OpenErg Based Erg Rower Generator

This pandemic has given all of us an opportunity to try something new or explore an idea that we would have otherwise not had time to do. Thanks to being stuck at home, here’s what I decided to do with my newfound time. For many years I’ve had an interest in human powered electricity, and my first adventures pursuing those interests resulted in my pedal generator designs (exercise bike, bike trainer and spin bike versions). I thought it might be a fun challenge to build a rowing machine generator.

Building a rowing machine generator has some unique challenges, mostly due to the 2 cycle nature of the rowing machine. Cycle one where the chain is pulled and power is created, and cycle two is the return stroke where no power is generated. In this design I do a couple of things to solve the challenge of smoothing out this cyclic power:

1. Use a heavy flywheel
2. Incorporate some ultracapacitors into the charging circuit

To make the story short: these two things did the trick and I’m able to generate 100-120 watts steadily and get in a great full body workout.

Supplies

e-bike motor: https://amzn.to/3w4j1Wr
ultra capacitors (3 or more): https://amzn.to/39hm5EB
charge controller: https://amzn.to/3flnmhR
diodes: https://amzn.to/3flnmhR
3 phase bridge rectifier(alternate option): https://amzn.to/3flnmhR
wago connectors: https://amzn.to/3flnmhR
wire: https://amzn.to/3flnmhR
banana connectors: https://amzn.to/3flnmhR
banana plugs: https://amzn.to/3flnmhR
heat shrink tubing: https://amzn.to/3flnmhR
project box: https://amzn.to/3flnmhR
meter: https://amzn.to/3flnmhR
sockets(2+): https://amzn.to/3flnmhR
laptop charger: https://amzn.to/3flnmhR
inverter: https://amzn.to/3flnmhR
USB charger: https://amzn.to/3flnmhR https://amzn.to/3flnmhR
automotive splitter: https://amzn.to/3flnmhR
jockey pulley: https://amzn.to/3flnmhR
wall mount pulley: https://amzn.to/3flnmhR
nite-ize carbiner(2): https://amzn.to/3flnmhR
chain(order 2): https://amzn.to/3flnmhR
exercise band set: https://amzn.to/3flnmhR
fixed casters: https://amzn.to/3flnmhR
freewheel: https://amzn.to/3flnmhR
hook: https://amzn.to/3flnmhR
velcro: https://amzn.to/3flnmhR
seat cushion: https://amzn.to/3flnmhR
oak dowel: https://amzn.to/3flnmhR
slotted flat bar: https://amzn.to/3flnmhR
duct tape: https://amzn.to/3flnmhR

Tools:

miter saw: https://amzn.to/31oQ6OF
router table: https://amzn.to/31oQ6OF
router: https://amzn.to/31oQ6OF
round router bit: https://amzn.to/31oQ6OF or https://amzn.to/31oQ6OF
drill press: https://amzn.to/31oQ6OF
forsner bit: https://amzn.to/31oQ6OF
cordless drill: https://amzn.to/31oQ6OF
flip drive: https://amzn.to/31oQ6OF
dremel multi-max: https://amzn.to/31oQ6OF
dremel knock off: https://amzn.to/31oQ6OF
soldering iron kit: https://amzn.to/31oQ6OF
helping hands: https://amzn.to/31oQ6OF
speed square: https://amzn.to/31oQ6OF
clamps: https://amzn.to/31oQ6OF

Step 1: Build the Frame

Let’s get into how I built it. A great big shout out goes to Jim Flood for coming up with and sharing his rowing machine plans that can be built with common lumber and wood tools and a few other parts. Check out his website at https://openergo.webs.com/
The first challenge I ran into, Jim's plans are in metric, and US lumber is in "imperial" dimensions. I did some quick conversion and was on my way.

I deviated from Jim's plans in a a number of ways.

1. Put grooves using a 1" round router bit in the top of the two main rails for the seat.

2. Added a cross brace to the front of the rower as I noticed when rowing the frame was twisting a bit.

3. I used a bicycle jockey pulley to route the chain.

4. I used an e-bike motor for resistance rather than a bicycle wheel with fins.

Step 2: Build the Sliding Seat

Once I had the frame built, I measured the distance between the two main rails to determine the spacing needed for the fixed caster wheels. I translated those measurements to a couple of 3/4" pine boards I put together as the seat. Find the center line of the boards, then split the distance to determine where to center the casters. Measure the distance between the caster mounting holes and split the distance again, mark, drill and mount. I was worried I might tip front to back or side to side with this seat, but in use I have not found this to be a problem. Adding a nice cushy pad to the seat was a much appreciated addition.

