It is often claimed that the UK possesses a disproportionate share of Europe’s wind resources (with figures like 40% being quoted) and about 50% of Europe’s tidal stream resources (Armstrong, 2010). Perhaps this is not surprising for a cluster of remote islands off the north-west coast of Europe. There has been much written (supportive and otherwise) about aspirations for large-scale deployment of offshore wind, with plans for 33 GW of capacity being mooted. Other marine energy technologies are under development but are at an earlier stage in the development process. Whereas the wind turbine industry has largely standardised on the basic concept of a three-blade, horizontal-axis device, there are 40 or more different concepts for what a wave or tidal stream power device should look like. Less progress, therefore, has in general been made in driving down the cost per MW. Consequently, other marine energy technologies tend to appear further down the timeline in the various renewable energy roadmaps. This issue of the Journal takes energy in the marine environment as its broad theme and looks at a number of different technologies.
The paper by Willis et al. (2010) looks at the potential for tidal stream turbines in the Bristol Channel off the coast of Wales. With tidal ranges of the order of 13 m, tidal flows in excess of 3 m/s and close proximity to national grid connections, the location offers some attractive prospects. Research involving marine engineers, marine biologists, environmental scientists and key stakeholders has enabled the selection of an ideal site for a demonstration tidal turbine. The work involved assessing the impact on other users of a busy stretch of water, evaluating the need for navigational aids, assessing the impact on particular fish species with high conservation value, and computational fluid dynamic modelling of individual turbines and arrays of turbines to estimate performance efficiency and the impact on marine life, the sea bed and suspended sediment levels. It is interesting to note the effective combination and phasing of desk-based research, computer modelling and field work in this wide-ranging assessment. They also look at how tidal stream and tidal barrage plans for the Severn estuary might conflict with one another.
A more detailed treatment of the proposed Severn barrage is provided by Ahmadian et al. (2010) in their development of a hydro-environmental model. They describe the well publicised adverse effects of a barrage (e.g. losses of intertidal mudflats) and point to environmental and ecological benefits (which are less well known), such as reductions in suspended solids levels and reductions in turbidity leading to better light penetration through the water column and reduced levels of enteric bacteria. The numerical model looks at dispersion, diffusion, decay, erosion, deposition and absorption, and can produce data on health risk assessment for bathing water and on flood hazard risk. Having validated the model against data for the status quo (viz no barrage), they go on to quantify the effect of a barrage on peak water elevations, suspended sediment levels, bacteria levels, etc. All of this is based on a barrage design which could supply 5% of the UK’s electricity consumption.
Returning to tidal stream turbines, O’Doherty et al. (2010) look in more detail at a horizontal-axis configuration for two locations – the Severn estuary (as above) and Anglesey. For each location they have collected high-resolution data on seabed depth profiles (using sonar beams) and velocity-depth profiles (using a vessel-mounted acoustic Doppler current profiler) in order to build their computational fluid dynamic models. The models show that power generation is reduced by 30–40% relative to simple theory as a result of the reduction in water velocity with distance down the water column. This is particularly significant in view of depth constraints to provide adequate clearance for passing ships, with power output being related to the cube of the tidal velocity. They conclude that the Anglesey site with its higher tidal velocities is likely to be economically viable while the Severn estuary location is unlikely to generate sufficient power.
For a very different perspective on energy in a marine environment it is instructive to look at energy supply on a remote northerly island where the costs of energy supply (in all its forms) are very different to mainland Britain. Martin and Spence (2010) look, in particular, at heat energy supply via a district heating scheme in the town of Lerwick on the Shetland Islands. Their fascinating paper traces developments from the first studies in 1997 through ten years of expanding operation through to the present day. It is interesting to learn how over time they solved the ‘Catch-22’ problem of how to grow supply and demand. Shetland has no mains gas, no connection to the national grid and some of the highest fuel costs in the UK. The installed scheme is based on Danish moving-grate technology which burns waste (from Shetland, Orkney and Shell offshore platforms) with 80% efficiency to supply a growing district heating network. The paper explains key decisions along the way such as choice of emissions clean-up technology, metering arrangements, training of local plumbers, backup arrangements to cover maintenance, retrofitting of a 300 m3 heat store and policy on incremental extension of the pipeline network. As for measures of success, the scheme saves half a tonne of CO2 per year per Shetlander, it delivers a net economic benefit to the Shetland economy and residents are lobbying for the scheme to be expanded.
So, faced with challenges such as reducing CO2 emissions by 80% by 2050, sourcing 15% of all UK energy from renewables by 2020, generating 35% of UK electricity from renewables by 2020 and other such targets, it would appear that there is a lot of potential in the marine environment. This potential goes beyond ambitious aspirations such as 25% of electricity from offshore wind to include (say) 5% from a Severn barrage and 3% from tidal stream. And if remote locations like Shetland can follow the district heating lead set by Scandinavia and some UK cities such as Sheffield, perhaps the targets are within reach after all.
