Motor's improved power-to-weight ratio suits atmospheric satellites

Controlled autonomously, this aircraft, called Solar Eagle, will eventually stay aloft for over five years on nothing but sunshine. The US Department of Defense (DOD)'s funding organisation, the Defense Advanced Research Projects Agency (DARPA), gave Boeing an $89m contract to develop its demonstrator vehicle which was to make its first flight in 2014 before flight trials were abandoned.

DARPA's High-Altitude, Long-Endurance (HALE) platform would have been capable of flying missions lasting between three months to five years. According to DARPA's original plan, this drone's 122m wingspan would carry solar panel arrays that would have been able to sustain the drone aloft in the stratosphere for at least five years.

Key suppliers for that programme include Versa Power Systems and QinetiQ of the UK. This solar-electric-powered aircraft was designed to cruise at altitudes above 60,000 feet at a speed of 70 to 80 knots while performing communications, intelligence, surveillance and reconnaissance missions. During the day, the solar power generation is used to power the aircraft and excess solar power generation is converted to hydrogen by the fuel cells as they operate in electrolysis mode. At night, the fuel cells run in fuel cell mode, converting the stored hydrogen to power. Solid Oxide Fuel Cell (SOFC) based energy storage systems have the potential to provide unprecedented round trip energy efficiency as the storage application of the technology is further developed.

DARPA is now refocusing on advancing critical energy management technologies - solar collection (PV) and fuel cells (energy storage systems). According to DARPA, these technologies are the least mature and are vital for enabling ultra-persistent HALE flights lasting multiple years. By narrowing the programme's focus, DARPA seeks to advance energy management technologies that would benefit a number of future HALE aircraft applications and should reduce risk for development of future very long-endurance aircraft programs.

It is also in the UK that Facebook has been testing similar aircraft entirely powered by PV with battery, in this case with the intention of bringing the internet to the two thirds of the global population currently denied it, an ambition shared by Google.

All this has much in common with work by Siemens on motors for manned electric aircraft, both pure electric and hybrid, with the two key objectives of improving power to weight ratio and simplifying or eliminating transmission. Siemens researchers have developed a 50kg electric motor for aircraft that delivers a continuous 260kW - five times more than comparable drive systems.

To implement the record-breaking motor, Siemens' experts scrutinised all the components of previous motors and optimised them up to their technical limits. New simulation techniques and sophisticated lightweight construction enabled the drive system to achieve a unique weight-to-performance ratio of 5kW/kg. Electric motors of comparable strength used in industrial applications deliver less than 1kW/kg. The performance of the drive systems used in electric vehicles is about 2kW/kg. Elegantly, the motor delivers its outstanding performance at only 2,500rpm so it can drive propellers directly, without transmission.

"This innovation will make it possible to build series hybrid-electric aircraft with four or more seats," said Frank Anton, Head of eAircraft, Siemens. "We're convinced that the use of hybrid-electric drives in regional airliners with 50 to 100 passengers is a real medium-term possibility."  The motor will begin flight-testing before the end of 2015. In the next step, the Siemens researchers will boost output further.

In 2013, Siemens, Airbus and Diamond Aircraft successfully flight-tested a series hybrid-electric drive in a DA36 E-Star 2 motor glider for the first time. The test aircraft had a power output of 60kW.