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%):
Scientific Gems 
Hi Tony, you are definitely addressing a crucial topic here. Is it doable to plot a graph for energy generation for Si at 24% and for Multi-junction at 35% against the different array temperatures, given the characteristics of these types of technologies?
I am sure the top teams with budgets to consider multi-junction have done many of these calculations, also bringing parameters like weight and aero into the picture.
I’m not sure exactly what you mean, Erik, but you can draw a line through the black dot at 35%/24% and the three red dots. Alternatively, take a look at this graph.
That graph is close to what I was looking for. But where now Relative Efficiency (not sure about the definition) is on the Y-axis, I am looking for the absolute amount of energy a 2.64 sqm 35% MJ array generates vs a 4 sqm 24% Si array at the temperatures on the X-axis. That should present some insight in how much energy cars with such arrays will have available at different temperatures.
The “relative efficiency” is just normalised to give 1.0 as the maximum value. Actual energy output is going to depend on latitude and on the amount of cloud cover, but taking 25 MJ/m^2/day as a rough guess for insolation, that would give 24 MJ/day as the maximum value.
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