Rowan's portable pedal power generator
|Kiswahili - English|
The idea behind the project is relatively simple. A stand allows the back wheel of a bike to spin freely (usually used for indoor exercise) supports a bike that powers a generator. The generated energy (actually it’s converted) is stored in a battery and used on electrical devices of an appropriate voltage.
Here is a list of the items used for this project:
- exercise bike stand
- 12v, 60amp*hour battery (deep cycle)
- 900 RPM DC permanent magnet generator
- 3 inch skateboard wheel (diameter)
- 12v Delco regulator
- blocking diode (45V, 35A)
- 375w inverter
- 15amp inline fuses (2x)
- Monacore analog volt gauge (0-15amp reading)
- 10 gauge wire (about 15 ft)
- scrap metal
- butt connectors
The first task of this project was to design a system that was appropriate in terms of price, availability, productivity and sustainability. Because I had never worked on a pedal power project, I consulted local expert Bart Orlando to help with the initial design. I used his advice as a template, and then adapted the design to fit various limitations and ideas of what I thought the project should provide. In addition, he had good suggestions on locating a permanent magnet generator of appropriate size and within the budget. Using his advice, I located and purchased an inexpensive, 900 RPM DC reversible permanent magnet generator from a company called C and H Industrial Supply.
A bike stand for indoor cycling was obtained for free from a relative leaving to join the Peace Core. This item not only took the back wheel off the ground, but it provided a frame in which to work. The stand was modified to fit the project by removing the factory-made friction wheel, allowing a place to mount the generator, and also reducing unnecessary friction.
Once the generator arrived, a skateboard wheel with a 3inch diameter was acquired in Arcata. The wheel and generator were taken to local machinist—Gene’s Machines—where the skateboard wheel was attached to the shaft of the generator (see Figure 1) to make the new friction wheel. This was done by boring out a metal sheath on a lathe just large enough to fit over the shaft of the generator. The sheath was threaded, allowing a bolt to go through the skateboard wheel, and was tightened with a rubber washer and ¾ inch bolt. Two setting screws were drilled into the sheath at ninety-degree angles to put pressure against the generator shaft. It was important that the friction wheel was true, and that it was tight enough to take the torque of the spinning bike wheel.
To mount the generator a 7 1/4 x 3 3/4 inch sheet of steel was acquired from a scrap pile. A grinder was used to remove the rust before a 1 x 3 inch groove was cut in the middle of the shorter side. This allowed the shaft of the generator to slide flush against the metal sheet. In addition, four holes were drilled around the groove in accordance with the mounting screws on the generator (see Picture 1). Later, the excess metal around the generator was ground off for aesthetics and to prevent injuries.
The next step was to take two 34 1/2 inch steel struts and weld them about 3 inches apart in parallel fashion (See Picture 2). Since I have no welding experience, I had a friend assist with this aspect. One end of the welded struts was attached to the bike stand where the factory-made friction wheel was removed. At the other end the generator was mounted. To align the bike tire with the skateboard wheel, the generator was mounted on the outside of the right strut (if one is sitting on the bike) so the generator produced a positive output. This was determined using a standard voltmeter. The struts were tightened onto the stand, but loose enough to rotate. This ability to pivot allowed the generator to keep contact with the back wheel of any bicycle secured in the stand (see Picture 3).
In addition, a 12v, 60amp/hour battery was bought at Interstate Battery Systems and the inverter was purchased at Redwood Electronic Supply (both in Eureka). These two items were mounted together, and are separate from the rest of the system. A thin piece of stainless steel was cut to shape and spray painted to house the battery and mount the inverter. Several holes were drilled, allowing bungee cords to secure the battery (see Pictures 4 & 5). Since the stainless steel mount is relatively thin and the inverter light, this entire unit can be carried by the battery handle. Ring connectors were used to join the generator to the positive and negative terminals of the battery, but can be easily detached by removing the wing nuts that keep them tight.
Wiring was the next step after a useable structure was fabricated. A 15amp fuse runs off the positive end of the generator and is followed directly by the blocking diode. This line then leads to the positive terminal of the battery and is attached by a ring connector. The line off the negative terminal contains the volt gauge and then runs back to the negative line of the generator. This line has extra length, allowing the volt gauge to attach to the handlebars of the bike with twist-ties and making it visible to the user. All the wires are 10 gauge except for a few inches of 14 gauge wire off the generator and surrounding the fuses. Various size butt connectors were used to connect pieces of wire that had been severed to add fuses and the diode. Additional wiring included the lines running from the battery to the positive and negative terminals on the inverter. On the positive line, a cut was made to insert a 15amp fuse. There are two AC outlets on the inverter.
