Production-storage-distribution: that is the infrastructure chain which must be created for the new hydrogen economy to operate. Like fuel cells, this research field triplet is a priority for European energy.
How can we produce the hydrogen which fuel cells need to operate? In the relatively short term, two broad solutions – electrolysis and hydrocarbon reforming (1) – provide the most feasible options. Nevertheless, the industrial expertise available in these fields requires technological and commercial adaptations before it can cater for fuel cells.
‘Chicken and egg’ electrolysis In the medium and long term, the ideal solution – the most ecological and able to produce a particularly pure form of hydrogen – would be water electrolysis. This process has already been perfected technically and is used without problems in a number of industrial applications. However, as it is costly and consumes large amounts of electricity it is rarely used to obtain large quantities of hydrogen.
In the context of the hydrogen economy, the use of electrolysis is reminiscent of the chicken and egg quandary. It would be absurd to produce hydrogen for non-polluting applications using electricity produced by conventional power stations which do pollute. This production method only makes sense if it is based on renewable energy available at competitive prices. These are slow to develop, however, and still have to prove that they are able to meet a (future) demand from the fuel cell market. In this connection, two electrolysis pilot units supplied by wind power generators are currently at the testing stage, in Greece and the Canary Islands, as part of the RES2H2 European project. The aim is to demonstrate a reliable capacity for hydrogen production. This test will be very significant for this resource which has a strategic potential along the length of Europe’s southern coastline.
Reforming, the best candidate The most common solution is the extraction of hydrogen from fossil resources – more specifically hydrocarbons. Given their importance in all energy distribution systems, this is likely to be a short- or medium-term solution for fuel cells.
Reforming can involve a number of approaches. One involves mixing the fuel (mainly natural gas, the least-polluting fossil fuel) with steam, in the presence of the appropriate catalysts and at a high temperature, to produce hydrogen and carbon dioxide. Another method is by oxidation which involves various purification methods to remove the sulphur and CO content, etc.
Reforming is mainly used on a large scale to meet the needs of the chemicals industry, in particular for synthesising ammonia, a process which currently represents almost half the total world demand for hydrogen. Therefore, the application of this technology in the supply of fuel cells primarily poses the problem of adapting the process to new dimensions.
New reforming methods must be developed which are more suitable for fuel cell power plants designed to meet regional or local needs. As the aim is to eliminate emissions of greenhouse gases and other polluting derivatives, one important aspect is the isolation of CO2 waste and operations to purify the hydrogen produced by the natural gas (or by other hydrocarbons such as methanol, methane, naphtha, etc.). More complex still is the development of small reformers, attached to the fuel cells and placed on vehicles, which would then be supplied ‘at the pump’ with these various fuels.
Plant potential In a more distant future, the plant world, which represents a vast reservoir of solar energy captured by photosynthesis, is potentially a major source of hydrogen. Biomass gasification makes it possible to produce bio fuels. Although, initially, the tendency has been to use these as such in internal combustion engines, they can also be reformed. Using thermo-chemical methods, biomass is able to supply hydrogen directly in the gaseous state. Therefore, the fields of research are wide and the options still need to be tested and validated, in terms of output, quality, dimensions and cost. The choices made also have implications for farming and forestry activities. Almost a dozen European projects have been launched in this field under the Fifth Framework Programme, for which the EU granted 50% of the total investment of €23 million.
No doubt the most futuristic of all the possible alternatives is direct hydrogen production from microscopic algae or bacteria. Recent research shows that during the process of photosynthesis brought about by these organisms, a complex enzymatic system, known as hydrogenase, can stimulate the formation of hydrogen molecules under certain conditions. This is a field with great potential but for which the feasibility and concrete applications remain very much unknown quantities.
Storing the volume
The option of solid hydrogen storage in new metal or carbonised materials is one of the key issues for the future of this energy sector. In the periodic table above, it is the alloys based on the lightest elements (in red) – especially magnesium, nickel and lithium, aluminium-based compounds (alanates) and borium-based compounds (borohydride) – which are the subject of most current research being undertaken by the Fuchsia, Historhy and Hymosses projects within the Fifth Framework Programme, and the Storhy Integrated Project under FP6, for example.
Although hydrogen offers all the advantages necessary to become a key energy vector, it faces two obstacles which could limit its use. It is the lightest of all atoms but is also the most voluminous in the gaseous state and at current pressure and temperature. And its ability to release energy also makes it a particularly inflammable gas.(2) A precondition for its use is to solve the very complex problem of its storage and distribution under conditions which are acceptable in terms of volume and safety, taking into account the cost of these operations. After the development of the fuel cells themselves, these aspects are most certainly the second technological hurdle to be overcome before the hydrogen economy can really take off.
A number of ‘solutions’ already exist but they fall far short of providing the technological and economic performances required for the storage and transport of hydrogen to be put into general use. Each of these strategies can have specific benefits but must also be evaluated in accordance with the energy consumption involved and –particularly in the case of transport applications – the increased weight which results.
Compression-liquefaction The most common form of storage involves compressing the gas and confining it in secure reservoirs. The pressures used range from 350 to 700 bars. This process, which is widely practised for industrial uses of hydrogen, requires an energy consumption equivalent to 10% of its calorific value (PCI). Storage can be in bottles of 10 litres or more, and the average size of fixed reservoirs (distribution stations) is 10 000 m3 . Much larger volumes are stored underground.
The receptacles used to contain the hydrogen, whether tanks or supply pipelines, are reinforced to withstand pressure. They must be made of rust-resistant metal (such as aluminium reinforced with carbon fibre) and exclude any possibility of infiltration by the very light hydrogen atoms.
