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Please Help With a Few Design Features for a Trompe

  1. Jan 15, 2014 #1
    Hi Everybody

    The plan is to build a trompe in rural Nepal.

    Has anybody here built one?

    Sure would love to hear from you.

    I believe we could generate enough psi to run a pneumatic motor that puts out 8.4 hp possibly more but i would like to start there and see what sort of output we could expect from an 8.4 hp motor.

    I'm struggling with some electrical questions, one being what alternator could produce the most energy from that 8.4 hp

    I have searched but can't find what i'm looking for, mostly because i don't really know what i'm looking for and specs on ones i found, when present, don't make much sense to me.

    We obviously want the maximum AC output possible.

    Another question is what happens after the alternator?

    What do i need to stabilize and distribute what was being produced?

    Any other aspects of the electricity side of this you could chime in on with suggestions that would be great.

    Thank You

  2. jcsd
  3. Jan 15, 2014 #2


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    What an interesting idea.
    The tromp is basically an air compressor and works directly on mechanical energy. I have a feeling that the overall efficiency may not be as good as you would get with a commercially designed hydroelectric alternator and a motor. You see, there are two energy transfers in your system (tromp and pneumatic motor) rather than just the one (turbine). Even if there were nothing fundamental to this, the fact that many more have been designed and built would probably make it a better proposition. (You could probably get one off the shelf).
    Nevertheless - as an interesting project, the Trompe beats my suggestion any day. It would all depend on your motives.

    Regulation would be along the lines of existing hydro systems, I imagine.
  4. Jan 15, 2014 #3
    Hi Sophiecentaur

    You're right about it essentially being hydro electricity but the turbine will possibly need much more flow of water than a trompe, to be honest i don't really know as finding a functioning trompe is not an easy task which also leads me to believe i might be on a fools errand. But at the same time i really want to give this a go, i guess you could say i want to believe.

    In this application we don't have a huge volume of water flow that is why i want to try with the trompe design.

    It will be an interesting experiment indeed.

    Just wish i had more knowledge on the subject as i said not much on the internet other than basic information and, i've yet to find one that actually creates electricity. Seen a few pulsar pumps that work, same principle only the energy is used to pump the water to a higher elevation, something that could also come in handy in Nepal.

    Any help would be greatly appreciated.

    Thank You

  5. Jan 16, 2014 #4


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    Is your goal to store energy in the form of compressed air, or to generate electricity from the flow of water?


    If the flow of water doesn't vary much, I'd stay with a low-flow turbine system. If the flow of water varies a lot over each day or week, then a compressed air system of energy storage may be practical.
  6. Jan 16, 2014 #5


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    Start with the primary engineering consideration:
    You first need to find whether there is enough flow and height to supply 8kW of continuous power output (with lists to spare to cope with an efficiency of, 80% (a reasonable figure for a small hydroelectric system). That means you need about 10kW of raw power from the flowing water. There are many combinations of height and flow rate but examples would be:

    100m height and 1l of water per second (I think that flow rate would involve much lower efficiency)
    10m and 10l per second
    Power in Watts = hmg Height in metres X flow rate in kg per second X g
    You need to check that you have that sort of supply capacity. It's not a vast amount and hydro power is surprisingly good value - if you have the free water in the first place.

    How is it looking so far?
  7. Jan 16, 2014 #6


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    A trompe is a great way of generating compressed air, but only if you have a continuous flow of water and sufficient head. The advantage of a trompe comes from it's mechanical simplicity. Unfortunately only a small part of the energy of the falling water is available in the compressed air.

    The compressed air does not have to be used at the base of the trompe. It can be returned to the head, or anywhere nearby, without significant loss of air pressure.

    The amount of air entrained as bubbles in the water flow decides the density of the water column. More air injected will therefore reduce the pressure at the air receiver. The flow velocity must be greater than the rate of bubble rise. If you do not already have access to both ends of a drill hole with a significant height difference, the cost of materials will be determined by the diameter and wall thickness required for the trompe pipeline.

