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.