On a weight basis, hydrogen has the highest energy content of any known fuel (almost three times that of gasoline). However, being the lightest gas, the density of liquid hydrogen is only 70.8 kg m-3, corresponding on a volume basis to an energy content about a factor of three less than gasoline. Nevertheless, liquid hydrogen is a compact form of hydrogen, making it in principle an attractive candidate for hydrogen storage, especially in transportation. In fact, it is in this form that hydrogen is used as a propellant for space vehicles. However, hydrogen, which has a boiling point of-253 °C is, after helium, the most difficult gas to liquefy. Complex and expensive multi-stage cooling systems are necessary to obtain liquid hydrogen. Typically, in the first step, hydrogen is precooled with liquid ammonia to -40 °C and then to -196 °C using liquid nitrogen. In the following step, helium is used in a multi-stage compression-expansion system to obtain liquid hydrogen at -253 °C. The efficiency of this complex process increases with the plants size and is thus more adapted for centralized production. The process is not only complex and expensive but also very energy intensive: about 30-40% of the energy content of the hydrogen is required for its liquefaction . Moreover, liquid hydrogen storage systems inevitably lose hydrogen gas over time by evaporation or "boil off". The rate of loss is dependent on the amount stored and the tank's insulation, and is generally lower for larger quantities of liquid hydrogen. In the case of automobile tanks with small capacities, the result is that 1-5% of the hydrogen content would be released to the atmosphere each day in order to avoid pressure build-up . Given the cost and energy invested into producing liquid hydrogen, this is unacceptable both from an economic and also from an environmental viewpoint. Guessing how much fuel is still in the storage tank after some days is certainly not something that most people would like to worry about! In addition to the cost of cryogenic storage being expensive liquid hydrogen, because of its extremely low temperature, must be handled with great care. But perhaps most important of all, hydrogen leaks can result in major safety hazards.
148 | Chapter 9 The Hydrogen Economy and its Limitations Compressed Hydrogen
In order to store sufficient amounts of energy in a given space, hydrogen compression is currently the preferred solution used in most of the hydrogen fuel cell-powered prototype cars. Because the same quantity of hydrogen can be stored in smaller tanks with increasing pressure, containers (tanks) were developed over the years able to withstand increasingly high pressures. Hydrogen can now be held under 350 or even 700 atm in tanks made from new lightweight materials, such as carbon-fiber-reinforced composites. However, even under these conditions, hydrogen has still a much lower energy content per volume than gasoline (4.6 times less than gasoline at 700 bar H2; Fig. 9.8) and thus requires several-fold more voluminous tanks. In contrast to liquid fuel tanks which can adopt any shape and easily be adapted to any vehicle, compressed hydrogen tanks have a fixed cylindrical shape necessary to ensure their integrity under high pressure. Vehicle designers and engineers will need to pay great attention on how and where to integrate the pressure tanks. Although hydrogen compression is less energy-intensive than liquefaction, depending on the pressure, it still uses the equivalent of 10-15% of the energy contained in the hydrogen fuel . Because of its small size, as mentioned, hydrogen is able to diffuse through many materials, including metals. During prolonged exposure to hydrogen, some metals can also become brittle. Consequently, because many parts of the fuel system in contact with hydrogen will be metallic, it is necessary to prevent material failure
which could have grave consequences, especially under high pressures. The risk of leaks is a major safety hazard as hydrogen is a highly flammable and explosive gas. This is of course an even greater concern in collision accidents. The high-pressure hydrogen storage systems made from high-tech materials are also complex and presently very expensive, and although technological improvements and larger-scale production will certainly reduce the costs, many of the concerns raised will remain.
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