Alta, a maker of thin-film photovoltaic cells, announced that Audi will be integrating Alta solar panels into the panoramic glass roofs of its new line of electric vehicles. Prototypes are expected by the end of 2017; solar-assisted Audis should roll out to dealerships by 2020.
Staying Cool in the Sun (or Warm in the Chill)
The first version of Audi's solar-assisted EVs will incorporate the PV cells into most of the roof area and use the solar-generated electricity to power the air-conditioning in the summer and the heated seats in the winter. While the batteries won't be charged by the solar roof, using PV to power some accessories will extend the car's range by easing the load on the batteries. Audi is integrating solar one step at a time - first to power accessories and later to help charge the battery bank.
Ballpark Estimates
Alta's thin-film Gallium Arsenide (GaAs) photovoltaics are among the most productive in the industry, with efficiencies as high as 28.8% for single-junction cells and 31.6% for dual-junction cells. They're semi-transparent, making them perfect for sunroof applications.
Assuming about one square meter of rooftop space covered in photovoltaics with a non-ideal tilt angle, we're looking at about 200 Watts of power at peak sun. In the northern US, with a daily average of five Peak Sun Hours, that's about one kWh of energy produced each day. A typical EV's energy consumption is 30 kWh per 100 miles (161 km), so that extra kilowatt-hour only adds a few miles to the car's range. (That's why Tesla isn't putting solar panels on its Model 3.) On the other hand, it allows drivers to use the HVAC without decreasing their driving distance, so that's a benefit - but a costly one.
GaAs vs Silicon
Solar cells tend to perform worse as temperatures rise; the temperature coefficient of power indicates how much of a decrease one can expect under various temperatures. Monocrystalline silicon solar cells - the kind used in most rooftop solar arrays and utility-scale solar farms - have typical temperature coefficients of -0.5% per degree C, meaning they produce about 0.5% less power for every degree C increase in temperature. Alta's GaAs cells boast a temperature coefficient of only -0.095% per degree C - about one fifth of silicon - so while those Audis are baking in the summer sun, the solar cells won't see such a dramatic loss of power. GaAs cells also tend to perform better under low and diffuse lighting conditions, so they'll produce power under cloudy skies. (Wondering why they don't use GaAs for all solar cells? It's because they're much more expensive - for now, at least.)
Is It Worth It?
Solar-assisted electric vehicles (SAEVs) are severely limited by the laws of physics - there's only so much power available in a square meter of sunlight. If they're inexpensive enough, then adding PV to the surface isn't a bad way to boost the car's range, but it's not all that affordable right now. We'll continue to see more solar on experimental cars and luxury vehicles such as Audis, which is good for the PV and SAEV markets in general. (As the rich kids buy the high-end stuff, the technology becomes better and more affordable for the rest of us.) On the other hand, innovative companies like Sono are already working on affordable SAEVs by putting lower-cost silicon PV cells around most of the car's body - not just the roof.
One thing I'd bet on: when the big automakers get involved, it's a sign of brightness looming on the horizon. Is this the dawn of a new era in SAEVs? We shall see.
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