53 Hydrogen Storage I Storage as Hydrogen

5.3.1 Introduction to the problem

The difficulties arise because although hydrogen has one of the highest specific energies (energy per kilogram), which is why it is the fuel of choice for space missions, its density is very low, and it has one of the lowest energy densities (energy per cubic metre). This means that to get a large mass of hydrogen into a small space very high pressures have to be used. A further problem is that, unlike other gaseous energy carriers, it is very difficult to liquefy. It cannot be simply compressed, in the way that LPG or butane can. It has to be cooled down to about 22 K, and even in liquid form its density is really very low, 71 kg.m-3.

Although hydrogen can be stored as a compressed gas or a liquid, there are other methods that are being developed. Chemical methods can also be used. These are considered in the next section. The methods of storing hydrogen that will be described in this section are: compression in gas cylinders, storage as a cryogenic liquid, storage in a metal absorber as a reversible metal hydride, and storage in carbon nanofibres.

None of these methods is without considerable problems, and in each situation their advantages and disadvantages will work differently. However, before considering them in detail we must address the vitally important issue of safety in connection with storing and using hydrogen.

5.3.2 Safety

Hydrogen is a unique gaseous element, possessing the lowest molecular weight of any gas. It has the highest thermal conductivity, velocity of sound, mean molecular velocity, and the lowest viscosity and density of all gases. Such properties lead hydrogen to have a leak rate through small orifices faster than all other gases. Hydrogen leaks 2.8 times faster than methane and 3.3 times faster than air. In addition hydrogen is a highly volatile and flammable gas, and in certain circumstances hydrogen and air mixtures can detonate. The implications for the design of fuel cell systems are obvious, and safety considerations must feature strongly.

Table 5.1 gives the key properties relevant to safety of hydrogen and two other gaseous fuels widely used in homes, leisure and business: methane and propane. From this table the major problem with hydrogen appears to be the minimum ignition energy, apparently indicating that a fire could be started very easily. However, all these energies are in fact very low, lower than those encountered in most practical cases. A spark can ignite any of these fuels. Furthermore, against this must be set the much higher minimum concentration needed for detonation, 18% by volume. The lower concentration limit for ignition is much the same as for methane, and a considerably lower concentration of propane is needed. The ignition temperature for hydrogen is also noticeably higher than for the other two fuels.

Hydrogen therefore needs to be handled with care. Systems need to be designed with the lowest possible chance of any leaks, and should be monitored for such leaks regularly. However, it should be made clear that, all things considered, hydrogen is no more dangerous, and in some respects it is rather less dangerous than other commonly used fuels.

5.3.3 The storage of hydrogen as a compressed gas

Storing hydrogen gas in pressurised cylinders is the most technically straightforward method, and the most widely used for small amounts of the gas. Hydrogen is stored in this way at thousands of industrial, research and teaching establishments, and in most locations local companies can readily supply such cylinders in a wide range of sizes. However, in these applications the hydrogen is nearly always a chemical reagent in some analytical or production process. When we consider using and storing hydrogen in this way as an energy vector, then the situation appears less satisfactory.

Table 5.1 Properties relevant to safety for hydrogen and two other commonly used gaseous fuels

Hydrogen

Methane

Propane

Density, kg.m-3 at NTP

0.084

0.65

2.01

Ignition limits in air, volume % at NTP

4.0 to 77

4.4 to 16.5

1.7 to 10.9

Ignition temperature, °C

560

540

487

Min. ignition energy in air, MJ

0.02

0.3

0.26

Max. combustion rate in air, ms-1

3.46

0.43

0.47

Detonation limits in air, volume %

18 to 59

6.3 to 14

1.1 to 1.3

Stoichiometric ratio in air

29.5

9.5

4.0

Two systems of pressurised storage are compared in Table 5.2. The first is a standard steel alloy cylinder at 200 bar, of the type commonly seen in laboratories. The second is for larger scale hydrogen storage on a bus, as described by Zieger (1994). This tank is constructed with a 6 mm thick aluminium inner liner, around which is wrapped a composite of aramide fibre and epoxy resin. This material has a high ductility, which gives it good burst behaviour, in that it rips apart rather than disintegrating into many pieces. The burst pressure is 1200 bar, though the maximum pressure used is 300 bar.2

The larger scale storage system is, as expected, a great deal more efficient. However, this is slightly misleading. These large tanks have to be held in the vehicle, and the weight needed to do this should be taken into account. In the bus described by Zieger (1994), which used hydrogen to drive an internal combustion engine, 13 of these tanks were mounted in the roof space. The total mass of the tanks and the bus structure reinforcements is 2550 kg, or 196 kg per tank. This brings down the 'storage efficiency' of the system to 1.6%, not so very different from the steel cylinder. Another point is that in both systems we have ignored the weight of the connecting valves, and of any pressure-reducing regulators. For the 2L steel cylinder system this would typically add about 2.15 kg to the mass of the system, and reduce the storage efficiency to 0.7% (Kahrom 1999).

