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
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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
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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 coil—coil fixed to the float, magnet fixed to rigid rod anchored to
bottom. See Figure T-1.
 Moving magnet—magnet fixed to the float, coil anchored to bottom. See Figure
T-2.
 Spring-mounted magnet—coil 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.
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Model Wave-Energy Generators 2.4.3
You are now done with the coil. See Figure C-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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Model Wave-Energy Generators 2.4.3
Figure W-4
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Model Wave-Energy Generators 2.4.3
Parts List – Current as of 8-15-2011
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Acrylic test tubes—16mm (30 cents each), 24mm (50 cents each), and 38mm (46
cents each)—from Lake Charles Mfg.,
http://www.lcmlab.com/SearchResults.asp?Cat=23
Most of the work will be done with the 16mm test tubes, so order enough of these
to go around. The larger tubes are used only to investigate the effect of a larger
coil diameter. You will probably need only a few of these.
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Rare-earth cylinder magnets, type D68 (3/8-inch diameter by 1/2 inch long, $2.16
each) and type D48 (¼ inch diameter by ½ inch long, $1.16 ea.). NOTE: Sizes
larger than 1/2 inch are not recommended, due to the risk of injury from these
extremely powerful magnets. Order from K & J Magnetics,
http://www.kjmagnetics.com/categories.asp
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Small magnetic necklace clasps, $1.25 a set. From Nye Cottage Beads, 208 NW
Coast St., in Newport, or any bead craft shop.
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Magnet wire, enamel coated, in the range of 20- to 30-gaugeMagnet wire, enamel
coated, in the range of 20- to 30-gauge. Radio Shack (store or website) has an
assortment of 22-gauge, 26-gauge, and 30-gauge wire (about $7.00). You can
also find 1-pound spools of 26-gauge wire at
http://www.amazon.com/b?ie=UTF8&node=310354011#. Price is about $18 each.
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Electric meter—zero center, -50-0-50 millivolts or -500-0-500 microamps or
similar. Having the zero in the center allows the needle to track the voltage or
current as it alternates between negative and positive. For example, see 7-1309-20
Galvanometer, -500-0-500 microamps ($9.50 each), from Ginsberg Scientific:
http://www.ginsbergscientific.com/0cat_details01.asp?cid=3092
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Wave tank—The 2-liter soda bottle with the top cut out will do for preliminary
testing, but it is quite small; more-extensive testing will require a larger tank. For
example, see the 20-gallon storage container at:
http://www.containerstore.com/shop/storage/totes?productId=10024301&green=2
445150072 (about $44 each). This container also makes a handy storage container
for all the components when not in use.
The following items can be purchased locally in most department stores or sporting
goods stores.
 Fishing bobbers, 2-3 inches. The kind that fasten to the line with a spring-loaded
hook make adjustments easier.
 Fishing line, monofilament or braided, 5- to 10-pound test. Heavy sewing thread
will also work fine.
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Model Wave-Energy Generators 2.4.3
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Fishing sinkers, 1 ounce.
Empty soda bottle, 2 liters.
Small fishing swivel snaps
Two 1” dowels, 2' long
Lead split shot, 1/4 inch
16 or similar gauge steel wire
Rubber bands
Glue gun & glue sticks
Packing foam
Fingernail polish
Medium sandpaper
Needle-nosed pliers
Yardstick
Black laundry marker
Paper or cloth towels
Depending on what you can beg, borrow or scrounge, the cost of these components will
run in the neighborhood of $150 to $160. A lower cost configuration using a 2-liter soda
bottle wave tank is shown below.
Economy Parts List
Item
16mm test tubes
24mm test tubes
38mm test tubes
3/8" magnets - D68
¼" magnets - D48
Magnetic necklace clasps
Radio Shack - wire assortment
meter - -500-0-500 uAmp
fishing bobbers
fishing sinkers
fishing line
Use 2-liter bottle
Misc Stuff
Sub-totals
Total
number
8
1
1
2
2
1
1
1
1
1
1
0
1
$80.75
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cost ea.
$0.30
$0.50
$0.46
$2.16
$1.16
$1.25
$7.00
$9.50
$3.00
$3.00
$3.00
total
$2.40
$0.50
$0.46
$4.32
$2.32
$1.25
$7.00
$9.50
$3.00
$3.00
$3.00
$20.00
$20.00
$56.75
shipping
$8.00
$8.00
$8.00
$24.00
Model Wave-Energy Generators 2.4.3
Operation
Setting Up to Use the Wave Tank
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Put the coil into the tank.
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Fasten the loop on the tube to a swivel in the bottom of the tank. Attach a flotation
collar to the top to keep the tube upright in the tank.
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Lead the wires out of the tank and connect them to the meter.
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Fill the tank up to about 2–3 inches above the top of the test tube.
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Put the float and magnet assembly into the tank with the magnet inside the tube.
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Adjust the line so that the magnet is just above the bottom of the coil. The waves
will go both above and below the fill level.
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Move the tank back and forth on the rollers to produce waves. Try to make the
waves as uniform as possible
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Observe the needle on the meter. It should move in time with the waves.
