Wave energy converter (WEC) systems have not yet made it to the stage of development of wind and solar renewable power systems, but wind-generated waves are a concentrated form of wind power covering over 70% of Earth and present a fascinating opportunity for the future of sustainable energy and power systems.
Figure 1. Point-Absorber Configuration of SurfWEC Device, Orientation of Components in 1 meter (3.3 ft) high, 8 second period Incident/Incoming Waves, Wave Heights Increase to 2 meters (6.6 ft) before Impacting the 3.65 meter-high (12 ft-high) Bobber Significantly Increasing Power Takeoff Performance
Figure 2. Internal Components of SurfWEC Device
Martin & Ottaway is developing a novel approach to commercially harness and convert the kinetic energy in waves to electric power called SurfWEC (www.surfwec.com). The novel part involves creating surging surf waves far offshore using a “beach” we can control from shore. Controlling the beach depth and slope autonomously and from shore (manual override mode) is critical to avoid damage to components while maximizing the useful work which can be done by waves.
- This SurfWEC configuration consists of two reactionary components; one prime mover component (float, prime mover, “bobber”, point-absorber, oscillator, flapper) and one moored component (variable-depth platform, “base”, beach).
The driving factors in the economics of ocean wave energy conversion are mass and velocity (kinetic energy). Where does the ocean move massive objects fast? The surf zone. The problem with harnessing the motion of massive WEC objects in the naturally occurring surf zone is the WEC operators have no control over the variable loading conditions due to changing tides and incoming waves. The SurfWEC design provides WEC operators with variable-depth offshore beaches to create the optimal surf conditions for WEC operations in mild and moderate sea states and avoid extreme loads from storm waves by lowering all components below the storm waves.
This SurfWEC system works by raising or lowering the base below the water surface until the passing waves achieve optimal height and surfing wave shape for prime mover operations. In the “point-absorber” configuration, the bobber is moved back and forth by surging surf waves and the wave energy is converted to electricity using a regenerative braking system inside the base.
We submerge the float when we need to, as during hurricanes, and leave it on the surface most of the time because that is where the most wave power is available for conversion to electricity.
The SurfWEC system causes the bobber to oscillate back and forth 10 to 30 meters per wave in average conditions (5 to 10+ second period waves) where the bobbers in other existing WEC systems only move 1 to 3 meters up and down per wave in identical offshore wave conditions. This extreme change in bobber motion is achieved by creating surging surf waves offshore.
Other configurations can be built to accommodate flapper or surge devices or oscillating water columns or various other existing designs to increase power takeoff (PTO) performance 10 times or greater in a wide range of sea states while still having a storm-load avoidance feature.
We consider our work as enabling for the WEC industry. We have no desire to put other WEC developers out of work. One of our goals is to integrate our patented variable-depth shoaling platform with other WEC systems to increase the average velocity of their prime movers by up to 10 times or more in a wide range of wave conditions for a wide variety of WEC designs.
The base can be placed at an optimal depth based on real-time wave conditions for various WEC systems which expands operational depths for flapper type devices from 8 to 20 meters water depth to enable projects in 30 meters to 1000s of meters water depth and enables most WEC designs to have optimal performance in offshore locations.
Unlike the surfer near shore, this system sets up an oscillating motion between the prime mover and base as the waves pass on their way towards shore. In the point-absorber configuration, when the bobber has surfed as far as the wave can push it, the PTO winch lines connected to the bobber are quickly rewound and pay out again as the bobber passes over the base heading towards the next incoming wave.
The majority of the system can be built using commercial off the shelf (COTS) or standard existing components. The motion control and energy conversion subsystem is the most complex assembly and is based on a modified regenerative braking system that is widely used in tractor trailer trucks that are achieving very high levels of reliability today. The other components are very typical marine components (semi-submersible barges, marine winches, synthetic lines, marine-rated generators, floating oil platform anchors, seafloor power cables used for offshore wind systems) The system is an integration effort, and the ocean engineering challenges, such as the power takeoff winches, have been met with innovative designs.
