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AREAS OF SCIENCE

Energy & Power

Design and build a solar-powered car.
  • Solar power
  • Fossil fuels
  • Renewable
  • Solar panel
  • Circuit
  • Chassis
  • Axle
  • Wheel
  • Bearing
  • Motor
  • Shaft
  • Transmission
  • Friction drive
  • Belt drive
  • Gear drive
  • Mass
  • Stiffness
  • Friction
  • Gear ratio
  • Transmission ratio
  • Drive gear
  • Driven gear
  • Rotations per minute (RPM)
  • Torque
  • Angle
  • What materials could you use to build the different parts of a solar car?
  • What are the different types of transmissions?
  • At a minimum, you will need to purchase an official solar panel and motor for your car. Decide if you want to build the other components from scratch, or purchase a kit that includes additional parts. The official vendors for the Junior Solar Sprint competition, Solar Made and Pitsco, have a variety of options (ranging from just the motor/solar panel to a complete car kit, including a balsa wood chassis, axles, gears, and wheels):
  • If you want to build your own parts from scratch, here are some suggestions for materials:
    • Chassis: corrugated cardboard, balsa wood, Styrofoam®, milk/juice cartons, cardboard tubes
    • Axles: metal coat hanger (cut a straight section), wooden skewer or dowel, any other thin metal rod
    • Bearings: drinking straws, eye hooks, holes drilled in chassis material
    • Wheels: bottle caps, circles cut from a rigid material (plastic, wood)
    • Tires: rubber bands, hot glue along the edge of the wheel
    • Transmission: gears salvaged from toys or an electronic device like a VCR or DVD player, small disks (like Dremel® rotary tool attachments), rubber bands
Solar power is power we get from the sun. Unlike fossil fuels, which can cause significant pollution and emit greenhouse gases when burned, solar power is clean and renewable. However, solar power is not without its drawbacks—it is not available at night or on a cloudy day. Solar panels convert sunlight into electrical energy, which can be used to power machines like motors. They must be connected to the motor by wires to form a circuit. A motor can be used to drive the wheels of a solar-powered car (Figure 1). In order to build a solar car, you need to be familiar with the basic parts that make up a typical car:
  • The chassis is the car's frame, or body.
  • The axles are straight, rigid rods that support the wheels.
  • The bearings connect the axles to the chassis and allow them to spin.
  • The motor converts electrical energy from the solar panel into spinning motion. It has a metal shaft that spins.
  • The transmission transmits rotating motion from the motor's shaft to an axle.

solar-car-parts-labeled
Figure 1. A solar-powered car (top) and the car with the solar panel removed (bottom) so you can see the other components. This car was built using a kit that supplies the solar panel, motor, axles, gears, and wheels (see Materials section). The chassis is a piece of corrugated cardboard, and straws act as the bearings (note that the straws are under the cardboard, so they are difficult to see in this picture). Paper clips hold the solar panel in place on top of the chassis.

There are several different types of transmissions that you can use for a solar car, shown in Figure 2.

  • In a friction drive, a disk on the motor shaft rubs directly against another disk on the axle.
  • In a belt drive, a disk on the motor shaft is connected to a disk on the axle by a belt (e.g. a rubber band).
  • In a gear drive, a gear on the motor shaft meshes with a gear on the axle.

transmission-types
Figure 2. Different types of transmissions.

There are different engineering and physics concepts you will need to take into account when designing and building a solar car. Some decisions may involve trade-offs between different factors. There is no single "correct" way to build a car. Some of the factors you should consider are listed below. Additional resources listed in the Bibliography can help you learn more about these topics.

  • Mass and stiffness: what material(s) will you use to make your chassis? Different materials have different densities (mass per unit volume) and stiffnesses (resistance to bending/flexing). In general, you want a chassis to be stiff and not too flexible. Your motor will have a hard time moving a very heavy chassis. However, if a chassis is too light, it could be blown around easily by the wind, or its tires might slip because there is not enough friction with the ground (see next point).
  • Friction is the force that resists two surfaces sliding against each other. Sometimes friction is bad—you want your bearings and axles to have as little friction between them as possible, so the axles can spin freely. However, sometimes friction is good—you want your tires to have a lot of friction with the ground so they do not slip.
  • The gear ratio is the ratio between the diameter of the drive gear on the motor shaft and the driven gear on the axle. This number tells you how many times the driven gear will rotate for each rotation of the drive gear. For example, if the driven gear is five times bigger in diameter than the drive gear, then whenever the drive gear completes one full rotation, the driven gear will complete 1/5 of a rotation. Note that this concept also applies to friction drives and belt drives, and more generally can be called the transmission ratio.
  • Adjusting the transmission ratio allows you to adjust the speed of your motor, measured in rotations per minute or RPM, and the torque of the motor. There is a trade-off between these two quantities (if you increase the RPM, you decrease the torque, and vice versa). You may need to experiment to find out what gear ratio makes your car go the fastest.
  • The angle of the solar panel relative to the sun's rays affects how much electrical power it produces. It will produce the most power when the panel is perpendicular to the sun's rays.