A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.
A recent thrust in the development of regenerative fuel cell systems has been led by NASA. Regenerative fuel cell systems provide energy storage at a scale that is larger than what is practical with advanced batteries.
In a regenerative fuel cell system, energy storage is achieved via the electrolysis of water to hydrogen and oxygen gas during the storage phase. Consumption of gases then occurs during the energy generation phase, with the subsequent generation of water.
It is envisioned that the energy for the electrolysis of water be supplied via solar power. The regenerative fuel cell systems can be used to power robots, mobility systems, and human habitats. This talk will provide an introduction of fuel cells and regenerative fuel cell systems and highlight the features of this technology for enabling future NASA missions to the moon, near-Earth asteroids and Mars.
Despite criticism and early technical failures, the taming of liquid hydrogen proved to be one of NASA’s most significant technical accomplishments. Hydrogen — a light and extremely powerful rocket propellant — has the lowest molecular weight of any known substance and burns with extreme intensity (5,500°F). In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant.
Because liquid oxygen and liquid hydrogen are both cryogenic — gases that can be liquefied only at extremely low temperatures — they pose enormous technical challenges. Liquid hydrogen must be stored at minus 423°F and handled with extreme care.
To keep it from evaporating or boiling off, rockets fuelled with liquid hydrogen must be carefully insulated from all sources of heat, such as rocket engine exhaust and air friction during flight through the atmosphere.
Once the vehicle reaches space, it must be protected from the radiant heat of the Sun. When liquid hydrogen absorbs heat, it expands rapidly; thus, venting is necessary to prevent the tank from exploding. Metals exposed to the extreme cold of liquid hydrogen become brittle. Moreover, liquid hydrogen can leak through minute pores in welded seams.
Solving all these problems required an enormous amount of technical expertise in rocket and aircraft fuels cultivated over a decade by researchers at the National Advisory Committee for Aeronautics (NACA) Lewis Flight Propulsion Laboratory in Cleveland.
Today, liquid hydrogen is the signature fuel of the American space program and is used by other countries in the business of launching satellites. In addition to the Atlas, Boeing’s Delta III and Delta IV now have liquid-oxygen/liquid-hydrogen upper stages.
This propellant combination is also burned in the main engine of the Space Shuttle. One of the significant challenges for the European Space Agency was to develop a liquid-hydrogen stage for the Ariane rocket in the 1970s. The Soviet Union did not even test a liquid-hydrogen upper stage until the mid-1980s. The Russians are now designing their Angara launch vehicle family with liquid-hydrogen upper stages. Lack of Soviet liquid-hydrogen technology proved a serious handicap in the race of the two superpowers to the Moon.4 Taming liquid hydrogen is one of the significant technical achievements of twentieth century American rocketry.
