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Feasibility Report of Fortissat Community Minewater Geothermal Energy District Heating Network

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Chapter 5: Development Options

5.1 Introduction

This chapter presents the Development Options which integrate the geothermal system design options appraisal (Chapter 3 and Appendix A3.2) and district heating network (DHN) design options appraisal (Chapter 4). Two alternative design options are presented, taking into account all subsurface and surface factors considered. The advantages and disadvantages of the two options - one proposing a passive treatment system, and the other proposing reinjection (with alternative locations for the production wells) - are compiled to compare the options.

This chapter also presents the risk assessment for the project, the opportunities, and a carbon audit which estimates the reduction in carbon emissions from the geothermal energy district heating network against business as usual.

5.2 Option 1 - 'Preferred'

Option 1 consists of a single production well and passive minewater treatment facility. The identified location for the production well is south of Allanton near or even at the Kingshill Mineshaft No.1. Pumping minewater in this vicinity offers the highest potential for lowering the local water table and reducing or preventing the minewater resurgence issues which affect this area and the village of Allanton which is downslope from there. It also allows the polishing wetlands to be located in an area already providing this facility, and contribute to the objectives of the Local Nature Reserve and Site of Importance for Nature Conservation. Minewater would be pumped from the WRSM seam at a depth of c. 340 m below surface level. This option has been modelled with three DHN designs - networks A, B and C - to assess the impact of the heat network's scale on the financial performance of the project.

Figure 5.1: Illustrative diagram of passive minewater treatment facility.

Figure 5.1: Illustrative diagram of passive minewater treatment facility. 

Option 1 is shown in Drawing 5.1.

5.3 Option 2 - 'Alternative'

Option 2 consists of a production well and two injection wells, with no passive minewater treatment. The geothermal system and heat centre components of this option are contained entirely within the JHI Hartwood Home Farm land boundary. This option produces and injects minewater from the WRSM seam at a depth of ca. 380 m below surface level. The heat centre in this option is situated between Allanton and Hartwood, so only network C, the largest DHN, has been modelled.

Figure 5.2: Illustrative diagram of doublet system with production and injection well(s).

Figure 5.2: Illustrative diagram of doublet system with production and injection well(s).  

Option 2 is shown in Drawing 5.2.

5.4 Options Summary Table

The options are outlined in Table 5.1:

Table 5.1: Summary table of minewater geothermal DHN development options.

Preferred Design Option

Geothermal System

District Heat Network

Option 1A

Production well with passive minewater treatment facility

Network A:

Allanton West only

Option 1B

Production well with passive minewater treatment facility

Network B:

Allanton West and East

Option 1C

Production well with passive minewater treatment facility

Network C:

Allanton West and East and Hartwood

Option 2

Production well with reinjection wells for minewater disposal

Network C:

Allanton West and East and Hartwood

5.5 Option Comparison - Passive Treatment vs Reinjection

There are advantages and disadvantages with both the Development Options under consideration, and Table 5.2 below, while not exhaustive, seeks to highlight the key considerations which have been identified at this stage in the process:

Table 5.2: Summary table of preferred and alternative minewater geothermal DHN development options.

Theme

Option 1 - Passive Treatment

Option 2 - Reinjection

Land Ownership

Advantages

Production well and passive treatment system are within the land ownership of NLC Greenspace Development Service, with potential for straightforward collaboration with NLC Housing and Social Work Services in progressing development.

Advantages

Production well and injection wells are within the land ownership of JHI and are therefore immediately accessible for testing at Development Stage.

Disadvantages

JHI role in progressing development will need to be clarified for Development Stage as they have no ongoing stake in project.

Disadvantages

JHI ownership will require position to be agreed with NLC as part of Demonstrator Stage.

Environmental constraints

Advantages

Minewater resurgence and drainage issues are ongoing environmental and financial burden to Council and residents in Allanton and could be mitigated by proposal.

Creation of new wetland areas offers potential for biodiversity gains, improving quality of water to local watercourses, and providing an attractive recreational area for the local community.

