Saturday, 25 April 2009

Tidal Power - David JC MacKay; the beauties of tide

extracted from
Sustainable Energy - without the hot air

Beauties of tide

Totting everything up, the barrage, the lagoons, and the tidal stream farms could deliver something like 11 kWh/d per person (figure 14.10).

Tide power has never been used on an industrial scale in Britain, so it’s hard to know what economic and technical challenges will be raised as we build and maintain tide-turbines – corrosion, silt accumulation, entanglement with flotsam? But here are seven reasons for being excited about tidal power in the British Isles.
1. Tidal power is completely predictable; unlike wind and sun, tidal power is a renewable on which one could depend; it works day and night all year round; using tidal lagoons, energy can be stored so that power can be delivered on demand.

2. Successive high and low tides take about 12 hours to progress around the British Isles, so the strongest currents off Anglesey, Islay, Orkney and Dover occur at different times from each other; thus, together, a collection of tide farms could produce a more constant contribution to the electrical grid than one tide farm, albeit a contribution that wanders up and down with the phase of the moon.
3. Tidal power will last for millions of years.
4. It doesn’t require high-cost hardware, in contrast to solar photovoltaic power.
5. Moreover, because the power density of a typical tidal flow is greater than the power density of a typical wind, a 1 MW tide turbine is smaller in size than a 1 MW wind turbine; perhaps tide turbines could therefore be cheaper than wind turbines.
6. Life below the waves is peaceful; there is no such thing as a freak tidal storm; so, unlike wind turbines, which require costly engineering to withstand rare windstorms, underwater tide turbines will not require big safety factors in their design.
7. Humans mostly live on the land, and they can’t see under the sea, so objections to the visual impact of tide turbines should be less strong than the objections to wind turbines.


Tidal power, while clean and green, should not be called renewable. Extracting power from the tides slows down the earth’s rotation. We definitely can’t use tidal power long-term.

False. The natural tides already slow down the earth’s rotation. The natural rotational energy loss is roughly 3 TW (Shepherd, 2003). Thanks to natural tidal friction, each century, the day gets longer by 2.3 milliseconds.
Many tidal energy extraction systems are just extracting energy that would have been lost anyway in friction. But even if we doubled the power extracted from the earth–moon system, tidal energy would still last more than a billion years.

Notes and further reading

page no.
82The power of an artificial tide-pool. The power per unit area of a tide-pool is derived in Chapter G, p311.

Britain is already supplied with a natural tide-pool . . . known as the North Sea. I should not give the impression that the North Sea fills and empties just like a tide-pool on the English coast. The flows in the North Sea are more complex because the time taken for a bump in water level to propagate across the Sea is similar to the time between tides. Nevertheless, there are whopping tidal currents in and out of the North Sea, and within it too.

83The total incoming power of lunar tidal waves crossing these lines has been measured to be 100 kWh per day per person. Source: Cartwright et al. (1980). For readers who like back-of-envelope models, Chapter G shows how to estimate this power from first principles.

84La Rance generated 16 TWh over 30 years. That’s an average power of 60 MW. (Its peak power is 240 MW.) The tidal range is up to 13.5 m; the impounded area is 22 km2; the barrage 750 m long. Average power density: 2.7 W/m2. Source: [6xrm5q].

85The engineers’ reports on the Severn barrage...say 17 TWh/year. (Taylor, 2002b). This (2 GW) corresponds to 5% of current UK total electricity consumption, on average.

86Power per unit area of tidal lagoons could be 4.5 W/m2. MacKay (2007a).