collective effects of the gravitational forces
What are tides? Tides are the rise and fall of sea levels all around the world caused by the collective effects of the gravitational forces exerted by the Moon and the Sun. Tides will change by the rotation of the Earth. Because of this outcome, there are two high and two low tides in one day, and is sometimes called the tidal cycle. This phenomenon is very attractive for energy capturing purposes, since tides are very reliable and only depends on the earth's orbit, unlike solar and wind, where there is no control from nature. The most famous commercial tidal plant is the Tidal Barrage in La Rance, France, where they use the idea of a hydroelectric dam to collect water from tides, close the gates, then releases it out again while creating electricity both in and out.
We decided to create a prototype to demonstrate the Tidal Barrage and show the audience just how tidal power works. This is done by several features from the prototype. Firstly, there is a mechanism in the prototype such that when the lever arm is rotated, with a fake earth in the middle, the gravitational pull of the moon creates changes in the sea level, and therefore will raise and lower the floor of the prototype. When the floor changes level, the water comes into a bay via a slot that contains a fan. This fan is spun and generates electricity. When the floor is then lowered, the water flows out of the reservoir and the fan turns again creating extra electricity. One rotation of the crank arm will create two high and low tides, like what the earth does now. To build this prototype, we had several innovative ideas, as explained in the report, to make it better and some concerns that couldn't have been addressed. After the creation of the while prototype the cost of it turned out to be $209.36.
To test how fans work and generate electricity, a drill was used that had several RPM levels, and was connected to a toy motor. The motor generated electricity and a voltmeter was used to measure the voltage given by the system. It turned out that the relationship between RPM and Voltage was linear and was proven correct by real life equations.
To conclude, tidal power is a gift from nature that should be used by humans to eliminate fossil fuels and cut our dependence on carbon fuels.
Final Report: Tidal Power
The MDP project's objective was to design an interactive display to demonstrate an environmentally friendly technology, suitable for use as any type of display. This objective leads to the design problem of raising awareness of tidal power. The problem definition was: “By using an interactive display, we will intrigue the population into understanding and possibly using tidal energy instead of a less efficient energy source.”
Other objectives were to construct a prototype that weighs less than 10kg, and must fit a maximum of 50cm by 50cm by 50cm cube. Also, the prototype has to cost less than $400. Considering these objectives and constraints, several concepts were thought of and discarded, and the best design that we came up with was the Tidal Fence - Raiser.
The Tidal Fence - Raiser had many advantages over other concepts since it incorporates basic science, the tides and the moon, and was also very interactive, was easy to make, easy to use, and looked very nice. Considering all these benefits, it was decided that our prototype will be the Tidal Fence - Raiser.
Tides and the Moon
Tidal Power is so called because—as many may have guessed—of the fact that tides are where this power is collected. Tides refer to the change in the level of sea water throughout the day. They are created by both the moon and the sun.
The moon is 1/4 the diameter of Earth and 1/81 it's mass, but it still has a strong gravitational effect on earth's water. Wherever the moon is around the earth, the water tends to be pulled towards it as shown in Figure 1 with the earth also being pulled to make the levels of the water equal on both sides of the earth.
The sun may be much more massive than the moon, but it is not as close and so has a small effect on the tides. Whenever the sun is aligned with the moon, when it is at full or new moon, it creates super high levels of water (Spring Tides) and whenever it perpendicular with the moon, at first or third quarter, (Neap Tides), it helps diminish the moon's affect, as shown in Figure 2
With both the moon and the sun creating forces that allow high and low levels of water (tides) to occur, assuming the moon and the sun are in a constant position over a day, a point on the earth rotating with the earth would get 2 times of high tides and 2 times of low tides.
To make electricity, a flow of electrons is needed. Electrons carry the energy needed to power homes. This is based on the electrons having a high level of kinetic energy. This energy is created when they leave the outer shell of the atom. A generator is used to excite the electrons. The generators are composed of a copper wire wrapped around a magnet which is rotated by a shaft, causing the electrons to gain energy. Once the electrons gain enough energy they leave the copper coil and are used as electricity. The magnet in a generator pushes a certain number of electrons along, which is called current and is measured in Amperes, and applies a certain amount of "pressure" to the electrons, which is voltage and is measured in volts.
