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.

Vattenfall Solar Team world record attempt

In 2017, Vattenfall (then Nuon) Solar Team in their car Nuna8S set a solar racing endurance record at the RDW (Netherlands Vehicle Authority) test track in Lelystad, NL of 882 km in 12 hours or 73.5 km/h (see Dutch video here).

On Saturday 8 August this year they hope to break their own record in the Nuna Phoenix which they originally intended to race at the now-cancelled American Solar Challenge (this car is, I understand, Nuna9S with a variety of changes made specifically for ASC, including a metal roll cage and a motor suitable for climbing mountains).

According to media reports, they will begin with a full battery, but then run only on solar power for the next 12 hours (this is in contrast to the 24-hour European Solar Challenge, where two recharges are permitted during the night). For their event, Vattenfall Solar Team will have four drivers, each taking on a 3-hour stint (Marloes Nanninga, Maxime Croft, Sylke van der Kleij, and Mees van Vliet). Nuna Phoenix will begin driving at around 7:00 local time. I understand from live streams that they are also water-cooling the panel at driver changes.

The weather forecast is for sun (with increasing clouds). This webcam and this one show weather at the harbour nearby (looking away from the track). For updates, see the team social media at 

Previously, Vattenfall won WSC 13; won WSC 15; won WSC 17; came 12th at WSC 19 (after their NunaX was destroyed by fire); won SASOL 14; won SASOL 16; and won SASOL 18.

Edit 1: This 12-hour endurance event also includes the first public reveal of Nuna Phoenix. The event has now begun.

Edit 2: Nuna Phoenix has clocked up approximately 126 km in 90 minutes so far (84 km/h).

Edit 3: Now it’s approximately 238 km in 3 hours (79.3 km/h).

Edit 4: Nuna Phoenix has now clocked up 355 km in 285 minutes (74.7 km/h, which suggests that they are being cautious about possible clouds coming in).

Edit 5: Now it’s approximately 538 km in 7 hours (76.9 km/h).

Edit 6: At the last driver change (9 hours) it’s 670 km (74.4 km/h).

Edit 7: After 12 hours, Vattenfall Solar Team have indeed set a new world record of 924 km (77 km/h), following a late sprint (reaching at least 97 km/h). Well done!

For teams interested in challenging this record, my proposal would be that the format used by Vattenfall Solar Team be followed, in a car designed for (and ideally, having passed scrutineering in) either WSC, ASC, or SSC within the previous 3 years (I exclude ESC from that list, because of its different format). Not that many teams have a car as fast as Nuna Phoenix, of course!

European Solar Challenge: modelling strategy

The iLumen European Solar Challenge at Circuit Zolder in Belgium is still expected to go ahead on 18–20 September (see my list of teams here). Following up on my earlier post about the sun at Zolder, here is a simplistic model of a hypothetical car under plausible (partly sunny) weather conditions:

The blue lines show energy output (in kWh) at three different speeds. Notice that the car is stationary during two charging periods (under regulation 3.10.1, each Challenger-class car may make two recharge stops, of at least one hour each; the timing of these is an important strategy choice). The car speed is assumed to be constant. In other words, variation in speed as the car drives around the circuit is completely ignored (see the circuit map below by Will Pittenger). For this hypothetical car, 52 km/h is the maximum speed.

The orange lines show corresponding energy input, from initial battery charge, two recharges, and solar panels. The iLumen European Solar Challenge (iESC) is a 24-hour race, with just under 12 hours taking place in the dark (grey area on the chart). On past trends, during September, the sun at Zolder shines for about 6 hours a day, and there is rain on one day in two. Only a few kWh therefore comes from the sun at the iESC. The race is, in fact, mostly won on aerodynamics, the rolling resistance of tyres, regenerative braking, and driving skill. The iESC organisers can, therefore, allow a mixture of different solar panel sizes without any unfairness, because smaller cars with smaller panels also have lower aerodynamic drag. I look forward to seeing how the various teams cope with this challenging race.

European Solar Challenge: the sun at Zolder

The iLumen European Solar Challenge at Circuit Zolder in Belgium is still expected to go ahead on 18–20 September (see my list of teams here).

Above is my calculation of the theoretical solar power for this event, assuming no clouds, ignoring diffuse sky radiation, and taking solar panels as 100% efficient (more realistic values are 20–24% for silicon and 35% for GaAs).

We will see, during the race itself, how much sun there will be (on past trends, during September, the sun at Zolder shines for about 6 hours a day, and there is rain on one day in two).

The Lost World by Michael Crichton: a book review

The Lost World, by Michael Crichton (1995)

Recently, because this is the season for extra reading, I re-read The Lost World by Michael Crichton (which was made into a 1997 film). This novel turns 25 years old in September. Its main plot needs no explanation, of course. Just like Jurassic Park, there’s action, there’s excitement, and there’s dinosaurs chasing people.

