Marine renewable energy
Commodore Steven Jermy RN, co-founder and managing director of Tidepod Ltd, looks to the future of a promising renewable energy source
As Western economies bounce along the bottom, with the shipping consequences reflected in the recent historic lows in the Baltic Dry Index, it is refreshing to know that there is at least one nautical sector where there is a sense of optimism – marine renewable energy.
Although governments and the press regularly conflate and confuse the three marine renewable energy types – offshore wind, wave and tidal – they each rely on very different sources with very different characteristics.
They are also each at very different commercial stages. Offshore wind energy is a reasonably mature sub-sector, with many if not most of the technological problems solved. Contrastingly, wave energy is still in a very early R&D phase, with no commercial device consistently anywhere in the world. In between the two is tidal energy, not yet deployed on a large scale, but with considerable momentum and poised for commercial take-off.
How does it work?
Tidal energy can be harvested in two ways: tidal range, where the energy is classically extracted through the use of large-scale barrages across lagoons or estuaries, with the enclosed waters then acting in a very similar way to water confined by hydroelectric dams; and tidal stream, where the energy is extracted using underwater turbines akin in many ways to wind turbines.
Essentially, tidal range schemes are coastal civil engineering projects, whereas tidal stream schemes draw heavily on marine operations expertise and will be of increasing interest to members of the Nautical Institute. It is this kind of scheme that will be discussed here.
The first key point is that, as with wind, the power that is available from a site varies with the cube of the tidal stream speed. The consequences of the cube law are important, and threefold. First, there is a minimum threshold below which, using most current technologies, the resource is likely to be uneconomic – as an early rule of thumb, the lower stream speed limit is in the 3–4 knots range. Second, the difference in harvestable power available between neaps and springs is very significant. Third, the difference in harvestable power available between medium- power sites, such as the races south of Portland and the Isle of Wight, and high-power sites, such as the Pentland Firth and Alderney Races, is again very significant, with the latter having levels of energy that are an order of magnitude higher than the former.
These issues accepted, tidal power clearly has inherent advantages over many other energy sources, be they fossil fuel or renewable. Unlike oil and gas, the tidal resource is already well mapped out, with no need for expensive exploration or drilling. Unlike offshore wind and wave, the resource is periodic, and predictable centuries ahead. And finally, because of the density of sea water, tidal sites have very high energy density, typically at least ten times those of wind sites, and very often much higher. For all these reasons, the resource is a particularly attractive prospect.
Putting theory into practice The UK is currently the world leader in the race to exploit tidal resources – maybe because, through oceanographic luck, the British Isles are host to between 10% and 15% of the world’s overall tidal energy resource. It currently hosts four times as many tidal energy companies as the US (the second major player), and has nearly a dozen universities engaged in cutting-edge tidal energy R&D.
A well-known example of the current technology is the SeaGen device, which is the first commercial-scale tidal stream energy device in the world. Thanks in no small part to the seminal design work of Peter Fraenkel, the design engineer, and the leadership of Martin Wright, a former naval officer, the MCT SeaGen turbine has been operating now for over four years in Strangford Loch. Although installed for R&D purposes, it has been delivering power to the grid with very high consistency. Informal sources suggest that the SeaGen’s load factor (the time for which the turbine is operating at its design loading) is close to 70%, which compares exceptionally well to typical load factors in the UK onshore wind sector – 22% in winter and as low as 10% in summer, according to OFGEM – and is further concrete evidence of the unusual potential of the tidal energy resource.
This is not to say there are not challenges to overcome in the sector; far from it. Securing turbines to the seafloor in high-energy tidal sites is extremely demanding in both an engineering design and marine operations sense. It is quite different from the challenges faced elsewhere offshore, for example, in the oil and gas sector or offshore wind sectors.
The navigator’s rule of thumb for wind and stream effects on a vessel during ship-handling gives a good sense of the scale and nature of this challenge. With 1 knot of tidal stream broadly equivalent to 10 knots of wind, it follows that trying to secure a tidal turbine to the seafloor in a 10 knot tidal stream, such as that at Pentland Firth, is akin to trying to construct a wind turbine in a hurricane. For this reason, marine operations are largely confined to slack water windows. And, thus, the expensive dynamic positioning (DP) vessels and jack-up barges that are used to deploy current technologies are inevitably sailing idle for much of their expensive working day. This has a large effect on tidal energy installation costs, which are still much higher than most other renewable sectors.
But, with the strategic and physical advantages of the tidal resource and the pressure on global energy prices in so many other areas, there is a will in the sector to confront and solve these challenges. Solutions are likely to be found in, for example, bespoke DP vessels and seabed foundation technologies that can be deployed within the confines of just a small number of slack water windows. These technological challenges are also investment opportunities. Any company that can solve them for locations in the UK, which has some of the most aggressive tidal sites in the world, will be extremely well placed to compete successfully for the installation, operations and maintenance, and supply chain opportunities in the global tidal energy market, which is expected to scale significantly from 2017.
It is for this reason that the sector is beginning to attract marine companies and people from a range of nautical backgrounds – marine geologists, physical oceanographers, naval architects, fluid dynamicists, marine engineers and, last but by no means least, mariners, particularly those with experience of the offshore construction sector, but from elsewhere too.
And it is for this reason that, as well as working hard on the design of tidal turbines and foundations, some of these companies and people are beginning to turn their minds to the ships needed to service this sector, not least because it is not at all clear that the vessels currently employed in other offshore sectors will adapt easily and economically to the tidal challenge.
Although the tidal energy sector still has real technological challenges to confront, and financial risks to manage, for those companies and people that do have the courage to take the plunge, and are then successful, there will be commensurate and consistent rewards, running into the long term. With apologies to William Shakespeare: ‘There is a tide in the affairs of men which, taken at the flood, leads on to fortune … and nautical fortune, at that!’
This is an edited version of an article first published in Seaways, the journal of the Nautical Institute. Reproduced by kind permission of the Nautical Institute.