Page 287 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

274 Fundamentals of Ocean Renewable Energy

FIG. 10.2 Variability of renewable energy resources at contrasting locations. Results are normal-
ized to the 98th percentile measured for each site. The solar sites are Botswana (top) and Sweden
(bottom); wind sites are South Africa (top) and Sweden (bottom); wave sites are Hawaii (top)and
Sweden (bottom); tidal sites are Pennsylvania—semidiurnal (top) and Louisiana—diurnal (bottom).
(Reproduced from J. Widén, N. Carpman, V. Castellucci, D. Lingfors, J. Olauson, F. Remouit,
M. Bergkvist, M. Grabbe, R. Waters, Variability assessment and forecasting of renewables: a review
for solar, wind, wave and tidal resources, Renew. Sustain. Energy Rev. 44 (2015) 356–375, with
permission from Elsevier.)

to either seasonal or diel demand for electricity, there is an underlying trend.
Because we measure time based on the Earth’s rotation (and hence the S2
constituent that has a period of 12 h), this means that at any one location, spring
(or neap) tides—which are a combination of the M2 and S2 tidal constituents—
will occur at the same time every lunar cycle. For example, in semidiurnal
regions, when the S2 phase is close to 0 degrees, then maximum values of S2
will occur near the time of solar transit (i.e. noon or midnight) [2]. Therefore,
mean high water spring (MHWS) will occur near noon and midnight, at times
when the S2 and M2 constituents are in phase. However, if the S2 phase is near
180 degrees, then S2 will have a minimum value near noon and midnight, and
mean low water spring (MLWS) will occur at these times. Therefore, particular
stages within the spring neap-neap cycle are linked to time of day, and could
be considered from an electricity demand perspective. Examining the S2 cotidal
chart for the northwest European shelf seas (Fig. 10.3), we can determine the
time of day that HW (or LW) spring tides will occur, and similar charts can be
used to determine at what time of day peak spring flood (or ebb) currents occur
(e.g. [3]).
Relating to variability, one issue that is shared by the majority of ocean
renewable energy resources is, although they have a high energy density and
may be predictable, they are not dispatchable. For example, if there is a sudden
increase in demand for electricity, then a pumped storage plant such as Dinorwic
(Electric Mountain) in North Wales can attain full power, from standby, in under