E-mail: arnulf@ite.no
arnulf@julo.com
Maeland A. J.
Institute of Energy Technology, Norway
The storage of hydrogen for vehicular use - A review and reality check*
INTRODUCTION
Environmental considerations have given momentum to the search for clean fuels to replace or at least offer an alternative to gasoline or diesel as the primary fuels in vehicular applications. Large amounts of carbon mono- and di-oxide, nitrogen and sulfur oxides, hydrocarbons and particulates produced in the combustion process of gasoline and diesel vehicles continue to pollute our atmosphere, sometimes resulting in dangerous levels of these pollutants in metropolitan areas. Devices to reduce the level of pollutants, e.g. the catalytic converter, have to some extent eased, but by no means eliminated the problem and while further improvements may be forthcoming, complete removal is an unrealistic expectation. However, there is another solution: employ a fuel which does not produce pollutants. Hydrogen with specific energy content of 33.3kWh/kg, nearly three times larger than gasoline or diesel, is such a fuel and it can be used in a conventional internal combustion engine with only minor modifications; the main product of the combustion process is water. (Combustion of hydrogen in air produces small amounts of nitrogen oxide (a pollutant), but the amounts are miniscule in comparison with the pollutants produced by the combustion of gasoline or diesel). Hydrogen may also be used with high efficiency in a fuel cell to generate electrical energy; the estimated efficiency is twice that of present day automobile engines. The fuel cell, however, requires very pure hydrogen for continued use while hydrogen purity is of much less importance for the internal combustion engine. An additional point to keep in mind is that exhaust heat is available from the internal combustion engine to liberate hydrogen, but not from the fuel cell. This feature is important if hydrogen is stored in a hydride. Both alternatives are being pursued for vehicular applications, but the current emphasis is on the use of hydrogen in fuel cells. Among the large automobile makers, the Ford Motor Company is working on a hydrogen powered internal combustion engine as an alternative to the gasoline engine until the automakers perfect the fuel cell power train. BMW plans to put a small fleet of cars on the road in 2000 designed to operate on a fuel cell/hydrogen combustion hybrid concept; the BMW hybrid will run on a hydrogen combustion engine and the fuel cell will power the car's on board electrical system [1].
Major difficulties to making hydrogen the clean fuel of choice in automotive applications exist. Hydrogen is the lightest of the elements with an atomic weight of 1.0079 and is a gas under ordinary conditions, making efficient onboard storage more than a trivial matter. The density of hydrogen gas at STP is 0.08988kg/m3 [2] (only
one-seventh that of natural gas) and onboard storage of quantities needed for practical driving ranges requires large volumes and high compression. Furthermore, hydrogen diffuses in and reacts with many materials, properties which must be taken into account when designing containers and which are reflected in the heavy and bulky steel tanks which have traditionally been used for storing compressed hydrogen. We will review recent work aimed at developing lightweight, high pressure containers for mobile application with hydrogen storage capacity > 10% hydrogen by weight.
Storing hydrogen as a liquid improves the volume efficiency greatly, but the liquefaction process is energy intensive, requiring cooling to 20K with a theoretical energy expenditure of 3.92 kwh/kg; in practice about 10 kwh/kg is needed. Well insulated, expensive, vacuum insulated containers are of course also required, and for prolonged storage of liquid hydrogen there is the additional problem of "boil off".
Chemical storage of hydrogen in the form of metal hydrides represents an attractive alternative which has received much attention in the past 30 years. The advantages of storing hydrogen in the form of metal hydrides include high volume efficiency, relative ease of recovery, indefinite storage capabilities and a high degree of safety. Our review will include a critical evaluation of this form of storage and prospect for further development.
Compressed hydrogen storage capacity can under appropriate conditions of temperature and pressure be augmented by adsoption (physi-sorption) on activated carbon. In addition, recent reports from Northeastern University (NU) in Boston, Massachusetts, U.S.A. have claimed large hydrogen storage capacities (as high as 75% by weight!!) of certain forms of carbon nanofibers. These reports have generated considerable interest, but also much controversy because confirmation by other laboratories are lacking. Our review will address this issue.