    The air receiver at the bottom of the trompe must remain at high pressure. This requires that a restriction be provided to regulate the flow and maintain the water pressure by limiting the escape of water from the bottom. The potential energy of the falling water appears across that restriction, through which the total flow of water passes. So if you want to generate significant energy, replace that restriction with a turbine. Milk off the compressed air for use in applications where air is needed such as for pneumatic tools and ventilation in mining. Use or sell the power generated from the turbine.

    Funny how things happen. There is an artesian water well in the top of a 30 metre hill nearby. Last week I was trying to work out how I could operate three trompes in series on the same hill to generate 100 psi for pneumatic tools and air cooling. That might generate compressed air whenever the well was permitted to flow. The problem is that the air flow needs to be in series while the water flow is in parallel. I see no way to achieve that simply, it needs pressure independent flow equalisers to operate in a steady state.

    There is a conceptual parallel here with Marx Impulse Generators, where capacitors are charged in parallel, then erected in series to produce the high voltage, which lead me back to the simple flyback diode. If I operate a hydraulic ram pump at the bottom of the hill, then it could impulsively increase the air pressure available from one single trompe pipeline. The hydraulic ram valve acts, (cyclically), as the trompe restriction. But more problems arise. The compressibility of the water column due to the entrained air reduces the pressure peak available. The air bubbles become slightly smaller so the water may become more “solid” as it approaches the ram valve. Unfortunately, a hydraulic ram prefers a bubble free water column in a non-elastic feed pipe, especially on final approach to the valve. A hydraulic ram already has an air chamber that acts as a capacitor in a low pass filter to push water slowly but continuously up the output line. The output line is not needed with the “trompe - hydraulic ram” chimera, but some form of water pressure release valve is needed.

    My investigation continues.
  8. Jan 16, 2014 #7


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    A trompe is great if you have loads of energy to spare but the energy transfer cannot be efficient because you can't design the bubbles as you can design a water turbine.
    Low maintenance though.
  9. Jan 16, 2014 #8
    Thanks Sophiecentaur

    Forgive me but i don't get all of that equation

    Power in Watts = hmg Height in metres X flow rate in kg per second X g

    what is the g factor?

    Unfortunately i'm not on the property now and have never been so i don't yet know how much water we are dealing with, i've been told it's a small stream and we have 100 m of head.

    Maybe like berkeman pointed out a traditional turbine could be more effective i will also look into this.

    I will be in Nepal in april staying for 5 months maybe more if i can sort out a visa. So i have time to figure out a plan and put it into action.

    So far the idea is to run the head down the side of the mountain and the return water going back up the hill to a lower elevation than the water/air intake in order to maintain flow. The difference in elevation between the intake and exit point will determine flow rate and the amount of psi i can create will be determined by a formula such as the one Sophiecentaur provided.

    Power in Watts = hmg Height in metres X flow rate in kg per second X g

    I'm curious if those numbers would change if the head isn't vertical, if slope would change the pressure?

    i found this chart, attached below that shows schedual 40 8" pvc will give me 950gpm i converted that to 126 cfm the reason i did that is the other chart for motor specs shows i would need about 187 cfm to run a motor that would give me 4.4 hp max output.

    with an 8" pipe i planned on trying to add a 3" air intake that would roughly move 47 cfm of air, a 4.4 hp motor would need about 187 cfm

    i would need 4 8" pipes with 3' air intakes to get 4.4 hp.

    or maybe not as the original flow was calculated at 0 gravity but if i had 100 meters of head the flow would increase, not sure how much Sophiecentaur's formula might help to figure that out, but it looks like a max of 2,800 gpm in an 8" pipe according to that chart i posted, but other charts suggest different amounts this was the chart with the more conservative results.

    If we could get a flow of 2,800 gpm thru the 8" pipe with a 3" pipe of air intake that would = about 140 cfm so somewhere between 47 & 140 cfm of air seems possible from an 8" water head with 3" air intake.

    So now i'm thinking 4.4 hp motor might be more a realistic target, powered by maybe 2 8" feeds with an undetermined head but hopefully less than 100 meters.

    Anybody know how much juice one could expect from a 4.4 hp motor?

    As i've stated before i've had no luck finding specs for alternators i only find things that are too big or a car alternator. Can anybody point me in the right direction to find the alternator i'm looking for to generate AC on this scale?

    Baluncore i would love to hear what you decide on and how it goes, it sounds like we are trying to accomplish the same thing.

