Performance of a WEC technology is undeniably one of the major criteria to demonstrate viability. The higher performance or electricity generated the higher project revenues will be.
Hence methodical design was understandably critical in maximising efficiency. The respected force by distance over time formula Pgenerated = Fwater x Lstroke / time was considered to be the most reliable theory to develop a high efficiency wave energy extraction system. A thorough investigation of different extraction concepts, and process viability was undertaken which determined a series of lightweight power take off (PTO) floats offered the highest density energy extraction possible.
Lightweight floats have a higher operating buoyancy compared to the weight to be moved, and as such are very compliant or easily moved in even small wave heights. Hence they offer high efficiency in small – medium wave height conditions whilst causing the least degradation to the wave possible.
This is essential to allow sufficient energy through to the rear PTO floats so they can also operate with a high efficiency.
Animated side view of the motion of 3 different weighted floats in waves. The light weight float at the surface has the greatest movement of the 3 floats. Compliments Wikipedia.
The animated image readily shows the 2 heavier floats are shown to have much less movement. The force over distance formula above, indicates less energy could be extracted by underwater systems.
In essence, light weight floats intrinsically have a high ratio of operating buoyancy to the weight of the float so they move up and down with greater ease in the waves. The higher operating buoyancy is in effect extractable energy and this feature also makes them very compliant which is significant especially in the small – medium wave heights.
OPERATE WHERE MAXIMUM ENERGY IS AVAILABLE
It is well documented that maximum energy is available at the surface of the waves. As wind and solar technologies have not been developed in low resource locations ie behind or under, buildings or trees, and have been typically developed where the energy resource is at its highest, there’s an obvious and reasonable expectation that commercial wave energy will also be developed where energy is highest, at the surface!
Having a number of linked floats provides multiple PTO energy extraction points. This is contrast to typical first generation technologies which feature single PTO energy extraction points.
It is believed that just like the Betz law which provides the theory behind maximum wind turbine efficiencies, there is a maximum efficiency that can be attained by a single wave PTO device. This is expected to be relatively low and may be in the order of approx 29%, the typical wave energy flux available at the surface. Hence if a number of PTO’s are provided in line than substantially higher efficiencies can be attained than can ever be achieved with a single PTO device!!
The Xtracta design is a Hybrid Attenuator – Point Absorber which as the name implies is second generation technology with a number of performance features and benefits over existing first generation technologies. The sequential energy pulses provided by an attenuator are combined with a wide capture width of a point absorber for unrivalled performance.
Not only do light weight floats offer the greatest density of energy extraction, they also have low impact to and result in low degradation of the waves. Reducing degradation of the waves is essential if a number of PTO floats are to work efficiently in a row behind each other, by allowing more energy through to the rear floats.
WIDE CAPTURE WIDTH
The Capture Width of a power plant is a critical factor in assessing the potential for a WEC technology to extract energy and produce commercial levels of electricity. The concept appears to be well understood with respect to wind and solar technologies, but the extended development of a number of wave energy devices which have had very limited capture width suggests this concept may not be so well understood with regard to WEC designs.
Wave energy is measured in metres of wave front. Hence the greater the capture width, the greater the addressed energy and higher electrical production will be.
Illustration of the swept area of a wind turbine in metres2 or energy resource capture area, and the comparable length of a wave front in metres or addressed energy resource.
The above FIGURE shows the larger the rotor blade of the wind turbine, the larger is the swept area and rated capacity of the turbine. Wave energy is comparably measured in meters of width of the wave front and the exciting Xtracta technology operates over a wide capture width to genuinely address, and deliver commercial levels of energy!
Demonstrating integrity in performance results is essential to building confidence of realistic projections of commercial projects.
The performance of the Wave Xtracta technology has been analysed during scale ocean testing using best practice methods. This involved measuring and recording average electricity produced over a window of time, for a particular wave state. Wave to wire efficiency is then determined by the percentage of electricity produced to the total contained energy in the waves over the same period.
In contrast, results presented by a number of other developers have included peak hydraulic and or air pressures which have often been shown to have little relevance to electricity production.