Science & Environment

Solar car put to wind tunnel test

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Media captionNuon technical manager Mike Hoogstraten explains the aerodynamic tests

The best in solar car engineering can be heard on the wind.

At the German-Dutch Wind Tunnels in the Netherlands, an entry into this year's World Solar Challenge is being put to its final tests - and the results are both visible and audible.

The bi-annual contest is a gruelling race straight across Australia, covering more than 3,000km between Darwin and Adelaide.

More than 40 teams from 22 countries have applied for the 2011 race in October. The teams are only allowed to travel between 8am and 5pm, and must follow Australian road regulations and share the road with conventional cars.

The Nuon Solar Team from the University of Delft has won four of the last five Challenges, and their latest effort - dubbed Nuna 6 - is the lightest yet, weighing in at just 145kg.

But getting the weight down is just part of the battle, said Pier von Zonneveld, the project's team leader.

"Aerodynamics is a very important part of the design process," he told BBC News.

"There's a rule of thumb that states that about 70% of the friction that the car needs to overcome comes from aerodynamics. Of course we tested in wind tunnels in our design phase, but now we've actually built the car we want to test and see if these measurements are still accurate."

Ribbon test

A gale of wind, completely silent, whips through the tunnel, simulating the conditions of travel at 100km/h.

The car is strapped to the tunnel's floor, and high-precision scales measure the how much lift the car is experiencing. But for all the technology gathering data from the scales, two low-tech approaches are part of the tests.

What the team wants to know is whether there is "laminar flow" around the car - smooth, uninterrupted sheets of air.

If the precise shape of the car's surfaces and edges aren't perfect, the flow can be turbulent - swirling and chaotic, and creating a great deal of drag that squanders energy.

The team uses a microphone and amplifier to quite simply listen to the air - laminar flow is silent, whereas turbulent flow makes a noise.

And a ribbon on the end of a stick can show where the flow is turbulent flow is too; as it is brought close to the car's surfaces, the ribbon whips around wildly where the flow is turbulent, and hugs the shape of the car where it is laminar.

Mike Hoogstraten, the team's technical manager said that preparing for the race would entail far more than fine-tuning the car's aerodynamics.

"The design is one part of the deal - you have to make a good car. But once you get to Australia there's another part - strategy," he said.

"To win the race you have to make lots of calculations and predictions. You have to use the energy as efficiently as possible, by knowing how much energy you're going to get, and how much you're going to give away."

Image caption The teams travel just nine hours a day and sleep alongside the Stuart highway in Australia

Those calculations even include weather modelling, a crucial aspect of a race that runs thanks only to the Sun.

The race's organisers tighten the regulations on the cars every year; the entries have become so refined that the contest simply needs to be made tougher.

This year, teams will have to use silicon solar cells - the type commonly found on roofs and used to power electronics - rather than the higher-efficiency gallium arsenide cells used previously.

The kind of engineering that is being worked out now, for nothing more than the glory of winning the race, will pay dividends in the development of solar-powered passenger cars.

"Our team doesn't believe that this kind of car is the commercial car - you can see by the size it'd be difficult to get your groceries from one place to another," Mr Hoogstraten said.

"But we're improving those technologies and using them and showing the world these cars are really good - they can go extremely fast with very little energy. It's just an inspiration to future technology."

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