Following the record breaking flight of Qinetiq’s Zephyr solar powered plane (EW 26/09/07), we spoke to Qinetiq technologist Jonathan Dixon to find out what is inside.
“Reducing mass is the key,” he told EW. “To save the cable runs back and forth we won’t usually have centralised systems. Instead there are layers of complexity reducing from hundreds of solar cells and hundreds of battery cells to two 
motors.”
The solar cells are flexible amorphous silicon types from US firm United Solar Ovonic. “They are very light and produce about 1.6kW at altitude at mid-day,” said Dixon.
To maximise power transfer to the aircraft’s systems, a ‘peak power tracker’ artificially matches the aircraft load to the solar cells. “Spacecraft do this kind of thing,” said Dixon. “We designed it here and it is the most incredibly efficient distributed peak power tracker: 92-97 
per cent efficient. There are a number of these on the aircraft.”
In the day, solar power is split between running the motors and charging the batteries. “There are two motors, each about half the height and half the diameter of a coffee cup,” said Dixon.
The organisation is working with several suppliers of motors. “They are custom brushless DC and are slightly unusual in that we are trying to do without a gearbox so they have to have high efficiency at low speed,” said Dixon.
Between 16 and 50V is available from the aircraft, according to Dixon. “Our speed controllers are really important. How they interact with the motor is a big thing to us.”
Peak power from the motors is a little hazy. “Lets put it this way, we can use all the power the aircraft can generate,” he said.
At night power is supplied by 3kWh of LiS cells from Sion Power. “We fly the best batteries in the world you can get,” said Dixon.
Operating temperature range is -70 to +30°C and the all-up weight of the 18m plane is 30kg.
Its record was to fly for 54 hours, and to almost 18km altitude.
Passing through the jet stream “it gets blown around a bit, and as soon as you are through it, no problem”, said Dixon.
See also: the Electronics Weekly focus on solar cells, presenting a roundup of content related to photovoltaic technologies, converting light sources to energy.