Demonstrating the kind of “stick-to-itiveness” that would make a high-school guidance counselor proud, Panasonic engineers have developed and the company is selling a full-length solar roof panel to charge both the 12V and the lithium ion drive battery (8.8 kWh) on the Japanese version of the 2017 Toyota Prius Prime plug-in hybrid. Panasonic’s heterojunction technology “HIT” photovoltaic panel adds extended range (about 10% of annual mileage) to the vehicle (the EPA estimated EV mode driving range of the Prius is 25 miles).
Toyota and Panasonic are quick to point out that because of seasonal variations in sunlight the amount of charge will fluctuate, meaning the average additional range gained from a single charge will vary from an average of 2.9 kilometers (1.8 miles) to 6.1 kilometers (3.8 miles) depending on the time of year. If a way can be found to fill all available body panels on the car with solar cells, it is estimated that range could be extended to 10 kilometers (6.2 miles).
When its HIT cells were introduced in 2014, Panasonic announced that it had achieved a conversion efficiency of 25.6%. In theory, 30% energy-conversion efficiency is the upper limit for traditional single-junction solar cells. The current efficiency record for mass-produced solar panels, set in March of this year by researchers from the Japanese company Kaneko, is 26.6%.
A key feature of HIT technology is its ability to reduce the recombination loss of charge carriers, particles of electricity generated by light, through laminating layers of amorphous silicon on the surface of the monocrystalline silicon substrate, where power is generated.
In addition to high conversion efficiency, there is said to be very little reduction in output power when the module increases in temperature, meaning it supports high output power even during high temperatures such as summer heat. Typically solar cells don’t do well in heat; their output suffers. According to Panasonic the rated output of the HIT panel is its value when the module temperature is 25°C (about 77°F), but in practical application a car roof can actually reach 80°C or higher, which would cause typical solar cell output to fall as much or even more than it would if mounted on a home roof. However, any output decline is minimized, according to the company, because the temperature coefficient of output of Panasonic's heterojunction solar cells is small compared to general solar cells.
The idea of car-mounted solar cells is not new: the 1992 Mazda 929 sedan was offered with optional solar cells built into its glass sunroof to power fans that cooled the car. A 2009 Prius had the option of a solar panel capable of producing 56 watts of power, which was also only used to power the fans for the AC/ventilation system.
In 2014 Ford Motor Co. unveiled its C-Max Solar Energi concept car. A plug-in hybrid, the C-Max had solar cells on its roof. The car employed lenses made of acrylic to concentrate sunlight on the cells, boosting the amount of electricity generated to recharge the battery. The Ford system tracked the sun as it moved from east to west, drawing enough power through the concentrator each day to equal a four-hour battery charge (8 kWh). To keep the car positioned in an optimal manner under the lenses as the sun moved across the sky, the car slowly moved forward on its own (no driver was required). Also in 2014 Nissan Motor Co. began to offer a photovoltaic solar panel rear spoiler option for its Leaf electric cars, providing a little extra juice for accessory systems.
To ensure a smooth flow of electric current Panasonic adjusted the way bypass diodes are configured in the solar panels. Filling in your knowledge gap, remember that bypass diodes are used to reduce the power loss solar panels experience due to shade (or because the cells have become faulty or open-circuited). When a solar panel has cells that are partially shaded, the current will go through the low-voltage shaded cells, causing the solar panel to heat up, resulting in severe power loss. Bypass diodes inside the junction box of a solar panel and wired in parallel with the cells make it easier for the current to go through the diode rather than through the shaded cell. This minimizes the heat gain, and reduces current loss.
To make the panels fit the contours of a car’s roof Panasonic solar cells for the Prius have a unique structure combining a crystalline silicon substrate and an amorphous silicon film into a three-dimensional laminate curved glass matching the new Prius’ body design. This technology features the thermal compression bonding of the upper and lower module surfaces with flexible material and is noteworthy in that it allows the manufacturing process to address three-dimensional curves in addition to conventional flat surfaces.
Panasonic is partnering with Tesla Inc. in making batteries at a factory outside Las Vegas, and given that Tesla CEO Elon Musk is on record saying that the Tesla Model 3 will “probably” have a solar roof option, expectations are high that Tesla will be the second car company to utilize the new solar panels.
For now, however, the option is currently not available on U.S.-market Toyota Prius Primes because the reinforced glass sheeting currently used for the solar panel will not pass U.S. rollover crash tests. Panasonic and Toyota are reportedly working on a solution, but as of now there’s no timeline for a U.S. debut.
