Solar Panels on Electric Bikes?

I've seen ideas brought up online about putting solar panels on electric bikes to recharge them, and have always seen them beaten down by the people saying it wasn't practical because you would need so much surface area it wouldn't fit on a bike. However, after seeing pictures of a light electric vehicle with a solar panel built into the roof last week, (if Mark or Griffin could remind me what that was, that'd be great) and being asked the same question today and giving the typical answer with no facts to back it up, I decided to see whether it was feasible.

First, the best case scenario. A bike rack over the rear wheel can hold a solar panel approximately 12" wide by 18" long. The sunniest place in North America is Inyokem California in the high desert, where it is sunny 355 days a year.

They receive 7.7kWh of sunlight energy per square meter on the average day, or about the equivalent of 7.7 hours with the sun directly overhead and no clouds. That means that the solar panel on our bike would receive 1070Wh of sunshine on a normal day in Inyokern. However, solar panels do not convert all of this to electricity. In fact, the most efficient solar panels in a lab are pushing 44% efficiency (awesome graph of this!), but the best commercially available solar panels are the SunPower X-Series which boast 21.5% efficiency. If we assume that our bike panel has the same efficiency, even though SunPower doesn't make any panels this small, that brings our energy down to 230Wh. A typical electric bike traveling at 20 miles/hr, a quick biking pace but somewhat leisurely for an ebike, uses about 17Wh per km, which gives us our average range of 13.8km, or 8.6 miles. This is actually fairly good, considering this is just on electric power, and you can always pedal to extend the range.

However, this is still assuming no losses in the electrical system, using solar panels better than anything on the market, and in the sunniest place in North America. Not all of the power our solar panel generates goes into making the bicycle move. The heat given off by the motor is taken into account in the bicycle efficiency number, but some goes into heat from the rest of the electrical system. Being generous, I would say that the charger has about 96% efficiency, the balancer 99%, the batteries 98%, and the controller 95%, bringing us down to 7.6 miles.
I managed to find a solar panel which is only barely over what I estimated could fit on a bike, and by far the most efficient I could find in this size, rated for 15W. This means that once we account for the wasted sunlight falling on the aluminum frame and the lines on the panel between the solar cells, this solar panel is at 10.6% efficiency, and our range is down to 3.7 miles.

It's not looking good for the solar panel...

The closest city to me right now is Boston, which receives about 3.8 equivalent hours of sun a day, bringing us down even further to 1.9 miles at 20 miles per hour.
Because this person has an electric bicycle, they might want to go a little bit faster, but the problem is that air drag increases with the cube of velocity, so going 30 miles per hour they will have slightly less than half the efficiency as 20 mi/h, so at 30 miles per hour they'd be able to go almost one mile per day in Boston before they need to start pedaling.
And I haven't even gotten to the cloudy days...

As promising as a solar panel looked at the beginning, it soon became apparent why we don't see solar panels on bicycles. They might be useful for people who are off the grid, except for the fact that there is not enough space on bicycles and solar is not reliable enough to count on for getting to work every day. If one has access to the grid, the solar panels on one's bicycle only serve to reduce the already miniscule cost of charging and extend the range of the bicycle, except for the fact that the wind resistance probably costs as much energy as the panel produces. If the aim is to make money by lowering the electrical bill, it makes much more sense to install panels at the house, where they can be the larger, more efficient type as well as perhaps being angled towards the sun, or if one is truly off the grid, an array of solar panels and a battery bank would provide electricity for both the household and transportation.

The one situation where solar panels on electric vehicles do make sense is on vehicles with large, flat roofs which can have panels embedded in them. Not in electric cars, because they use on the order of 20 times more energy than electric bicycles, but on the ultralight, aerodynamic, two person vehicles such as golf carts and the vehicle I saw a solar panel on last week.

Something to Show

Over the past couple of weeks, I've been slowly doing work on my bike. I installed and learned Autodesk Inventor, which is surprisingly similar to Solidworks, after trying to use AutoCAD and getting incredibly frustrated. A tip for anyone doing CAD work, the online stores Mcmaster-Carr and SDP-SI publish CAD models of most of their parts, so you never have to model a screw or bearing again. Inventor is pretty awesome, I played with the realistic rendering settings and attempted to use the amazing Finite Element Analysis.

This is a view from the front right of the motor unit, the main power switch and motor controller are the things on the front of the box. You can see the back of the motor through the large hole in the side of the box, and on the motor's shaft on the other side is the small pulley. The belt connects that to the large pulley, the big round thing in the back, which turns the shaft and then the sprocket on the side closer to the camera, which goes to the pedals with a chain.

Making the CAD model forced me to think about this more carefully, and I made a few changes, most notably moving the shaft backwards so it hopefully won't hit your leg while pedaling, and changing the belt tensioning system from motor mounting slots to a spring loaded idler. Last week I ordered the last of the electrical components I need to do some tests, just the motor (which still hasn't shipped yet!) and some connectors.

