We have been writing about the electrification of motorsport for some time now, but the direct use of solar power has not been covered in this section before. While Extreme E fundamentally draws its energy from solar energy via a complex and clever route involving hydrogen and batteries to charge a pretty conventional battery, there is a competition for solar-powered vehicles whose only means of charging is by solar panels fitted to the vehicle.

This is the solar car that holds the 12-hour distance record, seen in action at Lommel during the record run (Courtesy of Innoptus Solar Team)

The World Solar Challenge takes place on public roads in Australia, and covers a distance of just over 3000 km, from Darwin to Adelaide. It is becoming more relevant than ever, not only because of the increasing electrification of passenger transport and motorsport, but because some EV manufacturers are interested in augmenting their vehicles’ driving range through the use of solar energy. If the solar panels are exposed to sunlight, there will be a degree of charging regardless of whether the vehicle is being driven. While the likes of Aptera and Lightyear might only be familiar to ardent followers of EV developments, their vehicles are going to rely heavily on solar energy. However, that is not to say that only niche manufacturers are interested in solar energy being generated on passenger vehicles – Hyundai is also planning to introduce solar roofs into its roadcars.

Classes apart

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 first to arrive at the finish line. 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 minimising 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.

This image of the opened canopy shows the connections between the solar cells (Courtesy of Innoptus Solar Team)

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. However, to give an idea of what is possible in good conditions, the Agoria Solar Team from Leuven, in Belgium (now renamed the Innoptus Solar Team), broke the 12-hour record for distance travelled in 2022 by driving for 1051 km. The new record marked an improvement of almost 14% over the previous best, and was not broken in an equatorial location but in Belgium. The average speed over the 12 hours was 87.5 kph.

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. It remains to be seen if any records will be broken in the 2023 World Solar Challenge but, with an increasing focus on solar energy to augment the range of road-going battery EVs, this event will be closely watched.