I recently got a query from an engineer about an inventor who thought it was a brilliant idea to put a wind turbine on an to charge it while it moves. The engineer was rightly suspicious and sent the idea on over to me, the wind world’s resident skeptic.
Ok, it’s not original. It may be to the inventor but the general concept has been proposed many times. I fielded a similar query when I was working for Vaughn Nelson in the physics department at West Texas State University in 1979. Then it wasn’t EVs, but gasoline-powered cars. It doesn’t make any difference really.
My answer then as now was a simple “you can’t get something for nothing.” It’s a variation on a perpetual motion machine that engineers rightly scoff at.
First a disclaimer. I am neither an engineer nor a physicist, though I’ve studied engineering and physics. I like to think I can at least speak the language.
The idea behind this concept is that there’s a net benefit in powering the car with a wind turbine as the car moves through the wind as it drives down the road. This violates either the first or the second law of thermodynamics or both. It’s been so long since I studied physics I couldn’t tell you which.
Conversion Efficiencies
There are two ways to look at this. First, are the losses due to conversion inefficiencies. The wind turbine can’t capture all the energy in the wind that’s due to the car’s motion. According to Betz’s theorem, a wind turbine can’t capture more than 59.3% of the energy in the wind. (See Betz: Everything You Need to Know about Wind Turbines Was Written in 1927.) In practice, most modern wind turbines can capture only 40% of the energy in the wind over a very narrow band of wind speeds. Thus, we’ve lost about 60% of the energy imparted by the car moving through the air and that the engine had to expend to reach that speed with the wind turbine attached.
In other words, we don’t gain anything. We lose. The wind wasn’t free. We had to create the wind by powering the car through the air.
As Barry Commoner said in the 1970s, “There’s no such thing as a free lunch.”
Increasing Drag
We had to push not just the car through the air, but the car with the wind turbine on it. We’ve increased the drag on the car with the addition of the wind turbine. It’s there to capture the wind and thus it increases the car’s frontal area and associated drag.
Drag is a function of frontal area, the coefficient of drag, and speed. Increase frontal area, the coefficient of drag, or speed and you increase drag. Worse, drag increases with the square of speed. Double the speed and you quadruple the drag.
Drag is bad. Drag is the force that the car must expend energy to overcome to move through the air.
Drag is critical to EVs, less so gassers because their engines are so inefficient. The energy needed to push a gasser through the air is a small part of the total energy expended in a traditional vehicle as so much energy is wasted in heat that can’t be used to propel the car.
In contrast, an EV uses nearly all its energy to move the car through the air. Thus, drag is critical to an EV’s efficiency. This is the reason why analysts spill so much ink comparing the drag coefficient of one EV to another. It makes a big difference in the car’s effective range.
We drive a Chevy Bolt. When the car first came out the critics were aghast that the Bolt had a coefficient of drag Cd = 0.32 due to its boxy shape. The Bolt was clunky in comparison to the sleek Tesla Model 3 with a Cd = 0.23 or almost one-third better. However, the production version of the Bolt has a Cd = 0.308, still more than a Tesla, but better than the critics had thought. Even so, the Tesla Model 3 is a better road car than the Bolt because drag is so important at highway speeds.
Drag is so critical to the efficiency of the Bolt that car nerds argue over whether the utility of roof rails—not a full roof rack, just the rails—justify the increased drag. We opted for a Bolt without roof rails for example.
Imagine then the addition of a wind turbine. The wind turbine has to have a significant frontal area, or “swept area” in wind energy jargon, or it’s of no utility. Thus, the wind turbine would increase drag, robbing the car of the energy needed to push the car and wind turbine through the air despite whatever electricity the wind turbine could generate.
Now Solar, That Has Merit
Putting solar cells on the car are a different matter entirely. They can be incorporated into the skin of the vehicle. They neither increase frontal area nor the coefficient of drag and their performance is independent of speed. They work as well when the car is standing still as well as when it’s moving down the road.
Unfortunately, it takes a lot of energy to move a typical vehicle at highway speeds and that’s a lot more energy than solar cells themselves can produce. Even covering the surface of the car with high-efficiency cells is insufficient to power the car directly.
Solar cells can charge an EV’s batteries while the car is in motion. And therein lies an opportunity to slightly extend the range of the EV with solar.