Advantages

No environmental designations apply to site and borehole locations are not adjacent to existing watercourses or identified wetland areas.

Disadvantages

Production well and passive treatment system are within an area designated as a Local Nature Reserve, Site of Importance for Nature Conservation, and known to contain species of interest for Local Biodiversity Action Plan, which may constrain development locations, increase survey requirements and create resistance to proposals.

Disadvantages

Proposal does not incorporate the potential to mitigate the existing minewater issues affecting Allanton. While the system would be net neutral (i.e. the same volume of minewater would be reinjected as pumped), the presence of a minewater geothermal system accessing the mine seams from which the resurgence issues arise, would almost inevitably lead to association between the geothermal system and the ongoing environmental issues affecting the area.

Regulatory Framework

Advantages

Proposal will bring the existing minewater discharge from Kingshill within the CAR Licencing, with the consequence there will be a "Responsible Person" under the Licence.

Advantages

Proposal in-sync with pollution principles of returning abstractions to their source.

Suspended materials (Fe, Mn) remain within the minewater.

Disadvantages

Suspended materials will oxidise and will require to be disposed of to landfill once collected in settlement basin.

Information required to satisfy regulator will need to be scoped as it is not pre-defined and extended test period required.

Surface discharge will require Flood Risk Assessment to demonstrate acceptability.

Disadvantages

No improvement to the existing situation where Kingshill discharges are unlicensed.

Geothermal Technical Issues

Advantages

Passive treatment system eliminates the potential complexities of a reinjection system and increases the overall robustness of the system and the reliability of the DHN.

Avoids the possibility of thermal breakthrough.

Advantages

Open loop geothermal systems have precedent elsewhere.

Potential that this option has wider replicability as it does not require the same space requirements, which may not be available in more densely developed locations suitable for geothermal energy.

Maintains pressure in the mine system.

Maintains volume of water in mine system to avoid increasing ESP pumping costs.

Disadvantages

Extended testing period required to gather sufficient baseline environmental information to design system and secure CAR Licence.

Changes to minewater chemistry over time may affect design of system and its performance, requiring modifications.

Disadvantages

There are inherent risks associated with injection wells, as described in Chapter 3. It is generally true that injection of fluids into mines is more difficult than extraction. Several problems may arise:

  • In permeable rocks, re-injection is easier but the risk of cold water breaking through to the production wells is higher.
  • If injection is made into the collapsed longwall workings then there may be reduction in injection flow capacity due to collapse.
  • There is a high risk that geochemical reactions cause iron or other deposits to collect near the well and within the well, and these may eventually require cleaning or even re-drilling.

As the area around Fortissat has high groundwater levels higher pressure may be required to inject fluids into the already saturated substrate. This is a particular risk if the wellhead is located below 200 m OD and could lead to high pumping costs.

District Heating Network Technical Issues

Advantages

System is scale-able.

Energy centre is readily accessible to existing and gas electricity network.

Energy centre location allows potential to include visitor facilities related to the nature reserve as well.

Advantages

Allows energy centre to be located close to and between both target villages.

Disadvantages

Location of energy centre more distant from Shotts for potential future expansion of network.

Disadvantages

Location of energy centre less accessible to existing gas network for system backup, resulting in higher connection costs.

While it is evident from the above option comparison that there are both advantages and disadvantages to both systems, at this point Option 1 is identified as the 'Preferred' option due to the potential for it to mitigate existing minewater resurgence issues which currently blight Allanton. This is considered to widen the appeal of the project for all key stakeholders.

5.6 Risk Management

Throughout the project development to date, risks and opportunities that have arisen in project meetings and technical reports have been captured. The register in Tables 5.3, 5.4 and 5.5 document the current view of risks and identifies potential actions that can be taken in subsequent stages of the project to reduce risks. Following risks, there is a discussion on opportunities.