The reason for the low mass of hydrogen stored, even at such very high pressures, is of course its low density. The density of hydrogen gas at normal temperature and pressure is 0.084kg.m-3, compared to air, which has about 1.2kg.m-3. Usually less than 2% of the storage system mass is actually hydrogen itself.

The metal that the pressure vessel is made from needs very careful selection. Hydrogen is a very small molecule, of high velocity, and so it is capable of diffusing into materials that are impermeable to other gases. This is compounded by the fact that a very small fraction of the hydrogen gas molecules may dissociate on the surface of the material. Diffusion of atomic hydrogen into the material may then occur which can affect the mechanical performance of materials in many ways. Gaseous hydrogen can build up in internal blisters in the material, which can lead to crack promotion (hydrogen-induced cracking). In carbonaceous metals such as steel the hydrogen can react with carbon, forming entrapped CH4 bubbles. The gas pressure in the internal voids can generate an internal stress high enough to fissure, crack or blister the steel. The phenomenon is well

Table 5.2 Comparative data for two cylinders used to store hydrogen at high pressure. The first is a conventional steel cylinder, the second a larger composite tank for use on a hydrogen powered bus

2 L steel, 200 bar 147 L composite, 300 bar

Mass of empty cylinder Mass of hydrogen stored

Storage efficiency (% mass H2) Specific energy

Volume of tank (approx.) Mass of H2 per litre

2 It should be noted that at present composite cylinders have about three times the cost of steel cylinders of the same capacity.

known and is termed hydrogen embrittlement. Certain chromium-rich steels and Cr-Mo alloys have been found that are resistant to hydrogen embrittlement. Composite reinforced plastic materials are also used for larger tanks, as has been outlined above.

As well as the problem of very high mass, there are considerable safety problems associated with storing hydrogen at high pressure. A leak from such a cylinder would generate very large forces as the gas is propelled out. It is possible for such cylinders to become essentially jet-propelled torpedoes, and to inflict considerable damage. Furthermore, vessel fracture would most likely be accompanied by autoignition of the released hydrogen and air mixture, with an ensuing fire lasting until the contents of the ruptured or accidentally opened vessel are consumed (Hord 1978). Nevertheless, this method is widely and safely used, provided that the safety problems, especially those associated with the high pressure, are avoided by correctly following the due procedures. In vehicles, for example, pressure relief valves or rupture discs are fitted which will safely vent gas in the event of a fire for example. Similarly, pressure regulators attached to hydrogen cylinders are fitted with flame-traps to prevent ignition of the hydrogen.

The main advantages of storing hydrogen as a compressed gas are: simplicity, indefinite storage time, and no purity limits on the hydrogen. Designs for very high-pressure cylinders can be incorporated into vehicles of all types. In the fuel cell bus of Figures 1.16 and 11.6 they are in the roof. Figure 5.4 shows the design of a modern very high-pressure hydrogen storage system by General Motors, and its location in the fuel cell powered vehicle can be seen in the picture in the background.

5.3.4 Storage of hydrogen as a liquid

The storage of hydrogen as a liquid (commonly called LH2), at about 22 K, is currently the only widely used method of storing large quantities of hydrogen. A gas cooled to the liquid

Figure 5.4 General Motors very high pressure hydrogen gas cylinder

state in this way is known as a cryogenic liquid. Large quantities of cryogenic hydrogen are currently used in processes such as petroleum refining and ammonia production. Another notable user is NASA, which has huge 3200 m3 (850000 US gallon) tanks to ensure a continuous supply for the space programme.

The hydrogen container is a large, strongly reinforced vacuum (or Dewar) flask. The liquid hydrogen will slowly evaporate, and the pressure in the container is usually maintained below 3 bar, though some larger tanks may use higher pressures. If the rate of evaporation exceeds the demand, then the tank is occasionally vented to make sure the pressure does not rise too high. A spring loaded valve will release, and close again when the pressure falls. The small amounts of hydrogen involved are usually released to the atmosphere, though in very large systems it may be vented out through a flare stack and burnt. As a back-up safety feature a rupture disc is usually also fitted. This consists of a ring covered with a membrane of controlled thickness, so that it will withstand a certain pressure. When a safety limit is reached, the membrane bursts, releasing the gas. However, the gas will continue to be released until the disc is replaced. This will not be done until all the gas is released, and the fault rectified.

