SURVEYOR'S NOTEBOOK

Replacing Fossil Fuels with Ocean Wave Power

By Michael Raftery

Wave energy converter (WEC) systems have not yet made it to the stage of development of wind and solar systems, but most ocean wave energy is a concentrated form of wind energy covering over 70% of the Earth and presents a fascinating opportunity for the future of sustainable energy systems.

Martin & Ottaway is developing a novel approach to commercially harvest wave energy called SurfWEC.

Figure 1. Surf-making Wave Energy Converter (SurfWEC)

  1. The SurfWEC consists of two prime components; one freely moving component (float, point-absorber, “the bobber”) and one moored component (the variable-depth barge or “the base”).

The SurfWEC system works by raising or lowering the base below the water surface until the passing waves achieve optimal height and surfing wave shape. The bobber is moved by these surfing waves and the wave energy is converted to electricity using a regenerative braking system inside the base.

The SurfWEC system causes the bobber to move 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 per wave in identical offshore wave conditions. This extreme change in bobber motion is achieved by creating surf waves offshore.

Unlike the surfer near shore, this system sets up an oscillating motion between the bobber and base as the waves pass on their way towards shore. When the bobber has surfed as far as the wave can push it, the power takeoff (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 system can be built using off the shelf components. The motion control and energy conversion subsystem is the most complex component, but is based on a modified regenerative braking system that is widely used in hybrid cars and tractor trailers that are achieving very high levels of reliability. 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, but has no significant ocean engineering challenges.

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 puts into the energy storage and conversion subsystems.

The biggest problem in Wave Energy Conversion is that ocean waves are not easy to harness efficiently and safely. Martin & Ottaway’s SurfWEC converts offshore seas and swell to surf waves in mild to moderate waves to address the efficiency issues and lowers itself near the seafloor to avoid damage from waves 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 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 move 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 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 compartments are 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 on Earth. 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.

 

 

Figure 2. SurfWEC Full-scale internal components

This approach is remarkably cost effective. A 5 MW name plate capacity unit will cost about $7.5 million ($1.50/Watt) to build and deploy at commercial scale (200+ units). Deployed off New Jersey, such a unit would produce an average power output of 1 MW or more on an annual basis (8760+ MWh per year).

To achieve this average level of power output off the East Coast of the United States, the base from Figure 1. needs to be 60m long x 30m wide to create surf from all directions and the yellow bobber (essentially a non-hydrodynamic, large surfboard to absorb maximum wave energy) needs to be 50m x 20m x 2m.

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 broadside to the predominant wave direction 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).

A very wide range of wave conditions provide an average bobber velocity of 3 m/s due to the displacement to wave period relationship (This is one of those weird mathematical realities). 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 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 energy.

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, 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.

The SurfWEC design dimensions are optimized for US Atlantic and Pacific offshore conditions where Wave Energy Conversion is economical, which reduces the need for multiple SurfWEC sizes.

Harnessing and converting this energy to electricity at an overall conversion efficiency of 24% (taking into account the hydrodynamics, PTO efficiency, generator efficiency, and power use during rewind), results in the following output:

4,500,000 joules x 0.24/second = 1.08 Megawatts (MW) of annual average electricity production

This equates to 9460 Megawatt-hours (MWh) per year. This calculation accounts for flat sea periods as more than 1.08MW 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 then produces significant revenue (9460 MWh x $125 per MWh ($0.125 per kWh) = $1,205,000 per unit per year). That revenue allows for $205,000 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 SREC in New Jersey as of August 2018, 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 ($7.5M/5MW) at industrial scale (200 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 large power capacity, availability, and efficiency of each SurfWEC unit. 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.

SurfWEC inherently fits within the Bureau of Ocean Energy Management policies:

Programmatic Environmental Impact Statement (PEIS)

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 12 miles (North to South) by 2 miles (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 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 20 meters (66 feet). There is no maximum mooring depth, but installation costs increase with mooring depth, and most practical installation sites are over the US Outer Continental Shelf.

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.

Further background:

Concentrating Wave Power – A Function of Reduced Wavelengths

Determining and Controlling Peak Energy Density Location During Water Wave Deformation

Patent-Wave Energy Harnessing Device

Masters Thesis

Ocean Wave Energy Conversion Technologies

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