Wave energy challenges (Click figure for details)

To evaluate a wave energy device it is important to understand the major challenges in harvesting wave energy.

As the figure shows wave energy is much more than just converting the movement of waves.

FORCES - Wave energy is refined wind energy. When a storm has affected the ocean for days, the energy in a wave can be up to 1 MW/m wave width. These large forces will drive the cost of structure and mooring through the roof if the concept does not have a solution for this.

POWER CONNECTION - Power cables and connections at sea are very expensive. For Offshore wind power farms the internal power connection is 5% of the total cost and grid connection is 10%

LOGISTICS - Large structures are expensive to transport and deploy. A typical deployment vessel costs 50.000€ per day. That also goes for the days with bad weather and the vessel on standby.

MAINTENANCE & SURVIVABILITY - Seawater, cyclic forces and marine growth are a tough combination for mechanical structures. The correct choice of design and material is essential to build a low maintenance system.

How we handle the forces of the ocean

The Wavepiston concept captures the surge energy with vertical energy collectors (ECs) distributed on a horizontal string.

The defining and new feature of this concept over existing concepts is that many ECs are attached to

the same structure. The innovative aspect is that the mooring costs are reduced substantially, since many ECs can be moored using only two anchors.

The concept does, however, have another even more important feature: Due to the length of the string, and the oscillating nature of waves, ECs along the string will be subjected to forces in opposing directions. This is illustrated by men, all pulling a rope in different directions.

Although the situation for a single man is not affected by this situation, the net result of pulling in different directions is that comparatively small forces can anchor the rope. Like the men in the illustration the ECs are subjected to shifting wave forces in at any given time, hence resulting in a sharp decrease in the required anchor force.

This enables a slim, light and extremely cost-effective structure. Advanced simulations and tests by University of Aalborg (AAU) have proven that with more than 20 ECs connected in this way, the mooring needs are reduced to 1/10 in comparison with ECs moored individually, cutting the costs dramatically.

Mooring 10%

Structure 10%

Energy 100%

The idea of connecting several vertical ECs to a structure is issued as a patent in large parts of the world and marketed as “Force Cancellation”. Force cancellation does not affect the energy conversion.

How we handle storms

To optimise efficiency, cost and lifetime, a reduction of loads in storms is needed. We have chosen a double route strategy for proper redundancy:

How we avoid subsea power connections

Electric connections and power take of systems in water are notorious for being fragile and expensive.

For this reason, hydraulic and pneumatic power lines are always chosen over electrical power lines in off-shore structures.

Drawing on the experience from the offshore industry, the mechanical movement of the ECs is converted into pressurised water by hydraulic pumps.

The pressurised water is led to a turbine station and/or reverse osmosis system for desalination. This is placed on-shore for near-shore installations. For off-shore installations it will be placed on either a spar or a barge.

A commercial system will consist of many strings all leading the pressurised water to one turbine/reverse osmosis station.

Efficiency: turbine 80%

Efficiency: pipes/valves 94%

How we handle logistics

The low weight of a Wavepiston system enables the use of small vessels when deploying.

A Wavepiston system can be viewed as two parts; the main structure and the energy collectors.

The main structure consists of well proven standard offshore components such as mooring rope, chain and anchors. These are delivered by well-respected international companies. The energy collectors, which are the key components in a Wavepiston system, are modular. The nominal energy production of a single (full scale) EC will be in the range of 10kW. A commercial Wavepiston system will consists of hundreds to thousands of energy collectors. The large number of identical, simple, components enables, in the commercial situation, simple logistics and fully automated production. This strongly affects the production cost, but also speeds op optimization of the components.

The EC’s are designed to be fitted into 40 foot containers. The assembly process on site can be carried out using local workforce with just brief training, due to the simple mechanical structure. The simplicity also allows for maintenance with relatively little training required.

// Will the first plates not take the energy in the waves, leaving little or nothing to the next?    No - each plate has an efficiency of 7% - 29% depending on wave size. Energy passing beside or below the plates will spread and make up for the lost energy. The loss from one plate to the next is only 1% - 2%, depending on the distance between the energy collectors.

// Will the system produce energy when waves come from the side?   Yes and no - If waves come at an angle of 90° the system will not produce any energy. Test made with waves from different angels show an efficiency of 80% when the angle is 30°. The concept is designed to lay perpendicular to the coast thus waves directly from the side are not very big.

// Can the system handle sideways current?    Cross current can be an issue. In our test in the North Sea we experienced cross current up to 1.6 m/s. We have tested both with and without a side anchor attached. In locations with heavy cross current side anchors will be needed.

// The system looks very thin; will it not break during storms?   No - The best way to survive in the dynamic forces of the sea is being flexible. Static structures will experience forces that are much higher than flexible structures. As an example, you do not see large static plants in the sea. A tree would not survive in waves, but flexible plants raise 30 meters from the seabed in coasts with energetic waves.
The use of slack mooring and hinged connections give that flexibility. The weight of the anchor chain works as a spring when forces get too high.

// Will marine growth damage the system?     Biofouling has not been an issue in our tests so far. Pipe and pumps are opaque hence marine growth will only happen on external surfaces of the Wavepiston system. Our tests show that weight neutral growth is not affecting the function of the plates and we have mitigated extensive growths like mussels by using soft rubber padding. Further testing is needed in other locations to fully understand the biofouling effect on the system.

// Will the flexibility of the string and mooring not lower the efficiency of the system?     No - The structure will appear flexible in very big waves and in strong sideways current when seen from a distance, but from the perspective of the power converting modules it will appear stiff and static due to the high preload of the string.

// Will pressure loss in the pipes not impact on the system efficiency?  Yes, but very little. The pressure loss is a matter of pipe diameter. With current designs the pressure loss ranges from 0,2% - 7% and peaking when the energy production is at its peak.