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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
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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
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