system would be very small. A commercially viable system would be expected to have a capacity of a few megawatts, enough to provide heat for a few thousand houses, compatible with the scale of heat demand from the expanding heat networks at Hill of Banchory.
The feasibility of the scheme would depend on the geothermal resource that exists below the Hill of Fare. Sufficiently high temperatures would need to be found, alongside adequate permeability. In other words, the well must be able to sustain the production of a reasonable volume of hot water over the commercial lifetime of the project.
The Geology of Banchory and the Hill of Fare
The geological element of the feasibility study sought to identify how much heat might be available at the Hill of Fare site, how easily it could be extracted, and whether any aspects of the geology would create problems for a geothermal system.
The country rock throughout the Banchory district belongs to the 'Dalriadian Supergroup', a thick sequence of metamorphosed mudstones and sandstones (very little of which is exposed at the surface). Within this are found intrusions of cooled magma, which have formed crystalline granite bodies known as the 'Caledonian Supersuite'. These granites have long been known to host high concentrations of radioactive elements, which give out large amounts of heat as they decay over geological time spans. This heat production gives them, in theory, good geothermal potential. In this region, a sub-group of the wider 'Caledonian Supersuite' occurs, known as the 'Cairngorm Suite'.
Figure ES.2: Location of granite plutons assigned to the Cairngorm Suite (for full citation see here).
Contains British Geological Survey materials © NERC 2016
One of the constituent members of the Cairngorm Suite is the Hill of Fare 'Pluton', essentially a large mass of granite, which sits about 5 km to the north of Banchory. It is c. 8 km across on its longest (east-West) axis and has an area of around 37 km 2. While it can be expected to extend up to 15 km below ground, all studies of it to date have been confined to the surface. Observations suggest that the sides of the pluton descend vertically to depth, or may even slope inwards. There is little indication of faulting near the Hill of Fare Pluton, with the exception of one small area to its southeast. No zones of strongly fractured rock were identified, at least at the level of the present outcrop.
Other granites in the 'Cairngorm Suite' were the subject of geothermal research in the 1970s and 80s and were found to have high rates of heat production. Shallow boreholes were drilled to 300 metres in some of the plutons to measure thermal gradients, though these were surprisingly disappointing. However, this can be explained by a failure to correct for the long-term effects of the last ice age (which are still significant to the depths drilled), and the extent to which radiogenic elements are found at depth in the Cairngorm Suite remains poorly understood.
At Hill of Fare, the best exposures of granite are found in disused quarries on the south side of the pluton and that is where our tests were conducted. All of this rock can be classified as types of granite - and it is inferred that a deep borehole drilled anywhere into the Pluton would find only granite.
New results of fieldwork at the Hill of Fare - estimating the thermal gradient
Members of the project consortium made measurements on the exposed rocks (mainly in the quarries mentioned above) to establish the rate of heat production from the Hill of Fare granite. This is measured in micro-Watts per cubic metre (μW/m 3). The University of Glasgow used a hand held gamma-ray spectrometer to measure the concentrations of Potassium, Uranium and Thorium in the granite, from which heat production rates can be reliably calculated. Data were collected at 29 sites across the Hill of Fare, with great care being taken to ensure that measurements were as robust as possible.
In general terms, granites with heat production rates of 4 μW/m 3 or more are generally considered promising geothermal prospects. The Hill of Fare Granite was found to have a heat production rate of rate of 4.04 μW/m 3. This is lower than some other plutons in the Grampians (e.g. Cairngorm 7.3 μW/m 3, Ballater 6.8 μW/m 3, Mount Battock 4.8 μW/m 3, Bennachie 7 μW/m 3) but higher than others (Crathes Pluton 2.1 μW/m 3, Aberdeen Granite 2.2 μW/m 3).
