OWEC® Development

OWEC® Wave Tank Test

Three OWEC® Ocean Wave Energy Converter models were tested in a wave tank, May, 1982. The test was performed to observe mechanical and electrical responsitivity of working models under controlled hydrodynamic conditions. Models were placed near the middle of the tank and flotationally suspended in water with the buoys partially submerged and the rest of the structure totally submerged. Additional weights were distributed on module damper plates to achieve neutral buoyancy at preferred working depth. As each wave passed, the buoys were raised and lowered thus moving portions of linear electrical generators up and down within tubes. The tubes and other generator parts were maintained relatively stationary by the damper plates which strongly resisted vertical motion due to their placement at a depth where water particle movement from waves was essentially attenuated. The damper plates also countered structure tendency to drift off station. Motion of linear generators produced measurable electrical power from waves ranging 1" to 5" height. Although electrical outputs were slightly low for scale, waves that engaged the structure were only 30% of optimal design levels. The mechanisms of the apparatus functioned as intended and measurable electrical energy was generated from wave motion. This first test was valuable for successfully proving the concept and divulging considerations for subsequent OWEC® design development.

While promising, wave tank tests also revealed meritorious deficiencies. Non-resonant buoy actuation delay was promoted by high center of buoyancy and lack of tangential surface resistance to water particle motion. This condition indicated deriving possibly improved wave following capability from partially submerged buoy shapes having low centers of buoyancy and maximal planar contact with the hydroface. Fully submerged, however, these configurations typically embody added mass forces. A most efficient hybrid buoy shape incorporates smooth laminar flow and maximal buoyancy within design parameters. Whereas inclined reciprocation axes operated favorably to expand buoy capture distance by allowing simultaneous absorption of both the vertical force component of buoyancy and the horizontal time component of buoy/wave crest engagement with respect to wave procession, using buoy displacement for directly raising and lowering associated linear electrical generators caused wave-reciprocation frequency matching problems. Often, the time delay shortened or nullified reciprocation of the rods and associated generators due to their antiphasal movement with ambient wave fields. Although energy conversion was very direct, power generation was diminished during stroke reversal and start-up. Enhanced electrical output is obtained by relocating and improving generator components. The sacrifice of any peak value electrical outputs of the linear generator configuration should be acceptable in comparison to advantages of efficient output produced by a contemplated and proposed electromechanical assembly.

OWEC® Breadboard

The experiment evolved with construction of a breadboard built to add real hardware to the analytical part of this project. Loss factors are difficult to accurately assess and baseline operations of breadboards can shed much light on the problem. Also, integrated parts behavior and sizing are more easily understood and debugged. The board is provided an aluminum sheet for precisely affixing and changing energy converting components as transmission, flywheel, or generator. A wave simulator servo system is affixed to the board for controlling driveshaft reciprocation length and frequency. Due to horizontal driveshaft attitude, means for simulating effective weight and buoyancy forces were required. A wave simulator servo system of consideration would implement two springs, in series, acting on the driveshaft. One non-constant spring replicates a buoy under varying conditions of partial submergence and a constant spring models the weight of driveshaft and buoy. Instead, varying weights sling suspend from scaffold, by block and tackle, and connect to driveshaft end. A servo system of a 3:1 reduction block and tackle configuration is connected from the other driveshaft end to the post on a drive disk and motor. The motor is vertically affixed to breadboard by a steel plate. The motor axle is perpendicularly mounted with a 15" diameter steel disk having six threaded holes in radial alignment from hub to rim, for carrying a post. Placement of the post in a certain hole, relative to others, provides specific radii that translate disk rotation to sinusoidal motion and particular ranges of driveshaft stroke lengths (5.5' maximum). Reciprocation frequency is calibrated and speed adjusted as a function of simulated wave period. During testing with the wave simulator, it was quickly discovered that the motor was under powered. The 3:1 gear reduction ratio of the simulator, shafts, bearings, and generator wielded a large amount of force on the system. At the extreme settings, maximum driveshaft travel could not be sustained. In particular, the selected transmission was overscaled and exerted inordinate resistance on the drive train. The unloaded axle, downstream of the transmission and without the generator attached, resulted in rotation speeds of 20-70 rpm but could achieve barely 20 rpm when the generator was in line. In final runs, the transmission was eliminated to directly connect the flywheel to the generator. More favorable operation enabled electrical generation tests of 16-32 inch height waves. Assembly operational data provides power output values for modifying the design program. Program development has capability to correlate equations of motion for ocean wave height, length, period, and celerity for plotting vertical velocity of the hydroface as a function of time. Additionally, the sums of fundamental, secondary, and tertiary wave heights, etc. are described. Further development enables characterization of effective buoyancy from different buoy shapes, inclined driveshaft reciprocation relative to various wave procession directions, transmission calculations, and electrical output.