When looking at all the different concepts of wave power, it is oblivious that different inventors focuses on different issues. In general I think it is fair to say that projects that starts with a more fundamental approach, seems to be more successful than those that starts of with a 'bright' idea. When working with wave power in Intentium, we have set up a list of design guidelines. This list is not complete, and I don't expect everyone to fully agree in all of it, but here it is. What is your design guidelines?

  1. Floating system
  2. Make use of the dominating wave front
  3. Keep mooring lines and PTO separate
  4. Find a robust concept for the PTO
  5. Have a short time energy storage
  6. Ensure a favourable ratio between energy production and size of the structures
  7. Avoid risk of oil spill or other pollution
  8. Good concepts for installation, maintenance and survival of the system
  9. Use a control strategy for the wave oscillation if possible

Floating system:

An ocean energy device, be it wind, wave or tidal must either be onshore, bottom fixed or floating. The easiest development path is to take a land based device, like a wind turbine, and install it dry but close to the seaside. The second easiest path is to place a device in shallow waters, bottom fixed. As the world strives to find alternatives to fossil energy, it may seem attractive to jump for one of these two first paths. Pressure is on the coastline and the fishing banks. However, the price is high conflicts related to preservation of coastal scenery and local fishing interests. Moreover, most waters is deeper than what is possible to utilize with bottom fixed devices, and hence if a significant contribution towards a renewable future is aimed for, onshore or bottom fixed devices won't do the trick. Therefore, we believe ocean energy systems should be floating, offshore and relative independent to water depth. Another point about deep floating systems, is that mooring forces are reduced as depth is increased.

Make use of the dominating wave front:

It is said that waves are irregular. This is true, and maybe this is why most developers of point absorbers make them circular. Nevertheless ocean waves almost always have a dominant wave direction. When harvesting wave energy, it is a benefit if the device can absorb energy from a as large width of this wave front as possible. Further on, as waves and weather change direction, the device should be able to turn around and stabilize itself like a weather vane in relation to the incoming waves.

Keep mooring lines and PTO separate:

A wave energy converter (WEC) should be designed to absorb as much wave energy as possible, whereas a mooring-system should be designed to have as little mooring forces as possible. Failure in testing of wave energy prototypes is unfortunately not uncommon, many of which have power take-of (PTO) and mooring-lines connected. Another benefit when separating mooring lines and PTO, is a system that is less sensitive to tidal differences.

Find a robust concept for the PTO:

The repetitive surge and heave motions of the ocean waves, makes it natural and think of a PTO based on direct mechanical transfer, like chains, sprockets, racks, pinions and ratchets. Many wave energy prototypes have used these technologies, many of which have broken down. It is a classical problem; How to transform a powerful (relative) slow motion, to a fast (rotational) motion, preferred for generation of electrical power. By use of direct mechanical transfer, material stress gets very high, with fatigue as a likely result. It is a well known problem in the wind industry. In the case of ocean waves, the problem is even larger due to fluctuations and variations in the motions.

One step towards a better solution could be to use hydraulics. However conventional oil-hydraulics also have some critical downsides. One is high friction and high loss. Another is the risk of oil-spill and pollution. This issue becomes increasingly important if one considers a large scale industrialization of ocean energy.

We think the best solution is to use water as working fluid with a PTO with analogy from hydro-power. In practice this means that we pump water to a hydro turbine. The benefits for this is no risk of pollution from working fluid, and a very good track record from hydro-power for absorbing huge amount of energy without failure.

One should also find a sensible location of the PTO. If the plan is to pump working fluid onshore and have a PTO on land, then the project is either back in the conflict zone or have a long and high-loss pipeline. We think it is best to place the PTO out there either on the unit, or in the very close vicinity.

Have a short time energy storage:

Ocean waves have a certain amount of randomness. Subsequent waves differ in height, duration and shape, with a limited predictability. To even out the peaks and the dips in the wave energy, it is beneficial to have a short-time energy storage connected between the float (or other absorption element) and the PTO. With this it will also be easier to find a cost sensible nameplate rated power for PTO, and again increase general the overall energy absorption. This shows the main disadvantages with direct driven linear permanent magnet generators in wave energy. These generators gives no possibilities for short-time energy storage, but due to simple mechanical design, have received some attention in this industry. A system pumping water to a turbine, is well suited for using a pressure accumulator between the pump and the turbine as a short time energy storage. To avoid unnecessary loss, the energy storage should be placed as close to the absorption elements and the PTO as possible. The volume and weight of a accumulator-system is an issue, so this benefits larger WEC units.

Ensure a favourable ratio between energy production and size of the structures:

In renewable energy, it is often focused on efficiency. An analogous measure is energy cost or the ratio between energy production and size of the structures. From the early years after 2000, it has been an extensive focus of offshore floating wind energy. In Norway the interest was further fuelled by the world's first floating MW-turbine, Statoil's Hywind (2,3MW). However as we write the beginning of 2013, it still exists only two floating MW-turbines in the world. The reason is obviously high cost. The reason for the high cost, is the size of these structures. If one describes a land-based wind turbine as big, then the floating wind turbine is enormous. This is simply due to hydrodynamic stability, to avoid the turbine to capsize. This issue goes beyond economics. For any renewable energy system to be considered to be sustainable, it must be a requirement that it produces more energy than it takes to manufacture it within a reasonable amount of time, typically a few years. Even though the wave energy converter faces more severe technical challenges, it has a substantially more favourable ratio between size and power output than a floating wind turbine. The ratio between energy and size obviously also applies when comparing wave project against wave project. The start of 2013 marks a bit of irony, as many coastal countries starts to tag along after Statoils Hywind, while the Norwegian renewable industry seems to have abandon floating wind for good – not knowing where to go next.

Avoid risk of oil spill or other pollution:

For one single prototype of ocean energy converter, it may not seem that important to avoid all kinds of environmental impacts. However, if one imagines large scale industrialization of ocean energy, it becomes increasingly important. In that scenario even the smallest repetitive oil-leakage will make a big impact. To use water as working fluid, is therefore a safe choice.

Good concepts for installation, maintenance and survival of the system:

The practical solutions for this will differ from project to project. For the installation process, concepts that can be towed out fully assembled is beneficial. Passive survivability should is preferred to active survivability.

Use a control strategy for the wave oscillation, if possible:

A control strategy like phase control by latching, could considerably increase energy production for everyday moderate wave conditions. If not implemented immediately, this should at-least be planned for by having mechanical envelopes ready for this. A front-bouy for the mooring-system, makes a good platform for instrumentation and sensing of incoming waves.

   Lars Edvardsen,  Intentium as

 

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