Page 169 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

158 Fundamentals of Ocean Renewable Energy

TABLE 7.1 Tidal Streams at Fall of Warness (the EMEC Tidal Energy
Test Site), Based on Tidal Diamond Information Extracted From
Admiralty Chart 2249 (Orkney Islands—Western Sheet)
Time Relative Direction of Speed of Tidal Stream (m/s)
to HW (h) Streams (degrees) Spring Neap
−6 150 3.2 1.2
−5 144 3.7 1.4
−4 141 3.0 1.2
−3 116 1.4 0.6
−2 350 0.2 0.1
−1 308 2.0 0.8
0 329 3.3 1.3
1 329 3.3 1.3
2 320 2.5 1.0
3 325 2.0 0.9
4 324 0.6 0.3
5 160 0.9 0.4
6 153 2.9 1.2
Notes: Times are relative to HW (high water) at Aberdeen.

Such desk-based studies are particularly useful at early scoping stages of
tidal energy projects, and can help inform site selection, prior to investing in
detailed numerical model studies or costly field campaigns. However, the tidal
resource can only be truly characterized by conducting in situ measurements.

7.1.1 Water-Level Measurements
The most fundamental and easily quantified property of the tides, and one that
has been measured for hundreds of years, is the variation in water elevations over
time. The longest accurate time series of tidal elevations in the United Kingdom,
and indeed one of the longest in the world, is in Liverpool, extending back to
1768 [4]. Today, many tide gauge networks exist around the world (e.g. Fig. 7.1),
and hourly (or shorter timescale) data are widely available, generally over time
periods of at least a decade. In addition, tidal analysis of these elevation time
series has led to the development of tide tables, which report times and heights
of high water (HW) and low water (LW) for a location, generally published for
each calendar year (e.g. Table 7.2). However, when specific measurements are
required, for example, at locations that are not represented by an existing or
historic tide gauge (such as in an estuary or a region that is far from existing tide
gauges), several options are available.