Harvester - Building the Energy Forest

An Open Source project to build cheap linear generators to harvest the wind energy which moves trees in a forest (or waves in the sea)

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Harvester is an ongoing Open Source project. The goal is to build a platform which is able to harvest the energy inside oscillations of trees which are swinging in the wind. This eliminates the need to build big, ugly wind turbines and and lets the landscape unchanged. Switching to another object the system is mounted on, also other sources of energy, eg. waves, could be harvested.

The Problem

Fighting global warming is one of the biggest challenges for humankind. Therefore most of the industrialized nations try to increase their output of renewable energy. In Germany in particular this means despite other approaches to build wind turbines. A majority of people is in favor of wind energy, but in many places building a huge wind turbine is not possible because of the opposition of the residents, which usually don't like the look, sound or shadow of these turbines, or because of environmental reasons (e.g. nearby endangered birds).

The Solution

The solution for this problem is to use trees as a energy source. To be more precise: This project tries to generate electricity by using the bowing motion of trees which are exposed to wind.

Current Status

A first prototype has been built (see milestones below or video above), but to find the best solution, more research has to be done, which goes into three main directions:

► Energy: How much energy is trapped inside a swinging tree and how much can be recovered?

► Configuration: Where should the harvester be mounted? Between two or more trees or with connection to the ground? Directly at the ground or at the top? Shall the harvester be usable only with one tree (aka in a garden)?

► Engineering: How can the linear movement be converted into electrical power? Use of an hydraulic/pneumatic system, simple ropes or linear generators for direct electrical power generation?


First CAD Sketch
First data from the field
Harvester has a testing area
First parts ordered
First prototype
First wind energy harvested

  • Next steps

    Tobiwana day ago 0 comments

    Waiting on the parts I ordered I thought about what to do next. For the next week I see two major topics:

    • Research
    • the next level of Prototype

    For research I'm in the moment most interested in how the tree moves and how much energy is in the movement. So, besides some literature research, I want to build a arduinobased linear movement logger, which is basically two pipes which snuggly fit into each other, inside there will be ultrasonic or IR distance sensor and hooked to that is an Arduino which logs the data. I plan to build several of these to test different trees and different mountings.

    A second version will have a strong spring in it, so together with the position data I can calculate the force for every point in time, with that also the work done and therefore the minimal energy which the tree can give away.

    The endgoal for the working prototypes is to build something which fullfils this requirements:

    • easy to build (good documentation, no use of expensive tools)
    • versatile (so you can use it also for your wave energy power plant)
    • able to harvest power over a big range of wind velocities
    • as cheap as possible

    For the next working prototype I want to build a Harvester which can deliver >2W of peak power. I'm not sure yet what I will use as a generator and if I go the route with strings and a winch or rack and piston (see my brainstorming post), but I know that it will build in some sort of power/energymeter+logging and I want to integrate a battery so I can really use the power (e.g. for charging my phone). As a generator I look currently in bigger stepper motors, hover board or electrical bike motors, bycycle generators and DIY generators for wind turbines. But I think at least for the next prototype I will go the easy road and not build the generator myself.

    Also the concept with collecting the power with pressure in liquids and using a central turbine to generate the power has to be tested, but this is something which has a lower priority and I will probably only look into if I manage to get into the final round of the Hackaday Prize.

    Stay tuned for the next update!

  • First wind energy harvested!‚Äč

    Tobiwan4 days ago 0 comments

    Such a impressive moment! Today it is a light windy day (the forecast says 13km/h, in other areas this is probably not even considered as wind :D ) I connected the prototype I built yesterday to a big branch of our birch and it immidiately began to light up the LED I connected as a load. I made a short video of it:

    Here you can see the whole setup:

    Below there is a stone to hold the stool to the ground (otherwise the pull of the tree would tip it over), then you can see the red bungie cord, the stepper motor with the connected led and the wood holding everything together. The string ist tied to the bungie cord, winds around the stepper motor axle and goes the to branch. If the branch moves, the string is pulled and the stepper motor rotates and generates electricity. This is enough to light up a green led. When the branch moves back, the streched bungie cord pulls the string back and there is again energy harvested.

    The next interesting thing is to measure how much energy is collected over the day (and that with just an old floppy disk stepper motor as a generator!!!)

  • First (rough) prototype!

