Hydrogen and its competitors, 2004

36Risø Energy Report 3Hydrogen storage5.2kinetics or the thermodynamic stability of the hydride?If the process is rate-limited, the kinetics can beimproved by incorporating a catalyst [22], or by ballmilling and the introduction of defects.Second, what are the conditions for a reversible reaction?The reversible formation of LiBH 4 and most of the otherhydrides in this potentially very attractive group is achallenge. Desorption of hydrogen requires the formationand stabilisation of clusters of an intermetalliccompound containing the remaining metals. SiO 2 isknown to play a catalytic role, but these light-metalhydrides need much more research.Heat Management in Hydride OperationAs discussed above, a considerable absorption or releaseof heat is involved in metal hydriding/hydriding process.In several cases the reaction heat amounts to about 25%of the combustion energy of the stored hydrogen andthis fact calls for clever, dedicated solutions for exchangingheat with the hydride material. One example of suchheat management could be a hydride store, whichshould feed a fuel cell with hydrogen. In this case theheat produced by the fuel cell may be used to dissociatethe metal hydride and the systems may be tailored for anoptimal thermal match. Large Danish companies havelong experience in handling and exchanging heat, andfor such companies the topic of thermal management ofhydride stores could be of commercial interest.Other hydrogen-rich compoundsSimilarly to storing hydrogen in metal hydrides also theformation of other types of hydrogen-rich compoundscould be attractive for hydrogen storage. For instance itis possible to form methane by direct electrolysis of waterin the presence of carbon dioxide and indeed it ispossible to form liquid compounds like methanol byproperly reacting hydrogen and carbon dioxide. Bothreactions may be done in an environmentally friendlyway by taking the carbon dioxide from the atmosphere.The attractive features of such carbon based compoundsis that we have a long handling experience and that aninfrastructure is existing or at least well-known.Visions for the futureIt is reasonable to hope that improvements in theperformance of hydrogen storage materials will one daymatch those of high-temperature superconductors andpermanent magnets. In developing sustainable energypolicies, however, we must look at society, economicsand the way we think about energy, as well as the scientificand technical challenges.As mobility continues to increase, we must decidewhether prestige, size and power should remain importantin vehicle design. And do vehicles really needstrong, heavy structures to protect their occupants, or areelectronic control and collision-avoidance systems abetter approach? They could certainly make it possiblefor much lighter and more energy-efficient vehicles to bebuilt in those countries that do not consider energyavailability to be unlimited. Europe and Japan alreadyhave small, light cars whose engines consume as little as3 l of gasoline per 100 km. A single kilogramme ofhydrogen can fuel an engine-driven car for 100 km; witha hydrogen fuel cell the range is 200 km.