In addition, a voltage regulator was purchased from the internet, and will be received shortly. It will be spliced onto the positive line of the generator, between the fused and the diode.
The generation system functions properly as mechanical energy is converted to electrical and stored in the battery, but the system won’t work ideally until the electrical energy is restricted to 12 volts. The voltage in this system without the regulator is higher then the 15V scale on the voltage meter. No measurements will be made on how long it takes to completely charge the battery until a voltage regulator is integrated into the system. When the generator is rotated at the highest speed about 700 watts can be produced. This speed and output is extremely difficult to achieve and can’t be maintained for more than a few moments. 150 watts is a more realistic yield for any sustained period. A comfortable pace that could be maintained for at least 20 minutes would probably be around the 100 to 114 watt range.
In my opinion, the strength of the design is the fact that the generator is mounted above the bike wheel. This is what allows bikes to be interchangeable in the system and still be effective at producing electricity. While it adds some weight having the struts to bear the generator unit above the wheel, it also removes some of the torque off of the generator shaft as the friction wheel rests slightly. In addition, the struts provide a structure to run the wires through and house a fuse and the diode. The voltage regulator will also be mounted on a strut, once it has been received.
Another desired aspect of the design was a generation system that could be portable and universally powered. Unfortunately, the weight of the system limits some of the portability. The battery is a large portion of that weight—in the neighborhood of 60lbs—and isn’t attached to the bike stand because that would make it almost impossible for one person to move. The generator could have been mounted towards the bottom of the stand to remove some unnecessary weight, but as mentioned above, that would have reduced other beneficial aspects. The struts, generator, and bike stand weigh about 30 lb total.
While the weight of the system prohibits one from carrying the unit easily on foot or bike, it’s still practical to be moved by car. It’s fairly compact (see Picture 7) when folded together and the battery unit is detached. It would be easy to put in the trunk of a vehicle and take to a park or camping as long as a bike is available.
While any bike can power the system, there are certain bike abilities that work the system optimally. One factor that’s important is the tread on the tire. Bikes with light footprints have better contact with the friction wheel and keep bouncing to a minimum. This makes it easier to generate higher wattages and also puts less stress on the system. Similarly, since there is extremely low friction involved in spinning the generator, it can be difficult to pedal for any extended time. It is comparable to pedaling on flat ground in second or third gear, since it takes a lot of spinning to go a little distance. To compensate for this, it’s helpful to have low gears. I believe one would need to construct a custom sprocket to obtain a gear low enough for optimal pedaling comfort.
A third aspect that is beneficial would be to have a large back tire. The larger the tire, the faster the generator will spin at any given input. One could also achieve the same result by reducing the size of the friction wheel on the generator. But, at a certain point (which is unknown to me) there becomes too little contact between the bike and generator to create electricity (i.e. the shaft won’t spin). The bike that I ride daily (that the system was initially designed for) fits all these criteria for optimal efficiency, being a 24-speed hybrid possessing 26inch tires with almost no tread. Short of a high-end road bike, it’s probably the ideal bike to power the system.
The one aspect of the project that isn’t finished at the completion of this paper is incorporating the voltage regulator that has been ordered. It wasn’t factored into the initial design of the project, but is necessary and will be added. An applicable voltage regulator could have been fabricated, but time and my lack of electrical knowledge was a limiting factor.
In my mind, the most practical applications for this type of pedal power generator would be with someone living off the grid. The system can power any electrical device around 150W for a considerable period. In addition, one could pedal while using a given appliance and charge at a rate comparable to power use. If space was available, one could setup a bike permanently in the stand and hopefully not to move it often. Ideally, people could generate electricity while they were doing various energy draining, unproductive activities like watching television. This would not only give the viewer exercise and provide free electricity, but it might also dissuade people (especially children) from the brain-numbing activity.
