Harnessing the force of instability: inventions to improve wave energy
30 January 2014
Author: Patrick Duffy, managing director, Jospa Ltd
Wave energy is more complex than many realise. The maximum wave height is one-seventh of the wavelength before the wave self-destructs whatever the wavelength or period. In practice, when extracting energy, a slope of maximum only 4.50 is achievable, a very small displacement for the wave energy converter (WEC) to work off – albeit it has massive force behind it.
Slow movement, small displacement and huge forces are what WEC designers have to deal with and they are therefore always searching for any possible increase in this displacement. Jospa’s inventions to increase displacement, the buoyant fulcrum (BF) and the adjustable clutch fins (ACF) – both described previously in article three – have proven to be immensely successful. The ‘flip-flop’ (F-F) has also been invented to increase displacement, in a different way.
The F-F is so called because it is bistable, as the electronics analogy of that name. The mechanical analogy is a spring-loaded toggle where small perturbations have no effect but, with a larger force, it switches to the other state in a dramatic manner, in a rush. Fig 1 illustrates the concept of stability. Note it does not represent a wave – think of it as a curved wall. States 1 and 3 are stable, state 2 is unstable – a perturbation when in state 2 will trigger the body to go forward or backward.
When in states 1 or 3, if either their front or rear points slightly down, they will remain that way until strongly triggered to displace – this is the case whether the item is a Salter Duck or a Pelamis. If we now permit the diagram to represent waves, the bistable body can be set to trigger the flip and the flop at either equal levels of trigger force (say at 1 or at 3 as wished) or separately, i.e. to flip for the larger displacement and flop for the smaller.
This might be seen as useful for a long attenuator, where the first action had already removed much of the wave’s energy and there would be less energy available a little farther downstream to return it to the other state. Conversely, if a body at rest tended to droop at the front due to mooring details, or for other reasons, the trigger point could be set lower.
Any bladder containing air and water can constitute the bistable, as, correctly placed, the requisite perturbation will vigorously cause the air to rush to the highest point. In Fig 2, we see a representation of a Salter Duck with an air bladder fitted underneath. With the F-F, small waves in effect reaching the Duck at left are filtered out as they are too small to rotate it duck very far: it is in fact harder to move than if there was no trapped air in the F-F.
But when the larger wave arrives at the second Duck, this is sufficient to rotate it enough for the air to rush to the new position and in the process to cause a large forceful rotation. This is a very desirable gain for power extraction, but it is at the cost of ignoring small waves. However, as the energy in waves is proportional to the square of the wave height, this is a minor sacrifice.
Wave hollows occur as often in real seas as wave highs, thus resetting the flip to flop. Overall, the effect is that small low energy movements are lessened, while medium to larger useful movements are exaggerated to make energy extraction easier. This patented concept still needs to be experimentally proven.
USEFUL FOR MANY WECS
This idea can be applied to many WEC concepts and Fig 3 shows a multi-section WEC, such as a Pelamis, with trapped flip-flop air below the water line: a partially air-filled bag is located under each of the tanks. (In a practical design, the air would have to be prevented from moving sideways and up towards the surface – the air must remain trapped below the waterline).
This results in having two stable states in calm water, one with the nose up and one with the nose down and the situation where all the tanks lie level is an unstable equilibrium. Thus, small waves will not have enough energy to cause the WEC to flip or flop. However, when a wave of sufficient energy comes along, it overcomes the resistance of the entrapped air bubble and it flips or flops through the unstable level position with increased force and amplitude into its alternate stable state. This is because the entrapped air rushes to the upper end of the tank, pushing it upwards more vigorously, increasing the angular movement of the tanks and also the force.
The F-F trapped air forces the already higher ends of the tanks higher by resultant forces that are indicated ‘R’ . Of course, a flexible bladder or rubber bag could be substituted by a containing apron attached to the bottom of the WEC, like a hovercraft, open to the bottom but trapping the air.
