World Solar Challenge: about the Cruisers 2

Here is an alternate presentation of the World Solar Challenge Cruiser Class score calculation:


World Solar Challenge: about the Cruisers

To illustrate the World Solar Challenge Cruiser-class scoring for 2017, here is the calculation for Kogakuin’s 2015 car (above). Disclaimer: this is, of course, my personal interpretation of the regulations.

Notice that Cruisers are not in a race this year – any arrival time during the 11:00 to 14:00 time window on Friday is OK.

Arrival time

Friday 11:35.
Inside window? YES

Energy efficiency

Battery capacity, Q = 14.855 kWh
Number of recharges, n = 1 (at Alice Springs)
External energy use, U = (n + 1) Q = 29.71
Person-km, C = 3022
Energy efficiency, E = C / U = 101.7
Highest energy efficiency, E* = 203.6 (Eindhoven)
Relative energy efficiency, E / E* = 0.4996


Practicality P = 51.75
Highest practicality, P* = 84.5 (Eindhoven)
Relative practicality, P / P* = 0.6124

Total Score

Total score, S = 80 E / E* + 20 P / P* = 39.97 + 12.25 = 52.22

This is a massively lower score for Kogakuin than was actually awarded in 2015. This year, the World Solar Challenge Cruiser Class is all about energy-efficiency, carrying passengers, and practicality. Expect to see the four-seat and five-seat Cruisers (like the Polish car below) running with every seat occupied.

Silly Season in the Sky (again)

It’s apparently time for lunatic end-of-the-world prophecies again. The latest relates to a “great red dragon” in the sky (a reference to Revelation 12):

Turns out that this is a double image of Saturn, taken way back in 1983 by the Infrared Astronomical Satellite (IRAS). Saturn was then at RA 13h45m18.2s, DEC -08d13m07s.

It’s a false-colour image (i.e. not red at all), being taken at 100 microns, in the infrared region of the spectrum. But with enough spin, apparently it can be made to sound scary.

In the final version of the infrared sky survey, this artefact was blacked out, since it doesn’t reflect any actual stellar infrared sources (just a planet that moves around). Of course, that removal got the conspiracy-theory nutters going.

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.

World Solar Challenge: Challenger dimensions

MostDece has written a superb blog post on the WSC challengers. Based on that, I’ve updated my previous post on dimensions. The infographic above (click to zoom) shows the reported length and width of 16 WSC cars (Challenger class only, this time). The widest car (at 2.05 m) is the South African car from NWU (below), but of course that includes the outrigger wheels. The narrowest is the long narrow bullet car from Michigan. There are also short zippy little cars from Nuon, Principia, and Punch.

Update: The chart below clusters cars with similar length/width combinations. NWU is a visible outlier. Below NWU, we have big cars (ITU, MDH, Adelaide, Aaachen, JU – over 1.6 m wide and at least 4 m long), short catamarans (Nuon, Principia, Punch – 1.55 to 1.6 m wide and at most 3.5 m long), narrow catamarans (Nagoya, Stanford, Twente, WSU – 1.38 to 1.5 m wide and at least 4 m long), and monohulls (Tokai, Kogakuin, Michigan – at most 1.2 m wide and over 4.9 m long):

Update: Unfortunately, the two charts above reflect incorrect information from the Stanford team. The Stanford car is actually substantially wider.

World Solar Challenge: solar cells

Part of the rule changes for the 2017 World Solar Challenge was a change to allowable solar cell array areas. In the Challenger class, the limits became 4 m2 for silicon and 2.64 m2 for multijunction gallium arsenide (in the Cruiser class, 5 m2 and 3.3 m2, which is the same ratio). Depending on the efficiencies of the two technologies, we therefore get the following comparison:

There are two important caveats, however. First, the cars in the World Solar Challenge will be getting pretty hot. The performance of multijunction GaAs degrades less with heat than that of silicon, so this increases the benefit for GaAs beyond that shown in the chart. For example, if we assume a 24%/35% efficiency combination for Si/GaAs, with temperature coefficients of power of 0.4%/0.2%, then the red dots in the chart show a GaAs advantage above about 43°C.

Secondly, the use of a 2.64 m2 GaAs array allows teams to build a smaller (and hence more aerodynamic) car, as Nuon and Punch have done. This increases the benefit for GaAs even further. Consequently, the five favourites (Nuon, Twente, Tokai, Michigan, and Punch) are all capable of winning the race, but the teams that switched to GaAs might have made a good move.

Update – the graph below clarifies the temperature-dependence for the two technologies (assuming a 24%/35% efficiency combination for Si/GaAs, and temperature coefficients of power of 0.4%/0.2%):