The results of this modeling are shown on the left for:
On the right is an impression of wind and tidal turbines to scale in a typical Round 3 water depth of 60m.
The wind turbine - at 5MW - represents the largest yet built offshore, but would have only half the power of the 10MW tidal-stream platform on the left at the same scale. 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.
Audited cost modelling shows that tidal stream energy can become competitive with offshore wind, achieving a generating cost as low as 10p/kWh at the best sites.
The modeling also shows - in a result independently audited by Black and Veatch - that the multi-turbine platform approach to tidal more than halves the cost of energy achievable with single tidal turbine installations.
The cost model bases the economics of tidal on the weight and costs of components from offshore wind, combined with industry quotes for installation and operations requirements. It assumes that the technology and components for tidal stream have been driven to a similar degree of development as has wind energy, and in large farm quantities such as 100MW.
3MW Offshore Wind Turbine
The offshore wind turbine has, according to our cost model, an installed capital cost (Capex) of just under £3m per MW. This is the going rate (2012) for Round 2 wind turbine sites, while Round 3 sites - which are further offshore and in deeper water - are heading for £3.5 and even £4m per MW. For a good site and standard discounting assumptions, the CoE (Cost of Energy) is between 13 and 14p/kWh.
2MW Fixed Tower Raisable Turbines
The twin turbines on a fixed tower carry high Capex because of the long tower with high loads and moments, and high installation cost - similar to those of an offshore wind turbine. Most appropriate for shallower water sites. On the other hand access for maintenance is good, so operating costs are low - of the order of 17 to 18p/kWh at a good site.
1MW Subsea Turbines
Single turbines - especially if they have float-out installation - have the potential for lower Capex than the raisable turbines above, typically £3.5m per MW. However, they have the considerable disadvantage of needing to be returned to base for even the smallest maintenance task, for instance the replacement of an electronic system board, signal malfunction, defouling or the resetting of a manual trip. Given the difficulty of achieving 100% reliability (even offshore wind turbines require two servicing visits per year), this is likely to put operations and maintenance costs up considerably: typically to 25p/kWh.
3MW Triton T3 Platform System
The Triton T3 platform makes a connection to the anchorage point close to the seabed, whereas single turbine systems are mounted typically at 15m off the seabed, so exerting a considerable moment on the anchorage. The consequence of this is that the Triton platform, with three turbines, can use the same anchorage as the single cantilevered turbine. Furthermore, the 3MW Triton with three turbines can have a single cable connection, whereas three single turbines will require three separate cables or a ring main and jointing system.
These factors, combined with float-out installation for the Triton, gives it tremendous cost advantages, and the cost model shows that its Capex can be reduced to about £2.6 to 2.7m/MW. Simple access means that maintenance costs can be kept low, and the resulting CoE is predicted to be only 13p/kWh.
6MW Triton T6 System
With Triton T6, the cost benefits are taken even further - to a Capex of just over £2m per MW and CoE to just below 10p/kWh.
Renewable Tidal Energy - current and stream - TidalStream development
Halving the cost of tidal energy