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Darwin to Adelaide: a new speed record of 85km/h The 1993 World Solar Challenge was won by Honda, slicing almost nine hours off the time of the last race & achieving an average speed of 85km/h for the 3003km race. By BRIAN WOODWARD Honda’s win came about due to progress in solar cells, power management, electric motor and tyre design and is represen­ tative of the great strides forward since the 1990 race. Professor Martin Green of the University of New South Wales is 4  Silicon Chip acknowledged to make the best solar cells on earth but a relative newcomer, Richard Swanson of SunPower Corporation, can be confirmed as the maker of the best silicon cells for solar race cars. It was Richard Swanson’s cells which helped win the third World Solar Challenge for Honda. The secret to the cells’ design was hidden in a simple sentence or two on the specifications sheet published by Honda shortly before the race. Rumours abounded that Honda had bought a complete set of race cells (more than 20% efficient) from Professor Martin Green in Sydney at a cost of more than $1.2 million. The price may be subject to doubt but the fact isn’t. Honda did buy the cells but used them for a ‘mule’ (a mule is a test-and-discover race car built for trials before the real car turns a wheel in anger). The cells used on the race car which appeared for scruti­neering a few days before the November 7th race day were RIGHT: HONDA’S winning Dream car sliced nearly nine hours off the record & recorded an average speed of 85km/h over the 3003km distance. The second placegetter, the Spirit of Biel, is shown on the facing page at left. Both cars used brushless hub motors. sourced from a relatively new US company, SunPower Corporation, which was not named at the time. They were described as “intrinsic mono­crys­tall­ ine silicon cells with back surface contacts . . . laminat­ed with a silicon poly­­mer and covered with a textured acrylic sheet. This sheet is fabricated with parallel angled grooves to enhance energy collection at low angles of solar incidence.” Fresnel lens What the description didn’t explain was that the ‘parallel angled grooves’ actually comprised an elaborate Fres­nel lens of varying angle. This meant that in the early morning and late afternoon, the sun’s rays were diverted by the lens to hit the cells at the optimum angle. For best output, solar arrays are turned until they are perpendicular to the sun’s rays. This isn’t practical with a solar race car because of poor aerodynamics, but this factor has been ignored by cell makers, up till now. SunPower made a solar array for the 1993 World Solar Chal­lenge and Honda’s claim of more than 1500 watts from eight square metres was beaten on several days when ideal conditions saw almost 1700 watts generated. The array weighs only 19.5kg. The array used by the Engineering School of Biel was devel­ oped by Deutsche Aerospace and proved to be almost as effective as in the Honda Dream. The car’s better aerodynamics were offset by the compromise angle of the array facing the sun. Naturally, it was best during the middle of the day. Inoue and Michelin both developed tyres which reduced roll­ing resistance by 30%. At low speeds, a solar race car spends almost one third of its power simply rolling along the road. A 30% reduction is significant. Getting the most efficient array’s power to an efficient motor in the most THIS CLOSE-UP VIEW shows the brushless DC hub motor used in the Northern Territory University’s Desert Rose. The motor is controlled by a power management computer & has a claimed efficiency of 96%. January 1994  5 LEFT: THE Spirit Of Biel with its solar array raised to recharge its batteries at the end of a day’s run. Below: the cramped cockpit in the University of Michigan’s car. efficient manner has been the bane of solar race car designers since the first race in 1987. With arrays at 20% and motors usually at 83-85%, losses in the tracking and motor management systems are to be avoided. The big breakthrough The big breakthrough for 1993 was the brushless DC motor-in-hub. Three cars, the Spirit of Biel, the Honda Dream and the Northern Territory University’s Desert Rose all used a hub motor designed to run at 900-1100 RPM with every individual winding addressed by a very busy power management computer. The Honda Dream’s motor had a claimed efficiency of 95%, the Desert Rose’s 96% and the Spirit of Biel’s 97%. All lost about 1.5-2.5% The Honda Dream motor had a claimed efficiency of 95%, the Desert Rose 96% & the Spirit of Biel 97%. All lost about 1.5-2.5% in the tracking & motor con­trolling computers. in the tracking and motor con­trolling computers. Compare this with a best of around 83% from the 1990 race winner and you can see why so much excitement was generated. It is highly likely that this motor design will become the electric car motor of the future. The effectiveness of power management in solar race cars is such that, at the Speed and Stability test day in Darwin, Biel claimed the Spirit of Biel would achieve 130km/h. It managed 129.9km/h. Honda claimed that its energy balancing computer system predicted an average speed of 86km/h. Over 3003km (even allowing time for flat tyres), Honda’s car achiev­ ed 84.96km/h. As it turned out, both estimates were amazingly close. SC 6  Silicon Chip