Making a Wave Energy Converter Part 2: Stuck in a Design Loop

Energy & Sustainability Workshop
Making a Wave Energy Converter Part 2: Stuck in a Design Loop

This is part two of a 10-part series chronicling the R&D of a wave energy converter. Read part one.

WEC_prototype development

It had been four years since I first learned about renewable ocean power technology. I now had a team, funding, and the beginnings of a plan to build a wave energy converter (WEC) using off-the-shelf parts so that others could take our plans and improve them. Even before the project ever started, I had countless designs all worked out in my head. Yet, as painful as it was at the time, my initial vision would eventually have to be shaped and refined into a working physical prototype grounded in solid engineering fundamentals and teamwork. Over the duration of the project, the WEC design would go through three drastic design iterations before reaching the fourth and final design, WEC_004. Our grant sponsor, CITRIS, had given us one year to implement our proposal and spend the funds accordingly.

However, as senior engineering students at UC Davis we all had to participate in a senior design project before graduating. After speaking with the class professor, we decided to roll the grant funding into our senior design project with the understanding that this would accelerate the project timeline. This meant we would be able to focus all of our time and attention on one large project rather than dividing our time between a senior design project and the renewable energy grant project. The timeline was set in stone, and we had to finish the project in order to graduate. We would spend the first 12 weeks of the class researching and designing, and the second half of the class we would build the WEC in the student shop and test it in the ocean. The countdown began.

Initial WEC concept design.
Initial WEC concept design.

The first two weeks of the project we worked together daily. Even when there was no official team meeting scheduled, we would inevitably run into other teammates in class and the conversation would always drift back to the WEC project. It was unavoidable. We knew we wanted to build a WEC and were hoping we might be able to test it in Bodega Bay, so the next step was figuring out how to convert the motion of the ocean waves into electricity.

Two years earlier I found an in-depth research paper titled “A Review of Wave Energy Converter Technology” while researching wave energy for a class presentation. Since we had less that 12 weeks to design the WEC, it was understood that we weren’t going to develop some new, novel way to harness energy from waves. Instead we planned to incorporate existing technology into a more accessible platform that others might be able to replicate. The manuscript was a great place to start, and I highly recommend it to anyone interested in wave energy, as it’s packed with jargon and terminology related to ocean power.

Within the first week the team agreed to focus on designing a point-absorbing linear drive system that would bob up and down in the waves to produce electricity. Since we wanted our design to be readily adopted by others, we felt the device had to be able to operate in a wide range of wave conditions. Point absorbers are able to collect energy from waves regardless of where the waves are coming from. This makes them particularity suitable for locations like Bodega Bay, where the waves predominately approach from the northwest but can suddenly switch and approach from the south during large storms. By making the floating buoy circular in shape, the WEC could receive waves from any angle. Being that a linear drive system only has 1° of freedom, our design would be concerned with harnessing the heaving, up-and-down motion of the waves. This linear drive design made the mathematical modeling more manageable and would also be much easier to fabricate in the student shop.

WEC_001 early design of power-take-off system
Early design of power take-off system that fit inside the WEC_001.

Now that we had decided to use a point-absorbing linear drive system we then needed to select a power take-off system that would physically convert the heaving motion from the buoy into electrical energy. We could’ve used magnets and wire coils to directly convert the up and down motion of the waves into electricity. A few years earlier I had used this idea to build a wave energy converter model using neodymium magnets and coils of magnet wire. It’s the same principle used in the rechargeable flashlights that you shake: as the magnet moves in and out of the coil, an alternating electric current is induced in the coils. As long as the magnet is moving you can create electricity; however, the voltage and frequency of the electricity fluctuate based on the speed and displacement of the magnet, which makes collecting the energy more complicated.

The team’s WEC project was going to be much larger than my tiny PVC pipe model, and since none of us were electrical engineers the thought of building a circuit to rectify and condition the signal terrified all of us. Even without phase and rectification issues, the price of rare earth magnets was skyrocketing, which made the idea financially impractical.

