3.4- HYDROGEN

3.4.1- CHARACTERISTICS

Hydrogen is the first element of the periodic table, and the most abundant one in the universe, being the main composer of stars. Despite of that, it is nearby inexistent in its elemental state on the Earth, but it can be found forming chemical combinations, with water and hydrocarbons as the most interesting examples. At standard temperature it is a colourless, odourless, tasteless diatomic gas (H2) with a density of 0.08988 g/l (at 0 ºC and 1 atm) that releases 142 MJ/kg during its combustion.

This gas is susceptible of being oxidised, producing water through the following exothermic reaction:

This is the reason which makes hydrogen an interesting substance in the energy industry, added to the fact that it is the current best fuel for the PEMFC. Nevertheless, it is convenient to realise that hydrogen is not an energy source, as biofuels or fossil fuels, but an energy carrier, since its use doesn't signify an energetic gain given that the production of a certain energy value in form of hydrogen takes an equal or higher quantity of power.

3.4.2- OBTAINING

As has been commented before in this research, the most interesting hydrogen container compounds for the energy industry are water and hydrocarbons. Following the two current ways to synthesise hydrogen from that feedstock are exposed.

•  THROUGH SYNGAS PRODUCTION

Syngas is a mixture of CO, CO2 and H2 generated from the processing of organic hydrocarbons via certain methods:

If the prime material is a gaseous hydrocarbon, normally methane gas, its conversion is called steam reforming, consisting in the oxidation that occurs between this matter and water vapour at temperatures above 700 ºC and pressures between 10 and 20 atm with the presence of a catalyst (nickel on aluminium support).

In the case of non-gaseous organic compounds, what includes biomass but also coal and petroleum, this method uses the same gasification process that the one used in methanol obtaining: that is, the partial oxidisation of this feedstock in a gasifier.

Once the syngas is obtained by any of both ways, it is possible to recover more energy in form of usable H2 from the surplus carbon monoxide (and convenient, because it is necessary to avoid catalyst CO intoxication in FCVs) through water shift reaction: the oxidisation of this compound in contact with water steam at high temperatures.

After that, hydrogen must be separated from any trace of carbon oxide or any other impureness in order to achieve a purity grade greater than ~ 99.99%.

Nowadays, the steam reforming of natural gas is the most widely used method to obtain hydrogen, due principally to the fact that it is, with difference, the cheapest current available procedure, and also the most efficient (65-75% of the input energy), though it is obviously fossil-based and contributes to the greenhouse effect. The only carbon neutral way to produce this combustible from hydrocarbons is the biomass gasification; however, gasification is considerably more expensive and complicated than gas reforming, and even if it was adopted, coal would be a much more attractive feedstock, since it is a cheap, rentable, mature energy source.

•  THROUGH ELECTRIC WATER SPLITTING

This process, called electrolysis, consists in the application of an electric current on an electrolyte, a solution of water and certain chemical compounds (such as KOH) that have an electrically conductive behaviour, through an anode and a cathode made in conductive material. Actually, this system consists in reversing the process performed by a fuel cell. The reported industrial efficiency of this technology is of between 50-70% of the energy input.

Electrolytic water splitting scheme. Image: <http://www.theoildrum.com/node/4378>

The advantage of this process is its versatility, since it doesn't require complex structures and fits well on small and medium devices that can be used to store energy from decentralised, inconstant renewable sources such as wind, sun and geothermal heat.

•  THROUGH PHOTOBIOLOGICAL PROCESSES

Likewise as the 3G biofuel obtaining processes, a way to produce hydrogen from the activity of certain microalgae is being developed. The Chlamydomonas reinhardtii is an organism that switches from the generation of oxygen to hydrogen on certain environmental conditions.

Even though this hydrogen-obtaining system is still under development on laboratories, and far from commercial appliances of any type, its achievement would represent an actual way to produce hydrogen without any kind of harmful emissions, unlike fossil carbon based processes.

3.4.3- ON-BOARD STORAGE SYSTEMS

Even though that hydrogen has a high energy content per weight unit, its extremely low density (it is the lightest existing element) is an important inconvenience for its use in transport, since volume counts the most in automotion, given that the autonomy of a car depends of the quantity of energy allowable in a determinate space.

The energy density of hydrogen at atmospheric standard conditions is of about 12.7 kJ/L, while gasoline offers 34.656 MJ/L. Since the average FCV consumes 0.4 kWh/km, a viable storage system should afford about 3 kg of hydrogen in order to offer a minimal autonomy of 300 km. That would be attractive for consumers in general, so t his is the reason that makes essential to store this hydrogen in special conditions:

•  PRESSURIZED HYDROGEN

The cheapest, technically simplest and most mature way to delivery and store hydrogen is to compress and keep it in on-board tanks. It would employ the same technologies that CNG, butane and propane gases use, so research investments for developing this method would be unnecessary.

The most used pressure standards used in prototypes are 340 and 680 atm. To compress hydrogen at 340 atm it is necessary to spend 36 MJ per kilogram of this element, what represents 30% of its lower heating value (LHV), but that increases its energetic density up to 2.7 MJ/L. Hydrogen compressed to 680 atm offers 4.7 MJ/L, but its processing will consume 47 MJ, that is, around 40% of its LHV. With this last method, about 64 litres of fuel would be necessary to provide a 300 km autonomy.

It is easy to see that, still under high pressures, hydrogen offers a tiny energy per volume ratio, even lower than batteries' one. Furthermore, it takes relevant energy amounts to compact this gas, what drastically reduces the overall efficiency of the system. In addition to those technical problems, there is a safety concern about the risk of tank explosions in case of traffic accidents.

•  LIQUEFIED HYDROGEN

The liquefaction (or condensation) of hydrogen at very low temperatures (occurs at -253 ºC) is another purposed way to increase its energy density. Chilled hydrogen could virtually allow delivering and storing this fuel in a more versatile way than the gaseous alternative. However, the overall energetic cost of this process is about 50.4 MJ per kg of fuel, that is, a 42% of its LHV, plus the additional energetic expenses due to tank cooling. However, with this process, the energy density achieved is of 8.4 MJ/L, nearby the double than 680 atm pressurised fuel. This fact means that theoretically about 36 litres of fuel would offer 300 km of driving.

Anyway, the liquid storage of hydrogen has a critical issue: the unavoidable boil off of the in-tank combustible during inactivity of the vehicle, which requires evacuating the gas fraction in order to avoid dangerous pressure increments. In recent prototypes, the stored hydrogen lost per day is quantified in a 4% of the tank capacity.

•  METAL HYDRIDES

A large group of metal alloys can absorb hydrogen forming metal hydrides trough a reversible process. This absorbing capability makes those compounds to be considered as another interesting system for hydrogen containing.

The density of hydrogen stored in metal hydrides differs depending on the used compound; however, most of the present available techniques offer an energy density similar to liquefaction, and future researches may attain better results. Besides the concentration capability, the advantage of metal hydride storage is its low energetic cost, which is of about 15 MJ per kg of hydrogen (12.5% of LHV), mainly spent in refrigeration because of the heat released during the absorption. Anyway, the weight of a storage system based on this technology is a relevant problem, since common alloys can only assimilate about 1.8% of its mass in hydrogen, that is, several kilograms of metal are necessary to contain one kilogram of fuel.

In comparison to the rest of methods, this is the most promising one, yet it is still being developed and only applied in an experimental scale. In addition to metal hydrides, other cutting-edge solid-storage systems such as the ones based in carbon nanostructures have lately acquired interest due to their higher performance in energy per weight ratio.