In tidal power generator is very similar. Moving water spins through or across a blade or turbine system that turns a drive shaft connected to generator, usually by means of a gearbox. In some applications however, rotational force is transferred by hydraulic pumps; one pump is turned by the turbine, like the one in Figure 3, which is hydraulically connected to others that spin generators. The conversion of mechanical energy to electrical energy is done by inducing voltage into a wire by passing it through a magnetic field. In a generator, the armature (loops of wires), is spun by an external force (water) in a magnetic field causing the wires to cut through lines of flux, and by Faraday's Law of Induction, voltage is produced.
Tidal power is different from other green technologies since it does not experience unpredictability, like wind and solar, while tidal power is only dependent on the gravitational pull of the moon and the sun, and therefore is very reliable since orbits do not change. Lastly, tidal energy is an attractive alternative green technology because of the predictability of tides and its huge potential which worldwide is estimated to be about 500-1000 TW h/yr. There are many old and new designs out there for tidal power and the conversion from tides to electricity. The most popular options for capturing tidal power are: tidal barrages, underwater windmills, and ducted fans. Also there are new designs for creating “Tidal Bridges”, which is a bridge that has many fans under it in the water generating electricity. There are many setbacks in all of the designs made for tidal power, which also applies for wind. The main setback is that when there is a propeller in moving water it experiences limitation technically known as the Betz Effect. This effect states that no turbine underwater can capture more than 59.3% of kinetic energy. Water and other similar liquids tend to drift around rather than through energy capturing designs. Therefore, generating tidal power is very easy, but maximizing energy efficiency is the main setback.
Tidal Barrages are a direct adaption of a hydroelectric dam. A bay is close off by the dam that has movable gates. The incoming tides fill the bay behind the dam, then the gates are sealed at high tide, and the water is flown back out which uses turbines in the dam to make generate electricity. The first tidal barrage used in the world was at La Rance, France from 1966, in Figure 4. Tidal barrages have proven to work and generate power competitively with other power generation plants. When a bay is turned into a basin, its ecology and aquatic productivity are permanently altered. Another factor is that power can only be produced for half of the tidal cycle, and therefore cannot generate electricity 24/7 like other power plants
Underwater windmills receive the most attention since they are adaptions of popular wind technologies. They are propellers mounted on a fixed piling, chained by anchoring systems, like in Figure 5. Tidal stream turbines have the advantage of eliminating the high capital costs of large civil construction projects, little disruption to the environment and many locations can be used where there is no need for large tidal rise and fall. Several concerns is that the underwater windmill experiences the Betz effect. Since water is much denser than air, underwater windmills have short, low aspect foil sections and are incomplete in duration by structural requirements and water depth.
Ducted Fans are gigantic versions of air condition fans and heating fans. To increase the velocity of the water flow, a venturi-shaped duct is used in this fan, as shown in Figure 6. An outer housing forms part of the generator since it contains permanent magnet elements. This design is very sophisticated and nice in that it driveshafts and reduction gears that are major problems when it comes down to major maintenance and failure points. Concerns of this design are that marine growth becomes a problem after time since it eats away from the energy efficiency. Also long-term maintenance is a challenge, because any major services need the removal of the device from its mounting. However, the good thing is that maximum efficiency of energy conversion is not subject to the Betz limit of 59.3% of the energy incident on the swept area of an open turbine, since duct can draw in flow from a larger area and increase the available pressure drop across the turbine, generating about three times more power than normal turbines.
Features of Prototype:
For our prototype, we made a simple tidal barrage. At the front of the prototype is an earth attached to a rotatable pipe and a stationary moon. Rotating the pipe spins the earth and simulates the moon orbiting around the earth. Attached to the earth is a dart determining exactly where on the earth the tide is being produced. As the pipe is spun, it rotates two fixed wheels at 180° to each other. As the pipe makes a 90° rotation the wheels are at their highest point. Another 90° degree rotation and they are back at the lowest point. Rotating the pipe 360°, therefore, raises and lowers the wheels twice, representing the double cycle of the high tide in a 24 hour period.
