My daughter is trying to power a remote control car that currently uses 4 AA batteries as power. She is trying to convert it to solar. What does she need to do so?


Solar cars are really hard because solar panels are so inefficient.

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The energy in sunlight varies greatly by latitude, weather, and time of day. On a clear day, at solar noon, at equitorial latitudes, solar power at Earth"s surface can be a little over $1000:mathrmW/m^2$. However, it"s much lower at other times of the day and latitudes. For a lower bound, the WMO defines direct sunlight as anything above $120:mathrmW/m^2$. Clouds and smog of course can mean even less.

Additionally, we don"t have any really efficient converters of sunlight to electrical energy. Even the most efficient seem to be about 35%, with the types that aren"t research projects or only available to NASA coming in around 15% efficient, according to some quick research I just did.

So let"s assume this car is big enough to support a panel the size of an A4 sheet of paper. That"s $210:mathrmmmcdot 297:mathrmmm approx 0.06:mathrm m^2 $ of area available to collect light. How much energy is available? In the worst case:

$$ 0.06:mathrm m^2 cdot 120 :mathrmW/m^2 cdot 0.15 approx 1W $$

In the best case:

$$ 0.06:mathrm m^2 cdot 1000 :mathrmW/m^2 cdot 0.15 approx 60W $$

What can we do with about one watt? Maybe it"s best to compare to the battery in the car. It looks like the 7.2V battery packs used in many RC cars has a capacity of $3000:mathrmmAh$ (three amp-hours). We can get a very rough idea of the total energy in that pack by multiplying the voltage by the capacity:

$$ 7.2:mathrm V cdot 3:mathrmAh cdot 3600:mathrm s over 1:mathrm h = 77760:mathrm J $$

A watt is one joule per second. So, the rate of energy available from the solar panel is about the same as if you drained that battery constantly over a period of 77760 seconds or 21 hours. How slowly would you have to drive that car to make the batteries last 21 hours? That"s how fast it can go with the rate of energy available from the solar panel.

Using the best case of 60 watts is somewhat more favorable, but it"s a fair guess you will want the car to work in less than optimum conditions. In practice of course you will get something in between.

With some very careful engineering you can get something that works, but it"s never going to zip around like a battery-powered car can. It helps a lot to point the panels directly at the sun, but to keep them pointed at the sun as the car drives would require a very complex mechanism. Additionally you"d probably want to go through every part of the car and make it as efficient as possible. Minimize rolling resistance on the tires. Minimize any transmission losses. Use a very efficient motor. Make the car as light as possible, etc.

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These are rough calculations, of course, but serve to demonstrate the point: solar cars are hard. If they were easy, everyone would be driving them.