Three OWEC Ocean Wave Energy Converter® models were tested in a wave tank. 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.
The experiment evolved with construction of a breadboard built to add real hardware to the analytical part of this project. Loss factors are very 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 .5″x4’x8′ wooden board and frame is hinged on a base thereby providing underside access to fasteners. It accommodates a .5″x2’x4′ aluminum sheet portion having appropriate mounting holes for affixing energy converting components. The sheet includes two large holes for alternate flywheel or armature locations. Support components of the energy converting assembly are affixed to the aluminum portion and a wave simulator servo system, for controlling driveshaft reciprocation length and frequency, is affixed to the plywood portion.
Due to the horizontal orientation of the breadboard and driveshaft, means for simulating effective buoyancy and weight forces were required. A wave simulator servo system of consideration would implement two springs, in series, acting on the driveshaft. One non-constant spring models a buoy under varying conditions of partial submergence and a constant spring replicates the weight of driveshaft and buoy. This method was rejected in order to use gravity force of simpler 3, 5, and 10 pound weights sling suspended from a 5′ high scaffold, by block and tackle, and connected to a driveshaft end thereby enabling simulation of various driveshaft weights and gravity. 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. A .25hp, 120v synchronous stepper motor, with t of 720 oz-in @ 72 RPM, is vertically affixed to the breadboard by a steel plate. The motor axle is perpendicularly mounted with a 15″ diameter steel disk having six threaded holes, each 1″ apart 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 driveshaft stroke lengths ranging from 1.5′ to 5.5′ maximum. Reciprocation frequency is calibrated as a function of simulated wave period and manually adjusted with motor controls that allow speeds from 0-41.5rpm
During testing with the wave simulator, which was designed to exert linear forces of 150 pounds and simulate buoy and shaft weights of 15 pounds, it was quickly discovered that the motor was sorely 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 due to motor undersizing relative to simulator torque. 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. Reduced friction produced more favorable operation and the motor enabled tests of 16-32 inch height waves.
Compiled data from interrelated assembly operation studies provides input power point values for modifying a preexistent computer program already written for this overall design. 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 has enabled characterization of effective buoyancy from different buoy shapes, effect of driveshaft inclined reciprocation axes relative to various directions of wave procession, transmission calculations, and subsequent electrical output.