Generator at low RPM for a non-chemical battery - What to look for?

Thread Starter

Theo_KVA

Joined Dec 6, 2019
10
Hello!

I’m in the process of trying to build a non-chemical battery. I sick a bit of guidance as I tend to go down an engineer rabit hole, because of the complexity that the project is aiming.

My goal is to make some kind of object that could store enough energy to charge a smartphone’s battery. The idea is to have a heavy weight attach to some rope. As it would go down with its weight, the rope would drive a motor’s shaft that would then output energy. This idea ain’t new, I don’t want to break any laws of Thermodynamics.
With this simple setup I could complexity by adding reducing gears on the shaft, as well as a pulley system to augment the length of the rope. Obviously the weight would have to be heavier.

I would need this system to charge a smartphone’s battery. If it’s a bldc or stepper I would go by full bridge rectifiers and a buck/boost converter to output steady 5V (aiming 1A). I searched a bit and other than windmills enthousiastes, the subject is not that much documented.
The weight is probably going to hang from 2m high, so I can’t really be aiming for high RPM.

Generator with followed technical needs :
- Low RPM
- Output around 8W (with margins)

I made an experiment inspired by GreatScott’s video about producing energy with motors.
As I’m not as well equipped I did with what I had, a homemade, an oscilloscope and a drill.
I made the experiment with 2 stepper motors, which he stated as being best for low RPM.

Capture d’écran 2019-12-06 à 16.01.19.jpg

The results were really not as good as expected, the power generated was way too low. And I have no clue what to look for.
One of the comment under the video stated that hub motors for bicycle could be better.

Capture d’écran 2019-12-05 à 16.33.25.jpg

Capture d’écran 2019-12-05 à 16.47.46.jpg

Of maybe a stronger stepper motor, with a lower step angle. But again I don’t know what to look for. There are no clear data about energy generation, as there are not design for such thing.

Capture d’écran 2019-12-05 à 16.48.07.jpg

My question is : What do you think about this problem? What would be your recommendation of motor/setup to explore?

As I’m a designer, and not an engineer please be forgiving :)

Thanks !
 

Papabravo

Joined Feb 24, 2006
14,420
I don't see how this project has any prospect of being reasonable. To demonstrate this, there are museum displays that you have a geared hand crank to light a 12V, 8 watt lightbulb. It is almost impossible to keep it lit for more than a few seconds. So your mechanical system would need to repeatedly lift the weight and allow it to drop. I doubt you could do this more than a few times, certainly not for any appreciable period of time.
 

Tonyr1084

Joined Sep 24, 2015
5,080
Regardless of the gearing or motor/generator or anything else for that fact, there is a finite amount of energy in a weight lifted to a given height. I believe it is highly doubtful you can get enough energy from a hanging weight to charge a cell phone unless that weight was in the tonnage range. But then comes the question "How do you raise that weight?".
 

Papabravo

Joined Feb 24, 2006
14,420
Regardless of the gearing or motor/generator or anything else for that fact, there is a finite amount of energy in a weight lifted to a given height. I believe it is highly doubtful you can get enough energy from a hanging weight to charge a cell phone unless that weight was in the tonnage range. But then comes the question "How do you raise that weight?".
In fact the gravitation potential energy is given by the product of mass, height and the gravitational constant. Back of the envelope:

1 kg, x 9.8 m/sec/sec x 2 m. = 19.6 joules

which in fact is not a great deal of energy when you consider that 1 Watt hour = 3600 joules
Your falling weight corresponds to 0.00544 watt hours

By comparison a AA battery with a 2.4 Ah capacity at 1.5 volts for 3.6 watt-hours times 3600 joules/watt-hour = 12,960 joules

You mechanical 19.6 joules pales in comparison to a AA battery.
 
Last edited:

Chris65536

Joined Nov 11, 2019
227
I recently read an article about this type of thing. They were using it to power a small LED light, intended as a replacement for the oil lamps used in 3rd world countries. Solar + batteries was too expensive, and this was proposed as an alternative. Lifting 40lbs of rocks up 3 feet is about 163J of energy. That would run a 0.1W light for maybe 15 or 20 minutes. 50x that power would not be very practical though.
 

Alec_t

Joined Sep 17, 2013
11,649
More back-of-envelope maths suggests a cell-phone battery capacity is about 8Wh, or ~29000 Joules, so Papabravo's 1kg weight would have to be raised by 2m around 1470 times.
 

