TIDAL ENERGY - DESIGN TEST BED

The test bed was the heart of this senior design project. In the beginning, many design conceptions were considered and their attributes compared. Many drive systems, from bevel gear drive trains, to a flex-shaft driveline, to a belt or chain drive were analyzed. The properties of many strut designs, from the simple single pipe, to the twin foil design, were thought out.How the necessary measurements were to be taken and weather the system was to be water tight also needed to be considered. To help weigh the pros and cons of each of these system designs, a Tidal Turbine Test Bed Design Matrix was created with these major concepts:

Bevel Gear Drive
In-Line Submerged Drive
Chain Drive

After this initial analysis, it was determined that the best system would be a chain driven, twin foil strut design mounted on a dynamometer with small motor to provide torsional resistance. Next, the expected forces, including shaft torque and frontal drag load, had to be considered to achieve a reliable design. These two reports detail these calculations:

Power, Torque, Thrust, and Moment Calculations
Shaft Design

The design matrices and subsequent calculations lead to the develop of a chain driven system with an open water lower driveline, a streamline housing, and a dry upper driveline on a movable platform connected to a dynamometer. The chain drive was chosen after much analysis for its efficiency in energy transfer, resistance to the test-environment and ease of fabrication. The system was wired with instruments to read torsional load, rotational velocity, and frontal drag force on the turbine. The test bed was broken up into four major assemblies:

The dynamometer
Upper driveline assembly
Streamline struts
Lower driveline with streamline housing.

DYNAMOMETER

The dynamometer used was a 6-Axis direct force dynamometer. The system was designed and built by Matt Zeh (MMA). This dynamometer suspends a plate from long slender rods, essentially allowing the plat to move freely through small deflections. This allows for the measurement of X, Y, and Z forces and moments. For the test bed, only the axis which ran parallel to the velocity vector of the tow tank carriage was connected though a load cell to the upper driveline platform and was used to measure the frontal drag force on the turbine.

UPPER DRIVELINE ASSEMBLY

The upper driveline assembly was the heart of the test bed. This system coupled together all the instrumentation needed for data acquisition to the upper shaft, while keeping it out of the water. The upper driveline begins with the 0.500” OD stainless steel shaft being supported by two Rylon Linear Bearings. The bearings sit on aluminum pillow blocks and rest on risers to align the shaft. These bearings allowed the upper shaft to self align the chain drive system to the lower driveline. On the right end of the shaft, Rotary Incremental Encoder is fixed to the upper plate by a right angled mount and was coupled to the shaft. The encoder was used to get the rotational velocity of the upper shaft. On the left end of the shaft, a DC stepper motor was connected inline with the upper shaft to provide rotational resistance to the turbine. The rotational resistance was varied with a fixed (24 volt, 15 amp) Power Source and a Variable Resistor. A third Rylon Linear Bearing supported the other end of the motor through the motors rear output shaft and suspended the motor over a slot in the driveline plate. This allowed the body of the motor to rotate freely. A gimble device was then bolted to the upper plate and connected to the motor through a S-Type Load Cell pin supports on the plate and on the outer fringe of the motor. The gimble was used to obtain torque readings from the reactant force on the rotating motor. To transfer power between the lower and upper drivelines a Stainless Steel Roller-Less Chain was used in conjunction with a Nylon Sprocket. The upper plate was connected to the dynamometer housing by long, slender rods to allow the upper and lower assemblies freedom of movement so the forces could be measured by the dynamometer load cells.

STRUTS AND NACELLE

Two side by side streamline aluminum airfoil struts connected the upper driveline plate to the lower driveline. These struts were extruded in the shape of a streamlined airfoil and were purchased from Aircraft Spruce. The bottom ends of the struts were machined to match the 3.00” OD profile of the lower driveline housing. This allowed for a seamless connection when welding the struts to the housing. The top ends of the struts were squared off and welded to the chain-tensioner plate. The chain tensioner plate was bolted onto the driveline plate to connect the struts and lower driveline to the upper assembly. The use of twin side-by-side struts allowed for the chain to run up through the center of one of these struts and back down through the other while minimizing drag on the system as it moved through the water. Nylon tubing lined the walls of the strut to minimize friction from contact between the chain and the side wall of the strut.

LOWER DRIVELINE

The lower driveline was an open water system. Its body consisted of a 3.50” OD by 12.00” long aluminum pipe. This pipe was machined with internal threads on both ends to allow two UHMW Polyethylene bearing inserts, which housed unsealed stainless steel ball bearings, to be screwed into position. Two .825” holes were drilled in the top of the cylinder to allow for the chain to pass through. These inserts can be easily turned into the body and removed for occasional bearing maintenance. The two threaded inserts had a tapered 45 degree back edge that mated with a matching taper inside the pipe housing to center the inserts. The driveline housing was streamlined with a CNC machined UHMW front nose cone that had a standard tangent ogive profile and threaded into the cylinder. The rear of the housing had a similarly machined UHMW tail cone. These cones were constructed to minimize separation in the flow so the assembly would smoothly cut through the water creating little disruption to the flow. The lower drive shaft consisted of a 0.500” OD Stainless Steel Shaft. This rode on the ball bearings within the housing. To transfer the axial thrust and drag forces, the shaft was machined with a round nose to contact the flat face of the Rear Nose Cone. The front end of the shaft was machined with a 3/8”-16 thread to connect the turbine nose cone. A matching sprocket was attached to the shaft lower shaft just as it was with the upper shaft. This allowed the power and rotational motion to be transmitted from the lower shaft to the upper shaft through the chain.

PART FABRICATION

The design of the test bed included many parts that could not be purchased out of a catalog. SolidWorks was used to 3D model these parts, along with the entire test bed, and create drawings for parts so that the group could fabricate them. CNC machining was widely used, especially for part requiring precision.Many hours were spent doing both lathe work and mill work.