Sustainable expectations

While on travel in California, I drove past a car dealership with signage advertising a solar powered station for recharging the batteries of electric cars. Parked in the dealer’s lot was a trailer unit that evidently could be moved from location to location and contained the recharging equipment. The trailer held a large flat panel of solar cells that was pointed south and tilted up toward the sun. The signage was attention-getting, and the charging station seemed to be part of a marketing campaign intended to draw customers and highlight the dealership’s fleet and commitment to green vehicles. I gave some thought as I was driving by, and subsequently sitting in traffic, about the practicality of recharging electric cars in this manner:  drive around town for the day, park and recharge your car while grabbing lunch, and off you go with full batteries.

Under optimal conditions, sunlight strikes the ground with an intensity of about 1 kilowatt per square meter. The actual intensity is lower when weather and the sun’s motion are taken into account. The efficiency of photovoltaic solar cells has been improving recently, including work being done here in the College of Engineering by Professor Vik Dalal. He’s devoted nearly 40 years of research to improve solar efficiency, and his work with thin films is a major step. Still, efficiencies of inexpensive commercial solar cells are around 20-40%. The remainder of the sun’s energy is converted to heat or reflected back off the solar cells themselves.

The equipment I saw could produce about 1 kW of power, which is about the same as 1 horsepower. That’s the amount needed to run 10 incandescent household lights, and it’s not a great deal of power, particularly for a heavy car and when compared to an automobile engine that produces hundreds of horsepower. But to store a certain amount of electrical energy in the car’s batteries, you can make up for low power by keeping the car plugged in for a longer period of time. The equipment would be useful for charging an electric car that could be left and parked on the lot for hours or days, but it would be unrealistic for someone to think that they’d be able to pull up to the station and recharge their electric car with solar power as rapidly as a gas tank can be filled up.

As our society and economy move toward more sustainable energy sources, it’s important for engineers to play a role in explaining trade-offs and in anchoring expectations about what the technology can do.