Every morning I scan the headlines while sipping my coffee. This one - The Next Car You Buy Could Be Solar Powered - jumped out at me. Since my freshman students and my non-engineer friends often ask about solar powered cars, I decided it's worth writing about, if only to cut through the hype.
Prius with Solar Roof. Image Courtesy of Toyota
A Solar Powered Prius?
First, the headline in question was about the latest model of Prius, which integrates a 280 Watt solar panel into the roof. (The option is only available in Japan.) But if you look into the details, you'll see that the solar panel doesn't power the car directly, it simply helps charge the battery; under full sun (best case scenario), it could add about four miles to the car's range. FOUR MILES. Sorry, but that's not a solar powered car, and the extra four miles isn't likely to ease anyone's range anxiety.
It's All in the Physics...
The amount of sunlight reaching the Earth's surface is about 1000 Watts per square meter. Actually, that's a best case scenario that occurs when the sun is directly overhead. If you live in the tropical regions, that happens one day out of the year, at solar noon. For most of the planet, the rule of thumb that we use for estimating solar power is around 800 Watts per square meter. Again, that's the power reaching the Earth's surface in full sun.
… and the Math
Suppose we have a car with a combined area of six square meters on the roof, hood, and trunk. On a good day, about 4800 Watts of solar power will shine on the roof around noontime. Just for the sake of argument, let's say we have a solar panel that's 100% efficient, so all 4800 Watts could be used to power the car. You're probably not used to the term "Watts" (W) with respect to a car's power. That's because W is the unit of electrical power, where mechanical power is usually expressed in horsepower (hp). Here's the conversion:
1 hp = 746 W
Back to our hypothetical car with its perfect solar panels that deliver 4800W of power to the motors. Let's further assume that the motors are 100% efficient. How many ponies are under that hood?
4800W / 746 W/hp = 6.4hp
For comparison, my riding lawnmower has an 18hp engine. I've never measured it, but I'd guess that its top speed is about 10 mph.
The reality, of course, is worse. Nothing is 100% efficient; reasonably priced solar panels come in around 20% efficient, turning that 6.4hp into a mere 1.28hp
Solar Assisted Electric Vehicle (SAEV)
To be accurate, the Prius is a plug-in hybrid vehicle whose electric drivetrain is powered by battery. Solar is one option for charging the battery bank, making this a solar assisted hybrid electric vehicle, not a solar powered car. I suppose you could argue that the car could be charged using a home's solar array or a solar carport, but it's still a battery powered hybrid EV.
Will It Ever Be Feasible?
While Toyota engineers are offering a solar option that extends the Prius' range by about four miles, European engineering students built an EV whose solar panels can add up to 220 miles to the car's range, proving that although a solar powered car isn't feasible, a practical, solar assisted EV is, even with current technology.