    Thanks again you guys have been great.

    InJoy The Journey

  10. Jan 16, 2014 #9


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    g is acceleration due to gravity.

    Ignoring efficiencies...
    4.4 HP = 4.4 * 746 watt = 3.3 kW
    3.3 kW @ 230VAC = 14.3 amp
  11. Jan 16, 2014 #10


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    In theory, it is the height difference, not the slope, that decides the pressure and energy availability for power generation.

    But when a pipe is used as a trompe, the entrained bubbles are rising against the flow. If the pipeline is not vertical the bubbles will soon rise to the upper surface and merge into one long bubble. The bigger the bubble the faster it rises so you get a flow of water down the bottom side of the pipe and a reverse flow of air on the upper side, which will destroy the trompe effect. So for maximum efficiency the trompe must have a vertical shaft to prevent the air bubbles coalescing.

    There may be a compromise. If you insert very small bubbles from many pores at the top of a vertical tube, they will be compressed in the first say 10 metre vertical section. They then rise more slowly because they are compressed smaller and they may be able to tolerate a diagonal pipe section ending in the air receiver. I would stick to a vertical pipe for my early experiments.

    The best trompe situation would be a vertical drill hole into a gently up sloping adit below, where the air is needed and the water can flow out of the hillside.
  12. Jan 17, 2014 #11
    Thank You Balucore

    I'm actually surprised at how much power 4.4 hp will generate which has me even more eager to build a working model.

    I'm even more surprised more people aren't using a Trompe? Is that another warning sign i'm on a fools errand?

    The idea was to have air inlets coming from about 20 1/8" lines not just one 3" pipe, this way we could also easily add or subtract air input to find the optimal volume.

    The problem the slope creates is cause for concern thanks for bringing that to my attention.

    Can anybody chime in on how much head would be needed to maintain 100 psi using an 8" pipe.

    Would it be true, if taking into account that roughly 40% of the volume will be air on the intake my inlet should be about 40% higher than the outlet?

    Thanks Again

    InJoy The Journey
  13. Jan 17, 2014 #12


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    The pressure is dependent on air and water flow rate. If only a small amount of air is needed then each 10 metres will give you 14.5 psi. That requires a 70 metre = 225 ft head.
    If you want as much air as possible then you will need more height to compensate for the bigger bubbles at the top reducing the density of the water.

    It may be possible to compensate for a sloping trompe pipe by having an inner fixed spiral vane. That would cause the water to spiral down the pipe. Done right the bubbles would average out and not all end up on the upper wall. If you spin the water too fast all the bubbles will move to the centre, so maybe one turn every couple of metres could work in an 8” pipe.

    I don't understand how the input could be 40% higher than the outlet.

    Only the air near the injection point will have about 40% by volume. The bubbles will rapidly become less by volume as the pressure increases. For that reason the trompe inlet should be a trumpet shape with air injection close to the top where the hydrostatic pressure is least and the air can be drawn in by Bernoulli without additional technology. For economy, the wall thickness will need to increase down the pipe as pressure increases.
  14. Jan 17, 2014 #13


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    I am still concerned about some of your basic assumptions here. It is one matter to do calculations which assume a certain pipe dimension but the thing that counts is how much actual water flow is available at the top. Are we dealing with a lake or a small, conveniently placed, stream? Where does the water go at the moment?

    Your lack of the fundamentals is also of concern because it would be a shame if you were to spend a lot of money and effort on a project that has no possibility of working. For instance, you mention a 4.4hp motor. This would be a pneumatic motor, I assume. Where does this figure actually come from? You say you can't find suitable Alternators - so where are you finding a range of pneumatic motors (a very much smaller market)? I have seen alternators on alternative energy sites, which are suitable for domestic applications (feed in tariff etc.)
    Warning: There are plenty of suppliers of equipment who will be only too pleased to sell you something that is totally unsuitable for your purpose; you need to be fully confident about feasibility before spending what will amount to several thousand Pounds Stirling.
    Baluncore has made some interesting suggestions but I think they are angled at someone practical, like him. Have you the knowledge and facillites to experiment and develop this on the hoof? It bothers me that you didn't know what "g" stands for. It suggests that you haven't even looked at the Wiki article on Hydroelectricity. For any hope of success in this, you will need to become a mini expert. This is one reason why I am convinced that the hydroelectric solution is the one for you. There is so much more equipment and experience available and you could probably buy equipment for an off the shelf system.
    I am sorry. This may seem very negative and not at all what you want to read but your situation is not that unusual and the Trompe has not been used in more than a few (if any) projects like yours. There will be a reason for this (A good Engineering reason) and your level of expertise is clearly not high enough for you to supply the technical knowhow to drive such a project through to success.
    Engineering is just as much a matter of avoiding potential failure as completing successful projects.
  15. Jan 21, 2014 #14
    Thanks Sophiecentaur & Baluncore

