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Rainfall-powered alternator

  1. Sep 25, 2015 #1

    I know it seems awkward that I posted an earlier thread about this topic, but roll with it.

    I would like to share what I (our group) am going to make for our science fair project. It is an alternator that uses collected rainwater to rotate the rotor. It will work like this:

    At the top part, there is a collection vat. It's about 1m x 1m x 1m in size. Below it is a tube leading down to the water wheel of the generator. There will be a valve along the way, so you can set when you want to activate. The water makes the wheel spin, turning the rotor of the alternator. Right below the generator is another pipe, but wider in diameter. The 2nd pipe will lead to a basin so that the water can be reused, just in case the rain stopped.

    How are we going to present how it works at the fair if there's no rain, you ask?
    Well, we plan to use laundry water to make the water wheel move.

  2. jcsd
  3. Sep 25, 2015 #2


    Staff: Mentor

    Congratulations on your project, it is ambitious and not the easiest thing to do.

    First, raise the water tank as high as you can.

    Second, pay close attention to friction in the waterwheel and alternator. Some low friction bearings may help.

    Third, you might have a hard time making enough power to light a LED. Perhaps a voltmeter might be the best way to show that you are making electricity. An old-fashioned meter with a needle might be best because people can easily see the needle move, but a modern digital meter may also work.

    What about your alternator? Is it a spinning magnet and a loop of wire? The simpler and more visible everything is, the easier the audience can understand how it works.
  4. Sep 25, 2015 #3
    Exactly that.
  5. Sep 25, 2015 #4
    What you are proposing amounts to a minitureised model of a hydroelectric dam generator.
    While it's possible that you could prove the concept, the amount of electricity generated would be miniscule and as has been pointed out you would need to ensure that all bearings and other parts were selected to be as frictionless as possible so as to reduce energy wastage.
    In practice rainfall is only very rarely heavy enough that you could use it to produce a useful amount of power, unless your collection system is in the scale of hundreds of square kilometers, but then again that's exactly what a real full size hydro generator actually does.
  6. Sep 25, 2015 #5
    After all, there's where I got my inspiration from.
    Well, the only goal is to power a 5-volt light bulb.

    The purpose of this is to show how an generator (alternator) works. Might as well as introduce the magic of electricity and magnetism to 1st-4th graders.
  7. Sep 25, 2015 #6


    Staff: Mentor

    rootone is correct. It will be difficult to make enough power to light a bulb, even a 1.5 v flashlight bulb.

    That is why I suggested a voltmeter. It needs almost no power to make the needle move. Hopefully, you could see the needle move with every revolution. That means the water wheel can turn slowly.
  8. Sep 26, 2015 #7


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    Well, your collection vat will hold about 1 tonne (1000 kg) of rainwater when it's full, so raising it high above the generator will require some carefully designed structural supports.

    A heavy downpour will produce say 10-50 mm of rain an hour. It would take quite a deluge to fill the collection vat, one lasting almost 1-2 days. Some areas may not receive a meter of rain over an entire year, depending on the local climate.

    In some areas with the most rainfall, the open collection vat could provide a breeding location for mosquitoes, which is not desirable because they can carry disease.

    In the winter, any water in the vat is subject to freezing if ambient temperatures drop. This will be especially true if the vat is elevated and exposed to cold winds.
  9. Sep 26, 2015 #8
    Gears? Why not
    True, true.

    I will add a lid.

    I live in a tropical country, so I wouldn't mind.
  10. Sep 26, 2015 #9


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    Won't that make it inconvenient for the vat to catch any rain?

    A frame with some screen material on it should keep out mosquitoes but still let the rain in.

