Solar racing basics revisited

Last year I blogged this poster of solar racing basics. Things have changed slightly, but I think it might still be useful for new teams. Here are the 10 posts explaining it:

  1. Classes
  2. Aerodynamics
  3. Electrics
  4. Chassis
  5. Mechanics
  6. Race Strategy
  7. Logistics
  8. Sponsorship
  9. Media
  10. Map for the BWSC

Click to zoom / Image credits: Agoria Solar Team (wind tunnel), American Solar Challenge (chassis), Solar Team Eindhoven (Cruiser car), (battery & motor), Brunel Solar Team (race strategy), public domain (lower right 3), Anthony Dekker (remaining 7).

Solar Racing Basics: Map

Finishing up the analysis of my Solar Racing Basics Poster (see this tag), the central portion of the poster shows the route of the World Solar Challenge (although there will be no WSC until 2023). The race traditionally starts in Darwin and finishes in Adelaide, with nine compulsory 30-minute control stops in Katherine, Daly Waters, Tennant Creek, Barrow Creek, Alice Springs, Kulgera, Coober Pedy, Glendambo, and Port Augusta (solid white dots on the map). There is also rest time between 17:00 and 8:00 each night.

To read more about the route, see this detailed post.

Click to zoom / Image credits: Agoria Solar Team (wind tunnel), American Solar Challenge (chassis), Solar Team Eindhoven (Cruiser car), (battery & motor), Vattenfall Solar Team (race strategy), public domain (lower right 3), Anthony Dekker (remaining 7).

Solar Racing Basics: Media

Click to zoom / Public domain image

Continuing the analysis of my Solar Racing Basics Poster (see this tag), documenting the construction and racing of the car is important for the sponsors, for the fans, and for the team itself. Teams that want good sponsorship need a good media subteam. Good media helps a team convince potential sponsors that they are legitimate, and can provide a quid pro quo to existing sponsors. Solar Team Twente from the Netherlands is an example of a team with good media. In particular, they have a website and six kinds of social media:  .

Often the media subteam includes a specialist photographer, such as Jorrit Lousberg (Vattenfall/Delft), Hans-Peter van Velthoven (Vattenfall/Delft), Bart van Overbeeke (Eindhoven), Jerome Wassenaar (Twente), or Joseph Xu (Michigan).

In addition, media subteams will often “place” stories in local newspapers. It’s big news in Springfield, population 24,000 (to pick an imaginary example), that a young man or woman from the town is off to race a solar car on the other side of the world. Equally, it’s big news for Dutch-language media in Australia, such as SBS, that Dutch solar car teams have arrived in the country (and ditto for Italian, Turkish, and several other languages).

Media can also act as damage limitation in a crisis. In 2017, the Persian Gazelle 4 car from the University of Tehran was heavily damaged in transit, and was unable to race, leaving the team with very little to show for all their hard work. In 2019, NunaX from Vattenfall/Delft Solar Team was totally destroyed by fire during the race, but the team could still point to the copious media of the car and say, with justified pride, “we built that”:  .

It is, of course, important that teams not make classic media errors, such as constantly changing social media “handles,” retiring social media channels without proper announcement, not keeping the website up to date, or posting embarrassing photographs that you wouldn’t want a potential sponsor to see.

To read more:

Solar Racing Basics: Sponsorship

Click to zoom / Public domain image

Continuing the analysis of my Solar Racing Basics Poster (see this tag), the cost of logistics, and of building a car, mean that sponsorship is critical. There are three main kinds:

  • Cash sponsorship from companies
  • In-kind sponsorship (free or reduced-price products and services) from companies
  • Crowdfunding (donations)

Major cash sponsors are often acknowledged in the name of the team, e.g. Vattenfall, Agoria, or Top Dutch (the latter is a marketing campaign for the three northern provinces of the Netherlands). Alternatively, major cash sponsors can be acknowledged in the name of the car, e.g. Covestro Sonnenwagen from Sonnenwagen Aachen, thyssenkrupp blue.cruiser from Bochum, or Unlimited (Western Sydney University motto) from Western Sydney Solar Team.

In-kind sponsors often appreciate positive media coverage of the product or service, or photographs of the product or service in use.