As with all Crichton novels, there are technical and scientific themes that do not make it into the film. I had forgotten, for example, that the original mobile laboratory is solar powered: “He wants them light, I build them light. He wants them strong, I build them strong – light and strong both, why not, it’s just impossible, what he’s asking for, but with enough titanium and honeycarbon composite, we’re doing it anyway. He wants it off petroleum base, and off the grid, and we do that too. … The Explorer with the black photovoltaic panels on the roof and hood, the inside crammed with glowing electronic equipment. Just looking at the Explorer gave them a sense of adventure…” (pages 64 & 94)

Velociraptor skeletal cast at the Dinosaur Journey museum in Colorado (original photo by Jens Lallensack)

Another theme, naturally, is the changing scientific view of dinosaurs, and indeed other things, over time (in fact, the book and film are already out-of-date in some respects): “Back in the 1840s, when Richard Owen first described giant bones in England, he named them Dinosauria: terrible lizards. That was still the most accurate description of these creatures, Malcolm thought. … the Victorians made them fat, lethargic, and dumb – big dopes from the past. This perception was elaborated, so that by the early twentieth century, dinosaurs had become so weak that they could not support their own weight. … That view didn’t change until the 1960s, when a few renegade scientists, led by John Ostrom, began to imagine quick, agile, hotblooded dinosaurs. Because these scientists had the temerity to question dogma, they were brutally criticized for years, … But in the last decade, a growing interest in social behavior had led to still another view. Dinosaurs were now seen as caring creatures, living in groups, raising their little babies.” (page 83)

Tortuga Islands, Costa Rica (original photo by “rigocr”) – is this the mysterious Isla Sorna?

As with many Crichton novels, scientific hubris is a major theme. Other themes include the education of children (both dinosaur children and human children), information systems design, the theories of Stuart Kauffman about self-organisation and evolution, and the importance of what is now called the complex systems view.

Overall, this is a good solid action novel, with several scientific and philosophical themes to think about. Goodreads rates it 3.78. I’m giving it only 3½ stars, in part because it’s a little too much like Jurassic Park. But it’s certainly well worth a read.

The Lost World, by Michael Crichton: 3½ stars

How solar is that solar car?

Above (click to zoom) is a chart showing WLTP-standard solar-only driving ranges for the three solar cars from my last post (battery ranges not shown here):

  • Solar Team Eindhoven’s Stella Era, winner of the BWSC Cruiser class
  • Lightyear One, a commercial solar car from the Netherlands
  • The Sion electric vehicle from Sono Motors

These solar-only driving ranges are marked on a smoothed distribution of electric vehicle driving patterns reported in this paper (distance driven per vehicle-day on days when the vehicle was driven).

The sleek Stella Era has a solar-only range more than 4 times the mean 70 km driven. On more than 99% of trips, Stella Era can operate solar-only, and, on average, its solar panel produces substantial excess electricity which can be donated to other vehicles.

Lightyear One has a solar-only range less than the mean 70 km, but is still able to operate solar-only on 57% of trips.

The less expensive Sion is able to operate solar-only on 19% of trips, and has a useful solar boost to its battery the rest of the time.

So you want to buy a solar car?

Above (click to zoom) is a chart showing WLTP-standard driving ranges for four electric vehicles (brown for battery range, yellow for the boost due to solar panels). The four cars are:

  • Solar Team Eindhoven’s Stella Era, winner of the World Solar Challenge Cruiser class (not for sale, of course)
  • Lightyear One, a commercial solar car from the Netherlands which incorporates considerable know-how from solar car racing
  • The Sion electric vehicle from Sono Motors
  • The non-solar Tesla Model 3 Long Range

The sleek Stella Era has almost double the range of the Tesla, in spite of having a much smaller battery pack. This is due to the Dutch racing car’s extremely aerodynamic shape and light carbon-fibre construction. Lightyear One comes about as close to the performance of Stella Era as you would expect a normal-looking production car to come (and is about two and a half times as heavy).

The rather boxy Sion has a much smaller range than Lightyear One (but, at an expected €25,500, is much cheaper). Which solar car would you choose?

Albi Eco Race 2019 begins!

Image credits 1, 2, 3

The Albi Eco Race 2019 has begun. The solar-car segment (“Niveau 3”) includes Bochum University of Applied Sciences with their legendary 2011 car, SolarWorld GT (top left), as well as their sexy 2015 car, the thyssenkrupp SunRiser (top right; it will make a comeback at WSC later this year) and their 2017 car, the thyssenkrupp blue.cruiser (not shown). The French (or rather, Breton) team Eco Solar Breizh is fielding their challenger Heol and their new urban mini-Cruiser hx2 (bottom). I understand that the Lycée Jehan de Beauce (Project 28) is also participating, along with the fantastic Ardingly Solar team from the UK (who will take their Cruiser to WSC as well).

The actual solar-car race is from 9:00 to 16:30 on Saturday (French time), if I am understanding the timetable correctly. There are active Twitter feeds from Ardingly, Bochum, Eco Solar Breizh, and of course the race itself.

Sadly, it looks like rain.

Image credits 1, 2, 3

Edit: it seems that the SolarWorld GT suffered some damage, which means that only two Bochum cars are competing. Also, hx2 is not competing in the solar-car segment. Below are the speeds from the qualifiers.

