Hydrogen and its competitors, 2004

Risø Energy Report 3Hydrogen infrastructure 535.6systems can achieve very high overall efficiencies, anddue to the modular nature of fuel cell technologies theycan be configured to be distributed, centralised oranywhere in between.Robust hydrogen supply infrastructure solutions for roadtransport are needed urgently. Prototype hydrogenbasedfuel cell vehicles are already being tested anddemonstrated in Europe, Japan and the USA, and somemajor automotive manufacturers have announced thatthey will market fuel cell vehicles already by 2005.Large numbers of hydrogen vehicles, however, are notexpected before 2015-20. Considerable efforts are beingmade to develop infrastructure solutions to supportthese vehicles, both through national R&D and demonstrationprogrammes and by the car industry. Theaccelerating development of hydrogen vehicles brings anurgent need for common codes, standards and regulations.Filling stations for pressurised hydrogen gas are expectedto be an essential part of an initial infrastructure.Hydrogen stored as a compressed gas at up to 700 baralready gives today's vehicles a range comparable to thatof conventional vehicles, whereas infrastructures tosupport liquid hydrogen, metal hydride and otherstorage methods are not seen as viable at the moment.Road maps presenting the shorter-term aims and strategiesfor introduction of hydrogen for transport generallyagree on the steps towards an extended hydrogen infrastructure.First, hydrogen filling stations will be built inlarge cities to supply bus fleets and other vehicles inregular operation within a limited area. A single fillingstation in a densely-populated area can serve a largenumber of users and maximize the environmental andhealth benefits of hydrogen. This is an optimal way toincrease public awareness and acceptance of hydrogentechnology.Step two is to build filling stations on major roadslinking cities and large towns that already have theirown hydrogen filling stations. This allows existing fleetsof hydrogen vehicles to increase their range, and willhelp to increase the size of the hydrogen vehicle market.Hydrogen infrastructure and future supplyoptionsThe relevant hydrogen production options to supply thenetwork of filling stations much depend on the local orregional conditions. In an initial stage the hydrogenproduction and supply may take advantage of theexisting infrastructure for electricity and natural gas.Via electrolytic production hydrogen may be generatedand supplied at almost any location. The modularity ofthe electrolysis technologies means that both large andsmall-scale plants may be relevant. The individualhydrogen filling stations may be equipped with electrolysisunits, e.g. for high-pressure electrolysis, compressorand storage facilities, and dispensers for high-pressurehydrogen. Even very small-scale residential dispenserunits may be a future option based on electrolysis.Producing hydrogen via electrolysis means that attributesof the power production system in question e.g.primary energy inputs and CO 2 characteristics, partly aretransmitted to the transport sector. Fuel flexibility andsystem development options for the overall powersystem become options for transport as well.Where natural gas is available, reforming is anothersupply option for hydrogen. Reformer plants could besituated near, or in, hydrogen filling stations. Economiesof scale, however, may favour larger reformer plants,which in turn would require local pipelines to supply thefilling stations.Beyond the very initial stages of a hydrogen supplyinfrastructure build-up another option may be todistribute hydrogen mixed into the NG-grid in quantitiesup to about 15% volume, depending on pipelinematerial, pipeline pressure and end user technology.Hydrogen supplied from numerous sources may bedistributed via mixed gas grid lines.Hydrogen filling stations would be able to separate thehydrogen from the natural gas, purify and pressurise itfor use in vehicles. Most other users would simply be ableto burn the natural gas/hydrogen mixture without problems.The only difficulties would be with internalcombustion engines running on natural gas. Denmarkhas many such engines in small CHP stations, and it hasbeen shown that hydrogen levels above 1-2% in the gascan cause operational problems or reduced output. Newengines developed for natural gas/hydrogen blendsshould not suffer from these problems.Various other factors also limit the proportion ofhydrogen than can be added to natural gas pipelines. Theneed to preserve the combustion quality of the gas(Wobbe index) would limit the hydrogen content to 25%by volume, while restrictions on density would allow amaximum of 17% hydrogen. The most stringent limit isset by leakage and material compatibility. Above about15% hydrogen, problems of leakage, corrosion, degradationof polyethylene pipe and embrittlement of steels areexpected [8]. Tests and analyses of using the Danish NGgridfor transporting medium-pressure pure hydrogenhas been carried out during 2003 [15]. Results of this testcarried out during one year indicate that materialchanges may take place in the 4 bar distribution grid.Leakage measurements show that connections and sealingsmust be checked more frequently than for naturalgas.The energy content of hydrogen is only around onethirdthat of the same volume of natural gas. As a consequence,pipelines and underground storage caverns forhydrogen would have to be three times the size of theirnatural gas equivalents, at the same pressure, to coverthe same energy demand. For safety reasons, hydrogenpipelines will also operate at lower pressures than natural