Crowdfunding can be particularly promising when a team effectively represents a nation, region, or state, e.g. Agoria (Belgium), EcoPhoton (Malaysia), STC (Thailand), Hyadi (Mexico), or Top Dutch (three northern provinces of the Netherlands).

Click to zoom / Image credits: Anthony Dekker (Agoria’s sponsor logos on the side of their car and Western Sydney’s Unlimited 3.0 painted in their university’s colour)

To read more, see these posts from from team Arrow:

Solar Racing Basics: Logistics

Click to zoom / Public domain image

Even though the World SolarChallenge has been cancelled for 2021, I thought it worthwhile to continue the analysis of my Solar Racing Basics Poster (see this tag). There is an old military saying that “amateurs talk about tactics, but professionals study logistics.” The logistics of bringing a car and a team to Australia can be surprisingly difficult, and if you can’t do that, it really doesn’t matter how good the car is. Issues include:

  • Complexities of international freight (especially when batteries are classed as “dangerous goods”)
  • Australian customs and biosecurity regulations
  • Tickets and visas for the team
  • Covid-related regulations
  • Renting equipment and support vehicles in Australia
  • Having enough people legally permitted to drive in Australia (especially driving rental vehicles and trucks)
  • Accomodation in Australia (including tents for the Outback during the race)
  • Food in Australia (especially food in the Outback)
  • Insurance (of various kinds)
  • Other issues noted in the official Team Manager’s Guide

This comment from the Belgian team in 2017 highlights the complexity of just one issue:

“ ‘Voor mij is het de eerste keer dat ik voor zo een uitdaging sta,’ zegt logistiek manager Pieter Galle uit Leuven. ‘Het batterijpakket versturen is de grootste uitdaging voor het team. De batterijcellen die wij gebruiken zijn vaak niet toegelaten op vluchten. Om deze toch te kunnen versturen moeten er veel veiligheidsmaatregelen getroffen worden. Gelukkig heeft DHL Global Forwarding, in samenwerking met Deufol als verpakker van de goederen en batterijen alles tot in de puntjes kunnen regelen, zodat wij ons met het team volledig op het wereldkampioenschap konden concentreren.’ ”

(Translation: “ ‘It’s the first time I’ve faced a challenge like this,’ says logistics manager Pieter Galle from Leuven. ‘Transporting the battery pack was the biggest challenge for the team. The batteries we use are often forbidden on flights. To be able to send them, many safety measures need to be taken. Fortunately, DHL Global Forwarding, in cooperation with Deufol our packer, has managed all the details, making it possible for us to focus our attention on the world championship.’ ”)

To read more, see this post from Solar Team Twente and this 2016 solar car conference presentation.

Solar Racing Basics: Race Strategy

Click to zoom / Image credit: Vattenfall/Nuon/Delft Solar Team

Continuing the analysis of my Solar Racing Basics Poster (see this tag), it’s important to remember that the “best” car doesn’t always win the race. The work of the strategy subteam in the “chase vehicle” is also critically important. During the race, this subteam constantly calculates the best speed for the conditions, taking into account weather, road conditions, and existing battery charge. This includes deciding where it is worth speeding up to avoid upcoming bad weather. Making the right decision can be critical – the chart below (click to zoom) summarises the Challenger Class of the 2017 World Solar Challenge:

Road distance in this chart is from left to right, and the vertical axis shows how fast teams are (higher is slower, and the faint dashed lines show specific speeds). Teams 15 (Western Sydney University) and 88 (Kogakuin) were keeping up with the leaders, but made what in hindsight was the wrong decision when bad weather loomed, losing 6 or 7 hours as a result. Making the right decision under these circumstances is very difficult, however, and relies on good weather prediction services (or, as some teams have done in the past, on taking a meteorologist along).

Click to zoom / Image credits: NASA (unsettled weather across central Australia, 2013) and Vattenfall/Nuon/Delft Solar Team (interior of their chase vehicle for the 2011 World Solar Challenge)

In the Cruiser class, race strategy also includes deciding how many passengers to carry (more points, but more weight), and how much to recharge from the grid at stage stops (more energy, but fewer points). That makes Cruiser strategy an even more difficult problem.

