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By Katherine Webb,2014-11-12 00:54
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    Model Wave-Energy Generators 2.4.3

    Model Wave-Energy Generators

What?

    This document explains the theory, construction, and operation of a model wave-energy generator, used to convert ocean wave energy into electrical energy. The actual model is that of a wave-energy buoy, containing a linear electric generator. In a linear generator, the magnet moves in a straight line in and out of a coil of wire.

Why?

    The purpose is to further students’ and the general public’s knowledge of and interest in wave-energy generators. We plan to implement a student model first, specifically addressing the sixth- to eighth-grade science design standards. Ultimately, these models can be brought to the Hatfield Marine Science Center (HMSC) for evaluation using our wave tank and oscilloscope. The principal goals are

    ; to be as simple and clear as possible, so that students can see exactly what is

    going on and gain understanding of the basic principles involved

    ; to be inexpensive enough that teachers on a limited budget can reproduce the

    models for their classrooms

    ; to be functional enough to generate a detectable amount of electricity.

    Eventually, we want to scale the models up to the size required for a display in the HMSC Visitor Center.

Theory

    A changing magnetic field will induce electrical voltage into a loop of wire. This Faraday Law of Induction is the basic principle behind virtually every electric generator in the world. Whether the magnetic field is variable or the loop of wire is moving or rotating is unimportant; the changing strength of the magnetic field in the wire loop is all that matters.

    There isn’t a simple formula to calculate the expected voltage from this type of generator. The usual formulas deal with wire loops rotating in a uniform magnetic field, or loops in a magnetic field that is uniformly changing. In a linear generator the magnetic field is not uniform; it is strong at the surface of the magnet and weakens as it moves further out. The actual voltage from this model will probably be in the range of 10 millivolts to 1 or 2 volts, and must be determined by experiment.

Factors Affecting Voltage

    ; Number of turns in the coil

    ; Strength of the magnet

    ; Length of stroke of the magnet through the coil: controlled by wave height

    ; Speed of magnet through coil: controlled by wave period

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    Model Wave-Energy Generators 2.4.3

     Length of the coil: as the coil is ―stretched out,‖ with the number of turns ;

    remaining the same, the voltage will be reduced

    ; Diameter of coil: as the diameter of the coil grows larger, the voltage will be

    reduced

    ; Clearance between the magnet and the inside of the test tube: as clearance is

    reduced, fluid friction with the water is increased, and the magnet speed and

    voltage are reduced

    ; Amount of current flowing in the coil: as the current increases, it creates an

    opposing magnetic field, reducing the voltage

    Some Possible Configurations

    ; Moving coilcoil fixed to the float, magnet fixed to rigid rod anchored to bottom.

    See Figure T-1.

    ; Moving magnetmagnet fixed to the float, coil anchored to bottom. See Figure

    T-2.

    ; Spring-mounted magnetcoil fixed to the float, magnet anchored to the bottom

    and attached to the float by a spring. The intent is to keep the float from being

    entrained by the waves and dragged away from the magnet. See Figure T-3.

     Figure T-1

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    Model Wave-Energy Generators 2.4.3

     Figure T-2

     Figure T-3

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    Model Wave-Energy Generators 2.4.3

Construction

    The following discussion is focused on the moving magnet design of Figure T-2. Other

    designs follow similar construction techniques.

Winding the Coils

    Initially, wind the coils on the 16mm test tube. This size will accept all magnets up to 3/8 inch in diameter. The length of the winding should be a bit less than the expected wave height. The most voltage is generated if the magnet just clears the coil at the ends of each stroke.

    If the depth of water is limited, as in the soda bottle, you may want to shorten the test tube by cutting an inch or two off the top. One reason for using plastic test tubes is that they are easier to cut than glass.

    A good starting point is to wind a coil with 50 turns on the 16mm (smallest) test tube. It helps to fasten the wire to the tube with a hot-glue gun, leaving a couple of feet of wire free to make connections. Then wind the wire around the tube. When you have the desired number of turns, stick the wire down with the glue gun. Cut the wire, again leaving a couple of feet for connections. Apply a layer of fingernail polish to secure the

    windings. See Figure C-1.

    Attach a small loop to the bottom of the test tube. This will attach to a fastener in the wave tank to keep the tube from drifting out of position. Using needle-nosed pliers bend a small wire into a loop, with a small round base to glue to the bottom of the tube. Glue the base to the bottom of the tube. Then cover the wire with a coat of fingernail polish to prevent corrosion. The examples shown here are made of 16-gauge steel wire. See Figure C-2.

    When everything is dry, you must remove the insulation from the ends of the wires. The enamel insulation looks like part of the wire, but it must be removed to make an electrical connection to the meter. Drawing the end of the wire repeatedly through a fold of medium-grit sandpaper will do nicely. When the wire end becomes a bright copper color, the insulation is removed. If you have an ohmmeter, check to make sure there is zero resistance between the ends of the coil wires.

    To keep the tube upright in the tank, attach a flotation collar to the top of the tube. Wrap the top of the tube in several layers of plastic packing foam and secure with a rubber band.

    Drop a lead split-shot (fishing weight) into the bottom of the tube. This will help maintain stability and prevent the magnet from becoming stuck to the wire loop at the bottom of the tube.

You are now done with the coil. See Figure C-3.

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    Model Wave-Energy Generators 2.4.3

    Figure C-1

    Figure C-2

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Model Wave-Energy Generators 2.4.3

Figure C-3

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    Model Wave-Energy Generators 2.4.3

Magnet and Float Assembly

    Cut a piece of fishing line about a foot long. Tie one end to a magnetic necklace clasp. Then stick the clasp to a magnet. The magnetic clasp makes it easier to change magnets when doing comparative tests. See Figure M-1.

    Be careful not to get any magnet too close to any metal objects or other magnets, as they will jump very suddenly and forcefully. The large magnets will pinch most painfully if you get a bit of skin caught between them.

    Also try not to let the magnets jump together or strike each other forcefully. Severe impacts can knock chips out of the magnets, leaving rough, sharp edges.

Hook the float onto the line. See Figure M-2.

    Figure M-1

    Figure M-2

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    Model Wave-Energy Generators 2.4.3

    For testing without a wave tank, glue a small screw into the end of a plastic soda

    straw. Stick a magnet to the screw. The magnet can then be pushed back and forth

    through the coil to simulate wave action. The magnet can easily be changed for

    comparison tests. See Figure M-3.

    Figure M-3

Making the Wave Tank

    A wave tank could be made from a plastic storage container. Cut two pieces of 1‖ dowel

    about 2' long and place them under the tank, like rollers. See Figure W-1.

    To make waves, gently move the tank back and forth on the rollers. It takes a bit of practice to make decent waves without slopping the water out of the tank. The tank will have a natural period of oscillation. Once you find it, the waves will be much easier to control.

    Using the black laundry marker draw measured lines on the side of the tank to help estimate wave heights. See Figure W-2.

    Glue fishing swivels to the bottom of the tank to serve as anchors for the tubes. This will keep the tubes from being carried away by the waves. Place the swivels 3 4 inches

    from one end, where the waves will be highest. Make sure the swivel end is free to attach to the wire loop on the test tube. See Figure W-3.

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    Model Wave-Energy Generators 2.4.3

    Figure W-1

    Figure W-2

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    Model Wave-Energy Generators 2.4.3

    Figure W-3

    For preliminary testing, a tank could be made from a 2-liter soda bottle with the upper part cut off. Waves can be simulated by squeezing the sides of the bottle to make the water level rise and fall. Since the water depth is quite limited, you may want to cut an inch or two off the top of the test tube. See Figure W-4.

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