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of 9733 allis, 1, view 10 kW C211 2009 Published by Elsevier Ltd. chbegan conducted oscillating water-column device, Limpet, installed on the Isle of Islay off Scotland in 2000 8, and the Pelamis wave energy conversion system deployed off the coast of Portugal in the fall of 2008 5. In the United States, Ocean Power Technologies (OPT) has developed a point-absorber wave energy conversion buoy for the 2.1. Floating system ThefloatingsystemconceptfortheSeaBeavIconsistsofacentral cylindrical spar and an outer Taurus-shaped buoy. The spar is moored to the bottomwith the buoy freetotranslate relative to the spar. Having two floating bodies allows for the central spar to be restrained laterally using a mooring system without limiting the heaveof thebuoy. Thisallowsforaconventionalmooringsystemto be utilized without the need for subsurface floats to reduce the * Corresponding author. Tel.: 1 281 745 7343. Contents lists available Renewable Renewable Energy 35 (2010) 348354 E-mail address: (D. Elwood). 7andEvans2inGreatBritain andFalnes3 inNorway,amongst others. Several promising concepts were developed by 1980 including point-absorber wave energy converters such as the infa- mous Salter duck 7 and oscillating water-column (OWC) devices utilizingaWellsturbine1forpowertake-off.Withthedecreasein oil prices in the early 1980s much of the funding for ocean wave energy conversion was cut and no full scale demonstrations of the technology were constructed. Recently concerns about global warming and the increasing price of conventional energy has led to resurgence in research on ocean wave energy conversion. Currently several commercial developers are working to build grid-connected wave energyconversion systems. These include the 2. Concept design A systems design approach was used to develop the conceptual design of the SeaBeavI wave energy converter. The floating system, mooring, and power take-off concepts were optimized concur- rently in order to ensure that the combined systemwas as efficient as possible. Efficiency of the system was measured both in terms of the powercapture efficiencyand the overall cost of the system. The SeaBeavI conceptrepresents anoriginal approach totheconversion of ocean wave energy into electricity. Keywords: Wave Energy Direct-Drive Permanent Magnet Linear 1. Introduction Modernoceanwaveenergyresear the1970s.Muchoftheearlyworkwas 0960-1481/$ see front matter C211 2009 Published by doi:10.1016/j.renene.2009.04.028 duringtheoilcrisisof inEuropebySalter US Navy with a test buoy deployed off Oahu, Hawaii 4. OPT has plans to deploy an array of these buoys off the coast of Reedsport, Oregon representing the first grid-connected wave energy conversion plant in the United States. Available online 18 July 2009 ical, and ocean engineering. A systems design approach was used to develop the taut-moored dual-body wave energy converter concept with the detailed design focused on production and ease of maintenance. Accepted 30 April 2009 SeaBeavI project was an interdisciplinary effort bringing together researchers from electrical, mechan- Design, construction, and ocean testing wave energy converter with a linear generat David Elwood a, * , Solomon C. Yim a , Joe Prudell b , Chad Ted Brekken b , Adam Brown c , Robert Paasch c a School of Civil and Construction Engineering, Oregon State University, Corvallis, Oregon b School of Electrical Engineering and Computer Sciences, Oregon State University, Corv c Department of Mechanical Engineering, Oregon State University, Corvallis, Oregon 9733 article info Article history: Received 27 December 2008 abstract This paper presents an over in the ocean testing of a journal homepage: www.else Elsevier Ltd. a taut-moored dual-body or power take-off Stillinger b , Annette von Jouanne b , 1, USA Oregon 97331, USA USA of the SeaBeavI project which began in the fall of 2006 and culminated direct-drive wave energy conversion system in the fall of 2007. The at ScienceDirect Energy translate. The tensioned mooring line allows for effective force sectiontobelocatedinthehigherbuoyancybuoy.Withthearmature of the generator. Fig. 3 provides a rendering of the spar and buoy and their internal arrangements (Table 1). 3.2. Buoy structure The buoy structure was designed in a modular fashion to allow for ease of construction and maintenance. The buoy was con- Fig. 1. Dual-body wave energy conversion system concept. Fig. 2. Power take-off system concept. Energy 35 (2010) 348354 349 located in the spar, the power take-off cable is attached to a taut- mooredfloatingbody,limitingtheforcesonthecable.Fig. 2 provides transmission during both the upstroke and the down stroke of the buoy. A tensioned mooring system also limits the watch circle of the system allowing for tighter spacing in arrays of devices. 