To read more, see this post on Challenger strategy which I wrote in 2018.

Solar Racing Basics: Mechanics

Click to zoom / Image credit: Anthony Dekker

Continuing the analysis of my Solar Racing Basics Poster (see this tag), let us look at the mechanical aspects of solar cars. In order to drive, the car obviously needs steering (up until 2019, four-wheel steering was allowed, and it had some advantages). Brakes are essential too, of course. The car also needs a suspension, which allows individual wheels to move up and down (a double wishbone suspension is common for the front wheels). Attached to the suspension are shock absorbers and springs.

The mechanical parts of the car need to be strong, but not too heavy. They need to be able to survive the vibration that comes from driving more than 3,000 kilometres along the Stuart Highway (balancing strength and weight can lead to some roadside repairs, as in this Vattenfall/Delft video). And in small or narrow cars, considerable creativity is needed to fit all the mechanical components inside!

Click to zoom / Image credits: Agoria Solar Team (Agoria’s award-winning partial four-wheel steering, 2017) and Anthony Dekker (interior of Twente’s exceptionally tiny RED E showing the suspension, 2019)

To read more, see see this post about brakes and this post about suspension and steering by Nick Elderfield of the University of Calgary Solar Car Team.

Solar Racing Basics: Chassis

Click to zoom / Image credit: American Solar Challenge

Continuing the analysis of my Solar Racing Basics Poster (see this tag), solar cars have to keep their driver safe and the vehicle in one piece. There are two basic ways of doing this. First, a car can have a carbon-fibre-reinforced polymer body over a metal chassis. For example, Bochum’s thyssenkrupp blue.cruiser (below) is supported by a tubular frame of ultrahigh-strength steel. Second, a car can have a load-bearing “monocoque” body, possibly also of carbon-fibre-reinforced polymer. Carbon-fibre-reinforced polymer is strong for its weight, and this is significant, since a noticeable amount of energy in a solar car (though less than aerodynamic drag) is lost in rolling resistance. The rolling resistance of a car is proportional to its weight (it also depends on the quality of the tires), and so reducing weight makes the car faster. In 2019, the lightest solar car (from Western Sydney) weighed just 116.8 kg without the driver.

Cars may include a “roll bar” or “roll cage” to protect the driver in addition to the monocoque body. This “roll bar” or “roll cage” may be made of metal tubes, or it may also be made of carbon-fibre-reinforced polymer. A close look at unpainted carbon-fibre-reinforced polymer shows the “chequerboard” pattern of carbon-fibre “cloth” embedded inside transparent epoxy polymer (as in the body and roll bar of Durham’s Ortus, also below).

Click to zoom / Image credits: Anthony Dekker (Bochum’s thyssenkrupp blue.cruiser and the interior of Durham’s Ortus)

To read more, see see this post about car body and chassis by Nick Elderfield of the University of Calgary Solar Car Team, this Instagram post about composite materials by MIT Solar Electric Vehicle Team, and this UMNSVP wiki on Composite Chassis Design.

Solar Racing Basics: Electrics

Click to zoom / Image credit: Anthony Dekker (first two), (battery & motor)

Continuing the analysis of my Solar Racing Basics Poster (see this tag), solar racing cars are powered by the sun (and, in the Cruiser class, also by some external recharging at Tennant Creek and Coober Pedy). The major components of the electrical system include:

  • Silicon solar panels, up to 4 square metres in size for the Challenger class and 5 square metres in size for the Cruiser class. These will convert between about 20% and 25% of the sun’s energy into electricity, giving a maximum power level similar to that of a microwave oven.
  • A maximum power point tracker or MPPT (like this one) and other high-voltage electronics which will control the voltage and current of the panels (or of sections of panel individually) in order to give the maximum power output possible under different sunshine conditions.
  • A battery pack, made up of lithium-ion, lithium polymer, or lithium iron phosphate cells connected together (the first two kinds can catch fire if charged or discharged incorrectly; the third kind is safer, but twice as heavy). These battery packs are quite complex, including electronics to control charging, sensors to detect problems such as overheating, and cooling fans. Typically the total voltage of the battery pack is around 100–150 volts.
  • An electric motor. The most efficient solution is usually to mount a motor in one or both of the back wheels (often using a design developed by the CSIRO). This avoids wasting precious energy in gears or a transmission. The motor will also do “regenerative braking,” sending power to the battery as the car slows down.
  • A motor controller which controls the speed of the motor. This is in turn controlled by the throttle or accelerator pedal.
Click to zoom / Image credits: Anthony Dekker (Twente’s RED Shift showing solar panel, 2017), (battery from team SunSpec, 2015)

