DP Energy Ireland Ltd - Marine Energy Page 2

The potential for energy extraction from the rise and fall of the tide and its generation of local tidal streams is obvious. The degree to which this potential exists in any given location over the long term depends on the latitude, and the bathymetry, and over the short term by the relative position of the moon and the sun (i.e. lunar cycle, and time of year).
Unlike waves which are (generally speaking) driven by the wind acting on the ocean surface, tides occur as a direct result of the earth’s rotation and the effects of gravitational forces of the moon (and to a lesser extent the sun) on the ocean waters as the earth rotates.

These forces of gravitational attraction causes the ocean to sit in a slight 'bulge' shape facing the Moon, which results in water levels rising and falling twice in a day as the Earth moves through this bulge (see diagram). The Sun also creates a smaller bulge, which results in smaller or larger tides throughout the year depending on when the gravity of the Sun and Moon act in the same direction or against each over.

The largest tides and movements of water occur at new moon and full moon (spring tides), which is when the sun and moon’s gravitational pull is aligned. The lowest, occurs at the first and third quarters of the moon (neap tides), where the sun and moon’s gravitational pull are 90 degrees out of phase.

The effect of this movement of water is seen as the rising and falling of water level or tide as the waters go through various phases of ebb and flood.
Tides may be semidiurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). In most locations, tides are semidiurnal, however, because of the diurnal contribution, there is a difference in height between the two high waters on a given day. This is the case in Ireland and the UK and these tides are differentiated as the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are referred to as the higher low water and the lower low water. Since tides in Ireland and the UK are semidiurnal the potential for extracting tidal energy exists four times a day once from each flood and ebb tide.

In Irish and UK waters the flood tide flows from the west and splits at the west coast of Ireland to pass around the north and south coasts. It then continues to proceed along the west and north of Scotland, and South coast of England before notionally joining off the east coast. Some of this flood tide enters the Irish Sea via the North Channel between Antrim and the Mull of Kintyre and some via St Georges Channel in the South.

In open ocean these tidal flows are not that noticeable, however, they can be accelerated dramatically by the flows being funnelled through narrow passages or straits between islands or around protruding headlands. For this reason the locations where the energy can be feasibility extracted is limited to very localised areas of our coasts. The tidal atlas for the UK and Ireland illustrates (with hot spots in red) how few these locations are.

Because tides are purely the result of gravitational forces and not weather system driven like either wind or waves they can be predicted and this is one of the main advantages of tidal energy. In fact not only can we predict well into the future when high and flow tides will occur we can also predict what the tide rise and fall will be and with some further modelling what the tidal currents will be at any given location. In the UK the Admiralty has produced information of this type in the form of Tidal Stream Atlases for many years in order to aid safe navigation of vessels in areas where tidal streams are significant. The image below of the Pentland Firth (one of the highest tidal energy locations in the world) illustrates the tidal velocities likely to be experienced at 3 hours before high water at Dover. The numbers alongside the arrows indicate speed of tide in 10ths of Knots at Neaps, and at Springs.

Challenges in extracting the Energy
Like wave energy devices tidal energy devices also have to operate in a harsh marine environment, and although not usually quite so exposed to heavy wave loads they must be able to be installed, to operate and be maintained in areas of high currents. As illustrated by the extract from the Admiralty Tidal Stream Atlas above many areas of the Pentland Firth have flows in the order of 9knots Springs and 4knots Neaps. These are considerable currents and can impose large loads on vessels and devices operating in these waters. When the tide turns every six hours and ebbs the loads are reversed.

Tidal Technology
The following is not intended to be an exhaustive list of devices or manufacturers or an endorsement or otherwise of their technology but simply to provide a sample of devices to illustrate their general principles and a flavour of the technology and what these devices might look like.

Tidal technology devices can be classified in a number of different ways with overlap between categorisations. Probably the most obvious design element is the rotor configuration, of which there are three main categories:

Reciprocating Hydrofoils
Reciprocating hydrofoils do not rotate but are forced to move up and down repeatedly by the tidal stream as the blades pitch alters. To convert this liner movement into rotation suitable for a conventional generator the hydrofoils are connected to a linkage that is used to drive a rotating shaft, either mechanically or by using hydraulics. Two such devices are the Pulse device www.pulsegeneration.co.uk and the Stingray manufactured by Engineering Business Ltd (www.engb.com).

Horizontal Axis Devices
Horizontal Axis Tidal Turbines (HATT) – These turbines operate by the flow of the tidal stream acting on a number of pitched blades mounted on a rotor generating a rotational force in a similar manner to wind acting the conventional horizontal axis wind turbine. As the tidal currents flow over the blades, they are forced to rotate generating power. The majority of tidal stream devices under test fall under this category, and within this category there are two clear sub groups: