Solar-Powered Cars for Junior Solar Sprints

Building solar cars for the Junior Solar Sprint creates a hands-on opportunity for students to learn about many scientific and engineering concepts, ranging from solar energy, forces, mechanical efficiency, automotive design, and the engineering design process. This background section lays out some important features of the car that you and your students can explore, but feel free to tailor the lesson and topics to what works best in your classroom. You can also tie this lesson in with a unit about alternative/renewable energy sources.

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 which 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 the 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 connects the motor's shaft to one of the axles (it is called a transmission because it "transmits" power from one place to another).

A solar panel, motor, axles, gears and wheels are purchased online from Solar Made kits or Pitsco kits. The chassis of the solar power car is made from a rectangular piece of cardboard cut to a slightly smaller length than the solar panel. Attached to the bottom of the cardboard chassis are two plastic tubes similar to straws that will act as axles for four plastic wheels. A motor with two alligator clip leads is taped to the top of the chassis next to a rear wheel that has a gear which will interlock with a gear on the motor. Three paperclips are bent and taped to the top of the chassis to support the solar panel, two at the rear and one at the front.

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. 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 (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 pulley on the motor shaft is connected to a pulley 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.

Different transmissions change the way a motor transfers its energy to a wheel. In a friction drive a motor spins a smooth disk that is in contact with a smooth disk attached to the axle, the friction between the two disks allows the motor to spin the axle and wheels. In a belt drive a belt is wrapped around a disk on a motor and a disk on an axle, as the motors disk spins the belt is pulled along and rotates the disk on the axle. In a gear drive the motor and axle have gears instead of smooth disks. When the motor spins its gear, the force is transfered to the axles gear causing it to spin as well.

Figure 2. Different types of transmissions.

There are different engineering and physics concepts your students need to take into account when designing and building a solar car. Some of the factors are listed here. You can decide whether and to want extent you want to cover these topics in your class.

• 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 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 or transmission 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.)
• Adjusting the gear ratio allows you to adjust the speed of your motor, measured in rotations per minute or RPM, and the torque of the motor, or how "hard" it spins. 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.