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Optimization Chapter | 9 255

9.3 INTERARRAY OPTIMIZATION
Examining the M2 (principal lunar semidiurnal) cotidal chart of the northwest
European shelf seas (Fig. 9.12), Kelvin waves take a relatively long time to
propagate along a coastline. For example, the southern part of the Irish Sea
(around the Bristol Channel in Fig. 9.12) has a phase difference of around 3–4 h
compared with the northern part of the Irish Sea (e.g. Holyhead). Because tides
are predictable, it seems feasible that, with knowledge of the phase relationship
between tidally energetic sites, we could strategically station a series of tidal
power stations along a coastline, concurrently exploiting different parts of the
same tidal wave, feeding the generated electricity into a unified electricity grid,
and so reducing the variability that would result from a tidal power station
operating in isolation.

9.3.1 Phasing of Tidal Stream Arrays
A 2005 study commissioned by The Carbon Trust [27] examined the patterns
of energy availability from a range of tidal stream regions around the United
Kingdom, with a focus on opportunities for diversification to reduce the
variability of supply. The study is fairly comprehensive, but the results on
optimizing variability can be summarized by reference to Fig. 9.13. In this
figure, a range of development scenarios have been considered, from a relatively
low level of development (10% of the UK tidal stream resource) up to an
extreme scenario of 100% development. Within each of these development
scenarios, optimization modelling was used to determine the contribution of
each tidal stream site that resulted in the lowest average hourly variability
(expressed as a percentage of maximum output). Development of 10% of the
UK tidal stream resource across a range of sites would result in low variability.
However, at larger scales of development, the synchronized output of larger sites
(e.g. the Pentland Firth) dominates, increasing variability. For example, at 10%
development, the Alderney Race (Channel Isles) accounts for 25% of all output
compared with 10% for the Pentland Firth. However, at 100% development, the
percentage contribution of the Alderney Race reduces to just 6%, whereas the
Pentland Firth would account for 30% of the total output.
The issue of tidal phasing of tidal stream locations was revisited by Iyer
et al. [28], who extended The Carbon Trust study [27] to include energetic
sites in Wales: Anglesey (Skerries) and Ramsey Sound. Again, they concluded
that there is insufficient diversity between the sites identified across the United
Kingdom to be considered as a firm power source. In general, they found that the
high-energy sites were in phase with one another, with the exception of Alderney
Race in the Channel Isles. The problem was further investigated by Neill et al.
[29,30] who, rather than constraining their analysis to a limited number of
locations, considered the UK tidal energy system as a whole. By doing so,
they did not limit their analysis solely to high energy sites, but considered
a diverse range of current magnitudes in the analysis. In addition to seeking
sites with high energy, they penalized site selection where the combination of