Author: Patrick Duffy, managing director, Jospa Ltd
Engineering has placed roving vehicles to prospect on Mars and produced a spacecraft that can function as it exits our solar system. It has employed nanomaterials and even applied the elusive quantum effect. So, why can it not master wave power?
"I have been asked this question many times, but it is not easy to answer. Many good engineers have made great effort to extract serious power from the sea. Small, intermittent generation has been achieved, but generation that is commercially viable in terms of cost or reliability remains elusive. I suggest that the answer to the question is threefold – waves are very complex, the development of technologies for the sea is inherently very costly and of course, as we all know, the sea is a very harsh environment. Devices that overcome the harsh environment do so at the expense of very heavy construction, with associated higher mooring forces – incurring more costs as a result.
As engineers, we know that one day there will be viable wave energy. After all, wind energy took years to become competitive. In the meantime, however, stop-go energy prices that respond to oil fluctuations, i.e. short-termism, are a huge barrier to financing and hence to progress.
[caption id="attachment_9745" align="alignright" width="940"] Fig 1: Waves incoming from left accumulate an additive pressure to the right (λ is the wavelength)[/caption]
Jospa came about as Joss Fitzsimons, our technical director, mused over why a garden hose with air locks is so difficult to get going. He devised a diagram (Fig. 1) showing how the presence of airlocks builds head, in that case a back pressure, and he made up a static tube demonstration (shown as Test A under ‘Videos’ as an introduction only, as it is not maintained up-to-date for competitive reasons). Following this, he made a dynamic test (Test B), which most engineers tend to find counter-intuitive. That dynamic test showed that if you halted the action, equivalent to a lull in the seas, it resumed when re-started without having to disconnect or empty the tube – a vital real-life need.
The water in waves does not move forward: the wave’s momentum is passed on (as we see when a car impacts a line of stationary cars) and there is a forward rotation of water that is most active near the top of the wave. When depth starts to fall below 50m, the bottom drag has an increasing influence, leading to (pretty unusable) breaking waves near-shore. If water depth is maintained up to a cliff, for example, the wave reflection will make near-shore waves also largely unusable.
The point here is that the ideal situation for wave power would be large, regular, fast-moving waves right up to shore. Wave-energy devices (WECs) could then pump water at pressure onto land to generate power there. That would be relatively easy and very economical, but does not happen. In real life, the good waves tend to be way out, maybe 5km or more, making pumping friction losses too great.
Typical wave regimes vary by location – compare the Atlantic versus the North Sea, for instance – by height distribution (time/duration), period and even shape. Real waves are just not sinusoidal, they are quite mixed up (‘irregular’ waves). And that is just the beginning. There have been many devices that handled sinusoidal waves well, but the most important test concerns irregular, real-life waves.
WAVE ENERGY TECHNOLOGIES
Jospa has been working to develop wave energy technologies for five years. We have a lot of technology (all early-to-medium stage) and very different to what has gone before. It is still ‘early stage’ after five years because to take a technology to sea, even at small scale, is the really expensive bit. At that stage, any design changes are costly; the only wise course is to develop as far as you can at small scale.
Fortunately, there is a considerable amount of software that will fairly reliably help to examine parameters – particularly backed by or preceded by tests in water tanks that are equipped with wave generators and measuring equipment – and an artificial beach that absorbs waves at the end of the tank to avoid immeasurable mayhem.
[caption id="attachment_9741" align="alignright" width="1459"] Fig 2: First system design for the Irish Tube Compressor[/caption]
The Jospa idea was to use waves to push alternate slugs of air and water forward in a tube, accumulate in a receiver and discharge both elements through dedicated turbines with attached generators (Fig 2). Consider waves again: if you offer the mouth of a tube to a wave, you will not capture the speed of that wave in the tube. The maximum capture is one seventh of the speed – remember, the wave is about transfer of momentum, not of velocity. For this reason wave speed is termed ‘celerity’, rather than velocity.
As power is proportional to the square of speed, water at celerity has at least 49. So, this is essentially 50 times the power potential of the water entering the simple tube. This is vitally important to Jospa, as in filing patents, it transpired there were three prior patent applications on the concept (one of them a ‘lost’ Soviet one). However, all were missing this vital point – that a powerful feed is needed. All of this is covered in our first application.
[caption id="attachment_9738" align="alignright" width="2560"] Fig. 3: Early model of chuter with tracking lights attached ready for test at HMRC[/caption]
Jospa concentrated on getting the water in at celerity and succeeded by invention of (Fig. 3) the ‘chuter’, which looks like a chute for grain and shoots the water forward. The chuter receives the water’s power in a number of ways – it ‘cuts’, with a knife edge, the most energetic top of the wave so takes the forward momentum. But, with the addition of the rotation activity, it also uses forward wide arms to channel water in and rise up, as is the sole mode of some WECs called ‘overtoppers’. The pitching motion of the chuter also adds water head. That means four modes combine to take the water to celerity, of which pitching is vital. Now, how could we increase that power?
[caption id="attachment_9752" align="alignright" width="353"] Fig 4: Improved ITC - air feedback improves pressure and performance[/caption]
To do that, we added a standard rugged piece of mining gear, the air lift pump. Removing the air turbine, which is less efficient than a water turbine, we feed back its pressurised air using a small tube attached to or even integral to the large tube – and diffuse it through a perforated plate to let the water and air rise through a conical riser at the front of the tube (Fig. 4).
