Toyota may employ solar power to charge the Prius's high-voltage batteries eventually, but for now it is using the alternative energy source to run fans.
Developed by Japan-based Kyocera, the Prius solar roof module generates power for interior ventilation of a parked Prius that has heated up under sunlight. If activated by the driver before he or she leaves the car, the system turns on automatically when the outside temperature rises above 64°F (18°C) and there is sunshine. So that cool air already in the car upon turn-off is not vented out, the system does not kick on for at least 10 min. Fan speed depends on sunshine level.
Conversion efficiency of the solar roof module, which has 36 solar cells, is 16.5%, according to Kyocera. Average output of the 25-kg (55-lb) system is 56 W; the company would not reveal the figure for maximum wattage.
The Japanese automaker had explored many potential applications for the solar roof system, including recharging of the hybrid car’s high-voltage battery. But after weighing trade-offs, Toyota chose the fan application. The problem with recharging the 1300-W battery is electrical interference that affects nearby radios and mobile phones.
The system comes as part of a $3600 solar roof package. Included in the package, but separate in terms of technology, is a remote air-conditioning system that draws power from the battery to further cool the car before the driver enters.
Toyota would not say directly whether the benefit in terms of lower cooling costs offsets the added cost of the solar roof system but implied it does not. Cooling cost reduction is “very minimal,” a spokesman said. The real advantage is in comfort. Plus, the system “further accentuates the hybrid vehicle's concept of effectively utilizing limited resources.”
No maintenance of the roof module is required, says Toyota, but cleaner is better in terms of energy conversion efficiency.
For more than 30 years, Kyocera has been developing and supplying solar modules for a wide range of applications. The Prius is its first—and to date, only—automotive application. The solar cells and roof modules are produced in Japan. Kyocera uses the casting method to produce multicrystalline silicon solar cells. It carries out all production processes in-house from procurement of raw materials to silicon casting, cutting, wafer substrate production, cell production, and solar module assembly.
Since 2002, Kyocera has been offering high-performance solar cells that employ a unique reactive-ion-etching technology that creates microscopic ridges on the surface of the cell using plasma and reactive gases. The subtle ridges allow the solar cells to better absorb the sunlight reflected on the surface, therefore enhancing output and conversion efficiency. At the same time, a uniform dark navy blue color has been realized.