E 17 007
Examensarbete 15 hp
Juli 2017
Miniature wave energy converter
using dual rotating dynamic axes
Robert Antar
Daniel Gonzalez
Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student
Abstract
Miniature wave energy converter using dual rotating
dynamic axes
Robert Antar, Daniel Gonzalez
A seemingly everlasting problem that mankind is facing is that of sustainable energy solutions. Throughout recent history a couple of different renewable sources have been presented as possible endless sources of energy. One of these solutions is harnessing the natural motion of waves (as a result of the wind hitting the water's surface) in order to produce energy, often referred to as wave power. Difficulties arise with working offshore where the costs are increased and the fact that it's simply harder and a more tedious task compared to working inland. Another con is that environmental research needs to be completed prior to make sure that extracting wave power doesn't negatively affect fauna and flora in the water. Some countries even lack a coastline which concludes any possibilities of having wave power as a potential energy source. Due to the arisen difficulties, wave power has seen a lack of ambition in the investment front relative to other forms of renewal sources and has hence lagged behind in development and research. The natural benefit, as is the basis of renewable energy sources, of using wave power is that there will be an endless supply of energy ready to be utilised.
The project took advantage of resources available at Ångström Laboratory in forms of both software and hardware. OrCAD PSpice was used to dimension and simulate the electrical circuits for
AC/DC current conversion. The 3D-modelling of necessary parts was done in Solidworks and the 3D-printer was used to create the models in a relatively quick manner. Lastly, the workshop at Ångströms Laboratory hosted the water tank used for testing and the necessary tools to assemble the miniature wave energy converter. The dimensioning of the project was seen from the perspective of the motors and their specifications.
The execution of the idea worked as intended and could definitely be improved upon with sturdier materials, a grander scale and more time. As is, there are more efficient ways of harnessing wave power but this was one interesting miniature alternative. The idea worked and can definitely be improved upon. Whether or not it can be better than current alternatives is hard to predict but due to the energy converter's high reliance on mechanical parts and the arduous task of maintaining such a design, modern alternatives are most likely better options but the design could potentially act as a basis for a larger project.
Ämnesgranskare: Ladislav Bardos Handledare: Andrej Savin
Contents
1 Introduction 4
1.1 The project . . . 4
1.2 Scope and Limitations . . . 4
1.3 Wave power in Sweden . . . 4
1.4 AC-motor . . . 4
2 Experimental 6 2.1 Apparatus . . . 6
2.1.1 The Water tank . . . 6
2.1.2 3D-modelling . . . 7
2.2 Procedure . . . 7
2.2.1 Construction and assembly of the generator setup . . . 7
2.2.2 Tests with different loads . . . 8
2.2.3 Tests with the water tank . . . 9
3 Results 10 3.1 10Ω-load test . . . 10
3.2 100Ω-load test . . . 10
3.3 1,000Ω-load test . . . 11
3.4 10,000Ω-load test . . . 11
3.5 Water tank test . . . 12
4 Discussion 13 4.1 Results . . . 13
4.1.1 Tests with different loads . . . 13
4.1.2 Water tank test . . . 13
4.2 Faults and Improvements . . . 13
4.2.1 The wires material . . . 13
4.2.2 The wave energy converter as a whole . . . 14
1
Introduction
1.1
The project
In this project a miniature wave energy converter (WEC) will be built. The premise of the project is to assemble the miniature energy converter and to then test the converter. Various tests will be performed, foremost to see if any generation occurs and then testing will continue with different loads. The two mo-tors were chosen ahead of starting the project by supervisor Andrej Savin, who was fortunately looking at conducting a similar project to this one. As they were purchased before the start of the project the dimensioning of the energy converter will be based on the two motors as the focal point and it’s important that both motors are used as part of the energy converter. Further, some tests with wave generation and homemade buoys will be performed in order to see if the dimensioning of the energy converter was accurate enough to actually generate power using real waves. The generation of waves will be carried out within a controlled environment with the water tank available in the workshop at the Department of Electricity in Ångström Laboratory. The converter will consist of the two ac-motors, with their respective axes facing each other, attached to the dual rotating axes. The dual rotating middle axes will allow the motors to rotate, at the same time, in opposite directions without affecting each other. Solidworks will be used in order create customised parts for the converter seeing as special parts are required in order to assemble the converter for the specific dimensions of the two ac-motors available.
1.2
Scope and Limitations
As the wave energy converter is of a miniature nature the project should be seen as an interesting hobby alternative to modern WEC’s. Initially, It’s not intended to potentially replace or compete with current counterparts. The project was done for the sake of seeing if the theory behind this particular WEC could be realised and in the future be improved upon.