Mini WEC model built in 2010 using magnets and coils
Mini WEC model built in 2010 using magnets and coils.

Next we considered using pulleys and counterweights encased in a watertight housing: a network of pulleys, ropes, and weights all moving together, anchored to the ocean floor and tied to a floating buoy bobbing at the surface. Team member Alex Beckerman was especially fond of this idea, because in his engineering mind you can model anything in life as a system of masses, springs, and dampers and then use calculus to find the solution. However, with all the pulleys and bearings needed to make this work, we estimated that frictional losses would greatly decrease the overall power output and that the likelihood of a cable wearing down and breaking was too great.

Hydraulic circuit for linear drive system
Hydraulic circuit for the power take-off (PTO) system for WEC_001 – WEC_003.

Looking for a more simple design with fewer moving parts, I suggested that we use hydraulic components. Several research papers online referenced a basic hydraulic circuit that used a double-acting piston to pump fluid through a series of one-way valves and spin a hydraulic motor coupled to a generator. The parts were easy to find, and most of the fittings and components where rated for high operating pressures and could withstand the massive forces and corrosive ocean environment. There were even articles describing how to model these types of hydraulic circuits in a computer simulation to predict the potential power output as a way to evaluate the effectiveness of the system.

The only issue is that no one on the team had much experience working with industrial hydraulic components. Sure, some of us had experience fixing plumbing around the house or installing irrigation values, but designing a system with high-pressure accumulators and hydraulic motors, while factoring in pressure losses due to orifice restrictions, was foreign territory. Weighing all of our our options, the team determined that the seemingly relative simplicity of the hydraulics system and the ease of finding off-the-shelf parts made this the winning choice.

By the end of the second week we were feeling pretty good. We had made big decisions about the general layout of the design and now that we had agreed to use hydraulics to build our power take-off system, the team could split up and each of us could start working on the various subsections: hydraulics, electronics, buoy, heave plate, and spar. What we didn’t realize at the time was how each subsystem was interconnected to ALL the other subsystems.

In textbook problems, most of the variables are either given to you in the problem statement or you can look them up in a table somewhere. So when we sat down to start working on the calculations required to “solve” the WEC design problem, we found ourselves getting stuck in a massive design loop unable to make any significant decisions because there where too many unknowns. For example, team member Tom Rumble would try to determine how large of a hydraulic motor to purchase, but he needed Kevin Quach to select the hydraulic ram first, but Kevin needed to determine the bore of the hydraulic cylinder, which was directly related to the amount of pressure in the system, which led back to the question of the hydraulic motor. Alex was trying to design the large metal heave plate, which would provide hydrodynamic drag that would resist the upward pulling motion from the buoy, but first I needed to determine the total size of the buoy. But before I could calculate the total buoyant force required for the buoy, I needed to know the total weight of the WEC from Teresa Yeh and the weight of the heave plate from Alex. This continued for four miserable weeks and the team slid into a unproductive rut.

Heaving motion of the WEC-004 design in ocean waves.

By week six I was having trouble sleeping at night. I would lie awake in bed gazing at the ceiling, trying to untangle the entire project like a ball of string all knotted together. The magnitude of the project was weighing on me, and I could tell the other team members were feeling the pressure pile on. Midterms were upon us, which meant project deadlines went out the window. Team meetings became tense, as one person would try to explain that they could solve their portion of the problem if only someone else would give them the information they needed.

By now we had moved on to the WEC_002 design with a neutrally buoyant spar made from a 20″-diameter HDPE pipe and prefabricated roller assembly on top of the buoy, but the PTO had made little progress. To add the frustrations, I was trying to figure out how we would actually test this thing once we built it, but my emails to the marine lab were going unanswered and we really didn’t have a backup plan. Something had to change, and fast. Had we been doomed from the start, was this too big and too ambitious of a project for the five of us to tackle? I was beginning to have my doubts.

Tune in next week for part 3!

