How does a Solar Car work?

Mechanical

Frame

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 race car frames to ensure the highest quality construction.

The frame used by Fusion, our 2005 car, was built using thin-wall 6061 aluminum and was heat treated to make it as lightweight and strong as possible.

Body

The body of a solar car is designed with two goals: provide a large top surface area for mounting solar cells, and be as aerodynamic as possible.  Solar car drivers typically lay down in a solar car to reduce the height of the car and, consequently, its drag.  Many solar cars have a wing shape and covered wheels, which also help reduce drag.

There are a variety of materials that are used to make the body of a solar car.  Team PrISUm uses expired military grade carbon fibre donated by Boeing.  The carbon fibre we receive is normally used on F-18s and for rapid prototyping of developmental parts.  Resin is pre-impregnated in the carbon fibre, so it must be stored in a freezer until it is ready to be used.

The first step in building a carbon fibre body is to draw the design in CAD software.  Then a mold is constructed out of high density foam and taken to Remmele Engineering, where a giant CNC machine carves the body’s shape out of the mold.  The team then covers the mold in carbon fibre, which is a fabric before the resin cures.  Once the mold is covered with sheets of carbon fibre, it is put under a vacuum and baked in an autoclave.  When heated, the resin in the carbon fibre flows freely throughout the entire fabric and hardens.  The final result is a strong and lightweight body that can take complex shapes.

To make carbon fiber stronger, multiple layers of fabric can be used.  Strength is also greatly increased by placing Nomex, a light but strong honeycomb material, material between the layers.

Suspension

Fusion features a four-wheel independent suspension with custom Penske shocks, but it is not designed to provide a smooth ride.  Soft suspensions waste energy by absorbing the motion of the car as it travels over a bump.  Therefore, solar cars have a very stiff suspension, which is designed to prevent damage to the frame in the event of a large jolt.

Tires

To improve efficiency, solar cars use tires with very low rolling resistance.  Many teams choose to use Ecopia tires, which are single-ply racing slicks from Bridgestone.  Because they can be run at 90 psi and don’t have treads, they significantly reduce the amount of power required to move the solar car down the road.

Solar cars are the sole market for Ecopia tires, so Bridgestone only manufacturers them for NASC.  Every race, solar car teams combine their orders and purchase an entire production run of Ecopias.  Each tire costs nearly $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 much more durable.

Brakes

Most solar cars use hydraulic disk brakes like those found on a traditional automobile. For a solar car to qualify for NASC, its brakes must be able to bring it to a complete stop from 30 MPH in only 70 feet.  In addition, each car must have a redundant brake system, including a second set calipers and a second master cylinder.  Fusion's brake system uses calipers from a Harley Davidson drag bike and custom titanium brake discs.

Electrical

Solar Array

A solar car wouldn't be a solar car without its solar array.  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 monocrystalline silicon, and can reach efficiencies up to 20%.  Some solar cars feature gallium arsenide (GaAs) solar cells, which are nearly 30% efficient.  GaAs cells are typically produced for military and space applications, but there will often be surplus from government orders that solar car teams can purchase.

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 1250 to 2250 Watts of power for solar cars that compete in NASC.  This is approximately the same amount of power used by a hairdryer or microwave oven, but is enough to move a solar car down the road!

Power Trackers

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 level, and boost the voltage so that this energy can be used to charge the batteries.

Batteries

When it's not sunny, a solar car can drive using its battery pack.  Most teams use lithium ion batteries similar to the ones found in a laptop or cell phone.  Under NASC regulations, a solar car can carry 25 kg of lithium ion batteries, which hold enough energy to drive the car about 200 miles without sunlight.  The battery pack can be charged in about a day using only the solar array, or in roughly 3 hours using a 220 V drier outlet.  A typical battery pack can store 4.5 kWh, meaning that it costs less than 50¢ in electricity to charge the car.

Motor and Motor Controller

Like many solar car teams, Team PrISUm uses an NGM brand electric motor and motor controller.  The motor is built into the wheel and does not have a transmission; one rotation of the motor is the same as one rotation of the wheel.  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.