The wheels, at the highest point, then lift a panel which is hinged at one end. Hinging the panel lifts the center of the panel the same amount as a non-fixed panel but more steadily. Lifting the panel with the wheel represents the gravitational pull of the moon on the ocean floor. This shows that when the moon is perpendicular to the position on the earth, water is at its lowest point and, when the moon is parallel to the position on the earth, the water is at its highest point.
Above the panel is a water-proof containment area so, as the panel is lifted, water is able to rise. As the water rises, it is pushed out of the containment area and through a small slit. This slit contains a fan that is hooked up to a generator which powers a light. As the water rises, the fan is rotated and the light turns on. As the panel is lowered, the water flows back into the containment area and repowers the light. Having the fan work in both directions harnesses more energy since the power created by raising and lowing the tide would be double that of the power created by only raising the tide. This prototype simulates how tidal barrages harness energy.
Creation of Prototype:
There were many materials used to build our prototype which can be categorized as major structural materials, or finishing materials. The structural materials used were ¼ inch MDF sheet board, ½ inch “Styrofoam” Brand blue sheet insulation, thin sheet metal, 1 inch casters, 1 inch U-bolts, ½ inch cooper piping, 1¾ inch pipe restraints, 1½ dry-wall screws, and wood glue.
The finishing materials used were polystyrene 5 inch and 8 inch spheres, “Great Stuff” spray insulation, duct tape, commercial garbage bags, aerosol urethane, poster board, gold star stickers, paint, sand, pebbles, moss, and model trees.
There were many components to this design. The first is the ocean and estuary. The second is the base and cam shaft. The third is the crank light. The ocean and estuary is the largest component of the design. It is made entirely from the ½ inch insulation to reduce weight and forms a sloped bay and ocean, separated by a dam with a thin slit down the center. The bay slopes negatively from a beach towards the ocean half by approximately 12 degrees and the sides slope down from the shore line to the bay floor. To achieve the proper shape and slope five trusses where constructed evenly spaced across 35 cm, the total length of the bay half. These trusses were cut to the shape of the bay at its supporting location, forming the largest flat-bottom shaped ‘V' on the last truss closest to the dam, and the smallest flat bottomed ‘V' closest to the top, as shown below in Figure 7.
Then sections were cut to lie across the trusses and form the bay. Sides and a bottom were attached and the gaps were filled with spray foam insulation.
The parts were screwed together using 1½ drywall screws with the spray foam also acting as an adhesive. The bay was waterproofed using duct-tape.
The ocean side of the design has a moving floor to raise the water level which at its lowest lies right below the level of the dams slit and bay floor. As the ocean floor is raised, the water level rises and flows through the dam's slit, filling the bay. This is accomplished with our cam shaft design detail shortly. The ocean floor is also a piece of foam insulation, backed by a piece of sheet metal for added rigidity. This proved not to be strong enough with the weight of the water and resulted in poorer performance of the model. The floor is hinged at one side and the other is free to move up and down. Above the floor is a bladder made using an industrial strength garbage bag. This was duct-taped to the wall of the ocean side forming a waterproof pool of water. The ocean floor swings upwards pushing on the bladder thus raising the water level and pushing water into the bay via the slit in the dam, as seen in Figure 9.
To raise the ocean floor we designed a cam shaft with copper piping, casters, U-bolts, and MDF sheet board for the base. The cam shaft has two lobes 180 degrees apart from each other causing two up and down cycles per rotation, or two tides per rotation of the earth, as shown in Figure 10 and in Figure 17, in appendix. A copper pipe is positioned horizontally just below the ocean floor along the front view center line. U-bolts and casters form the lobe. As the cam shaft rotates, the caster rotate around the pipe, rolling against the ocean floor, raising and lowering it.
The copper pipe is attached to the MDF base by 1¾ inch pipe restraints in which it can spin. The ocean and bay assembly is attached to the base by risers to accommodate for the swing radius of the cam shaft. The base was made using screws and wood glue. The casters were attached to the U-bolts and the U-bolts were tightened to the copper pipe by nuts. The casters had to have new holes drilled to fit the U-bolts. One problem during presentation was that the nuts tightening the U-bolts to the copper pipe were not tight enough and slipping occurred with the weight of the water. This was a simple adjustment.
