Solar Roadways for Rocky and Bullwinkle

The solar roadway is as impractical an idea as running cars on water or powering phones and computers with urine. These ideas are appealing, because we would like to think we can keep our industrial economy going without killing people to keep the oil flowing, or otherwise wrecking the earth. In reality, they are as as bad as the idea that vaccines are useless, except for causing autism.

The basis for a viral crackpot idea is that it sounds plausible to people who tend to believe what they read on the internet and see on TV. It’s much a much better practice to check the facts behind a plausible-sounding claim, and do a little critical thinking.

I’m an engineer who retired from over 30 years manufacturing solar panels. My inclination is to support solar panel applications that are known to work, and not to waste time or money on applications that are likely not to work. Solar roadways, if any are ever built, will be bad as roadways and poor producers of electricity.

First off, there has been no demonstration of a solar panel which will actually work as a replacement for paving. But let’s put that aside for the time being, and get some estimate of what it might cost if it is possible to manufacture a panel suitable for the purpose. Some math is unavoidable, but we can use round numbers and make it fairly painless.

Maximum Earth surface insolation equals approximately 1000 watts per square meter. That’s how much power the sun delivers through the atmosphere at high noon, directly below the sun. That’s what we have to work with.

Solar panel efficiency of 15% equals approximately 150 watts per square meter. There are roughly 10.6 square feet/square meter, so that’s about 14 watts per square foot.

A standard solar panel costs on the order of $14 per square foot. That’s a dollar per watt at 15% efficiency. A standard panel can actually be cheaper, but there’s no way a standard panel will last a minute with trucks driving over it. A paving panel will cost more than a dollar per watt, certainly not less.

Installation on a roof or ground mount is approximately equal to panel cost. Wiring for use also is approximately equal to panel cost. Installed cost then is approximately equal to $42 per square foot. Again, costs for a rooftop panel can be less, but the installation is for wind and rain, not for truck traffic.

Large-scale installation and mass production would tend to drive cost down. So would more-efficient panels, but there is a limit to efficiency, and whatever the starting efficiency, driving on the panels is not going to make them better. Design and production of panels robust enough for cars and trucks to drive on would tend to drive cost up.

In round figures, $40 per square foot is a reasonable starting estimate for the cost of replacing normal pavement with solar panels. It could be much more if design and build is especially difficult, but it could not be much less. We can put the range of possibilities at $30 to $60 per square foot. If the cost is significantly more than $60/square foot, solar roadways will not be built at all.

Freeway lane width equals 12 feet. A minimum freeway equals 2 lanes each direction. Pavement 48 feet wide x 5280 long x $30 to $60. This yields a cost of $7,603,200 to $15,206,400 per mile. In very round figures, that’s on the order of ten million dollars per mile of freeway, just to replace paving with solar panels.

Average daily production in watt-hours in Michigan is roughly 4 x maximum rating for a fixed solar panel oriented south and tilted up 45 degrees. A roadway surface is approximately flat, so the “4” becomes a “3” or less. For something like $10 million per mile, you get 48 x 5280 x 14 watts/square foot x 3 or less. That comes out to about 10 million watt-hours per day, or 10,000 kilowatt-hours – and that’s with no cars driving on the road during daylight hours.

10,000 kilowatt-hours is a power production equivalent to 2-3 thousand residential solar roof installations. Roof installations are something we know we can do for the same amount of money or less, with standard panels that are manufactured now, panels that are expected to last for 30 years or more with little or no maintenance needed.

Every car or truck that is on the road casts its shadow. That blocks production of 2000 to 5000 watts (varies with size of vehicle and angle of the sun) for however long the car is on that stretch of road. That is, the more the solar highway gets used as a road, the less electricity is produced. The pounding from cars and trucks ruins non-solar paving surfaces in 20 years, and applications of salt in the winter in Michigan makes the lifetime of pavement closer to 10 years. Salt is going to be especially bad for corrosion of electrical contacts and such. There’s no reason to believe solar roadways could last longer than 10 years.

The two uses – roadways and solar panels – contradict each other in more ways than one. For instance, a smooth glass surface is good for easy cleaning of solar panels. That would be terrible for traction – but a rough surface, good for traction, would collect dirt, lowering the panel’s electrical production.

Overall, from a practical engineering point of view, the solar roadway is just another bad idea. It might work in a cartoon, but not in the real world.

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