vol 6 issue 4

The test of tides

15 April 2011
By Sean Davies
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Rolling waves

Control of a wave device is essential for maximum efficiency

Wave Star's floats

Waves raise the Wave Star's floats, making the pistons force hydrolic oil into the machine's transmission system

Wave Star machine

An artist's impression of the Wave Star machine

It has taken eight years of development but a wave device is finally ready to deliver economical power to the grid

The battle is on to channel the power of the waves to deliver reliable and clean electricity, but it is proving a onerous one. While wind and solar energies have been grabbing the headlines with large and powerful devices and installations, wave energy appears to have been drowned out.

Many devices have hit the headlines only to founder in the turbulent seas required for energy generation, or to fall short of the funding requirements to cross the so-called 'valley of death' – the funding black hole between a proven prototype and full-scale production models.

However, one device is closing in on delivering renewable energy. Since 2003, Wave Star has been dedicated to the development of reliable, commercial wave energy technology. With its new 500kW prototype, the company has reinforced its position as a world-leading wave energy developer.

After more than three years of constant operation in the sea, Wave Star has proven the patented concept with a 1:10 scale test machine. Day in and day out the machine has been working – through more than 15 storms – with a minimum of maintenance. This in itself is a milestone in international wave energy.

In September 2009, a research section of the commercial 500kW wave energy machine was installed in the North Sea near Denmark. An impressive 40m long, with two 5m diameter floats, it represents a shortened 'test' version of the complete machine. Located 300m off Hanstholm in the North Sea, at a water depth of 7m, it is already connected to the grid.

Working with the waves

'Unlike other wave power concepts, the Wave Star machine, which is based on standard offshore technology, does not form a barrier against the waves but instead sits at right angles,' Laurent Marquis, technical director at Wave Star explains. 'This way, the waves run along the length of the machine and the energy is used in a continuous process.

'Projecting from each side of the Wave Star machine there are a number of half-submerged hemispherical floats. When a wave rolls in, the floats ride the crests and are raised, one after the other, until the wave subsides. Each time a float is raised, a piston forces hydraulic oil into the machine's transmission system at a pressure of up to 200 Bar. This oil flow drives a hydraulic motor, which is connected to a generator that produces the electricity.'

As the machine is several wave lengths long, the floats will work continuously to harness energy and produce a smooth output. The machine also exhibits efficient storm protection concept – when the waves reach a certain height, the floats are automatically lifted out of the water.

Although the concept sounds simple, the random nature of the waves in terms of height, frequency and length means that an advanced control system was required in order to extract as much energy as possible in the most efficient way. The control system has to measure and manage multiple variables in the hydraulic system – such as speed, movement, acceleration, pressure and loads – and does so by using an advanced algorithm, developed by Wave Star, running on an industry and off-shore-proven Allen-Bradley ControlLogix PAC from Rockwell Automation.

'At the start I don't think that Wave Star thought that automation was important, they were focused more on the physical equipment and how they were able to survive conditions at sea such as storms,' Per Thyme, Rockwell Automation's project leader, explains. 'But they did have a lot of good ideas that needed to be implemented – they had a lot of formulas that they had developed about how to position the floaters in the water and they knew that they had to manage a lot of mathematics to develop the most efficient machine; this is where the control system comes in to help increase the efficiency of the machine.

'When we first met them they were not sure about how we could help with our control systems. This was back in 2004 and, as I said, they already had a lot of good ideas. One of the things that makes this wave machine unique is some basic technology that means that it is not going to fail in stormy conditions. Over the years they have conducted numerous experiments with the machine through 15 to 20 big storms out in the North Sea while most of the competition has been hugging the coast.'

Single supplier solutions

The solution employed by Wave Star, although simple, is extremely powerful and is a good example of how a suite of control, visualisation and I/O components from a single supplier can work in harmony to achieve the desired objective.

Not only does it deliver the closed-loop capabilities needed by Wave Star to manage the energy generation operation, but it also gives clear visibility of the process, informing the operator of all important variables and functions.

'Our solution is pretty much a standard industrial solution – using common industrial calculation, control and automation products,' Thyme adds. 'Where it is different is that within this machine we have to control a lot of hydraulics systems. At the moment they have a half scale prototype running but the full control system is installed on this.

'The intricate calculations that calculate the floaters' position in the waves take a lot of processing power to ensure that they are positioned both safely and able to extract the most power. To achieve this efficiently requires a very fast response time to calculate the correct position and control the hydraulics to achieve this. This requires a closed-loop system.'

'The regulation that the PAC has to achieve is extremely complex,' explains Marquis. 'With multiple floats and multiple inputs there is a huge amount of data traffic, which the PAC has to manage as efficiently as possible.' Allen-Bradley Flex I/O is also a major part of the architecture, connecting many of the data points to the PAC, with process visibility, database values and historical data capture being provided by FactoryTalk View on a central HMI.

'The power plants are designed to work in remote locations around the world, maybe offshore small islands, and there will never be any people on the devices to operate them,' Thyme says. 'It is therefore very important that these plants can be maintained remotely so we have put a lot of effort into the visibility of the system to ensure that all the information is available to an operator who will be some distance away. He needs to be able to check the parameters of the drive and control system. It has always been very important for Wave Star that the plant can be maintained remotely.'

Completing the automation solution is Allen-Bradley Powermonitor technology, which is used to collate information on the power being generated, such as load and power quality, which is then fed back to the PAC to help achieve optimum generation.

Rockwell Automation has been an intrinsic part of the project since 2004, with its Allen-Bradley ControlLogix PAC being the brains behind the process throughout its development.

Control systems

Complex as the control system is, Thyme confirms that they did not have to develop it specifically to control the wave device. The components are off-the-shelf and form part of the well-proven factory automation systems. 'It's true that there is a lot of data flowing around but it is not a huge problem because the control system that we are using was designed for large factories,' he says. 'This system was not developed specifically for use in this sort of application: it is a standard control system in many industries.

'There are some applications in other renewable applications and offshore in oil and gas facilities. One of the most important things is that it was robust enough to be used offshore and this has been proven over many years both in oil and gas and shipping applications.'

When Wave Star enters full operational service in 2011, the complete machine will be 70m long with 20 floats. The technology is already proven and the control system well established, all that remains is to ensure that the system is commercially viable. 'What is really going on right now does not involve technology or the control systems, but is really aimed at reducing the cost by optimising the technology.

'When you launch a powerplant it has to be able to generate electricity economically compared to other renewable energy sources.' Marquis says. 'They have been developing it for eight years so between us we have the control system well developed. It may impact on the control system: whenever they change something it will have an impact on the control system but the infrastructure is in place with Ethernet connectivity.'

The vision from Marquis is that Wave Star can be combined with offshore wind and even solar to develop offshore renewable energy parks. 'Our machine is not only a wave machine it can be combined with wind energy and we are thinking a lot about these combinations.

'Denmark has worked for many years producing wind turbines and is one of the leading wind developers in the world. We are thinking about producing an energy park, rather than just a wind park which I think is a very interesting idea for the future.

'We need to think a bit more about offshore energy. We think that this combination is very promising for the future: we can learn a lot from each other. So instead of competing against each other to 'try and find out who has the best technology we should be working together on' this new concept.' *

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