Compared to other forms of motorsport, speeds in the Solar Challenge are slow and the main focus is on the capture and efficient use of solar energy. The winner is the fastest car around the lap. There are three classes, two of which – Challenger and Cruiser – are competitive while the third allows older solar cars that no longer conform to the present rules to take part. Many of the teams have older cars that are still in serviceable condition.

The Challenger cars are the most extreme and aerodynamic; their drivers sit in a relatively cramped compartment and they have the same focus on efficiency as Shell Eco-marathon cars. Where mileage marathon car design is an exercise purely in minimizing drag, the Solar Challenge cars have a large plan area in order to accommodate their solar arrays. The Cruiser class is aimed at a combination of efficiency and practicality. The cars must carry the driver and at least one passenger.

The technical regulations for both competitive classes mandate various parameters, as per any motorsport class. Challenger cars are allowed a maximum of 4 m2 (43.05 sq ft) of exposed solar cells while in the Cruiser Class it is a maximum of 5 m2. There are also limits on the mass of the battery cells, with different limits applied to different battery chemistries in order to provide parity of energy storage capacity. For instance, a team might choose to use 36 kg of LiFePO4 cells, 20 kg of lithium-ion or lithium-polymer cells, or 15 kg of lithium-sulphur cells.

In order to prevent a competition to see who can find the lightest cylindrical cells, the organisers have applied a ‘deemed’ mass to the most popular lithium-ion cells, the 18650, 20700 and 21700 geometries. The figures allow 420, 315 or 285 cells of these respectively. While there is no merit in finding slightly lighter cells, there is in choosing those with a wide operating range in terms of cell voltages, charge and discharge currents, and operating temperatures. Under the rules, the cells are not allowed to operate outside the range specified by the manufacturer. That prevents teams from trying to find an advantage by pushing their cells beyond safe limits. The Challenger cars are designed to run solely on solar energy produced by their onboard cells, while those in the Cruiser class, with their higher energy consumption, are allowed to charge overnight.

There is a balance between speed and duration. A Challenger car must be able to travel at 60 kph and at a minimum of 50 kph on roads with a 100 kph limit. The car can however travel at a speed that is dictated by the power being generated on the car, plus the maximum rate of discharge of the battery. 

Energy and power

If we take a figure of 240 Wh/kg for a representative lithium-ion cell of 18650 geometry, we can see that a typical battery with 20 kg of cells would have 4800 Wh of energy, which is equal to 17.28 MJ of energy. If we compare that to gasoline being burned in a very efficient engine, at 40% efficiency, we could extract that amount of energy from around 1 kg of gasoline. Averaged over the course of 12 hours, that equates to 0.4 kW.

For a domestic PV array, a typical nominal output is 500 W/m2, so we can expect the Challenger cars to have only a few kW of power available. Given a specified maximum area of exposed solar cells, the competition is effectively one of powertrain and aerodynamic efficiency. Since the competition finishes for the day at sunset, the competitors are not permitted to drive onwards after that for as long as their batteries allow. The aim is to achieve the greatest distance in a day, which means travelling at the highest speed. Higher speeds increase drag, with the power requirement increasing as the cube of speed.

Reference

https://www.raceenginetechnology.com