A number of prototype fuel cell vehicles are equipped with compression tanks, but the low volumetric density of the hydrogen stored limits their autonomy. Attempts have been made to introduce polymers so as to reduce the weight of the tanks for vehicles. Research is also being conducted into shock resistance and technologies to adapt other accessories (valves, regulators, etc.).
Another solution in overcoming the problem of storage volume and preventing the risks of inflammability is to liquefy the hydrogen. This can only occur at very low temperatures – minus 253°C – or under very high pressure. Such cryogenic technologies are commonplace in industry,(3) but involve a real cost in terms of energy (between 25 and 30% of the hydrogen PCI). If the tank material does not have the same resistance constraints, it must have thermal insulation properties (double-wall tanks). Prototypes produced by BMW, Opel, and DaimlerChrysler already have this liquid hydrogen option and pilot service stations exist in Munich, Germany.
Solid storage solution The most promising way forward – and one that would be decisive for the development of fuel cells in the transport and portable applications sector – would seem to be solid storage. Some new materials (consisting of metal alloys or carbon nanotubes) have the capacity to absorb, at current temperatures, hydrogen atoms in the interstitial spaces of their basic metal structure. Under the appropriate catalytic conditions and following slight heating – to around 80°C, which can be achieved by using the heat emitted by the fuel cell itself – a phenomenon of disabsorption subsequently releases the hydrogen which could be used as fuel.
Major hopes are being put into the development of this technology. ‘Solid storage offers a solution to the security issues raised by compression in the gaseous state,’ stresses Jiri Muller, researcher at the Institutt for Energiteknikk (IFE) in Kjeller (NO), a participant in the StorHy project. Equipped with a nuclear reactor enabling very detailed analyses of the positioning of hydrogen atoms in a vast range of metal hydride compounds, this organisation is at the core of many European research projects in this field. ‘For the same volume, we could develop tanks with a capacity comparable to that of storing hydrogen in liquid form. Furthermore, access to the tank, for filling, would not pose any problem of impermeability. The challenge is also to find solutions which would provide optimal performances in terms of stability, easy reversibility of the storage, tank weight and, of course, cost.’
(1) Along with reforming, another means of hydrogen production is gasification through the partial combustion of fossil fuels (coal and heavy hydrocarbons). In this case, too, the technologies are only applied on very large scales and result in costly purification operations. Research on low-capacity mechanisms does not offer prospects of feasibility in the near future. (2) However, the highly volatile gaseous hydrogen disperses very rapidly in the atmosphere – making it somewhat less dangerous. (3) The European leaders are Air Liquide (FR), Linde Gas (DE), Air Products (UK), etc.
Seven integrated projects covering the full spectrum of hydrogen options have already been selected following the first 2003 calls for proposals under the Sixth Framework Programme. On the production front, the Chrisgas project will be working on the ...
Nuclear, thermal solar and hydrogen power
With zero CO2 emissions, nuclear power plants are in the running to supply the electricity needed for water electrolysis – production at off-peak times, in particular – alongside renewable energies. However, hydrogen can also ...
Using hydrogen as a fuel vector in the transport sector requires new developments in the field of safety standards, industrial specifications and inspection procedures, whether in terms of on-board components and systems or fuel supply stations. It is ...
H2 and FP6
Seven integrated projects covering the full spectrum of hydrogen options have already been selected following the first 2003 calls for proposals under the Sixth Framework Programme. On the production front, the Chrisgas project will be working on the potential offered by biomass gasification. On the storage front, the Storhy project is placing the emphasis on compressed and liquefied hydrogen technologies but it is also planned to further develop current research (1) on solid storage.
The Hysaf Network of Excellence is making a horizontal study of safety issues at all stages of the chain, while the Hyways project will draw up a European road map to achieve a sustainable energy system for hydrogen. Upstream research also includes an Integrated Project, known as Zero Regio, which is looking at the development of vehicle fleets powered by fuel cells. Finally, the Hyice project is concentrating on the direct use of hydrogen as a non-polluting fuel for internal combustion engines, a method which constitutes a rapidly applicable first stage in the development of the hydrogen economy.
With zero CO2 emissions, nuclear power plants are in the running to supply the electricity needed for water electrolysis – production at off-peak times, in particular – alongside renewable energies. However, hydrogen can also be extracted from water by another method, known as thermo-chemistry: at temperatures above 1 000°C the water molecule can ‘split’ under the effect of the intense heat.
Plataforma solar, the European thermal solar energy plant in Almeira (ES).
In this respect, present projects for the fourth generation of high-temperature power plants of the future foresee a nuclear industry which produces electricity and heat. The most frequently cited application for the use of the latter is the desalination of seawater, but this resource could also be used in the hydrogen production sector.
Another interesting possibility for the intensive production of heat is the thermal solar energy sector. The European Heliosol project – involving Greek, British, German and Danish partners – is looking at complex catalytic processes that could make this approach interesting.
Using hydrogen as a fuel vector in the transport sector requires new developments in the field of safety standards, industrial specifications and inspection procedures, whether in terms of on-board components and systems or fuel supply stations. It is an area currently being explored by the EIHP2 (European Integrated Hydrogen Project– Phase II) project to which a wide range of interested industrial sectors are contributing. The aim is to lay the foundations for the European – and international – harmonisation of these key aspects.
Modelling of an inflammable gaseous cloud combining air and hydrogen (in a 4% concentration) ten seconds after a leak from a tanker vehicle in an urban neighbourhood.