Step 3: Make the Foot Rests

Rather than use Jim's fixed mounted foot rests, I decided to use the other 16" left of the 1.25" oak handle to make some movable foot rests that rotate with each rowing stroke. The approach works better than I expected. I drilled a 1.25" hole through the frame with a Forsner bit, filed a bit to make the hole a little bigger, then put the dowel through the frame. I then used the same Forsner bit to drill out a couple 2x4 base plates with a hole right at the edge of the boards and mounted those to the dowel on each side. On top of those I put a 3/4" pine foot rest boards, along with a 2x4 heal stop. Adding a couple Velcro straps to each side with some screws and washers and the foot pedals are complete.

Step 4: Chain Routing, Return and Handle

I used 2 full lengths of bicycle chain for this rower, and that seems to be just enough. I routed the chain using a bicycle jockey pulley from the e-bike motor to the underside of the rower. Connecting the end of the chain to an exercise band with a Nite-ize carabiner. The exercise band then routes through a wall mounted pulley and back to the front of the rower where I used a hook to secure the end of the exercise band.

The handle for the rower was made with 1.25" dowel, 20" long with an eye bolt through the center, also connected with a second Nite-ize carabiner to the bicycle chain. I put some duct tape on a couple wood screws and use those as a rest/stop for the handle.

Step 5: Mounting the E-bike Motor

To mount the e-bike motor, 2 flat iron slotted steel mounts were made. I measured the width of the flat sides of the e-bike drive shaft and used my Dremel Multi-Max, Dremel knock off, and metal file to create the proper size slot on each mount. I marked center line for these mounts. Drill and mount with a couple bolts and nuts on each. I used a Shimano freewheel instead of the 7 speed cassette that came on the e-bike motor. I went with a single speed freewheel to keep the chain line straight and limit chain derailment. I did go through a couple freewheels, though. The first one was really noisy, so I picked up a Shimano one, which is much quieter.

Step 6: Flywheel

I used 2 steel flywheels together for my flywheel. They each weigh 28 pounds, total of 56 pounds. I had a local water jet cutting company make these. The dimensions I gave were:

1. Just inside of the e-bike side plate mounting bolts
2. A little less than 18" outside diameter

I had originally had these cut for a bicycle based pedal generator, but repurposing for this project, and they worked out very well!

Some alternatives:

1. You could make a wooden flywheel, check this out for how to make a circle:

With the extra cross brace, it looks like a 32" diameter flywheel will fit, but you could extend the length of the motor mount braces to go bigger on the flywheel if desired.

2. You could get a fully build e-bike wheel, and fill the tube with water using a garden hose and an adapter

Step 7: The Electronics

The circuit is basically this:

E-bike motor->3 phase bridge rectifier->meter->ultracapacitors->charge controller

The charge controller is a 24v->12v step down controller that connects to a couple automotive sockets which can be used to charge phones, laptops or power a TV or other devices with a DC to AC inverter.

From my experience with building pedal generators, I found that the off the shelf 3 phase bridge rectifiers can get pretty hot and call for ventilation and/or a cooling fan. One of the guys that built my plans suggested I use Schottky diodes instead, as they are more efficient. I've made a couple bridge rectifiers with these diodes, and the do run much cooler, eliminating the need for added ventilation and cooling. I use some 4mm banana connectors and heat shrink tubing to make the rectifier.

Next, to add a buffer during the "return stroke" on the rower to keep the electricity flowing, I added three small ultracapacitors to the circuit. These make the rower generator possible, but are also the limiting factor to how many watts I can currently produce with this rower. Once I can afford it, I will be picking up some larger ultracapacitors to really see what the upper limit is of this design (and me!).

Next in line is the charge controller, which is a component typically used battery powered golf carts to run 12 volt devices. The one I'm using is a 24v to 12v step down controller. Input voltage range is 18-36 volts, right in line with what I'm seeing from the e-bike motor while rowing. If I really pull on the rower, it will go higher than that so I use the meter as my guide to stay under the high end. This Drok meter has a setting that will blink the screen when the voltage goes over (or under) a set limit. Besides current volts, it also displays current amps, total watt hours and current watts.

Step 8: Finished

This was a fun project for me and now provides me a great full body workout alternative to my pedal generator. I am able to get a good workout in, while charging my phone, laptop, iPad and other devices - I can even power a TV to watch while I work out! Another plus of this - if the power goes out, I have an option to charge my phone or other devices until the grid comes back online. Those that live off grid, this could be used to supplement your off grid battery bank.

Thanks for checking out my project - I welcome any questions as you plan to build your own!