Further edit: The thyssenkrupp blue.cruiser won the event on points. The thyssenkrupp SunRiser came second, with 119 laps in 8 hours, i.e. an average of about 53 km/h. Heol from Eco Solar Breizh came third.

Gender and Solar Car Teams

Solar Team Twente, led by Irene van den Hof, arrives at the World Solar Challenge 2015 finish line in 2nd place (photo: Anthony Dekker)

As a keen follower of international solar car racing, it’s interesting to explore the so-called “Gender-Equality Paradox” in Science, Technology, Engineering, and Mathematics (see Stoet and Geary, 2018) as it relates to solar car teams – although I realise that this is a controversial subject.

In countries with high gender equality, such as Sweden, female participation in the STEM professions is paradoxically low. In part, this seems to be due to the fact that young women with STEM skills and interests often have other skills and interests as well, and these drive their educational and career choices (and within STEM fields, women appear to preferentially choose medicine over engineering). One can hardly force women to make other choices, though!

Solar car racing is in some ways engineering at its most intense – a difficult challenge requiring a substantial sacrifice of free time (much like an engineering start-up company). In the chart below, I plot the UN Gender Inequality Index for various countries against the average percentage of women in the engineering segment of solar car teams from those countries (I include team leaders in the count, but not dedicated media or public relations personnel). The colour of the dots for each nation indicate whether team leaders are mostly women (pink) or mostly men (blue).

The results are not statistically very significant (p = 0.05 and 0.09 for the two coefficients at the individual team level), but there is an interesting inverted parabolic fit here. For countries with high gender equality (Sweden, Belgium, Germany, and the Netherlands), only about 6.7% of the engineering segment of solar car teams is female. This is compared to 11.3% for other countries. On the other hand, Germany and the Netherlands do have mostly female team leaders.

In part, these results may reflect the fact that when a team attempts to make an optimum assignment of people to roles, the best people to carry out public relations and leadership roles are often the female team members (some people have suggested psychometric reasons for this). In fact, women are exactly twice as likely to be team leaders as you would expect based on the composition of the engineering segment of teams.

Obviously this small-scale study doesn’t settle anything, but it does raise some interesting questions for further investigation. And, of course, it would be fatal to believe that the man or woman building the car’s suspension was doing a more worthwhile job than the man or woman raising the sponsorship money that the team needs to survive. Success requires being good at everything, and that requires a diverse team.

Edit: This analysis may have missed a few women who were not included on team web pages.

Solar Car World Rankings Revisited

Nuon at WSC 17 (photo: Anthony Dekker)

Below is my personal world ranking of the top 21 Challenger-class solar car teams (revised with new data from an earlier list). It was produced entirely algorithmically by using linear regression on historical data to build mappings between WSC rankings and those of other races, and then applying those mappings to the results of four recent events (WSC 17, ASC 18, ESC 18, and Sasol 18). For example, this is the mapping between Sasol placings and WSC placings. It was used to map all Sasol 18 teams to expected WSC placings:

There is as yet insufficient data to rate Cruiser-class teams (apart from the actual WSC 17 results: 1 Eindhoven, 2 Bochum, 3 Arrow). But here is the table of Challengers:

Rank Previous Team WSC17 ASC18 ESC18 Sasol18
1 1 NL  Nuon Solar Team 1 1
2 ↑ 3 NL  Solar Team Twente 5 1
3 ↓ 2 US  University of Michigan 2 2
4 4 BE  Punch Powertrain Solar Team 3 6
5 5 JP  Tokai University 4 2
6 ↑ DE  Sonnenwagen Aachen P 3
7 ↓ 6 AU  Western Sydney Solar Team 6 1
8 ↑ 18 CH  Solar Energy Racers 3
9 ↓ 8 HU  Kecskemét College GAMF (Megalux) 4
10 ↓ 7 JP  Kogakuin University 7
11 ↓ 9 SE  JU Solar Team 8
12 ↓ 10 US  Stanford Solar Car Project 9
13 ↑ ZA  Tshwane University of Technology (TUT) 4
14 ↓ 11 CL  Antakari Solar Team 10
15 ↓ 13 CA  University of Toronto (Blue Sky) 11
16 ↓ 14 CA  ETS Quebec (Eclipse) 3
17 ↓ 15 JP  Nagoya Institute of Technology 12
18 ↓ 12 ZA  North West University P 5
19 ↑ FR  Eco Solar Breizh 7
20 ↓ 17 CA  Poly Montreal (Esteban) 4
21 ↓ 19 US  Massachusetts Institute of Technology 5

Note that Cruiser teams like Eindhoven, Bochum, and Arrow are excluded from the list. The letter P marks cars that participated in WSC 17, but did not finish, and thus were not ranked at the time. It must also be said that Western Sydney, Eclipse, Esteban, and MIT should probably be ranked higher than they are here – the algorithm is not taking into account the dramatic improvement in ASC teams this year. However, good ESC and Sasol performance has bumped up Aachen, SER, Eco Solar Breizh, and South Africa’s new champion team, TUT.

Michigan at WSC 17 (photo: Anthony Dekker)