Trying to run an electric car from solar cells on the car’s roof makes no sense—or so I thought. Earlier this week, I met with Daniel Theobald, the CTO of healthcare company Vecna, who has been driving a solar-powered Volkswagen bus for a year.
His project won’t transform the auto industry overnight. But by taking on what most engineers would consider foolishly ambitious—a solar-powered car—he’s created a proof of concept that could lead to commercial solar vehicles, at least for some uses. In the process, he and his colleagues have created power electronics technology that could be applied to other products for Cambridge, MA-based Vecna, including its delivery robots for hospitals.
Certainly, combining solar power and electric vehicles isn’t new. Many EV owners have rooftop panels on the homes. And some Toyota Prius models are equipped with solar cells embedded into the roof, which power a fan to cool the car. But making car-attached panels serve as the primary energy source is very hard. The big challenge is that on-board panels put out a tiny amount of juice compared to the power a car needs to run.
Theobald was also under the impression that a solar-powered car was essentially impossible. But he wanted an electric or hybrid car that could transport his large family, so he decided to challenge his assumptions and see where it would lead. He bought a 1966 Volkswagen minibus off Craigslist, converted it to electric power (in an afternoon), and started reengineering the vehicle for solar.
His conclusion: “You can have a practical vehicle that runs completely on solar.”
To be sure, the VW bus looks funny—there are ten solar panels affixed to its roof, some of which hang over the front and back. The cost of the vehicle and equipment, including very efficient, but pricey, solar panels is about $30,000. Driving on solar also restricts his range to about 20 miles if there’s no sun.
But in taking on the challenge, Theobald and other Vecna engineers working on the project have created technology they’re seeking to patent, which could lead to new business opportunities. For example, airport service vehicles are outside for long periods of time and don’t need to go long distances. This is a situation in which having a solar-powered vehicle—one that wouldn’t necessarily need to be charged by plugging in—could make sense, Theobold says.
“Making practical solar vehicles is something most people wrote off so in many ways, it’s a fairly unexplored problem,” he says. “But there are a lot of applications where solar-powered vehicles make a ton of sense.”
There’s some crossover with Vecna’s current business, too. Engineers are now working to outfit the bus with a lithium-ion battery, which will be used in Vecna’s healthcare robots.
Optimizing for the Solar+Mobility
Engineering-wise, Theobald’s project uncovered some conventions that make solar-powered passenger cars such a tough challenge. For example, the electronic components normally used in electric cars, such as the battery packs, are not optimized for the voltage that solar panels put out. Also, a significant portion of energy is lost in charging and discharging a battery, but that can be minimized.
In early tests, Vecna engineers tried to have the bus run directly off the solar panels, rather than charge the battery and run off the stored energy. Powering a car largely from solar panels limits the acceleration significantly, but it can be done in city driving, they found. Driving the car at highway speeds (it can go 80 miles per hour) requires much more power than what the panels can generate in full sun.
Theobald also found that the topography of a given route makes a huge difference. Driving up a big hill to bring his kids to school would kill his battery, but taking a slightly longer yet flatter route used about one-quarter of the energy. “This idea of being aware of the energy as we travel is going to become much more relevant as we start using electric vehicles,” he says.
The solar-powered bus also uses a number of tricks to lighten the weight and reduce aerodynamic drag, such as using thin tires and lightweight wheels and bumpers.
With more tinkering, the energy use and efficiency can be optimized further, Theobald says. For example, the lithium-ion batteries are lighter and last longer than their lead-acid cousins now installed.
Another possibility is to use batteries in conjunction with ultracapacitors, a type of storage device that can very quickly discharge power to accelerate a car, for example. Engineers are also trying to find lighter-weight versions of some of the electrical components, such as the motor and battery charger, by converting products used in other industries.
Theobald’s ride certainly won’t work for everyone—even with solar charging, the car has a range of about 30 miles. But he’s made something that electric car enthusiasts could only dream about: an electric car that rarely needs to be plugged in.
“Given my normal driving, I don’t need to charge it all—the sun gives me all the energy. It’s incredibly convenient. As long as I’m not parking under a tree, I’m covered,” he says.
Updated on December 18 with a new price figure based on new information from Theobald.
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