Sometimes, this concept is mistaken for already existing ideas in which water ‘sloshes’ to and fro. There is a very important difference. Air, as in the Jospa case, moves towards the high part of a wave and has insignificant mass, while sloshing water has a great deal of mass and moves away from the wave crest. Waves propagate very fast in Atlantic, perhaps at 15m/sec (54 kph).
As the waves pass, the large mass of sloshing water would have to accelerate, slow down and reverse every 10 seconds or so. The turbulence losses and forces would be large. Trapped air therefore has a great advantage.
The F-F lends itself to simple control. Differentiation between choosing, say, 1 and 3 as triggers for flipping or flopping could be achieved by shape variation or by position of the flip-flop body. Air could be injected or bled of-F – in storm conditions, all air could be bled from the F-F to avoid adding to displacement. Bleeding of-F some air will also activate the F-F for smaller waves.
In Fig 2, the flexible bladders could be replaced by air bubbles trapped inside an upturned U shape under every tank. The air moves back and forth as before amplifying the amplitude & force. The tanks would be plumbed in pairs, 1 and 2 being interconnected via a flexible with butterfly vale (3 and 4 likewise). Thus, for longer wavelengths, two tanks can be made to work like one long tank, assuming a suitable control system is used to control the opening of the valve.
In another embodiment, all four tanks have valves and feed to a common manifold so that air from any tank can leave and enter any other tank. This opens up the possibility of a suitable control system being used for extra energy extraction.
When Diesel was developing his cycle and engine, did he ever foresee that one day it would be used by adding a gearbox, clutch, differential, various types of suspension and a raft of electronics? Chances are he did not. Likewise, it can be expected that WECs in the future may be somewhat complex, using a variety of technologies. Quite apart from the fact that there may be ‘horses for courses’ – different types of WEC for different seas – they will likely use an assembly of technologies and ideas from different suppliers.
There may not be just one winner then when wave energy becomes a reality. Quite apart from the potential of either or both of Jospa’s two WECs being ‘winners’, there is a possibility of the Chuter itself, the BF, the ACF or the F-F also individually or together being winners.
The three improvements of BF, ACF and F-F may end up being used separately or together in a variety of combinations. There are many ways to configure their principles for different WECs. Much further work needs doing on them alone – just think of the amount of work done on pistons, and injection systems for internal combustion engines. Between them, many power improvements and many better controls may be achieved.
Note that although the buoyant fulcrum and the adjustable clutch fins have been clearly shown to not only work, but to be very effective, the flip-flop has not yet been tested. Tests are scheduled, however, and a WEC developer says he will also test to see what extra it delivers for him.
THE FUTURE OF WAVE ENERGY
All of this suggests that there should be cooperation in the industry. So far, too many WECs have been hyped, promised too much and achieved little. One lesson hopefully learnt is the need to test and develop almost ad nauseam at very small scale before going up to even one-quarter scale, or full scale.
Recently, some 11 developers of wave-energy technology in Ireland came together to discuss common problems and needs. Already, there has been mutual assistance, exchange of ideas and even an amount of co-operation agreed, which is a wonderful development. One joint need agreed and now being discussed with the authorities is for a ‘nursery slope’ – a test facility at sea or a lake at about 1:15 scale as a stepping stone between tank testing and one-quarter scale (previous articles by Graham Brennan of the Sustainable Energy Authority of Ireland have described the one-quarter scale facility operated by SmartBay in Galway Bay).
Recent statements from Brussels on governments’ obligations regarding renewables are not seen by the wave energy fraternity as threatening to them. On the contrary, Brussels appears to be considerably increasing its support for wave energy. The SEAI, the responsible body in Ireland, is also expected to very shortly announce an increase in its funding for wave energy development.
Jospa is now embarking on a development programme for most of 2014 that is planned to fill out details on the chuter and its potential, the power potential of the vortex turbine, and the ability of the ACF to double the power outputs of other WECs and to stabilise service vessels. We plan to move either or both of the WEC technologies into a collaborative effort with other partners at that stage, and to move the ACF with its two different market opportunities to or close to a market-ready stage.