    Tobiwan5 days ago 0 comments

    Again I couldn't wait to see the idea coming to life, so I hacked together a quick prototype: I used a stepper motor as a generator, wound a string around its shaft and used a rubber bungie cord to pull the string back after a pull. This string I tied to the apple tree in our garden. Unfortunately today is a day without any wind, so I had to test it by moving the tree manually. As you can see the LED I connected to the stepper motor was lighting up. Yeah! :) Credits for the idea with the string goes to Florian, who posted in the comment section!

    Here you can see the tree:

    And finally the first energy* harvested! :)

    * = Unfortunately only Tobiwans energy, but a update when it will get windy the next time will follow soon!

  • Parts ordered!

    Tobiwan5 days ago 0 comments

    First I thought I should wait until I gathered enough data from my tree data collection, but I can't wait to build something, so I ordered some parts in advance :)

    • First I want to build a low RPM axial flux generator, so I ordered some magnets and copper wire on Ebay. It will be not the version I will use in the field (since I don't know the optimal RPM range yet, this depends on the most common swing frequency of the trees), but I think I can learn how to design and build such a generator :) Fortunately I have the rest of the materials needed (bearings, silicon for the mold, resin for stator and rotor already in my workshop).
    • Second I want to build a energy collection system based on liquids (see also my Brainstorming project log), therefore I ordered some tubes and pipes. I plan to compare these both methods based on tne price per watt of output power.

  • Access to 3D files

    Tobiwan6 days ago 0 comments

    One of you guys asked about the 3D files of my first 3D sketch, so here it is:

    I used Fusion360, so feel free to download the files and do whatever you want with them. But remember: That is just a sketch for illustration, so neither the dimensions nor the technical details (number of magnets, coils, etc) are how they will be in the first prototype!

  • Harvester is going into the woods!

    Tobiwan6 days ago 0 comments

    Yesterday I had a talk with a friend of mine who owns a small forest here in Bavaria, Germany. He got excited about my idea and offered that I can mount data collection systems and prototypes at his trees. Its a working forest with mainly spruces and it is situated at a hillside, which is very good for wind energy harvesting :) As soon one of my prototypes is ready I will bring it there and test it. If I come to the final round of the Hackaday prize I plan to use some of the money to build and test a small (<10 units) batch of the most promising prototype, but more about my future plans and the next steps in one of the next project logs.
    Above you can see the aerial view of the forest in question :)

  • Research: First data from the tree

    Tobiwan07/12/2018 at 07:56 0 comments

    Yesterday I took the phone back off the tree (see yesterdays project log entry) and today I look for the first time into the data. The graph shows the acceleration values parallel to the ground over time:

    First obvious statement: We had some wind in the evening and during the next day, but the night was very calm. By the way: This is not the raw data, I used the calm period at night to get rid of the baseline (which is caused by gravitation, which is a constant acceleration), so the whole graph is shifted to zero (the unit to the top are m/s2 by the way, to the left there is the number of 500ms timesteps, which means 2*3600=7200 is one hour) . Besides that there seems to be some osscilations so lets look closer at the red rectangle:

    There are smaller oscillations, but not around zero, interesting... I think the bow of the stem brought back the constant influence of gravity (perhaps we can use that to run a running average and then have an idea to which angle the stem bowed?). Nevertheless, the oscillations we can see here are in the order of minutes, which is not what I expected from just watching the trees swing in the wind (there I estimated swing periods below 30s). My first guess is that the wind is changing in this time period, but so regulary? Second guess would be that the swing period of trees is indeed in the minutes, I have to do more research about that :) We also have to keep in mind that this is just the x-direction, so if the wind direction changes, this can also have a influence on the data, since this is only the projection of the movement on the x-axis.

    All this questions the method to use a phone as a monitoring device for low frequency oscillations, so either I use a more sensitive accelerometer/more sophisticated data analysis in the future or I try to measure the amplitude, which means the distance from the place where the stems is without wind. More about that coming soon!

    If one of you guys wants to look into the raw data, you can find it on Github. A second opinion/approach about analyzing the data would be much appreciated!

  • First 3D Sketch

    Tobiwan07/11/2018 at 17:32 0 comments

    Today put a little bit more thought into solution 1 (see last project log entry, the one with rack and pinion). I think I will go with a rack, which slides in a tube and rotates a small gear. Connected to this gear is the rotor of an axial flux generator. This type of generator are well suited for low RPMs, you can see the principle at the end of this project log post.

