Tidal movements result from an interaction of the gravitational fields of the sun and moon with the earth’s water masses. The global potential for tidal power is an estimated [22] 6 TW, but there are just a few special locations for its economic exploitation. Specifically they have the appropriate orientation to access the Coriolis forces created by the earth’s rotation and a shape whose natural oscillatory frequency closely matches that of the tides [23]. Depending on the situation, then principally[4] tidal range or tidal stream systems are the flexible means of extracting the energy. In the former mature technology, water flows at a flood tide are trapped behind a dam or lagoons and in the process drive horizontal low-head turbines during parts of both flood and ebb tides. Suitable sites include the Severn Estuary (UK), La Rance (France), Bay of Fundy (Canada) and some others where the tidal range exceeds the necessary 7 m for an economic development [22]. Though lagoons as a means of reducing upstream ecological damage were considered for the Severn Estuary scheme, they were subse­quently rejected for the induced scouring of the interstitial seabed and their cost-effectiveness [23]. In fact there are no tidal lagoon schemes presently in the world.

Tidal stream devices extract a portion of the kinetic energy from relatively fast tidal currents as for example at the UK sites of Pentland Firth, Strangford Lough, and Alderney. Various blade designs are under development for the Kaplan-type turbines, but as yet no real choice exists with regard to efficiency and cost-effectiveness [23]. Individual units can provide up to 1 MW, so “farms” of as many as 30 are planned in order to justify the provision of substantial new Grid connections to prevent transmission “bottlenecks” that would otherwise exist between these generating units and the centers of largest electricity demand [23]. Other issues can be identified by assessing the potential of two powerful tidal steams in New Zealand to provide a material portion of its 13 GW demand [116]. These particular steams peak around 5h apart, so their combined power profile assumes the form

P = P0[2 + cos 0 + cos (0 + f)] (1.3)

where

0 = at; a = p/6 and f = 5p/6

Extreme values occur at

tan U = —b/a; b = sin f and a = 1 + b giving the ratio

Pmax : Pmin = 1.33 : 1.0 (1.4)

Granted sufficient tidal energies, equation (1.4) shows that even at minimum flow enough Kaplan turbines could meet a specific quota in any 24—h period. The excess power at higher flows could simply be rejected by de-exciting individual generators. Though apparently very promising, some 5000 of the presently largest 1 MW turbines would be required for 5 GW. Moreover, powerful tidal streams (~5 m/s) offer very restricted safe diving windows[5] and are likely to entrain amounts of grit that could erode turbine blades. Because installation, intercon­nections and maintenance do not benefit from scale only much smaller stations appear viable. In this respect the 30 MW trial at Strangford Lough should be definitive in terms of reliability, scale, and costs. In contrast the proposed 16-km-long Severn Barrage between Cardiff and Weston would have produced a material maximum output of 8.46 GW that is closer to the largest demand centers and 4.7% of UK national consumption.

Flood and ebb tides each occur twice daily and repeat about 1 h later each successive day. Also tidal ranges and stream speeds vary periodically over the 28-day lunar cycle due to the changing align­ment of the sun and moon. The electrical output of a tidal generator varies in like manner so it is frequently out of step with a grid network’s daily demand. Nevertheless tides are regular and largely predictable [24] so tidal generators can be readily incorporated into the largely predictable daily load schedule of a Grid network. Furthermore the relatively low height of a tidal barrage results in low mechanical stresses so that unforeseen outages are rare. Indeed the La Rance 240 MW plant has performed without a major incident for some 40 years since its construction in 1966. At 2006 prices, maintenance costs for the Severn scheme are estimated at £139M or $214M per year which is about 1% of total capital costs, and

Tidal barrages affect the physical, chemical and ecological features of an estuary. La Rance is the one materially sized plant in the world, but studies of its environmental impact have been limited [23]. However, insight can be gained from observations on harbor walls, jetties, bridges, or breakwaters. Though a barrage mitigates upstream flooding by tidal surges, its associated storage would regularly flood marshes used for previous centuries by wildlife, and it may also reflect tidal surges to damage downstream areas. Water management at La Rance creates the advertised “largest whirl­pool in the world” and has become a frequently videoed tourist attraction. Also changes in hydrodynamics, dissolved oxygen and salt concentrations can produce radical changes in local flora and fauna as well as to sources of public drinking water. At La Rance for example sand eels and plaice have been replaced by sea bass and cuttlefish. Sediment accumulated behind a barrage not only requires regular dredging as in a conventional hydro scheme, but the estuary’s topography and shipping channels could be altered by the modified silt deposits. A subdued water flow behind a barrage may also concentrate human and industrial effluents. The “footprint” of a tidal barrage scheme is highly significant for the densely populated United Kingdom, and that for La Rance is 9.38 hactares per installed MW.

Though La Rance was originally intended as a prototype of many tidal stations supplying a material portion of France’s electricity requirements [27], nuclear power development became the final choice. Reasons for EDF’s decision have apparently not been published [27], but the above environmental issues must have entered the argument. Whatever the truth, the nuclear choice became very profitable. After the unification of Germany in 1990 and with Chernobyl probably influenc­ing matters, the Russian RMBK reactors and heavily polluting lignite­burning generators in the DDR were decommissioned. The resulting energy gap was filled by energy from France’s largely[6] nuclear plants, which also buffer the UK’s grid network by the 2GW cross-channel ac connector. In the case of the Severn Barrage, BBC News announced on 9th February 2010 that the estuary would be “devastated” and on the 5th September the Guardian newspaper wrote that governmental support had been withdrawn.