Author: Patrick Duffy, managing director, Jospa Ltd
At the end of the first article, Jospa had developed a WEC (wave-energy converter) based on the chuter feeding alternate slugs of air and water to a large-diameter, long and flexible tube. The chuter had been tested and shown to work: proof of concept had succeeded with a tube containing air and water – a test proving that waves gradually built pressure. Subsequent mathematical analysis indicated the Irish Tube Compressor (ITC) had immense power potential. Patents had been applied for many features of the technology and Jospa planned to advance the ITC further.
That was not what happened. As William Dick, physicist and inventor of Wavebob, said: “As there are no commercially successful WECs, then surely there can be no true experts. I have seen demonstrably incorrect ‘expert’ opinions taken as fact.” The ITC suffered from an assessment, since proven to be incorrect, based on errors and incorrect assumptions, as a result of which Jospa needed a Plan B. Fortunately, there was an obvious one to hand.
Measurements and observation of the chuter in the Hydraulics and Maritime Research Centre wave tank – remember it was designed to throw water forward energetically into the tube of the ITC – had suggested that this was so successful it could drive other power take-offs (PTOs). We rapidly came up with an idea for a PTO that might best fulfil requirements (see below).
To commence design of that PTO, we needed to know more about the performance of the Chuter and then to optimise it. Tank tests were arranged at HMRC. Infrared tracking balls were fitted to the chuter at key points and it was subjected to a variety of wave regimes while being filmed by seven synchronised calibrated cameras. The tracked motions were fed as data into a computer, then HMRC’s experts, having witnessed the tests, studied the outputs and produce graphs, curves and tabulate and wrote an independent report on their findings.
We are very fortunate to have such a world-class facility in Ireland. As detailed in the recent article by Graham Brennan of the Sustainable Energy Authority of Ireland (SEAI), this is now merging into the Irish Maritime and Energy Resource Cluster programme that will be one of the top marine research and development centres in the world.
FURTHER DEVELOPMENT OF THE CHUTER
[caption id="attachment_10054" align="alignright" width="715"] Fig. 9: Scatter diagram showing preliminary chuter Mark I response to various wave frequencies and heights (click to enlarge)[/caption]
The chuter proved to suffer from a weakness common to most WECs: Fig 9 shows how it was effective at (and tuned to only) limited-period waves and high waves, but performed poorly for others. There was a serious need to extend the period bandwidth at lower wave heights. To improve this response, we first attempted to have the chuter numerically simulated by using a powerful computational fluid dynamics programme called ANSYS.
Jospa uses translucent materials, insofar as is possible, to study flows and we hoped the simulation would replicate what was observed, thereby giving confidence in using it to predict the usefulness of variations. ANSYS, perhaps the most sophisticated such programme available, threw a net or mesh of over 300,000 elements over the chuter and solved a number of simultaneous equations for each mesh before moving to the next.
It was a truly massive computing task and the computing power used was not really up to it (it was later suggested that 3,500,000 elements would have been more appropriate!). Jospa had to leave simulation for the time being, but we shall return to it backed if possible by parallel processing.
[caption id="attachment_10056" align="alignright" width="691"] Fig. 10: Mk II chuter achieves excellent performance as desired over a far wider range of frequencies with waves up to 3m high (click to enlarge)[/caption]
We therefore relied on more regular engineering to design and make a Mark II version of the chuter in our workshop, and further tank tests were performed. The results were very pleasing, as is shown in Fig. 10 (the HMRC scatter diagram of these later tests). Jospa could not have hoped for more – a ‘tuning’ barrier facing many WECs had been overcome. To highlight the comparison of ‘before’ and ‘after’ in the figures, compare red to red directly (note the positive operation over 3m significant wave height in Fig 9 has been changed from red to grey, as in order to conserve funds the ‘after’ tests were confined to those waves of particular interest, i.e. 3m and under).
In its conclusions, the HMRC stated: “The chuter now operates at wave heights that correspond to mean values for the west coast of Ireland. This is an important advance, as it was shown that the previous chuter design could only operate for a very limited range of conditions.” Another important conclusion: “In addition to the improvement indicated in terms of wave heights, the wave period band in which the device operated broadened significantly. For the original chuter, the wave periods for which it produced a flow was extremely limited, but the current version does not seem to have this limitation as it operated for a wide range of conditions (5-13s)”.
Further, the HMRC stated: “Another aspect in relation to the original chuter was that it never worked for irregular sea states, whereas the modified design now seems to be very robust and produces flow for a variety of Bretschneider wave conditions.” An EU patent for this valuable invention, the chuter, was granted in November 2013.
