Peter Kydd, Parsons Brinckerhoff
May 28, 2014 | 1 Comments
Tidal power remains one of the great engineering challenges yet to be fully exploited in the modern world and yet it was first mooted more than a century ago. The oceans hold enormous quantities of potential energy that can be developed with very low greenhouse gas emissions.
There are three main types of energy that can be captured from the oceans: wave, tidal stream, and tidal range. Wave energy holds significant global potential but is the most challenging in engineering terms. Tidal stream and tidal range rely on high tidal currents and tidal range respectively and their potential is limited by the availability of suitable sites. Nonetheless, in the U.K. alone, there is some 15-20 GW of tidal energy that could be captured. Other global tidal power hotspots include the U.S., Canada, France, Russia, and Korea.
Tidal range is the most mature tidal technology. Essentially it uses modified hydropower turbines and significant civil engineering works to impound a body of water and generate power using the head difference created as the tide ebbs and floods. France led the way with tidal range development back in the 1960s and La Rance (near Saint-Malo in Brittany) was the world’s first project of this kind. Until 2011, the 240-MW La Rance project was the largest tidal power plant in the world. Some 45 years later, the original turbines are still in use and the plant is one of the cheapest forms of power generation on the EDF Energy network. Since then, tidal range power development has progressed somewhat erratically. The U.K. has studied many sites, in particular the Severn Estuary (see Figure 1). Parsons Brinckerhoff led the consulting consortium undertaking the Severn Tidal Power Feasibility Study in 2008 – 2010 and also acted as technical advisor for the SETS programme looking at embryonic technologies that could be suitable for deployment in the Severn and elsewhere.
The Canadians (at the Bay of Fundy) and the Russians have built smaller pilot plants, but it wasn’t until South Korea moved forward with its 254-MW Sihwa plant in the last decade that a modern commercial-scale tidal range project was developed.
Tidal range projects have two forms:
Figure 1: Plan on the Severn Estuary showing shortlisted options in the UK government’s feasibility study. Credit: Parsons Brinckerhoff.
Power can be generated only on the ebb tide or on both ebb and flood tides and can be augmented by adding sluicing and pumping variances to further increase generation heads.
The power generated from a tidal range turbine is calculated in the same way as a hydropower plant with the power produced being directly related to the head of water and the flow through the turbine. The power, at any point in time, is dependent upon the depth / storage relationship of the impounded basin, the difference in level between the impounded basin and the natural tide, the corresponding head and flow, and the efficiency of the turbine at that head and flow.
Normally a simple 0-D (flat basin) or 1-D (a long section through the basin) hydrodynamic model is used to take a first estimate of the power yield likely from a site, but as tidal range sites exist in dynamic estuaries where water profiles vary longitudinally and horizontally, accurate estimates require a 2-D hydrodynamic model that simulates the effects on the water profile horizontally and longitudinally. 3-D modelling is now also possible where currents are modelled at different depths rather than averaged. Figure 2 shows how energy generation changes the natural tidal cycle within the impounded basin.
Figure 2: Variation in natural tide and tidal range power generation water levels on spring tide in ebb and flood operating mode. Credit: Parsons Brinckerhoff.
Such changes in the natural tidal regime require serious consideration and the 2-D and 3-D models are used to assess the environmental and geomorphological effects that will result as a consequence of developing tidal range projects in an estuary. As will be obvious from the above, tidal range feasibility studies can become quite complex with long development cycles. However, intelligently executed tidal range projects can be a sustainable form of predictable low-carbon generation with the potential to reduce the cost of electricity over time.
Figure 3: Aerial view of La Rance Tidal Barrage. Credit: EDF.
Tidal range turbines are large and have to be housed in even larger concrete structures. A 40-MW bulb turbine would typically have a rotor diameter of about 8m, rotating at around 60rpm (revolutions per minute). They also require substantial civil engineering works to form the impounding basin. Figures 3 (above) and 4 (below) show the La Rance tidal barrage and a computer generated image of a tidal lagoon respectively.
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