Solar Cells and Fuel Cells for Solar-Powered ERAST Vehicles

NASA had first acquired solar cells from Spectralab but chose cells from SunPower Corporation of Sunnyvale, CA, for the ERAST UAVs. These photovoltaic cells converted sunlight directly into electricity and were lighter and more efficient than other commercially available solar cells at that time. Indeed, after NASA flew Helios, SunPower was selected to fur­nish high-efficiency solar concentrator cells for a NASA Dryden ground solar cell test installation, spring-boarding, as John Del Frate recalled subsequently, "from the technology developed on the PF+ and Helios solar cells.”[1546] The Dryden solar cell configuration consisted of two fixed – angle solar arrays and one sun-tracking array that together generated up to 5 kilowatts of direct current. Field-testing at the Dryden site helped SunPower lower production costs of its solar cells and identify uses and performance of its cells that enabled the company to develop large-scale commercial applications, resulting in the mass-produced SunPower A-300 series solar cells.[1547] SunPower’s solar cells were selected for use on the Pathfinders, Centurion, and Helios Prototype UAVs because of their high – efficiency power recovery (more than 50-percent higher than other com­mercially available cells) and because of their light weight. The solar cells designed for the last generation of ERAST UAVs could convert about 19 percent of the solar energy received into 35 kilowatts of electrical current at high noon on a summer day. The solar cells on the ERAST vehicles were bifacial, meaning that they could absorb sunlight on both sides of the cells, thus enabling the UAV s to catch sunrays reflected upward when flying above cloud covers, and were specially developed for use on the aircraft.

While solar cell technology satisfied the propulsion problem during daylight hours, a critical problem relating to long-endurance backup sys­tems remained to be solved for flying during periods of darkness. Without solving this problem, solar UAV flight would be limited to approximately 14 hours in the summer (much less, of course, in the dark of winter), plus whatever additional time could be provided by the limited (up to 5 hours for the Pathfinder) backup batteries. Although significant improvements had been made, batteries failed to satisfy both the weight limitation and long duration power generation requirements for the solar-powered UAVs.

As an alternative to batteries, the ERAST alliance tested a number of different fuel cells and fuel cell power systems. An initial problem to overcome was how to develop lightweight fuel cells because only 440 pounds of Helios’s takeoff weight of 1,600 pounds were originally planned to be allocated to a backup fuel cell power system. Helios required approximately 120 kilowatthours of energy to power the craft for up to 12 hours of flight during darkness, and, fortunately, the state of fuel cell technology had advanced far enough to permit attaining this; ear­lier efforts back to the early 1980s had been frustrated because fuel cell technology was not sufficiently developed at that time. The NASA – industry team later determined, as part of the ERAST program, that a hydrogen-oxygen regenerative fuel cell system (RFCS or regen system) was the hoped for solution to the problem, and substantial resources were committed to the project.

RFCSs are closed systems whereby some of the electrical power pro­duced by the UAV’s solar array during daylight hours is sent to an electro­lyzer that takes onboard water and disassociates the water into hydrogen gas and oxygen gas, both of which are stored in tanks aboard the vehicle. During periods of darkness, the stored gases are recombined in the fuel cell, which results in the production of electrical power and water. The power is used to maintain systems and altitude. The water is then stored for reuse the following day. This cycle theoretically would repeat on a 24-hour basis for an indefinite time period. NASA and AeroVironment also considered, but did not use, a reversible regen system that instead of having an electrolyzer and a fuel cell used only a reversible fuel cell to do the work of both components.[1548]

As originally planned, Helios was to carry two separate regen fuel cell systems contained in two of four landing gear pods. This not only disbursed the weight over the flying wing, but also was in keeping with the plan for redundant systems. If one of the two fuel cells failed, Helios could still stay aloft for several days, albeit at a lower altitude. Contracts to make the fuel cell and electrolyzer were given to two companies—Giner of Waltham, MA, and Lynntech, Inc., of College Station, TX. Each of the two systems was planned to weigh 200 pounds, including 27 pounds for the fuel cell, 18 pounds for the electrolyzer, 40 pounds for oxygen and hydrogen tanks, and 45 pounds for water. The remaining 70 pounds con­sisted of plumbing, controls, and ancillary equipment.[1549]

While the NASA-AeroVironment team made a substantial invest­ment in the RFCS and successfully demonstrated a nearly closed system in ground tests, it decided that the system was not yet ready to satisfy the planned flight schedule. Because of these technical difficulties and time and budget deadlines, NASA and AeroVironment agreed in 2001 to switch to a consumable hydrogen-air primary fuel cell system for the Helios Prototype’s long-endurance ERAST mission. The fuel cells were already in development for the automotive industry. The hydrogen-air fuel cell system required Helios to carry its own supply of hydrogen. In periods of darkness, power for the UAV would be produced by combining gaseous hydrogen and air from the atmosphere in a fuel cell. Because of the low air density at high altitudes, a compressor needed to be added to the sys­tem. This system, however, would operate only until the hydrogen fuel was consumed, but the team thought that the system could still provide multiple days of operation and that an advanced version might be able to stay aloft for up to 14 days. The installation plan was likewise changed. The fuel cell was now placed in one pod with the hydrogen tanks attached to the lower surface of the wing near each wingtip. This modification, of course, dramatically changed Helios’s structural loadings, transforming it from a span-loaded flying wing to a point-loaded vehicle.[1550]