When the LH2 tank is being filled, and when fuel is being withdrawn, it is most important that air is not allowed into the system, otherwise an explosive mixture could form. The tank should be purged with nitrogen before filling.

Although usually used to store large quantities of hydrogen, considerable work has gone into the design and development of LH2 tanks for cars, though this has not been directly connected with fuel cells. BMW, among other automobile companies, has invested heavily in hydrogen powered internal combustion engines, and these have used LH2 as the fuel. Such tanks have been through very thorough safety trials. The tank used in their hydrogen powered cars is cylindrical in shape, and is of the normal double wall, vacuum or Dewar flask type of construction. The walls are about 3 cm thick, and consist of 70 layers of aluminium foil interlaced with fibre-glass matting. The maximum operating pressure is 5 bar. The tank stores 120 litres of cryogenic hydrogen. The density of LH2 is very low, about 71kg.m-3, so 120 litres is only 8.5 kg (Reister and Strobl 1992). The key figures are shown in Table 5.3.

The hydrogen fuel feed systems used for car engines cannot normally be applied unaltered to fuel cells. One notable difference is that in LH2 powered engines the hydrogen is often fed to the engine still in the liquid state. If it is a gas, then being at a low temperature is an advantage, as it allows a greater mass of fuel/air mixture into the engine. For fuel cells, the hydrogen will obviously need to be a gas, and pre-heated as

Table 5.3 Details of a cryogenic hydrogen container suitable for cars

Mass of empty container 51.5 kg

Mass of hydrogen stored 8.5 kg

Storage efficiency (% mass H2) 14.2%

Specific energy 5.57kWh.kg-1

Volume of tank (approx.) 0.2 m3

Mass of H2 per litre 0.0425 kg.L-1

well. However, this is not a very difficult technical problem, as there is plenty of scope for using waste heat from the cell via heat exchangers.

One of the problems associated with cryogenic hydrogen is that the liquefaction process is very energy-intensive. Several stages are involved. The gas is firstly compressed, and then cooled to about 78 K using liquid nitrogen. The high pressure is then used to further cool the hydrogen by expanding it through a turbine. An additional process is needed to convert the H2 from the isomer where the nuclear spins of both atoms are parallel (ortho-hydrogen) to that where they are anti-parallel (para-hydrogen). This process is exothermic, and if allowed to take place naturally would cause boil-off of the liquid. According to figures provided by a major hydrogen producer, and given by Eliasson and Bossel (2002), the energy required to liquefy the gas under the very best of circumstances is about 25% of the specific enthalpy or heating value of the hydrogen. This is for modern plants liquefying over 1000 kilograms per hour. For plants working at about 100kg.h-1, hardly a small rate, the proportion of the energy lost rises to about 45%. In overall terms then, this method is a highly inefficient way of storing and transporting energy.

In addition to the regular safety problems with hydrogen, there are a number of specific difficulties concerned with cryogenic hydrogen. Frostbite is a hazard of concern. Human skin can easily become frozen or torn if it comes into contact with cryogenic surfaces. All pipes containing the fluid must be insulated, as must any parts in good thermal contact with these pipes. Insulation is also necessary to prevent the surrounding air from condensing on the pipes, as an explosion hazard can develop if liquid air drips onto nearby combustibles. Asphalt, for example, can ignite in the presence of liquid air. (Concrete paving is used around static installations.) Generally though, the hazards of hydrogen are somewhat less with LH2 than with pressurised gas. One reason is that if there is a failure of the container, the fuel tends to remain in place, and vent to the atmosphere more slowly. Certainly, LH2 tanks have been approved for use in cars in Europe.

5.3.5 Reversible metal hydride hydrogen stores

The reader might well question the inclusion of this method in this section, rather than with the chemical methods that follow. However, although the method is chemical in its operation, that is not in any way apparent to the user. No reformers or reactors are needed to make the systems work. They work exactly like a hydrogen 'sponge' or 'absorber'. For this reason it is included here.

Certain metals, particularly mixtures (alloys) of titanium, iron, manganese, nickel, chromium, and others, can react with hydrogen to form a metal hydride in a very easily controlled reversible reaction. The general equation is:

To the right, the reaction of (5.8) is mildly exothermic. To release the hydrogen, then, small amounts of heat must be supplied. However, metal alloys can be chosen for the hydrides so that the reaction can take place over a wide range of temperatures and pressures. In particular, it is possible to choose alloys suitable for operating at around atmospheric pressure, and room temperature.

The system works as follows. Hydrogen is supplied at a little above atmospheric pressure to the metal alloy, inside a container. The reaction of (5.8) proceeds to the right, and the metal hydride is formed. This is mildly exothermic, and in large systems some cooling will need to be supplied, but normal air cooling is often sufficient. This stage will take a few minutes, depending on the size of the system, and if the container is cooled. It will take place at approximately constant pressure.

