Wave energy power systems have not (yet) made it to the stage of development of wind and solar, but wave energy is much more concentrated than wind and solar and, therefore, is a excellent source of sustainable energy, and there is so much of it out there too.

 

The biggest problem is that waves are not easy to harvest. Some systems work well in big waves and others only in small waves. There is a way around that problem by inducing waves to break. Once a wave breaks, the energy is further concentrated and energy capture is easier to accomplish in breaking waves than in smooth offshore waves, just like surfers can only surf in breaking waves. 

 

The Surf Making Energy Harvesting Device (SMWEHD) produces breaking waves by creating an adjustable bottom beneath ocean waves.

 

 

Figure 1. Surf Making Wave Energy Harnessing Device (SMWEHD) with a "point-absorber" configuration. The yellow point-absorber is a low-density polymer to maximize performance. A wide range of existing designs can be integrated with the variable-depth barge.

 

While it is a somewhat more complex system, it effectively addresses a number of the traditional wave recovery system constraints:

 

  1. 1.       A “regenerative braking” control subsystem is continually tunable, which allows it to harvest all waves of all sizes and frequencies more effectively
  2. 2.       A “barge and  power takeoff” subsystem is non directional so it can capture waves from all directions
  3. 3.       The barge is universally deployable. It fits into existing marine infrastructure allowing multiple units to be towed to mooring sites with one vessel
  4. 4.       Integrating the mooring subsystem into the barge allows the barge to avoid extreme loads during extreme weather events by lowering the barge near the seafloor during storms. This allows the SMWEHD to continue to harness wave energy without the need to tune waves
  5. 5.        The depth control subsystem allows the variable-depth barge to optimally convert wavelengths into wave heights which increases the efficiency of the power takeoff system and consolidates incoming wave frequencies using shoaling and refraction effects in mild to moderate wave conditions
  6. 6.       The energy storage subsystem in each unit is capable of storing over 500 kilowatt-hours of energy, so energy harnessed during storms can be used at later dates

 

The subsystem approach consists of two components; one freely moving component (point absorber, flapper, or oscillating water column, “the bobber”) and one moored component (the variable-depth barge or “the base”).

 

The combined system works by raising or lowering the base below the water surface until the passing waves achieve optimal height. The bobber is moved by these waves and recovers energy through a power takeoff system.

 

The system can be built using off the shelf components. The energy recovery system is the most complex component, but, in essence, is 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, so the system has no significant ocean engineering challenges.

 

 

Figure 2. SMWEHD Full-scale internal components

 

This approach is remarkably cost effective. A 5 MW name plate capacity unit will cost about $5 million to build and deploy. In an US East Coast deployment, 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 dimensions need to be approximately 60 m long x 25 m wide x 4 m high, and the bobber dimensions need to be 50 m x 20 m x 2 m.

 

The average wavelength off the East Coast is approximately 100 m (8 second period “deep water” waves). Mooring the units broadside to the predominant wave direction and tuning the average 1m-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), every wave cycle for an average bobber velocity of 3 meters per second (m/s) (24m per 8 seconds).

 

At a one meter draft, the bobber displaces approximately 1,000,000 kg of water. The kinetic energy in this oscillating motion 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 24 meter total displacement from the still water location is all calculated from surge.

 

Harnessing and converting this energy to electricity at an overall conversion efficiency of 24% (25% power takeoff efficiency x 92% generator efficiency) on a per second basis would result in:

 

4,500,000 joules x 0.24/second = 1.08 Megawatts (MW)

 

This will provide an annual average power production per unit of approximately 9460 Megawatt-hours (MWh) per year.

 

A billing rate of $120 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 $120 per MWh ($0.12 per kWh) = $1,135,200 per unit per year) and revenue will increase with improvements in conversion efficiency. That revenue allows for $135,200 per unit, per year for maintenance and $1 million per unit, per year for amortization and revenue.

 

A unit like this would cost about $1 per watt of name plate capacity to build and deploy ($5M/5MW) at industrial scale (1000 or more units), which is cost effective against wind (at $3 per watt) and PV solar (at $5 per watt). This includes installation costs for these systems at industrial scale, but the best part is that the wave units do not require land purchase and have no visual impact, except to possibly reduce shore wave impact, which could be a desirable feature.  The subsystem units can be built one by one and are scalable both in single size unit construction and in a variety of sizes. As such, there are tremendous economies of scale if multiple units are installed as farms.

 

It is always important to study environmental impacts of any engineered system. Removing energy from waves before they hit the beach reduces the size of waves once they hit the beach. This could be quite helpful in a global warming environment, But at the same time, there is so much wave energy offshore that that it is unlikely that all the waves on a coast will be measurably reduced by even a massive offshore installation.

 

A truly powerful wave farm is analogous to building a man-made island 10 miles wide and 50 miles offshore from New Jersey which will still have very little effect on sediment transport along beaches. This system can be adapted for surfing offshore, but the primary design function is industrial scale electricity production.

 

The reason each SMWEHD unit is built with 5MW of name plate capacity (maximum generator output) is to make them sufficiently large to take advantage of the vast energy available during storms and extended periods of large waves.

 

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 SMWEHD, pronounced "Some Wed", 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.

 

 

Figure 3. The modified Bosch-Rexroth regenerative braking system in this rendering of a scale model SMWEHD was developed with a great deal of help from engineers and technicians from Bosch-Rexroth and Airline Hydraulics. 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: 

 

Ocean Surface Wave Energy Harnessing Development at Stevens Institute of Technology (SIT) 

 

Determining and Controlling Peak Energy Density Location during Water Wave Density Location during Water Wave Deformation 

 

 

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