Wave Power: Stealing the Juice?
Posted on February 01, 2007 @ 2:34 PM
THE POWER BUOY
Similar to a wavebuoy that measures wave height and direction, the Power Buoy converts wave power to electrical power by utilising the vertical motion of the buoy. It does this via a piston connected to an electrical generator. An interesting feature is that it has sensors to continuously monitor the performance of the system and the surrounding wave conditions, with the data transmitted in real time to a shoreline receiving station – again, very much like a wavebuoy. In the event of very large waves, the system automatically shuts down to avoid damage.
The type of device likely to be installed on the Wave Hub will have a large ‘collar’ to keep the thing afloat; but apart from that, most of the workings will be beneath the surface. There is also another type, fixed to the seabed, which the manufacturers are presently testing in Hawaii: this has virtually nothing showing above the surface apart from an antenna and some lights. Although this type won’t be attached to the Wave Hub, it’s worth mentioning because of an environmental impact assessment done by the US Office of Naval Research, whose overall conclusion was: “No Significant Impact”. And: “Minimal impacts on shoreline conditions, no alteration to currents of wave directions and no adverse effects on shoreline erosion or change in sand deposition patterns”.
THE FRED OLSEN FO3
This floating structure, looking rather like a miniature oil platform, consists of a series of vertical hydraulic cylinders containing piston-like ‘point absorbers’ whose up and down movement is transformed into electrical energy. Working in both directions, the absorbers pump hydraulic fluid into electrical generators.
The structure is made from modern composite materials, and the design is based on well-proven technologies. The mechanical and electrical characteristics can be tuned according to variations in the wave climate. Physical models of the device have been tested in a wave tank with equivalent wave heights of 14 metres (46ft), and a 1:3 scale model has been tested in the sea.
WILL IT AFFECT THE WAVES?
We all know that, if we put a wall or any other structure directly in the path of the oncoming waves, it will either block them off or cause some sort of interference as they refract around it. This is what seawalls, groynes, piers and breakwaters are designed to do. By the same logic, the presence of the Wave Hub and its associated wave-energy devices is bound to affect the waves in some way or other. After all, the whole idea is to extract energy from the waves.
The question is – and this is what all the fuss is about – whether the effect will be significant enough to reduce the quality of the waves that break on the shore, and hence make them less suitable for surfing. And, if the effect does turn out to be significant, whether the loss or degradation of our surfing waves is too much of a price to pay for a step towards meeting the need for renewable energy.
In most western countries, before a large project such as the Wave Hub can be put into practice, a proper environmental impact assessment needs to be carried out. In order to pre-empt any complaints and fears based on lack of information, the results of this study should be published and made available to the public. At the time of writing, the possible effects of the Wave Hub on Cornwall’s coastal hydrodynamics and morphology have been assessed by two different groups, with a third study possibly underway. The results of these studies are directly relevant to how the surfing waves along the North Cornish coast might or might not be affected.
First, a comprehensive independent study was done by Dean Millar and colleagues from the University of Exeter, using a mathematical model to simulate waves propagating towards the coast. The model was run first with, and then without, the Wave Hub in place. A paper describing the results of this study has been published in the peer-reviewed journal Ocean Engineering.
The Hub and its attached devices were represented by a single object 4km (2.5 miles) wide, which absorbed some or all of the wave energy. Since the authors didn’t know which devices will be deployed on the hub, or how much energy each one absorbs, they repeated the tests with different absorption coefficients. These were: 100, 60, 30 and 10%. The last example, with one-tenth of the energy being extracted, was estimated to be the most likely, based on calculations on the efficiency and physical characteristics of the devices themselves. Nevertheless, to highlight and make easily identifiable any shoreline effects, they published all the results including those assuming a total, 100% block of wave energy. It must be stressed that this is assuming all incoming energy across the entire 4km is absorbed, which is clearly impossible.
