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A solar panel on a robocar

I often see people say they would like to see solar panels on electric cars, inspired by the solar-electric cars in the challenge races, and by the idea that the solar panel will provide some recharging for the car while it is running and without need to plug it in.

It turns out this isn’t a tremendously good idea for a variety of reasons:

  • You’re probably not going to get more than a couple hundred watts of PV peak power on a car with typical cells. Even properly mounted on a roof in a sunny place like California, each peak watt delivers an average of about 5 watt-hours in a day, so 200 watts gives you 1kw-h. That’s good for around 4 to 6 miles on today’s electric cars. Not a huge range boost.
  • While thin film panels don’t weigh a lot the power they provide during actual driving would normally be only a minor boost. My math suggests they weigh more than the battery for the power they will provide while operating.
  • Panels on a car will instead be mounted flat, cutting about 30% of their output. Normally you want to tilt to the angle of the sun.
  • Cars are often in the shade, even parked indoors. Unless you work to pick your parking to have sun all day, you’ll only get a fraction of the power.
  • If you do leave your car in the sun, in many places that means it will get quite hot, you’ll burn up some of the solar energy cooling it down. (Indeed, the solar panels sometimes found on today’s hybrids and EVs don’t charge the battery, they just run a cooling fan.)
  • The worst one: If your battery is not somewhat discharged, it doesn’t have any place to put the solar energy, and so it is just thrown away. But due to range anxiety, people prefer their electric cars be kept full. It takes careful planning to use that energy.
  • A car is a very bumpy place, so you need more rugged panels than what you might put on a roof.

It is possible to get more than 200w on a car — some of the solar challenge cars that exist to be nothing but panels have gotten around a kw by using high price, high-efficiency panels. But it’s still generally much better to just put the panels on a roof where they will realize their full potential, and feed the grid, and charge from the grid.

However, on Friday I was teaching a class on the future of Robocars to my students at Singularity University and in the exercises some students wondered if they might do something for solar powered cars. (I was impressed since the students, having had only a short time to think about the issue, have to work to bring up something new.)

Robocars might solve some of the problems above, and thus possibly make sense as a place to put panels.

  • A robocar parks itself and can move. So one with a solar panel can move around to make sure it’s always in the sun, and that the sun is striking it from the right angle. It can’t move too far or too often without wasting some of the power, but it can do something.
  • When the batteries get so full that they are not making proper use of the solar energy, a robocar can find a charging station, not to charge but rather to sell excess power back to the grid and other cars. (This presumes charging stations are set up this way.)
  • Robocars could dock with other robocars that are more discharged and offer them the extra solar power, no charging stations involved — though fancy robotics are needed on the charging interface, or human beings who can do the connections.
  • If a robocar has an actuator that can tilt the panels, it can do even better. While an ordinary car could have this, an ordinary car would not have the ability to rotate in the plane of the ground to track the sun without another actuator.

It’s still not great, but it might improve things. Generally it still may be better to have the panels on rooftops and get the most from them. However, when we start thinking about super lightweight cars, cars that travel for under 100 watt-hours/mile, as well as higher efficiency panels, we might get some value if the panels are light.

It’s also expensive to install panels on top of existing facilities. Turns out that while panels are dropping below 1$/watt next year thanks to cheap Chinese capital and manufacturing, the cost of install is still over $2/watt. Cost of install on newly manufactured buildings — or cars — can be cheaper because it’s designed in from the start. The car already has the complex electrical system, while houses need to add them if they go solar.

People really are in love with the idea of a solar powered car. It’s not really possible to go green this way right now, but the future might bring something interesting.