Why solar cars are really about efficiency

The World Solar Challenge have been held since 1987. This is a great competition where people often ask: why doesn’t my car have solar panels? Long hasn’t this been an option, but since 2012 Toyota is offering a solar roof on their Prius. This has mostly been a gimmick, where you at most charge to keep the air condition running. In normal cars it just doesn’t attribute enough.

In the specially designed solar cars it’s proven that decent speeds are possible. The Tokai Challenger, winner of 2009, drove at an average speed of 100,5 km/h. And even though the maximum size of the surface of solar panels was lowered from 6 to just 2,64 square meter the last winner of the world solar challenge still managed a speed of 81,2 km/h. This is done, because every aspect of the car is design for efficiency. This means special tires, low air resistance and above all low weight.

Here are the weight of the car from the winners of last three editions:

Nuna 7 (2013): 190 Kg

Nuna 8 (2015): 150 Kg

Nuna 9 (2017): 141 Kg

The cars have very small batteries (only 20 kg) which means that sunlight is put directly into forward speed. The cars themselves are from the ground on developed for efficiency, not for practical means. The drivers are mostly packed into a small cockpit without air conditioning. Every bit of sunray needs to be converted to energy.

This ‘solar’ approach doesn’t go well on electric cars. Take for example Tesla who have been touted to make efficient electric cars. Those cars are developed for normal usage with four persons with decent top speeds. Also, very important is to have enough range. This leads too big heavy batteries and a heavyweight car:

Tesla Model S: 2200 kg (90 Kwh)

Tesla Model X: 2440 kg (90 Kwh)

Tesla Model 3: 1611 kg (60 Kwh)

The Tesla’s are more than ten times as heavy, so the roof size is just too small to have any impact. See also:

https://qz.com/1482588/why-teslas-dont-and-cant-have-solar-roofs/

The cruiser class

So we have super efficient solar cars on one side and fast and practical electric cars on the other side. In 2013 a new racing class was introduced to combine these two things. The cruiser class. Cars are judged not only on speed, but on practical aspects as well. Since its inception this competition have been won by team of the technical university Eindhoven. They designed a practical four-seater car which was solar powered and weighed only 370 kg.

Now the original team formed a startup company “Lightyear” taken things a step further. They created a full practical car with a 60 kWh battery pack and a solar roof. Why is this special? Tesla’s were designed to bring electric cars to the masses. So, they didn’t took the approach (like General Motors did with the EV1) to optimize the car to get as far as possible on a small battery. Tesla’s goal was a car that looks good, drives good and has a decent range. Besides the electric motor self it uses a lot of standardized components.

Lightyear came from superefficient background of the World Solar Challenge. It reinvented the car from the ground up with low weight and low air resistance in mind. The total weight with a 60 kWh (same as Tesla) battery is 1300 Kg. Because of the low air resistance (Cw is below 20) with a moderate weight the range is a decent 725 km (WLTP). The efficiency also improved, because it uses four independent in wheel motors.

In car making it’s an old recipe to make lightweight cars with a decent engine (Mazda MX5, Lotus), but now applied to range instead of speed. It’s like a certain moment it doesn’t add up to make motor/battery bigger anymore. Instead, make it heavier to get more power, make it lighter to take full advantage. So the Lightyear with 60Kwh has a longer range than Tesla with a 100 kWh battery.

Based on this the batteries shouldn’t become bigger, but lighter. Think of battery improvement like

a) Nano sized silicon: https://www.innovationtoronto.com/2019/01/10-times-more-capacity-for-lithium-ion-batteries/

b) Fluoride based batteries: https://newatlas.com/fluoride-battery-caltech/57563/

This is of course a two-sided blade as the article on nano sized silicon states:

“Imagine a car having the same size battery as a Tesla that could travel 10 times farther or you charge 10 times less frequently, or the battery is 10 times lighter.”

Check more on Tesla:

Could Tesla be as big as Volkswagen and Toyota in the next decade?

Check latest Prius Solar roof:

Toyota is testing a much more efficient solar roof for its electric cars

https://cleantechnica.com/2019/07/15/are-solar-vehicles-becoming-viable/