This is the first of a series of pages intended to give readers from all backgrounds and roles an overview of a solar car's systems, their design, and manufacturing.

The Competition

Competitive solar cars are generally split between two classes:

• Challenger
• Cruiser

These classes are defined by the World Solar Challenge (WSC), which is the forefront international solar car competition held every two years across Australia. There is another competition held every other two years in the USA, the American Solar Challenge (ASC), which is starting to recognize the definitions outlined by WSC and more closely aligning its regulations with those of WSC.

Challenger Class

The Challenger class comprises ultra-lightweight and aerodynamic endurance race cars carrying a single driver, usually in a small cockpit with a canopy. The shape of these cars is designed for maximum efficiency and solar energy collection, usually resulting in a large flat surface to maximize solar array area. This was the original solar car class and remains today the most well-established competition category, containing the majority of solar cars teams around the world. The competition style is a time challenge where the team that reaches the finish line with the least time expired wins.

Midnight Sun has traditionally designed Challenger Class vehicles, but in recent years has shifted to the newer Cruiser class.

Cruiser Class

The Cruiser Class is a fairly new class (started in 2013) that focusses on energy-efficient, practical vehicles designed to carry at least two people. The WSC competition is structured as a "regularity trial" in which teams must reach the finish line within a certain time window but are scored solely on energy consumption and "practicality", which is determined by judges. This is intended to mirror the design goals of consumer passenger vehicles.

As of 2018, the American Solar Challenge holds a separate class for Cruiser vehicles, with special rules and scoring for them. However, the scoring system is still under review and is subject to change.

Frame Design: Chassis & Body

Building a solar car requires extensive design to be put into the vehicle's chassis and body. These terms are used in automotive engineering and are described below:

Chassis: The structure on which the vehicle's primary components are mounted. In traditional automotive engineering, the chassis provides the majority of the vehicle's structural strength.

Body: The structure (commonly panels) that form the exterior of the vehicle, directing airflow and determining the appearance of the vehicle.

These terms originate from a time when most vehicles had a separate chassis, in the form of a heavy flat steel frame (onto which the engine and suspension were mounted), and body, in the form of metal panels attached together. This type of design is referred to as a "body-on-frame" and is rarely seen today on anything except trucks.

Modern Automotive Frames

In modern vehicles, automotive frame design has evolved to the point where in some cases the chassis and body become much harder to identify as separate entities. Three common design patterns for vehicle frames are:

Unibody: The frame and body are a single integrated piece. This is the most common design used on modern consumer cars. Steel unibodies form most of the body of the vehicle with an integrated steel structure and may have plastic paneling for aerodynamics.

Space frame: The vehicle has a skeletal frame constructed of steel beams or tubes of varying diameter and wall thickness. Non-structural panels are attached to the frame solely for aerodynamic purposes but otherwise provide no additional strength. This design is used on some high-performance consumer cars and smaller, older aircraft. The strict technical definition for a space frame requires that the frame be sufficiently triangulated such that all forces within the frame are turned into compression or tension, and not flexing.

Monocoque: The body of the car provides all of the strength through distributing loads through its "skin". This is a design typically only seen in aircraft, F1 motorsports, and some high performance motorcycles, where the entire body of the vehicle is formed with a strong shell made of either carbon fibre composites or aluminum panels. Some high performance consumer cars use a monocoque for the passenger cell but extend metal subframes from the front and back for mounting the engine and suspension. Monocoques have the advantage of theoretically achieving the greatest strength-to-weight ratio, but are very hard to design because stretched-skin structures are difficult both to simulate and modify.

Solar Car Frames

Solar cars can be found that use steel space frames and composite unibodies. Composite unibodies can be formed by building a strong composite chassis using thick composite panels and attaching to it a shell made of thinner composite material. The differences between Challenger and Cruiser class vehicles become relevant here: for example, Cruiser class vehicles must incorporate a much larger passenger cell which can made designing a strong composite frame difficult. Furthermore, all solar cars must have a metal roll cage, which can potentially be challenging to attach reliably to a composite frame.

Midnight Sun's MSX was designed with a steel space frame, whereas MSXI was designed using a carbon fibre monocoque.

Manufacturing

Creating composite panels for the body shell of a solar car requires molds to be made, which have historically started from CNC'ed polystyrene or medium density fibreboard (MDF). Flat panels do not require molds and can be made using a mirror. Composite panels can be made of fibreglass or carbon fibre, the latter of which has seen the most use by our team in MSXI. Structural panels that need high strength are usually made by sandwiching a core material, which can be a honeycomb or foam material, between layers of carbon fibre.

The manufacturing process of composite panels is usually referred to as a "layup". A "wet" layup involves laying sheets of carbon fibre or fibreglass onto a surface, applying epoxy resin,  and curing the soaked composite under a vacuum seal. This process is known as "vacuum bagging", because the film used to seal the vacuum is pressed against the panel by the ambient air pressure. A more advanced process involves using carbon fibre which has already been "impregnated" with resin, called "prepreg", which must be stored in a freezer to prevent the resin from curing before use. Once the material is removed it must be applied within a certain time and cured in an industrial oven or autoclave (pressure chamber).

Solar Car Powertrains

A vehicle's powertrain encompasses everything that generates power and propels the car by transferring it to the wheels. In a traditional gasoline automobile, this means the engine, gearbox, drive shaft (if present), differential, axles, suspension, and wheels. Solar cars, as electric vehicles, forego the engine, gearbox, drive shaft, and differential, since most solar cars use electric motors that directly mount behind the wheels. However, solar cars must have a large battery for energy storage and, of course, a PV array for energy generation.

