Compressed cryogenic technique

Recent approaches aimed at improving the unsatisfactory gravimetric and volumetric capacities aie based on compressed cryogenic technique [110], In particular by cooling a tank to nitrogen liquefaction temperature (77 K) the volumetric capacity results three times higher with respect to conventional high pressure tank. 

Table 2.4 displays critical constants Tc, Pc, Vc and critical compressibility factor Zc for a number of common gases. (Accurate determination of the critical point is experimentally challenging, and quoted values are generally uncertain in the final decimal.) One can see from the table that many common gases (including N2, 02, and CH4) are actually supercritical fluids ( permanent gases ) under ambient temperature conditions, incapable of liquefaction by any applied pressure whatsoever. (Aspects of cryogenic gas-liquefaction techniques are discussed in Section 3.6.3.)... 

The refrigeration techniques required to reach cryogenic temperatures are different than those of conventional vapor-compression refrigeration, which is used for most cooling applications closer to ambient temperatures. Most domestic refrigerators and air conditioners use the vapor-compression cycle. Figure 4a shows a schematic of the vapor-compression cycle, and Figure 4b shows the path of... 

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