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Minimum Storage Strategy

The first scenario focused on minimising the amount of pumped storage whilst optimising the potential energy generated from the MCTs. Thus producing an optimal baseload supply strategy that selected the maximum installed MCT capacity possible for a low pumped storage implementation.

Energy analysis results   Economic analysis   Socio-economic analysis  

Emissions analysis   Environmental impact analysis  


Energy analysis results

  1. First Generation Scheme


    1. Scheme Size


    2. Location No of units Installed capacity (MW)
      Islay, South of Orsay 40 40
      Kyle of Rhea 32 32
      Pentland Firth, Inner Sound 120 120
      Pentland Firth, South Ronaldsay 30 30
      Pentland Firth, South of Skerries 15 15
      Kintyre, South of Mull 30 30
      Total 267 267

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    3. Power profile using mean tidal speeds


    4. The first graph shows power profiles of each farm and the total varying power output. The power output fluctuates between 80 and 133 MW within the day. Using pumped storage a baseload of 105.5 MW was achieved and the pumping and generation cycles to achieve this are shown in the second graph.




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    5. Variation in power profile during spring and neap tides


    6. The nature of the total varying power output for springs and neaps are shown as below with the corresponding pumping and generating cycles of the pumped storage plant to achieve a baseload. The baseload for spring and neap tides was found to be 139 and 49 MW respectively with corresponding energy lost of 2 and 6.5 %.

      Power profiles using mean spring tidal speeds





      Power profiles using mean neap tidal speeds




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    7. Summary of results


    8. The table below presents a summary of the results for this strategy. An installed capacity of 267 MW resulted in an annual energy yield of 823.4 GWhr. The low capacity factor (0.35) is as a result of compromising on a relatively low speed location (Pentland Firth, Inner Sound) with greater number of units (120) to achieve a less varying power output and hence low storage.

      Description Units Mean Spring Neap
      No of units   267 267 267
      Installed capacity MW 267 267 267
      Range of varying power output MW 80 - 133 107 - 169 17 - 84
      Baseload MW 105.5 139 49
      Daily energy lost MWhr 54.3 67.4 82.3
      Percentage energy lost % 2.1 2.0 6.5
      Daily energy stored mwhr 164.0 219.5 269.7
      Percentage of energy stored % 6.3 6.5 21.4
      Percentage of cruachan used % 1.9 2.5 3.1
      Annual energy GWhr 823.4
      Power factor 0.35


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  2. Second generation schemes


    1. scheme size


    2. Location No of units Installed capacity (MW)
      Islay, south of orsay 120 120
      Kyle of Rhea 48 48
      Pentland Firth, Inner Sound 120 120
      Pentland Firth, South Ronaldsay 209 209
      Kintyre, South of Mull 209 209
      Pentland Firth, South of Skerries 165 165
      Total 871 871

    3. Power profile using mean tidal speeds


    4. The graphs below show the total varying power output for a day for 871 units installed on six different locations. The resulting baseload of 433.5 MW together with the pumping and generation cycles are shown in the proceeding graph.





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    5. Variations in power profile during spring and neap tides


    6. The graphs below show the power profiles for each farm and the resulting total varying power output during springs and neaps. A higher baseload of 522.8 MW was produced during springs and a lower baseload of 191.6 for neaps representing a 63 % drop.

      Power profile using mean spring tidal speeds





      Power profile using mean neap tidal speeds





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    7. Summary of results


    8. The summary results are as shown below. In this case 871 units resulted in an annual energy yield (AEY) of 3129.1 GWhr and a capacity factor of 0.41. The higher percentage of energy lost (7.8 %) during neaps is because of the higher percentage of energy storage (26 %).

      Description Units Mean Spring Neap
      No of units 871 871 871
      Installed capacity MW 871 871 871
      Range of varying power output MW 223 - 653 375 - 768 45 - 379
      Baseload MW 433.5 522.8 191.6
      Daily energy lost MWhr 413.7 341.5 391.0
      Percentage energy lost % 3.8 2.7 7.8
      Daily energy stored MWhr 164.0 219.5 269.7
      Percentage of energy stored % 12.7 8.8 26.0
      Percentage of Cruachan used % 15.6 12.9 14.8
      Annual energy GWhr 3129.1
      power factor 0.41
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Economic section

The cost of electricity with such an installation is shown for the first 14 years below:

cost of electricity

This is based on the assumption of a loan for each year of installation, taken out at the start of each installation year, plus annual costs and pumped storage costs. The yearly loan repayments, for the total expenditure of 500,000,000 are shown below ():

repayment distribution

For an explanation of the economic calculation, click here

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Socio-economic section

The job potential as a result of this strategy is shown in the graph below : job potential chart
For an explanation of the socio-economic calculation, click here

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Emissions analysis

The following table shows the emissions saved by using marine current energy instead of conventional thermal power, at the first and second generation stages:

emissions table showing savings



Note: These figures are estimates based on average emission values from several sources. No distinction is made for generation using specific pollution abatement technologies etc.

