When is a Solar Bus Not A Solar Bus?

By Bill Moore

Posted: 01 Sep 2010

It's been billed as the "world's first solar-powered public bus." Built as a joint project between Sanyo Electric Co. Ltd and Ryobi Group for the latter's 100th anniversary, the "Sorabi" SOLARVE is described in the Japanese language press release as a 'futuristic hybrid bus' with two different types of high-efficiency Sanyo solar-electric panels mounted on its roof: 420W HIT cells, and 378W Amorton cells: totaling 798W.

From such headlines it would be easy to assume that the bus is being run on solar energy produced from the 800W of photovoltaic cells affixed to the roof. You would, of course, be wrong. What's being run isn't the bus, itself, but the LED lights inside the bus, and possibly a Sanyo-developed air filtration system, similar to that incorporated into the early "Sai" bus, circa 2006.

If the panels were to supply electric power, how far could it drive? Assuming for the moment that that the bus is extremely lightweight and very energy efficient, a trait not all that common in public transit buses, and it takes 800W hours to drive one mile (1.6km), in an average 8-hour shift, it might be able to go 8 miles with 100% system efficiency, or one mile for each hour of absorbed solar energy. In actuality, it probably takes considerably more energy to move than 800W hr per mile, as we'll see in a moment. The average electric car can consume 250 Watts per mile, and the newly proposed EPA/DOT vehicle stickers use 345W per mile as their base line for an electric car.

The first acknowledged "solar bus" to be put into operation is the "Tindo" bus in Adelaide, Australia. It's electric power, which is stored in 11 ZEBRA sodium nickel chloride batteries capable of storing better than 260kWh of electrical energy, comes from a AUD$550,000 Uni-Solar array mounted on the roof of South Australia citys' central bus station. Those panels generate an estimated 70MWh of electric power annually. And in reality, these panels don't actually directly charge the bus, but appear to feed into the local grid, which then is used, through a fast charger rated at 386V DC at 200a and 70kW, to charge up the bus' battery pack at a rate of a mile per minute. Tindo is said to have a range per charge of 200km (124 mi).

Based on Tindo's performance specs, it takes nearly 2 kilowatt hours of energy to propel it, nearly two-and-half-times the energy output of the panels on top of the SOLARVE. So, while it is a nice gesture to call it "solar," it is that in name only; and it certainly isn't the first solar bus in the world. Maybe the first with solar-charged LED interior lighting. No, the real motive power for the bus likely comes from a form of ancient sunlight stored in decayed plankton and called petroleum fueling a diesel-electric hybrid drive: perhaps that's why there's also an air filtration system on it.

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