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LORC tests and demonstrates technology for harvesting renewable energy offshore

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Meeting industry’s needs for environmental testing of large components for offshore use is now one step closer with the ordering of a climatic chamber for a new LORC test centre.

Read more about the environmental testing of structures here

 

 

 

Struggling with the waves

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onsdag 20. april 2011

FACT BOX

Explanation of the factor 11
Froude’s Law says that the effect will increase according to the size uplifted to the power of 3.5. Why is this so? When the Wave energy converter is built in double size, it will receive a wave front that has double the width. If the device is placed in a wave climate with an average wave height that is doubled from 1 to 2 meters, the power in the waves will be raised by 2 uplifted to the power of 2.5 which equals 5.56. The double width device then receives 2 x 5.56 = 11.3 x more power.

Facts on the Wave Dragon
At Nissum Bredning in Denmark a 1:4.5 scale Wave Dragon prototype launched in 2003 was the world’s first floating grid-connected wave energy conversion device. It weighs 200 tonnes, is 57 meters wide and is made of steel. The test unit has accumulated over 20,000 hours of experience supplying electricity to the grid. At the moment, the Wave Dragon is not producing due to a breakdown. EUDP funding allows the team behind Wave Dragon to pursue the modeling and design of the full-scale device in spring 2011. The demonstration project will be installed off the Danish coast with an almost twice as large device to follow in Wales.

Wave energy concepts

There are four overall types of wave energy extractors.

1. Oscillating water column - These devices contain a chamber where the water level changes as waves go in and out. As the water level changes so does the pressure, which causes air to flow through the turbine. This generates electricity.

2. Attenuator or linear absorber – These devices, also called attenuators, use less than the full amount of energy in a wave. The linear absorber structure is oriented roughly parallel to the direction of wave propagation and is composed of multiple sections that rotate in pitch and yaw relative to each other. This motion is used to pressurize an internal hydraulic fluid. This fluid turns a turbine that is coupled to a generator to produce electricity.

3. Overtopping terminator - A terminator is a device that reflects or absorbs all the energy in a wave. One type of terminator is an overtopping device that uses a floating reservoir structure, typically with reflecting wings to focus the wave energy. As waves arrive, they overtop the device and are restrained in the reservoir. The head of the collected water as it flows back out to sea turns conventional low-head hydro turbines, and the turbines are coupled to generators to produce electricity.

4. Point absorber - These devices capture energy from the ”up and down” motion of the waves. They may be fully or partially submerged. They are small compared to a typical wavelength and can absorb energy from all directions.

Read more about wave energy devices, the challenges and opportunities at LORC Knowledge lorc.dk/knowledge/wave-energy

Waves converted into green energy have the potential to cover the global energy demand. Yet the first commercially competitive device is still to be seen in full scale demonstration.

By Anne Korsgaard
For many years, wave energy has been a good idea in the renewables sector. And there is a certain simplicity about the idea of converting the endless rolling waves into energy. But as it has turned out, creating competitive energy is not simple at all – and the wave concepts need to break through one important barrier: funding of fullscale projects that can convince both venture capital and traditional investors to put their money into the embryonic sector of wave devices.

For the wave device Wave Dragon, the major challenge is to raise around €8 million to demonstrate a 7 MW fullscale device deployed off the Welsh coast.

- We were granted €7.4 million from the Welsh Assembly as an Objective One project. The money was ready to collect at the bank if we could raise the same amount ourselves, says Erik Friis-Madsen.

Photo: Wave Dragon

The energy potential in waves
The difficult part is when two worlds meet: the financial funding experts and the engineers.

The venture capitalists are standing on one shore looking at wave energy and comparing it to the mature technology of wind energy. What the investor sees are small experimental wave energy devices producing from 0.01 to 0.75 MW – much smaller than today’s offshore wind turbines. On the other shore stands the engineer at the wave energy converter, trying to explain that if the device was doubled in size and deployed in a wave climate with the double average wave height, the generated power would be 11 times higher. But so far the message has not got through to the venture capitalist.

- It is a general problem for the developers of wave energy converters worldwide that venture capital is not adventurous enough toward waves. We simply need to prove that wave energy can deliver considerable MWh at a competitive price in order to move on, says Erik Friis-Madsen, the inventor of Wave Dragon and one of the founders and board members of the Danish Association for Wave Energy. He points out:

- And that takes a successful full-scale deployment of wave energy plants of multi-megawatt size.

It is a great challenge to explain the mathematic modeling that makes it plausible that the power output of Wave Dragon, for instance, will rise by a factor of 11 when doubling up the length and width, and exposing it to a wave height of 2 meters instead of 1 meter.

- It is mathematic modeling of energy conversion at a quite high level, which can be diffcult to explain. In the world of engineering we also know that scaling up devices from small models tested in wave basins to full scale needs more than one step. Weak points in performance, as well as robustness, only become obvious in real life conditions with prototypes matching the actual wave climate at the test site. As technicians we are aware that testing and demonstration is important, explains Erik Friis-Madsen, but we need to explain to the capital market that their investment horizon has to be longer than 2-3 years. Development in several steps is necessary and this can take 10 years or more if you start from scratch.

Upscaling
Wave energy technology is a complicated area where simple problems can be easy to cope with in a small-scale test device, but might be critical if you move to full scale in one step. Forces in the construction rise with a scale factor to the power of 3, and waves will hit a device up to 5 million times a year, which may turn out to have destructive effects if intermediate-scale test results are not at hand before beginning the hard core engineering of a full scale device.

- As an engineer it is hard to understand the skepticism when it comes to the financial calculations in the scaling-up of wave devices. We also have to prove the construction robust enough to withstand waves of 10 to 25 meters depending on the deployment site. If you aim at sites with a wave climate of 40 kW/m or more you need to design your device for a wave power of 1000 kW/m in the highest waves, Erik Friis-Madsen points out :

- The answer yet again is demonstration and testing, which is quite costly.

In comparison if you double up the rotor diameter of a wind turbine the effect will rise by a factor of 4. In a financial environment used to the arguments of the wind energy sector there is a clash between these two calculations (see the explanation of the calculation of the wave factor 11 page 35). This fact generates skepticism towards the calculated 11 times increase in production with a similar 2 times scaling-up of the wave energy device.

Money, money, money
The good news is that Wave Dragon has recently received an EUDP appropriation for phase 1 of a 1.5 MW Wave Dragon demonstration project, which is three times the size of the existing installation at Nissum Bredning in Denmark. The demonstration project in Wales that will construct and deploy a 7 MW Wave Dragon is however still underway. According to the experts, it is necessary to see the first large-scale installation in action before the money markets and the utilities show any serious interest in wave energy.

- I am convinced that the first full scale project will crack the curve and the potential in waves is huge, says Erik Friis-Madsen.

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