The project will keep to the specific water tank at the workshop in Ångströms Laboratory. This is because it had modifiable settings in terms of wave amplitude and frequency. Effectively, this means that parts of the project might have to be scaled down (which could lead to worse results) to accommodate the water tank. The energy converter will be dimensioned using the motors as focus whilst keeping the limitation of the water tank in mind.
1.3
Wave power in Sweden
There is a certain number of sites in Sweden that have a sufficient enough average wave energy potential that can be successfully utilised as energy farm sites. As it currently stands, the low number of sites that can be used sufficiently ensures that Sweden’s wave energy potential is rather low in comparison to the other Scandinavian countries. Norway has favourable wave conditions with numbers of 40-60 kW/m in a lot of sites along the coastline. Sweden on the other hand possesses very few good sites with the best ones being at around 6kW/m.
1.4
AC-motor
The AC-motor is an electric motor that consists of a stationary stator and a rotational rotor with the stator being cable winded in order to generate and lead alternating current. There are different possible ways to design the rotor where the rotor in these particular ac-motors are permanent magnets. In a motor setup the stator windings are fed with alternating current. The permanent magnet will want to rotate it’s position relative to the alternating magnetic poles that the current in the stator windings are inducing. The motor
can also be used in a generator setup where the rotor is rotated which in turn induces alternating current in the stator. Naturally, the two ac-motors in the project will be used as generators as to fulfil their purpose. The ac-motors that are going to be used for the wave energy converter have a mechanical to electric ratio of 1:50. What this essentially means is that for every full rotation (360◦) that the rotor completes, 50 electrical turns (meaning 50 periods) will be produced in the stator. The upside is that shorter rotations of the motor axes should still produce electrical periods of sufficient amounts, which is foreseen with the fact that the waves for this project are realistically a couple of centimetres high. Coupled together with some electrical components to charge and discharge energy a uniform electrical output should be attainable.
2
Experimental
2.1
Apparatus
The static part of the assembly included the following: A rotating steel axis, two cassette sprockets on either side of the axis to achieve the desired mechanism, two AC-motors restricted in specifications, two different shaft coupling for connection between the cassette sprockets and their respective motor axis, plastic blocks as means to raise the motors to the appropriate height to align with the steel axis.
The dynamic part of the assembly included the following: Hooks big enough to fit through pre-extruded holes on the steel axis, wires that were to bind to the hooks and transfer the power, a feather strong enough to keep the wire strained at all points of oscillation, a large piece of styrofoam to use as buoy and flat plastic and wooden pieces to distribute the force in wire on the buoy.
To keep the assembled parts in place, bar clamps and duct tape was used. To measure our voltages an oscilloscope was used. To filter our output signal a rectifier in cascade with a resistor parallel with a capacitor was used. To measure the pulling force in testing outside the water tank a feather with accompanying measurement (i.e. a spring balance) indicators was used.
2.1.1 The Water tank
The water tank is that of a larger one which unfortunately lacks details and specifications. An estimate shows that it has a volume corresponding to circa 540L (540,000cm3). The water tank has a built in electronics system that mechanically controls a board. The system has buttons for inputs in terms of wave amplitude and frequency. The buttons includes manual (where you set wave amplitude and frequency, unfortunately this setting didn’t work), Preset 1, Preset 2 and Preset 3. The presets had set wave amplitudes and frequencies so these were not modifiable and preset 3 was used throughout the water tank tests as the previous two presets weren’t sufficient in order to rotate the axes. It is however very hard to know the specifics of the presets and ultimately, due to the limited tests in the water tank, these weren’t particularly relevant.
2.1.2 3D-modelling
Solidworks was used in order to model the customised parts necessary for the project. It was chosen out of convenience, both in terms of available licensing and previous experience as it was part of an earlier course. Also, the file format is supported in the 3D-printer. Hence, it was very suitable for the task at hand. The parts that were modelled in Solidworks were those that would be too difficult to attain due to the lack of readily available counterparts on the market that would fit the required dimensions of the setup. This ended up being shaft couplings and motor mountings.
2.2
Procedure
2.2.1 Construction and assembly of the generator setup
The first half of the practical part of the project was construction. The second half was measurements. For the construction, time was mostly spent on preparation. Not all parts needed were available in a timely manner, and some had to be designed. The parts available from the start of the project were the AC-motors (that were used as generators), the water tank, the wave generating machine, and most material needed to assemble most of the components including the buoy and the wire. The parts that were designed and printed were the bracket mounted on the motors to keep them in place, and the shaft coupling between the motors and their respective cassette sprocket. No construction started until these parts were satisfactory, which required a couple of iterations.
