Spacecraft cells

Batteries of fuel cells have been used in spacecraft but, in general, the potentially high efficiency of the fuel cell has not been realized industrially. 

Action of Vacuum on Spacecraft Materials. For service beyond the atmosphere, the vacuum environment allows materials to evaporate or decompose under the action of various forces encountered (1,18,19). These forces include the photons from the sun, charged particles from solar wind, and dust. The action of space environment on materials and spacecraft can be simulated by a source—sink relationship in a vacuum environment. Thus, for example, the lifetime of a solar panel in space operation may be tested (see Photovoltaic cells). 

Although the principle of fuel cells has been known since 1838, practical applications arc fairly recent. The first applications were in the space program, where fuel cells powered the Gemini and Apollo spacecraft. In the 1960s and 1970s, fuel cells... 

Dry cells (batteries) and fuel cells are the main chemical electricity sources. Diy cells consist of two electrodes, made of different metals, placed into a solid electrolyte. The latter facilitates an oxidation process and a flow of electrons between electrodes, directly converting chemical energy into electricity. Various metal combinations in electrodes determine different characteristics of the dry cells. For example, nickel-cadmium cells have low output but can work for several years. On the other hand, silver-zinc cells are more powerful but with a much shorter life span. Therefore, the use of a particular type of dry cell is determined by the spacecraft mission profile. Usually these are the short missions with low electricity consumption. Diy cells are simple and reliable, since they lack moving parts. Their major drawbacks are... 

Fuel Cells Chemical. -High output -Stable current -Water supply as a sub-product. -Complex design -Bulk -Life-span is limited by onboard fuel supply. Used in some piloted spacecraft for short duration missions. 

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

Photovoltaic (PV) solar cells, which convert incident solar radiation directly into electrical energy, today represent the most common power source for Earth-orbiting spacecraft, such as the International Space Station, where a photovoltaic engineering testbed (PET) is actually assembled on the express pallet. The solid-state photovoltaics, based on gallium arsenide, indium phosphide, or silicon, prove capable, even if to different extents and with... 

Alkaline fuel cells (AFCs). The electrolyte is 40 to 70% KOH, the working temperatures are 60 to 240°C. Such systems were used in the spacecraft of the Apollo program and in the U.S. space shuttle. 

Aqueous, alkaline fuel cells, as used by NASA for supplemental power in spacecraft, are intolerant to C02 in the oxidant. The strongly alkaline electrolyte acts as an efficient scrubber for any C02, even down to the ppm level, but the resultant carbonate alters the performance unacceptably. This behavior was recognized as early as the mid 1960 s as a way to control space cabin C02 levels and recover and recycle the chemically bound oxygen. While these devices had been built and operated at bench scale before 1970, the first comprehensive analysis of their electrochemistry was put forth in a series of papers in 1974 [27]. The system comprises a bipolar array of fuel cells through whose cathode chamber COz-containing air is passed. The electrolyte, aqueous Cs2C03, is immobilized in a thin (0.25 0.75 mm) membrane. The electrodes are nickel-based fuel cell electrodes, designed to be hydrophobic with PTFE. 

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the US space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C. However, more-recent AFC designs operate at lower temperatures of roughly 23°C to 70°C. 

There is also the possibility of propelling vehicles. This means not just providing electricity, as with NASA spacecraft, but also providing the means of propulsion. Space-faring rockets require a bit too much power for this to be practical as yet, but cars and small airplanes travel at much more attainable speeds. Fuel cell engines are an extremely active area of research. 

The sulfonyl fluoride groups were hydrolyzed after the polymer had been processed into the membrane. One of the earliest applications of Nafion was in hydrogen/oxygen fuel cells such as those which provided electrical power for the Apollo spacecraft used for the manned expeditions to the moon in the early 1970s. 

Sealed cells also have many important military and aerospace applications where absence of maintenance may be important. The battery for the Viking Mars orbiting spacecraft, consisting of 26 sealed 30 Ah cells, is shown in Fig. 6.10. 

Fig. 6.10 Battery for Viking Mars orbiting spacecraft, comprising 26 sealed 30 Ah nickel-cadmium cells, which was placed in Mars orbit in 1976. (By courtesy of Jet Propulsion Laboratory,)...

Fig. 6.23 Battery for Ranger lunar photography spacecraft, comprising 14 sealed 45 Ah zinc-silver oxide cells, which impacted on the moon in 1965. (By permission of Jet Propulsion Laboratory,)...

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