Moving the shaft back allows the controller to go inside the box, against the front wall, which I think would be a better spot. The hole on the side is for putting the motor through when assembling it, because I'm not sure if it make the turn if you put it through the bottom. I was planning on putting a fan over that hole, but I realized that would conflict with the chain going down to the crank, so right now I'm thinking of just covering it with a thin plate. I moved the motor so close to the bottom that I might not even need to drill the hole, I'll see once I get the motor as the dimensions were a little vague.

I'm still not done with the design, I need to figure out the belt tensioner, cooling fan, and rain/splash covers for the top and bottom that still let air through. I have no idea how to design torsion springs for the tensioner so that'll be fun. Now that I have dimensions to what I'm doing I'm going to start drilling holes in the tube this weekend.

MIT Mini Maker Faire

A couple of weeks ago, on October 4th, I traveled down to Boston to compete in a Combat Robotics competition at the MIT Mini Maker Faire. It was hectic and tons of fun, I met some awesome people and didn't have time for any pictures, but somehow won the Antweight Rumble and came home with Best Rookie. After the competition was over I was able to explore the Maker Faire for a few hours. There were tons of 3D printers, tesla coils and robots, but surprisingly the most common thing people brought were electric vehicles. There was a little racetrack set up for the Power Wheels Racing Series as well as more general EV racing. So without further ado, here are all the crazy things MIT students are building, in order of the number of wheels. Sorry for my terrible camera skills.

Flying Nimbus, a segboard

Following the trend of DerpyBike and Herpybike this is known as eNanoHerpyBike 

There were a surprising number of tricycles

Most of these are made at MITERS, aka the MIT Departament of Silly Go Karts

A seriously overpowered and scarily low RC motor tricycle

DriftTrike is somehow even more scary than the last tricycle. That's half an bike with a hub motor on the front.

What makes it scary are those back wheels. They're casters, meaning they can spin around horizontally, so when you go around a corner they turn sideways.

A very narrow tricycle powered by an electric chainsaw. I don't understand how it goes around corners.

Another Tricycle. The whole red front half tilts side to side for cornering

LOLrio Kart. Yes it does use that wheelie bar.

Check out that custom differential!

Yes, this is a wooden go kart

Chibi-Mikuvan, a miniature 1987 Mitsubishi Delica with a giant RC boat motor, angle grinder gearbox and 2010 Ford Fusion Hybrid battery 

There were also a number of practical vehicles. Scooters are a fairly popular way of getting around MIT because the campus is fairly compact and you can bring scooters into class.

So many scooters

Cruscooter, built in the scooter class taught by Charles Guan, the guy who build LOLriokart, eNanoHerpyBike and Chibi-Mikuvan above

In front of the race course with a dubiously practical motorcycle

All Terrain Scooter - Redux hiding behind a bicycle with a homemade carbon frame

One of the few electric bicycles. The motor in the center of the photo connects to the wheel on the left side of the bike, without going through the bike's gears, so it's neither mid-drive nor a hub motor

EtekChopper, built by the Daniel Gonzalez, the same guy as Cruscooter

Named after the gigantic brushed motor at it's heart

Another motorcycle conversion, a lot more polished and sporting a more efficient AC motor

The trunk of a 1980s Porsche conversion that I forgot to take a full shot of

And under the hood. Where's the motor?

Last but not least, the 5 wheeled recumbent with a trailer full of batteries that goes several hundred miles 

It's another chain driven non-middrive setup. I don't know why these are so popular, they get the worst of both worlds.

There were a lot of fascinating vehicles at the Maker Faire, and these were not even close to all of them, as I only realized I should be taking photos about halfway through. Even more interesting, however, was talking to all of the people who built these, many of whose blogs I've been following, some of them for years. Jamison Go helped me troubleshoot my robot and after the competition was over, I had a lot of time to kill so I spent a while talking to Shane Colton, Ben Katz, and people from the Cheetah and NASA Rover Challenge teams at MIT.

Next up, progress on my bike.

Hello World!

So if you're reading this you probably know me already, but for those of you who don't, I'm Adrian Kelly, a high school senior at the Vermont Commons School and someone fascinated by problem solving, technology, and engineering.

I have another blog that hasn't been updated in a few months, but this one is devoted to my project to build a modular electric bicycle system for my Social Studies class. I chose this project not only because it thought it would be fun to build and ride, but also because climate change and air pollution are going to be increasingly large problems in the coming years. As huge numbers of people, particularly in China and India, rise out of poverty, they need a mode of transportation and cars, even electric ones, are much too heavy and wasteful for everyone to have. As prices have dropped over the last couple of years, electric bicycles have taken off in developing nations as a cheap method of personal transportation, in large part because of the restrictions on the numbers of gas vehicles that can enter Chinese cities each day. From what I've seen, there is an absence of this type of electric bike, that is cheap, relatively easy to install, powerful, and efficient. The final reason I am doing this project is that I think I will learn a lot about the designing and building process, and this is something that I intend to actually use, so hopefully I will also learn how to make this bike more reliable than my projects have been in the past.

