NUON SOLAR CAR TEAM NETHERLANDS

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Delft University of Technology - Technische Universiteit Delft

Founded in 1842, the Delft University of Technology, in Delft, the Netherlands, is one of the oldest, largest, and most comprehensive technical universities in the Netherlands, with over 13,000 students and 2,100 scientists (including 200 professors). The university is a member of the IDEA League.

Nuna is the name of a series of manned solar powered vehicles that won the World solar challenge in Australia three times in a row, in 2001 (Nuna 1 or just Nuna), 2003 (Nuna 2) and 2005 (Nuna 3). The Nunas are built by students of the Delft University of Technology.

Nuna 3 - superb solar car design

Nuna 3 (2005)

Nuna 3 was one of the favourites for the 2005 edition of the World Solar Challenge with a pre-race test-drive speed of 130 km/h. The final result was that the 3021 kilometers between Darwin and Adelaide were covered in a record 29 hours and 11 minutes, averaging about 103 km/h.

It has improved solar cells of a type that is otherwise only used in artificial satellites (as had the previous Nunas), and it has better aerodynamics and a lower weight than its predecessors.

It was designed and built by 11 students from different disciplines of the Delft University of Technology, who have partly put their studies on hold for this. They used the hightech labs and workshops of the University and, as with the Nuna 2, they received advise from Wubbo Ockels, the first Dutch astronaut and professor at the University.

Main specifications

Design criteria

To have a good chance to win, the car has to:

• collect as much solar energy as possible

• use as little energy as possible to drive at a certain speed. This means special attention to:

  • the efficiency of transferring electrical energy to the wheels, and

  • minimizing friction, constituted by:

   

1) air friction (air resistance), and

2) rolling friction, which in turn is affected by the weight, among other things

The Nuon 2005 team at race course Zandvoort

Solar cells

The solar cells are made of Gallium Arsenide (GaAs) and consist of three layers. Sunlight that penetrates the upper layer is used in the lower layers, resulting in an efficiency of over 26%. This type of solar cell is among the best available at the moment. Apart from efficiency, size also matters, so the entire roof of the Nuna 3 is covered with them, except for the cockpit.

Efficiency is optimal when the cells are hit by the solar rays perpendicularly. If not, output is reduced by roughly the cosine of the angle with the perpendicular. Because the race is held in September this year (as opposed to October or November in previous years) the sun will be lower in the sky (it's earlier in spring). To compensate for this, as many cells as possible are placed at the sides, most notably on the wheel caps.

A solar cell gives a certain amount of current for a certain amount of sunlight. The voltage depends on the load (more precisely the resistance of the load). The power is the product of voltage and current and therefore also depends on the load. Over a certain voltage the current of the solar cell quickly drops to zero, as the graph illustrates.

However, the batteries have a fairly constant voltage, which also has a rather different value than that of the solar cells. So a voltage transformation is needed. Because this is direct current, a normal transformer, which uses alternating current can not be used.

Also, the DC-DC converter has to make sure the load the solar cells see is such that the solar cells give maximum power, so also at the top of the green line in the graph. The machine that does this is called the Maximum Power Point Tracker (MPPT). Here too, the goal is to have this conversion achieve maximum efficiency (>97%).

Aerodynamic design

The aerodynamic resistance is an important part of the total resistance. Important are the frontal surface and the streamline. Any deviation from the ideal streamline will cause turbulence, which costs energy. The ideal streamline is achieved in various stages:

1. Through computer simulations of the design

2. Through testing of a scale model in a wind tunnel. For example, liquid paints can be applied to see the flow of air over the surface. The photo shows is taken during one of those tests in the Low Speed Laboratory of the TU Delft.

3. Through testing of the full scale car in a wind tunnel. For this a German-Dutch wind tunnel in Emmeloord will be used.

From meteorological data from the area where the contest is to take place, it can be concluded that there will likely be a strong side-wind. The wheel caps of the Nuna 3 are designed such that a sidewind will have a propulsory effect.

Engine

The engine is totally encased in the rear wheel to minimise loss through mechanical transmission from engine to wheel (such as in a normal car in the gear box and cardan). The engine is an improved version of a Swiss engine by Drivetek, a spin-off of the Hochschule für Technik und Architektur Biel. The improvements are a 30% lower weight (10 kg), 50% more power (over 2400 W) and a 45% higher torque. The efficiency of the engine is also improved and is now over 98%. But as the graph shows this depends somewhat on the speed and increases with speed.

Test drive

During one of the test drives in the Netherlands the Nuna 3 achieved a speed of 130 km/h. On the first day of the race the car achieved a top speed of 140 km/h.

The race

Winning the race requires not just a good vehicle but also a clever way of driving it, in accordance with the characteristics of the track. Which is why this has been researched for two months prior to the race. Height differences are mapped and linked to GPS data. From this, during the race, the optimal speed can be determined.

Despite all testing and other preparations, one uncertain factor remains; the weather. Any clouds would strongly influence the amount of sunlight that can be captured. So any weather changes along the track will have to be constantly monitored. All these data are analysed by a computer model that constantly computes the ideal speed for that moment. This equipment is built into (petrol powered) pilot cars. Through telemetry these constantly receive data about the condition of the batteries and the amount of captured sunlight.

Important opponents

The winner of the American Solar Challenge from the University of Michigan (USA) was considered to be one of the most important opponents. Other important contestants were the MIT (also USA) and the Japanese Ashiya University team. In 2005 there were also two other European contestants, the Dutch Raedthuys Solar Team from the University of Twente and the Belgian Umicore Solar Team from Leuven.

2005 Race monitor

• 5 August 2005: the team arrives in Adelaide.

• 2 September 2005: The road permit is granted.

• 16 September 2005: During a test drive Nuna 3 strands in the rough next to the road. A defective wheel suspension turned out to be the cause. The damage was limited and repaired after a few days.

• 22 September 2005: The Nuna 3 is approved by the organisation.

• 24 September 2005: The Nuna 3 qualifies for the 8th starting position, which is better than the starting positions the previous two models got.

• 25 September 2005: The Nuna 3 covered 827 km holding first place, leading the next-placed Michigan team by approximately half an hour.

• 26 September 2005: On the second day the Nuna 3 covered 835 km, at an avarage speed of 105 km/h, which is a new single-day record for the World Solar Challenge. The Michigan team is now 132 km behind.

• 27 September 2005: Nuna 3 covered 858km, beating yesterdays record. They extended their lead to two hours. 500 km to go.

• 28 September 2005: Nuna 3 arrives as first car in Adelaide, thus scoring a hat-trick. The overall average speed of 103 km/h over 3.010 km means an improvement by 6 km/h of the 2003 record.

LINKS:

• www.nuonsolarteam.com

• The weblog of the Nuna 3 team (largely from Australia)