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VOLUME -22 NUMBER 10
Publication Date: 10/1/2007
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Fuel Cells Power UAVs and Manned Aircraft
Thomas Bradley and Reid Thomas go through the procedure of starting up the fuel cell aircraft during a test flight at the Atlanta Dragway. Georgia Tech Photo: Gary Meek
Atlanta, GA — Georgia Tech researchers have conducted successful test flights of a hydrogen-powered unmanned aircraft powered by a proton exchange membrane (PEM) fuel cell using compressed hydrogen. In tests, the aircraft has to date reached an altitude of 25 meters, followed down an automotive track by a chase car at the Atlanta Dragway in Commerce, Georgia.
The fuel-cell system that powers the 22-foot wingspan aircraft generates only 500 watts. "That raises a lot of eyebrows," said Adam Broughton, a research engineer who is working on the project in Georgia Tech's Aerospace Systems Design Laboratory (ASDL). "Five hundred watts is plenty of power for a light bulb, but not for the propulsion system of an aircraft this size." In fact, 500 watts represents about 1/100th the power of a hybrid car like a Toyota Prius.
A collaboration between ASDL and the Georgia Tech Research Institute (GTRI), the project was spearheaded by David Parekh, GTRI's deputy director and founder of Georgia Tech's Center for Innovative Fuel Cell and Battery Technologies.
Parekh wanted to develop a vehicle that would both advance fuel cell technology and galvanize industry interest. While the automotive industry has made strides with fuel cells, apart from spacecraft, little has been done to leverage fuel cell technology for aerospace applications, he noted.
"A fuel cell aircraft is more compelling than just a lab demonstration or even a fuel cell system powering a house," Parekh explained. "It's also more demanding. With an airplane, you really push the limits for durability, robustness, power density and efficiency."
"For starters, fuel cells emit no pollution and unlike conventional UAVs, don't require separate generators to produce electricity for operating electronic components," noted Tom Bradley, a doctoral student in Georgia Tech's School of Mechanical Engineering who developed the fuel cell propulsion system. "Another plus, because fuel cells operate at near ambient temperatures, UAVs emit less of a heat signature and would be stealthier than conventionally powered UAVs," he said.
Manned Fuel Cell Flight
In another scenario, a European group is testing a manned fuel cell powered aircraft. The systems integration phase of the Fuel Cell Demonstrator Airplane research project, under way since 2003 at Boeing Research and Technology-Europe (BR&TE), was completed recently. Thorough systems integration testing is now under way in preparation for upcoming ground and flight testing.
The demonstrator uses a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which is coupled to a conventional propeller. The fuel cell provides all power for the cruise phase of flight. During takeoff and climb, the flight segment that requires the most power, the system draws on lightweight lithium-ion batteries.
Not surprisingly, the demonstrator aircraft is a lightweight vehicle — a Dimona motor glider, built by Diamond Aircraft Industries of Austria, which also performed major structural modifications to the aircraft. With a wing span of 16.3 meters (53.5 feet), the airplane will be able to cruise at approximately 100 kilometers per hour (62 miles per hour) using fuel cell-provided power.
Flight tests, which have been planned to take place in Spain, will demonstrate for the first time that a manned airplane can maintain a straight level flight with fuel cells as the only power source. The Madrid-based avionics group Aerlyper performed airframe modifications, as well as the mounting and wiring of all components; SAFT France designed and assembled the auxiliary batteries and the backup battery; Air Liquide Spain performed the detailed design and assembly of the onboard fuel system and the refueling station; the Electronic Engineering Division of the Polytechnic University of Madrid (School of Industrial Engineering) collaborated in the design and construction of the power management and distribution box; post-integration bench testing is being conducted in a facility that belongs to the Polytechnic University of Madrid (INSIA); and SENASA (Spain) will provide a test pilot and facilities for flight tests. Other suppliers for the Fuel Cell Demonstrator Airplane include UQM Technologies Inc. (United States), MT Propeller (Germany), Tecnicas Aeronauticas de Madrid (Spain), Ingenieria de Instrumentacion y Control (Spain), GORE (Germany), Indra (Spain) and Inventia (Spain).
High-Flyer from NASA
NASA has begun testing a hybrid all-electric aircraft using a combination of fuel cell and solar power. The Helios Prototype is an enlarged version of the Centurion flying wing that flew a series of test flights at Dryden in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, and 2-1/2 times that of the Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft. The remotely piloted Helios Prototype first flew during a series of low-altitude checkout and development flights on battery power in late 1999 over Rogers Dry Lake adjacent to NASA's Dryden Flight Research Center in the Southern California desert.
During 2000, more than 62,000 bi-facial silicon solar cells were mounted on the upper surface of Helios' wing. Produced by SunPower, Inc., these solar arrays convert about 19 percent of the solar energy they receive into electrical current and can produce up to 35 kw at high noon on a summer day.
The second milestone established by NASA for its development ? a long-endurance demonstration flight of almost two days and nights ? required development of a supplemental electrical power system to provide power at night when the solar arrays are unable to produce electricity. AeroVironment developed an experimental fuel cell-based electrical energy system combining advanced automotive fuel cell components with proprietary control technology designed for the harsh environment above 50,000 feet altitude.
The first version of this system combines gaseous hydrogen from two pressurized tanks mounted on Helios' outboard wing sections with compressed oxygen from the atmosphere via a series of proton-exchange membrane fuel cell "stacks" mounted in the central landing gear pod. The system produces more than 15 kW of direct-current electricity to power Helios' motors and operating systems, with the only by-product being water vapor and heat. The system will increase the Helios Prototype's flight weight by about 800 lb to about 2,400 lb.
Two other versions of the system are contemplated: One, employing liquid hydrogen, would enable the Helios to fly for up to two weeks in the stratosphere anywhere around the Earth, not limited to temperate or equatorial latitudes. Another version, a closed or "regenerative" system, uses water, a fuel cell, and an electrolyzer to form a system similar in function to a rechargeable or "secondary" battery, but with much greater efficiency than the best rechargeable battery systems.
A production version of the Helios with the regenerative fuel cell system is of interest to NASA for environmental science, the military and AeroVironment for various roles, primarily as a stratospheric telecommunications relay platform. With other system reliability improvements, production versions of the Helios are expected to fly missions lasting months at a time, becoming true "atmospheric satellites."
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