Fake solar bike exposed

Bottom line: These "solar panels" are fake. Even if such wheel-disc solar panels were possible, they would require nearly 20 hours of charging for every 1 hour of riding.

I've been contemplating publishing my impressions of this "solar powered" bike since I started this blog over a year ago. I've held back because I didn't think I could talk about it without coming across as petty and jealous. In the interest of fair play, I'll start by discrediting myself and confessing that I'm cranky about this project being the top Google result for the query term solar bike. I don't think it deserves this level of attention.

Here are two major design problems the inventor would need to overcome to build a working prototype.

1. How do you transfer power from the moving solar cells in the wheel to the stationary forks? Magnetic induction? Inefficient and heavy. Brushed commutator? Inefficient, vulnerable to dirt and moisture. Hub motors solve this problem by having the wires attached through a hollow stationary axle and the permanent magnet parts of the motor (armature) rotate around the electromagnetic windings. This doesn't work if your current source is moving and your batteries are stationary. And it certainly doesn't work if you have a quick-release mechanism on the wheels like the bike pictured. The skewers in the center of the hollow hubs are right where the wires would have to be. Now that I think about it, the bike pictured above cannot have a hub motor hiding in the wheel (as the inventor claims) because hub motors cannot use quick-release levers.
2. Where do you get photovoltaic thin film which can be cut into a circle? I'm not saying it couldn't be manufactured but I attended the InterSolar trade show in San Francisco three weeks ago. 444 exhibitors showing off the latest solar technology and I didn't see one product which can be cut like this. In the Discovery Channel video on this page, the inventor holds a real PowerFilm thin film cell in his hand but the shiny metallic film in the wheels appears to be cheap reflective mylar held together with scotch tape. But it was on TV so it must be real, right?

Over two years have passed since this tantalizing photo surfaced and the website still doesn't offer to sell anything resembling the bike with the shiny wheel discs. The owner appears to be offering a cheap Chinese e-bike with a 200 or 300 watt front hub motor and a 5 watt solar panel strapped to the rear rack for a bargain price of $1800. A comparable ebike sans solar sells at Wallmart for under $300. Something tells me the shiny wheel idea didn't work out.

But let's be generous and assume that our intrepid inventor somehow overcame these two engineering hurdles. The numbers show that this idea still doesn't work. For the purposes of this exercise, I'll define "work" as having a design where the solar charging adds enough value to the non-solar enhanced ebike to justify the added cost and expense.

Let's start by calculating the surface area of the wheel discs. Measuring one of my 26" wheels, the inner diameter is 21". The area of a circle is πr² so we have 3.14 x (21 inches/2)² = 346 in² = 0.223 m². I'll be charitable and pretend that the parts of the panel obscured by the front fork, chain stay and seat stay don't reduce the panel's output.

At 1000 W/m², we would have 1000 x 0.223 = 223 watts theoretical output at 100% efficiency. Thin film modules have efficiencies from 3.8% to 5.9% (see the full-screen table in this post for details). Using the best-case scenario of 5.9% this gives us 223W x 5.9% = 13 watts per wheel or 26 watts total. Note that I am not counting both sides of each wheel because only one side can face the sun at any given time.

Now, let's consider the vertical orientation of these solar panels. Using the U.S. Department of Energy redbook data on flat-plate solar collectors for my locale (San Francisco Bay Area), we get an average of 2.7kWh/m²/day for a 90° tilt collector facing South. That means our shiny wheel discs would produce 2700 Wh x 0.223 m² x 5.9% panel efficiency x 2 wheels x 90% charging efficiency = 64 Wh/day. At an estimated 20 Wh/mile for a gearless hub motor on flat ground without pedalling, this gives us a whopping 3 miles on an all-day solar charge. Maybe twice that if you pedal.

If you lay the bike flat on the ground (0° tilt) from sun up to sun down in the summer, you get an average of 7.1 kWh/m²/day or 7100 Wh x 0.223 m² x 5.9% panel efficiency x 2 wheels x 90% charging efficiency = 168 Wh/day. Good for about 8.4 miles.

So there you have it, 3-8 extra miles per day under average charging conditions. You'll get a little more on clear, sunny days and a lot less on cloudy, rainy days. Put another way, you need 15-20 hours of charging for every 1 hour of riding. If this was your only solar option, your money would be much better spent on upgrading to slightly larger and/or more efficient batteries and skipping the shiny wheel discs. Luckily, there are many other solar options out there.

Security warning: The ordering page on this site is not secure (does not use SSL). Unacceptable. Never order anything online if you don't see HTTPS at the beginning of the page address.

Best solar panels for your solar powered bike

I've recently been asked to recommend solar modules for use with a solar powered electric-assist bicycle project. It's not the first time the question has come up so I decided to organize some of my old research, do some new checking around. Here's my ranked list of about 20 different solar modules which may be suitable for a bike. They are ranked by weight, cost and size.

View full screen | download as MS Excel file

Short Answer

Get the SunWize Sol-Charger SC24-12V. It looks rugged, is relatively light, small and is priced reasonably well for this kind of module. Here is the spec sheet in PDF. I don't have any hands-on experience with this product but if anyone out there takes my advice, please let me know how it works out for you.

Long Answer

There's a confusing array of solar products out there. Most of them making the same claims about being "high efficiency" and whatnot which makes it difficult to sort them out. I approached this problem by identifying the following criteria:

• Weight If you're putting a solar module on your house or boat, you're probably not very concerned with how much it weighs.* But if you're going to be hauling it around on your bicycle it really makes a difference when your batteries run out and you need to pedal home up a long steep hill. I ranked all the different modules I found by how many watts you get for each lb/kg of module you have to haul around and found a huge difference between the best and worst examples. At the top of the list was the PowerFilm 28W rollable thin-film module with nearly 16 watts per pound (34 watts per kg) but at 3.75% efficiency it was the worst of the bunch in terms of size. A lower efficiency module needs needs more surface area than a higher efficiency module to produce the same amount of energy.
• Cost If cost is your primary concern you should have no trouble finding solar modules for under $3/watt on eBay. The trade-off is that your module will weigh about 4 times as much as the winner in the weight category above. If you're carrying 100W of solar panels, this difference adds up to 18 extra pounds. This is because the high volume production lines that make these lower prices possible churn out solar modules built around a big piece of heavy tempered glass surrounded by a chunky aluminum frame. It's a cost-effective solution for use on a roof but is far from ideal on a bicycle. If after reading this you are still considering glass/aluminum framed panels, pay extra attention to how you mount them to your bike. You will need to protect them from vibrations and mechanical shock.
• Size Solar real estate is scarce on a bicycle so the more watts per square foot/meter your module can produce, the better. The highest efficiency modules available are very large 200+ watt Sanyo N-series HIT modules and SunPower's slightly less efficient modules. Both are about 16-18% efficient and too big and heavy for bicycles. If you can get your hands on some Sanyo or SunPower cells and build your own lighter module, that would give you the best of all worlds.

I hope some of this was helpful. Leave comments if you would like clarification on any of this and I'll be happy to update the post.

* Yes, I know about concentrated point loading and rafter spans in light frame construction.