13 thoughts on “Making a Wave Energy Converter Part 2: Stuck in a Design Loop

  1. Ian says:

    This seems like a strange problem for an engineering student to be stuck in for weeks… Although the interconnected design variables might keep you in a loop, they are also connected to manufacturing variables which many times force you to narrow your design. Then you identify the characteristics you want to optimize (lifespan, electrical output, weight, cost…) and pick the design variables that give the best embodiment if that characteristic. Just keep looping through that decision cycle and eventually you are left with small design choices that are usually inconsequential to form, fit, or function.

    Hopefully the next installment leads you down that exact path!

    1. Nick Raymond says:

      Ian, you bring up a great point. While it took us much longer than I would like to admit, this is indeed how the design loop was eventually broken (spoiler alert!). For me personally, the equations and book problems from class do not always directly correlate to real world applications in an obvious way. That’s why student projects and collaborative efforts are so critical to the learning process; unlike the professional realm students are allowed to make mistakes and use their experience to further their education. They force you to realize that there are very few “perfect” solutions to any one problem, and that the field of engineering uses the iterative process (something students may be resistive to appreciate until you get your hands dirty) where you have to continue to refine the design until it satisfies the criteria and expectations.

      It took us weeks of being stuck in a design loop to finally reach that “Eureka” moment when we finally broke the vicious cycle, but THAT was a valuable lesson and experience that we will never forget.

      Thanks for the comment and insight.


  2. David Lang says:

    NICK! This is AMAZING!! I love this series. Can’t wait for the rest of the posts.

    1. Nick Raymond says:

      Thanks David. I appreciate the feedback, especially coming from you.

  3. Dominic Le Prevost says:

    Looking forward to reading more. I spent months obsessing about wave power a few years ago. The only idea I came up with that I didn’t easily discover proposed or trialled elsewhere was rather than using an expensive inefficient network of cables it might be a good idea to convert the electricity to hydrogen and have a robot ship go from generator to generator collecting the compressed gas. Fancy turning your generator into a mini floating Hindenburg?

    1. Nick Raymond says:

      Hi Dominic,

      You bring up a great point. Instead of an expensive and inefficient network of underwater cables, this technology could be used for localized hydrogen production. I especially like the idea of the robotic ships, but as you stated, safely transporting large volumes of hydrogen would be yet another design challenge to overcome.

      During a guest lecture at UC Davis, Professor Belina Batten of the Northwest National Marine Renewable Energy Center, estimated that it cost one million dollars per mile to install the underwater sea cable. This was specifically for the new test center being installed in Oregon, but I can image that this would be comparable to most installation costs for wind and wave energy. Getting the power back to shore is a serious engineering challenge, which is why large farm networks are often proposed. This way, large clusters of generator can all share the same line used to send power back to the grid. Thanks for the comment.

  4. says:

    Greetings! Very useful advice in this particular article!
    It’s the little changes that will make the most important changes.
    Thanks a lot for sharing!

  5. Joe Burnham says:

    Glad to see students interested in the most practical “high denisity” form of alternative energy available on the planet. It is a crying shame that we / US are not the leaders inocean wave / tidal energy. I have a an idea I have had for years, sort of along the lines you are following , However my idea , I call it kiss ( keep it simple stupid …) I am not an edcuated person but I own a facility on the gulf of alaska , on the 3rd highest energy denisity beach on the planet.
    My idea as yours is to use a heaving bouy and hydraulics, execpt I am using a weighted piston
    wiithin a cyclinder with three intake flapper valves , ( something like old diaphram trash pump valves, ) and one discharge flapper valve , wave falls three flapper valves open weight of piston draws cyclinder full of water, wave rises three flapper valves slam shut and discharge flapper opens allowing pressurized seawater to be pumped to shore where it will first hit an accumlator to eliminate the hydrulaic shock and then to a conventional pelton wheel and generator. Essentially a heaving bouy seawater pump ??? why not? we already pump millions of gallons of oil and natural gas allover the place sub sea, why not high pressure sea water ??? Idea here is that your generating facility will be land based so it is easy to access to effect repairs ,moniter ect. I look at all these damn dumbass ideas what with putting the generating equipment at sea, or subsea and I roll in my grave and I ain’t even dead yet….
    You folks want ot get serious about this I have a 20,000 square foot facility on the Gulf of Alaska that I will make avialable to you ( free of charge ) for you to do your practical research from . Thanks for allowing me to comment .
    Ps, the down stream is to take the surplus electricty crack water , make Hydrogen and oxygen
    also these units or multiples of can be situated so the generating station can be hooked into the existing grid . Good Luck with your project :) Joe

