Power from Tidal Currents — grossly undervalued

The Carbon Trust’s Future Marine Energy (January 2006) gave a low figure for the total UK resource and relatively high costs for power from marine currents. How reliable are its projections?

The report did admit the costs are uncertain and conclude that

  • tidal streamtidal stream The flow of water through channels or around coastlines as a result of tidal water movement energy could become competitive with the current base costs of electricity within the economic installed capacity estimated for the UK, 2.8 GW.

Graph showing tidal current costs

Indeed, the ultimate price given of 3p/kWh is very competitive [see diagram]. That is reached without the present 3.5p allowance via the RO and CCL. This makes it a very good or even the best renewable option. Yet in the Report, the figures given up-front are off-putting – central estimates of 12-15p/kWh, within an uncertainty band of 9-18p/kWh.

One question is why such a negative presentation? Unfortunately the BWEA repeated the pessimistic figures with little reservation in “The Path to Power. Delivering confidence in Britain’s wave and tidal stream industry” www.bwea.com/pathtopower [Note 1].

Total Marine Current Resource

The Carbon Trust’s figure of 18 TWh/yr for the UK resource comes from a Black & Veatch study for the Trust

  • this should be compared with the DTI’s previous estimate of 70 TWh/yr in the UK Tidal Stream Review (1992), totalled over the eight largest sites. A definite figure like 18 is of course not to be trusted.
  • Total resource is normally related to a price level. As a very good price is found for the best 18 TWh/yr, a lot more is likely to be accessible for the target cost of 6p/kWh.
  • Prof. Salter says the resource is far under-estimated, from his studies of the Pentland Firth; speaking at the Bristol WATTS2006 conference he said the consultants had not asked experts in the field and their estimate was out by a factor 10.

Studies of the Pentland Firth north of the Scottish mainland give Prof. Stephen Salter a figure for the tidal power lost in “bed friction” though that channel of 120GW. He argued that 10% of that figure could be extracted by tidal power devices. This number of 12GW is over four times the figure of 2.8 GW for the total UK resource given by the Carbon Trust/Black & Veatch. Unfortunately, the Sustainable Development Commission in their tidal power assessment accepted the low figures without examination.

Cost of Marine Current power

EDF’s speaker at the Bristol conference Simon Merriweather reported that they are backing the MCT (Marine Current Turbines) design. Following testing of the small version ‘Seaflow’ in the Severn estuary off Lynmouth, Devon, the commercial 1.3 MW prototype ‘SeaGen’ recently erected in the Strangford Loch narrows is expected to supply power at ~9p /kWh (compared with 18 p/kWh at Lynmouth), while a 20MW array of them scheduled for 2007-8 will supply power at ~6p/kWh. EDF’s presenter said they are “absolutely clear” they will get to 4p/kWh (even 3p/kWh shown with an improved design). They plan from 2009 to be building commercial installations of over 50 MW wherever they can and look to a global market of thousands of MW.

While EDF predict they will achieve 6p /kWh with their first 20MW array, the Carbon Trust’s consultants ENTEC predicted 6p /kWh would not be reached until after much learning and economies of scale, at the point when between 500 and 1500 MW had been installed (their Figure 10).

What’s wrong with ENTEC’s assessment in the Carbon Trust report?

ENTEC did not consider the most promising devices, but considered the range of costs of many planned devices (9 full-scale prototypes and perhaps 11 part-scale models).

  • ENTEC accept this can be “misleading due to the influence of unpromising high-cost devices”
  • ENTEC use prototype costs, while admitting they are a poor indicator of costs
  • Costs of first production models (of which few are available) are much higher than batch costs
  • ENTEC infer O&M costs from offshore wind farms and oil/gas installations (far bigger engineering structures)
  • ENTEC assume a very high rate of return, 15%, to reflect the risk of new devices. This doesn’t reflect government/EU grants or envisage that supply companies like EDF are willing to support loss-leaders and win PR.

ENTEC admitted these problems to some degree, but proceeded by assuming figures for economies of scale and of learning, which they took from experience with wind power development. They give a schematic (their Fig.5) for performance characteristics for a tidal current generator analogous to that of a wind turbine – despite the crucial engineering difference that the former operate under the highest current speeds while the latter are designed to cut out at high wind speeds.

The ‘learning rate’ of 5-10% given to ENTEC by Black & Veatch is not validated; much of the improvement in wind power economics has been through larger and larger scale turbines. Second, what is taken as the start cost for new devices injects a lot of uncertainty – the EDF figures show the start could have been 18 p/kWh (the highest in ENTEC’s range) or 9 p/kWh (the lowest in their range).

Of course ENTEC looked at key sensitivities in order to derive a range of predictions. They showed start costs make a lot of difference in projecting wave power costs (Fig.10c), but not for tidal cost projections. And, as is common, they didn’t consider the full range of uncertainty because that would admit to such large uncertainty as to render their report near useless.


Politicians have taken as definitive the Carbon Trust’s figures of high costs and small potential contribution from tidal current turbines. The high costs in its summary give a false impression of future costs in the text. But even these ENTEC figures are overly pessimistic. The EDF costings coming so soon after surely show us that “consultants” cannot be trusted. We remember that consultant economists (AEA) some 15 years ago killed off the UK’s wave power programme with faulty costings (Power From the Waves David Ross, Oxford, 2nd ed 1995, ISBN 0 19 856511 9).

Max Wallis, Cardiff, December 2008

Note 1: The BWEA’s steering group of managers adopted the Carbon Trust’s figures in their June 2006 Path to Power with hardly a reservation – just a comment that “some device developers believe … the costs ranges … are too high.” It seems the steering group failed to ensure independent critical scrutiny, having ENTEC’s senior consultant among their number. Npower Renewables funded the Path to Power project and were also represented on the steering group; they have since become a partner to MCT, planning to erect the first marine turbine cluster off NW Anglesey.

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