Author: Patrick Duffy, managing director, Jospa Ltd
In the first of this series of articles, we described how we developed a very promising wave energy technology, the Irish tube compressor (ITC), but were set back by an erroneous report. To feed the ITC energetically, we developed a key new invention, the chuter.
In the second article, we described how we then developed another new wave energy convertor (WEC) called the Vortex Turbine, which also relies on the chuter to feed it. Wave tank tests on the chuter were disappointing – like many WECs, it had a very narrow productive band, particularly for low waves under 3m significant height. The chuter is so basic to our making progress that we concentrated on measures that would improve its production performance in bandwidth for small waves.
This was completely successful, as was evidenced in the second article’s ‘before’ and ‘after’ output graphs. We did not explain, however, how that was achieved. It stemmed from the use of ‘buoyant fulcrum’ (BF), one of what we have termed our ‘three improvements’. These ‘three improvements’ are methods of improving the performance of the chuter that can also benefit other devices (remember that the future winning wave-energy technology may be a combination of a number of ideas).
If you were to ask a wave-energy developer what improvements they would wish for, it is more than likely they would wish for better amplitude, bandwidth, controllability, low cost and survivability. Both of our WEC technologies score well on cost and survivability, due to their low weight and limited surface exposure to the elements when deployed. Jospa therefore concentrated on improving the first three 'wishes' – amplitude of displacement, bandwidth of usable waves and controllability – by means of these improvements, all of which are the subject of patent applications.
Those three are the ‘adjustable clutch fins’ (ACF), the ‘buoyant fulcrum’ (BF) and the ‘flip-flop’ (FF). Each offers improvements in the three areas desired, so the choice of which to deploy relates to specific WECs. All use simple, passive effects derived from differences of density and gravity: all are proving to ‘do what it says on the tin’ and more.
FIRST IMPROVEMENT – THE ACF
The ACF could more than double output of many wave energy devices. It is exciting because, as tested, the ACF is expected to more than double the output of many WECs at little added cost. It could also stabilise service boats for the offshore wind market (and for WECs). The ACF has already been through two series of tank tests – first for proof of concept and, more recently, for power measurements. The results were so good that it will be the subject of more exhaustive characterisation tests.
[caption id="attachment_10803" align="alignright" width="390"] Fig 1: Particle motion of wave energy[/caption]
The rigid fins are designed for amplitude and bandwidth improvement: the circular diagram of wave motion shown in Fig 1 is essential to understand how it functions.
At the top of the wave, the water particles move in nearly circular orbits whose diameter decreases rapidly with depth. These orbits are propagated onward by the next particles, the wave moving at ‘celerity’ with the upper water particles moving in the same direction as the wave, albeit more slowly. That is why the chuter has its ‘top-cutting’ shape and we speak of it having a cutting ‘knife’.
At the hollow part of the wave, the particles move backwards with respect to the advance of the wave. This gave us the idea to use these opposing forces as a couple at opposite ends of a device to increase its displacement and to so make more power available.
[caption id="attachment_10801" align="alignright" width="1024"] Fig 2: Detail of a fin[/caption]
To test the concept, we made up two identical sealed tubes, one functioning as reference tube, the other fitted with fins that may be set at various angles (as per Fig 2) for tank tests, to rotate freely side by side in each particular type of wave on a common axis. Motion was tracked and plotted as usual using infra-red reflecting balls and a set of synchronised cameras linked to a database. Adding fins fore and aft dramatically increases the pitching movement of a floating tube device by means of much improved power capture by the device (~∆H2). The extra pitching proved to be 30 per cent or more.
Just as dramatic was that simply reversing the couple force of the fins (by rotating the fin angle by approximately 90+ degrees) delivered nearly 50 per cent increase in stabilisation. To increase power, we had used the fins to force the model WEC upward at crests and downward at troughs, but when we reversed the angle of the fins, we reversed the ‘up with down’ to ‘down with up’, resisting the motion, i.e. stabilising. Each of the two effects, power mode and stabilisation mode, offers to open a distinct market.
ACF IN POWER MODE
The fins mounted on shafts dramatically increase the amplitude and bandwidth in definable normal seas, but can be set to release and rotate freely when they encounter chosen levels of excessive force – thus increasing energy extraction but without adding significantly to weight or maximum mechanical loads. The fins then relock automatically when they swing back to the desired angle, akin to the anti-lock braking mechanism of a car.
[caption id="attachment_10806" align="alignright" width="1024"] Fig 3: ACF applied to a four-section attenuator such as Pelamis: circular orbits show action of couples[/caption]
When rotatable fins (to be releasable at predefined loads) are mounted on shafts at an angle on a Pelamis WEC (Fig 3), the water motion causes a ‘lift’ at both ends. However, in the wave hollow (see middle of Pelamis), the water motion is now backward compared to celerity. This causes the fins, which are at the same angle as before, to be pulled downward.
Extensive tank testing has proven this effect works over a wide range of wavelengths and also in irregular waves. At the correct angles, the fin-effect acts as a couple, pushing an object upwards at the crest of a wave and pulling it down at the hollows, thereby increasing the amplitude and force and so the energy extraction from the wave.
[caption id="attachment_10809" align="alignright" width="768"] Fig 4a: Measurement system at HMRC[/caption]
[caption id="attachment_10808" align="alignright" width="1024"] Fig 4b: HMRC test rig[/caption]
The next task was to measure this exciting gain of power to find how significant it is. To measure the power produced, we used a caliper brake with strain gauge (Fig 4), at the suggestion of Brendan McGrath of Ocean Renewables in Wexford. The idea, slow in execution by its nature, is to equate the motion of the two tubes by use of the brake and to read the force that required.
When first and second attempts wrecked the brake’s disc, we knew there was real power available. Following tank tests at HMRC, their report was extremely positive. With their permission, we quote:
“Wave energy devices can typically take 10-20 per cent of available energy from the sea... but by using the ACF: