Saturday 7 December 2013

Monday 21 October 2013

The tube map dynamic model

The London tube map is a strange and wonderful thing. Compared to most maps, it is not information dense. Yet it contains exactly enough information to allow passengers to easily navigate a complex system of interchanges. Compared to most maps, it is not geometrically similar. The one criteria that usually determines the usefulness of a map has been flouted: indeed a geometrically similar tube map would be more difficult to follow, and hence less useful.

The London tube map is an example of how the measure of a model's quality and information content depends on its purpose. There is no one model that is the best choice in all contexts. It is also a useful reminder that a model is distinct from the concept it represents.

Friday 11 October 2013

Why not use wavemakers?

"If you are trying to make a good wavemaker, why not use wavemakers?"

If we take the statement 'a good wave absorber is a good wave maker' to its logical conclusion, then why not consider the wavemakers in our wave tanks? Most tanks for simulating deep water waves have a series of bottom-hinged flaps, mounted on the edge of the tank.

Monday 30 September 2013

EWTEC - Inore group discussion session

After the INORE workshop panel discussion, the panel joined the audience, and we broke into several discussion circles. The most interesting point that came out of the group that I joined was a discussion of a point Peter Frigaard had made in the panel session. Peter had mentioned that for wave power, the extreme loads are much greater than the operational design loads – he estimated a factor of 30. I've heard this concern voiced by several people, including Peter Fraenkel. Peter Frigaard pointed out that wind turbines shed loads by pitching their blades once they reach rated wind speed. They also have the option of yawing to reduce loads. In the break-out discussions, Prof Falcão emphasised Peter's point. He spelled out Peter's implicit suggestion: that wave power needs to have some element of adaptable geometry to reduce this gap between operational and extreme loads.

Monday 23 September 2013

EWTEC – Inore panel session

Thankyou to INORE for organising an engaging and enjoyable workshop on the last afternoon of EWTEC 2013, Aalborg. If you've not heard of INORE, they are fantastic – enough said! See their website for details of fantastivity. The workshop had a questions-and-answers panel session, followed by discussions in break-out groups. The panel consisted of António F. de O. Falcão (IST in Portugal), Bruce Cameron (Nova Scotia Department of Energy), Chul H. Jo (Inha University in South Korea), Peter Frigaard (Aalborg University), Jens Peter Kofoed (Aalborg University), Enrique Vidal Sánchez (Wavestar), and Ian Masters (Swansea University in Wales). The workshop was peppered with discussions around my current favourite topic – technology development paths. I'm going to pick out some of these points, as they tell a story when brought together.

Tuesday 10 September 2013

EWTEC 2013 presentations: The long and the short of it

The weird and the wonderful 

There were a couple of new wave power concepts that caught my eye.

Monday 2 September 2013

What cost renewables? - interview with Sue Jane Taylor.

At this year's All Energy Exhibition, I was somewhat surprised to see an artist exhibiting pictures of some offshore wind turbines. They were part of Sue Jane Taylor's exhibition 'Beatrice Works', which will return to Aberdeen in September. Recently I spoke to Sue Jane Taylor about the stories told by the landscapes she draws and paints, and about her plans to depict Pelamis.

Monday 26 August 2013

Comparative tank test challenges: ...... Part I ...... systemic errors and bias

A USA funding body (Wind and Water Power Technologies Office) recently called for advice on an innovation prize challenge for WECs. The proposed approach was tank-testing of ten short-listed concepts. I was very encouraged by this proposal, and this got me thinking about the practical challenges of comparing ten very different concepts using tank tests. There is a risk that the concept which does best in the tank may not be the concept which does best at sea. The reasons for this include:
  • scale and conditions insufficient to identify full-scale sea-trial problems
  • systemic tank testing errors
  • systemic bias due to choice of test program
  • systemic bias due to practical restrictions on test program
  • practical difficulties associated with direct comparison of different types of device
  • bias due to non-blind testing

Friday 16 August 2013

Beautiful and wild

Health warning: do not look at this painting too long if you suffer from sea-sickness. As you focus your eyes on the quilt of waves on the horizon, you stomach will eventually get dragged down into the wave trough in the lower left hand corner. Yes, the sea is beautiful, but it is not tame. The sea's power leaves us feeling small; it even dictates our horizons.

Monday 12 August 2013

In a nutshell: Peaks and Troughs (Yemm)

The Peaks and troughs of wave energy is a treasure trove of gems awaiting discovery. There are associated papers, but there are also audio recordings of all the talks. Richard Yemm's (Pelamis) talk is particularly good. He shared his experience of "the coal-face" (oh dear), and gave his candid opinion of mistakes Pelamis had made, as well as things that they felt they'd got right. It was clear that the pitfalls experienced by Pelamis await others following in their path, and his talk was peppered with advice for other developers.

Monday 5 August 2013

Design challenges - Oscillating Wave Surge Converters

In this article I outline my thoughts on the design challenges and opportunities associated with WECs known as 'Oscillating Wave Surge Converters'. As discussed previously, this term is used to describe seabed-mounted pitching flaps.

Monday 29 July 2013

USA shows some pioneering spirit

Well done to the USA (Wind and Water Power Technologies Office) for acknowledging that game-changing innovations are essential for speeding up the commercialisation of WEC technology, and that researching these innovations is most cost effective at early stages of technology development. A recent announcement proposed an innovation challenge, to be judged by numerical modelling and tank trials.

Market incentives for sea trials

For a long time, the value of a prototype marine energy technology, and hence the investment available for its development, has been measured on a 1D linear scale: technology readiness level (TRL). Consequently there has been market pressure to attain high TRLs at the lowest overall development cost.

Monday 22 July 2013

Pretentious? Moi?

There is a reluctance to criticise people

It is hard not to be aware of the strong reluctance in the wave energy community (valid use of the much-abused term 'community') to be seen as critical of “other people's work”.  I think the problem is less the criticism of the work, and more the implied criticism of the people who identify with this work - there is concern that critique could cause offence. Perhaps more importantly, being critical of previous work bears a risk to one's reputation of being perceived as "pretentious". This is because criticism can be interpreted as an attempt to show that one is better than the person whose work is being criticised.

Tuesday 16 July 2013

US Wave Energy Converter Prize Consultation

The US federal government (The Wind and Water Power Technologies Office) is holding a public consultation about a proposed funding scheme for wave energy technologies. The public are invited to share their views either on a webinar to be held this Thursday (9am-11am British summer time), or by email (closing date 25 July). Full details can be found at:

Monday 8 July 2013

Falnes's diagram: just a bunch of old maths?

What does Falnes's well-known diagram depicting the principle 'to absorb a wave you must make a wave' (Fig 1) actually show? Is this achievable or just a mathematical slight of hand, a bit of physics-sudoku with no relevance to the real world?

Monday 1 July 2013

It is human to model

René Magritte's "The Human Condition II" (1935) is not about waves, even though it has got waves in it. This is a painting about modelling.

Inside the room stands an easel bearing a painting, which depicts the waves in the landscape outside the room. The painting on the easel gives the illusion of blending in with its subject: only close inspection reveals where one starts and the other ends. This wry joke rings true for engineers: unless we pay attention, we could risk confusing a model with the real system.

Monday 24 June 2013

Peter Fraenkel on fundamental technology challenges - - - - - - - - - - - - - - - - - - a discussion

The fundamental challenge of large torque

At the quickfire seminar series at All Energy 2013, tidal energy veteran Peter Fraenkel (Fraenkel-Wright) gave his views on fundamental engineering challenges facing both tidal stream and wave energy: the lower the speed of the prime mover, the higher the torque, and the bigger the machinery. He estimated that tidal turbines were an order of magnitude slower, and WECs (wave energy converters) two orders of magnitude slower, than fossil fuel turbines.

Peter Fraenkel’s summation of the fundamental engineering constraints carried the implicit suggestion that wave energy would always be more expensive than tidal energy. Perhaps some insight might be gained from examining the reasons why tidal energy has progressed faster than wave energy. Of course I naturally feel compelled to defend wave energy as a matter of principle, so while I acknowledge that tribalism is a motivating factor for me, I shall do my best to prevent this from affecting my objectivity.

Monday 17 June 2013

AEO - fundamental challenges

Technology comparison

At the quickfire seminar series at All Energy, tidal energy veteran Peter Fraenkel (Fraenkel-Wright) gave a talk comparing ocean energy technologies. He gave his views on which technologies were the most likely to become commercially competitive. He did not think that OTEC, tidal barrage, or shoreline wave were close to becoming competitive in the near future. Perhaps unsurprisingly, he thought that tidal stream stood a good chance of being competitive with offshore wind. He thought that offshore wave had a chance of becoming competitive, but that it faced greater challenges than tidal stream.

Tuesday 11 June 2013

AEO trends and titbits - cost of energy

Here’s what wave and tidal developers had to say about improving the cost of energy (CoE), at the recent All Energy Opportunities conference. There were some interesting pointers to where the technology is heading.

Monday 3 June 2013

AEO Trends and Titbits - commercialisation

It was simply not possible to see everything I wanted to at All Energy Opportunities 2013. Quite apart from half the exhibition hall being prematurely closed due to an excessively mobile tent, some of the talks were on concurrently. This year there was a new quickfire seminar series where wave and tidal developers could report on their progress. I spent as much time at these talks as the official conference (sessions Wave and Tidal 6&7, and Offshore Wind 3). There were some interesting comments about the commercialisation process. Here are the highlights:

Thursday 30 May 2013

RenewableUK and SI Ocean supply chain workshop

RenewableUK hosted a pre-All Energy workshop that reported SI Ocean's work to date, and discussed interaction between the supply chain and the wave and tidal sector.

Several reports were unveiled:

In the supply chain discussion, there was a real sense of urgency in getting the sector off the ground. Conflicting viewpoints were expressed. Sometimes this lead to insight and consensus; sometimes to an almost tangible frustration about impasses lying on the road ahead.

Tuesday 14 May 2013

Travel Info for All Energy

Why I'm not buying carbon compensation for the All Energy Opportunities event:

The Venue and Travel page on the All Energy site is a good example of how far we as a society have to go to achieve a low carbon economy.  

Monday 13 May 2013

Wave and Tidal Knowledge Network

The Crown Estate have launched a website that gathers together useful information for wave and tidal developers:

This includes the most complete list of resource data to date.

Wednesday 8 May 2013

Sleepwalking into an energy crunch

Featured talk from IET 'Wave and tidal: towards commercialisation' seminar.

Speaker: Martin Wright - Managing Director Aurora ventures Ltd, Chairman of Mojo Maritime and the Renewable Energy Association, a past Chairman of the Ocean Energy Group, and one of the founders of Marine Current Turbines Limited.

The talks in the IET seminar were all very good, but this one in particular left the audience stunned. Apart from the shock of hearing a venture capitalist tell us that venture capital was unsuitable for funding wave and tidal, he presented the audience with a compelling argument for future economic mayhem in Britain if marine renewables are not funded now, and then went on to explain why the funding was not forthcoming. 'All is not rosy' was a running theme. His claim that we are 'sleepwalking into an energy crunch', although bleak and somewhat apocalyptic, was based on solid arguments. The principle one being:

Those making the decisions about the UK's future energy mix do not shoulder the risk of hydrocarbon price volatility.

Monday 29 April 2013

IET Towards Commercialisation seminar

This thought-provoking seminar provided much pragmatic optimism and frank discussion of often taboo subjects.

Monday 22 April 2013

Mysterious waves on a living room wall

For a change, here is some art that is not on the internet or in a gallery. It's on the wall in my step-uncle's playing-about-on-t'internet room. He inherited it from his step-dad, who ran a pub on the harbour front in Ramsgate, and had accepted it as payment for a bar tab. The artist, Paul Teschinsky, is thought to be a German Jew who came to Ramsgate just before the Second World War.

A quick search on the google showed that his favourite subject during the First World War had been German warships and submarines. Many were of a patriotic nature, perhaps approaching propaganda; some illustrated post cards issued by the German army postal service. Ironically, the ever present enemy alluded to in these painting would later give him refuge from his own country: paintings were titled 'War against England stormy day in the North Sea', 'German submarines off the coast of England', and 'The turbine cruiser Dresden chasing the English steamer Mauretania'.

Monday 15 April 2013

The radiation paradox

Ya cannae get aroond conservation o' energy, ya reekie oxtered sassenachs yae. (Translation from the Scots: You can't get around conservation of energy).

Conservation of energy from a far field perspective
Consider an arbitrary boundary around a big bit of sea. In order for a wave energy converter (WEC) to absorb energy from within that boundary, the waves leaving the boundary must have less energy than the waves entering the boundary. The only mechanism to capture useful energy while ensuring less energy in the waves leaving the boundary, is to create waves that destructively interfere with waves that would leave the system in the absence of energy capture. To create waves while absorbing energy involves the radiation of waves by damped (by a power take off system that is) wave excited motion.

Monday 8 April 2013

Opportunities for sites with low wave resource

Many commentators have pointed out that locations with low average wave power tend to have a smaller ratio between average and extreme wave powers. This ratio between average and extreme wave powers is one of the underlying challenges of extracting wave energy. This suggests that lower energy sites could give cheaper wave energy.

Monday 1 April 2013

The importance of big industry in wave power

An understanding of the importance of large industrial partners to the wave energy industry can give important insights into some of the challenges currently facing wave energy. The main reason large industrial partners are essential is that demonstration projects typically have large capital and operating costs. 

Monday 25 March 2013

Why do investors want high nameplate power ratings?

There is a lot of focus on the nameplate power ratings of wave and tidal projects. A good example of this is Aquamarine’s choice of name for their second prototype. The ‘Oyster 800’ is not the most obvious sequel to the ‘Oyster 1’. The name does however draw attention to the 800kW power rating, which happens to be higher than the 750kW of competitor Pelamis. One could almost imagine that there was a race of some sort!

Tuesday 19 March 2013

Outstanding waves from Escher

M.C. Escher’s Phosphorescent Sea (1933) is possibly the most beautiful picture of waves I have seen. It is possible that I like it so much because I am familiar with Escher’s work, so I expect optical illusions, allusions to the impossible, interplay between background and foreground, and playfulness from a mathematical perspective. The Phosphorescent Sea has none of this obvious trickery. I admit that it is possibly my own imagination that applies what I expect from Escher to this picture, but let me share it with you anyhow.

Monday 11 March 2013

Free tanks tests and sea trials

Marinet is an excellent resource for marine renewables developers. This EU funded project provides free access to 45 experimental facilities across Europe, as well as expert advice from the staff based at these facilities. The experimental facilities can model waves, tidal currents, wind, and even combinations of these. Additionally, there are facilities to model power take off, including two operational shoreline wave energy converters for full scale turbine testing. There are two offshore sites for sea trials of wave energy converters. There are options for testing moorings, corrosion, and materials.

In order to be eligible for funding, the lead and most of the team members must be from the EU (or a country associated with the Framework 7 program), the team must test in a different country from their own, and the team must be willing to publish results in some form (arrangements can be made to protect IP and commercial sensitivities).

If you know of anyone who might benefit from this resource, please tell them about the MARINET website. The deadline for the 3rd call is the 27th March.

MARINET also offer free training courses that are available to both industry and academia.

Monday 4 March 2013

RenewableUK 2013 Wave round-up

Worst joke: In the present financial climate we are in the unfortunate situation of only being able to back one horse. Let's hope it doesn't become horse meat.

Closest to fisticuffs: Martin McAdam (Aquamarine) and Guy Newey (Policy Exchange). The chair Juliet Morris swiftly changed topic when Guy and Martin entered into a lively discussion about whether the global wave energy resource was large enough to justify investment in the sector. 

Doom and Gloom: Apprehension about the electricity market reform dampened the party spirit for both wave and tidal energy: the lack of a stable regulatory framework does not provide the desirable long term strategy message. Likewise, longer-term planning is hindered by uncertainty surrounding austerity and Scottish independence.

There was however a cautious optimism for tidal energy that was notably absent from the wave energy discussions. Perhaps there was a sense that, with limited budgets, the recent industry investment in tidal had dried up the funds available for wave energy? The fear of 'winner takes all' was again expressed in discussions of the outcome of recent EU funding rounds: the two NER300 marine projects in the UK were both tidal. 

Monday 25 February 2013

In a nutshell: technology readiness and performance matrix

Weber (2012), 'WEC Technology Readiness and Performance Matrix - finding the best research technology development strategy', 4th International Conference on Ocean Energy, 17 October, Dublin.

Fig 1: Readiness-performance matrix (reproduced with permission from Weber)

The technology readiness level (TRL) is the de-facto metric for describing the development stages of wave energy technologies. The TRL indicates a technology's maturity, and is strongly related to the amount of investment. Weber's paper proposes a second metric, the technology performance level (TPL). Arranged like TRL on a scale of 1 to 9, it measures a technology's economy, and is inversely related to the cost of energy.

In a two-dimensional graphic these two metrics give us the readiness-performance matrix. The development trajectory of a wave energy technology can be plotted against it. Fig 1 shows three hypothetical trajectories:
  • Orange: Readiness before performance
  • Green: Performance before readiness
  • Black: Performance and readiness developed together

Monday 18 February 2013

Liberazione di Andomeda (Piero di Cosimo 1461) - they don't make art like this anymore!

They don't make art like this anymore. Partly because no one would take a sea monster that looked like the grandfather of the Luckdragon from the Never Ending Story seriously. And partly because our expectations have been raised by the ubiquity of fairly decent physics to represent fluids in computer games and animated film.

Monday 11 February 2013

Complex conjugate control without the equations

Many people would like to know about complex conjugate control of wave energy converters, and the problems with its implementation, without wading through masses of equations. Ok, lets not kid ourselves, only a few people want to know this, but I cater to all tastes, including highly refined ones.

While I won't use equations directly, I will need to make reference to a well-known mathematical model, if you're an engineer, that is. Different mathematical models suit different purposes. For explaining how complex conjugate control works, a simple linear mass-spring-damper model, with excitation at one frequency only, is a good place to start. If you need to brush up on this model, there are some reminders here.

The natural frequency of the system (fn) is where the response amplitude is highest, which happens to be the only frequency where the response phase is zero (with respect to the excitation phase). A system that is being excited at its natural frequency is described as resonant.

The basic idea behind complex conjugate control is to have a small (= low cost) absorber that has a natural frequency higher than the power-rich frequency components of a typical wave spectrum, and to then force the system into resonance by doing clever things with the power take off (PTO) force.

Wednesday 6 February 2013

Doctor doctor! I had to tie it down to stop it thrashing about!

Q: We had to tightly moor our device, otherwise its movement would have resulted in waves being radiated away. What could be causing this problem?

First we need to define what we're looking at. We'll dismiss the trivial case of a single body absorber: clearly preventing its motion will give us zero power capture! Hence we must be talking about a floating system with more than one wave activated body. Furthermore, as we talk about tightly mooring the device, it makes sense that one body is a large structure and the other smaller body/ies (e.g. oscillating water columns or flaps) move/s with respect to this main structure.

Thursday 31 January 2013

Doctor doctor! I see waves before my eyes!

Q: If I can see waves being radiated, that's good ... right?

Fig 1: Duck radiating a wave

We've established that wave energy converters need a radiated wave to cancel out the wave being absorbed.

Question is, if you are watching a device operating in a wave tank or a sea trial, and you can see a wave being radiated, does this prove that the device is doing a good job of power capture?

Monday 28 January 2013

A quick recap of linear mass-spring-damper models

In the spirit of not frightening away readers with equations, I shall refer to the Wikipedia (1) (2) and wave in the general direction of Wolfram (1) (2) (3). A user-guide follows:
  • If we know how a linear system moves in response to an excitation by a single frequency (a sinusoid) we can use superposition to find the response to an excitation composed of many frequencies; so let's make things simple and consider single frequency excitation.
  • The response to such an excitation is the sum of a transient which dies away, and an underlying steady state response: a sinusoid with the same frequency as the excitation, but a different amplitude and phase.
  • Both the amplitude and phase of the steady-state response depend on the forcing frequency. They can be plotted against excitation frequency (Fig 1).
Fig 1: Universal resonance curve

Monday 21 January 2013

Does Falnes's wave maker diagram apply to all WECs ?

Fig 1: Falnes's wave absorber = wave maker diagram
Falnes's assertion that 'a good wave absorber must be a good wavemaker', and his diagram depicting the amplitude and phase conditions for optimum absorption have been around since the early days of wave energy theory. I'd always thought that the wave absorber = wave maker diagram (Fig 1) was general, and applied to all wave energy converters. Would you be surprised if I told you that it applied to point absorbers only? (by point absorber I mean a body that is at least an order of magnitude smaller than the wavelength of the incident waves)

Monday 14 January 2013

Botticelli's Venus: nice goddess, pity about the waves.

You know you've reached the room that houses Botticelli’s Venus by the crowds of people obscuring the view of the painting. It is certainly stunning from up close though.

Like a beauty spot, this painting is all the more beautiful for it's flaws. Consider the half-shell.

Monday 7 January 2013

Wrong turns on the way through the 96% capture maze

The photo below (Fig 1) was used to show that 96% of the energy in the wave coming from the right had been absorbed by the duck. In a previous post I showed how we can draw this conclusion from the photo alone. Like navigating through a maze, my first attempts at solving this problem took me up several dead ends. It is worth discussing these, as I'm sure I'm not the only one to have made these mistakes?