Battery

Most modern solar cars, like modern electric cars, use Lithium-ion batteries for their main energy storage. This type of battery chemistry provides the greatest energy-to-mass ratio, which is also why they're found everywhere in consumer electronics like phones and laptops. The smallest unit in a battery is known as a cell, and a battery is (strictly speaking) an arrangement of multiple cells to provide a greater total capacity and/or voltage. An extremely common Li-ion cell size is known as the 18650, which is economic to purchase due to their popularity.

Cells that can be recharged are known as secondary cells, whereas cells that cannot be recharged are known as primary cells (AA alkaline cells).

The most important point to understand about rechargeable batteries is that they do not store electricity. Batteries store energy in the form of chemical energy, and it is chemical reactions occurring within the battery that either produce or consume electrons, giving the appearance of charge being stored and released. Thus, batteries cannot be thought of as a water balloon that is only at risk of being damaged and rupturing if too much "water" is put in them. Furthermore, it is not valid to assume that batteries are fully "depleted" when they reach 0V. Voltage is not equivalent to remaining capacity!

In reality, because batteries only contain chemical reactions, causing the voltage between the terminals to go above or below its operating limits, regardless of the current state of charge, will cause damage to the battery and potentially fire or explosions. Batteries also have a temperature operating range and leaving this range will also result in damage to the cell and dangerous results. These voltage and temperature operating regions are especially important when using Li-ion cells because they have both a very narrow safe region and a very high energy density.

Virtually all Li-ion batteries are monitored in real-time by a battery protection system (BPS). The BPS is usually its own PCB and is an embedded system located typically inside the battery enclosure. It monitors parameters such as cell voltage and temperature, taking appropriate action such as quickly disconnecting the battery from the car in the event that the battery leaves its safe operating region.

Motors

Solar cars use electric motors, usually mounted directly to the wheels, to move the car. Motors are large, heavy electrical devices that contain conductor coils and permanent magnets. In order to generate force, current must be passed through different coils at a time, which is the job of a motor controller. The motor controller acts as an interface to the motor by taking high voltage power and control signals from a digital control port and applying the appropriate current to the motor to make it move properly.

Steering

The typical design of a solar car's steering system uses a mechanism called a rack and pinion to transfer the rotational motion of the steering wheel into linear motion to pivot the front wheels. The steering rack is mounted between the front wheels and attaches to both using tie rods. When the pinion is shifted left of right, the tie rods push the front wheels to rotate them accordingly.

The steering wheel is attached to a long rod called a steering column, which mounts to the rack to couple it to the steering wheel.

Suspension

For simplicity, and because solar cars don't need to transfer the motion of a gas engine to the wheels, most solar cars do not have axles. Each wheel is mounted directly to the vehicle. Each wheel mounts to its suspension system which then mounts to the frame. There are many different types of suspension systems, each with different geometries.

PV Array

The photovoltaic array, arguably the most characteristic component of a solar car, converts solar energy to electricity both while the car is driving and when it is parked. PV arrays absorb a wide spectrum of light, with a large part coming from infrared frequencies. This means arrays will usually not work behind windows as many windows have films that reject most IR light.

Under full illumination, a solar car's array is designed to generate voltages exceeding 100V and thus is a safety hazard. Solar arrays should be treated in the same way as battery packs or wall outlets. Arrays are also extremely fragile and can scratch or crack when touched on their surface.

Electronic Control Units

The term "Electronic Control Unit" (ECU) is used in automotive engineering to refer to electronic embedded systems within a car that control everything from driver controls to ABS braking and power door locks. They are commonly connected together on a shared digital interface known as a Controller Area Network (CAN). This interface can deliver both data between boards and supply moderate amounts of low voltage power. The primary source of low voltage power comes from DC power supplies in the car that convert battery voltage (typically 120V) to 12V.

See Intro to Solar Car Electrical Systems for an overview of how a solar car's electrical system fits together.

Midnight Sun's ECUs

Midnight Sun's current electrical system has 5 types of ECUs to control the primary vehicle systems:

Power management: Controls power switches connecting high voltage system components like the battery, motor controller, and array. It performs the vehicle start up and power down sequences.

Battery protection (BPS): Monitors the main battery pack and signals a BPS fault in the event of cell over-voltage, under-voltage, over-temperature, under-temperature, and over-current conditions.

Driver controls: Contains driver interface with buttons and switches for toggling vehicle power, turn signals, horn, etc.

Lights: Routes and controls power to the vehicle headlights, braking lights, and turn signals, by receiving commands from the driver controls board.

Motor controllers: Routes and controls power to vehicle motors (sources current into battery pack during regenerative braking), by receiving commands from the driver controls board.

Maximum power point tracking (MPPT): Power converter that transforms PV array voltage to a constant value, while adjusting the current draw from the array to maximize total power output available to the car.

Our electrical system also has a non-critical ECU for telemetry, which is equipped with a radio transmitter to broadcast vehicle diagnostic information to a team member in a chase vehicle during a race.

All ECU boards except for the motor controllers and MPPT boards are designed and manufactured by the electrical team, which also contains a large software group dedicated to writing the embedded firmware for all ECUs. Midnight Sun has a relationship with a high-quality PCB fab located in Markham, but also uses various other fabs with faster lead time. All soldering is done by team members, which requires good training to ensure reliable solder joints are made consistently on our boards (debugging hardware manufacturing problems later takes far longer than getting it right the first time).