Comments

[nbakoc99]

Hi! I am trying to recreate this project. I am having trouble with the ebike motor. notably when its is not attached to the circuit is spins freely. As soon as i attach 2 or more phases to the same circuit (the 3 phase diode rectfier) you built the motor is incredibly hard to turn. I suspect it is back emf. How did you solve this? I bought a gearless brushless rear hub motor thats 48v 1500W. Please let me know!

here is the link for the motor:
AW Electric Bicycle 26" Rear Wheel Electric Bicycle Motor Kit 48V 1500W Cycling Hub Ebike Conversion Kit https://a.co/d/h2xoDjU

[Gene'sG]

I've not run into this before - what do you have hooked up after the bridge rectifier? Are there any shorts/wires crossed anywhere? Did you build the 3 phase bridge rectifier as I did? If so, are the diodes oriented correctly? Maybe a picture would help clarifiy.

[nbakoc99]

Amazing project! How did you figure out what exact e-bike motor to use? How did you know what voltage/torque/power specs to look for?

[Gene'sG]

Actually it was just the motor I had :)
I would recommend any 24, 36 or 48 volt brushless, gearless ebike motor. Make sure it's a rear hub motor. I like this one because it can accept a cassette: https://amzn.to/3Ul29H0
As far as torque, power, etc - most of these e-bike motors start at 500 watts, and typical humans will produce somewhere between 50 and 400 watts, so any e-bike motor you pick will handle whatever you've got! I've been able to get about 250 watts for a short period of time (as a non-rower 50-something year old), but am most comfortable closer to 100 watts. YMMV.

[nbakoc99]

Amazing, thanks so much for the reply. Another thing, you suggested that an alternative option to the steel flywheel would be to get a e-bike motor that is attached to a bike tire and then to fill that tire with water. Will that be enough? The steel plates are around 50-60 pounds and I doubt that the bike wheel would be close to that. Will that difference drastically impact the power generation during the backstroke?

[Gene'sG]

I’ve actually knocked it down to just one steel plate (27#) and it works just as well. A 2” mountain bike tire would be about 300 cubic inch of volume, filled with water would be almost 11 lbs, add the wheel, tube and tire weight and you’re probably at 12 lbs. probably not as much effect, but better than just the e-bike motors momentum. It would also help that the tire would be at a further out radius, so will be moving faster, further assisting with maintaining momentum. Been a while since college physics, I could be off base here!

[Cjchester]

what is the experience in terms of the pulling force exerted?

[Gene'sG]

It really depends on the load. If I plug in around 100 watts of resistance, for example my laptop (65 watts), iPad (12 watts), iPhone (12 watts) and a battery pack (13 watts) its comfortable and consistent with what a typical rowing machine feels like. Less than that and it’s too easy, more than that just makes it a more challenging workout 🙂

[Cjchester]

thank you for you response 🙂 what if you will use 500W of brushless gearless hub motor? will there be a difference in the pulling force exerted?

[Gene'sG]

The force required to pull is directly related to the watts required to power or charge what you have connected. A smaller motor won’t make much difference. The TV you see in the video only needed about 35 watts and was easy to pull. Plugging in a laptop (65 watts), iPhone (12 watts), iPad (12 watts) and a battery pack (13 watts) makes for a good workout.

[Cjchester]

Thank you 🙂 how did you identified the (13 watts) of a battery pack?

[Gene'sG]

Good question! I used an inline USB meter https://amzn.to/3tSB8wd

[AdrienR]

Really cool project!
Why do you need such big capacitor? I think that they are here to limit the ripple and help the charge controller to stabilise.

[Gene'sG]

The capacitors are needed as a buffer to keep a steady flow of power during the return stroke on the rower when less power is being generated.

[Stevens Workshop]

Great project, I have always wondered why this isn't a thing in gyms.
In every single gym there are people burning energy often even measuring their output in Watts, seems like such a waste of all that effort.

[Gene'sG]

I totally agree!! Much of the reason I’ve built these human powered generators, I felt like I’m wasting perfectly good energy as heat that could actually power something.

[Stevens Workshop]

Keep up the great work sir.

[Felix_H]

Very cool project and great outcome.
The flywheel looks a bit dangerous though. Maybe you shoud consider to attach a housing for it.
Anyway thanks a lot for posting and have fun rowing! :-)

[Gene'sG]

Agree, if you’ve got kids around that flywheel could be a hazard. A shroud would be a nice add on.

[pjdelys]

Very cool project! My electronics skills are a bit old, but I think if the capacitors were in parallel instead of series, they'd hold more charge and provide better smoothing. Of course, you have them in series to split the voltage since they're only rated to 15v. If one never rows past 24 volts, one could add a fourth capacitor bank and reconfigure them as two sets of series capacitors across the output. That would approximately double your supercap capacity to better absorb impulses, smooth the output, and increase captured rowing energy. I should look into doing this with an old Concept II rowing machine I have.