Technology Status of Hydrogen Road Vehicles
Technology Status of Hydrogen Road Vehicles
Technology Status of Hydrogen Road Vehicles
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Refilling procedures are more complicated than for GH 2, because <strong>of</strong> the thermal management required for the<br />
exothermic reaction. However, the refill times seem to be acceptable, especially if less than 100% full is<br />
requested, but this <strong>of</strong> course reflects itself directly into a reduced range.<br />
3.3 Liquid <strong>Hydrogen</strong><br />
This storage method is adopted in the space industry because it is a clear winner on the basis <strong>of</strong> energy density<br />
per unit weight.<br />
Supplies <strong>of</strong> bulk hydrogen for most applications are presently made by LH 2 tanker (except for isolated cases<br />
in which a GH 2 supply is located close to the application), so on-board LH 2 for road vehicles has a logistical<br />
advantage. The preferred method <strong>of</strong> shipping merchant hydrogen is as LH 2 (Moore and Raman 1996). The<br />
major drawbacks <strong>of</strong> LH 2 are the energy <strong>of</strong> liquefaction (about one-third <strong>of</strong> the lower calorific value), and the<br />
need to maintain very low temperatures (-253 o C at 2 bar).<br />
But the following should be kept in mind:<br />
! Some prime movers give better performance on cryogenic than on ambient hydrogen; this comes about<br />
because in the ICE the combustion must be cooled to avoid backfire, knock, and excess NO x; cold H 2<br />
displaces less air than ambient H 2 resulting in increased power.<br />
! Most industrial nations have climates that need air conditioning for at least several months per year.<br />
McKenzie (1994) states that about 50% <strong>of</strong> all new vehicles sold worldwide have air conditioning, and<br />
refrigerated goods transport is constantly on the rise; while CFCs with their disproportionate contribution<br />
to the greenhouse effect are being phased out, their replacement HFCs still emit about half the CFC effect.<br />
Amann (1992) claims that air conditioners in the United States will contribute more CO 2-equivalence than<br />
the tailpipe emissions until about 2015. Using LH 2 as a refrigerant as well as a fuel avoids all that, as<br />
amply demonstrated by the Musashi-9 van (see Table 1, item 15).<br />
! Scott et al. (1996) point out that the 10% thermomechanical energy present in LH 2 total energy, because<br />
<strong>of</strong> the processing down to cryogenic temperatures, could be largely recovered by, for example, using the<br />
LH 2 as heat sink in a cryonic heat engine.<br />
The cryogenic temperature drawback needs super-insulation to keep the boil-<strong>of</strong>f losses within acceptable limits<br />
(below 2%/day for vehicles). This will always add to the costs, and aggravates the already low volumetric<br />
density. Boil-<strong>of</strong>f gases can sometimes be productively used, which in turn would eliminate the potential safety<br />
problem <strong>of</strong> vented hydrogen. But there is no escaping the boil-<strong>of</strong>f problem, especially for parked vehicles,<br />
although Hynek and Moore (1995) tend to minimize it. Michel et al. (1996) show how LH 2 now need suffer<br />
no boil<strong>of</strong>f for several days.<br />
LH 2 refueling for vehicles has received considerable attention during the past decade, especially in Germany.<br />
Industrial systems are being used for the relevant projects described in the next chapter. One thing is already<br />
clear: LH 2 refueling must be almost fully automatic, with little human intervention except at the beginning and<br />
end. Expert supervision will be required, although Hynek and Moore (1995) again tend to minimize the<br />
matter. However, in this case it can become an acceptably fast procedure (Tachtler and Szyszka 1994).<br />
Nothing in the literature deals with the question <strong>of</strong> initial filling, i.e., with the vehicle tank at ambient<br />
temperature, presumably because ad-hoc measures were taken. However, vehicles that run out <strong>of</strong> LH 2 will<br />
be almost as common in the future as vehicles fueled today with conventional fuels. Driving to the service<br />
station will be a problem, and the filling procedure will be more complicated and time-consuming. This<br />
disadvantage seems particularly applicable to LH 2. The hybrid vehicles described in section 4.3 can partially<br />
solve this problem.<br />
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