The best evidence to date is that tidal stream energy could become competitive not just with offshore wind, but with onshore wind - eventually achieving 5p/kWh at the best sites.

Early estimates - for instance in the 1993 DTI report - were rather higher at around 10p/kWh for the best sites. However, these studies did not take full account of learning curves - eg the way in which wind turbine costs have plunged with experience - and were also flawed in appreciation of the proper rating of turbines (optimising rotor diameter to power ratios) and the dramatic effect of this on costs.

More recent studies - for instance the 2001 AEAT Study - estimated 4p/kWh for the first 1.5 TWh or so of energy, with the cost rising sharply after that as the (few) shallow, high flow sites are all used up. However, this study assumed bottom-mounted devices such as the MCT, and did not take into account development of floating concepts such as the SST proposed here. The same study gave estimates of 4½ - 5 p/kWh for offshore wind.

A 2005 study published by The Carbon Trust indicates a broadly saucer-shaped curve for the progress of costs over time.Initial costs will be high - of the order of 7p/kWh - because the technology is new, even though the best and most shallow sites are being developed. The cost then falls to around 3p/kWH after 12 TWh of energy (or 3000 MW of power capacity) is installed. Costs then rise again as only the lower flow sites are left.

The SST Concept
The next relevant question to ask is whether the SST concept proposed here can be cost-effective in comparison with the best tidal stream concepts studied above. There are three elements to the cost equation that determine cost effectiveness:

   1. The 'effectiveness' of the technology
   2. The capital cost of the installed devices
   3. The operating cost of the devices

The effectiveness of horizontal axis rotors to capture energy is not really in question. The fact that almost all wind turbines use this approach in preference to others (vertical axis rotors, flapping aerofoils, ducted rotors etc) is a good enough indication.

However, compared to wind, tidal stream has the advantage of a better velocity distribution - energy-producing flows are present for a greater percentage of the time than for wind. In areas of high tidal flow, ack water is very short-lived - there are very few 'calm days'. This means that the capacity factor (ratio of rage power to rated power) at a site such as the Pentland Firth can be of the order of 50%, whereas for wind sites it rarely exceeds 40%. Of course, this is also a function of the rating of the device, but the broad conclusion must be that tidal stream capacity factors are significantly greater than for wind energy.
 

The capital installed cost can be broken down into:

 
   - the cost of the device itself
   - the cost of everything else required to make it operational: - foundations, cabling, installation,

     grid - connection

The device cost for an onshore wind turbine is something like 75% of the capital cost; offshore this proportion is less than half. The device cost for horizontal axis tidal turbines will probably be similar - all need blades, hubs, transmissions and generators, support structures; and all need to be marinised for prolonged underwater use. In comparison with wind turbines, the rotors are not so large, but the steady loads are much higher. On the other hand, the structures do not have to cope with extreme storm conditions - tidal flow is not extreme in that sense. Marinising will undoubtedly increase costs relative to wind, but marine and offshore oil & gas technology is well developed, especially in the UK. Overall, it is likely that tidal turbines such as the SST will be more costly than equivalently rated wind turbines, but similarly cost effective.

              

Costs

A scaled comparison of offshore tidal stream and wind turbines in typical water depths (60m and 25m respectively). The wind turbine - at 5MW is the largest yet built, but would have only half the power of the 10MW tidal-stream platform. With float-out installation and accessible maintenance it is not hard to see that the tidal-stream system could have significantly lower capital and operating costs than the wind turbine.

TidalStream TRITON

The enabling technology for tidal turbine deployment

Provides the most cost effective and  efficient turbine deployment system

Up to 10MW from a single installation

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