    You both are bringing up good and valid concerns that i am definitely taking into consideration.

    Sophiecentaur you are correct when you say i don't have the background or knowledge to pull this off but i'm not alone, we do have an electrical engineer who will be helping. This is a guy who has made his own charging system with a bicycle and he made his own permanent magnent alternator by fitting magnets and coiling copper wire into an old ceiling fan, that charges a 12volt battery, he powers his bungalow led lights and a laptop with a 20 min stationary bike ride each day. He has built his own solar panels as well so we do have other options. He is a clever guy and has loads of ideas. I may not have the necessary knowledge but i will learn what i can and get help where needed.

    I love challenges and have done many different things in my life like buying a 110 year old house that hadn't been lived in for over 50 years. I had never mixed a bag of cement, tiled anything, plastered walls (do you know how hard it is to plaster when you've never even seen it done?) or built kitchen cabinets from scratch but i did it all on my own with only the help of 2 other workers to help with structural construction for 6 weeks. Completion took me 3 years mostly coz i had no money and banks had already loaned a large sum but it was the housing crisis in Spain so getting more was not possible but i plugged on as i do.

    In the end i made much more money than i thought i would when i started the project because i did the work as well if not better than "professionals" would have done. That may sound cocky but the mason that helped with structural stuff was amazed at what i did and how well i did it even built a fire place, they are tricky but i did it. Called the mason to help with the fireplace but i had actually finished it and wanted to show him, like i said he was impressed. A local plaster guy gave me a few tips over beers but his final tip was when you realize you can't do it just give me a call and i will do the job, for a fee of course. I called him he came with his tools but the job was done and he couldn't believe i had no cracks it was done properly after 8 years it still all looks good. Opting to build cabinets, windows, doors from scratch instead of buying prefab crap was also a huge plus to the finish product. The only thing i had was self taught basic carpentry skills the rest i learned on the job i plumed the house with just the information on what size pipes to use and what the grade had to be for the sewage. A professional told me what size wires breakers and such i ran all the cables, to be honest i could have finished the job but he wanted to connect everything to be sure and then he signed off on the electric and plumbing. I see this new project as just another challenge, a much bigger one and new skills but that does not scare me, neither does failure, but i will try.

    We may very well decide not to build a trompe but we want to at least research it, and yes the fact that i can't find a working model on the web is of concern, yet how cool would it be to put up some youtube videos of our success and show people that it can be done, if you have the right amount of water and head on your property. There is a mine in Canada that has a large one with just over 100 meters of head they run demanding pneumatic tools and also supply fresh air to the miners. We won't have 100 meters of vertical head but if my theory is right about the sloped head we could have 100 meters of head. This should generate enough constant air pressure to generate enough power to run a 4.4 hp motor.

    About the air bubbles not working on a slope i was thinking that it could be possible to have the 1/8" air lines actually go just short of the separation tank. The flow of water will suck the air in and it won't have much time to rise in the way Baluncore described. Maybe i'm wrong i'm in Thailand now and have no tools or materials to work with so i can't really play around with a model just yet but i'm just getting together a plan and trying to learn what i can so i'm ready to dig in when i get there in April.

    The 40% figure i came up with was because the feed will have roughly 40% less water that the outlet that has no air, that airless water will be heavier than the the feed so less distance will be needed to create the same pressure. Since the feed will be roughly 40% air i thought that was a good place to start. Maybe my theory is wrong but it does seem to make sense, if anybody can point out what is wrong with that line of thought i'm all ears.