Ocean wave energy conversion involves the simple relationship between mass and velocity. The term “marine hydrokinetic” is often used to describe these systems and abbreviated “MHK” systems. The more water mass a WEC displaces per second, the more power it can put into the energy storage and conversion subsystems. Once a system is moored in place, the operator must maximize the motion of the bobber to create as much electricity as possible from the available wave power.
The biggest problem in wave energy conversion is that ocean waves are not easy to harness efficiently and safely. Martin & Ottaway’s SurfWEC units convert offshore seas and swell to surf waves in mild to moderate waves to address the efficiency issues and the units lower autonomously near the seafloor upon detecting big waves to avoid damage from in storms to address the safety issues.
Once mild to moderate waves are converted to surf, the energy is concentrated and the bobber can be moved forward and backward much farther per wave than in swell or seas. When storm waves begin impacting the bobber, the base automatically lowers itself near the seafloor which gives the bobber more line to move and eliminates almost all of the wave energy impacting the base. The bobber can be flooded and lowered beneath the surface in less than 10 minutes to continue harnessing wave energy in extreme storms and raised back to the surface in less than 4 hours using bilge pumps after the storm passes.
The SurfWEC design addresses a number of the traditional Wave Energy Converter problems:
1. A “regenerative braking” control subsystem is continually tuned, which allows it to harvest all waves of all sizes and frequencies more effectively.
2. The base-and-bobber, two-part system, creates surf and the bobber moves one direction while the base moves the other direction which creates maximum power takeoff. This is called Two-Body Harmonic Motion where the parts oscillate 180 degrees out of phase. The Two-Body Harmonic Motions were discovered during my wave tank testing at Stevens Institute of Technology and submitted as part of the “continuous phase control feature” in the Wave Energy Harnessing Device, US patent 8093736B2 which is the basis for the SurfWEC design.
(To see Two-Body Harmonic Motions go to: https://web.stevens.edu/seahorsepower/video/ )
3. SurfWEC units are easy to deploy. SurfWEC units fit into existing marine infrastructure allowing multiple units to be towed to mooring sites with one standard tugboat.
4. The depth control subsystem allows the variable-depth base to convert a very wide range of incoming waves to surf. The four-point mooring allows the base deck to be sloped to create optimal surf conditions for power conversion. The only conditions where the base will not convert incoming waves to surf are in completely flat seas and in extremely large waves. In extremely large wave conditions, the base will lower itself near the seafloor which extends the distance between the base and float to give the float a wider range of motion to keep generating power. Converting incoming waves to surf is not needed for power conversion in large wave conditions.
5. The base compartment containing the generators is pressurized to compensate for surrounding pressure from seawater. Pressure sensors continuously monitor the surrounding seawater and inert gas is used to keep pressure inside the base slightly higher than seawater pressure outside the base to stop seawater from getting into the base. The inert pressurized gas and hydraulic fluid distribution system is a closed-loop system which connects pumps, motors, compressors, gas cylinders, accumulators, valves, filters, and storage tanks. We have designed a large system with the best components available. The large storage tanks allow us to operate for long periods of time, up to a year, without routine maintenance. Major overhauls at five years have been included in the operational budget.
6. The energy conversion system incorporates an energy storage subsystem in each unit capable of storing over 500 kilowatt-hours of energy. In some scenarios, energy storage is commercially important and enables SurfWEC operators to sell both electric power and electric power capacity.
7. The units do not require specialized vessels or large staging area for installation and maintenance like offshore wind turbines. Standard tugboats are sufficient for all installation and maintenance operations, and installations from 20 to 100s of miles from shore are commercially viable based on the wave climate.