For an explanation of the emissions calculation, click here

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Environmental impact

This strategy, with the aim of reducing the storage required to an acceptable level, would therefore have a reduced impact on the environment when compared to the maximum capacity strategy.
The number of units installed at each site compared to the total area of seabed, considered available for installation of the turbines in the Site Analysis phase, can give an indication of the level of impact that would be experienced:

Pentland Firth, South of Skerries

165 MCT units would be installed out of a maximum possible 260 units. This means that 63% of the area would be used, leaving 37% unaffected. This site is classed as a high collision risk area (experiencing over 5 vessels per day [1]), therefore exclusion zones required around the farms and additional traffic routing measures would be required to allow shipping to safely navigate around the Skerries. As no units would be installed the north of the Skerries, traffic could be diverted around this route. The closest point-of-connection to the transmission network is at Thurso, therefore over 19km of sub-sea cabling would be required which would result in further disruption of the seabed and wildlife due to the digging of trenches and laying of cable.

Pentland Firth, South of South Ronaldsay

209 MCT units would be installed out of a possible 244 turbine units. 86% of the available area would therefore experience significant impact. These waters are also classed as high navigational-risk and would require major navigational buoyage to be laid to warn traffic. The channel of water to the north of the Skerries and south of this site, could be used as a dedicated traffic route e.g. to allow safe passage to and from the Scapa flow oil terminals and fishing grounds around Orkney. Over 20 km of sub-sea cabling would be necessary to connect the farm to the Grid Supply Point at Thurso. This would cause significant damage to seabed and habitat located close to the cables path.

Pentland Firth, Inner Sound

120 units would be installed out of a possible 211 meaning 57% of the available area would be developed. This would leave a significant area unaffected and as the risk of collision is smaller in this area, shipping would be less effected compared to other sites in the Pentland Firth. If alternative routes were required the Outer Sound could be used. The transmission grid is located closer, meaning over 13km of sub-sea cable would be required, therefore less of the seabed would be effected compared to other sites located further out, such as Pentland Skerries and South Ronaldsay.

West Scotland, Kyle of Rhea

A total of 48 MCT units would be installed exploiting the total 2.8 km2 area available. Therefore 100% of the area would receive significant impact. As this is a narrow channel between the Isle of Skye and the mainland, careful navigational marking and exclusion zones would be required to allow fishing and recreational vessels to pass through safely. The Kyle of Rhea is designated as a Site of Specific Scientific Interest by the SHN. This could prevent monopile support structures being drilled into the seabed, therefore alternative second-generation MCT technology, such as a mooring system, may have to be employed to secure the 48 1-MW turbine units specified.

West Scotland, Southwest of Orsay, Islay

120 MCTs would be installed out of a possible 519 units, so only 23% of the available area would be affected. The installed locations could be carefully selected to reduce damage to any wildlife and reduce the risk of collision.

Kintyre, South of Mull

209 MCTs units would be installed out of a possible 1298, resulting in only 16% of the available 74.1 km2 area being covered by the MCT farm, leaving a large proportion of the seabed unaffected. This would allow significant room for shipping traffic, fishing vessels, recreational craft and large mammals to navigate safely around the scheme. The site is located close to the coast therefore only 2 to 4km would have to be covered by sub-sea cable, however, the closest connection to the transmission network lies at Carradale, some 100km north. Over 100 km of overhead lines would have to be constructed causing massive visual impact along the Kintyre peninsula. (this line is scheduled to be reinforced by Scottish & Southern Energy). Alternatively a marine transmission line could be laid from the Mull of Kintyre, west to the link up with the network infrastructure at Hunterston Nuclear station. This could have significantly less visual impact but be much more expensive.

For an explanation of the environmental impact analysis click here

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