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.
“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 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.
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:
1. Descending the system (in locations with high wave power density / large extremes): The string with the ECs is placed near the surface where the energy is most dense. During storms, we can flood parts of the system to bring the EC’s to greater depths and calmer waters (illustrated below) until the storm has passed.
2. Collapsing plates: If the plates hit the end stop a flipping mechanism makes the plates collapse taking the loads of the plates. When the loads decrease the plates flip back into position. This idea is very similar to the pitching of the wind turbines blades in high winds to reduce the loads.
The extreme wave conditions are present in less than 1 % of the year. Designing for energy absorption in these short periods will drive up the cost of any system, and most likely lower the average power output. Hence a load control system is a necessity. The figure on the left shows the reduction of critical stresses when the plate is flipped.
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.
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 DanWEC test area is located 2Nm SouthWest of the Port of Hanstholm.
Significant wave height up to 8.2 meters and single waves up to 15 meters.
Cross current up to 3.1 knots.
The prototype project ran from 2015 to 2019 and included several development iterations increasing the energy production and durability in each iteration.
In the first iteration two energy collectors were each fitted with a 4 m2 plate. In the second and third iteration a total of 4 energy collectors were each fitted with a 7 m2 plate. In our last iteration the energy collectors were each fitted with a 8.5 m2 plate.
The last version was a 100-m string with the possibility of attaching 6 energy collectors. In the last iteration we tested 4 energy collectors simultaneously.
Different storm protection systems were tested in parallel on the energy collectors. The storm protection system reduces the plate area when forces get too strong.
Data from the loads and the energy production was collected on a computer on the inner buoy and uploaded to a server. The data was analysed and compared to the predictions from the Wavepiston Load and Energy Tool developed in collaboration with the Technical University of Denmark. The identified improvements in the design will be implemented in the next projects at full scale in Gran Canaria.
The PLOCAN test area is located 6 Nm South of the Port of Las Palmas.
Significant wave height (Hs) up to 5.8 m and single waves (Hmax) up to 9.30 m.
Cross current up to 1.4 knots.
The project period is December 2018 – February 2022.
The full-scale demonstration system consists of a single Wavepiston WEC (the string) with 24 energy collectors (length 200 m, width 8 m) and a turbine generator on the PLOCAN platform for conversion to electricity.
The purpose is to demonstrate a full-scale Wavepiston system with electricity conversion connected to a grid and prepare for the first commercial projects.
The system is expected to have a peak effect of 200 kW, being able to produce 547,000 kWh per year (equal to the electricity consumption of 140 standard households).
This Project has received funding From the European Union ́s Horizon 2020 Research & Innovation SME Instrument Programme Grant Agreement 830036
Isola Piana is a small tourist Island located in the Mediterranean Sea, 3 Nm West of Portoscuso, Sardinia.
The island's consumption pattern and wave climate is used as a case study for potential commercial systems at Sardinia and other island communities.
The project period is 2019 – 2022.
The project consortium consists of Vryhof Anchors (Mooring), Wavepiston (Wave Energy), Fiellberg (Hydraulics and Desalination) and Ener.Med. (Grid, Monitoring and Control). Together we assemble the multidisciplinary technologies needed to demonstrate the benefits of a combined system of using wave energy for electricity conversion and desalination.
A full-scale Wavepiston wave energy converter will be installed at the test site PLOCAN, Gran Canaria, with the combined system for both power production and desalination to supply clean electricity and desalinated water to island communities.
The system is expected to have a peak effect of 150 kW, being able to produce 350,000 kWh and 28,000 m3 desalinated water per year.
This Project has received funding From the European Union ́s Horizon 2020 Research & Innovation FTI Instrument Programme Grant Agreement 831041