This approach is remarkably cost effective. A 6 MW name plate capacity unit will cost about $9 million ($1.50/Watt) to build and deploy at commercial scale (100+ units). Deployed 25 miles off New Jersey, such a unit would produce an average power output of over 1 megawatt (1MW) on an annual basis (8760+ MW-hours per year). Farther offshore from New Jersey or New England (50 to 200 miles), we can install units with 10MW of name plate capacity at a cost of $15 million per unit (still $1.50/Watt) and produce over 1.5MW per unit on an annual basis.
To achieve 1MW+ average level of power output 25 miles off the East Coast of the United States, the bobber/point-absorber/prime mover have a “capture width” of at least 30 meters (100 feet) as shown in Figure 1.
The capture width is basically the width of the prime mover being impacted by waves. This large of a capture width makes this a diffraction based design as the capture width is a significant portion of the average wavelength in most sea states. Without the ability to create surging surf wave conditions, it would not be possible to get a prime mover this large to oscillate in surge and input megajoules (MJ) of kinetic energy into the power takeoff system in mild to moderate wave conditions.
The average wavelengths off the East Coast are approximately 60 meters to 100 meters (6 to 8 second period “deep water” waves). Mooring the units in depths of 30 meters or greater for safety, and converting 0.5 meter to 1 meter-high waves to surf waves would displace the bobber approximately 24 meters from the still water position, in an oscillating motion (12 meters forward then 12 meters back) in 8 second waves for an average bobber velocity of 3 meters per second (3 m/s). The bobber would displace approximately 18 meters from the still water position, in an oscillating motion (9 meters forward and 9 meters back) in 0.5 to 1 meter high 6 second period incoming waves (also 3 m/s).
The prime mover will oscillate east-west/north-south/northeast-southwest/ or northwest-southeast at average velocities of 3 m/s or faster over 80% of the year depending on the incoming wave directions.
Using the SurfWEC base to create optimal surf waves, a very wide range of wave conditions off New Jersey will provide an average bobber velocity of 3 m/s or more due to the displacement to wave period relationship (This is one of those weird mathematical realities where the bobber surfs for about 1/4 of the wavelength). This movement (Mass with Velocity) provides power that is harnessed by the PTO system. A small portion of the energy stored during payout of the PTO winch lines is used to rewind the PTO winch lines as the wave is not pulling on the bobber after the wave crest passes the bobber.
At a one-meter draft, the bobber displaces approximately 1,000,000 kg (1000 tons) of water.
The average kinetic energy in the oscillating motion of the bobber-base system will then be:
1,000,000 kg x ((3m/s)^2)/2 = 4,500,000 joules of average energy input to the SurfWEC PTO system.
The reason we can use the linear equation for kinetic energy for this calculation (KE = (mv^2)/2) is that the calculation is only accounting for wave surge and bobber oscillations, a forward and backward motion along a line. Like a surfer, the bobber is lifted by the surf wave then surged forward. The lift is a small percentage of the total motion, so we do not use it in the power conversion calculations.
This water displacement goes directly into rotational motion of the winch drums so the average load on the winch lines is equivalent to a free-hanging float swinging like a pendulum. If one of the winches seizes up, the winch lines are designed to separate from the float to avoid damaging the winches or platform. The lines are buoyant in seawater. In this case, the Supervisory Control and Data Acquisition (SCADA) system would send a notice to operators the winch had seized and request deployment of a repair crew. The other three PTO winches are capable of sustaining normal operations until the repairs are completed.
The SurfWEC design dimensions are optimized for US Atlantic and Pacific offshore conditions where Wave Energy Converter (WEC) systems are economically viable, which eliminates the need for a wide range of SurfWEC sizes. Most of the kinetic energy causing the bobber to move will go into winch drum rotations which results in average winch line tensions much lower than in static conditions.