In this project a risk is any issue that may lead to a loss in value; an opportunity can add value to the project. Risks and opportunities may be discrete events, such as removal of the RHI; or broader uncertainties, such as unpredictable minewater chemistry over time. Risks and opportunities have been recognised in all aspects of the project and organised by three categories: geothermal supply (Table 5.3); district heating network (DHN) (Table 5.4) and other (Table 5.5). Risks have been ranked relatively from Low to High, where the ranking indicates both the potential impact on the project value if the risk materialises and the probability of the risk occurring. Risks are also ranked relatively according to the ability of the project to manage the risk, again from Low to High.

Mitigating actions are described, and the point at which the project will need to address the risk is identified. For the geothermal supply the project stages during which to address the risk are either Licensing, Testing, Design or Operation. For the DHN they are Engagement, Design or Operation. For Other they could be any of the above.

Table 5.3: Risk Register - Risks and mitigating actions associated with geothermal supply

Geothermal Supply

#

Risk

Risk = Prob * Impact

Manageability

Mitigating actions

When to address risk

Risk after mitigation

1

Faults are sealed

ie fluids will not migrate across or along the fault

Low

Moderate

Probability of fault bisecting the mine being sealed is 50:50 due to lack of knowledge of regional fault behaviour. There are at least 6 major faults which bisect the WRSM and WNMA worked seams. Fluid migration across the faults can be avoided by designing system so that production and injection wells are on the same side of the main fault. Impact depends on which side of the major fault is selected, because if the faults are sealed this will limit the volume and depth of mine from which heat is extracted.

Design

Low

2

Recharge from aquifers to the mine is low, limiting sustainability of minewater and therefore heat extraction.

Moderate

Low

Aquifer transmissivity is quite speculative, as it is unknown where the source of water flooding the mine is. Monitor production well for temperature and flow rate with submersible pump power requirements. Can be mitigated in medium term with reinjection. If no aquifer recharge impact on longevity of geothermal resource without reinjection is catastrophic, so reinjection well would need to be drilled.

Operation

Moderate

3

Minewater temperature at low end of range ie <15°C

Moderate

High

Produce from deepest portion of mine. Select heat pumps based on known minewater temperature following testing, to ensure COP >2.9. Monitor temperature during operation and test chemistry at intervals to assess whether there is mixing with shallower or surface waters.

Design & Operation

Low

4

Unpredictable water chemistry over time

Moderate

Moderate

Deliberately over-size treatment facility or ensure that land available to expand treatment facility at later stage if desired.

Design & Operation

Moderate

5

High Fe concentrations in minewater

High

High

Utilise prophylactic heat exchanger. Maintain sealed, anoxic conditions in abstraction-heat exchange- reinjection system. Dose with benign reducing agents to maintain Fe in solution.

Design

Low

6

High Mn concentrations

High

Moderate

Utilise prophylactic heat exchanger. Maintain sealed, anoxic conditions in abstraction-heat exchange- reinjection system. Dose with benign reducing agents to maintain Mn in solution (if reinjected).

Increase size of treatment plant and consider active chemical dosing (alkali addition) to remove Mn (if treated)

Design

Moderate

7

High H2S concentrations

High

Moderate

If reinjection practised, maintain sealed system. If minewater treated, ensure H2S is vented or scrubbed from minewater to prevent odour issues.

Design

Moderate

8

Minewater quality dramatically reduces lifespan of submersible pump and heat exchanger

Moderate

High

Assess water chemistry during well test. If engineering solutions can resolve any prohibitive chemistry issues schedule annual maintenance with deep clean and possible replacements scheduled. Accounted for in financial model.

Testing & Design

Low

9

Artesian conditions below 200 m OD

Moderate

Moderate

Difficult to manage if the ground level at the drilling site is below 200 m OD. Can be addressed with high injection pumping capacity and multiple, injection wells, but this will increase costs.

Design

Moderate

10

Production (or injection) well fails to encounter permeable roadway

High

Moderate

Withdraw bit and deviate to re-attempt to encounter roadway. Careful quality control of mine plans and drilling process. May be worth testing for permeability and yield before drilling again as naturally permeability may match required geothermal supply requirements.