    You'll have to keep the collector in the open so that leaves or any other plant debris won't block the screen.
  11. Sep 27, 2015 #10


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    Are you primarily doing project to illustrate how a hydroelectric dam generator functions; or, are you intending to present it as a potential method of providing supplemental power in a residential area. Your tank seems to be very large for a simple demonstration. Remember, the amount of water in the tank only determines how long the generator will run. It is pressure from the height of the tank and volume of water flow that totally determine how much electric wattage your generator will produce to light a bulb (preferably a LED due to its very low wattage requirement or activate a volt meter. As to the torque required to create the power, as stated above you need the lowest friction possible; but to some degree that can also be addressed by the water wheel diameter and the water tank height.
    A few questions:
    1. Do you already have; or, have built your generator?
    2. Do you have or have access to a voltmeter for testing even if you prefer to use a light for the project demonstration?
    3. How much time do have before the science fair starts?

    If you will give me that feedback on the above then I will try to help you to see if we can make your project work.
  12. Sep 27, 2015 #11


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    Have you thought of doing some simple calculations to give you a realistic idea of the energy available? It is always worth while doing that in advance of any practical project - even a very basic one.

    The Gravitational Potential Energy (mgh) for 103 kg at a mean height of 10m ( more than you could expect to find unless you have a nearby cliff or hill) would be around 105J That would provide 1W of Power for105seconds (27 hours) ideally. You could expect to have a much smaller container, in practice (say 1/10 of the volume) and have it at perhaps at five metres (a first floor room, on a table with the generator on the ground floor floor). Also, the efficiency of your system would very likely be no better than 10%. That would give you 1/200 of the original calculated energy which is about 8 minutes for the demonstration. Quite good enough to prove a point. No need to use a 1W bulb; you could use an LED which would consume less than 1/10 of that - giving at least 10 times the running time.
    Actually, my suggested 10% efficiency may be over optimistic if you are intending to make your own turbine and alternator. But you should definitely have enough in hand to provide you with a result. You can get LEDs that work on 10mA (in fact, they were all as low power as that in the good old days)
    Using two LEDs, each in series with a 'non return' diode and connected in opposite ways could be better than using a full wave rectifier.
    Your down pipe needs to be capable of delivering plenty of water (several litres per second) with no pressure loss (hard to measure so you need lots in hand). Go for smooth sided tubing as fat as you can get hold of (say 2cm internal diameter).
  13. Sep 28, 2015 #12
    1. Currently building it.
    2. Yes.
    3. Submission is in 4 weeks.
  14. Sep 28, 2015 #13


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    1. Are you designing your generator using calculations based upon the wattage you will need for the light bulb or LED you intend to use, including a realistic efficiency rating, ? Have you done this?
    2. OK, from time to time I am probably going to ask what you will think are trivial questions like this one; but, to best understand your challenges I need to get an understanding of your available resources.
    3.. That means you need to aim for three weeks, because there is always problem that will pop up at the end. That that means you need to work in the most organized manner possible without getting ahead of yourselves and building components before their actual requirements are determined.
    Trust me on this, many decades ago on my team's second Science Fair project, a table top high performance recirculating wind tunnel, we had finally completed the project, we thought, two nights before we needed to deliver it to the fair we ran our final extended test run and our selected 1/4 HP electric motor when up in smoke due to the fact that we underestimated the load requirements of the fan. The next day, we scrabbled around and finally found a 1/3 HP motor that we hoped would solve the problem and installed and successfully tested it by working until dawn the next morning. We then delivered the unit that morning to the fair for judging that same day; unfortunately, we were so exhausted from our last minute efforts that we did not stay to demonstrate the unit; and, we placed second when there were many people convinced that if we had been there for the demonstration and questions we would have placed first. Sorry for the long story but I really wanted to make my point because even on the best planned corporate projects, successfully meeting deadlines is always a struggle.

    OK, at this point I am going to actually present my initial recommendations on how you should proceed in the effort to complete your project successfully and on time.

    1. The first thing you should have or need to do, in addition to no. 1 above, is make preliminary list of all of the physical constraints of the location where your unit will be installed and for example: the limit on the height available for your tank, since this will limit the water head height and therefore the energy available to drive your generator; and how will you support the, now very heavy, tank you are going to use, what you are going to use to collect the discharge water from the tank and how will you recycle it to the supply tank, if required, etc.

