(article adapted from Photovoltaics International magazine - June 1986)
To demonstrate the principle of photovoltaics at an open-air fair, Greg Johanson wired some PV cells onto a toy electric train. Seeing the attention this little solar train got as it sped around its track, Greg and I thought that a solar vehicle would attract more interest in PV. That's what inspired us to build our first solar vehicle.
Our first vehicle was not strictly solar powered. It was a lightweight three-wheeler with a 1/2 horsepower, 60-volt, direct current motor. Five small, 20 amp-hour, 12-volt batteries wired in series were used for motive power. Five 35-watt Arco Solar modules, mounted like a canopy, trickle charged the batteries. The vehicle could travel (very unsafely) over 40 mph (64 km/h), but under 30 mph (48 km/h) it was fun to ride. Under direct solar power in full sun, the vehicle would go 6 mph (9.6 km/h).
To charge the batteries, we chained the three-wheeler to a pole outdoors in the sun. The steering wheel was removed to prevent joy riding or theft. Unfortunately, one evening, the vehicle was stolen. Despite its unique appearance, a police investigation, and a $500 reward, it was never found.
The theft was a shock and a setback. All the time and money put into the vehicle was lost, but we had been bitten by the solar vehicle "bug." We wanted to build something bigger and better. Our experience and calculations indicated that a solar-only vehicle was possible. Besides, battery vehicles are fairly commonplace. We wanted to do something new.
After research and discussions with bicyclists and engineers, we sketched plans. Sunrunner, our aptly named vehicle, was designed for speed. Although some of the design ideas came from work developed by members of the International Human Powered Vehicle Association, compromises had to be made to accommodate the solar array. Also, we wanted the array to double as a portable power plant for more practical demonstrations.
Construction proceeded slowly as we were paying for the project out-of-pocket with no sponsor or grant. We used lightweight chrome-molly steel for Sunrunner's frame. The design provided enough flex to eliminate the need for suspension hardware. We used wheels designed for tandem racing bicycles and light-weight wheelchairs. No batteries were used on Sunrunner; the solar array would be the entire power plant.
To simplify the steering mechanism, we made the turning radius large. As the speed run would be a straight track, high speed maneuvering was unnecessary. Our experience with the first solar vehicle taught us to design for the open, quiet roads in the desert, not the city. There are no restrictions on experimental vehicles, but city driving is out. The danger of an accident caused by curious drivers and onlookers was too great. Every time we took Sunrunner on the street, traffic would stop and people would stare.
Our first brakes were racing bicycle disc brakes which worked too well. They locked the wheels on our lightweight vehicle that weighed less than 600 pounds (272 kg) driver included, causing tire burn-through. We compromised and used bicycle caliper brakes, which we jokingly called "slower-downers."
The power train was a simple one-wheel chain and sprocket drive from the motor shaft to one axle. We used the same model 1/2 horsepower motor as our first vehicle because it was so efficient. Our experience with the motor on solar powered water pumping systems and our first solar vehicle indicated that it would give us the best performance.
For power, we used 24 44-watt 1 x 4 ft (305 x 1219 mm) Arco Solar modules with no metal frames, back sheets or interconnect boxes to minimize weight. The array was connected in series parallel for nominal 60-volt operation. Early street testing was a problem as the motor did not have the starting torque necessary to get us across intersections in traffic. In addition, terminal velocity would not be reached until we traveled 5/8 mile (1 km). Again, it was time for a design compromise.
We changed to a less efficient 1 horsepower motor. Brad O'Mara made a dc-to-dc converter for the vehicle which changed the current/voltage of the array to provide high current and low voltage needed to get the motor started. Once rolling, the converter would shift to higher voltages. Brad also made the rheostat speed control.
For the lowest drag coefficient, the driver should be recumbent, face forward, stomach down. We opted for a more conventionally seated recumbent position, giving the driver better visibility, more comfort, and somewhat better safety. Lying back in the solar array, speeding along close to the roadbed was a wonderful feeling.
To transport the vehicle, we modified a boat trailer by extending the frame 5 feet (1.5 m) and adding a plywood deck to which Sunrunner was strapped. Also on the deck were 16 golf cart type deep cycle batteries and a 2500 watt inverter. The combined solar vehicle array and trailer provided approximately 5 kWh/day of electricity with 16 kWh battery storage for loads up to 2.5 kW. The batteries and the inverter were not a part of the vehicle, but were used for demonstrations.
Time and money were running out. Testing was limited to one or two days per month on the relatively quiet desert roads near Palmdale, California. Friends were always willing to give a hand in exchange for a ride. Cruising along in the solar vehicle at over 30 mph (48 km/h) with a safety car in front and behind was like a fantastic voyage.
The vehicle performed as predicted. Wind resistance was not a factor as the frontal area was very small. Drag from the top and bottom surface of the solar array did affect speed, but nothing could be done about it. The tires were inflated to 120 pounds per square inch (psi) so the vehicle rested on only 4 square inches (25.8 sq cm) of tire-to-road, reducing resistance significantly.
It was time for clocking by an official independent timer. The International Human Powered Vehicle Association, official timer for DuPont's award for the first bicycle to reach 65 mph (104.6 km/h), was willing and available. Unfortunately, the track they used was in one of the smoggiest places in Los Angeles. To make things worse, their timing device was set up right after two underpasses and the track was one-way going into a 12 mph (19.3 km/h) head wind.
Although conditions were far from optimal, on July 1, 1984, at Bellflower, California in 70% of full sun light, Sunrunner was driven by Mary Anne Reynolds, achieving 24.7 mph (39.8 km/h). This speed was well under our near 40 mph (64.36 km/h) desert runs, but was officially clocked by independent observers needed to submit result to the Guinness Book of World Records. The 1986 edition of the Book of Records listed Sunrunner as the world's fastest "Solely Solar Powered Vehicle." During 1984 and 1985, we had an open invitation to race any other solar vehicle on land or in the air, but no one accepted our challenge.
By 1986, a few other people had gotten into the fun of solar vehicles. Peter Rubie (are you still around?) had an aborted attempt at a cross-country run. Art Boyt made a cross-country run, although his vehicle used battery storage. In Europe, Mercedes Benz spent a lot of money to build a racer. The solar vehicle craze had caught on.
Sunrunner appeared at energy trade shows and outdoor exhibits. We couldn't afford to participate in international shows, but Sunrunner was on television and in movies worldwide. Expo 86 organizers showed Sunrunner on a 360 degree dome screen in a film on travel. In keeping with the travel theme, Expo organizers thought a solar vehicle would make a good exhibit. Greg jumped at the chance to build another solar vehicle for the Vancouver Expos. The third solar vehicle had 12 Arco Solar modules, six deep-cycle batteries and a 2 horsepower series wound motor. Batteries give it a range of 40 miles (64 km). It took the solar array three days to recharge the batteries, but batteries gave the vehicle "punch." It could "lay rubber" and go over 60 miles per hour (96 km/h).
What's the future for solar vehicles? Greg thinks that someday solar vehicles will be practical, but I doubt it. As long as people want to speed in large, heavy vehicles, physics is against solar cell powered cars. We both agree that more solar vehicle research is good for PV. It teaches how to squeeze the most amount of power from an array. And there's always the thrill of speeding along on a virtually untapped energy source.