Once all the metal has reacted with the hydrogen, then the pressure will begin to rise. This is the sign to disconnect the hydrogen supply. The vessel, now containing the metal hydride, will then be sealed. Note that the hydrogen is only stored at modest pressure, typically up to 5 bar.

When the hydrogen is needed, the vessel is connected to, for example, the fuel cell. The reaction of (5.8) then proceeds to the left, and hydrogen is released. If the pressure rises above atmospheric, the reaction will slow down or stop. The reaction is now endothermic, so energy must be supplied. This is supplied by the surroundings; the vessel will cool slightly as the hydrogen is given off. It can be warmed slightly to increase the rate of supply, using, for example, warm water or the air from the fuel cell cooling system.

Once the reaction has completed, and all the hydrogen has been released, then the whole procedure can be repeated. Note that we have already met this process, when we looked at the metal hydride battery in Chapter 2; the same process is used to store hydrogen directly on the negative electrode.

Usually several hundred charge/discharge cycles can be completed. However, rather like rechargeable batteries, these systems can be abused. For example, if the system is filled at high pressure, the charging reaction will proceed too fast, and the material will get too hot, and will be damaged. Another important problem is that the containers are damaged by impurities in the hydrogen; the metal absorbers will react permanently with them. So a high purity hydrogen, at least 99.999% pure, must be used.

Although the hydrogen is not stored at pressure, the container must be able to withstand a reasonably high pressure, as it is likely to be filled from a high pressure supply, and allowance must be made for human error. For example, the unit shown in Figure 5.5 will be fully charged at a pressure of 3 bar, but the container can withstand 30 bar. The container will also need valves and connectors. Even taking all these into account impressive practical devices can be built. In Table 5.4 gives details of the small 20 SL holder for applications such as portable electronics equipment, manufactured by GfE Metalle und Materialien GMBH of Germany, and shown in Figure 5.5. The volumetric measure, mass of hydrogen per litre, is nearly as good as for LH2, and the gravimetric measure is not a great deal worse than for compressed gas, and very much the same as for a small compressed cylinder. Larger systems have very similar performance.

One of the main advantages of this method is its safety. The hydrogen is not stored at a significant pressure, and so cannot rapidly and dangerously discharge. Indeed, if the valve is damaged, or there is a leak in the system, the temperature of the container will fall, which will inhibit the release of the gas. The low pressure greatly simplifies the design of the fuel supply system. It thus has great promise for a very wide range of applications where small quantities of hydrogen are stored. It is also particularly suited to applications where weight is not a problem, but space is.

Table 5.4 Details of a small metal hydride hydrogen container suitable for portable electronics equipment

Mass of empty container 0.26 kg

Mass of hydrogen stored 0.0017 kg

Storage efficiency (% mass H2) 0.65%

Specific energy 0.26kWh.kg-1

Volume of tank (approx.) 0.061

Mass of H2 per litre 0.028 kg.L-1

Figure 5.5 Metal hydride stores can be made quite small, as this example shows

The disadvantages are particularly noticeable where larger quantities of hydrogen are to be stored, for example in vehicles! The specific energy is poor. Also, the problem of the heating during filling and cooling during release of hydrogen becomes more acute. Large systems have been tried for vehicles, and a typical refill time is about one hour for an approximately 5 kg tank. The other major disadvantage is that usually very high purity hydrogen must be used, otherwise the metals become contaminated, as they react irreversibly with the impurities.

5.3.6 Carbon nanofibres

In 1998 a paper was published on the absorption of hydrogen in carbon nanofibres (Chambers et al. 1998). The authors presented results suggesting that these materials could absorb in excess of 67% hydrogen by weight, a storage capacity far in excess of any of the others we have described so far. This set many other workers on the same trail. However, it would be fair to say that no-one has been able to repeat this type of performance, and methods by which errors could be made in the measurements have been suggested. Nevertheless, other workers have shown fairly impressive storage capability with carbon nanofibres, and this is certainly one to watch for the future (see Chapter 8 of Larminie and Dicks (2003)).

5.3.7 Storage methods compared

Table 5.5 shows the range of gravimetric and volumetric hydrogen storage measures for the three systems described above that are available now. Obviously these figures cannot be used in isolation; they don't include cost, for example. Safety aspects do not appear in this table either. The cryogenic storage method has the best figures.

Table 5.5 Data for comparing methods of storing hydrogen fuel

Method

Gravimetric

Volumetric

storage efficiency,

mass (in kg) of

% mass hydrogen

hydrogen per litre

Pressurised gas

0.7-3.0

0.015

Reversible metal hydride

0.65

0.028

Cryogenic liquid

14.2

0.040

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