DEXAWAVE converters

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

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

Anyone who has visited an ocean beach has witnessed the vast amount of debris that floats around in the water. Bottles, fishnets, wood, containers with shoes, rope, fishcrates, oilspills and anything else that can float. Things that are dumped or lost from passing ships.

Inevitably these thing will drift into a wave energy park, and most likely during a storm. How will the wave converter withstand this attack of floating debris, and how much will it affect the operation, and survivability of the wave park? If a wave converter type with numerous small exposed parts are subjected to a debris attack, it will either jam down, and require maintenance, or it will be damaged. In both cases costing money and production time. In the DEXA project, we have adressed this problem from the start. The construction is simple, and indifferent to debris attack. It will produce normally under any debris attack, and after, with no service attention required.

A wave power converter will after a few years be covered with shellfish, seaweed, and marine parasites. 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 motion of power take-off systems. Therefore poisonous coating, and huge maintenance expenses are necessary to keep those wave plants free from marine growth.

Not in the DEXAWAVE converter! Continous marine growth will result in a 3-5 cm layer of shells, and large amounts of attached seaweed. From the DEXAWAVE converter perspective this only adds up to a slight weight increase, that is already calculated into the pontoon construction (Anticipated ageing process). This will not reduce the energy production.
Marine growth is allowed to continue freely for the powerplant life cycle, 50 years, and no poisonous coatings are used. Turning the wave plant into a safe habitat for a small community of plants and animals, that will attach to the wave converters wet side.

The DEXAWAVE converter has inherent storm survival capability, through a special mechanical feature. It is not absorbing the water level, but rather the water angle. This means that the higher the waves get. the less percent of the wave power is absorbed by the mechanical system, keeping it safe, even at 50 m high waves. (Which has been verified with model tests).
Other wave power systems, based on a conventional bottom fixed technology may shut down production at high storms, but every 50 or 100 years a huge wave will pass through the wave park. Since the power in the wave is defined as the wave height ^ 2, the power quadrouples with every doubling of wave height. In open ocean, the normal wave height could be 5 m, containing 50 kW per m but every 50 years or so, the wave height could rise to anourmous 30 m swells. And now the wave power is around 2000 kW per m. This tremendous power will rip the bottom fixed wave plant a part, regardless of the safety system used. We refer to this as weather limited life expectancy.

The DEXAWAVE converter will only absorb the water angle, which regardless of the wave height, can never exceed 50 deg. This is because as waves get higher, they also get longer, and thus tending to keep the angles constant.
This means, that as the wave height increases during severe storm, the efficiency of the DEXAWAVE converter drops. And the converter still only absorps 50 kW per m, even when the 30 m waves are pumping in 2000 kW per m. The rest of the wave power just passes on in the ocean, with no mechanical impact on the wave generator.

A concrete construction has very long service life in the marine environment. Known from bridges, dammes etc, where life expectancy is 100-125 years. And this with no significant maintenance. Concrete simply doesn't corrode very fast, when exposed to the salty ocean environment.
Steel does. And many other wave converters are made of steel, which requires painting, and maintenance with 5-6 year intervals, to avoid corrosion. This is very costly, and reduces the up time. An even more severe problem 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 marinegrowth, and thus can kill marine life, also after it has flaked off. More about that later.

Let's look at what happens to a structure left to ocean environment without maintenance.

This photo shows a steel hull, that has been subjected to the ocean environment since ww2 without maintenance. (From Fraser Island Australia). As it should be obvious the ship is all but oblitorated.

If we take a look at concrete ships from the same era, they have withstood the ocean environment far better, also without maintenance since ww2.

These concrete ships have been analysed, to establish the condition of the concrete after 55 - 80 years exposure to the sea environment.

The simple construction of the DEXAWAVE converter allows 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 avoid corrosion, like steel or other materials normally associated with wave energy systems.

Ship paint after 3-4 years exposure to ocean environment. About 30% of the paint has flaked off.

As the DEXAWAVE converter is mainly made of concrete, only small exposed areas are steel, and they are not required to use bio repellant paint.

Marine growth or ocean debris does not have to be removed from the DEXAWAVE converter system at any time during the entire lifecycle of the converter.
All mechanical systems are placed above water, and thus ealisy accessible for technical maintenance. Other wave systems have generators, gears, hydraulics etc. placed underwater, making service and maintenance costly, as it now has to be performed by divers, or ROV's (Remotely Operated Vehicles). Furthermore the subsea mechanics are much more prone to damage, than if the same hardware - as in the DEXAWAVE converter - in placed in a dry, protected environment above water.

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. Again keeping the maintenance cost down. Other wave systems have to serviced on site by special crane ships and an engineering team, or 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 it generates profits for longer time. Compared with solar energy, wave power is more stable, as there is no ageing slope of energy production, like the 10% power drop per decade associated with solar cells.

A DEXAWAVE converter is very simple to install. The converter and mooring block is tugged from the shipyard to location with a tug boat.
The mooring block is sunk into the ocean at location, and the DEXAWAVE converter attached (with power cable). After that only normal startup procedures are required, and then the converter is in operation, producing power.

The converter can easily be moved to shore for maintenance overhaul, by use of a tug boat. It can also be moved to other location, in case ship routes, or environmental preservation plans change over years. The converter can also be traded and moved, without significant expense.
Lastly the converter can be decommissioned after end of 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 take injury from any part of the construction. It has soft corners, and no confined spaces. Instead the DEXAWAVE converter or wave park becomes a safe haven for a small community of marine animals and plants.

A DEXAWAVE converter uses no materials that are harmful to plant or animal life in the ocean. The hydraulic system only uses plant based oil, which does not pollute the environment in case of a leakage. No poisonous surface coatings are used.
In offshore steel constructions (other wave converter systems), poisonous ship paint is used to prevent marine growth. A biopoison (TBT) is mixed into the ship paint to inhibit shellfish and sea weeds from attaching to the structure. No constructions or materials are used, that require use of such poisonous surface coating in the DEXAWAVE converter.

Ship paint flake off after 5-7 years, and the surface has to be painted again to prevent corrosion. With surface areas on typical wave energy converters, of 1000 to 5000 square meters (10000 to 50000 sq ft), per converter, considarable amounts of paint flakes are submitted to the environment every year.