Solar Car Fundamentals
A solar car's frame is designed to be as minimal as possible, while still able to protect the driver and support the rest of the car's components. To verify the safety of the frame, it is simulated in various collision and rollover situations using finite element analysis (FEA). In addition, the frame is welded by a professional who specializes in building race car frames to ensure high quality construction.
The body of a solar car is designed with two primary goals: providing a large top surface area for mounting solar cells, and being as aerodynamic as possible. Solar cars often have a smooth, wing-like shape and covered wheels, which helps reduce aerodynamic drag.
Team PrISUm uses military grade carbon fiber donated by Boeing to build its bodies. The carbon fiber we receive is normally used on F-18 fighter jets and for rapid prototyping of developmental parts. The first step in building a carbon fiber body is to model the design in CAD software. Then a mold is machined out of huge pieces of high density foam. The team covers the mold in carbon fiber sheets and bakes it in a large autoclave oven. When heated, the resin in the carbon fiber sheets flows freely throughout the entire fabric and it hardens. The final result is a strong and lightweight body that is ideal for solar cars.
Suspension is a necessary system for solar cars because it protects the frame and other on-board components from large jolts encountered along highways. If the suspension is too soft, energy is wasted by absorbing the motion of a car as it travels over bumps. For increased efficiency, most solar cars use a suspension that is stiffer than normal.
To improve efficiency, solar cars use tires with very low rolling resistance. Many teams choose to use Ecopia EP80 tires, which are single-ply racing slicks from Bridgestone. Because they can be run at 110 psi and don't have treads, they significantly reduce the amount of power required to move a solar car down the road.
Solar cars are the sole market for Ecopia EP80 tires, so Bridgestone only manufacturers them for the American Solar Challenge every two years. Each tire costs over $100, which makes them impractical for anything other than competitive events. For testing and exhibition, Team PrISUm uses $10 moped tires, which have higher rolling resistance but are far more durable.
Most solar cars use hydraulic or mechanical disk brakes similar to those found on traditional automobiles. For a solar car to meet race regulations, its brakes must be able to bring it to a complete stop from 30mph in just 70 feet. Additionally, each car must have a redundant brake system, including a second set of calipers and a second master cylinder in case of failure.
The defining feature of any solar car is its array of solar cells. The solar array can consist of anywhere from hundreds to thousands of individual solar cells, which convert sunlight into electricity. A typical silicon cell for a solar car is made of ultra pure mono crystalline silicon, and can reach efficiencies of over 20%. Some solar cars feature more expensive gallium arsenide (GaAs) solar cells, which are nearly 30% efficient.
As a general rule, 1000 Watts of sunlight fall on every square meter of Earth's surface during the peak afternoon hours. When the efficiency of the solar cells is factored in, this equates to a theoretical maximum of about 1300 Watts of power for solar cars that compete in the American Solar Challenge. This is approximately the same amount of power used by a microwave oven, but is enough to move a solar car down the road at upwards of 40mph!
An important part of any solar array is the maximum power point tracker (MPPT). Due to the characteristics of solar cells, they produce the most power when operated at a specific current. This ideal current varies over time, depending on the amount of sunlight, temperature, and other factors. The MPPTs continually adjust the current to maintain the optimal efficiency, and boost the voltage so that this energy can be used to charge the car's battery pack.
When it's not sunny, a solar car can drive using power from its battery pack. Most teams use lithium ion batteries similar to the ones found in laptops and cell phones. Under race regulations, a solar car can carry 20 kg of lithium ion batteries, which hold enough energy to drive a car over 100 miles without sunlight. The battery pack can be charged in 6-8 hours using only the solar array, or roughly 3 hours using a 220 V drier outlet. A typical solar car battery pack can be charged with less than 50 cents of electricity.
Motor and Motor Controller
Many solar car teams use an NGM brand electric motor and motor controller. This motor is built into the wheel hub and does not have a transmission. The motor controller converts DC power from the batteries and solar array into three-phase AC electricity for the motor. The motor operates similarly to a synchronous AC motor, so controlling the speed of the motor is done by simply adjusting the frequency of the AC output.