Thread Starter

Theo_KVA

Joined Dec 6, 2019
10
Thanks for the replies ! :)

@Externet Thanks for the links, but solar energy is another topic I'm not interested right now. The idea is not to produce energy but to store it.

@Papabravo & Tonyr1084 & Chris65536 & Alec_t
Okay I see your point. Now I understand why EnergyVault (https://energyvault.com/) are having that large installations.
But isn't it any way to make it small enough to power a phone?
 

joeyd999

Joined Jun 6, 2011
4,429
I recently read an article about this type of thing. They were using it to power a small LED light, intended as a replacement for the oil lamps used in 3rd world countries. Solar + batteries was too expensive, and this was proposed as an alternative. Lifting 40lbs of rocks up 3 feet is about 163J of energy. That would run a 0.1W light for maybe 15 or 20 minutes. 50x that power would not be very practical though.
 

Papabravo

Joined Feb 24, 2006
14,420
Thanks for the replies ! :)

@Externet Thanks for the links, but solar energy is another topic I'm not interested right now. The idea is not to produce energy but to store it.

@Papabravo & Tonyr1084 & Chris65536 & Alec_t
Okay I see your point. Now I understand why EnergyVault (https://energyvault.com/) are having that large installations.
But isn't it any way to make it small enough to power a phone?
Unfortunately I think the answer is that it will be exceedingly unlikely. Any time you have two quantities multiplied together, mass and height in this case, you have an infinite number of possibilities to choose a pair of numbers to get a particular result. They will not all be practical. The definition of 1 horsepower is doing 550 ft-lbs of work in 1 second. There are many horses that can lift 550 lbs 1 foot in 1 second, but there are no horses anywhere that can lift 1 lb. 550 ft. in 1 second.

If you are still interested in mechanical power generation, consider falling water. It drove the industrial revolution along with steam.
 

Tonyr1084

Joined Sep 24, 2015
5,080
More back-of-envelope maths suggests a cell-phone battery capacity is about 8Wh, or ~29000 Joules, so Papabravo's 1kg weight would have to be raised by 2m around 1470 times.
@Theo_KVA you could increase the weight by 1470 times (using Alec-t's numbers) OR you could increase the height by 1470 times. If you raise it to 2940 meters - good luck finding a building that tall to house your rig. If you double up the pulley's then you have to double the weight. Quadruple the pulley's - you quadruple the weight. Of course, by quadrupling the pulley's you reduce the height by 75%. But then you'd only need a rig 735 meters high. If you double the weight you can reduce the height to 367 meters. Still a tall building to say the least.

If you take the other approach and increase the weight to 1470kg's that's a pretty heavy rig. Now, instead of having a height problem you have a structural problem. How to manage all that weight. The ropes and pulleys would have to be able to handle that kind of pressure. And assuming you came up with some compromise between height and weight, you still have the problem of how to lift that weight. Remember, in a perfect world, you can only get as much energy out of a machine as you put into it. Hand cranking a gear induction generator to charge a cell phone could be done in an emergency, but it will take a whole lot of human power.

Search YouTube for "Olympic cyclist versus toaster"
 

Chris65536

Joined Nov 11, 2019
227
But isn't it any way to make it small enough to power a phone?
The key is to find an efficient low power generator. I don't really think of steppers as being that efficient, and from your experiment, it looks like you'd still need some gearing to increase the RPM. The bicycle hub motor sounds perfect, but maybe too expensive.

As far as your 8W power requirement, I could imagine something where the user steps up onto something, and is lowered to the ground over about 30-60 seconds. If the user is 70kg and the step is 0.5m, that would be 343J per step. Repeat 100 times, and you've got at least a partial recharge for your phone.
 

Tonyr1084

Joined Sep 24, 2015
5,080
Repeat 100 times, and you've got at least a partial recharge for your phone.
PARTIAL charge.
My goal is to make some kind of object that could store enough energy to charge a smartphone’s battery.
Thread Starter (TS) did not as for a partial charge.

One thing I've experienced as a smart phone user. Charging it when not at home. You have to bring a charger with you. Some of my job assignments disallowed smart phones on the facility. Others were more liberal, but you had to get permission to attach ANYTHING to the computer (i.e. charging a phone). You COULD bring in a charger, but you had to have an extra charger to bring in. And many times people were seen asking their co-workers if they had a charger and cable for their phone they could use.

The point is that it is a lot easier to carry a charger in your pocket than it is to carry a rig of so many meters high or with such a weight or a stepper to charge the phone. I'd have to liken the task as proposed to that of draining a kiddy pool using just a small sponge. Gonna take a whole lot of squeezing to get the water out.