    2. Compare and evaluate the available water energy limit to the required output of your generator. Based upon this you will be able to make a preliminary determination of the limits as to what you will be able to use to demonstrate your generators operation, ie., LED, standard small flashlight bulb (for example: I have one that only requires 2.2 volts w/ 0.25 amps).

    3. Adjust your generator design to meet those parameters. As a general additional rule " always include safety factor in the design of all of your components".

    4. Important: Perform a preliminary analysis of each of each component of the system starting at the light bulb and working from the end point backwards. In other words, determine the requirements of each unit before deciding what is required of the component(s) before it. In your case that order should be: light bulb selection; then the generator; then the combined waterwheel diameter, water pipe size (you will already have established the maximum height of the tank and therefore the approximate pipe length) required to drive the generator; then the size of the tank based upon what is required to provide a reasonable running time for your demonstration(s) and upon the supporting structure load and stability limitations and you should definitely seek professional assistance in the selection of the supporting structure and its load limits because safety is the most important issue here. One add note regarding the tank, its height should be limited to allow the maximum pipe length if required.

    OK, I think that is plenty for you to consider at this point. So review all of the above items and get back to me with any questions. While I have been extensive in my initial input here because of your time restraints, it is important for you to do as much of the work, analysis. etc. on your project; so, as the project proceeds, I will be limiting my responses to only what is necessary to help you through the sticky points that you encounter. But I will always be here to lend support when needed.
  15. Oct 6, 2015 #14
    After much thinking with my groupmates, I designed another.

    Instead of being its own structure, it should be attached to the gutter of the house. The roof of the house gets rained on. The water goes to the gutter. From the gutter goes to a storage where the water can stay if switched off. When you open the valve, the water flows to the waterwheel and spins the generator.


    Note: I'm just trying to power a small LED.
  16. Oct 6, 2015 #15


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    I agree that an installed system should use the runoff from one (or more) rooftop drains to take advantage of the increased area(s) for water collection.; and a valve for control should be a part of the system. Of course, by using this system you will be restricting the height of the collection tank; but, based upon some preliminary calculations, as long as the maximum water height of the tank is on the order of about 8 ft above the waterwheel then lighting an LED for a sustained run should not be a problem.
    The valve type used should preferably be a "full bore" ball valve so that when open it will not restrict the water flow rate in the downpipe.

    In addition, one tip I am going to give you at this point is that downpipe should be relatively large with a reduced diameter discharge nozzle at the end. This will allow you to optimize the flow rate to the waterwheel by changing the nozzle bore without worrying about increasing the piping flow loss.
  17. Oct 31, 2015 #16
    Holy crap, it's been 25 days!

    Apparently, we're having problems with the energy it generates. It only produces 100 millivolts.
    I think that we should increase the voltage by increasing the speed the alternator spins. I suggest using gears.

  18. Oct 31, 2015 #17


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    Before making any modifications to your setup, you should see if turning the alternator faster has any useful effect. (Good engineering is economical with time and effort. Planning and experimenting is important.)
    Have you tried rotating the alternator manually or with an electric drill? Holding it against the tyre of a bicycle would make it go pretty fast. If that has no effect then you need to look at your alternator design. How many turns of wire are in the stator winding? How fast would you estimate that it is turning with the water wheel?
  19. Oct 31, 2015 #18


    Staff: Mentor

    That is what I expected. Go back and read posts #2 and #6.

    Lighting an LED is too ambitious. If you measure 100 millivolts, that demonstrates that you made electricity. Be happy with that and use the voltmeter to display your success.
  20. Oct 31, 2015 #19
    Ah, forgot to tell about this.

    I ditched the alternator and decided to use a motor (used as a generator).
  21. Oct 31, 2015 #20
    I can simply stick the voltmeter later when attempts at lighting the LED fails.

    Right now, I'm trying to focus in trying to light the LED with a motor (used as a generator).
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