Wave Group Reports

 

 

In response to the 1973 oil crisis, Professor Stephen Salter (1938 – 2024) nudged the UK Department of Energy into funding wave power research. This led to the 1974 Wave Energy Programme, which funded several designs of wave energy device. Due to political unpopularity, funding of specific devices was abruptly and controversially terminated in 1983. During the years that followed, the Edinburgh Wave Power Group worked on projects for generic technologies. In the absence of academic funding, there was also more focus on commercialisation of technologies that had been developed during the funding programme, such as wave makers and digital hydraulics pumps.  

Professor Salter’s research lab, ‘the Portacabin’, had copies of all the documents associated with the Edinburgh Wave Power Project. As a PhD student, these reports were revered artefacts. They all had a particular look about them, even those authored by his researchers a decade after the end of the funding programme. Access involved knocking on the Portacabin door; promising to put them back into the right place; etc. A few years ago, Professor Salter decided to make this collection available to the wider world via the Edinburgh Research Archive. I had promised a link from my blog, but I didn’t get around to this, as I was disturbed by subsequent additions to the archive that weren’t part of the Portacabin collection. To get around this, here are the links to the archive items that I associate with the Portacabin. It’s worth looking at the main archive page if you wish to search by author or subject.

 

Old Skool Significant Wave Height Calculator

Sometimes you just want a quick and dirty calculation of significant wave height and period. Back in the day, these were calculated using the zero crossings method, and this can still be used if you want to avoid Fourier shenanigans.  Here is a spread-sheet that will calculate zero-crossings period (\(T_z\)) and significant wave height (\(H_{1/3}\)). Now it's never been easier to check what scale of model yatch you should be using on your local duck pond! 8¬)

Marine Renewable Energy for beginners

I’m very excited to welcome the participants of ‘One planet – one ocean’, a Massive Open Online Course. From watching earlier Q&A sessions, I can see that you come from all around the world, and from all walks of life. For those of you who would like to dig deeper into Topic 10.5, ‘Sustainable Energy from the Ocean’, I have put together this fact sheet (liberally sprinkled with opinion). I used the questions and comments you left next to this lecture to decide what to include. I will be a guest speaker on the Q&A session on Marine Renewables this Thursday. I am hoping that this blog post will spark the curiosity of those who were still deciding whether to listen in, and inspire thoughtful questions from those who already know they will be joining us.

This article covers the following topics: making the case for marine renewables, a list of the main features of the different types of marine renewables, a technology status update, a discussion of the current challenges, and some thoughts on how marine renewables could fit into a future low carbon energy mix.

Six little questions

At a recent speed-networking event, I found myself at a table with the founders of a new wave technology development company. They were armed with a list of 6 substantial questions. As there was no way these topics could be covered in our 15 minute slot, I noted down the questions for further consideration. While writing the email reply, I realised that my answers might be of interest to other technology developers – no so much as advice, but as a starting-point for internal discussions about the questions posed.

Doctor doctor, is negative spring like a flux capacitor?

Many thanks to the reader who asked me the following question about the post on negative spring:

' I am a bit puzzled as to how two springs in series can exert a force at right angles to their longitudinal axis. Didn't the pair of springs (providing the negative spring) in the CorPower device sit at an angle in a Back to the Future "flux capacitor" type arrangement?'

Negative spring is a mysterious thing

Negative spring may well be the answer to making wave energy economic, so it would be good if it were less obscure and mysterious. This post describes what negative spring is, and how it can be used to control wave energy converters.

Tank check-list for wave energy

What type of tank is needed to test a wave energy absorber? Standard tanks for testing ships are a good start, but wave energy has 'special needs'. Here is a check-list of specifications, including a wish-list of features that are nice to have. They are grouped under 7 headings: repeatability, depth, working area, get the waves you want, know the waves you get, get the data you need, don't let scale muck things up.

Comparative tank test challenges: Part II - Reducing systemic errors & bias

Part I discussed some of the inherent difficulties with comparing different concepts of wave energy converter using tank tests. Some of these problems we are stuck with; others can be avoided if we know about them in advance.

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.

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

Definitions in depth: Oscillating Wave Surge Converters

The term 'Oscillating Wave Surge Converter' (OWSC) is primarily used to describe devices that absorb wave energy by moving in …

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.

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 (f_n) 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.

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

How to design a device that absorbs no wave power

1. Do not provide a point of reaction for the PTO. e.g the PTO opposes surge motion between two floating vertical plates, one behind the other; ensure one of these plates is light and porous so it offers little resistance to the PTO force; ensure they are close together to minimise excitation phase difference.
   
   
2. Avoid restoring forces in the degree of freedom (DoF) where power is extracted. There is no buoyancy restoration in surge, sway or yaw, so extract power from these DoF, ensuring that extra spring is not inadvertently applied. Moorings provide spring in these DoF, so just let the device drift where it wants.
   
   
3. Avoid useful wave radiation. Ensure that when the power take off (PTO) system is run in reverse (motoring), the waves generated are as small as possible. Better still, generate large waves that travel in the opposite direction, or have a similar phase, to the waves you're trying to cancel out. The next three points describe pragmatic methods for achieving this goal. 
    
4. Get the level of damping wrong. Light as a feather or heavy as a brick.
   
   
5. Mismatch natural period and wave period. Design the device so that the natural (resonant) period is as far as possible from the energy period of the waves you want to capture. 
   
   
6. Make it really really small. This will make the bandwidth of the capture curve really narrow; combined with a mismatch between natural period and wave period this will ensure very little energy is captured indeed.
   
   
7. Pump up those losses. Induce turbulent flow by lining exterior surfaces with mesh beaches similar to those used in wave tanks. Use a leaky transmission system. Design the bit just above the water-line so that small motions induce slap and slam loads. Install the biggest generator possible to ensure you stay at  the low end of the generator efficiency curve during normal operation.
   

Seven key principles for converting wave energy

1.  Provide a point of reaction. Power is captured when wave induced motion is damped. The force resisting the motion results in an equal force in the opposite direction. This has to be opposed by something; no sky hooks allowed.
    
2. The DoF where power is extracted must have a restoring force. There must be a way to restore the change in position due to wave induced motion. Gravity is the simplest method; buoyancy does the job for heave, pitch and roll motion. If power is captured from surge, sway or yaw motions, a restoring force must be supplied by mechanical springs, or by coupling to a motion that has buoyancy. 
    
3. Radiated waves capture waves. When the power take off (PTO) system is run in reverse (motoring), the waves generated indicate the potential for capturing ocean waves. Good performance results from radiation of waves that have the same direction of travel, the same amplitude, and the opposite phase, as the waves you want to capture. These are the waves that the incoming wave splits into once it reaches the device: the waves reflected and/or transmitted by the wave energy converter.
    
4. Get the level of damping just right. Like baby bear’s porridge.
    
5. Match natural period to wave period. Radiating a wave which is out of phase with the waves you want to capture is important. At resonance the phase takes care of itself. The options are to design for a natural period that equals the design wave peiod, to have a range of operating modes with different natural periods, or to use the PTO force to trick the system into behaviour with the desired natural period.
    
6. Size matters. The smaller the device, the narrower the bandwidth of the capture curve, and the lower the natural period. These disadvantages trade off against cost, so clearly size is an important cost of energy consideration.
    
7. Manage losses. Reduce losses (PTO, overheads) in low energy seas; shed loads and power in high energy seas.