Is a Self Powered Solar Car Realistic? Nope!

Apr 28, 2016

I was having a friendly chat with one of my pro renewable energy friends. Mind you, you wouldn’t tell by reading some of my blog posts but I happen to be pro renewables. My only requirement is that any future source of renewable energy be continuous, readily available, and affordable. Solar and wind power fail miserably in all three categories but I digress.

Solar Car

My friend suggested that soon we’ll be able to coat an electric car with a polymer photo voltaic (PV) paint and supply the car with all the energy it needs. With that statement, it was time to bring him back to our planet. It can’t be done and it will never be done. Mean old Mr. Physics guarantees that statement.

There are two basic theoretical limitations in converting sunlight into electricity in solar panels. First, is the limitation of the energy of sunlight reaching the ground. The energy works out to around an average of 1,000 Wm2. Assuming 100% conversion efficiency, the perfect PV cell would require an area roughly 3 ft. x 3 ft. to produce 1kW of power. Over an hour the PV cell would produce 1 kWh (kilowatt hour) of working energy - what the EPA uses to measure electric car efficiency.

However, there is a theoretical limit in the conversion efficiency of a PV cell. It was first calculated by William Shockley and Hans-Joachim Queisser in 1961 that the maximum conversion efficiency of a PV cell is 33.7%. This is known as the Shockley Queisser Efficiency Limit or SQ Limit. [1] The limit is one of the most fundamental to the solar energy industry, and is considered to be one of the most important contributions in the field.

Modern commercial mono-crystalline solar cells (the most efficient available) provide about 24% conversion efficiency. Experimental Polymer-fullerene bulk heterojunction solar cells, still in the lab, have a conversion efficiency of 8% with a possible efficiency of around 10%. [2]

We made it through most of the technical jargon. Let’s try to put the above into practical terms. The Tesla Model S - one of the more practical electric cars in my opinion, has an EPA-rated efficiency of 350 Wh/mi, which happens to convert nicely to roughly three (3) miles per kWh. Say that we paint the Model S with some future polymer PV paint with a conversion efficiency of 10%. The car would need a surface area facing the sun of 10,000 square meters or 30,000 square feet to produce enough energy to drive it three miles. That’s assuming you don’t mind driving at three miles per hour. That, or parking the car for an hour every three miles, which gets you the same distance down the road either way.

The last I checked, your average 4 bedroom, three full bath home is around 2,200 to 3,000 square feet. The car would have to be 10 times the size of a house but remain the weight and have the aerodynamics of a Model S. A car or truck that size wouldn’t be allowed on the road.

My friend felt that I had no faith in future technology. No, I have much faith in future technology. I’m a technologist and well published researcher. I think that future technology will ultimately solve our need for endless and affordable power but I try more than my friend to stay grounded in reality. If you don’t define the problems, there will be no solutions. I don’t present the problems to be a nay sayer. I present them to put them front and center to be considered and solved. It’s the dispassionate researcher in me.




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