Putting a recharge on your roof
Cars with builtin solar panels move from theory to reality
Panasonic sees the future of solar power on car rooftops. The Osaka- based electronics maker has started producing a 180-watt array of solar cells that can be fixed to a vehicle’s roof. In February, Panasonic announced that its photovoltaic module would be used on the roof of Toyota’s latest Prius plug-in hybrid.
Cars represent a potentially lucrative new outlet for solar cells, in an industry where intense competition from Chinese manufacturers has pushed down prices sharply.
That’s prompting some manufacturers to adapt solar cells for everything from home roofing tiles and the outer skins of buildings to backpacks and tents.
“Car roofs have the potential to become a new market for solar panels,” says Shingo Okamoto, the general manager at Panasonic who was in charge of developing the technology. “We made history in the auto industry and in the solar industry with the sun powering massproduced cars for the first time in the world.”
Cars could hold the promise of a giant new market for solar panels from Panasonic, which has also partnered with Tesla in making batteries at its gigafactory outside Las Vegas.
Others are noticing the potential. Tesla chairman Elon Musk tweeted in November that his company’s Model 3 car may come with a solar roof. He is also beginning to sell a type of roof tile for homes featuring embedded photovoltaics. Nissan offers an add-on solar panel option for its Leaf electric cars, giving extra charge to systems such as the air conditioners and radios, says a spokesman for the company.
Chinese company Hanergy Holding Group unveiled four concept solar cars last year but has yet to begin commercial production. It’s seeking to develop car roofs using thin-film solar chips.
By the end of April, in Japan Toyota had sold 1350 of the new Prius plug-in equipped with solar panels, nearly 9 per cent of the total, says the car maker.
The latest Prius is the first massproduced model with solar panels that provide juice for the car’s main battery, according to Toyota.
Because of seasonal variations in sunlight, the amount of charge will fluctuate, meaning the average distance travelled on a single rooftop charge will range from an average of 2.9km to 6.1km, depending on the time of year.
“That means you get about 10 per cent of annual mileage from solar just by letting your car sit,” says Okamoto.
While that’s modest, developers say the amount of charge will improve over time and the range could be extended by adjusting a car’s design and the area on which the panels can be fixed.
“We are aware that the panels are supplying only a small amount of electricity,” says Shoichi Kaneko, chief engineer for Prius. “But this system is still a breakthrough as we are making use of the energy we would be wasting otherwise.
“By filling all available space with cells, it is possible to extend the range easily to 10 kilometres.”
In its current iteration, the solar rooftop represents a step forward for Toyota as it aims to reduce carbon dioxide emissions from new cars by 90 per cent by 2050, from 2010 levels.
This is Toyota’s second attempt to outfit a Prius with a solar roof. A Prius released back in 2009 had the option of a solar panel capable of producing 56 watts of power, but it was only used to charge the ventilation systems.
To come up with the panels for the new Prius, Panasonic engineers first began compiling a set of conditions and constraints they figured solar panels would face when attached to the roof of a car instead of a house.
The l i st i ncluded constant vibration, the unpredictable appearance of shadows and the limited amount of space for installation. Engineers also considered how to make panels fit the contours of a car’s roof instead of the flat surfaces found on residential rooftops.
To handle the variable sunlight expected from shade, Panasonic also adjusted the way bypass diodes are configured in the solar panels to ensure a smooth flow of electric current, says Panasonic’s Okamoto.
“You’ll see panels become more common as an option, particularly on high-end electric sedans and SUVs,” says Colin McKerracher, an analyst at Bloomberg New Energy Finance. “But the range benefits are always going to be limited. This is mostly about feeling smug.”