During the preparation many design challenges had to be overcome, with relatively little time and resources. The only means of designing were 3D-modelling, which led to restrictions in the dynamic movements of the parts. For example, no self-made sprockets were able to be easily incorporated. A prototype was developed but quickly scrapped in favour of pre-built cassette sprockets. The cassettes were originally intended for bicycles, but they were re-purposed for the project.
Once all parts were ready, assembly began with fitting the respective parts. Firstly the cassettes were mounted on the steel axis and fitted so that they wouldn’t slip off. Although one sprocket was pre-mounted, the added one required a decent amount of pressure to fit tightly. Thereafter each shaft coupling was fitted to its respective sprocket. Once this was done, the whole part was ready for loose mounting. A piece of pvc-plastic, circa 1.5 centimetres thick and 40-by-30 in area, was used as a board for which the apparatus was to be mounted and screwed to. Since the cassettes on the sprocket were several times wider in diameter than both the motors and the rest of the apparatus, there was need to suspend the axis high enough for the cassettes not to impact the mounting board. Hence, the motors needed to be suspended too to align to the coupling - this was done with blocks of pvc-plastic, too.
As all parts were getting raised the right distance from the board to align the holes on the shaft coupling to the motor axes, the whole assembly was taking form. When all the axes were connected, the static part of the construction was finished. The remainder of the construction was concerning how the wire was to be threaded for its path to prove the most efficient (i.e with the least frictional losses) and how the wire was to be fixed to the buoy and the steel axis. The end result proved to be satisfactory, with the wire passing through several pulleys and attaching using custom made hooks. The spring pulling the steel axis back from its eccentric state (i.e when the buoy is at its highest point) was attached in the same manner and then threaded over a bent screw.
2.2.2 Tests with different loads
Once the assembly and construction was completed the measurement-tests could begin. The whole setup was fastened to the table using bar clamps (as seen in figure 2) with one end of the first string being attached to the middle axis of the assembly and the other end to the spring balance. The spring balance measures the weight (the force can be calculated) that the motor is pulling with. The second string has one end on the middle axis and the other end is loose and meant to be pulled periodically in order to "simulate" waves. It’s imperative that both strings are as tense as possible and to ensure this. The setup is used for all non-water tests. Note that due to the string being a simple thread-string used for sewing it couldn’t be pulled too much without snapping so only waves with lower wave heights could be simulated.
Figure 2: The generator setup attached with a bar clamp to the workbench used for testing. With the testing station established the tests could begin. The two generators had separate electrical circuits. Note however that the two circuits were identical and had the same valued resistances and capacitors. The three phases of a generator was connected to a three phase rectifier that was then connected to the load running in parallel with a capacitor.
Four different tests were performed with each test having a different sized load. The loads used for the four different tests were; 10Ω, 100Ω, 1,000Ω and 10,000Ω. Note that higher resistances means a smaller load (since the current flow is lowered) and the most interesting test to see how well the energy converter generates is at 1Ω, this was not available however and 10Ω was chosen as a close alternative to simulate a very large load. These were coupled in parallel to 4700µF capacitors that remained the same throughout testing. The voltages across the loads were measured. Attempts at measuring the current were made with hall sensors and a multimeter. The low currents coupled with the somewhat significant noise ensured that the current graphs were unreadable. Naturally, the multimeter didn’t work either because it can’t record the fluctuating voltage values. Instead the voltages across both loads were measured. This means the rectified voltages the generators were producing which in turn would allow the calculation of the power that was generated. The spring balance was video-taped throughout testing so it’s possible to recap and more accurately read what force the motor pulled with for each test. An offset of 2kg was noted for the spring balance prior to testing
and this will be accounted for in force calculations.
The tests were performed by pulling the string with one non-fixed end short distances (3-4cm), in turn rotating the rotating axis, and letting the spring balance pull the axis back into position. This was done as close to periodically as possible.