    1. Nick Raymond says:

      Hi Joe,

      Thanks for the comments. Always nice to hear about other people’s projects and gain a new perspective about the same problem. The sea-water pump is a fantastic idea, I also have thought about using ocean water as the working fluid to spin a pelton wheel turbine and produce energy (although I have some concerns about using the corrosive salt water in the pump mechanism) . I’ve always been fascinated with the pelton turbines due to their high efficiency and interesting double spoon configuration. In fact, one of my first ideas for the hydraulic power takeoff system was to use a the hydraulic fluid from the piston moving up and down to spin a small pelton turbine installed onboard the WEC. Very similar to your own design.

      As you noted, having the power generation system on land and not on the WEC makes maintenance much easier/cheaper/safer. The trade off is that you have to run long lengths of pipe instead of power cables, which may prove to be a substantial pressure drop depending on the size of pipe and length the sea water has to travel and the speed of the moving fluid. Our own system was relatively small so even additional pressure drop of only 5 PSI was enough to warrant concern. From the scale of your setup, you may have much larger pressures during the down stroke as the weight of the buoy forces the water through the 800ft-1000ft of pipe, and the pressure drops in your system may be negligible, not sure but worth investigating.

      One design challenge our team had was properly sizing the buoy for the WEC 003. If I am understanding your design correctly, as the wave height increases the buoyancy of the buoy causes the piston to rise, and as the wave heigh decreases the weight of the buoy causes the piston to compress and force the sea water to shore. In this way, the volume of the buoy and the weight of the buoy are two critical design parameters. If the buoy is too heavy, it will not float but if it is not heavy enough it will not be able to compress the piston. Like wise, if the volume of the buoy is too small it will not displace enough water during the up stroke and become submerged. One way we tried to solve for this was to determine the total force required to compress the hydraulic cylinder when it was fully extended (for your design this would be pushing sea water to shore after the wave crest, for our design it was pushing fluid through the hydraulic hoses and through a hydraulic motor).

      To get an estimate for the total force required, we approximated all of the pressure drops in the hydraulic system to determine the total change in pressure. For example, we knew that we would lose 5 PSI of pressure just from the “cracking pressure” requirement of the check valves. Additionally, there would be small pressure loses due to friction in the rubber hydraulic hoses and the hydraulic pipe fittings we were using. There is a niffty free online tool that can approximate pressure losses for various pipe flow conditions and sizes, and even inlet-outlet conditions if you have converging and diverging connections.

      So, once we had the summed up all these little pressure drops and included the large pressure drop from the hydraulic motor (this value came from the total power output of the motor for a specific flow rate as found in the data sheet) we had a total change which accounted for the drop in pressure from the face of the piston all the way to the outlet port of the hydraulic motor. Lets say we calculated that it was 1000 PSI ( I can’t recall off hand what the exact value was). From here, we treated the problem as a hydro-statics problem, using the area of the piston face to solve for the necessary downward force of the buoy. Our piston face was 2 inches in diameter, giving a total cross sectional area of approximately 3.14 inches^2.

      Pressure = force / area

      Solve for force:

      force = Pressure*area = (1000 Pound/in^2)*(3.14 in^2) = 3140 pounds of force

      BUT – since the buoy is in sea water, it will also have a buoyancy which means that the weight of the buoy must be increased so that when you put it into the water the NET weight of the buoy in water is 3140 pounds (according to this approximation). This was our team’s dilemma, there was no way could build a 3000+ pound buoy, it would cost too much and require a crane to move it. However, this is why I believe large scale wave energy converters are more effective and practical, as the size of the entire project is increased so too is the power output so the cost per watt should decrease. Not sure if any of this was helpful to you specific project, but your comment inspired me to start typing and this is what came out.

