Why don't we have solar cars?

Wherever I give a speech about the automotive industry someone always asks about why we don't have any solar powered cars other than those fragile things we see competing for scientific prizes each year. I enjoy this question because it allows me to utilize the knowledge I gained during 25 years working in the laser applications field. The physics of lasers is the physics of light, so we laser-jocks got to know a lot about the subject.

While solar power is a very real thing and useful in certain applications, don't confuse the physics with the hype. Allow Albert Einstein, if you will, to go over the basic physics of the sun's energy as it hits the earth. He received the 1921 Nobel Prize for physics not, ironically, for the Theory of Relativity (arguably the most important scientific breakthrough of all time - no pun intended with the "time" thing...) but for his discovery and explanation of the photoelectric effect.

Einstein explained that the sun delivers approximately 1,000 watts of total energy per square meter (roughly 100 watts per square foot) on the earth, and that's really only when there's direct light, at noon, on a clear day. He demonstrated that photons (the sun's energy output consists of photons of all wavelengths, from X-rays to radio waves) hitting the earth can be converted to electrons if they hit the right kind of atomic surface, such as today's photovoltaic cells. Einstein, as you know, was a very smart guy.

Anyway, if you could convert all that solar energy to electric power you'd need 7.43 square feet for each horsepower (there are 743 watts in a horsepower) in your motor. You need at least 50 horsepower (37,000 watts) to safely move a car in real-world traffic, so you'd need at least 371 square feet of surface area to generate the electricity. That's a square about 19 feet on a side, so your car would have to be very large or have a huge solar sail on it to capture the light.

It gets worse, because solar photovoltaic panels waste most of the sun's energy. The best solar panels on the market today are less than 20% efficient at conversion of energy, so you really need panels 5 times larger than the one in the example above to create enough electricity to run the car. Remember also that we're talking about "perfect" conversion of energy at midday when it's clear outside. As the daylight goes down so does the amount of electricity. If this isn't difficult enough, how do you compensate for those periods when the car is driving in the rain, cloudy weather, through tunnels and at night? What we've got here is a fundamental problem of capacity: There's simply not enough surface area on a car to generate sufficient power from photovoltaic cells.

Add to these pressures the fact that photovoltaic cells cost at least $6 per watt of output, making these things prohibitive for most people even if size weren't a consideration. As you can see, solar electric vehicles are a complex problem of physics, engineering and economics that have no foreseeable solution.

Are there any ways around these limitations? Well, solar panels on cars might be utilized to help power accessories, thus relieving the alternator from consuming engine power. This would result in slightly higher fuel mileage. Hood, roof and trunk panels could have their output directed to the battery  - or battery pack, in the case of electric cars - and could help partially charge the system. A typical car's surface area is around 60 square feet, so at 20 watts per square foot it's possible to generate 1,200 watts (a little less than 2 horsepower). That's not much, but it's enough to consider if the devices become more cost-effective.

That doesn't mean we shouldn't work on any - and every - potential alternative fuel or drivetrain we can envision. Cars of the future will certainly be electrically powered, be it from fuel cells or other energy storage devices, because dwindling fuel supplies and escalating pollution concerns will kill the gasoline engine. I'm betting that by 2050 the only large-scale use of internal combustion engines will be in heavy trucks, and even those will be diesel-electric.

Albert would be very happy about that.

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