In my opinion, this approach to charging a phone is a non-starter. I would have to say "Good luck in this endeavor. I'd LOVE to hear how it turns out."
 

MisterBill2

Joined Jan 23, 2018
7,031
A very good and possibly cheap generator would be an automotive radiator cooling fan motor. If you can rescue one of those from a wrecked car that did not get destroyed in the front. They are made to last a long time and they are in a higher power class. But they will need to spin faster so a crank and two sprockets or a belt and two pulleys for a good speed multiplication is what you need. About a 10:1 speed increase will be what works. And your setup will be able to charge more than cell phone batteries.
 

Thread Starter

Theo_KVA

Joined Dec 6, 2019
10
Thank you so much for all these replies ! :)

The idea is not so much about producing, it's about storing.
As nowadays other than chemical batteries you can not really "store" energy. All the excess is wasted. So there are these projects on large scale that emerge (like the Dinorwic pumped storage scheme in North Wales, EnergyVault or Nasa's flywheel studies) that transform excess energy into kinetic energy by lifting stuff and then when needed you transform it back into electricity via a generator. Right now I was just focusing on the second part.
Anyway I had this idea to take it to a small scale, but with all your replies I finally understand what are the challenges of such project. It is exactly the kind of answers I needed.

@joeyd999
How can I never came across this?
The light gravity seem to be running a DC motor. I counted 4 gears changes. 100 teeth to 21, 63 to 16, 100 to 15 and around the same for the belt. So it is a ratio of around 650:1. For one bright LED, damn.

Well probably this project won't go further.
@Chris65536 If I take your numbers it would mean 70kg at 2m high is about 4% of a phone's battery. Which does not make any sens to make that kind of a rig for that small amount.

Thanks again for all the replies guys :)
 

BobaMosfet

Joined Jul 1, 2009
1,208
@Theo_KVA

First, the math doesn't lie. That is what engineering is about. Identifying limits and finding a solution that satisfies all, or an exceptable number of those limits (a compromise) to achieve an end goal.

What you are attempting to do is simple physics. You have to work within the physics box. What I can tell you is this- the only acceptable storage medium you're going to find is a super capacitor, for a number of technical reasons (or bank of them), and you need to re-evaluate your requirements as follows:

  • Higher suspension height of weight (easiest way to add energy)
  • Heavier weight
  • flywheel to capture energy
  • higher RPM (easier to drive)
  • higher current motor, low voltage (5VDC)

One way or another, you have to alter your design to satisfy the real world in order to achieve your objective. The good thing is, the sky is the limit. Sometimes, the best path is not the obvious one, but it will be the most elegant and simple solution. Think about what levers you have to pull on:

  • Drop height
  • Fly-wheel diameter <- this is probably your most useful aspect

However, your enemy is time. If it's a one-shot, one-time release and you capture and convert whatever energy you can from that, then efficiency of conversion (even multiplication- as with the flywheel), You can only make a flywheel so big (based on falling weight powering it), turn so fast, and turn for so long, and once it drops below a certain point, the ability to take energy off of that will diminish or cease to be useful quickly.

At the end of the day, it still comes down to physics- Find that compromise to achieve your objective, or do something else, instead.
 

shortbus

Joined Sep 30, 2009
7,942
A very good and possibly cheap generator would be an automotive radiator cooling fan motor.
While I agree they are powerful, those fan motors are BLDC motors, with a built in driver. Don't think they will put out a voltage when turned by some other means. If they did they would cause havoc to the electrics of a car. From the fan turning while going down the road at speed.
 

Tonyr1084

Joined Sep 24, 2015
5,080
@shortbus I have a couple of those fans from my wife's wrecked 1989 car. If I short the leads and try to spin the blade the result is a very very heavy load (dead shorted). So yes, they would produce a significant amount of current. However, I still think the idea of a stored weight hanging like an old clock mechanism is just not practical.
 

MisterBill2

Joined Jan 23, 2018
7,031
While I agree they are powerful, those fan motors are BLDC motors, with a built in driver. Don't think they will put out a voltage when turned by some other means. If they did they would cause havoc to the electrics of a car. From the fan turning while going down the road at speed.
I have played with three automotive cooling fan motors that delivered a useful amount of power being spun by hand, and they would light a 12 volt bulb a bit as well. While the current ones may be of the electronic type, none of the ones that I played with were. and probably an electronic motor with an external driver would work as a generator with a three legged diode bridge added. Also the motors on my two older cars were quite obviously brush types.
 
Top