Hydrogen and its competitors, 2004

Risø Energy Report 3Hydrogen storage 315.2Hydrogen storageLOUIS SCHLAPBACH, EMPA MATERIALS SCIENCE AND TECHNOLOGY INSTITUTION OF THE ETH DOMAIN AND UNIVERSITY OF FRIBOURG; ANDREASZÜTTEL, UNIVERSITY OF FRIBOURG; ALLAN SCHRØDER PEDERSEN, RISØ NATIONAL LABORATORYHydrogen is a gas at ambient temperatures and pressures,but it can be stored as a gas, a liquid or a solid. In the caseof solid storage, the hydrogen exists as a chemicalcompound, and not as a pure substance.None of the existing methods for storing hydrogen areefficient in terms of energy density, neither on a volumenor a mass basis, and release rate at the same time.Compared to fossil fuels such as gasoline, hydrogen hasa very obvious shortfall in the amount of energy it canstore in a given volume or a given weight. The US Departmentof Energy has set up a target of 6.5% hydrogen byweight or 62 kg H 2 /m 3 and several storage techniques areable to meet these requirements (magnesium being anexample, se later). These techniques do not, however,show other needed features like release or filling rate atproper temperature or even reversibility.This is a critical barrier to the wider use of hydrogen inthe transport sector (Figure 9). In stationary applicationsthe problem is less severe.Storing hydrogen as a gasHydrogen is the smallest and lightest molecule known.As a result, hydrogen in the gaseous state has anextremely high ability to diffuse through solid materialsand to escape from even the smallest openings in jointsand seals.Storage of hydrogen as a gas dates back more than acentury. Today the most widely used technology forcommercial distribution of hydrogen is steel oraluminium cylinders pressurised to 200-250 bar. Suchcylinders are heavy and the energy density is relativelylow (Table 8). High-pressure cylinders are available involumes up to about 50 litres; larger volumes are storedat much lower pressures, and the largest compressedw-%h g H/l kJ/ml Kj/gHydrogen at 200 bar 100.0 17 2.4 141.0Magnesium Hydride 7.6 101 14.4 10.9Hydride of Fe-Ti 1.8 96 13.7 2.7Hydrie of La-Ni 1.4 89 12.7 1.9Liquid Hydrogen 100.0 70 10.0 141.0Methanol 12.5 99 19.0 22.7Gasoline 33.4 47.6Lead/Acid Battery 0.2Advanced battery 0.5Liquid Methane 25.0 106 25.0 55.7Liquid Ammonia 17.6 120 17.9 25.2Fly Wheel 0.5Table 8. Physical and chemical properties of hydrogen, methane andgasolinehydrogen tanks in the world (about 15,000 m 3 ) use pressuresof only 12-16 bar [1]. Pressurised gas is still thepreferred technology for storing and distributinghydrogen because it is well established and has an extensiveinfrastructure.If gaseous hydrogen is to replace stores of liquid fossilfuels, the technology needs to be improved significantly.Intensive development taking place at present includesefforts to create lighter storage cylinders made fromfibre-reinforced composites. The Canadian companyDynetek Industries Ltd., for example, claims weightsavings of 20-50% for its composite cylinders operatingat 200-350 bar. Dynetek has also developed hydrogencylinders that can operate at 825 bar for stationary useand 700 bar in transport applications [2].Figure 9: The volume of 4 kg of hydrogenstored in different ways, relative to thesize of a car.Mg 2 NiH 4 LaNi 5 H 6 H 2 (liquid) H 2 (200 bar)