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Is it possible to use solar panel(/s) as primary power for an electric bike, HPV or light velomobile (without batteries, mandatory assisted mode only) ? Are there any limitations or obstacles ? Example could an "solar backpack", panel 300Wp 1.6x1m size, weight <20kg help to keep at least reasonable constant speed on a flat road. There could be 5x output power difference depending on sun light, season, etc. (home installation webs) Suppose a single vehicle - no large separate trails, etc.

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  • Possible or practical? All things possible, I think you are underestimating just how big 1.6x1 meters is when talking about trying to mount it on a normal bicycle without trailer in a way that catches the sun but does not turn into a sail. Velomobile sure, but pretty much everyone uses a battery for good engineering reasons, would be more like for the sake of showing you can? – Affe Jun 24 '20 at 16:52
  • There were, in the past, several "competitions" for solar-powered vehicles, though most relied in part on batteries -- you might want to research those. Your scheme might sorta work on a recumbent tricycle or some such, but the weight, wind resistance, and effect on balance (especially in a crosswind) would make it quite impractical on a standard bicycle. – Daniel R Hicks Jun 24 '20 at 17:34
  • Maybe enough for assistance only. EON says there is aprox. 140kWh/m2/year. It would be 613Wh/day/panel - eScooter can run more than hour from whole day sun ? Another web say 1500 working hours per year and 2/3 only 30% as not best oriented. That would be 4h/day, 1 panel may produce 50-150W depending on orientation ? – Tom Jul 5 '20 at 18:59
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A human produces 100-300 W (100 W is trivial even when exhausted, 300 W can be produced only up intermediate-length hills). Unless you want the bicycle to be unstable up hills, you want the full 300 W up steep hills. Solar power constant is about 1000 W / square meter, when there are no clouds. Of those, common silicon solar panels produce about 200 W / square meter, the rest is losses. You need more than a square meter of panels and continuous sunny weather to get up hills at any reasonable speed.

If multijunction cells are an option (expensive), you get 400 W / square meter perhaps. So, less than a square meter is enough in sunny weather. You however need to handle cloudy weather somehow.

My advice? Use multijunction cells that track the sun and don't ditch the pedals. The pedals come handy when it's cloudy. I would also advise to not ditch the battery. It helps you up hills, so that even 100-150 W average power produced is enough.

A problem is where to mount the cells and how to ensure they track the sun when you turn. If they don't track the sun, you need more cells.

Also, how to ensure that the solar cells don't cause excessive air resistance is a problem. The mount needs to be very sturdy so that they don't fly away in the wind.

I think you'll find that common backpacks rarely have more than 0.2 square meters of area. Multijunction panels that are 0.2 square meters produce only 80 watts when they are perfectly oriented and there are no clouds. A backpack most of the time is not perfectly oriented, and often there are clouds.

A better approach would be to connect the solar panels to electricity grid (then cheap single-junction silicon cells are enough) to produce hydrogen during the daytime. The hydrogen is then compressed. During the nighttime, when you sleep, the battery of the e-bike is charged using fuel cells powered by the compressed hydrogen. With large enough battery, the system might be enough for riding one day at a time.

You can also refine the grid-connected idea by adding wind power to complement the solar power during windy but non-sunny (cloudy) days.

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  • +1. The only practical application is if you are travelling on an e-bike in a sunny area and have lots of time off the bike during noon to optimally position your solar panels (at least 0.5m^2 of them) to charge your batteries. Just draped over your backpack or rear rack while riding the efficiency will be too bad. – Michael Jun 25 '20 at 5:44
  • My idea was a 3-4 wheeler, 3-4 panels on top and sides (ev. back), longer side in drive direction (>1.6m long, >1m width - minimals based on standard panel size). There must be pedals anyway (law). Such setup can give interesting additional power and may not increase drag. 60+ kg (panels) vehicle + driver, ramp areas on sides will probably not fly away in a wind. But there is nothing like that available (?) except some "Sci-Fi" lab vehicles. Something simple shaped like Tesla Cybertruck ;-), no batteries, pedals, it would be bicycle by law. – Tom Jun 25 '20 at 8:53
  • See as soon as you talk about multiple panels you have multiple power sources at different voltages with different current sourcing capacity to somehow reconcile into a single motor controller. The common solution is to put that energy into a battery which is a stable and predictable energy source that well understood motor controller designs know how to use. Totally feel you on the fact that batteries are also made out of finite stuff mined from the ground! But you are talking about doing some boundary pushing engineering here, not wiring up stuff you can buy off the shelf :) – Affe Jun 25 '20 at 16:39
  • In case you would have education in this area, you know it is even common to connect multiple panels in series, but it may not be so efficient. Final efficiency depends also on how much electronics you use. In theory classical motor can stand wide range of voltages, but there is overal power limit. I was not thinking about details, but in case you use separate driver for each panel and another for motor... Basic question was is average energy produced by panel(s) and consumed by motor worth their weight, size, drag, etc. – Tom Jun 27 '20 at 14:36
  • @Tom I think you'll find that today, even cheap silicon panels are used with maximum power point tracker devices. I know this because I have invested some money into solar power companies, and one of my investments is SolarEdge that makes those devices. – juhist Jun 27 '20 at 18:13
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There is a race, the Sun Trip, for solar-powered bikes (and there have been a few other one-off efforts). Looking at the entrants, there are some that use huge PV roofs to avoid using trailers, but as @Affe points out, those could turn into sails.

Most e-bikes use 250-W motors (if not larger). Depending on the speed you're satisfied with, you could get away with half that power. Weight isn't what will make it hard to maintain a decent speed with these: aerodynamics are vastly more consequential, and a big flat PV array will make it hard to optimize aerodynamics.

Depending on how many hours of the day you're riding the bike, how many hours your PV array is in the sun, and your battery capacity, you may be able to run the bike without ever plugging it into a wall…but you could extend that logic to using a stationary PV array as your power source and running only off batteries—that is, a regular e-bike—when on the road. This might turn out to be more efficient than trying to charge while you ride due to the inefficiencies of having a bike-mounted PV array.

My own take is that solar-powered bikes are possible as interesting experiments, but impractical for day-to-day transportation.

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