“For WECs, the key to successful commercialization is the right combination of low cost, good conversion efficiency, and survivability. Wavepiston, because of its unique design is poised to offer a breakthrough in all three categories.”
“…Wavepiston have come up with what is in my opinion likely to be the first commercial viable wave energy harvesting device.”
Wave power challenges
To evaluate a wave power device it is important to understand the major challenges in harvesting wave energy.
As the figure shows wave power 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 doesn't have a solving 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 forces
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, themooring needs are reduced to 1/10 in comparison with ECs moored individually, cutting the costs dramatically.
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
The structure with vertical ECs (referred to as string) is placed near the surface where the energy is most dense.
Since the string is neutrally buoyant it is possible to flood parts of it to bring the EC’s to greater depths and calmer waters. This will protect the system during storms and when the storm has passed, air is forced into the string hereby increasing buoyancy and returning the system to the surface again.
Storm protection is a must. Most test sites today bring the test systems to shore when storms are forecasted. This will not be necessary with the submerged ECs.
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 Wavepiston system is designed to run at 10 bars, which ensures sufficient lubrication of the pistons and allows the use of standard polyethylene water pipes for transporting the pressurised water along the string.
The pressurised water is led to a turbine station on-shore for near-shore installations. For off-shore installations the turbine station 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 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 attached EC’s.
The main-structure consists of well proven standard offshore components such as steel cable, chain and anchors. These are delivered by well-respected international companies. But the (EC’s), 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, but a commercial Wavepiston system will consists of more than 100 identical EC’s. 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 2% -13% 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%.
// 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? Sideways current can be an issue. We aim to do the first test in an area without heavy current. We believe 1- 1.5 knots won’t be a problem as we have made calculations on that. The test will reveal if we must have an extra mooring point at the middle of the string to handle currents above 2 knots.
// 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. The use of slack mooring gives that flexibility. The weight of the anchor chain works as a spring when forces get to high.
A steel cable is the most efficient way to make a structure between the two mooring points. In the project design the steel cable has a break load of 118 tons and is stretched between the two mooring points with a preload of 10 tons. It might look thin, but it can handle the dynamic forces of the sea a lot better than any of its “bulky” competitors.
// 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 covering the plates with industrial rubber.
// Will the flexibility of the steel cable 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 steel cable.
// 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.
The concept has been tested in the wave tank of the University of Aalborg. Tests in irregular waves at 4 different wave states proved the system efficiency with various numbers of collectors, various distances between collectors, various loads on the collectors and waves from different heading angles.
The bars show efficiency for 1 plate at different wave states (1-4m)
Wave state 1 13%
Wave state 2 7%
Wave state 3 4%
Wave state 4 2%
The decreasing efficiency at higher wave states is a huge advantage, as this will allow for a more cost efficient design, since forces are reduced in strong waves where the available energy exceeds the handling capacity of any reasonably dimensioned power take-off system.
Oblique wave lowers the efficiency. The circles shows efficiency of waves at 30 degrees compared to 0 degrees.
Wave state 2 - 30°
Wave state 3 - 30°
The impact of oblique waves is less than we had expected and decreases when waves are bigger.
The full test report can be found under documents.
A 1:9 scale model was operated successfully for 7 months at Nissum Bredning in 2013. The figure below shows the 50 m string with 8 ECs before deployment from the beach at Nissum Bredning.
The 8 mounted EC’s maintained their efficiency during the test period without maintenance, despite heavy biofouling and millions of wave cycles. Much was learned about the hydraulic cylinders, valves, and the use of seawater as a hydraulic fluid.
Furthermore lessons were learned on the handling of a full string in deployment and mooring.
Although Nissum Bredning is excellent for testing systems, deployment and influence of biofouling, the waves at Nissum Bredning are very calm. Hence, neither realistic energy production, nor testing the mechanical limits of a concept is feasible at this test site. On days with heavy wind the energy output was however close to the calculated forecasts.
On the chart below these days are marked along with the predicted curves.
The successful testing in the wave tank as well as in Nissum Bredning generated a host of useful information which has been utilized in a design study used to understand the forthcoming test in energetic waters.
The test relies on a sturdy main-structure designed by Vryhof Anchors and normal sized hydraulic components.
Low project risk has been focus in the test.
The structure alone will hardly be affected by the forces of the waves due to its limited surface area and weight compared with its design loads and tension. The test will start with one small collector plate (1:3 scale) and generate data from “real” conditions, but with very limited forces and risks. Scaling up the plate area and numbers will happen gradually based on system data and design optimization in a stepwise approach. The plan is to end with energy collectors in 1:2 scale.
Destruction of ECs in rough weather and by unforeseen wear and tear is the main risk. But the ECs are low cost and can easily be replaced. The structure serves as a platform for rapid optimization of efficiency and life time. The key to this process is the rugged structure but also a hydrodynamic model developed by the Technical University of Denmark for fully understanding the system behavior.
Levelised cost of energy is estimated to be best in market
Patent on the force cancellation principle is issued in countries around the world
The technology is assembled using rugged components and standard offshore technologies.