WPE: Projects

We do not know any successfully commercialised technology converting ocean wave energy into electricity. There is a long list of wave energy projects based on the different designs but none of them could produce commercially valuable power.

Note: we review only ocean wave energy converters. We do not consider designs using tidal or any other form of ocean energy. Also, we do not consider any submerged WEC designs. We do not believe that submerged WEC can produce commercially valuable power. Our concerns:

The cost of any submerged devise in times dearer than cost of the device with the similar power output located above water level. Even when alternator located at shore it does not make the cost of installation less expensive.
The cost of installation and maintenance is much higher.
Ability to interact with waves and absorb energy from large ocean area is limited.

There are 4 main types of floating wave energy converters

Point absorbers
Terminators
Attenuators
Oscillating Water Column (OWC)

A point absorber is a floating structure with components that move relative to each other due to wave action (e.g., a floating buoy inside a fixed cylinder). The relative motion is used to drive electromechanical or hydraulic energy converters.

Terminator devices extend perpendicular to the direction of wave travel and capture or reflect the power of the wave. These devices are typically onshore or nearshore; however, floating versions have been designed for offshore applications. The oscillating water column (OWC) is a form of terminator in which water enters through a subsurface opening into a chamber with air trapped above it. The wave action causes the captured water column to move up and down like a piston to force the air though an opening connected to a turbine.

The common disadvantage of known designs is unstable output. They interact with wave crest and idle in trough.

Theoretical limit of wave stability approximated by a wave height to wavelength ratio of 1/7

If wave period is 7 seconds, ideal device will work only 1 second and will idle rest of the time. A low efficiency is perfectly embedded into all these designs.

The lack of ability to produce stable continuous output during the wave period is one of the major reasons that makes wave energy projects unsuccessful.

There is only one solution to make wave energy converter to provide continuous output during the wave period is using an array of devices deployed along the wave length. Each unit will work only a second and idle rest of the time. Obviously, this solution has enormous capital and operational expenditures, which makes cost of power completely uncompetitive on electricity market.

An oscillating water column (OWC) is most promising concept for ocean wave energy development, because it does not have any mechanical parts and can be inexpensive in mass production.

What is the main advantage of OWC against the other concepts, such as a float or buoy? Simplicity and reliability. OWC pushes air out and sucks it back and can be considered as a single cylinder air pump where oscillating water inside OWC plays role of piston. To be able to produce continuous output during the wave period a long line of rigidly connected OWC’s has to be deployed.

To be able to synchronise direction of airflow among multiple OWC’s we added inlet and outlet flaps to each OWC.

Two air ducts allow to create unidirectional air flow to feed wind turbine.

We adopted a concept of reciprocating air pump to our design. OWC with inlet and outlet flaps connected to inlet and outlet ducts works like air pump, where OWC’s are cylinders and oscillating water inside OWC is piston. Please, welcome the Wave Mill!

The Wave Mill's design is based on the concept of rigidly joined assembly of oscillating water columns (OWC’s) working together as a reciprocating air pump.

A conventional OWC is a semi-submerged chamber containing trapped air. Oscillating water inside OWC pushes air out and sucks it back.

To adopt concept of the reciprocating air pump we added inlet and outlet flaps to each OWC. Inlet and outlet flaps allow to connect each OWC to air ducts – to inlet and outlet duct accordingly. When wave rises, water inside OWC pushes air out via outlet flap. When wave falls, water inside OWC draws air in via inlet flap. In each air duct air goes only in one direction. The air turbine located between two ducts receives unidirectional air flow and generates continuous power.

As a wave passes along the WM, the wave crest sequentially passes through the each OWC. Water oscillates sequentially inside each OWC and pushes air out into the outlet duct and draws air in from the inlet duct. Watch animation

Unlike all other designs, WM creates unidirectional air flow through the air turbine and thus it is highly efficient without the need for advanced turbine systems. The average air flow velocity through the turbine, and thus the amount of electricity generated, depends on wave conditions, but the direction of air flow is constant from the high-pressure duct, through the air turbine and then as it is drawn into the low-pressure duct.

If the length of the rigidly joined OWC’s are longer than length of passing along waves, at least one wave crest will always interact with OWC’s and WM will generate power. The key advantage of the Wave Mill is continuous uninterruptable power generation.

Another advantage is simplicity. Single WM unit is a row of rigidly joined OWC’s as shown on the picture above. WM does not have any moving mechanical parts. The wind turbine located between inlet and outlet duct generates power.

The Wave Mill design is mass production ready. Inexpensive and off-the-shelf materials and parts can be used to build WM units with the long lifespan. Multiple rows can be joined together in one large and solid floating construction as shown on the picture.

Another advantage is a large deck completely opened for customer applications, such as

Floating support facility with embedded power supply
Floating foundation for offshore wind. WM doubles generating capacity
Aquaculture
Breakwater which converts wave energy into electricity
Floating marina

The modular and scalable nature of the Wave Mill design allows to create fully customizable solutions with generation capacity ranging from 10kW to greater than 50MW. In terms of meeting consumer demand, this equates to powering between two and several thousand average households.

Period: July – August 2009. Location: Moreton Bay, Brisbane, Queensland, Australia

The Wave Mill dimensions
Length: 30 m;
Width: 1.5 m;

Platform has two ducts and two 600W generators;

Wave conditions
Wave period (T): min 2 sec; max 3 sec;
Wave height (H): min 0.4 m; max 0.6 m;
Wave Power(Wp) = 0.5 * H² * T (kW)
Min: 0.14 kW; Max: .81kW;

Performance

Pneumatics, air flow, air velocity
Fan diameter (D): 0.5 m
Air velocity (m/s) (V): min 8 m/s; max 15m/s;
Air density (AD) = 1.22
Power (W) = 0.5 * AD * (D² * 0.7854) * V³
Power: min 50 W; max 330W;
Platform has two ducts (inlet and outlet) and two fans:
Total measured pneumatic power: min 100W; max 660W;
Pneumatic efficiency: min 41%; max 81%;

Output
Generator:

Rated power: 600W;
Measured output: min 15 W; max 145W;

Platform has two ducts and two generators:
Total measured power output: min 30W; max 290W;
Efficiency from wave to wire: min 22%; max 36%;

Conclusion

Test data proves that the Wave Mill is the most efficient ocean wave energy converter.
The Wave Mill absorbs 90% of wave energy which makes it suitable for breakwater and coastal protection.
Using WM as a floating foundation for wind turbines or solar panels increases total generating capacity. Wave energy is well predictable and unlike wind or solar, WM unit generates power almost 24/7.