More importantly, most passenger vehicles sit idle much of the day. It’s during this down time that the embedded solar can trickle charge the EV, gradually building up the charge. If the EV is used periodically for short trips to the grocers, for example, the solar cells could effectively charge the car sufficiently over time to power just such short commuter trips.
There’s a lot of fervent in the field as inventors tool up to build solar-augmented EVs. To make on-car solar charging as effective as possible various groups on both sides of the Atlantic are pushing to improve the EVs aerodynamics and lightening the vehicle to improve the overall efficiency.
Dutch startup Lightyear recently signed an agreement with supercar manufacturer Koenigsegg to build their expensive, but sleek EV. Lightyear claims the car can gain 70 km (40 mi) from the 5 m² of solar cells imbedded in the car’s skin per day based on summer insolation in the Netherlands.
Not to be outdone by the Dutch, German startup Sono Motors has embedded 7.5 m² of solar cells in the skin of its Sion. Some of the Sion’s solar cells are on the vertical panels of the doors as well as the roof and hood (or bonnet to the British). This gives the Sion 1.2 kW of solar capacity in what the company is calling Vehicle Integrated Photovoltaics (VIPV).
Sono Motors claim that the Sion will be able to charge the equivalent of 15-35 km (10-20 mi) per day under German conditions.
The Sion will be produced by contract manufacturer Valmet Automotive.
While Lightyear and Sono’s designs are otherwise conventional four-wheel vehicles, San Diego’s Aptera takes solar-powered EVs in an entirely different direction. Their teardrop shaped three-wheeler looks like an airplane without wings.
Aptera’s claims are also bolder than their competitors. The company claims its two-passenger EV won’t require regular charging during a typical daily commute because of the 0.7 kW in solar charging capacity.
With its front wheel fairings and bug-like shape, the Aptera looks like a tadpole driving down the highway with a claimed Cd = 0.13 or half that of the Tesla Model 3.
None of the solar claims by Lightyear, Sono, or Aptera have been independently verified. Despite that there is promise here.
Running the Numbers
Solar panels with optimal alignment typically can produce ~1,000 kWh/kW/yr at the best sites in Northern Europe or ~1,500 kWh/kW/yr in Central California.
Lightyear doesn’t report how many kW its 5 m² of solar cells represents, but if it’s proportional to Sono Motors’ Sion, it should be ~1 kW. One kilowatt of solar cells in an optimum alignment should be able to generate ~1,000 kWh/yr in Northern Europe and ~1,500 kWh/yr in Central California.
Solar cells in the skin of an auto will seldom be in optimum alignment. We don’t know how well they’ll perform in practice, but let’s discount their maximum production 50%.
So a car like the Lightyear could produce ~500 kWh/yr in Northern Europe and as much as ~750 kWh/yr in California.
How far will that get you? I can get about 4 miles per kWh in our Bolt. (Note that this is more than the EPA rating.) That would get the Lightyear 2,000 miles in Germany, say, or ~3,000 miles in California.
The Bolt is far from the most efficient EV, but it is relatively small. Even so, lightweight, aerodynamic vehicles should be able to do much better than the Bolt or even a Tesla Model 3.
In a recent Youtube video of the Aptera by Fully Charged (see Aptera – this Hyper-Efficient ‘Car’ Costs HALF as much as a Model 3!) the reviewer says the Lightyear can get 7 mi/kWh while the Aptera claims to achieve 10 mi/kWh.
None of the claims by Lightyear or Aptera have been confirmed by an independent testing laboratory. Let’s temper the claims somewhat and assume that these new solar cars can get 6 miles per kWh or an improvement of 50% on our Bolt.
If these assumptions are reasonable, the Lightyear would be able to drive ~3,000 miles per year on solar alone in Germany and as much as ~4,500 miles in California.
On average, you could drive the Lightyear 8 miles (13 km) per day in Northern Europe and 12 miles (20 km) per day in Central California on solar alone. Of course you could drive further in summer, but less in winter.
The gist? Solar in the skins of EVs makes sense. Will the solar cells on the EVs from these startups meet their projections? Probably not, but the solar electricity they can generate represents a not insignificant amount of the electricity needed to power an EV for an entire year. For our use case, solar embedded in the skin of an EV could meet from 30% to 40% of the 10,000 miles (15,000 km) we drive per year.
Now we just need some real world experience to show just how much electricity these vehicles can actually produce and how efficient they really are.