Cost was a big factor in the construction of this project. There were certain expensive items that were necessary for construction— I spent upwards of $300—although the total was reasonable for what was accomplished. The most expensive aspect of the project was having the custom friction wheel attached to the shaft of the generator. That totaled $90, with only about $10 going to parts. If someone had access to a lathe and basic fabrication skills, they could save almost a third on total expenses. Another unavoidable item was the battery, which cost about $60. As mentioned earlier, the generator was ordered from a company that specializes in cheap mechanical equipment. Most of the equivalent generators I located on the web were between $200-$350. I acquired the 900RPM DC permanent magnet generator for a nominal $45, plus an additional $6 to ship. Needless to say, this was the largest saving in the project.
Other substantial costs included the inverter ($40), volt gauge ($21), diode ($12), regulator ($13) and another miscellaneous parts ($20—wires, connectors, several bolts, and various small items I didn’t use). The only real cutback that I could have made with these items would have been finding a broken appliance that contained a volt gauge, and adapting it to my project. But, the gauge wasn’t obtained until the project deadline neared, and spending money seemed easier and less valuable then my time and energy. Similarly, the diode obtained wasn’t ideal for the project. Lack of accessibility (and the fact that I’d already bought two cheap, but wrong diodes) helped me justify the extra expense. As previously mentioned, the bike stand was acquired for free. After the generator, this was the second largest savings, as similar models I’ve seen cost over one hundred dollars. All metal used was scrap and free.
Using a battery was something that I initially debated, because they contain hazardous chemicals and various non-biodegradable materials. And, if the overall idea of the project was to make something appropriate, I had to consider if its function made it justifiable. Besides being able to store amazing amounts of energy for long periods of time, the battery in this system also works as a regulator. If I were to run the energy produced from the generator directly to an electric device, it would be extremely difficult to maintain an even output. In addition, one would need to pedal constantly or the device in use would turn off.
One option to replace the battery would be to create a mechanically powered device. There are both positives and negative to this design. The benefit is that as energy changes form, some of it is dispersed, and therefore less available to do work. As a human powers a bike, their chemical energy is converted into mechanical energy. This is usually a fairly efficient transfer, but there is still energy being lost to heat, light and sound (but mostly heat). The biggest drawback to designing a mechanically powered system is that it would probably be limited to one function (powering a blender, transporting a person, making ice cream, etc.). While there is additional energy lost between the conversion of mechanical to electric energy (which is also a less efficient conversion), the product allows for a much broader range of functions. Therefore, the battery was justifiable because it allowed for the long-term storage of an extremely universal and practical energy source.
Something that needs to be considered in any project is the long-term maintenance of the product. On the system that I built, there are various moving parts, and areas that obtain more stress than others. With that said, the maintenance should be limited to keeping nuts, bolts and screws tight, in addition to keeping the generator bearings lubricated. The structure is mostly solid steel and isn’t going to be exposed to the elements. Most of the wires are located in places where they can’t be damaged easily. Fuses may need to be replaced if too much current is produced at any time.
It is worth mentioning that Bart Orlando was very helpful with developing the design of the friction wheel and the appropriate specs for the system. I had extremely limited fabrication experience and very limited electrical knowledge prior to this project. And, while I was initially apprehensive about using Bart as a resource (he can be abrasive and difficult to work with), he was helpful and more than willing to share his knowledge. Something to keep in mind when trying to get Bart’s time and energy is to provide him with foodstuffs for his solar cooking. For additional help (beyond picking his mind), he actually requires payment in eatables, which is pretty minimal in comparison to the resource he provides.
This project was extremely challenging, consuming huge amounts of time, energy and resources. However, the knowledge and experience obtained was tremendously valuable and has given me confidence to partake in more complicated fabrication and electrical projects. It is important in projects like this to be willing to make mistakes, and to take setbacks in stride. Most aspects of the construction took more time then initially expected, and it seems like there are always additional steps that can be taken to make the system better. I know next time I will be less paralyzed by the enormity of the project and will have a better understanding of what steps should be taken to design and construct a complicated system.
I plan on using this project to power music in places without electricity (either on CD players or amps), charge batteries, my ipod, and dissuade my roommate from watching so much television. In addition, it will allow for light and access to various appliances when there are power-outages. Hopefully it will be a valuable tool for all these reasons, and justify my time and cost.
- By Rowan Steele
- ENGR305 Appropriate Technology
This project is similar to Steven Vromman's pedal power generator
View this page in Kiswahili at Jenereta la nguvu ya pedali la Rowan