    In the following you can see an overview of the design I came up with. Point A and B are connected to the trees and the linear power generation unit is in its fully compressed state:

    A more detailed view into the core. Please ignore the design of the housing :-). On the left is the tube where the rack sits in during full compression, in front you can see the pinion which is connected to the (transparent) rotor with 16 permanent magnets. Behind that is a red stator which holds 8 coils. The size and number of magnets and coils is not defined yet, this is just for illustration.

    Last but not least the difference between radial and axal flux in generators:

  • Brainstorming

    Tobiwan07/11/2018 at 08:30 0 comments

      Today I made some sketches brainstorming different configurations and technical solutions:

      The main technical challenge is to use the motion of the tree, which is most of the time very powerful, but also very slow and with low amplitude. Therefore a mechanism has to be found, which can convert this motion into electricity.

      1. The most likely solution, a connection between two trees where a rack and pineon transform the linear movement into a rotational one, which then through some gear drive powers a generator.
      2. There is a central post between several trees, where some weights or springs pull at ropes which are connected to the trees. The ropes have a chain element which drives a gear which then drives the generator.
      3. There are two chambers mouted to the tree. When the tree bends, one chamber is compressed, the other is stretched. The compressed chamber pushes a liquid through a one-way valve into a central high pressure tank from which the fluid goes through a turbine into a low pressure tank. When the tree bends to the other side, the compressed chamber streches and gets new fluid through a one-way valve and a seperate connection from the low pressure tank. The previously stretched chamber gets compressed at the same time and the cycle continues.
      4. Same as the principle above, but instead of a compressable chamber there is a piston which generates the pressure (also two one-way walves and to connections to the high and low pressure tank).
      5. This is just another configuration, it would work with a linear generator as in Type 1 or with a piston as in Type 4. The difference to all other concept is, that it uses a connection not between two trees, but between the tree and ground.

  • Research: Data is coming!

    Tobiwan07/10/2018 at 14:00 0 comments

    I managed to put the phone up to our big birch and enjoyed the awesome view meanwhile. It sits now at 10m height and collects accelerometer data from all three axis. It is a waterproof phone, but just in case I put a protective plastic bag around it. Better safe than sorry! I will put the phone down again in 24 hours and make a preliminary analysis then.

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Florian Festi wrote 3 days ago point

Nice prototype! In case you want to want to put more work into the stepper idea:

Put a rectifier on each coil. So you need 2 single-phase rectifiers. Just connect their outputs. It might be worth using Delon rectifiers which doubles the voltage - depending on the output voltage you need.

Obviously having a gearbox for higher RPMs at the stepper would allow harvesting higher forces from the tree.

  Are you sure? yes | no

Tobiwan wrote a day ago point

Thank you for the tips. In the next prototype I will propably use steppers again, then I will put in the rectifiers. A gearbox is something I want to avoid because it adds complexity, costs and reduces the overall efficiency. But lets see :)

  Are you sure? yes | no

Florian Festi wrote 7 hours ago point

Oh, as you have limited motion you could also just use a lever. Attach the rope from the tree in the middle and attach a rope to the motor at the end.

  Are you sure? yes | no

Tobiwan wrote 07/12/2018 at 13:50 point

Good idea with the cables and the the winch! Perhaps one could use a second rope running from the other side of the tree to the generator to use the second stroke also. I already thought about the problem of two trees running syncronous, but I think it will be very unlikely that two trees have exactly the same natural frequency. But their are coupled then, which could cause syncronous swinging. I think I have to try that out.

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Florian Festi wrote 07/12/2018 at 11:36 point

Not sure if this is feasible at all. But if it is I would suggest the following setup:

Use a generator on or near the ground that is powered by a winch that is hooked up to the top of a tree with a cable or rope. The winch would need a return spring and a ratchet towards the generator to allow pulling the cable back in.

Yes, this wastes the power of the return stroke. But as the tree is a spring to some extend you may get some of that energy back on the next stroke.

Using a cable avoids using push rods which are very difficult to get stiff enough to transmit the relatively high forces over the long distances.

This setup is also much easier to maintain as everything but the rope is on ground level. It also avoids the issue of both trees doing the same movement and keeping the push rod at the same length.

  Are you sure? yes | no

AndreasW wrote 07/12/2018 at 09:40 point

Great idea, want to see more!

  Are you sure? yes | no

Tobiwan wrote 07/11/2018 at 11:26 point

Hehe, you got me, Im building this just to phone home ;)

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