2.2.3 Tests with the water tank
Figure 3: The generator setup sitting atop the water tank’s wooden case attached with a bar clamp. The generator setup was fastened atop the water tank using bar clamps as seen in figure 3. In order to make sure that as little energy as possible was lost the friction losses that the wire could experience had to be removed. This was done by measuring where the buoy would be when the water is stationary in relation to where the generator setup was. The measurements were used to drill holes for two pulleys on a pvc-plastic board to ensure that the wire was simply going in straight lines between the buoy, pulleys and the wood case. This solution ensured that the wire going between the generator setup and the buoy doesn’t experience any unnecessary friction. There were some issues with the plastic board as it moved around at the bottom of the tank because it lacked friction. As the strings were being tensed the plastic even rose. Therefore weight had to be applied to the plastic board in order to keep it in check. A lack of weights available in the workshop meant that a temporary solution was needed and the plastic board was simply pushed down during testing (as advised by the workshop supervisor) with a larger bar clamp. It was imperative that the strings we as tense as possible which was a difficult task. A lot of re-stringing had to be done for the string to be tense enough to pull with enough force to overcome the generators inertia.
The large surface area of the buoy meant that in order to evenly spread out the force applied to the buoy (when waves are forcing the buoy upwards) a hole in each corner of the buoy was made. Each hole was then threaded with some wire and a screw with a larger piece of wood across each hole on top of the buoy. The screw was attached to the piece of plastic using screw-nuts and the plastic was the glued onto the top of the buoy. The four wires were then tied to a fifth wire. This would help with spreading the force across the buoy evenly when the wire pulled the buoy from beneath. This can be seen in figure 1 with the four wooden
3
Results
3.1
10
Ω-load test
Figure 4 below shows that with the largest load (lowest resistance) the generator being pulled generated a voltage of circa 1.6V peak value with the second generator (the one attached to the spring balance) generating circa 750mV.
Figure 4: The oscilloscope-generated graph shows the voltages across both 10Ω loads for the two generators. Note that the vertical axis is 500mV per square.
The pulling force in this test was measured to 58.9N and gave a combined peak power of 552mW. The total power was not able to be obtained due to a data corruption.
3.2
100
Ω-load test
Figure 5 below shows that with a resistance of 100Ω the generator being pulled generated a voltage of circa 7V peak value with the second generator generating around 6V.
Figure 5: The oscilloscope-generated graph shows the voltages across both 100Ω loads for the two generators. Note that the vertical axis is 5V per square.
The pulling force in this test was measured to 78.6 N and gave a combined peak power of 1.69W. The total power was not able to be obtained due to a data corruption.
3.3
1,000
Ω-load test
Figure 6 below shows that with a resistance of 1,000Ω the generator being pulled generated a voltage of circa 13.5V peak value with the second generator generating around 9V.
Figure 6: The oscilloscope-generated graph shows the voltages across both 1,000Ω loads for the two genera-tors. Note that the vertical axis is 5V per square.
The pulling force in this test was measured to 98.2 N and gave a combined peak power of 506.2mW. The total power was not able to be obtained due to a data corruption.
3.4
10,000
Ω-load test
Figure 7 below shows that with the smallest load (highest resistance) the generator being pulled generated a voltage of circa 11v peak value with the second generator generating around 6V.
The pulling force in this test was measured to 147.3 N and gave a combined peak power of 28.9mW. The total power was not able to be obtained due to a data corruption.
3.5
Water tank test
Figure 8 shows the graph generated for the water test running on 1,000Ω loads in parallel with 1000µF capacitors.
Figure 8: The oscilloscope-generated graph shows the voltages across 1,000Ω loads for the two generators. Note that the vertical axis is 5V per square.
The impedance was calculated with a resistance of 1000Ω in parallel with 1000µF and a frequency of 1 Hz, calculating to 0.1592Ω with an angle of -89.99 degrees. The peak power was calculated with a peak voltage of 4 volts, calculating to 100.5 VAR (per generator).
4
Discussion
4.1
Results
Overall, the results coincided with what was expected. As is, they stand as a first step to realising the mechanism of two generators being used in cooperation in this manner. We believe there to be value in this mechanism, but its cost effectiveness has yet to prove itself.
4.1.1 Tests with different loads
Here we can conclude that the appearance of the graphs coincide with theory, and serve as basis for desirable specification. For example, were the design to be implemented as a source for direct current, the capacitance would need to be decided based on the resistance of the application. A good pairing would yield a voltage across time similar to that of figure 7.
4.1.2 Water tank test
These tests were meant to give some sort of indication of what kind of power one could expect from the setup, but to measure it empirically proved difficult due to the lack of specifications. Anecdotal measurements tell us that in order to have the steel axis move, a lot of force had to be applied - for us doing it manually it took quite some force. Since the buoy managed to move it under limited conditions, the prospects for a full-scale system at an ocean shore seems promising.
