Liquid Hydrogen

Lowering the temperature of hydrogen to its boiling point at 20.39 K (-252.76°C/-422.97°F) at atmospheric pressure requires approximately 39.1 kJ/g (Ullmann, 1983, p. 312) or 79 kJmol-1. To put this value into perspective, this energy amounts to a third of the lower heating value (LHV 242 kJ/mol) and over a quarter of the higher heating value (HHV 286 kJmol-1) of hydrogen. In other words, the overall energy efficiency has already significantly dropped by the time the cryogenic tank is filled.

Part of this energy, approximately 6 kJ/g (Ullmann, 1983, p. 312), is consumed because of a quantum mechanical phenomenon, the nuclear spin. Because H2 contains two atoms, spin-parallel so-called ortho-hydrogen (o-H2) and antiparallel para-hydrogen (p-H2) species exist. Although at ambient temperature,

FIGURE 5.2 Liquid hydrogen tank inside a BMW 7 series family sedan powered by hydrogen internal combustion. (Source: BMW.)

hydrogen consists of 25% p-H2 and 75% o-H2, p-H2 is the stable form at cryogenic temperatures. Unfortunately, the ortho-para conversion occurs on a timescale of days (Ullmann, 1983, p. 245) and is highly exothermal, leading to excessive losses of hydrogen by evaporation, in addition to the boil-off due to heat leaks discussed below. Therefore, spin conversion to p-H2 is carried out during the liquefaction process over catalysts of iron oxide, hydroxide, or chromium oxide supported on alumina.

Another problem with cryogenic storage is hydrogen boil-off. Despite good thermal insulation, the heat influx into the cryogenic tank is continuously compensated for by the boiling off of quantities of the liquid (heat of evaporation). In cryogenic storage systems onboard cars, the boil-off rate is estimated by most developers at approximately 1% per day, which results in further efficiency losses.

Cryogenic tanks are now available from a number of companies such as Linde and Messer. They consist of multi-layered aluminum foil insulation. The tank used by BMW with its hydrogen internal combustion engines stores 120 liters of cryogenic hydrogen or 8.5 kg (Reister et al., 1992), which corresponds to an extremely low density of 0.071 kgdm-3. The hydrogen tank inside a BMW internal combustion car is shown in Fig. 5.2. The empty tank has a volume of approximately 200 liters and weighs 51.5 kg (Larminie and Dicks, 2000). This corresponds to a hydrogen mass fraction of 14.2%. Figure 5.5 later in this chapter shows a comparison of various storage options. Technology is being developed to fit a similar tank inside the new BMW MINI Cooper Hydrogen (see Fig. 5.3.)

In General Motors' HydroGenl, 5 kg of hydrogen are stored in a 130-liter/50-kg tank, giving the vehicle a 400-km (250 mile) drive range. The future target is a 150-liter tank that is lighter yet, holding 7 kg for a range of 700 km (438 miles), as well as reduced boil-off time via an additional liquefied/dried air cooling shield developed by German industrial gas producer Linde (H&FC Letter, 2001).

Perhaps surprisingly, the safety of cryogenic hydrogen storage is not a major concern; hydrogen tanks have obtained technical approval by TUV, the German safety authority.

The actual handling of cryogenic hydrogen poses a problem to the filling station, requiring special procedures. A fully automated, robotic filling station for liquid hydrogen was installed at the Munich Airport in a collaboration between Linde and BMW. It is shown in Fig. 5.4.

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