      Thank you for the generous offer, your facility looks impressive ( I looked it up on google maps) and being that it is right next to the coast it should make for a very convenient test site for your future ventures. One thing I noted, are those large tree trunks along the shore line? If so, any thoughts on how to prevent the tree trunks from hitting your buoy? Look forward to hearing more, and thanks for the email and comments! Best luck on your own wave energy projects.


      1. joe burnham says:

        hey Nick thanks for the comments and info
        however , i am trapping the heave energy
        of the wave on the up stroke , as the buoy
        starts up the in take flapper valves slam shut
        and the discharge to shore flapper valve is forced
        open and the water is moved shoreward to the
        accumulator and then pelton wheel .

        Do the math , the displacement lift less
        its own weight is still significantly
        more than
        the weight of the falling piston
        I weight the piston so heavy , to 1.
        help keep the buoy centered over
        the cylinder 2. assure that the piston
        travels the maximum extent possible
        on the falling wave , and its constantly loaded
        so no extreme shock factor and also
        keeps it in time with the waves .
        later on i may add a low pressure side
        to recover the energy on the down stroke.

        i am making a practical working system
        here not some bullshit in a wave tank
        i am dunbfounded by all the research
        funds pissed away and so few practical
        systems that will stand up to a 10 year
        or 100 year storm event .

        yes those are logs on the beach some 130
        ft long many in the 80 to 100 ft range .
        as a rule they come along as single logs
        not in packs, they form dense log jams as
        they are thrown up on the beach .
        another reason to have the buoy/pump
        assembly as far offshore as possible

        also another reason to use a sphere
        or very least round cylinder buoy
        the average life span of your
        channel marking sea buoy is 50 years
        with very little maintence , i don’t really
        envision logs as my problem …

        The thing i haven’t quite worked out yet
        is storm waves , as I said 3rd highest
        energy density beach on the planet
        thing i have to figure out is a system to
        reel in the pennant line to draw the buoy
        under the wave during storm events and
        also allow the system to continue to
        produce power.
        I am think of a hydraulic / water pressure
        driven winch mounted on or in the buoy
        which during storm conditions will reel
        its self in . governed by wave height ?
        or frequency ? or heave pressure ?

        this is where you geniuses/ engineers in
        any thoughts on how i control this
        beast ?

        as far as pressure losses due to friction
        in the line ?? who cares ? this thing will
        will supply more power than i need by
        a long shot . Bigger buoy , larger pipe
        no muss no fuss .

        I fly and have many friends who do and some are so
        anal about getting there plane to its very
        lightest so they can carry an extra
        few pounds ….. most of which they could just
        look in the mirror and figure out real quick
        how they could carry that extra ten or
        twenty pounds !!!!!
        anyway friction and line losses are the least
        of my concerns . Its the damn storms is
        i have to design for .

        Good luck with your project , I look forward
        to updates as it moves along .
        remember , the kiss rule and don’t worry
        too much about losses , mother nature has
        already allowed for that , it aint like is going
        into space and has to be super efficient
        just build something thats practical and
        can be up sized to usable application size
        in a real world ocean environment .

        the place is there if you decide to build
        something in real world conditions
        good luck.
        joe :)

  6. Joe Burnham says:

    looking forward to your next blog…

  7. jagjit says:

    hello Nick i m doing this project for my final yr can u help me

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Nick is a recent UC Davis mechanical engineering graduate. He enjoys building and programming CNC machines, surf trips along the California coast, and home brewing belgian ales.

Most recently Nick has become fascinated with the field of ocean engineering, and is working to combine sensors with mechatronic systems for ocean research projects.

Contact me at nraymond(at)makermedia(dot)com

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