    On the pneumatic motors i attached a screen shot of a chart that has specs for a variety of motors.
    They also have larger ones but it seems like a 4.4 hp one would be a good one to shoot for.

    My friend will probably build the permanent magnet alternator from scratch but if i can find something off the shelf it would be nice to know we have another option.

    Again you guys have been great i really appreciate the help.

    InJoy The Journey


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  16. Jan 21, 2014 #15


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    It's good that you have someone practical to help you here. There are the other issues that still need to be addressed. What is the available water flow from your reservoir? You have not said, yet and it is a make or break figure. You need a minimum of what I calculated in my other post. If you insist that the tromp is the solution then I have no idea how efficient the energy transfer from hydraulic to pneumatic energy is likely to be. As it is inherently very turbulent and you don't seem to have design knowledge, you could be aiming at something very low - like 20% or less (?). Whatever the efficiency, it will be far from the value of a well designed water turbine and will have serious repercussions on your possible supply power. A hydro system will not be affected much by any slope.

    All this is fine if you just want a fun project. If you really want to power your house electrics from it then the sums have to be right. For instance, the power of those motors is quoted at 6.3 Bar. That is Six Atmospheres, which is the equivalent of 60m of static water pressure. Hopefully, your friend will help you sort out things like that but it certainly can't be ignored. Also, do those motors work with saturated air with actual water droplets in it? That could be what the pressurised air from your tromp is probably like. Those pneumatic motors are surely for specialised use with a source of clean air from a compressor.

    I have to ask you again- why do you think that alternative power schemes all seem to use hydroelectric generation? There are a lot of very experienced Engineering* types who are involved in alternative energy. Do you know of any examples of a tromp being used for your type of project? The fresh air for that mine seems a very good application of the 'free energy'. What tools do they use and at what power? The details really do matter.
    If I were you, I wouldn't choose to do it this way on the grounds that you could impress some people on YouTube (the YouTube audience can be impressed by a dancing dog - more possible hits there, in fact!). It would be a big stake for not much return.

    *The definition of and Engineer is someone who can do, for £5, what any idiot can do for £50. Talk to (employ) an Engineer before embarking on this.
  17. Jan 21, 2014 #16


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    There are a couple of engineering principles here:
    1. It is better to couple power through a solid, than a liquid, than a gas. Boats are more efficient when powered by propellers in the water, not the air. Cars are propelled by wheels on the road rather than propellers in the air.
    2. Every energy conversion involves loss. Why generate pneumatic energy unnecessarily, if you then need to convert it again. Compressed air is quite unnecessary in this electrical power generation situation.

    At the bottom of your trompe there must be a restriction to maintain hydrostatic pressure in the water. Without that restriction the air pressure will be very much lower. The vast majority of the energy available from the drop is available at that restriction where it takes the form of mass flow rate multiplied by the pressure difference. That is where you should put your turbine.

    My guess is that you cannot get 5% of the available energy from a trompe as air pressure. You could easily get 85% from a water turbine at the restriction. The cost of the pipeline would be significantly less when using a hydraulic turbine, rather than a pneumatic turbine.
  18. Sep 20, 2014 #17
    Trompe I am on the same path, for creating Alternative Energy. I would love to speak with you, kindly reply.
  19. Jan 10, 2015 #18
  20. Jan 10, 2015 #19


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    I am confused where conservation laws come into the nitonal thing. That video can't seem to make up it's mind whether we are talking about an 'energy source' or an energy converter. Many words but very few figures - which are needed before anything is a serious Engineering proposition.
    For answers to your questions regarding the troupe, you would have to specify just how much water is dropping down this column. and how high it actually is. You will not get more power out than the gravitational potential times the mass flow per second will allow - minus the various losses. (More conservation laws, here, too)
    Don't consider investing any money in such a project until you have done the sums.
  21. Jan 10, 2015 #20


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    When I see references to a Tesla Turbine or a Nitinol Engine, I know that economy and power are not being seriously considered. A trompe is likewise, a way of generating compressed air while wasting huge volumes of water. If a waterfall is handy then by all means use it to produce compressed air if compressed air is what you really need. But considering using a trompe in a desert is a really poor engineering solution.

    Maybe it is time to define the engineering problem that needs to be solved.
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