Sorry, but solar cars will always remain a romantic sci-fi fantasy

Toyota engineers will be nervously checking the weather forecast at the end of the month, as the company begins trials its latest solar-powered Prius in Japan. Solar panels have been available on production models of the plug-in hybrid car since 2010, but this experimental model – developed in conjunction with Sharp and NEDO – is a leap forward. It’s the first iteration of the technology that lets the car’s battery charge while on the move, rather than just when parked in a sunny spot.

Toyota isn't the only one. In June, Lightyear One unveiled a prototype of its solar-equipped electric car and began taking pre-orders on a limited run of the vehicles for €120,000 (£107,000) each, with delivery in 2021. But this doesn’t mean an end for trips to the petrol pumps or charging stations. Despite their promise, solar panels are unlikely to ever provide enough energy on their own to power anything but the shortest, sunniest trips.

The first solar-powered Prius produced 180 watts (W) of electrical power per hour with 22.5 per cent conversion efficiency from the solar panels to the battery cell, for a range of 6.1km after a day of charging. It contributed nothing to moving the car, and simply powered internal systems such as cooling.

The demo car for the new edition promises 860W at 34 per cent efficiency, with Toyota claiming 44.5km on a full charge — and 56.3km if it's recharging while driving. But that doesn't mean the car can run forever so long as it's sunny.

"They claim to generate 860W by solar cells — that is 1.15 horsepower, while the total power of the car is 122 horsepower," says Giangiacomo Minak, professor of industrial engineering at the University of Bologna. Instead, the car's battery needs to be recharged from an electrical point or left in the sun for several hours before a (very short) road trip, with the roof-top panels only topping up the battery minor amounts while driving — and that's only if it's sunny.

The 860W cited by Toyota is only achievable in full sun, so the car will get less power when the sun is low or the sky is overcast, adds Peter Pudney, a professor at the University of South Australia and the chair of the technical committee the World Solar Challenge, a solar car race. "Even with full sun, the power generated is about half the power required to run a hairdryer, and nowhere near enough to directly power the car’s electric motors, which use up to 53,000W of power when accelerating," he explains. "So the car also has a battery that can store the solar energy collected over a day and then use it over a short period to assist with the driving. The car also has a petrol motor to provide power when the battery runs low."

Still, if Toyota's system works, it would be possible to run a car entirely on solar power — though only for short trips in specific conditions, says Erik Mayer, associate professor at Pittsburg State University. "If you could get 860W out of the solar panel while you are parked at work for eight hours, it would generate... 6.88 kWh," he says. "This might be enough to fully charge up your battery while parked at work. And if your commute was less than 12 miles each way, you could possibly run your car totally on solar energy with no additional gas or having to plug in your car." However, he notes that would depend on the efficiency of the battery charger and whether the sun was directly shining onto the panels — which often won't be the case.

That may be enough to get to work and back for some commuters — so long as they aren't delayed in traffic — but it's a short drive versus the 3m000km that solar-powered cars traverse to complete the biannual World Solar Challenge, an endurance race held in Australia that runs from Darwin to Adelaide. The last three cruiser classes of the competitions have been won by students from Eindhoven University of Technology — some of whom are behind Lightyear One.

Those cars look very different from a vehicle like the Prius. They're not really cars as we would usually imagine. To accommodate more solar panels, the roof is often flat and massive; to save on energy use, they're built with ultralight materials. "The single-seater Challenger class cars will do the trip without topping up from the electricity grid," says Pudney, with the next competition happening in October this year. "The more practical Cruiser cars will carry more than one person and are allowed to top up from the grid, but will have to drive almost 1,200km without plugging in. The secret to this high performance is that the cars have less than 20 per cent of the mass of conventional cars, and much lower aerodynamic drag."

The winning designs don’t leave much room for some of the amenities we’ve come to association with driving – comfort is an afterthought. The Prius, on the other hand, still looks like a normal car, and is made from standard materials approved by safety regulators — though previous versions were initially banned in the US because the solar panel glass wasn't deemed sturdy enough for a vehicle.

The solar film on the latest demo car is only 0.03mm thick, allowing Toyota's solar panels to be applied across the roof, bonnet and boot without adding much weight. The design is compelling not only because it allows solar panels to be added to cars without changing the physical design, but also because the solar film is close to world-record efficiency levels, says Pudney. "[They are] triple-junction cells — effectively three solar cells sandwiched together where each layer responds to different wavelengths of light," he explains.

But they’re still not as efficient as standard, thicker solar panels installed on the roof of a building, Pudney says. These can be situated to get the full force of the sun and angled to avoid shade. Plus, cars have smaller batteries and may be unable to make the most of a day of blazing sunshine. "Once the battery in a solar car is full, the solar panels shut down — there is nowhere else for the energy to go," he notes.

Of course, any solar power generated is energy that doesn't need to be drawn from another source, renewable or otherwise. Just because cars won’t be able to get all their energy from the sun, it doesn’t mean solar panels won’t be useful for cutting demand on the grid when more cars do go electric, says Minak. "Moreover, the more solar power you put into batteries, the less solar power heats up your vehicle," he adds. The power is there, we may as well use it, even if it's only for a small slice of required energy.

That said, solar-powered cars will depend on better batteries that are yet to be developed and sunny weather we don't have, and are less efficient than other ways to power electric vehicles. Plus, if Lightyear's example holds true, they may well be exceptionally expensive when they do arrive.

And yet, they aren't a wholly foolish idea, as not only could they extend the range of electric vehicles and reduce the burden on the grid as more cars go electric, but — if designed correctly — could also help power our homes. "If the solar panel is connected to the grid, by plugging the car in or by having the panel on a building, solar energy not required by the car can be used for other purposes," says Pudney. That means that even though wholly solar powered cars may never exist for most of us, if these trials prove positive, we could get yet another reason to celebrate sunny days.