If diffused at, say, 10m depth, this effectively adds close to 1 bar to the system. As we estimate the receiver will reach 1-2 bar, depending on the tube and its length in particular, this is boosted to an operating pressure of 2-4 bar that facilitates use of a standard Francis-type turbine. The Irish Tube Compressor (ITC), as we have named the system, now benefits from a smaller receiver if you wish but from a turbine reduction to one-quarter size, another square law, with cost and survivability benefits.
Survivability is a vital consideration for the sea. Sean Lavelle, a versatile engineer in Belmullet who is also developing a WEC and who operates work boats, buoyed us up (sorry for the pun) by saying that as a studier of relevant websites, he found the Jospa technology to be “the most survivable I have seen”. It is a great thing that engineers, essentially competing with each other, can be generous enough to give technical advice and tips to each other.
[caption id="attachment_9739" align="alignright" width="651"] Fig 5: Hose features longitudinal and spiral reinforcements[/caption]
We approached Dunlop Oil and Marine, the largest suppliers of hose to the offshore industry, and gained a technical co-operation agreement with them. The company produces very hefty hoses, while we are looking for very light, pliable ones. A tube under pressure tends to inflate; we want the tube to conform and bend with the waves. Spiral reinforcement, as in a standard garden hose, will confine the pressure but will still tend to rigidity.
The answer is longitudinal reinforcement (Fig. 5) along the neutral axis. This patent has, of course, also been applied for as without it, there would be a decided reduction in the desired efficiency of the ITC. The tube would be extruded either out onto a jetty into the water to the desired length for towing to a particular site, or the extruder would be in an ocean-going small vessel (think of the various tax benefits given to shipping) to extrude at the site, in-situ.
Spools of spiral and longitudinal reinforcements would feed into the extrusion, which is straightforward engineering. The tube could be made of recyclable elastomers – here Dunlop’s programmes to calculate for hundreds of additives will be beneficial. Pat Murphy then joined our group. Murphy has installed fifteen pipe lines up to 2,100 metres long, from 400-2500mm diameter. He has drilled, blasted and dredged rock, and has also designed and built breakwaters and general marine works.
During this time, we performed tank tests with 50mm bore tube, mainly at University College Cork’s HMRC (Hydraulics and Marine Research Centre). Along with the chuter, it is translucent in order for us to be able to watch what is happening. There are reliable formulae available for scaling up these test results and there is a rule of thumb that scaled-up performance is usually better. In this regard, Jospa was very fortunate, as a bore tube of any smaller than 50mm would not have worked because friction losses are exponentially related to power. We were at the workable limit, with larger diameters also bringing exponentially less friction losses.
We found the chuter performed well – including for irregular waves – but within a restricted range of heights, so it achieved only a medium ‘bandwidth’ (how that was solved will be explained in the third article in this series). We did a successful ‘proof of concept’ test on the tube in waves, also at HMRC (artificially driven, as it was too early to marry with the chuter). We now had a very survivable system, with none of it protruding more than maybe 1.5m above the mean water level. The chuter has been designed to dive if hit by a freak wave and its tube is known to be highly survivable through Dunlop. At this point, Dr Willi O’Connor of UCD's Department of Mechanical Engineering married us with a very capable master’s student, Simon O’Callaghan.
[caption id="attachment_9743" align="alignright" width="1920"] Fig 6: Illustration of sink-float effect[/caption]
Simon plugged a very important numeric modelling and fluid dynamics gap for the ITC. He produced curves linking output, tube diameter and length. We then also realised the obvious, noting the ‘sink-float’ effect (Fig 6). There is a relationship between wave height and the maximum height of tube one can use. As the air-filled parts of tube sit up on the wave crests and water-filled conversely per sink-float, we can greatly increase the diameter. As well as offering more volume, this can conservatively offer a valuable extra 0.5 x diameter head. Simon simulated this also.
From this work, the optimal theoretical tube diameter is impractical. Tube length is a straight-line relationship up to about 1000m that is easier to accommodate. As associated curves do not include the important airlift pump contribution, we are confident the projections are conservative for more practical 1.5m or 2m diameter tubes approaching 1000m long.
Tom Thorpe, a respected figure in the industry who has been technical assessor for the Carbon Trust in the UK, has reviewed the foregoing. Claire Lambe, engineer and world-class champion Irish oarswoman, reviewed the economics of the ITC based on the protocols then being developed by Gordon Dalton of HMRC (that are now becoming the world standard) – again before the airlift pump, showing the ITC and one other WEC as best.
In their comments, neither Thorpe nor Lambe (as neither was asked to) commented on our planned ITC array of three units feeding one receiver and turbine. One chuter would be open-cycle and two closed-cycle (air) in the array to cut costs. On top of this, we believe Jospa may perform surprisingly best nearer to shore, as reflected waves reduce the incoming wave speed and thereby friction losses, while increasing the head – that could exactly fit the ITC’s strengths.
[caption id="attachment_9746" align="alignright" width="3264"] Fig 7: Newer design of chuter under test in HMRC’s flume tank[/caption]
It is survivable. It has no moving parts that are essential although a valve to shut off a turbine, to maintain pressure between wave trains, is a desirable benefit. It is easy and cheap to make and has a high output. The development of the ITC is looking good. In our next update, we will look at another approach to wave energy using the latest version of the chuter (Fig. 7) in the fastest offshore waves, a Vortex Turbine.
Patrick Duffy is an electrical engineering graduate of UCD and a Fulbright scholar at University of California, Berkeley, who has been a director in marketing and technical roles, as well as managing director and chairman of a number of companies in the UK and Ireland.