Toyota is trying to use the power of the sun to boost its Prius

TOKYO – How can Toyota make the world's best-selling green car, the Prius, even greener?

Cover the top with solar panels and let it run in the sun.

It's the goal of a Toyota trial that starts this month in Japan. The system generates enough power to drive the Prius PHV for about 56 km.

Toyota will test until February on public roads, in collaboration with solar cell manufacturer Sharp Corp. as part of an initiative supported by the Japanese Government, called the Organization for the Development of New Technologies and Industrial Technologies.

Toyota already has a Prius with solar panels. But technology is being offered as a Japan-only option on the Prius PHV, the plug-in variant sold in the US under the name Prius Prime.

The new demo car dramatically improves performance.

Unlike the current system, which only charges when the car is parked, the new version also charges while driving.

The existing Prius PHV system generates approximately 180 watts of electricity while the vehicle is parked, enough to drive within 4 miles of battery-only driving. The new system provides 860 watts, nearly five times more energy – enough for around 28 miles if the car is parked, or 35 miles if charged while driving.

Part of the gains come from more efficient solar cells. But the new technology also uses a lot more panels.

The hood, roof and rear window are covered with black solar cells 0.03 mm thick. Because they are so thin, they can be adjusted to the silhouette of the Prius and its rear hatch.

A spokesman for Toyota said the rear visibility was not a problem because the driver could use the side mirrors while driving and the rear view camera to park or retreat.

Toyota said its test would help determine the amount of solar energy that will improve the driving range and energy efficiency of electrified vehicles.