Heat flow at depth can only be established reliably by drilling a borehole and measuring it directly; though this was not possible at the Hill of Fare, the heat flow measurements done at the other East Grampian granites in the 1970s can be used as a proxy. When the original data were corrected for 'paleo-climate' we find heat flows in those four plutons are in the range 87 to 95 mW/m 2, consistent with the conditions required for direct geothermal heat generation.
Thermal conductivity is a measure of a material's capacity to conduct heat; granites are usually good heat conductors on account of their high content of quartz - one of the most conductive minerals. Using a 'portable electronic divided bar' at the University of St Andrews, Town Rock Energy Ltd measured 19 plug samples from the Hill of Fare which were found to have conductivity values ranging from 2.176 to 3.682 W/mK, which is consistent with previous observations for other high heat producing granites.
Using these figures for heat production, heat flow, and thermal conductivity, well established analytical methods were used to predict how the temperature is likely to vary with depth in the Pluton. For each quantity figures were selected to represent three different scenarios - 'most favourable', 'intermediate' and 'pessimistic'. These were used to make three estimates of the thermal gradient as shown in the figure below.
Figure ES.3: Geothermal Gradient Prediction Scenarios for Hill of Fare Granite
These three scenarios produced thermal gradients of 29.0°C/km, 25.9°C/km and 21.1°C /km respectively. If we use these to calculate the depth of borehole required to reach 75 and 90°C, we obtain the following table.
Table ES.1: Depth to Typical Target Temperatures
Target Temperature (°C)
Depth to target temperature (km)
This suggests that a borehole 2,000-2,500 metres deep would encounter temperatures between 65°C and 82°C. This depth of borehole is entirely practical, and the temperatures are consistent with supplying heat to a heat network.
In granite the flow of water is concentrated in fractures and fracture networks, as the permeability of the rock itself is very low. It is hard to predict the extent of fractures at depth in granites.
The range of flow rates observed in similar deep granite settings elsewhere (e.g. Soultz-sous-Forêt, France, and Rosemanowes and Weardale in the UK), indicate that the likely flow rates for a successful borehole-double in fractured granite are in the tens of litres per second. Continuing with the three scenario methodology used above, three flow rate scenarios have been assumed ranging from a most favourable case of 50 litres/second (most favourable) through 15 litres/second (intermediate) to 5 litres/second (least favourable). These figures were used for the analysis of the commercial potential of the Hill of Banchory geothermal scheme.
The range of flow rates observed in similar deep granite settings elsewhere (e.g. Soultz-sous-Forêt, France, and Rosemanowes and Weardale in the UK), indicate that the likely flow rates for a borehole-double in fractured granite are in the tens of litres per second. Continuing with the three scenario methodology used above, the flow rates for a Hill of Fare borehole were estimated to range from a most favourable case of 50 litres/second (most favourable) through 15 litres/second (intermediate) to 5 litres/second (least favourable). These figures were used for the analysis of the commercial potential of the Hill of Banchory geothermal scheme.
The geothermal system will require a minimum of two boreholes of very similar design. Essentially deep water wells, the boreholes must be able to stand open without allowing fine particles to cause clogging, yet allowing the groundwater to enter the borehole at the target depth. This is achieved using the appropriate lining, or 'casing'. (When drilling through a very strong rock like granite, it may be possible to dispense with casing though this will depend on local conditions.)
This report sets out a typical geothermal borehole design. The diameter at the bottom of the borehole is proposed as 9 5/8", with a series of casings widening back to 20" casing at the surface.
Output and input temperatures
An issue which the study identified as being of crucial importance was the need for the output temperature of the geothermal well to be compatible with the input temperature of the network. The existing Hill of Banchory network designed operating temperatures are 85°C flow/60°C return. The network's heat customers in Banchory have heating systems that are compatible with this temperature range. Delivering heat at lower temperatures would require the operator to make some adaptations to the existing system. Supplying lower temperature heat to future new build properties would have the advantage that suitable changes can be made cheaply at the design stage.
