Nuon Solar Team

Nuna is the name of a series of manned solar powered race cars that have won the World Solar Challenge in Australia six times: in 2001, 2003, 2005, 2007, 2013 and 2015. The vehicles are built by students who are part of the "Nuon Solar Team" at the Delft University of Technology in the Netherlands, sponsored by N.V. Nuon Energy.

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

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 currently. Apart from efficiency, size also matters, so the entire upper surface 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.

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. A special type of DC-DC converter is used to ensure the load resistance presented to the solar cells is such that the solar cells give maximum power, so also at the top of the green line in the graph. This is called a Maximum power point tracker (MPPT). Here too, the goal is to have this conversion achieve maximum efficiency (>97%).

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Wikipedia