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GM’s SunRaycer still holds the record for the Darwin to Adelaide World Solar Challenge. It had a “full cockroach” shape but most of the cars in the 1993 race will have flat solar pan­els. Darwin to Adelaide: technology makes it faster The leading cars streaking south from Darwin on 7th November in the World Solar Challenge are expected to be able to cruise at more than 80km/h using just Sun power. With battery assist, they will be much faster, perhaps running at up to 140km/h. By BRIAN WOODWARD The progress in solar car technology since the last WSC in 1990 has been dramatic. Aerodynamic drag has been drastically reduced, rolling resistance lowered, electric motor efficiency improved significantly and power management exceeding 98% effi­ciency has been achieved. Solar cells have experienced the most dramatic change in the threeyear period. The last race was won by the car entered by the Swiss Engineering School of Biel. This car used sili- con solar cells developed by Professor Martin Green of Sydney’s University of New South Wales. The revolutionary ‘Green’ cells offered a huge increase in power, but at a premium. Since the last race, Professor Green’s team has been working with BP Solar to bring these new cells to a stage where they can be mass pro­duced. The result is dramatic. For a time it was thought that Australian innovation would once more miss the boat. Green cells may be efficient, acknowl- edged many industry commentators, but they were prohibitively expensive. Much less efficient, but very much cheaper amorphous silicon cells being developed in Japan were set to upstage the Green cells before they could reach mass production. Now, Martin Green’s breakthrough photovoltaic cells cost 15% less to make and offer 30% more power than conventional mass-produced solar cells. Five years ago, mono or poly­ cryst­alline cells would have cost in excess of $20/watt to make. Costs have dropped to about $3/watt which relates to a retail price of about $10/ watt – less than one third the price only a few years back. BP Solar has supplied more than 40 kilowatts of cells to cars racing in the 1993 WSC. Unisearch, the research arm of the University of New South Wales, has supplied sufficient (more than 21% efficient) cells to power four November 1993  53 During the race, telemetry will be all important in monitoring the car’s systems in order to extract the maximum performance from the solar cells. cars. These are likely to be the front running favourites in the race because to clothe a race car in the very best cells costs more than $1 million! A single seater three-wheeled race car will have an array of about 7.9 square metres of cells which, at close to 20% effi­ciency, will develop around 1500 watts in full sunlight. A few car syndicates are claiming more than this. Cells need to be managed and “power trackers” do this. One of the best is the Australian made AERL tracker with a claimed efficiency of better than 95%. Some teams with lavish research laboratories able to construct one-off equipment are expected to have a tracker offering 99% efficiency at full power (when the semi­conductor’s temperature remains below 30°C). Batteries are the big disappointment in solar racing. No significant improvement has been made in the past three years. One team has managed to improve the number of recharging cycles for red-hot racing batteries, but little else has happened in battery development. There are two categories in the race – one for cars with lead acid batteries and one for cars with more elaborate batter­ies. Cars are permitted to carry 54  Silicon Chip five kilowatt-hours of stored power. 5kWh of silver zinc batteries are worth about $40,000 and under race conditions, these can take 10 recharges. Carefully managed, they may manage 30 cycles. After that, they’re scrapped. To be competitive, a team will need, say, three sets of these batteries – or about $120,000 worth – to cope with develop­ ment, training and testing before the race itself. Motor developments Real progress has been made in electric motor design. At least three cars will have the motor inside the drive wheel’s hub. A truly leading edge design will be a DC brushless motor weighing about 12kg and offering 2.5kW of continuous power or a staggering 11kW peak power (think of those poor MOSFETs under full load!). Three teams are claiming effic­ iencies along the lines of 98% for the motor controller, 99% for the tracker and better than 96% for the motor. This will mean that these advanced vehicles will be able to claim better than 92% efficiency from the solar array to the road wheels. One such car, the Northern Territory University’s Desert Rose, has a motor of such efficiency that the team’s lead- er, Dean Patterson, can state that the car is the most efficient motor vehicle ever built. Drag & rolling resistance The last two factors which influence the success of a car are its aerodynamic drag and rolling resistance. The Swiss Engineering School of Biel’s team claims a reduc­ tion in rolling resistance for its tyres of 30% compared with conventional heavy duty bicycle race tyres. A section of the Northern Territory’s Stuart Highway race course was moulded and shipped to Switzerland where an elaborate rolling road was con­structed. At temperatures soaring above 40°C the new tyres were tested to destruction. At one stage the “Stuart Highway” broke, but the tyre survived! Aerodynamics is the last area of technology to contribute to a win. With a frontal area of about 1.1 square metres and a drag coefficient (CdA) of 0.11, these cars have less drag than a fighter airplane. One Australian car, the Aurora Q1 from Victoria, has just shifted the goal posts. Its frontal area is just 0.75 square metres and its CdA an amazing 0.095. This is almost certainly the first road registered vehicle to have an aerodyna­ mic drag of less than CdA 0.1. The performance which comes from this technology should result One of the contenders in the 1990 WSC, this entrant from Hoxan really looked the part but it did not win. This is a preview shot of the Aurora, from Victoria. This car is claimed to have a CdA of 0.095, an unheard figure up till now. in a cruising speed of 80-90km/h in clear sunlight and perhaps 140km/h with the batteries approaching meltdown. A good set of batteries will take a solar race car 200-300km in cloudy weather, or even in rain. So power management tactics will play an important in the 3004km race from Darwin to Adelaide. Part of the tactics is telemetry between the race car and its support vehicles. This technology has been accepted since the first event when the GM Holden SunRaycer’s driver was told when to take on water and when to stop to visit the ‘loo’. The support vehicle monitoring crew knew because telemetry was used to moni­tor the ambient temperature and humidity inside the race car. How much do solar race cars cost? They start at $15,000 for some of the Holden-sponsored school teams and range up to an estimate of $20 million for some of the Japanese teams. At the end of the last race, observers said that an improve­ment of 10% would give a car the winning advantage. At least half a dozen cars would appear to have made more than 30% progress over the 1990 race cars. SC It will be interesting race. November 1993  55