There may be an opportunity here to create heat customers that can use the lower temperatures from the geothermal well. The future refurbishment of council properties could be carried out to make their heating systems compatible with lower input temperatures.
The financial viability of a geothermal system
A key objective of the feasibility study was to determine the commercial feasibility of the overall project -what were the business fundamentals of a geothermal well at Hill of Fare feeding heat into the Hill of Banchory heat network? A financial model was created by Cluff Geothermal and Ramboll Energy to test the relationship between total heat sold, return on capital required, and heat price achieved.
An important income element for the geothermal well - and for the existing biomass heat plant - is the Renewable Heat Incentive ( RHI). This is the UK Government's subsidy regime for renewable heat generators. Eligibility to receive the RHI is dependent on repaying any public sector grants that have been received, and receiving RHI income will normally be preferable to keeping the grants. The financial modelling conducted for this project therefore assumed that any grant would be repaid and RHI eligibility restored.
Financial analysis: methodology
There were two elements to the financial analysis. Firstly, a financial model for the geothermal well itself was constructed and used to predict the heat sale price required to achieve various rates of investment return. (A key factor here is the level of annual heat sales; the unit cost of heat supplied is dominated by the up-front capital expenditure, so supplying higher levels of heat increases income while having only a minimal impact on production costs.)
'Central case' assumptions were used for the parameters of the geothermal system (cost of drilling, well heat capacity etc.). There is inevitably some uncertainty in this analysis (as a robust prediction of well output is difficult) but the analysis suggests that a geothermal system at the Hill of Fare would be commercially viable if it could sell around 10,000 MWh of heat a year at 2p/kWh - lower than the market cost of heat from gas and heating oil.
Secondly, a study of the existing heat network established the price at which it made economic sense to purchase heat from an external supplier instead of using the current (biomass) fuel source.
Finally, the outputs of the two analyses were brought together to explore the conditions under which both the geothermal well and the heat network could operate commercially.
The (indicative) graph at figure ES.4 below illustrates this analysis. A geothermal project at Banchory is commercially viable in the part of the annual heat demand range where the green line (the heat price required by the heat network price) rises above the blue line (the heat sale price required by the geothermal well).Figure ES.4: Indicative graph illustrating the 'Cost of Heat' model outputs
Image: Ramboll Energy
Results of the financial analysis
The Hill of Banchory network currently generates heat from biomass sourced from local forests. This leads to a low heat production cost, competition with which would be challenging for almost all heat technologies. The study concluded that under its current configuration, competing against this low cost biomass, it would not be commercially rational for the Hill of Banchory network to purchase heat from a geothermal system, given its likely characteristics.
However, the fuel arrangements at Hill of Banchory are not representative of space heating more widely in NE Scotland, where natural gas and heating oil dominate.
The consortium therefore also considered whether a geothermal well would be competitive against natural gas heating, either in a future phase of the Hill of Banchory network where gas becomes the dominant fuel (it is currently used for back-up), or in a more typical 'generic ' gas powered heat network. The conclusion was that it would be competitive against gas. Consideration was given to how the future evolution of the Hill of Banchory heat network might lead to the use of gas as a fuel, permitting the geothermal heat resource to be exploited.
The Hill of Banchory network does, in fact, use 'peaking' gas boilers to meet periods of high demand, and the proportion of heat supplied in this way will increase as the network expands. However, the seasonal nature of peak demand makes it commercially unattractive for the geothermal developer, as heat from the geothermal well will be produced at a steady rate.) On the other hand, the supply of cheap biomass is necessarily limited, meaning that network expansions are likely to utilise gas fuel to meet a high proportion of heat demand.
Future expansion options for the Hill of Banchory network, therefore, could well be compatible with heat from a geothermal well. For example, an expansion of the Banchory town scheme will be taken forward as a low temperature network. This could be connected to the existing Hill of Banchory scheme via a sub-station on the Raemoir Road, which would provide peaking and back-up heat supply. This would allow the network to be developed and supplied by the Hill of Banchory energy centre in the short term, but still enable the supply of heat from the geothermal well in the future (see Figure ES.5, below).