ALTERNATIVE TO THE FIRST WEC TUBE
What PTO should we now put forward to avail of the proven capability of the chuter, being that our first WEC, the ITC, is ‘parked’ for the moment? Off the Irish west coast, the calculations showed that the chuter could ‘shift’ approx 90-100 tonnes of seawater forward with each wave. At the speed of 14 m/s associated with a wave period of 8-9 seconds - 50 km/hr. That averages 817 kW of kinetic energy.
If we could capture even a very conservative 20% net to drive a generator, we would already be equivalent to the best world WEC performances but at a small fraction of the weight, the cost and even the mooring forces of our rivals.
This left two challenges to be overcome: first, the intermittent nature of the waves (i.e. the wave periods) that call for a flywheel effect, and second, flows of ‘headless’ volume with high kinetic energy are not suited to conventional turbines. As regards the first challenge, an HMRC study of sea states off Belmullet summarises: “For most of the year, wave conditions fell within a significant wave height range of 1-3 metres and a wave period range of 5-8 seconds.” Vortex bowl smoothing with added field current regulation of the generator would enable a reasonably steady output, particularly when the spacing of a number of devices can be added.
[caption id="attachment_10050" align="alignright" width="1754"] Fig 11: views of the chuter - vortex turbine with draft tube (click to enlarge)[/caption]
This suggested the use of a vortex with vortex bowl, so that the flow could remain at a comparatively constant level through the wave periods. Fig 11 represents the arrangement. A bowl thrown about by seas would dissipate much of the vortex energy, but this is strongly mitigated by the slight gyro stabilisation of the vortex and by the draught tube. This drawing also shows the studs on the top of the chuter to discourage significant boarders!
To satisfy the second challenge, the turbine itself will be purposely designed. Although most engineers who refer to turbine selection charts tend to assume it will be a Francis-type, the flows are in fact quite different and require different blade shapes and entry angles. Vortex turbines already exist, but our job will be to tailor-make one for this application.
[caption id="attachment_10062" align="alignright" width="345"] Fig. 12: Test piece being assembled[/caption]
How big is this vortex turbine with chuter likely to be? That will depend to an extent on where it is to be situated. For the world’s most energetic seas – to be found off Belmullet – it is likely to be about 50m long x 10m wide overall, with a runner (impellor) diameter of about 3.5m. Important for survivability, it should protrude comfortably less than 1.5m above mean water level. A version for the North Sea will be considerably smaller.
How far advanced is this vortex turbine, which is now the second Jospa WEC technology (the ITC is the first technology)? Fig. 12 shows the best we can do at little cost assembly of a chuter/vortex turbine combination being made up in our workshop for small-scale tank testing. It passed the proof-of-concept well, yielding plenty of video footage of the revolving but unloaded relatively unsuitable makeshift turbines (we tested two very different readily available little impellors, one of them a hot-end turbo from an HGV – see Fig. 13). Although the power has not yet been measured, it is evident it will be significant when scaled up.
[caption id="attachment_10058" align="alignright" width="640"] Fig. 13: Truck’s turbo-impellor as tested[/caption]
The wave regimes applied in the test tank can be related to real wave conditions (including tests using irregular waves, on which many aspiring WECs fall down). Jospa is about to embark on measuring outputs, backed by SEAI grant funding, of first the chuter alone and then of the combination. This is extremely difficult because of the intermittent, varying velocity profile of the water flow in the chuter.
POSITIONING AND RELATIONSHIP BETWEEN JOSPA’S WECS
This project will include the optimisation of the chuter for a Mark III version and the design and improvement by iteration of the turbine’s runner, all of which Jospa will make except the runner that will be rapid-prototyped (i.e. 3D printed). Measured outputs are the main objective of the project.
That should place Jospa within the year 2014 with two viable WECs well into developmental stage. Further progress will depend on resources, i.e. funds. There is a consensus emerging on some aspects of future successful WECs, at least in two respects. The first of these is that successful WECs will be in a ‘horses for courses’ situation, where different designs will favour different locations (for Jospa, the vortex turbine is definitely best suited to offshore energetic seas while the ITC is, for technical reasons, expected to perform well closer to shore but not quite ‘inshore’).
As both share the chuter as driver, development work can remain focused. Both WECs score more than well compared to others in the world scene with regard to their low weight, low profile offered to the waves and their failsafe design-to-dive for later recovery in the case of a rogue wave.