Sunday, July 19, 2009

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.


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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.