Figure ES.5: Simplified system schematic for direct geothermal heating in Banchory
Image: Ramboll Energy
The conclusion that heat from the proposed geothermal well would be competitive against natural gas, and by extension heating oil, is important as these fuels are used to supply the majority of space and water heating in the NE of Scotland. This suggests that deep geothermal heat systems of the type proposed for Hill of Banchory could be competitive in many locations across the region.
A register of risks and opportunities was compiled. The highest risks were the following, in order of decreasing magnitude:
- insufficient permeability or connectivity to sustain flow;
- RHI cutback or abolition;
- district heating return temperature too high; and
- lack of public support.
Carbon emission savings
Deep geothermal energy is widely recognised as a very low carbon energy technology. However there are few reference figures available for its carbon intensity, partly because this will vary from site to site. This feasibility study calculated the carbon intensity of a geothermal heat 'doublet' at Banchory, taking into account the 'embedded' carbon emissions involved in its construction as well as the carbon emitted as a result of operating the pump over the project's lifetime (assumed to be thirty years in this context). The conclusion was that heat from the well would have a carbon intensity in the range 3.5 to 5.4 KgCO 2/MWh - a very low level.
Using the midpoint of this figure it was estimated that over thirty years a geothermal heat system at Banchory would save around 71,000 tonnes of CO 2, on the assumption that natural gas heating was being displaced.
Using the Scottish heat demand map, an estimate was also made of the approximate carbon savings available if geothermal heating from radiothermal granites was rolled out to its maximum extent across NE Scotland. The calculation - which necessarily involves considerable assumptions and uncertainties - suggested that radiothermal granite could ultimately meet 5% of Scottish heat demand, and deliver annual savings of around 900,000 tonnes of CO 2 per year.
HOBESCO, the operators of the Hill of Banchory heat network, held a community event for local people at which a questionnaire survey was conducted. Around two thirds of respondents thought they had a 'medium' knowledge of geothermal energy, with the rest in the 'low' category: indicating that local knowledge of deep geothermal energy is (unsurprisingly) relatively sparse. However, support for developing geothermal energy in Scotland was very high, as was the idea that this should be supported by Government. There was also widespread enthusiasm from respondents for the idea that the heat they received should come from geothermal energy.
This feasibility study has confirmed that there is a promising prospect for a geothermal heat generating system at Banchory, increasing confidence that generating renewable heat from Scotland's radiothermal granites is viable. It represents a positive strategic development in the context of Scotland's wider quest for affordable low carbon heat.
The geothermal potential of the Hill of Fare granite has been found to be greater than had been supposed, though some uncertainty inevitably remains. Heat from a geothermal well at Hill of Fare would probably not be competitive with the unusually cost-effective biomass fuels currently used by the Hill of Banchory heat network, but it would be competitive against natural gas and heating oil, the dominant heat sources in the NE of Scotland. Furthermore, as the Banchory network expands it is likely to become more gas-dependent, creating the necessary conditions for the Hill of Fare geothermal project to become commercial.
There is scope to use this project's findings as a template for other appraisals in similar locations across Scotland. The high-level analysis of the full technical potential of radiothermal granites suggests that as much as 70% of the total head demand in the adjacent regions could come from this source. This is equivalent to around 5% of total Scottish heat demand, and this figure could increase further as other potential geothermal sources elsewhere in Scotland are explored.
The next steps for the project should include:
- pilot drilling to improve understanding of the thermal gradient and granite permeability;
- further geophysical studies;
- identification of funding;
- achievement of regulatory approval for the drilling and geophysical testing mentioned above;
- liaison with local landowners to identify possible drill sites and wayleaves for pipelines; and
- events to increase public engagement with deep geothermal energy locally and nationally. This should include opportunities for the public to experience the drilling of a test borehole at first hand.