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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
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