To read more, see see this post about battery packs by Nick Elderfield of the University of Calgary Solar Car Team, this IEF Solar Car Conference presentation on the same subject, and this page on electrical systems in the Solar Car Wiki.

Solar Racing Basics: Aerodynamics

Click to zoom / Image credit: Anthony Dekker (top three), Agoria Solar Team (wind tunnel)

Continuing the analysis of my Solar Racing Basics Poster (see this tag), perhaps the most important issue in solar car racing is aerodynamic efficiency. In the Challenger class, average power is limited by the size of the solar panel to about that of a microwave oven. This is about one hundredth of the engine power in a typical small car.

Solar cars must therefore eliminate as much aerodynamic drag as possible. Aerodynamic drag is the most important limiting factor on speed.

The aerodynamic drag force, which is given by the formula F = ½ Cd A ρ v2, acts to slow the car down. At maximum speed, the drag force (plus also other factors) exactly balances the force of the motor, which of course acts to speed the car up. In the formula, v is the speed, Cd is the drag coefficient, A is the frontal area of the car, and ρ is the density of air (which we can’t change).

If Fmax is the maximum force that the motor can deliver, then the maximum speed is given by rearranging the formula: vmax = sqrt (2 Fmax / (Cd A ρ)). One way of speeding up the car is by making the shape more aerodynamic (that is, by reducing Cd). Challenger class teams should be aiming at drag coefficients Cd under 0.1. In the Cruiser class, values under 0.2 would be appropriate (for comparison, Cd in ordinary cars ranges from 0.25 for a modern streamlined sports car to 0.6 for an SUV).

Click to zoom / Image credits: Anthony Dekker (Electrum from Michigan, Green Lightning from Top Dutch)

We can also speed up the car by making it narrower (that is, by reducing the frontal area A). In 2019, the 1-metre-wide Electrum from Michigan finished ahead of the 1.2-metre-wide Green Lightning from Top Dutch, in spite of having had problems (79.6 km/h compared to 78.4 km/h). On the other hand, if you make the car too narrow, it will roll over (which means that the car fails pre-race scrutineering). Alternatively, we can reduce the frontal area A by making a “gap” for the air to flow through. If we can make both the drag coefficient Cd and the frontal area A one sixth of the values for a typical small car, then we can travel at 60% of the speed of that car, even though we have only one hundredth of the engine power.

In the past, successful Challenger class designs have included:

  • Catamarans: double hulls, one of which holds the driver, held together by a “wing” on which the solar panel is fixed, with a “gap” between the hulls (this design has won every race since 4 wheels were made mandatory in 2013)
  • Monohulls or “bullet cars”: long, narrow cars with a tapering rear like Electrum or Green Lightning (this design came second in both 2017 and 2019)
  • Outriggers: like monohulls, but with the wheels outside the main body and more widely spaced for stability (this design has not performed quite so well, because of aerodynamic drag from the wheels)

Three wheels are now allowed in 2021. This allows slightly better monohulls (with two wheels inside the front of the body and one wheel just inside the tapering rear of the body). It also allows noticeably better outriggers (only two wheels need to be outside the main body). In addition, there are a number of new designs that teams have been thinking feverishly about for several months.

Click to zoom / Image credits: Anthony Dekker (a catamaran and an outrigger car, both Swedish)

During design, aerodynamics is typically assessed using computational fluid dynamics software, but “ground truth” is a wind tunnel or an actual race. In the illustration on the poster, Belgian team Agoria is using a green laser to reveal airflow around their car in a wind tunnel (see also the video here).

To read more about aerodynamics, see this brief post from 2018, this 2015 Solar Car Conference presentation, and this 2021 Solar Car Conference presentation from Durham.