HYDROGEN STORAGE TECHNOLOGIES
Compressed Hydrogen Gas
The problem of onboard storage of gaseous hydrogen can be appreciated if we consider the fact that a small fuel cell powered automobile requires approximately 3 kg hydrogen to have a modest range of 500 km. The volume occupied by this amount of gas at room temperature and atmospheric pressure is 36,000 liters! The volumetric density (weight of stored hydrogen/volume of gas) can be improved by compression; e.g. compression to 20 MPa
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*Presented as report IFE/I-99/012 on the First International Seminar on safety and economy of hydrogen transport, Russia, Sarov, 23-30 july 2000: received 25 july 2000.
(200 bar) reduces the volume to 180 liters. Compressed hydrogen is commonly sold in 50 liter steel cylinders pressurized to about 20 MPa. Four of these provide a little over 3 kg of hydrogen, but they also weigh approximately 270 kg! The challenge for this storage technology is to design and develop lightweight containers with significantly better performance. Early results from research in several laboratories indicate the challenge is being met with noteworthy success. The performance requirements for storing compressed hydrogen are in many respects similar to those of storing compressed natural gas for vehicular use: light weight, low cost and increased safety. Many aspects of the technology developed for natural gas storage have therefore been adopted and modified when neccessary for compressed hydrogen storage. Dynatek Industries of Calgary, Alberta, Canada, for example, makes a light weight cylinder designed for roof top mounting and is used in the Daimler Chrysler Nebus bus recently
Pused in a demonstration project in Oslo. The storage tank is constructed of a thin aluminum liner (about 3mm wall thickness with a burst strength of 7.7 MPa) wrapped with a composite of carbon fiber in an epoxy resin; the cylinder has a burst strength of about 63 MPA [3]. The Nebus has seven 150 liter tanks of this type, pressurized to 30 MPa and delivering 21 kg hydrogen for a 250 km range. In the U.S.A the Lawrence Livermore National Laboratory (LLNL) in California together with their industrial partners, Thiokol and Directed Technologies, Inc., are fabricating high performance prototype tanks which meet the U.S. Department of Energy (DOE) goals for the year 2000. The goals are 4000 Wh/kg, 12% hydrogen by weight, 700 Wh/liter, 34.5 MPa at room temperature, and $20/kWh in high volume production [4]. These pressure vessels use technologies that are easily adopted and instead of four tanks weighing 270 kg as in our example above, one tank weighing 25 kg and with an internal volume of 143 liters would do. The LLNL prototype tanks are made of carbon fiber composites wrapped around a thin metalized plastic liner which provides the permeation barrier for hydrogen. The volume and placement of the storage tank(s) pose major problems in retrofitting passenger cars, but is much less of a problem in a vehicle designed from the ground up. However, a nightmare of safety and regulatory issues remain unresolved. A particular safety concern is in my opinion the potential energy stored in the gas due to the high pressure. The Nebus tank, for example, represents 4.5 million newton meter (4.5 Nm) of force stored as potential energy in the gas pressure alone!! Such a tank, if punctured, could become a dangerous projectile. A frightful thought indeed.
Compression obviously requires energy. To compress from 0.1 MPa to 30 MPA takes approximately 15% of the total energy available in hydrogen. Onboard storage of hydrogen as compressed gas is currently the most common form for storage used in the many vehicular demonstration projects now in progress throughout the world [5]. The technology is here and readily available. While compressed hydrogen storage may be adopted in the public transport sector (buses), I believe it will be hard to sell the general public on the safety of having a high pressure tank of hydrogen in their own cars.
Liquid hydrogen
The volumetric density of hydrogen can be further improved by liquefaction. Liquid hydrogen is 788 times more dense than the gas at STP, i.e. 70.8kg/m3 at the b.p.(2). 3 kg of hydrogen can be stored in a cryogenically designed
system weighing about 45 kg and having a volume of about 100 liters [5], a bit large, but managable. Liquefaction, however, is energy intensive requiring cooling to 20K and consumes 30% or more of the energy contained in hydrogen. Liquid hydrogen is the choice fuel for launching of space vehicles because it stores 3 times the energy of jet fuel by mass; it can be loaded directly just before take-off and consumed within the short time it takes to launch. Due in part to the space program the liquid hydrogen technology is quite advanced. However, several drawbacks are encountered in use as a fuel for automotive applications. The volumetric density, although better than that of the compressed gas, is still unfavorable as compared to gasoline. It takes a volume 4 times that of gasoline to produce the same powe
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