Hype Aside, Real Solar Cars Are Far Off—But Don’t Tell Sono Motors That ""
Posted on: August 14, 2017

Sono Sion solar car

Munich-based Sono Motors is moving ahead to develop what it bills as a solar car, the Sion (above), after a highly successful $650,000 crowd-funding campaign on Indiegogo last year. While that’s small beans compared to the number of Tesla Model 3 reservations—and very small beans for developing an all-new vehicle that is compliant with regulations—the Sion teases some compelling numbers.

Its maker touts a price of about $18,000 for a little hatchback sized about like the Kia Soul. Features include a 107-hp electric motor capable of taking the 3080-pound Sion up to 87 mph, a 1650-pound towing capacity, a 155-mile driving range, and a solar array built into the car’s exterior that can recover up to 19 miles of range per day.

Scrutinize the Sion’s specifics a bit more, and it’s clear why that price is so low. “Batteries sold separately” used to be the norm for toys and small electronics in the days before rechargeable everything, but who knew it would ever apply to cars? The Sion will include Level 3 (50 kW) fast charging and even a 7.6-kW power takeoff, but the 35-kWh water-cooled lithium-ion battery—by far the most expensive component in an electric car—would be something you’d lease or buy separately, at a price yet unannounced.

“From now on, you’ll never have to worry about range,” proclaims Sono in its marketing. So if it’s this easy, why aren’t solar cars a thing? The answer is that automakers don’t think they can generate enough range from existing solar tech to make it worthwhile. Bloomberg New Energy Finance (BNEF) didn’t even bother to include solar in its projections of future vehicle electrification technologies.

It Doesn’t Add Up—Or Does It?

“The limitation is area,” not price, said Hugh Bromley, BNEF’s solar analyst. “Solar cells are a commodity, and they can be put anywhere on the vehicle; there just isn’t enough space on the outside of a vehicle now to charge a battery much or be the primary source of power,” he said. Solar race cars, which are extreme engineering exercises, also typically require support teams of several ordinary cars and trucks just to complete their limited missions.

Current solar-cell efficiency is around 20 percent, and with very slow gains in the technology in recent years, there’s no obvious inflection point in sight for solar technology to suddenly make sense, according to Bromley. “We’re simply not going to get to the point where a truly solar-powered passenger vehicle is viable through improvements in the solar technology,” he told C/D.

Sono Sion solar car

The Sion isn’t completely solar powered, of course; it just recoups some range while parked out in the sun. And to that end, Sono Motors claims a few tricks. Plastering the hood and doors, as well as the roof, of its earliest prototype with solar panels covered 81 square feet. The company noted that these are the newest-tech SunPower cells with a claimed 24 percent efficiency. There are 330 individual solar cells, each capable of generating 3.65 watts—adding up to a 1.21-kW peak and that claimed 19 miles of recovered range “under favorable conditions.”

Favorable conditions implies a lot more than just avoiding the path of a total solar eclipse or a cloudy day. Solar cells are very particular about how the light hits them. Catch the sun at an oblique angle, and they may put out a small fraction of their theoretical output. And then there are safety and durability considerations, Bromley noted. You have to cover the delicate solar cells with something like glass (or, in the Sion’s case, polycarbonate), and that in itself reduces the output. And then you have issues with a real foe of solar efficiency: superheating. “Solar doesn’t like to be hot. If it’s under clear protection, and the paint and the cabin are heating up over the day, efficiency is going to drop from that as well.”

Other Companies Cautiously Exploring Solar

Although putting solar cells on concept cars is quite common, very few other models have offered solar roofs—the Karma Revero is one niche entry—and the cells have claimed a small fraction of the Sono’s recovered range. The previous-generation Toyota Prius offered an optional solar roof that merely operated a small auxiliary ventilation fan, to help keep the cabin cooler when the vehicle was parked out in direct sun. This idea has even been adopted in the aftermarket to fit in existing cars.

In Japan, a solar roof has been available since earlier this year on the Prius Prime for the equivalent of about $2500; the Prius Prime is claimed to be the world’s first mass-production vehicle with a range-extending solar roof. About 9 percent of buyers there are choosing the option, according to a Bloomberg Technology report. Toyota noted that its system can generate a theoretical maximum of 3.8 miles of range per day. A daily average rate is listed as 1.8 miles, although last year Prius Prime chief engineer Koji Toyoshima told us that the real-world max is somewhere in the vicinity of two miles.

“We continue to study the solar roof here in the U.S., but there are still no current plans to offer it in the U.S.,” said Craig Taguchi, senior manager for Toyota’s U.S. product communications.

Toyota Prius Prime solar

Panasonic, which supplies that 180-watt roof (pictured above) or the Prius Prime, started producing a modularized array of the same output earlier this year to offer other automakers, hoping they will add them to the roofs of vehicles. Not even Tesla, with close ties to Panasonic and its much hyped solar roof for homes—an important part of the Tesla Motors master plan—has slapped solar panels on a car yet. CEO Elon Musk has so far given some mixed signals (via Twitter) about the possibility of a solar roof on the Model 3 or any of Tesla’s other models.

According to BNEF solar analyst Bromley, the reason you see Panasonic exploring this market is that, in solar, it’s more of a high-end boutique manufacturer (unlike China’s Trina and Korea’s Hanwha, two volume leaders) and is looking for higher-value applications.

Solar might not add up right now in most cases; yet we’re likely to keep seeing it as a feature in electrified vehicles as a way of adding perceived value and green “bling.” That’s because it’s extremely cheap—in the vicinity of 30 cents per watt, at most, or a few hundred dollars for the entire Sono array.

Where solar could produce much more significant gains, perhaps, is on buses or large trucks—such as the electric semis that are in the works from Tesla and others. And, as Tesla’s master plan suggests, large-scale arrays mounted on roofs or in fields could generate enough electricity to recharge electric-car batteries. That’s a far different case from a “solar car” powered by onboard cells, although the environmental and economic math may be similar.

Meanwhile Sono has developed a driving prototype with Roding Automobile of Germany and is currently doing an 11-city test-drive tour. It also plans to sell the Sion in the United States; if its solar claims live up to the hype, more than one industry may be quite surprised.


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