Setting Up to Use a Soda Bottle Tank
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Fasten a weight to the bottom of the test tube and a flotation collar to the top to
keep the tube upright in the tank.
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Put the coil into the tank.
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Lead the wires out of the tank and connect them to the meter.
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Fill the tank up to about 1 inch above the top of the test tube.
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Put the float and magnet assembly into the tank with the magnet inside the tube.
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Adjust the length of the line so that the magnet is just protruding from the bottom
of the coil. This is because the waves will never go below the fill level.
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Squeeze and release the sides of the tank to make the water level go up and down.
Try to make the wave heights as uniform as possible
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Observe the needle on the meter. It should move in time with the waves.
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Model Wave-Energy Generators 2.4.3
Appendix 1:
Basic Electrical Measurements
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Digital meters are not much use here. The wave frequency is usually 3–5 Hz, but
most digital meters take 1 or 2 seconds to settle on a reading. The meter may
show a flicker of digits, but nothing even close to a true reading.
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Analog meters with 0 at the center of the dial are much better. In this case we are
using a microamp meter capable of measuring from -500 microamps to + 500
microamps with 0 at the center. Having the 0 at the center allows the needle to
swing either direction, tracking the current as it goes positive or negative. The
needle response is usually fast enough to get a rough idea of the current.
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The analog meter cannot give exact readings, but the magnitude of the needle
swing can give a fair idea of relative readings. This allows us to compare different
configurations, like different-sized magnets or different sizes of test tubes.
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Model Wave-Energy Generators 2.4.3
Appendix 2:
Other Things to Try
Larger-Diameter Coils
Wind a coil onto larger test tubes, using the same number of turns. Figure A2-1 shows
three coils of 50 turns each, wound on 16mm, 24mm, and 38mm test tubes.
Figure A2-1
Length of Coil
Wind a coil on the same size test tube, using the same number of turns, but increase the
spacing between the turns. This will increase the overall length of the coil. You will need
larger waves for the magnet to span the entire length of the coil. See Figure A2-2.
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Model Wave-Energy Generators 2.4.3
Figure A2-2
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Model Wave-Energy Generators 2.4.3
Number of Turns
Wind a coil on the same size test tube, using a greater number of turns. To keep the length
of the two coils the same, wind the extra turns on top of the previous ones. It doesn't
matter if the larger coil looks messy. This is one occasion where messy doesn't matter!
Multiple Layers
Wind a coil with the same number of turns, wound in multiple layers. See Figure A2-3.
It is difficult to wind neat coils with more than one layer of windings. The windings of
the next layer tend to force those of the first layer apart and slip down between them. If
you want to wind a multilayer coil and have it look neat, wind one layer but don’t cut the
wire, then apply a coating of fingernail polish. When the polish is dry and hard, continue
winding the next layer, followed by another coat of nail polish, and so on.
Figure A2-3
Size of Magnet
Compare the 3/8-inch magnet with the 1/4-inch magnet. Use the same coil with both
magnets.
How do you think these changes will affect electrical output?
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Model Wave-Energy Generators 2.4.3
Appendix 3:
Electrical Measurements Using the Oscilloscope
The setup described previously can give only a rough idea of electrical energy being
produced. To get accurate figures, we must use a digital storage oscilloscope (DSO). This
instrument measures voltage vs. time and displays the results on a computer screen.
Unlike the microamp meter, the DSO measures voltage, not current. See Figure A3-1.
This instrument will make measurements and do calculations, including maximum and
minimum voltage, RMS voltage, and frequency. It also gives a trace on the screen
showing the actual waveform of the voltage. See Figure A3-2. A DSO is more expensive,
running in the neighborhood of $200.
See the following websites for more information.
For MS Windows:
http://www.amazon.com/Hantek-Based-Digital-Storage-Oscilloscope/dp/B0036FZRU4
cost: $191
For Macintosh:
http://www.syscompdesign.com/CGR101.html
pick CGR-102 package
cost: $220
Figure A3-1
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Model Wave-Energy Generators 2.4.3
Figure A3-2
To measure the voltage, connect the probe of the oscilloscope to one of the coil
connections. Connect the ground connection alligator clip of that probe to the other coil
connection.
To find out how much power the generator will produce, we must find the voltage and the
current through a known resistance. This will allow us to calculate the power.
The DSO can be used to measure the current as well as voltage. To measure current,
measure the voltage across a known load resistance. See Figure A3-3.
The current is calculated by Ohm’s Law:
current (amps) = RMS voltage (volts) / resistance (ohms)
Use the RMS value for the voltage. This is a good average value, and can be calculated
automatically by the DSO.
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Model Wave-Energy Generators 2.4.3
Figure A3-3
Once you know the current through a known resistance, you can calculate the power
generated.
Power (watts) = current (amps) squared x resistance (ohms)
This will generally be in the microwatt range.
Authors
William Hanshumaker, Ph.D.
Public Marine Education Specialist
Oregon Sea Grant Faculty
Alan Perrill
Volunteer
Hatfield Marine Science Center
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