Converters

There is no visual damage to the sea view from a DEXAWAVE converter park. The profile is low, and the wave park is invisible from more than a few kilometers. A DEXAWAVE converter park has no noise impact on human living space. At the end of service, the whole wave converter and mooring system can be removed from the environment.

In Danish waters, tide changes of 1.5 to 2 m are normal. During stormy weather, surges of up to 3.5 m above normal water level can occur. A wave energy plant should be able to accommodate this tidal fluctuation without damage, and preferably without loss of production time. In other parts of the world, tidal fluctuations of 8 to 10 m are normal, putting high demands on the technology if wave energy converters are to operate there. Because the DEXAWAVE converter is slack moored, it can tolerate tidal fluctuations of up to 40 % of the total water depth. This means that in most places up to 10 m tidal fluctuation is no problem, and production can continue uninterrupted. 

Anyone who has visited a beach will have seen evidence of the huge amount of debris that floats around in the water: bottles, fishing nets, wood, shoes, rope, fish crates, waste oil and anything else that can float.    
 
Inevitably, debris will drift into a wave energy park, most likely during a storm. The design of the wave converter needs to take into account how it will be affected by such an attack. If a wave converter with numerous small exposed parts is subjected to a debris attack, it is likely to either be damaged or to jam and require maintenance. In both cases, this will cost money and production time.
 
In the DEXAWAVE project, we addressed this problem from the start. The construction is simple and indifferent to debris attack. Electricity production will continue normally during any debris attack, as well as after, with no additional maintenance required. 

After a few years, a wave power converter will be covered by shellfish, seaweed and marine parasites. These are also known as marine growth, or bio-fouling. This is a huge problem for many other wave energy technologies, as the marine growth blocks water intakes, or impedes the motion of power take-off systems. To keep these wave plants free from marine growth, poisonous coatings and high maintenance expenses are required.  
 
However, marine growth is not a problem for the DEXAWAVE converter. Continuous marine growth will result in a 3 to 5 cm thick layer of shells, as well as large amounts of seaweed. For the DEXAWAVE converter, this is only a slight weight increase, and is included in the calculations for the pontoon construction as ”anticipated aging process”. Therefore, marine growth will not reduce the energy production. Marine growth can be allowed to continue freely for the power plant life cycle of 50 years, and no poisonous coatings are needed. The DEXAWAVE converter's wet side will thus be a safe habitat for a small community of plants and animals.

The DEXAWAVE converter has inherent storm survival capability, due to its mechanical design. The converter does not absorb the water level, but rather the water angle. This means that the higher the waves get, the lower the percentage of the wave power that is absorbed by the mechanical system. This keeps the converter safe, even in 50 m high waves. Model tests have verified this storm survival capability. Other wave power systems, based on a conventional bottom-fixed technology, may shut down production during high storms, but every 50 to 100 years a huge wave will pass through the wave park. Since the power in the wave is defined as the wave height squared, the power quadruples for every doubling of wave height. In the open ocean, the normal wave height could be 5 m, with a power of 50 kW per m. However, every 50 years or so, the wave height could rise to enormous 30 m swells, and the wave power would be around 2000 kW per m. This tremendous power would rip the bottom-fixed wave plant apart, regardless of the safety system used. This creates a weather-limited life expectancy.  
 
The DEXAWAVE converter only absorbs the water angle, which, regardless of the wave height, can never exceed 50 degrees. This is because as waves get higher, they also get longer, which tends to keep the angles constant. As the wave height increases during a severe storm, the efficiency of the DEXAWAVE converter drops. For example, a converter designed for 5 meter waves still only absorbs 50 kW per m, even if 30 m waves are pumping in 2000 kW per m. The excess wave power just passes by, with no mechanical impact on the converter.

Concrete constructions can have very long service lives in marine environments. This is well known from bridges, dams and other civil constructions, where life expectancy is 100 to 125 years. No significant maintenance is required for concrete constructions. Concrete simply doesn't corrode very fast when exposed to the salty ocean environment. Steel, however, corrodes rapidly. Many other wave converters are made of steel, which requires painting, as well as maintenance at 5 to 6 year intervals, to minimize corrosion. This is very costly, and reduces the converter's production time. An even more severe problem for steel structures is the pollution caused by the poisonous paint as it flakes off and settles on the ocean floor. Ship paint is mixed with TBT poison, to reduce marine growth. This kills marine life, even after the paint has flaked off the structure. See the section on “Environmental issues” for more information.  
 
