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)


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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.


Solar car team composition

The chart above shows 2017 team composition for the Eindhoven and Bochum solar car teams (divided by study major, not team responsibility). Not surprisingly, electrical and mechanical engineering students are the core of both teams (about half in each case) Yet there is also considerable diversity, because the business side of a solar car team requires other skills too. The Bochum team also includes a media unit, which explains the large “other” category (one of the team photographers is a biology student, for example).

The chart was constructed by parsing web pages, which may have introduced errors (also, I guessed a bit with the German words). But the main point stands – solar car teams require a diverse set of skills.


The Bochum car (photo: Anthony Dekker)


WSC: final Cruiser results

Based on the official results, the chart below (click to zoom) shows the final scores for the WSC Cruiser class. Each team has three coloured bars: first the number of person-kilometres, which should be large (black icons show occupied seats and white icons empty seats), then the energy usage, which should be small (number of charges, which is 6 in each case, times battery capacity), and finally the overall efficiency score, which should be large again (it is the ratio of those first two numbers). The rule for the efficiency score bar is: first bar divided by second bar, then scale so that the largest result is 80%. The scaled practicality scores out of 20 (grey bars) are then added. Eindhoven is the clear winner, with Bochum second.

The chart below (click to zoom) shows the raw practicality scores for all Cruisers (finishing, non-finishing, and non-starting).


WSC: three gem awards


Nuna9, the car from Nuon Solar Team

It has been my tradition to hand out “Gem Awards” after major solar car races. This WSC, the “Faster Than Lightning” gem again goes to Nuon Solar Team, the undefeated Challenger champions.


The 2017 “Faster Than Lightning” gem goes to Nuon Solar Team

 


Stella Vie, the car from Solar Team Eindhoven

The “Solar Family Car” gem again goes to Solar Team Eindhoven. They completely dominated the Cruiser class.


The 2017 “Solar Family Car” gem goes to Solar Team Eindhoven

 


Western Sydney Solar Team

The “Solar Car Family” gems go to Western Sydney Solar Team, for the way that they welcomed international teams passing through Sydney. Western Sydney Solar Team are, of course, also Australian champions in the Challenger class.


The 2017 “Solar Car Family” gems go to Western Sydney Solar Team


WSC: The Cruiser class

The chart below (click to zoom) shows the current state of play for the WSC Cruiser class, using data from the official website (although it seems to me that there are some roundoff errors in those numbers). Each team has three coloured bars: first the number of person-kilometres (black icons show occupied seats and white icons empty seats), then the energy usage (number of charges, which is 1, times battery capacity), and finally the overall score (which is the ratio of those two numbers). The black number in the final bar shows the ranking. All bars are scaled to a percentage of the maximum. It can be seen that Eindhoven has a solid lead (and they will display their own performance in detail here).