2.3. Power take-off system Conversionoftherelativelinearmotionbetweenthesparandthe buoytoelectricityisachievedthroughtheuseofapermanentmagnet linear generator. The generator consists of a stack of permanent magnets housed in the buoy and an armature composed of copper wireandbackironinthespar.Asthemagnetstranslaterelativetothe copper wires, current is induced in the wire due to the changing magneticfield.Thegeneratortopologyallowsfortheheaviermagnet vertical forces on the system. Fig. 1 provides an illustration of the two-body floating system concept developed for the SeaBeavI. 2.2. Mooring system In a two-body wave energy converter concept, the relative velocity and force transferred between the spar and the buoy are used to extract energy from the ocean waves. To maximize the power extracted by the system, the relative motion between the two bodies must be maximized while allowing for effective force transmission. The SeaBeavI concept uses a tensioned mooring to restrain the heave of the spar while still allowing the buoy to D. Elwood et al. / Renewable an illustration of the power take-off system concept. 3. Detailed design 3.1. Principal dimensions Dimensions of the buoy and spar were constrained by the limitations of the long wave flume at the O.H. Hinsdale Wave Research Laboratory. Prior to testing in the ocean, the system was intended tobe tested in thelaboratory undercontrolled conditions. The maximumwater depth of the long wave flume is 10 feet, which limited the overall draft of the system. The diameter of the buoy was constrained by the width of the flume in addition to depth. Since the flume is 12 wide, the maximum diameter to ensure a blockage of less than 33% is 4.95 feet. The final configuration of the system had an overall draft of 8.17 feet with a buoy diameter of 5.08 feet. A buoy diameter greater than the initial target was required to provide adequate stability based on the as-built weight structed of glass reinforced plastic (fiberglass) with high density foam to provide extra buoyancy. Because the linear generator depends on magnetics for the contactless force transmission and the generation of electricity, the use of magnetic materials in the structure of the buoy and spar is not desirable. In addition, fiber- glass does not corrode in salt water making it a preferred material for salt water applications. The magnet section of the linear generator was integrated into the buoy structure so that it was modular and removable for later testingofthegeneratoronalineartestbed.Akeywayatthetopand bottom of the magnet section transfers the lateral forces between the magnet section and the hull of the buoy. Rubber o-rings were used to seal the joint between the magnet section and the keyway. To hold the magnet section in place and provide compression for the o-rings, a heavy fiberglass lid was attached to the buoy struc- turewith12halfinchstainlesssteelbolts.Ageneralarrangementof the buoy structure is included in Appendix 1. Fig. 3. Rendering of spar and buoy general arrangement. provided by two fiberglass compression rings tied together using 3.5. Linear bearings To enable efficient conversion of the linear motion between the spar and the buoy into electricity, the gap between the magnet sectionandthearmaturemustbesmall(5 mm).Thisgapneeds to be uniform around the entire circumference of the spar in order to ensure that the magnetic normal forces between the magnet section and the armature are balanced. If the alignment is not precise, the normal force between the two sections becomes extremely large and increased friction will result. Thin strips of a laminated plastic material hold the gap between the buoyand the spar and provide a smooth bearing surface to ride against the stainless steel tube adhered to the inside of the magnet section. Twelve 1/2 inch wide strips are glued into grooves evenly Energ 3.3. Spar structure The spar structure was designed in three sections allowing the armature to be removed for maintenance and testing. Like the buoy, the spar is made entirely of fiberglass with stainless steel fasteners. Havinganon-magneticsparisimportantsincethesparlieswithinthe field of the magnet section. The bottom section of the spar houses a ballast tank used in the tensioning of the mooring system. A half sphericalbaseonthebottomoftheballasttankprovidesastrongpoint for attaching the mooring line. The power take-off cable attaches to a wet mateable waterproof connector at the base of the spar. The center section of the spar houses the armature of the linear generator along with the battery to power the ballast control and data acquisition systems. Twelve sections of inch stainless steel threaded rod join the center section to the top and bottom compartments of the spar. Compression plates with two sets of rubbero-rings ensure that the entire spar is sealed. A bilge pump in the lower section of the spar provides both dynamic ballast control and protection against leaks in the operating condition. The top section of the spar houses the ballast control and data acquisition systems. A linear position sensor utilizing a magnetic pickup recordstherelative displacement between thebuoyand the spar. Thermocouples mounted on the inside of the armature record the temperature of the coils during operation of the generator. Wireless communication is used to transmit the measured data to a nearby research vessel along with providing control signals for thepumpingsystem.Afiberglasslidattachedwitha12boltpattern and sealed with a rubber o-ring isolates the top section of the spar from the sea. A general arrangement of the spar structure is included in Appendix 1. 