Testing

Moderate

11

Extraction flow rate is not achievable from mine roadway

Moderate

Moderate

This is unlikely. A contingency plan which involves planning to drill a second production well should be considered.

Testing & Design

Moderate

Option 1 only

12

Mine subsides on minewater extraction

Moderate

High

Producing water from mine where matrix likely to hold up bedrock, although this may be challenging to predict prior to drilling. Monitoring surface seismicity (if deemed necessary in design stage). Reinjection well may be required to inject some minewater to retain pressure within the mine system.

Design & Operation

Low

13

Treatment facility is not deemed better than business as usual by SEPA

Moderate

Low

System will be designed with injection wells

Design

Low

Option 2 only

14

Insufficient permeability to allow reinjection of minewater

Moderate

Low

Risk can be mitigated through installation of multiple injection wells, a minimum of two are suggested for one production well. Risk unknown until injection tested. Injection test and core samples collected from test well may be advised to collect aquifer transmissivity, porosity and permeability data. Installing a passive treatment facility will avoid this risk.

Design

Moderate

15

Thermal breakthrough occurs

High

Moderate

Perform hydraulic testing and modelling of mine system prior to designing injection well doublet. Maximise fluid flow pathway between injection well and production well. Avoid direct roadways linking wells. Potentially avoid injection, although in this case be careful of changing pressures within the mine.

Design

Moderate

Table 5.4: Risk Register - Risks and mitigating actions associated with the district heating network.

District Heating Network

#

Risk

Risk = Prob * Impact

Manageability

Mitigating actions

When to address risk

Risk after mitigation

16

District heating network requires railway line crossing

Moderate

High

Ensure long lead time (up to 18 months) to consult with Network Rail to agree suitable crossing point, negotiate wayleave agreement and liaise as necessary to secure acceptable design solution.

Engagement

Low

17

Private heat customers do not connect to network

High

Moderate

Engage private householders early in project design. Make connecting to network economically and socially attractive.

Engagement

Moderate

18

Heat demand too low, so cost of heat is too high per kWh

High

Moderate

Select the design option with highest heat density for least pipework and largest feasible network. Structure delivery model ESCo willing to make a low return or not for profit. Engage with NLC. Assess national value of minewater geothermal as demonstrator project.

Engagement & Design

Low

19

Testing of production well results in "no go"

High

Moderate

Drilling test well is fundamentally riskiest part of project development. Sunk costs of c. £500k potentially lost if well is unused for heat supply.

Testing

Moderate

20

Design not optimised and integrated across generation/ network/supply to reflect key operating conditions (such as compatibility of network flow/return temps with operating temperatures in secondary heating systems)

High

High

Clearly defined basis of design that forms the technical specifications and delivery by a competent contractor. Generation, network and customer interface must be integrated in the delivered solution with suitable supporting investment.

Design

Moderate

21

Development trajectory of network and connections is delayed and installation and connections resulting in oversized plant and lower than predicted efficiency

Moderate

High

Focus on customer engagement and planning of network installation and connections. High priority on customer satisfaction in contract specification and performance requirements.

Engagement & Design

Moderate

22

Poor customer satisfaction due to mismanaged customer service - particularly relating to installation of heat connection and heat interface unit: poor customer communication at planning stage; disruption to customers; quality of service; quality of finishing/making good

Moderate

High

Focus on customer engagement and planning of network installation and connections. High priority on customer satisfaction in contract specification and performance requirements.

Engagement

Moderate

23

Alternative heat sources developed that outcompete geothermal on cost of supply

Moderate

Moderate

Model economic benefit of alternative heat sources to supply district heating in next phase. Will be a greater risk to future projects following establishment of geothermal demonstrator.

DHN is "energy agnostic" so energy centre could run on alternative fuel source, retaining value for installed infrastructure.

Design

Moderate

Table 5.5: Risk Register - Risk and mitigating actions associated aspects of the project other than geothermal supply and the district heating network.

