Peter Kydd, Parsons Brinckerhoff
May 28, 2014 | 1 Comments
Figure 4: Computer generated image of a tidal lagoon in the Severn Estuary. Credit: Parsons Brinckerhoff.
Tidal stream technologies differ from tidal range in that they exploit the tidal currents rather than the tidal range. They have emerged over the last fifteen years and, in the simplest terms, can be described as an evolution of the wind turbine, adapted to operate in the more challenging environment where the fluid density is an order of magnitude greater, and suitable locations in terms of depth and current are more challenging to find.
Tidal currents of 2.5m/s (metres per second) or more are required to generate power efficiently.
Tidal stream turbines have a much lower power density than conventional tidal range turbines and the power calculation is also quite different, being related to the swept area of the turbine blades and the cube of the current flowing through the turbine. As power is proportional to the cube of the current, tidal stream sites require significant currents to generate power economically. The greater the current, the smaller the rotor diameter for a given power output. However, rotors for relatively small tidal stream devices can be 12 to 20m in diameter, rotating at around 15rpm. The largest tidal stream device currently being developed is 1.5 MW. This illustrates two of the main engineering challenges with tidal stream turbines – the structural design of large diameter turbine blades and placing of turbines and cables in fast flowing waters.
Tidal stream devices can be deployed singly (e.g., at Strangford Lough in Northern Ireland), in arrays (as proposed for two sites in the UK), or as a fence (as was proposed for the SETS programme in the UK). An array is comprised of a number of turbines co-located in a grid, whereas a fence is comprised of a number of turbines set out in a single line across an estuary.
Some of the world’s largest turbine manufacturers have now entered the tidal stream turbine arena with Siemens purchased of Marine Current Turbines (MCT), the technology used at Strangford Lough; Alstom’s acquisition of Tidal Generation Limited; and Andritz’s Hammerfest Strom turbine. Figure 5 shows Siemens’ MCT installation at Strangford Lough.
Figure 5: Cutaway of the Strangford Lough tidal stream turbine. Credit: Sea Generation Ltd.
Tidal Power in Practice
One of the largest tidal ranges in the world occurs in the Severn Estuary bordering South West England and Wales (see Figure 1 above). At its highest point, near Bristol, the tidal range is 14.4m, or over 47ft.
In 2006, the government reviewed its energy policy and, with the enactment of climate change legislation that committed the U.K. to an 80 percent reduction in carbon emissions by 2050, commissioned new studies on low carbon generation, including tidal power. The initial studies were undertaken by the government’s Sustainable Development Commission, which recommended that further, more detailed studies should be undertaken on the Severn.
Following an invitation to submit proposals, a consortium led by Parsons Brinckerhoff was appointed to lead the two-year study, and a long list of over 10 potential “significant” options were evaluated before reducing this to a shortlist of five, three barrages and two lagoons. “Significant” in this context meant large – typically more than 1 GW of installed capacity. However, the study concluded that the size of the five feasible options was directly related to the adverse environmental impacts, particularly for the barrages. One of the land-connected lagoons, Bridgwater Bay (L3d in Figure 1), however, came out of the study with some significant benefits in terms of reduced impacts on navigation and with potentially acceptable environmental impacts.
Completion of the study in 2010 encouraged a fresh look at the evidence base that had been created to identify what opportunities should be followed up, if any. The Bridgwater Bay lagoon was an obvious starting point but had a capital cost of more than £10 billion [US $16.7 billion], which was not particularly attractive in those days shortly after the credit crunch.
The focus changed instead to smaller tidal lagoons, with better ground conditions and more innovative impoundment designs, so that the potential of lagoons could be demonstrated. Parsons Brinckerhoff’s concept was that a series of smaller tidal lagoons could form the basis of an incremental approach, whereby the first lagoon is built, and experience gained from the first is then used to build a second, and so on.
Parsons Brinckerhoff, assisted by Black & Veatch, worked up a conceptual design for what has been called the Stepping Stones Tidal Lagoon (the semi-circular lagoon in Figure 1). This was, at £1.7 billion [US $2.85 billion] capable of being developed by the private sector. It had a rated power output of 600 MW and an annual energy yield of 1.2 TWh/yr (terawatt-hours per year) with a lower cost of energy than the previously favored barrage proposals. In addition, tidal lagoons had a bigger advantage, namely, they did not interrupt the main navigation routes and environmental impacts were largely confined to the area within the impounded basin. As such they don’t require extensive navigation or environmental mitigation measures.
At the same time, Regen SW, the regional energy advisory organization, along with the newly created South West Marine Energy Park (a government initiative to focus marine energy resources within a geographical region) published a balanced technology approach for the Severn Estuary that promoted the concept that different technologies could be deployed collaboratively with both existing and emerging technologies, at appropriate scale, offering a more sustainable approach than one single mega-project. It was a simple matter to integrate Parsons Brinckerhoff’s incremental approach, based on tidal lagoons, with the balanced technology approach and its mix of technologies to produce a single best-practice approach, with practical project examples and consideration of how a phased development could use both existing technology today whilst still allowing scope for emerging technologies to be used in the future.
Both the UK government and the select committee on energy and climate change endorsed the balanced technology approach in early 2013. Many environmental and commercial stakeholders have also endorsed it as a more sustainable and appropriate approach than a large barrage.
Even though to date no technology has been ruled out on the Severn, including a large barrage; nonetheless, Parsons Brinckerhoff’s role in searching for a more sustainable alternative, exemplified by the Stepping Stones Lagoon, informed by its work on the Severn Tidal Power Feasibility Study and the associated SETS program, has been helpful in informing stakeholder perspectives. Working with the South West Marine Energy Park, the company has also been active in assisting local authorities and stakeholders in planning how energy from the Severn could be successfully developed in an environmentally sustainable and economically beneficial way.
Although the Severn is one of the largest tidal energy resources in the world, there are also many others that are being investigated to see how and when tidal energy may be generated. For such a nascent industry, there are many organizations, large and small and including Parsons Brinckerhoff, taking a keen interest in marine energy in parts of the U.K., France, Canada, the U.S., and Korea, to name but a few. The stakeholder engagement model being used on the Severn to develop a transparent, incremental, and evidence-led approach could and should be applied to other potential marine energy development zones around the world.
With many countries actively seeking to reduce carbon emissions from their energy use, the coming decades are likely to see a significant increase in the amount of marine energy being used worldwide.
The author would like to thank the many stakeholders of the South West Marine Energy Park who have worked collaboratively to promote the benefits and challenges of marine energy, as well as those colleagues at Parsons Brinckerhoff who have worked on the various tidal energy studies undertaken in the UK in recent years.
With over 52,000 subscribers and a global readership in 174 countries around the world, Renewable Energy World Magazine covers industry, policy, technology, finance and markets for all renewable technologies. Content is aimed decision makers...