Let's look at what happens to a structure left in a marine environment without maintenance.   

This photo, from Fraser Island, Australia, shows a steel hull, which has been exposed to the marine environment since World War Two, without any maintenance. The ship has been all but obliterated.  
 
If we look at concrete ships from the same era, they have withstood the marine environment far better, even though they have not been maintained since World War Two.

These concrete ships have been analyzed, to evaluate the condition of the concrete after 55 to 80 years of exposure to the marine environment.  
 
The simple construction of the DEXAWAVE converter allows the use of concrete as the main structural building material. This gives a very long service life, with low maintenance costs, and no negative environmental impact.  
 
Concrete doesn't have to be painted to reduce corrosion, unlike steel or other materials normally associated with wave energy systems.

Ship paint, after 3 to 4 years of exposure to ocean environment. About 30 % of the paint has flaked off.  
 
As the DEXAWAVE converter is mainly made of concrete, there are only small exposed areas of steel, and these do not require the use of bio-repellent paint.

Marine growth and ocean debris do not have to be removed from the DEXAWAVE converter system at any time, for the entire life cycle of the converter. All mechanical systems are located above the water, and thus easily accessible for maintenance. Other wave systems have generators, gears and hydraulics placed underwater, making service and maintenance costly, since this has to be performed by divers, or Remotely Operated Vehicles (ROV's). Furthermore, sub-sea mechanical parts are much more vulnerable to damage than the same hardware in a dry, protected environment above water, as in the DEXAWAVE converter.  
 
When heavier service tasks are required, the whole DEXAWAVE converter can simply be detached from the mooring system, and tugged to a shipyard for an overhaul. This feature keeps the maintenance cost down. Other wave systems have to be serviced on site by special crane ships and an engineering team, or by divers.  
As seen above, the service life expectancy of a concrete construction is at least 50 years. This is 30 years longer than a typical wind turbine. A long service life reduces the cost of energy production, as the initial investment generates profits over a longer period. Compared to solar energy, wave power is more stable, since there is minimal aging for energy production, unlike the 10 % power drop per decade associated with solar cells.

A DEXAWAVE converter is very simple to install. The converter and mooring block are tugged from the shipyard to the location, using a tug boat. The mooring block is sunk into the ocean at the location, with a buoy attached to the mooring block. The DEXAWAVE converter and its power cable are then attached at the buoy to the mooring block. After that, only normal startup procedures are required for the converter to start operating and producing power.  
 
Using a tug boat, the converter can easily be moved to shore for maintenance or an overhaul. It can be moved to another location, if shipping routes or environmental preservation plans change over the years. The converter can also be traded and moved without significant expense. Finally, the converter can be decommissioned at the end of its life and removed completely from the ocean environment, using the same equipment used to install it.

A DEXAWAVE converter is constructed in such a manner that sea animals cannot get squeezed, trapped or injured by any part of the construction. The converter has soft corners and no confined spaces. Instead of harming marine life, a DEXAWAVE wave park will be a safe haven for a small community of marine animals and plants.  
 
A DEXAWAVE converter will not use materials that are harmful to marine plant or animal life. The hydraulic system uses water, which does not pollute the environment if there is a leakage. No poisonous surface coatings are used. For offshore steel constructions (including other wave converter systems), poisonous ship paint is used to prevent marine growth. A bio-poison (TBT) is mixed into the paint to prevent shellfish and seaweed from attaching themselves to the structure. The DEXAWAVE converter uses neither constructions nor materials that require the use of such poisonous surface coatings.  
 
Ship paint flakes off after 5 to 7 years, and the surface has to be painted again to prevent corrosion. With surface areas for typical wave energy converters of 1000 to 5000 square meters (10000 to 50000 square feet) per converter, a lot of paint flakes are released into the environment every year.