3.4. Linear generator The generator consists of an 1196 mm long magnet section Table 1 Principal dimensions of the SeaBeavI wave energy converter. Buoy Diameter 5.08 feet Draft 6.00 feet Depth 7.50 feet Freeboard 1.50 feet Displacement 6473.90 lbf SPAR Diameter 2.00 feet Draft 8.17 feet Depth 10.71 feet Freeboard 2.54 feet Displacement 1616.42 lbf D. Elwood et al. / Renewable350 housedinthebuoyanda286 mmlongarmatureinthespar6.The magnet section is composed of over 900 individual high density neodymium iron boron magnets held in place using aluminum retainers. The back iron on the magnet section is radially laminated to reduce eddy current losses. A composite structure utilizing stainlesssteelrods,aluminumendpieces,andafiberglassshellwas used to provide structural support for the magnets and back iron during construction and installation. This structure also enabled the generator to be tested without the support of the buoy struc- ture. A thin stainless steel tube adhered to the inner radius of the magnet section isolatesthe permanent magnets fromthe seawater and provides a smooth surface for the linear bearing system. The armature also utilizes radially laminated back iron with slots filled with windings of 14 gauge copper wire. Each coil of the armature consists of 77 turns of wire with 4 coils for each of the 3 phases of the generator. The structural rigidity of the armature is inch stainless steel rods. Each phase of the generator was terminated into acentraljunctionboxandthepower was fed outof the spar through the power take-off cable. A rendering of the magnet section and armature is included as Fig. 4. Fig. 4. Crosssection of the magnet section and armature. y 35 (2010) 348354 spaced around the circumference of the outer shell of the spar. The material used to manufacture the bearing strips is designed to be water lubricated making it ideally suited for marine applications. 3.6. Mooring The tensioned mooring system consists of a single-anchor-leg mooring with a mushroom anchor. The buoyancy of the spar is greater than its weight, generating 650 pounds of pretension in the mooring line. This pretension is sufficient to ensure that the line will not go into compression during the down stroke of the buoy. Mooring pretension is achieved through the use of spar water ballast during installation. The spar ballast tank is filled with water prior to installation and the mooring line is attached slack to the base of the spar. Slack in the mooring system is removed using to achieve the desired pretension. having been built separatelybytheir respective manufacturers. The structural components were built at Plasti-Fab Inc., a structural fiberglass manufacturer in Tualitin, Oregon. While the fiberglass components were being fabricated, the magnet section and arma- ture were being constructed by graduate research assistants at OregonState.Simultaneously, themooringwas beingassembled by the staff rigger at Englund Marine in Newport, Oregon. 4.1. Structural components The outer shell of the buoy was constructed using a filament winding process on a largediameter mandrel. Both the bottomring frame and the inner shell of the buoy structure were molded using a vacuum driven resin transfer process. To provide extra buoyancy and stability for the buoy, high density structural foam was added totheexteriorof theoutershellandsealedwithalayeroffiberglass mat. The lid for the buoy was constructed of fiberglass with a foam core. Keyways for the top and bottom of the magnet section were also molded and grooves for the o-rings machined using a 3DOF computer controlled router. Pictures of the buoy structure can be D. Elwood et al. / Renewable Energy 35 (2010) 348354 351 The mooring line is composed of steel bottom chain and synthetic rope joined together using forged anchor shackles. A cluster of trawl floats is used as a mid-column float to remove the weight of the bottom chain from the spar. A pear link at the top of themid-columnfloatallowsforthetensionersystemtobeattached by divers during installation of the device. Two 3:1 polycarbonate blocks and 300 feet of synthetic line make up the block and tackle system used to tension the mooring. Once the tensioner has been attached to the mid-column float bya dive team, the system can be tensionedanddetensionedfromthesurface.Ageneralarrangement of the mooring system is included in the Appendix. 