World Solar Challenge; inmiddels in 2014!

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Why is Tokai so good?

Let’s do some numbers based on Day 3 results. Tokai departed Wauchopa at 08:00 and reached Kulgera - 667 km down south - at 16:02, having stopped twice at the 30-minute control stops along the road. So – with 7:02 hours driving – they averaged at 95 kph.

Nuna 6 departed at 08:08 and arrived at 16:37 – thus having averaged 89 kph. This 6 kph difference with Tokai leads to a gap of about 30 minutes between the two cars.

Since this race is about energy, let’s do some energy calculations. I will take this site as example – you can calculate power consumption based on weight and aeroshape.
A typical solar weighs in at 230 kg (150 kg + 80 kg driver) and has a CdA (a measure for aero drag) of 0.09.

Fill this in on that site and look at the resulting tables: 89 kph leads to a consumption of 1315 W. Nuna drove for a total of 7:30 hours, so total energy consumed on this 667 km stretch is 1315 x 7.5 = 9.9 kWh.

As a reference, Nuna used the equivalent energy of 10 dishwashers to travel over 600 km at normal commuting speed.

Tokai’s 95 kph with the standard specs lead to a consumption of about 1550W and since they drove for 7 hours, their consumption amounts to 1550 x 7 = 10.9 kWh.

In other words, to create a gap of 30 minutes to Nuna, Tokai had to consume 1 kWh of energy more than Nuna 6, or 10% more energy.

Obviously, energy flows into the car via the solar panel, but where does it go to? First of all, there are losses from solar panel to electric motor, like in the motor controller and other electronic equipment required for operating the solar car.
Secondly, the aerodynamic shape requires energy – the shape of the car is important but also surface smoothness.
Thirdly, there is rolling resistance, related to the type of tire and the weight of the car pushing on those tires. Less weight decreases rolling resistance.

So, let’s see how good Tokai needs to be to be able to spend 10% more energy create that 30-minute gap.

Keep in mind that 1% of energy consumption is about 12W, similar to a small lightbulb.

A 10 kg lighter car leads to 1.5 % less energy consumption.

5 % improvement in aerodynamics lead to 2 % less energy consumption. 5% is the difference between a clean car nose, and one which has flies on it, or other irregularities. So the car needs to be as smooth as possible at all times. Small things have great impact.

If your panel is able to give a mere 2% more solar power, the car has 2% more energy to spend.

If you are able to make your electronics between panel and motor just 1% more efficient, there is a direct relation and you’ll have 1% more energy to spend.

Let’s talk a bit about the battery. A typical solar car battery can hold 5kWh energy. Solar cars take into account what energy the sun delivers and what they can take out of the battery. Battery energy level is called State of Charge (SOC), and you may finish a Race Day with a certain percentage of SOC. Suppose you have 2 similar cars – if one drives faster on a race day, he will have covered more distance but will have less Battery SOC.

Now suppose Tokai has taken more out of its battery SOC than Nuna, when rolling into Kulgera. Let’s say that number is 3.5% of the total Energy Consumption, which comes down to 0.35 kWh. On a battery with 5 kWh capacity that’s 7% SOC less than Nuna, which is really not that much.

In short, for Tokai to gain 30 minutes on Nuna, they need to have taken 7% more out of their battery, have a car that weighs just 10 kg less, have an aero that is just 5% better (which could already be achieved with keeping the car smooth and clean), a panel that can power 2 small lightbulbs more and a little more efficient electronic system.
That totals the 10% of energy that Tokai got to spend to increase their lead by 30 minutes.

Is Tokai that mystical energy powerhouse that people claim it to be?

No. It’s all in the small numbers as described above. A little bit better here, and a little bit better there can already lead to fantastic results.

Tokai’s technology is not lightyears ahead but just a few lightbulbs in every area.

Tokai is not magic, not supernatural, not sent from the future and they don’t overpower their competitors with special technology.
They are just a very good team with great attention to detail. And that’s how you win races.

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Op dinsdag 18 oktober 2011 21:45 schreef Velocitas het volgende:
Goed artikel over de race en de leider vd race hier:
http://www.solarwebsite.nl/2011/10/how-good-is-tokai-really/

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cpheineman Says:
oktober 18th, 2011 at %H:%M
Hi,

Good article! But remember that tokai buys a solar array, buys batteries and more and let it put together. For example nuon makes the most of the car by itself, which has the consequence that its never that good as bought. But you do get a lot of experience and learn a lot from it. That is the most important goal of the race.