ASC 37: Road Race Day 7


ASC, Day 7 (picture credits: 1, 2, 3, 4)

Day 7 of the American Solar Challenge was full of drama. An incredible race between Western Sydney and Michigan saw average speeds to the checkpoint in Mountain Home of 90.8 km/h (56.4 mph). In the Cruiser class, there was a tragic breakdown by Minnesota, which I think will leave Onda Solare the winners by default.

I am, however, giving my “Most Desirable Car Gem” award to PrISUm. Their car did not qualify for the road race, but I still think that their “solar SUV” is a fantastic concept.

And here are the night-time car positions (unreliable in the case of Michigan). I have overlaid them on my elevation map to emphasise the 850 metre downhill run into Mountain Home, and then the climb back up into Burns.


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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 Cd 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 Cd is the drag coefficient, a number which indicates how aerodynamic (and therefore, how energy-efficient) the shape of the car is. For Challengers, minimising Cd allows the speed v to be increased, while for Cruisers (for which the average v is essentially given), minimising Cd 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 Cd, 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 Cd 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 Cd = 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 Cd = 0.07 (photo: Hans-Peter van Velthoven)

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


Bochum’s 2011 car, SolarWorld GT, with Cd = 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 Cd = 0.16. Appalachian State (Sunergy) have stated that their newly-built ROSE has Cd = 0.17. PrISUm’s Penumbra has a higher value (Cd = 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 Cd values for other ASC cars.


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


ASC 8: About Cruiser Practicality

The American Solar Challenge Cruiser class is a contest for multi-person solar vehicles, each powered by 5 square metres of silicon solar cells (or 3.3 m2 of multi-junction cells), with the option of recharging from the grid. The contest is not actually a race – cars must get to the finish line on time, carrying as many people as possible, and drawing as little power from the grid as possible.

Cars are also scored partly on practicality. This can mean different things. Eindhoven’s 2015 car (Stella Lux), for example, was designed as a four-person family car, and the team took photos of it doing family things like shopping, going on holiday, and picking children up from school. A big feature was that, for an average family in the Netherlands, the car would produce more electricity than it used. The car scored 84.5 for practicality at the 2015 World Solar Challenge.


Eindhoven’s Stella Lux (photos: TU Eindhoven, Bart van Overbeeke 1, 2, 3, 4 – click to zoom)

Bochum’s 2015 car (ThyssenKrupp SunRiser), on the other hand, was a luxury two-person sports car, with leather seats and an incredibly beautiful interior. It was an almost perfect example of the car it was trying to be, and scored 80.5 for practicality (far higher than the next car, which scored 63.5).


Bochum’s ThyssenKrupp SunRiser (photo: Anthony Dekker)

One of the highest WSC 2017 Cruiser practicality scores went to PrISUm for their four-seat Penumbra, which was intended as the kind of practical SUV that you might take on a fishing trip. The car has plenty of room for carrying your esky, tackle box, etc. PrISUm deliberately made some aerodynamic compromises in order to achieve their practicality goal, and the car scored 79.8 for practicality at WSC 2017.


PrISUm’s Penumbra (composite image)

This year at ASC, PrISUm’s Penumbra is again a strong contender. Minnesota (UMNSVP), Appalachian State (Sunergy), and Waterloo (Midnight Sun) are entering two-person solar sports cars, while Onda Solare from Italy seems to be inspired by Eindhoven (see my annotated teams list). It promises to be an interesting field.


Midwestern Solar Challenge

On June 3, two of the top Cruiser-class solar cars held a Midwestern Solar Challenge, racing south from St Paul, MN to Ames, IA. The teams were:

Race news

Scoring

Here is my (totally informal) scoring of the race (see the chart below):

  • Person-kilometres is the race distance times the average number of people carried (which I believe was 4 for PrISUm and 2 for UMNSVP). The first coloured bar shows this, scaled so that 100% is the highest value.
  • Energy input is the number of charges (1, in this case) times battery size. The second coloured bar shows this, scaled so that 100% is the highest value (this bar points downward, because smaller is better).
  • The third coloured bar shows the ratio of these numbers, scaled so that 80% is the highest ratio.
  • We add on (in grey) the practicality scores (I’m estimating 9 for PrISUm and 7 for UMNSVP), scaled so that 20% is the highest practicality.
  • This gives final scores of 100 for PrISUm and 82 for UMNSVP. Congratulations, PrISUm!
  • But will PrISUm still have the advantage when it needs to carry passengers uphill?


Solar Car Racing Status Check

In solar car racing news, preparations are beginning for the SASOL Solar Challenge in South Africa (September 22 to 30). It seems that both Nuon and Tokai will attend this event, along with local teams.


Nuon at WSC 2017 (photo: Anthony Dekker)

Thirteen teams have registered so far for the 24 hour iLumen European Solar Challenge in Belgium (September 19 to 23), and Twente will be defending their title there. I am maintaining an information page and teams list for this race. See also the official iESC social media at  


Twente at WSC 2017 (photo: Anthony Dekker)

The American Solar Challenge is a lot closer than those two races, with scrutineering beginning on July 6, track racing on July 10, and the road race running from July 14 to July 22. I am maintaining a detailed information page and teams list for this race. At last count, 34 teams were registered, with Anderson, UCSD, Principia, UC Irvine, Phoenix, and UT Austin having, sadly, dropped out.

Six teams are attending with cars that raced at WSC 2017, although these cars will require adjustment to satisfy ASC rules (Michigan, Western Sydney, Principia, and Illini, plus the Cruisers PrISUm and Minnesota). Six other teams are attending with cars that previously raced at ASC.


PrISUm at WSC 2017 (photo: Anthony Dekker)

Twenty-two other teams are frantically building cars for ASC. Car unveils that have been announced include team 42 (Missouri) on 18 April, team 55 (Esteban) on 23 April, team 101 (Eclipse) in mid May, team 828 (AppState) in mid June, and team 65 (Calgary) on 16 June.


Missouri’s unfinished car (picture credit)

See my detailed information page and teams list for this race for more information and for social media links. I will continue to update that page as news comes in.


WSC: One more gem award


Penumbra, the solar SUV from PrISUm

A belated “Huntin’ and Fishin’” gem award goes to the to PrISUm team from Iowa State University, for their roomy solar SUV capable of carrying an esky, tackle box, and other fishing gear.


The “Huntin’ and Fishin’” gem goes to PrISUm


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.