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Initial Strategic Decisions   The Two Chosen Strategies  

Initial strategic decisions

The aim of this stage was to select several MCT sites and combine the varying potential power output together into a supply strategy with the objective of a producing a combined source of energy that could provide a proportion of Scotland’s baseload energy demand. The combined power flow must therefore be as close to a constant output as possible.

However, it was identified at an early stage of phasing different energy output that a constant-flat power flow would be impossible to produce given the geographical spread of the sites identified in the Resource and Site Analysis stages of the project.

It was decided that a storage technology would have to be employed to fill in the short term variations i.e. the variation in power output due to the fluctuations in tidal speed experienced over the 12.4 hour tidal cycle, and in the longer term i.e. the large changes in potential generation due to the large variations in tidal speeds experienced over the 14-day alternation between Spring (maximum tidal range) and Neap (minimum tidal range) tides.

Pumped Storage was selected to fill in the gaps in generation and enable a constant source of energy to be produced. Scottish Power’s 400MW Pumped Storage Hydro plant at Ben Cruachan was selected for the purpose of the project.

The sites were selected to ensure an even distribution of maximum tidal stream peak speeds. The phased output of the potential power available from each site would thus give a more constant supply strategy, which always generates power at any given time. Some sites, such as the Pentland Skerries, are much larger areas than other, such as the Kyle of Rhea, therefore the available capacity that can be installed at a site was also considered when selecting the sites.

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The two chosen strategies

Initially, the phasing of multiple sites showed that combinations of several sites with very large installed MCT capacities generated highly fluctuating output and required much more pumped storage to produce baseload. Reducing the installed capacity at certain sites would reduced the fluctuation in the combined output and therefore reduce the storage requirement. After careful selecting different numbers of MCT unit (and many hours of trial and error) optimal strategies were developed.

Two different strategies were chosen as targets for the experimental process of combining the power output from several sites to form a baseload supply strategy:
  1. Minimum Storage Strategy (Best Baseload)

    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.

  2. Maximum Storage Strategy (Maximum Baseload)

    The second scenario focused on maximising the installed capacity of MCTs, which uses the maximum the level of pumped storage available. Thus producing a maximised baseload supply strategy that selected the maximum number of MCT units that could be installed at each site selected.
Each scenario was then tested using Spring (maximum) and Neap (minimum) tidal current velocities to show the variation in the potential baseload supplied and identify the changes in Pumped Storage required to fill in the gaps.

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