We recommend a solar array created from individual solar cells as opposed to one made of prefabricated solar panels. It enhances the students' learning and can result in a lighter solar array. Cells can be bought from either Siemens or ASE Americas. Both sell the terrestrial-grade cells that are permitted in the Winston Solar Challenge, and the cost for terrestrial-grade cells are much lower than space-grade cells, though terrestrial-grade is less efficient. Each solar cell should produce .5 volts at about 3 amps at peak sunlight. The number of cells to use depends on their size and the allowable solar area per Winston rules. Solar cells should be wired in series on a panel and should be divided into several zones. For example, if you have 750 solar cells, you might want to wire 3 sets of 250 cells, each zone producing about 125 volts. If one zone fails, two other zones are still producing power. The solar array voltage does not need to match the system voltage of the motor if you use power trackers. Power trackers convert the solar array voltage to the system voltage. They are essential in a solar car. Be sure to verify with the power tracker vendor the necessary array voltage to feed the power trackers. If the car drives underneath shade, the power trackers automatically adjusts the power to match system voltage, allowing the system to run as efficient as possible. Power trackers are available from AERL.
Batteries
The batteries store energy from the solar array and makes them available for the motor's use. Many different types of
batteries are sold. Most high school teams use lead-acid batteries because they are inexpensive, but some teams use
lithium-ion or nickel-cadmium. We recommend that you stick with lead-acid batteries because they are readily available and
inexpensive. Another choice teams must make is running with flooded-cell batteries or gel-cell batteries. Flooded-cell
batteries are the standard automotive batteries filled with liquid sulfuric acid. They are preferred because they can be
overcharged without risk of blowing up, but they weigh more than gel-cell batteries. Gel-cell are sealed and lightweight,
but when charging the batteries, check the battery voltage often. The number of batteries to choose depends on the motor
(system) voltage. If the system voltage is 72 volts, you will need 6 12-volt batteries. Also be sure to check the rules
for weight or watthour requirements. Buy batteries with as many amphours as allowed by the rules to maximize the amount of
energy you can store.
Motor & Controller
Most teams use DC brush permanent magnet motors to drive their solar
cars. Inexpensive and easy to hook up, these motors are desirable for high school teams with little financial support.
Expect a maximum efficiency of 80-90%. For teams with more money, brushless motors increase the efficiency of the motor to
the 94-99% range. Also, some motor and controller setups allow for regenerative braking, which allows the solar car to put
energy back into the batteries when going downhill. For the beginning team, DC brush motors would be sufficient to get a
solar car up and running. Another variable in choosing a motor is how much power it has. We have found that there is
little need to have more than 5hp continuous power output on our motors. There are two manufacturers who supply most teams
with motors and controllers: Solectria and Advanced DC Motors. Many college teams buy their motors from Solectria, but
Advanced DC Motors have less expensive motors. Controllers usually drive a particular motor. Once you choose the motor
that suits your needs, the same vendor would most likely have a matching controller.
Instrumentation
One of the most important pieces of instrumentation is a state-of-charge meter. A state-of-charge meter gives
information about system voltage, amp draw, battery energy remaining, and estimates the how much time remains until the
battery is out of energy. We found that the E-Meter, manufactured by Cruising Equipment, served out purpose well. It has
a digital display and accurately counts the number of amp-hours remaining in the battery. The E-Meter is the do-it-all in
instrumentation. Another instrument that may be useful is a speedometer. Instead of using a regular speedometer drive,
use magnetic contact speedometers, found in many sports equipment stores. This option does not add drag to your car. To
ensure that your batteries are running properly, you may invest in getting a voltmeter for each of your batteries. A failed
battery may show the proper voltage when the car is not running, but while the battery is under load, the voltmeter will
show a lower than normal battery voltage.
Steering & Suspension
We strongly recommend front wheel steering as it tends to be more stable and safer. A solar car uses energy frugally if
it is to be competitive. If there are two front wheels, it is therefore advisable to work out the geometry so that they run
parallel when the car is going straight ahead, but when the car is turning, the front wheels turn at different radii. If
the car is turning left, the left front tire is making a smaller circle than the right front tire. If the tires remain
parallel while turning, they will cause unnecessary drag, decreasing tire life and overall performance.
The only advice we can offer with respect to suspension is that it should be soft enough to protect the car and solar
array from unnecessary jolts and firm enough to provide a stable ride.
Brakes
Disc brakes are desirable as they are predominantly hydraulic. Having hydraulic lines running to the wheels can be
easier than mechanical brake arrangements. The most significant problem with disc brakes is that the brake pads do not
back away from the brake rotors when pressure is released, they just relieve braking pressure. Because the pads don't
normally back away from the rotors, they continue to have a small amount of drag. While this drag may not be noticeable on
the family car, it is very inefficient on solar cars. Go kart shops now have brake calipers that are spring loaded to move
the pads away from the rotors. We have found these very worthwhile.
Tires & Hubs
Tire selection will affect rolling resistance which affects how far the solar car will travel with the energy available.
Tires with thicker rubber and wider tread tend to have higher rolling resistance (a bad thing). Thinner tires with higher
pressuer have less rolling resistance, but are more susceptible to flats. The best tires we have found are the Bridgestone
Ecopia tires made for solar cars. They are very thin and operate at over one hundred pounds/inch pressure. Unfortunately,
they need to be mounted on specially made wheels and require custom made hubs. On the good side, these tires and wheels are
very light. Some colege teams have experimented with bicycle tires but report limited success (bicycle tires, rims and
spokes are not designed for the forces placed on them by non-tilting vechicles that weigh several hundred pounds).
Motorcycle tires tend to have more resistance, although there may be high pressure tires with low resistance that we don't
know about yet.
Bearing resistance can be reduced by light minimal lubrication. Bearing seals can be cut away at the contact lip to
leave most of the seal protection while removing most if not all seal drag. It is a good idea to get the rolling chassis
operational months before your schedule gets critical. Run the chassis as many miles as possible to prove that your bearings,
axles, steering and suspension can survive.
Please direct comments and questions to:
William Shih, Northview Solar Racing Team
Last modified: September 2, 1998