4. Construction a tensioner system and the water is pumped out of the ballast tank Fig. 5. a) Buoy lid b) Buoy inner shell and ring frame c) Buoy outer shell with added foam. Construction of the SeaBeavI wave energy conversion system was completed during the summer of 2007 with the subsystems Fig. 6. a) Spar structure showing the ballast tank, armature, and electronics compartment showing fiberglass ring frame at the top of the armature. seen in Fig. 5. The outer shell of the spar was constructed from two sections of 24 inch fiberglass water pipe with a 1/2 inch wall thickness. The outer diameter of the pipe was machined and slots for the bearing stripswerecut usingaCNClathe. Amoldedhalfsphericalfiberglass bottom plate was fixed to the bottom of the spar to provide an attachment point for the mooring line. One inch thick fiberglass ring frames were molded for the top of the lower spar section, and the top and bottom of the upper section. O-ring grooves were cut in the ring frames using the computer controlled router. Pictures of the spar structure during construction can be seen in Fig. 6. 4.2. Linear generator The linear generator components were assembled by hand by graduate research assistants (9). Each of the laminations for the armature and magnet section was hand shimmed using high temperature shim tape. The armature laminations were stacked using the fiberglass compression plates to hold the laminations in place during construction. The magnet section was constructed in quarter round sections that were assembled in a specially made jig. After the laminations were assembled, the quarter rounds and b) Bottom plate with wet mateable connector and strong point c) Interior structure armature were dipped in high temperature epoxy resin to protect the iron from corrosion and provide insulation between the lami- nations. The permanent magnets were fixed to the quarter rounds individually and then the sections were assembled to form the completed magnet section. The copper was wound onto the armature using a specially built winder with fiberglass insulation between layers of windings to prevent internal shorts between wires. Pictures of the armature and magnet section build can be seen in Fig. 7 a and c. The completed generator components can be seen in Fig. 7 b and d. 4.3. Mooring system Eyelets werespliced intothe ends of the spectra rope in order to prevent abrasion between the rope and the connecting shackles. A marker float for the mooring was built using a surplus 1 m steel float fitted with a mast and a 2 nautical mile solar powered light programmed to flash 15 times per minute. The anchor for the system was an 8200 pound steel mushroom anchor purchased as surplus from a scrap yard in Seattle, WA. The tensioner system consisted of two Harken 75 mm Carbo triple blocks run with 300 owned by the Oregon State Marine Mammal Institute. The 80 foot vessel is outfitted with a 10,000 pound crane used to launch the two rigid inflatable boats used for tracking and tagging whales. With the net reel and stern ramp removed, the working deck of the vessel is quite large, making it well suited for deploying oceano- graphic equipment. 5.2. Mooring installation The mooring system for the SeaBeavI was designed to be installed prior to the installation of the floating system. After the anchor, mooring line, and mid-column float were installed, SCUBA divers would be used to attach the tensioner system to the mid- column float during installation of the floating system. A steel Before testing the SeaBeavI in the open ocean, the system was D. Elwood et al. / Renewable Energy 35 (2010) 348354352 feet of Sampson Dura-Plex line. The tensioner was attached to a titanium eye nut anchored to the base plate of the spar. A rope clutch was used to enable the tension of the system to be adjusted during operation. Fig. 8 provides photographs of the as-built mooring system. 5. Installation and ocean testing The installation and testing of the SeaBeavI wave energy converterwas completed betweenAugustandOctoberof 2007.The systemwas installed in two phases, with the mooring system being installed in mid-August and the floating system in early October. The SeaBeavI was tested on the pier at the Hatfield Marine Science Center and in Yaquina Bay followed by testing in the open ocean. 5.1. Installation vessel The Research Vessel Pacific Storm was used for both the mooring installation and the ocean testing of the wave energy conversion system. The Pacific Storm is a converted fishing trawler Fig. 7. a) Laminations being assembled for one of the magnet section quarter rounds b) Completed armature with terminations and fuse box c) Armature laminations being assembled between the fiberglass compression plates d) Magnet section being removed from the buoy after testing. tested on the pier at the Hatfield Marine Science Center and in Yaquina Bay. The ballast control system, data acquisition system, and mooring tensioner were all tested in controlled conditions prior to ocean testing. The linear generator was tested on the pier by placing the buoy on concrete p