4.2
Faults and Improvements
In this section we will discuss the shortcomings of the results, how it differed from expected and/or wanted results, and introspect to give feedback and recommendations to future research and/or development. In order to maintain structure, firstly we will address improvements in time-management. As individually tailored as this topic is, we hold the belief that the expectations on the length of planning should be properly extensive. Planning and preparation proved to take the longest time, something we did not count on at the start. Realistically speaking, the time one spends planning one gets back tenfold during execution and by letting different scenarios brainstorm one can be better prepared for unwanted events, when and/or if they occur. One may realise that what seemed like a fine idea initially may very well not be realistically applicable, be it because of time-restraints, budget-restraints or exceeding the decided size of the project. We will speak to several of these types of non-realistic expectations that happened during this project, in the coming paragraphs.
The majority of the setbacks originated from the project being overambitious, sometimes to an unrealistic extent. The examples one could give account for here are too many to list them all, but a couple stand out: the wires place in the dynamic mechanics, the kinetic energy to harness in the waves with the given tank and wave-generator, how late time-restraints would change the nature of our results.
sword since it makes in unnecessarily impractical to lace around the axis because it naturally wants to stay straight. Also, if this type of wire were to be implemented, a surrounding case would need to be as well. The case would ensure that the wire stays in place and that the force the wire will apply to the case will see to the axis being rotated in the other direction too. Finding a wire of a suitable material and stiffness and designing and producing a case in time proved to be too much of a hassle, especially when considering the diameter of the steel axis might be forced to be widened a considerable amount; something that would take a lot of time in itself. Instead, the solution that was chosen was to use cotton wire in combination with a second wire laced counter-wise relative to the first, and tied to a spring mounted to a practically stationary point. The problem with this solution is that the whole setup is reliant on the elasticity of the spring not being too tight or too loose. Also, additionally to finding a perfect elasticity this must be paired with a perfect length that distributes the tension over a long enough distance for the wire to remain very tense for the whole oscillation of the wave; too tight and the wire snaps, too loose and the axis will not spin back on the buoys downwards motion. Another undesirable consequence of this setup is, of course, that it introduces more friction losses.
4.2.2 The wave energy converter as a whole
Secondly, due to our lack of experience with wave power, we underestimated the energy we would be able to utilise from the waves. The generators have an inherently large inertia and therefore required quite a bit of force to be moved. The wave energy converter was over dimensioned in relation to the water tank available for the project. Hence, a larger tank was necessary in order for the energy converter to consistently perform in a real life scenario. From either perspective, the energy in the waves were barely enough to turn the axis and induce the motors. When we did get it working, it worked more or less as expected but the moments were few and far between and it was obvious that it was right on the brink. Of course, had the execution been flawless and the friction losses perfectly optimised it would have been easier and ideally the induction would have been less spotty. But even then we would conclude that the whole setup, with the given motors and the given mechanism, was too heavy for the given wave-generator and the size of the tank. Since we did few if any real analytic estimations that had physics theory and algebra behind it, one could critic that we could have known sooner (e.g to our point earlier about the planing phase and its importance). But given that the motors were already acquired resources when we presented our proposal, we worked within the conditions we were placed in.
Initially, the middle axis was designed to have a stand which would reduce the load that was being put on the two generators axes. Due to design this introduced unnecessary friction in the system and we noticed that the stand wasn’t actually doing too much in lightening the load for the generators and it was therefore scrapped. We felt that the drawback of having this friction on the system was larger than the benefit of potentially reducing the load on the generators axes. A good solution to this would be to have bearings on the middle axes which would allow the stand to reduce the load and introduce only minimal friction (as long as the bearings are good enough) but this was not feasible with the current middle axis and time restraints. It was simply impossible to attach bearings to it the middle axis and in order for us to implement this solution the middle axis had to be redesigned to allow for this modification. In the end the load of the middle axis was put on the generators axes which was simply a short-coming that we had to accept.
5
Conclusion
In conclusion the project was successful since we were able to implement the dynamic we had designed and have it work as intended in the water test environment. Though it is a proof of concept, it is lacking in empirical results due to lack of time, lack of specifications, the motors being too heavy to move and human error. Accounting for the faults and issues discussed in section 4.2 the project can be greatly improved upon in order to achieve solid empirical results. An issue that the design faces is the fact that it would be hard to maintain due to the heavy reliance of moving mechanical parts which is generally a design flaw that is avoided in market competitors. On top of this the design faces a lot of tension when under working under load. Therefore this type of wave energy converter is interesting in its current stage and should be credited for the creativeness in its design but it’s not going to be a market competitor in it’s current stage due to the smaller intricacies of the design maybe being a bit too hard to maintain in a water environment. With that said, with the improvements to both design and execution to sort out the intricacies the design could potentially act as a basis for large projects.
