The source of this energy is not even indirectly associated with the sun. Instead it draws on the rotational energy of the earth:moon system. Drawing on it is extracting energy from the orbital motion of the moon, thus offsetting by a minuscule amount, the steady outward spiral of the moon. This is an inexhaustible energy source though it is very hard to tap and cannot provide continuous output. The consequence of this is that all tidal schemes require a solution to the bulk electricity storage problem or the provision of backup generation capacity because electricity is only generated when the tide is running.
Using tidal power requires uncommon geographical features. For instance, La Rance Station, near St.Malo on the Brittany peninsula in France is a barrage across an estuary with a large tidal range. The barrage directs the tidal flow through a set of turbines for a peak output of 240 mW.
The proposed Severn Barrage offers around 17 TWh annual capacity (6% of the UK's annual consumption) or 2 GW average output (8.6 GW peak), but there's no sign of bulk energy storage in the plans other than that provided by the large Severn tidal baisin. The design is not capable of producing output at slack water.
In New Zealand Cook Strait, Foveaux Strait and French Pass have unexploited tidal currents of 8 kts or above. The kinetic energy in these flows is large, which in turn makes large amounts of electricity available from relatively small plant that doesn't require dams or barrages. Neptune Power is a NZ startup promoting the Cook Strait tidal turbine project, which would anchor up to 7000 twin turbines in Cook Strait. They hope to install a test unit during 2009. An advantage of these schemes is that they are modular and can start generating as soon as the first turbines are in place. The main disadvantage is that they would need a lot buffering electricity storage because the output from even one of them would be a large fraction of the total national generating capacity. Fortunately New Zealand's electricity base-load comes from hydro generation, which can be started and stopped easily. Although current hydro schemes are now at the full capacity permitted by lake levels they can be used as buffers. If tidal turbines were fitted to Foveaux Strait, for example, it would be possible to run the Manapouri power station at full power during slack water and reducing its output as the tidal system takes over the load during tidal peaks. This will increase the effective capacity of the buffering hydro schemes by conserving their lake levels.
Americans and Canadians may be able to exploit tidal flows in the Inland Passage, which runs up the Canadian west coast from Vancouver to Alaska.