The wave characteristics between the Hub and the coast were calculated over an 11-month period using a model called SWAN (Simulating WAves Nearshore). The inputs to this model were from ‘real’ offshore waves obtained from the NOAA WaveWatch III model every 12 hours between December 2002 and November 2003. Using data covering almost a whole year would give a good spread of different wave conditions. The difference between the wave height with the Hub and without it was then calculated for a number of points all along the coast, and for the four different absorption coefficients.
They found that, over the entire 11-month period, the maximum reduction in wave height at the shoreline due to the presence of the Wave Hub, assuming the most realistic scenario of 10% absorption, was 4cm (1.5in), and the average reduction was less than 1cm (0.3in).
In their conclusions, the authors state that: “The results suggest that any signal could easily be swamped by natural wave climate variability year-to-year”; and: “It also appears unlikely that the effects of the Wave Hub will be felt by shoreline sea users”.
Another consultancy, called Halcrow, some more simulations. Again, they used a mathematical model to estimate the changes in wave height between the Hub and the shoreline. This time, however, they were more specific about the devices deployed on the Hub. They repeated their tests with two different layouts: (a) One Wave Dragon, two Fred Olsens, 30 Power Buoys and six Pelamis, and; (b) Four west-facing Wave Dragons. (This was considered the ‘worst-case scenario’.)
Rather than using ‘real’ wave data, Halcrow mathematically ‘generated’ various different types of waves and ‘propagated’ them in. These included large, long-period waves, small shortperiod ones, very large waves and what they considered typical waves for surfing. Heights ranged from 1-4m (3-13ft) and periods ranged from 4-16secs.
The results showed an absolute maximum reduction at the shore of between 5% (long-period waves, with layout ‘a’) and 13% (short-period waves, with layout ‘b’). Note that the main difference in the height-reduction figures between layouts ‘a’ and ‘b’ is due to the number of Wave Dragons, suggesting that this is the device producing the most effect. Travelling along the coast, the effect will range from zero to the maximum then back to zero again, depending on your position relative to the wave shadow. While not shown on the model, it is worth remembering that the position of the wave shadow (and therefore the area of maximum reduction) will ‘sweep’ up and down the coast in accordance with the swell direction and period. This means that one single beach will not be directly in the lee of the hub all the time.
“ONCE THE TESTS HAVE BEEN CARRIED OUT, THE INTENTION IS TO DEPLOY THEM IN GREATER NUMBERS SOMEWHERE ALONG THE COAST OF THE UK, EUROPE, OR THE REST OF THE WORLD. IN THIS CASE, THE EFFECTS ON THE COASTAL HYDRODYNAMICS AND MORPHOLOGY MIGHT BE MORE SERIOUS.” - TIM NUNN
Halcrow then went on to model a typical sea state consisting of many waves of different periods and directions, all mixed up together, which is more characteristic of a stormy sea than of a clean groundswell. In this case, the attenuation in wave height at the shore was only around 3%.
Finally, to see if the devices would cause any morphological changes to the shore, they did simulations on the effects on shore sediment regimes using a simple mathematical model that predicts the sediment transport in response to the waves and currents. They ran the model for 48 hours using the same ‘storm’ wave conditions as above. This was repeated first with four Wave Dragons in place, and then without them. The results showed a difference in sediment transport virtually undetectable against the normal background variations.
As I have already mentioned, there are no Wave Dragons actually planned to be plugged in to the Wave Hub. So, why did Halcrow decide to model the waves using four Wave Dragons? Well, it is envisaged that, over the years, several different devices will be plugged into the hub, each one having a different effect on the waves. Halcrow, not knowing which devices might be on the Hub in the future, just assumed that nothing could be worse than a Wave Dragon. In reality, since each company is only allocated one slot, there couldn’t ever actually be four Wave Dragons.
If the manufacturers of the Wave Dragon want to connect in the future, they will only be allocated one of the four spaces available, just like any other company. Also, because of the large size of the Dragon (both physically and in terms of power production), they will only be able to put one device in. Therefore, the maximum number of Wave Dragons on the hub can only ever be one. The equivalent effect of four Wave Dragons would only occur if four separate companies each connected a device with the equivalent effect of a Wave Dragon, and then all tested them at the same time. The chances of this occurring are very slim.
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