ASC 9: Aerodynamics

For both the Challenger class and the Cruiser class, solar car racing is to a large extent about aerodynamic drag. That’s overwhelmingly what the hard-earned solar energy is being wasted on, and therefore it’s what teams need to concentrate on minimising. The drag force on a car is given by the equation:

F = ½ C_d A ρ v²

Breaking that down, v is the speed of the car, ρ is the density of air (about 1.2), A is the frontal area of the car, and C_d is the drag coefficient, a number which indicates how aerodynamic (and therefore, how energy-efficient) the shape of the car is. For Challengers, minimising C_d allows the speed v to be increased, while for Cruisers (for which the average v is essentially given), minimising C_d allows non-solar energy use to be minimised. Of course, minimising frontal area is important too (and that is the motivation behind asymmetric Challenger cars).

To give a feeling for the all-important C_d, here are some vehicles with values ranging from 0.19 to 0.57:

Drag coefficients for a selection of vehicles. Clockwise from top left: 0.57 – Hummer H2 (photo: Thomas Doerfer); 0.30 – Saab 92 (photo: “Liftarn”); 0.26 – BMW i8 (photo: “youkeys”); 0.19 – General Motors EV1 (photo: Rick Rowen)

Because C_d is so all-important, it is the one thing that solar car teams are really secretive about. Challengers generally have values under 0.1. With no need for practicality, they chase their way towards the impossible goal of C_d = 0, trying to come up with the perfect race car, which will slice through air like a hot knife through butter:

Nuon’s 2005 car, Nuna 3, with C_d = 0.07 (photo: Hans-Peter van Velthoven)

Cruisers, on the other hand, have to balance aerodynamics with practicality. Bochum’s early SolarWorld GT had C_d = 0.137:

Bochum’s 2011 car, SolarWorld GT, with C_d = 0.137 (photo: “SolarLabor”)

Eindhoven’s recent Stella Vie, with its sleek aerodynamic shape, does much better than that (but they won’t say how much better):

Eindhoven’s 2017 car, Stella Vie (photo: TU Eindhoven, Bart van Overbeeke)

I understand that Sunswift’s 2013–2015 car eVe had C_d = 0.16. Appalachian State (Sunergy) have stated that their newly-built ROSE has C_d = 0.17. PrISUm’s Penumbra has a higher value (C_d = 0.2), because of the blunt end which they chose for practicality reasons (although they did do a few clever things to reduce the impact of that blunt end). I’m not aware of the C_d values for other ASC cars.

Appalachian State’s beautiful ROSE, with C_d = 0.17 (image credit)

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World Solar Challenge: dark horses

Recently I made a poster of the favourites (based purely on 2015 performance) for the 2017 WSC. Here is a somewhat more subjective list of new, innovative, and rising teams. All worth watching! For more details, see my annotated list of teams.

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World Solar Challenge: the favourites

I will need to re-do this at some point, but the poster below shows the favourites (based purely on 2015 performance) for the 2017 World Solar Challenge (click to zoom). There is a very interesting mix of designs this year! For more details, see my annotated list of teams.

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World Solar Challenge: lighter and lighter

The chart above shows car weights (in kg) for the World Solar Challenge Challenger class, since 2001. In spite of the increasing safety standards and the shift from 3 wheels to 4, weights have trended steadily downwards, which says something about the strength-to-weight ratio of modern composite materials.

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Which is the best World Solar Challenge team?

Recently, I saw that someone had asked on the Internet which the best team in the World Solar Challenge was.

For the WSC Challenger class, this is not a difficult question. Nuon Solar Team owns the race, and has won six times out of eight this century (although “uneasy lies the head that wears a crown”). The more interesting question is: who is second? There are four main contenders for that honour.

A few years ago, I would have placed Tokai University second. They won the race in 2009 and 2011. However, unless they can reverse the trend, their star seems to be falling.

Michigan are very definitely the best US team. However, they have pointed out themselves that they suffer “the curse of third,” and thus far lack the je ne sais quoi that it takes to win (of course, when they find it, Nuon had better watch out).

The star of Solar Team Twente is rising. They worked their way up to second place in 2015. They could win this year.

Finally, the Belgian team from KU Leuven is also moving up, and I expect them to do very well this year also.

In the WSC Cruiser class, “best” is a fuzzier concept. However, Eindhoven, Bochum, and UNSW/Sunswift have all done consistently well, with Eindhoven winning the last two races.

