Three developments to improve wave energy technologies
16 January 2014
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.
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.
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.
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.
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:
- where that’s 10 per cent, it could increase it to 34 per cent; and
- where it’s 20 per cent, it could increase it to 44 per cent (as measured at six to nine-second periods).”
HMRC understandably insists that such an amazing increase must be verified in turn by tests on particular devices, but it can be seen that the ACF is likely to more than double the output of many wave-energy devices.
ACF FINS IN REVERSE
ACF fins in reverse, i.e. stabilisation mode, opens a new, non-wave market.
- There is an increasing commercial interest in stability-docking for tender vessels in the offshore wind industry;
- ACF control can be used in the counter sense to stabilise floating structures;
- The Hydraulics and Maritime Research Centre (HMRC) is currently carrying out research on stability docking and regard the results of ACF as significant in this area;
- JOSPA has secured significant patent rights in this area of research.
It has been predicted by Peter Gifford, president of Extreme Ocean Renewables of Canada, that more than 19,800 offshore turbines will be installed worldwide by 2025, with up to one million crew transfers per year by 2030, and that stabilisation in waves up to 3m can offer boarding at sea for 310 days a year – bringing major savings in operating costs.
This, Gifford suggests, will require some 775 new purposed service boats. HMRC has suggested – and, of course, we have agreed – that it would undertake study of this aspect itself by means of tank testing and simulation. The ACF offers the opportunity of addressing this market for both new and refits very effectively, at very low cost, without sacrificing the speed of the boats.
Maintenance and the weather window is also a major problem and cost for WECs, as they are designed to move, even in low seas. The ACF, when attached to such a WEC, simply de-clutches and releases the fins, relocking them to stabilise at the new angle approximately 90 degrees apart.
But why did we just say that we do not want to increase the peak power? Remember, almost all WECs tend to be tuned, to have quite sharp power peaks at particular wavelengths/periods. All of the equipment must be sized to that peak. Therefore, using the ACF to greatly increase power across the rest of the spectrum, so that there is reasonable power generated most of the time, is far more important than doubling the peak.
Doubling the peak is normally very undesirable: in fact, the ACF would likely be adjusted to feather at or just above the peak, so that the whole generation/transmission system does not have to be doubled in size at great cost. This also means that mooring forces and thereby mooring installation costs are not increased and, finally, that in the case of a freak wave the already-feathered ACF does not contribute to the destruction of the WEC.
In fact, the ACF is tunable by rotation to different angles to produce different required results: it is controllable. The ACF thus ticks – remember them? – the wish list boxes of amplitude, bandwidth and controllability. What of cost? It really scores most there – only some 20 tonnes of steel for a leading WEC such as a Pelamis. Survivability has already been discussed. The ACF thus helps to tick all the boxes for WECs and it offers to open the service boat market.
FURTHER DEVELOPMENT OF THE ACF
The promise of the ACF is such that a much larger test programme is crucial. In studying the power contribution in the crucial irregular seas, we found significant increased power but were not able to measure it as the caliper system gives sub-optimal power extraction compared to a loading system which is variable through the wave cycle. Braking force is essentially constant force and must be set at a single setting, regardless of wave height and slope. This power must be measured so we have to decide on a practical, inexpensive alternative for planned further test series.
Our options are to develop either (a) a viscous dashpot mounted on a load cell or (b) an electrical/electronic system which the doyen of wave energy, Donald Salter (professor emeritus at Edinburgh University and inventor of the Salter duck) strongly recommends.
It was the buoyant fulcrum BF that delivered the improvement in performance of the chuter described in article two, dramatically increasing its performance particularly with respect to wave bandwidth for small waves. The secret of the BF is that it uses the mooring force actively, rather than only as the usual means of keeping the WEC in location.
Anyone in a small boat will be aware of the strong pull on the mooring line when being lifted on the oncoming wave, and how it then goes slack. Originally, the chuter had buoyancy rigidly tied to its body. Now, with the BF, the mass of the chuter is pivotally attached to the buoys, like a see-saw floating in the water. The mooring force acts on the end of the see-saw and the floating barrel acts as a fulcrum.
The load, the chuter body, is attached to the other end of the see-saw. As the tension increases on the mooring line, the see-saw tends to move away in front of the wave so the moored side of the chuter is pulled deeper and the working end is lifted higher.
Its use with the chuter is shown in the three pictures of Fig 5. The pivoting part or ‘saddle’ bridges over the chuter at left, mounted on the pivot plates that are attached to it at both sides (one end of the see-saw). It pivots on the upper red fulcrum axis (fixed but free to rotate) to which the buoyancy ‘barrels’ – bottles in this case – are strapped. The mooring line is attached to the other end of the pivot plate on the free-moving axis marked green, i.e. the other end of the see-saw.
In the 45o position (middle picture), there is little mooring tension so that little of the barrels is in the water. With the oncoming wave from the left side increasing tension on the mooring, the wave drives the chuter before it and then up the wave front. This forces the buoyancy saddle with buoys attached to rotate clockwise around its red axis, causing the buoyancy to sink deeper and flatter to the mean water level (right-hand picture), thus lifting the chuter higher.
This lifting coincides with wave crests, so exaggerating the effect of wave height. It increases the bucking motion, the key to the chuter’s ‘swallow then shoot’ action that enables it to deliver power.
There are a number of variations in how the BF can be applied to suit a particular device. Fig 6 shows, for instance, the BF acting on a ‘heaving buoy’ similar to Wavebob. An approaching wave crest forces the buoy to move away and upward in the normal manner. The second mooring prevents this, and instead forces the buoy to rotate and sink deeper, thereby increasing the upward displacement force and stroke, hence the power, acting on the pump as shown.
The buoyant fulcrum has now been proven to tick ‘wish list’ boxes for amplitude and bandwidth and will be developed further. Nonetheless, it would benefit from modelling and simulation.
Having now described two of the ‘three improvements’, we have left one, the ‘flip-flop’, for the fourth and final article, together with some more general comments and observations and responses to readers’ questions.