Other

#

Risk

Risk = Prob * Impact

Manageability

Mitigating actions

When to address risk

Risk after mitigation

24

Renewable Heat Incentive reduced or eradicated by government

High

Moderate

Continue to engage with DECC and Scottish Government in the first instance

Engagement

Moderate

25

Well licensing delays project

Low

High

Work closely with SEPA during application. Do not commit to any CAPEX expenditures until license granted.

Licensing

Low

26

Housing stock upgrades and minewater geothermal system timescale are out of sync

High

High

Keep open dialogue with NLC regarding timing of housing upgrades

Engagement

Low

5.7 Opportunities

Many of the key opportunities have already been mentioned, the potential wide-ranging benefits of installing a passive minewater treatment facility. These include:

  • The Limestone Coal (LSC) mined seam reaches depths greater than 500 m, meaning temperatures above 18°C may be present, and may allow the system to access the Geothermal RHI tariff.
  • The fractured bedrock above the long-wall workings is likely to be productive, which increases the area through which minewater can migrate to the well, increasing the heat potential of the system from "mined-area only" estimates.
  • Working closely with SEPA throughout the well design process could reduce the 2'' grouting annulus illustrated in the well designs in Appendix A3.7 in turn reducing the cost of the wells from the current estimates.
  • The Allanton community are enthusiastic about renewables and very enthusiastic about any prospect of mitigating current surface leakage of minewater.
  • The Allanton community has extensive local knowledge and experience of the mines.
  • Shotts has an economically attractive heat market for district heating, and should be explored in more detail in future studies.
  • The downturn in the oil industry may result in lower costs for services and potentially drilling.

5.8 Carbon Audit

Switching from domestic gas boilers to a minewater geothermal district heating network will reduce carbon emissions. The carbon emissions are compared to a business as usual (BAU) case assuming that existing heat is supplied from individual gas boilers. The carbon emissions are based on DECC figures and assume that the Gas Carbon Emissions Factor is 0.18407 kgCO2/kWh and the Electricity Carbon Emissions Factor is 0.46219 kgCO2/kWh. The BAU is compared to the heat pump scenario based on the gas boiler supplying 5% of the annual load and the heat pump supplying 95% of the annual demand. These are the same figures that have been used in the financial model. The analysis is based on a heat pump COP of 3.5 and gas boiler efficiency of 90% that are also used in the model.

Table 5.6 shows the BAU emissions, Table 5.7 shows emissions with minewater geothermal system installed, and Table 5.8 shows the emissions saved.

Table 5.6: The BAU carbon emissions with 100% of heat generated from domestic gas heating systems.

Option

Total Heat Demand (MWh)

Gas Consumption (kWh)

tCO2 for Gas Boilers

1A

3,860

4,541,176

836

1B

5,713

6,721,176

1,237

1C/2

9,670

11,376,471

2,094

Table 5.7: The projected carbon emissions associated with electricity consumption of the minewater geothermal WSHP system (providing 95% of heat) and gas consumption of the back-up gas boiler (providing 5% of heat) in the heat centre serving the district heating network.

Option

Total Heat Demand (MWh)

Gas Consumption

(kWh)

tCO2 for Gas Boilers

Electrical Consumption of WSHP (kWh)

tCO2 for WSHP

1A

3860

214,444

39

1,047,714

595

1B

5713

317,389

58

1,550,671

880

1C/2

9670

537,222

99

2,624,714

1490

Table 5.8: A low estimate of the carbon emissions savings associated with the four different design options based on the 2015 UK electricity mix. Carbon savings will increase further as the UK electricity mix becomes less carbon intensive.

Option

Carbon emission savings (tCO2/year)

1A

312

1B

462

1C/2

782

These figures are approximate as they only take into account the emissions saved from replacement of the heating gas with electricity to run the minewater-source heat pumps. In addition, the electricity carbon emissions factor is based on the 2015 UK electricity mix, which is projected to become less carbon intensive over time. Therefore, the CO2 emissions from the minewater geothermal DHN will decrease significantly over the projects lifetime, whereas emissions from gas heating will only marginally decrease if a 100% efficient boiler is installed in every house.