Harnessing and converting this energy to electricity at an overall conversion efficiency of 25% (taking into account the hydrodynamics/diffraction/reflection, PTO efficiency, generator efficiency, and power used during winch rewind operations), results in the following output:
4,500,000 joules x 0.25/second = 1.125 Megawatts (MW) of annual average electricity production
This equates to 9855 Megawatt-hours (MWh) per year. This calculation accounts for flat sea periods as more than 1.125 MW will be produced during times when wave heights exceed 1 meter which will compensate for time when seas are flat. Historically, hours per year with waves over 1 meter high off New Jersey (at potential SurfWEC locations) are more than 10 times the hours per year when seas are flat.
A billing rate of $125 per MWh is a reasonable near-future projection for a carbon-free electric power production source.
The electricity produced by this system will then produce significant revenue (9855 MWh x $125 per MWh ($0.125 per kWh) = $1,231,875 per unit per year). That revenue allows for $231,875 per unit, per year for maintenance and $1 million per unit, per year for amortization and profit.
Since ocean wave energy conversion is a carbon-free form of electricity production, it should also qualify for renewable energy credits, which are near $200 per MWh for the solar renewable energy certificates (SREC) in New Jersey as of January 2019, which would make the system even more cost effective. The Offshore Renewable Energy Certificates (ORECs) currently being negotiated for Offshore Wind Farms connected to the grid in New Jersey will have similar values to the New Jersey SREC program, and WEC systems should be included in this program.
A unit like this would cost about $1.5 per watt of name plate capacity to build, install, and connect to the U.S. power grid ($9M/6MW) at industrial scale (100 or more units), which is very cost effective against wind (at $3 per watt) and PV solar (at $5 per watt). This lower cost is due to the high availability of the wave resource and large electric power output of SurfWEC units in mild to moderate wave conditions. The SurfWEC units do not require land purchase and will have no visual impact from shore based on proposed installation locations off New Jersey. There are also tremendous economies of scale if multiple units are installed as farms. The global market for SurfWEC units is on the order of 100 million units.
SurfWEC inherently fits within the Bureau of Ocean Energy Management policies:
The BOEM PEIS pertains to requirements for permitting, leasing, and licensing offshore renewable energy systems.
A truly powerful wave farm is analogous to positioning a swath of 200 SurfWEC units over an area of 10 miles (North to South) by 1 mile (East to West) and 20 to 200 miles offshore from New Jersey:
While the primary design function is industrial scale electricity production, harnessing wave energy reduces wave energy behind the wave farm; therefore, wave impacts behind the wave farm are reduced, which is something to consider in a global warming environment. The units can also be used as offshore recreational surfing platforms by using very small bobbers designed to only power the platform depth control system. Another application is offshore crypto-mining platforms with larger bases to house and provide cooling for computer server systems. The ocean is Earth’s largest heat sink. The most extreme application would be shoreline protection systems which would require mooring systems designed specifically for the extreme wave loads which occur during hurricanes and tsunamis. The cost of the shoreline protection systems are likely only justified in locations such as Fukushima, Japan or other locations with nuclear power plants near a coastline.
The SurfWEC offshore wave energy conversion system is designed to remain fully operational in waves over 10 meters (33 feet) high off New Jersey. The minimal mooring depth for industrial scale electricity production is currently 20 meters (66 feet) for mild wave climates. There is no maximum mooring depth, but installation costs increase with mooring depth, and most practical installation sites are over the Outer Continental Shelf in water depths ranging from 20 meters to 200 meters.
Very often inventions cannot be commercially developed because there are missing technological components, but this system is an assembly made completely of existing technologies. All the supporting technology for the SurfWEC is readily available today, which means that today is the time to put the pieces together and to add wave energy recovery to the commercially viable sustainable energy basket in the United States and globally.
Figure 3. The modified Bosch-Rexroth regenerative braking system in this rendering of a scale model SurfWEC was developed with a great deal of help from engineers and technicians from Bosch-Rexroth and Airline Hydraulics corporations. I am especially grateful to Marshall Reid and Alex Benham of Stevens Institute of Technology, Pete Loscalzo of Airline Hydraulics and Daryl Walbert of Bosch-Rexroth for their countless hours developing this system.
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