By the end of the year, we hope to be speaking commercially with some leading WEC(s) developers to greatly improve their performance, and with leading service vessel companies about stabilised service vessels. We could and would be delighted to make faster progress if favoured by greater resources: apart from having a number of very promising unique technologies (all patented or patent-applied-for), we also have near-market potential that we consider exciting.
- Reader’s query 1 – why does the Jospa vortex turbine include a draft tube?
We have queries on the merits of the draft tube for the vortex turbine. Engineers who look at old mills are often taken with the idea of re-installing the old waterwheel to generate electricity. If they look into it, they learn that waterwheels are very inefficient compared to a turbine with draft tube, as old waterwheels ‘throw’ out the exhaust water at high velocity. If this kinetic energy could be captured, the waterwheel efficiency would almost double.
The normal reason to use a draft tube for a turbine discharging into water is to squeeze the last available energy from the flow. The velocity of water at the turbine runner outlet is very high. By employing a draft tube of increasing cross-sectional area, the discharge takes place at a much lower velocity: part of the kinetic energy that would go to waste is recovered as a gain in the pressure head, thereby increasing the efficiency of the turbine. (Per Bernouilli’s theorem states that the draft tube reduces the kinetic component of the equation and converts it into an equal increase in the pressure component).
Indeed, turbines with high specific-speeds such as a Francis or our vortex turbine (where the runner will have C-shaped vanes) must be particularly designed to recover their velocity energy and typically 35% of their total head is recovered by the draft tube.
Jospa intends to run the vortex water level at typically about 2m above MWL (mean water level), but this will vary according to wave conditions. The shape of the turbine blades will guide the water down and out through the runner, even when the water level outside becomes temporarily higher than the level in the vortex.
A problem with draft tubes is that while they need to be long to extract the energy, if the cone angle is too steep, the water breaks away from the edges and starts turbulent counter-rotating eddy currents. The maximum practical cone angle is 7 degrees (difficult to achieve in rivers). When the draft tube’s outlet diameter is double compared with the inlet, it will capture 15/16 of the kinetic energy in the flow, leaving the small balance as a swirl component of kinetic energy on exit which cannot be captured.
WECs will include PLCs to optimise output at all times, to monitor for problems. Jospa will monitor the swirl component mentioned above and also the pressure at the outside bottom of the draft tube, which varies within passing waves. Vane angles and positions may be varied accordingly, likewise generator field current.
The draft tube is particularly important for Jospa’s vortex turbine WEC, as it will serve to stabilise the bowl as it is thrown about by the sea, reducing the dissipation of the energy in the flow before entry to the turbine runners.
Occasional, big waves could threaten to reverse flow within the turbine. A long draft tube will greatly avoid and reduce this effect (ocean waves are often called ‘surface waves’, as so much of their energy is concentrated near the top). Other active measures will also be used.
- Reader’s query 2 – Why not go to sea with prototypes to save time?
For those not familiar with wave energy, 1:15 scale may seem almost ridiculously small, even ridiculous or impractical. This would be a major error, as there are proven reliable formulae for scaling up from model scales to full size that are routinely accepted on their own as the legal basis of performance guarantees in shipbuilding and power and process plant.
The costs of testing at one-quarter scale are huge. They become massive at full scale, so good testing at small scale is essential. A power measurement taken of, say, only 10W when measured at 1:15 scale can thus be reliably projected to measure 130 kW when transposed to full scale.
WEC developers, in fact, regard 1:15 scale as reasonably big and most development is done at scales ranging between 1:50 and 1:25.http://www.engineersjournal.ie/2014/01/30/harnessing-the-force-of-instability-inventions-to-improve-wave-energy/http://www.engineersjournal.ie/wp-content/uploads/2014/01/455897353-1024x678.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/01/455897353-300x300.jpgElecenergy,renewables,UCC,wave