There are a lot of electric bicycles in the world now, from the commercial bikes with custom-designed frames to the front wheels you can buy with motor and battery integrated into the hub. I wanted to make it modular so that it is possible to add the motor, gearbox, and batteries to a bike fairly easily and without permanently damaging it, making it accessible to more people and allowing components or even the whole bike to be upgraded easily. The vast majority of ebikes sold today use hub motors, which have the motor built into the wheel because that is easy and cheap to implement, but after a lot of research, I found a different solution that made a lot more sense.

Specialized Turbo S with a hub motor in the rear wheel and the battery hidden in the frame. Specialized.com

Copenhagen Wheel with everything in the hub. Uncrate.com

The basics of my design is based around maximum efficiency and power on a budget. This means using high speed motors going through speed reductions, and then using that to drive the crank, which is known as a mid-drive system. The first half of my design is the fast motor. The hub motors have to spin at the speed of the wheel, on the order of several hundred rpm, which is quite slow by motor standards. The faster motors use more voltage and less current for the same power and therefore lose less energy to heat, and the fast speed, generally 5-20 thousand revolutions per minute, is then reduced to a usable speed and torque with gear, belt, or chain reductions. I am going to use a brushless motor, which is more efficient and more powerful than brushed because there is no sparking on the brushes, and a combination of chains and toothed belts. The only commonly available high speed brushless motors are for the remote control airplane industry, some of which are actually quite large.

Two large brushless outrunners. Charles Guan

The second part of the design is the mid-drive setup. Hub motors are commonly found on electric bicycles because they are fairly simple and cheap, but besides being usually low power they are stuck in one gear ratio. Every motor, like every person, has an optimal speed at which to spin their rotor or legs. Hub motors are like like riding a fixed gear bicycle, meaning that it is difficult to get started or go up hills because you must push so hard and it is difficult to go fast because you must spin your legs so quickly. With the ability to change gears, you can always pedal at the optimal speed, climb hills much quicker and have a higher top speed, as well as the fact that your legs aren't burning as much when you're done. By connecting the motor to the cranks instead of directly to the wheel, I am utilizing the existing bicycle drivetrain to provide the adjustable gear ratios, something that would be tremendously difficult to build.

A homemade mid-drive bicycle. ElectricBike.com

In the last year or so, the electric bicycle manufacturers and high end companies looking to get into the business have started to see the advantages to mid-drive high rpm drivetrains. A number of polished looking mid-drive bicycles have come onto the market recently from companies such as Bosch, Trek, Daum, Panasonic, and Audi but they all cost thousands if not tens of thousands of dollars.

Brose 450w mid-drive with belt. ElectricBike.com

They nearly all have the battery built into the frame and the motor and reduction system built into the crank spindle (called the bottom bracket), things that are not possible without a custom frame.

Trek/Bosch mid-drive with gears. ElectricBike.com

Only a few have released details, but it seems there is a split between using gears or belts for the reduction inside the bottom bracket, with gears being stronger and smaller while belts are cheaper, quieter, and more resistant to dirt and grime.

Exploded CAD view of Brose belt reduction. MTB-News.de

Bosch gearbox cutaway view. ElectricBike.com

I thought up this bike project in July 2013 while taking a CAD (Computer Aided Design, like the diagram above) class at UVM, and came up with a quick CAD model and parts list. That August, I bought a cheap mountain bike (A Mongoose Torment if anyone cares) off Craigslist, and have been slowly working on it slowly ever since. I replaced the shifters and grips because they felt super cheap, replaced the cables while I was at it, removed the decals, and replaced the rear derailleur because it was bent. This summer I bought the parts to make the box that will house the motor and controller, and machined the HDPE clamps that go around the seatpost and can be seen in the picture.

The design in my head and parts list has continued to evolve, and I made some simple CAD models with OpenSCAD, which is horribly inferior for most things compared to software like Solidworks which I learned at UVM, but I need to make a better CAD model so I know everything fits and have something to work off of. This means either getting Windows to work on my desktop so I can install and learn Autodesk Inventor or spending lots of time in the UVM computer lab. I also have a spreadsheet for listing the parts and costs as well as doing gearing and speed calculation.

I need to figure out whether to use Lithium-Ion batteries, which are lighter, more powerful, and store more energy, or Lead-Acid batteries, which I got for free, and also whether to upgrade the chainrings and freewheel or leave the current ones. I'll probably just make it with the cheapest components possible and upgrade later if it's not good enough.

Right now I have a small pile of parts, shown below in my first attempt at knolling. The square aluminum extrusion is the frame for the motor and reduction system, with shaft collars as well as radial and thrust bearings on top. I have five lead acid batteries, 12V 7.2Ah, from old UPS's at my dad's work which have unknown life left, and will choose the best three to use. Also shown is my controller, a Mini Jasontroller from the supremely cheap and somewhat shady ELifeBike.com, and my throttle on the right.  Finally, there's a precharge resistor and switch.

In terms of actually building, I could work on either the electrical or mechanical side first, because neither depend on the other. The electrical side just needs the motor and some large connectors for the power before I can get the motor running. The mechanical side, on the other hand, needs the chain, sprockets, shaft, pulleys, and belt, and I need to get some holes and slots accurately cut in an aluminum extrusion which will probably require a machine shop.

But first, another blog post about all the awesome stuff I saw at the MIT Mini Maker Faire!