Cost Effectiveness

The best evidence to date is that tidal stream energy could become competitive not just with offshore wind, but with onshore wind - eventually achieving 3p/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 estimated 4½ - 5 p/kWh for offshore wind.

The most recent study has been 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.

In all of these studies, an 8% discount rate has been assumed - this is standard for the renewables industry, but of course ignores the fact that interest rates go up and down, and that some developers have access to cheaper financing than others. However, comparisons are what counts, and what is evident is that tidal stream offers at least equivalent cost effectiveness to offshore wind. As offshore wind represents one of the major planks in the UK Government's renewables strategy to limit global warming, tidal stream development should be taken seriously.

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, slack water is very short-lived - there are very few 'calm days'. This means that the capacity factor (ratio of average 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.

An impression of the comparability of tidal stream turbines with offshore wind turbines can be seen from this image of two turbines of equivalent power rating (4 MW), the tidal turbine at a 60m water depth site such as the Pentland Firth, and the wind turbine offshore in 25m of water.

The everthing else cost for tidal stream power is likely to dominate the capital cost, however for the best concepts should be no more than for offshore wind. Power/area densities for tidal stream are roughly four times greater than for offshore wind - you can get the same energy from a much smaller field area - so cabling costs will be correspondingly less.

Foundation costs are one of the largest unknowns - gravity bases have been used for offshore wind in the Baltic (where loadings are low), but piled structures are preferred for North Sea wind. Piling is almost certainly impractical in the Pentland Firth, where the seabed is rocky and the tides too strong for piling barges, and gravity bases are the only likely option. The great advantage of the SST is that the foundation carries the thrust load from the rotor directly, and not as a cantilevered moment. This means that a smaller foundation base can be used and its cost reduced.

The installation cost of tidal turbines - as for offshore oil platforms - will depend on the ingenuity of the concept designer. For any design requiring barges, floating cranes or jack-up rigs, costs will sky-rocket - and their use may not be possible in strong tidal flows anyway. The SST concept has been developed from the start not to need such support for installation, and will be subject only to the costs of cable-laying and tow-out vessels, and small handling cranes. Compared to offshore wind, where barges, jack-ups and cranes can be used but add substantially to the charge-out and availability cost, floating tidal devices such as the SST will be at an advantage and should incur lower costs.

Operating costs for all marine turbines are likely to be substantial. For tidal, maintenance access in channels such as the Pentland Firth will be difficult and potentially dangerous, as will large equipment handling support. The SST concept has been developed to reduce these difficulties and dangers - and therefore their costs - as far as possible. Although a floating device, it is connected to the seabed by a rigid arm that will give it considerable stability when on the surface. 'Landing' on it for routine maintenance or minor servicing purposes would be similar to landing on a fixed oil platform or semi-submersible rig. Given the flow regime of the Pentland Firth, such access is likely to be more difficult than for offshore wind turbines. For larger tasks, for instance changing out a generator or blade, a special-purpose lifting rig could be docked on to the SST at slack water. In comparison, major component change-out on a 60m offshore wind turbine requires a floating crane with a massive height capability, of which only a few exist.

Conclusion

At this stage, absolute cost assessments probably do not have great validity. Assessment is best made relative to a more advanced technology such as offshore wind. The indications are that:

1. Tidal stream devices in deepwater channels such as the Pentland Firth should be more effective - ie gather more energy for the power rating - than offshore wind turbines

2. Tidal stream cost per kWh for floating devices such as the SST should be roughly equivalent to that for offshore wind (more for the hardware, less for installation)

3. Operating costs for devices such as the SST should be equivalent to offshore wind (greater for routine maintenance, less for major component change-out)

4. Overall, the cost effectiveness of the SST - ie cost of energy - should be equivalent to offshore wind: the most recent study puts the cost of energy from the first 12 TWh of tidal stream energy at 3p/kWh.

5. Tidal stream turbines have several advantages over offshore wind:
       - 100% predictability of output
       - greater capacity factor, so better upstream grid economics
       - low visibility
       -  no noise
       -  higher density, so less 'land' use

6. Compared to other tidal stream devices, the SST offers:
       -  lower loads, so less material / cost
       -  simple fixed pitch rotors
       -  self-aligning stable platform for turbine operation
       -  inexpensive installation / removal
       -  stable surface platform for maintenance access

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