Cars powered entirely by solar cells sound like a dream come true, but physics and power numbers may indicate a different scenario.
All-electric vehicles (EVs) are a hot topic these days in terms of design, manufacturing, and sales for many reasons. While only about two percent of the cars sold in the US in 2020 were pure EVs, that number is expected to increase significantly over the next five, ten and twenty years. How much that increase will be is anyone’s guess, as the crystal balls of the market analysts and pundits who make these forecasts are all over the place, with predictions ranging from a modest gain to a significant one.
But why just think about EVs which are recharged via the grid or an advanced home solar/hydro/geothermal system? Why not go all the way and put solar cells on the roof of the car, thus potentially eliminating the need to connect anything? It’s personal, it’s portable, it’s the ultimate type energy harvesting from a never-ending free source—what could be better than that?
As ambitious as it sounds, a few companies are working on cars which charge via such panels (they also include an on-board charger for conventional EV charging as well). These vehicles are not the ones you may have seen for the well-known Australian World Solar Challenge which has been running for over 30 years with three classes of cars (Figure 1), none of which are even close to “street legal.”
Figure 1 The Australian World Solar Challenge has been running every two years since 1987; its entrants are one-of-a-kind and nowhere near street legal. (Image source: Australian World Solar Challenge)
Among the companies working on viable solar-powered cars are Lightyear (based in the Netherlands) with their Lightyear One (Figure 2). It has about 50 square feet of solar cells (4.6 m2) and four lightweight electric motors (one in each wheel) to minimize weight and extend range, rather than a single motor and gearbox.
Figure 2 The Lightyear One looks like a conventional sedan, but it is capable of fully self-contained solar charging. (Image source: Lightyear)
The Lightyear One prototype claims a range of more than 440 miles (700 km) on a full charge; a full day in the sun provides a range of a little over 40 miles (64 km). If you are wondering “how much does it cost?” and “when will it be available?” the answers are “$175,000” and “next year” for production. Before you say “that’s crazy,” the company claims that more than 160 vehicles have already been reserved in Europe, with most of them paid for upfront (US sales are planned for a later time).
Another developer of these solar-powered vehicles is San Diego-based Aptera Motors Corp. with the Luna, a two-person, gull-wing, three-wheeled vehicle (Figure 3). With about 24 square feet (2.2 m2) of solar cells, they claim a range of up to 40 miles after a full day of charging in the summer sun; the 350-V battery pack is supposed to be good for 250 to 1,000 miles on a full charge depending on pack capacity, which can range up to 700 W-hr), whether from solar or grid.
Figure 3 The Aptera Luna is a three-wheel solar-powered vehicle which claims a 40-mile range on a full day of optimum charging. (Image source: Aptera Motors Corp.)
As for price, they say they’ll be delivering a version with 400-mile range to U.S. customers next year, with prices for a basic model starting at $29,800 — significantly less than the Lightyear One. They are classifying it as a three-wheel motorcycle rather than a car, to avoid some of the regulatory mandates such as airbags. (They say that it most states in the US, such vehicles do not require a motorcycle license, only a regular driver’s license.)
In addition to the lower rolling resistance of three wheels versus four and extremely light weight due to use of highly advanced composite materials, Aptera claims one of their keys to success is far lower drag coefficient (wind resistance) than conventional vehicles (Figure 4). Note that drag coefficient is extremely critical as the impact of drag increases with the square of the vehicle’s speed).
Figure 4 The drag coefficient of the Aptera Luna is about half that of standard vehicles but twice that of a typical World Solar Challenge vehicle (which designed for a unique contest scenario). (Image source: Aptera Motors Corp.)
All these promises are nice, but maybe it’s time for a reality check. While I don’t doubt the sincerity of these start-ups, I do wonder about the optimistic assumptions they may be making: the basic physics numbers are tough to beat. Under best-case conditions of sun, season, and Earth latitude, the available surface area of these PV cells just cannot deliver much power; even if their efficiency increases significantly, the amount of solar radiation reaching the Earth is modest at best.
Here’s the situation: approximately 99% of solar, or short-wave, radiation at the earth’s surface is in the region from 0.3 to 3.0 µm, between ultraviolet and near infrared. Above the earth’s atmosphere, solar radiation has an intensity of approximately 1380 W/m2, called the Solar Constant. At 40⁰ latitude, the value at the surface is approximately 1000 W/m2 on a clear day at solar noon in the summer months due to atmospheric losses (see US Dept. of Energy, “Solar Radiation Basics”).
Do your own rough, “back of the envelope” calculation (Figure 5) using numbers you feel are realistic now and in the near future on efficiency of the PV cells, power-conversion and -management circuitry, inverters, battery, and the motors. You’ll likely see that the amount of solar power that can be captured and is subsequently available is fairly low even under “ideal” conditions of full summer sun (and remember that one horsepower is about 750 watts).
Figure 5 Using this custom, one-of-a-kind “back of the envelope” pad encourages broad estimates in the numerical analysis and reminds you that precision is not always a good thing; it’s often better to be roughly in the ballpark than imply unrealistic accuracy of the final answer. (Image source: author)
Of course, that situation quickly degrades due to clouds, shade, shorter days, and many other factors. Further, increasing potential range by adding more batteries to capture more when the sun is fully available and the car is not in use means added weight and range penalties.
What’s your view on the viability of these solar-powered cars? Among the many questions are these:
So many questions, so many views—looks like we’ll have to check back in a few years!
This article was originally published on EDN.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.