The next system was the crank light. We wanted to gear a crank flashlight to the cam shaft however finding the appropriate gear proved difficult with and impending timeline. A remote-control model boat prop was positioned in the dam's slit so that when water flowed through, it would spin. Instead we attached the crank assembly to the side of the bay assembly with a crank arm extending out, as shown in Figure 11. Another prop was attached to the axis of the crank. The idea is to associate the two fans rotation. When the crank arm is rotated power is generated by a generator and LEDs are illuminated. The flashlight was dissembled and the crank assembly and gearbox was placed in the side of the bay. Additional wiring was needed to be able to relocate the LEDs to the top of our display. Installation was simple. We cut out a space from the foam for the gear box and arm and duct-taped it in flush. The lights we simply pinned into the foam at the top of the assembly. After all major components were attached together, the final details were completed. The ocean floor was sprayed with glue, sprinkled with sand, and clear coated/waterproofed with spray urethane, as shown in Figure 16 in appendix. Other features were painted and decorated. All the sides were finished in poster board with holes for the light crank and cam shaft.
The cam shaft extended out through the front face of the assembly and then was connected to a 90 degree elbow. This created a crank arm also made of copper piping to spin the crank shaft. At the elbow was a model earth, Figure 12. The earth was a painted polystyrene 8 inch ball, painted and split and hollowed to fit around the elbow connection. Another painted polystyrene 5 inch ball was placed on the front ocean wall and represented the moon. One major building error was inserting the dam after the sand had been glued to the bay floor. When the dam was inserted it was hard to waterproof the dam to the bay walls and floor because of the sand. If we had waited until after the dams could have been sealed to the floor with duct tape prior to gluing on the sand, we are confident our design would not have leaked.
The costs of the materials were as follows:
Testing and Modelling:
Testing was conducted based on how tidal energy may work. Tidal Power, as said before, is the use of tides to turn fans, turbines, or anything the water can turn. Tidal fans basically take the mechanical energy they receive from the water to turn copper inside of a magnetic field. Once this happens electricity is made and collected. Based on this concept, many ideas of how to make an experiment based on the prototype were made. First, the idea of using a computer fan with deionized water to test how much electricity is generated when the water is run through the fan, but the idea did not work as there was not enough pressure in the water to move the computer fan to create electricity.
It was then decided to use an axle and the end of a drill as the tidal fan. The drill's set speeds from 5000-35000 rpm were used to simulate the power of the running water that is created from the tides. The axle that was used was part of a “toy motor”. The motor gave good values for the voltages and so a relationship was created between the motor and the speeds of the drill. The voltage was measured by a voltmeter. The voltages were small, since the toy motor was small and was not intended to create electricity.
As you can see in Figure 13, the relationship between speed and voltages turned out to be linear. Therefore, for Voltages vs. Drill RPMfor a “toy motor”, the equation turns out to be best at y = 0.1765x with a deviation value of 0.9819. This relates to the real life equation that is V = 0.142(T)(RPM)( i )(Eff.), where “T” is torque, “i” is current, and “Eff.” is efficiency. Therefore, x is RPM, which is linearly proportional to voltage. The slope of 0.1765 is equal to 0.142(T)( i )(Eff.), with the slope being constant, but the torque, current, and efficiency may change accordingly.
The prototype developed for the interactive display met all the requirements and constraints that we set. It also cost $209 which higher than we wanted to pay however it was worth every penny. It was also very interactive, and simple, so as to get children more interested. The design leaked during the showcase, so the process does require some improvements. In conclusion, the project was successful, despite the leak at the show case. But overall, tidal power is a great way to go for in the future because of its reliability, mass amounts of energy, and can sustain us for the years to come.
The process needs to be changed so that the sand in the estuary is either not applied or applied once the prototype is built. Also, the bottom of the wave pool should be made from a stronger, more flexible material. The raiser in the wave pool should be reinforced so that it does not bend with the weight of the water. The U-bolts also need to be fastened tighter or a more secure way to fasten the camshaft to the spinner.