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Praising UNSW

UNSW lower campus (photo: Jinbo Bu)

The University of New South Wales (UNSW) – located in the suburb of Kensington in Sydney – was founded in 1949. Its original focus was on engineering and technology, and it still excels in those fields, being ranked 68^th in the world on the Times Higher Education list of engineering institutions. UNSW is home to the Sunswift solar car team, who came fourth in the 2015 World Solar Challenge Cruiser class in a beautiful solar sports car called eVe:

UNSW’s solar car Sunswift eVe crosses the 2015 WSC finish line (my photo)

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World Solar Challenge: Day 7

Today in the World Solar Challenge, the last few non-trailered teams came in, causing me to reflect on how poorly I’ve covered Principia (team 32, USA). They weren’t the fastest car, nor did they run into spectacular problems – they just reliably covered 3,022 km of outback driving.

See the chart above for other cars, and click to zoom (data in the chart is taken from the official timing board, with the most obviously wrong numbers removed, and with some numbers added from twitter photographs taken at control stops). The three remaining non-trailered Cruisers also arrived: Sunswift (team 75, Australia), Bochum (team 11, Germany – who drained their battery dry getting to end of timing last night), and Minnesota (team 35, USA – who made it to end of timing with three minutes to spare).

A large number of trailered cars also arrived. It will be impossible to rank these until the official results are released. Among the six trailered Cruisers were Lodz (team 45, Poland – their vehicle would make a great city car) and ITS (team 31, Indonesia – which has same national colours),

We also had the “practicality judging” for Cruisers today. Sadly, the results have not been released. In the absence of official results, I’m just going to hand out my own awards – the Scientific Gems “gem awards.” And the first of these is the sexy car gem, which goes to Bochum, for building the car that several hundred drooling engineers (even those who had successfully optimised their own vehicle for speed) wished that they had built. Bochum even had real wood inlays in the dashboard!

The “sexy car gem” goes to Bochum, for the ThyssenKrupp SunRiser

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WSC: the areas of competition

In the World Solar Challenge, there are – as I see it – five key areas of competition. Winning the WSC requires doing well at all five.

Solar Cells

Solar cars run on solar energy, so the efficiency of the solar cells is critical. Going by what the teams report (in the histogram above) about 23% is typical (unfortunately, efficient solar cells are expensive). To get the best out of the solar cells, good MPPT electronics are also important.

Aerodynamics

Nuon Solar Team’s very aerodynamic Nuna8 (photo: Jan Willem de Venster)

At cruising speed, virtually all the solar energy goes into combating aerodynamic drag. The speed of the car at cruise is determined by the balance between electrical energy and drag. Reducing the drag coefficient is therefore an essential part of designing a fast car. Nuon Solar Team have been particularly good at this, partly because of input from aerospace engineering students at the Delft University of Technology. Computational Fluid Dynamics and wind-tunnel testing are two important techniques here.

Reliability

The UNSW team perfecting their car (photo: UNSW Solar Racing Team)

Designing a fast car is not enough to win the World Solar Challenge, however. To travel 3,000 km along the less-than-perfect road from Darwin to Adelaide requires an extremely reliable car. Designing a car to survive the conditions means paying very careful attention to mechanical design.

Testing

University of Michigan running their practice race in 2015 (photo: University of Michigan Solar Car Team)

To develop a reliable car, extensive testing is important. To quote Rachel Abril from Stanford, “Test it. Test it again. Test it more.” However, testing is equally important in developing efficient race procedures, which is why teams like Michigan and Twente will run simulated practice races.

Race strategy

As part of managing the uncertainty about future sunshine, Solar Team Twente took a military meteorologist along for the 2013 WSC.

Finally, good race strategy is essential to doing well in the World Solar Challenge. This includes the psychological aspects of race strategy common to all forms of racing, together with choosing the optimum speed for the conditions (and also, in the Cruiser class, the optimum number of passengers). Managing the uncertainty about future sunshine is also critically important.

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World Solar Challenge: Sunswift eVe

Very happy to see Team 75’s fantastic car, on its travels around Australia.

World Solar Challenge: Team 75

75  UNSW / Sunswift (eVe)

The Australian team from the University of New South Wales introduced their sporty eVe in the 2013 World Solar Challenge, and were the first Cruiser across the line, although they came 3^rd on points. This year they are racing an updated version of the car, redesigned inside (below) and out (above). The car also holds the record (with solar cells disconnected) for fastest electric vehicle over 500 km (averaging 106.966 km/h). I am sure that many people would love to purchase a copy of this nifty solar sports car, and we wish the Sunswift team well. Good luck, team 75!

For up-to-date lists of all World Solar Challenge 2015 teams, see:

• Country list
• Cruiser list
• Challenger list