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?


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%):


World Solar Challenge: Solar Panels

Teams in the World Solar Challenge can sometimes be a little coy about the technical details of their pride and joy, but 12 teams have reported the efficiency of their solar panels, ranging from 18% to 24%, and the histogram below plots these reported values. The top cars all seem to be using solar panels with efficiencies around 23%.

Culture




Folk culture (left), popular culture (centre), and “high” culture (right). The photo of the blue handmade pottery cup at top left is by “Wildfeuer,” and the photo at bottom right is by Jessica Spengler.

I recently read a book discussing the now-traditional distinction between folk culture, popular culture, and “high” culture (see pictures above). Folk culture includes traditional arts and crafts, hand-made objects, and fairy tales or recipes passed down from generation to generation. Popular culture is characterised by mass-produced objects made from cheap materials or ingredients, aimed at immediate gratification and at the lowest common denominator of taste. “High” culture consists of things that only the well-educated cognoscenti can appreciate. “High” culture should be distinguished from what is simply the more expensive end of folk culture – craftspeople have always been able to produce more sophisticated items, made with more expensive materials and more elaborate decoration, if they were paid for the extra time and cost (“for you, my lord, I can fletch the arrows with eagle’s feathers instead of hen’s feathers”).


Bristol Cathedral

Now, I think this classification is missing a few things. For a start, there’s the important category of religious culture, which includes things like the great cathedrals (or, elsewhere in the world, temples and mosques) and the religious music of, for example, J. S. Bach. Such cultural artefacts were aimed at ordinary people (not the cognoscenti), but they were dedicated to God. They were meant to inspire devotion, and quite literally to point to heaven.

The Agnus Dei from Bach’s Mass in B minor, sung by Kathleen Ferrier

In Bach’s case, this intent was genuine – he was a deeply religious man, who ended his musical manuscripts with the phrase Soli Deo gloria (Glory to God alone), or with the initials S.D.G. In religious culture, it is neither the artist nor the listener/viewer to whom honour is intended to accrue. In a weaker form, this attitude can be found in, for example, movie soundtracks, whose primary purpose is not to be appreciated on their own, but to help the audience enter into the story.

In many cases, what we call “high” culture is actually folk culture or religious culture that has lost its original context. Folk-culture artefacts from the past (like the Greek vase at top) first enter the antique store and then, as they become even older, move to the museum, where they become objects of “high” culture. Folk-culture artefacts from other countries appear to become objects of “high” culture as soon as they are transported from their place of origin. Religious music becomes “high” culture when it shifts from the cathedral to the concert hall. When a degree of context is restored, some objects of “high” culture can actually become extremely popular, as in movie adaptations of Shakespeare or of classic novels. Other objects of modern “high” culture bemuse even well-educated individuals outside the cognoscenti.


Traditional slit drums from Vanuatu in the Australian Museum, Sydney (my photo)

Movies are a key part of popular culture. The chart below relates the “percentage of professional critic reviews that are positive for a given film” from rottentomatoes.com to total revenues (as at a few years ago – data is from here). It is not clear, however, exactly what the “professional critic reviews” are measuring. The relationship between revenues and quality is hazy as well, although some really terrible movies do seem to make a great deal of money.

For several decades now, a growing rebellion against popular culture has been emerging. There was the Arts and Crafts movement around 1900, and since then a steadily increasing interest in traditional forms of music, hand-crafted objects, and home cooking. There have also been a number of other interesting movements. The slow food movement attempts to resurrect a comprehensive folk culture of food, and offers a superior alternative to “fast food.” The maker movement (as defined by e.g. Make magazine) merges modern technology with traditional crafts. It seeks to ally modern technology with folk culture, rather than with mass-produced popular culture. At the simple end, maker culture includes minor customisations of high-tech devices, like this Macbook sticker:


Macbook sticker (photo by Denis Dervisevic, slightly modified)

At the more sophisticated end, there is this classic steampunk computer monitor and keyboard by Jake von Slatt:


Steampunk computer monitor and keyboard (photographer & maker: Jake von Slatt)

Steampunk culture also rebels against the cheap plastics used in so many popular-culture artefacts (although Robert M. Pirsig notes that “Mass-produced plastics and synthetics aren’t in themselves bad. They’ve just acquired bad associations. A person who’s lived inside stone walls of a prison most of his life is likely to see stone as an inherently ugly material, even though it’s also the prime material of sculpture, and a person who’s lived in a prison of ugly plastic technology that started with his childhood toys and continues through a lifetime of junky consumer products is likely to see this material as inherently ugly”). Steampunk culture prefers older materials like brass and copper, as in this 1994 Jules-Verne-inspired steampunk makeover of the Arts et Métiers Métro station in Paris:


The Arts et Métiers Métro station (photo: Stephen Butterworth)

Aspects of the emerging maker movement can also be seen in the solar cars developed for the World Solar Challenge, where high-tech electronics and solar panels are combined with carefully engineered and hand-crafted car bodies made of quality materials like carbon-fibre composites, resulting in vehicles of aesthetic beauty as well as practicality and speed.


Solar Team Twente’s solar car Red One (photo: Jérôme Wassenaar)

Milestones in Materials

Here is an experiment with using R to produce infographics – in this case, milestones in materials technology. The top line shows glass and ceramics and the second line metals, while the third and fourth lines are miscellaneous. The time axis is logarithmic. Click on the image to zoom.

A Century of History

Solar Team Twente recently posted this photograph (by Jérôme Wassenaar) of the almost-100-year-old grandfather of a team member test-driving one of their older solar cars. This is what else the man has seen in his life:

  • 1997: the first Toyota Prius goes on sale in Japan
  • 1998: the first MP3 player goes on sale in the USA; also Google is founded
  • 2005: the first autonomous vehicles complete the DARPA Grand Challenge; also Solar Team Twente races in the World Solar Challenge for the first time
  • 2015: the New Horizons space probe makes the first visit to Pluto, 85 years after Pluto was discovered

Happy 100th Birthday in advance, Opa Mulder!