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IE Volume29 - Number3March 1990Pages 223-328International Edition in EnglishActive MgH,-Mg Systems for Reversible Chemical Energy StorageBy Borislav Bogdanovii., Alfred Ritter, and Bernd SpliethoffDedicated to Professor Gunther Wilke on the occasion of his 65t h birthdayAn overview of the importance of and methods available for heat storage in the form ofsensible and latent heat is followed by a discussion of the advantages and disadvantages ofreversible thermochemical energy storage compared to conventional energy sources such asfuels, i.e. irreversible chemical energy carriers. Of the reversible metal-hydride-metal systems,the MgH,-Mg system is particularly attractive as a hydrogen and a high-temperature heatstorage material because of its high hydrogen content and the high energy content of theMg-H bond. Th e advances made in this area over the past few years, namely in catalytichydrogenation and the doping of magnesium powders, have led to the development of activeMgH,-Mg systems for energy storage. The first experimental results on high-temperatureheat storage (also with cooling) by coupling a MgH,-Mg storage system with a low-temper-ature metal hydride storage system are presented.

1. IntroductionFor thousands of years--ever since the discovery of fire-

mankind has conveniently satisfied its energy needs by burn-ing wood or fossil fuels, thus liberating, in the form of highgrade heat, solar energy which had been stored chemicallyfor years (or millions of years) by photosynthesis. The energyproblem currently facing us primarily arises from an incorri-gible disparity between the storage of solar energy in natureand our energy consumption, which will lead, on the onehand, to a serious shortage of fossil fuels and, on the other,most probably to a climatic change through a build-up ofcarbon dioxide in the atmosphere (greenhouse-effect).This problem may be resolved, however, by: 1) reduction of[*I Prof. Dr. B. Bogdanovii, DipLIng. B. SpliethoffMax-Planck-Institut fur KohlenforschungKaiser-Wilhelm-Platz 1, D-4330 Mulheim an der Ruhr 1 (FRG)

Dr. A . RitterMax-Planck-Institut fur StrahlenchemieStiftstrasse 34-36, D-4330 Mulheim an der Ruhr 1 (FRG)

the use of fossil fuels in favor of other energy sources, suchas nuclear power, water power and wind power; 2) intelli-gent, more economical energy consumption;] 3) the searchfor, and use of energy storage systems which obtain theirenergy from a primary energy source, e.g. from the sun, andwhose energy transformations occur in a closed cycle andhence are environmentally unobjectionable.[ The followingreview article describes, inter alia, an experimental chemicalcontribution to the last of these three possibilities.

2. The Importance of Energy and Heat Storage,especially of High-Temperature He at Storage r3 1The necessity of energy storage in general, and of heat

storage in particular, arises from the fact that there is atemporal and intensity mismatch and also a spatial separa-tion between the energy supply from an available energysource and our need or consumption of energy. Energy stor-age can offset this incongruity, improve the efficiency of an

A n g e w Chem. /n l . Ed . Engl. 29 ( 1 9 9 0 ) 223-234 8 CH VerlugsgesellschajimbH. 0.6940 Weinheim, 1990 0570-0833/90j0303-0223 $02 .50/ 0 223

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energy supply system and hence reduce the energy costs .Consequently, energy storage is a major factor in energysaving an d conservation. Th e storage of energy f rom a per i-odic or intermittent energy source, such as the sun, is ofpr ime importance.

The type and m agnitude of the mismatch between energysupply and need are exemplif ied in Figure 1.[41 Figure 1 (a )shows the case of constant energy supply with, however , abrief peak load which must be covered by energy storage. Incase (b), the energy supply is periodic (solar energy), whilethe energy requirement remains con stan t; a s ignificant par tof the energy must , therefore, be s tored dur ing daytime inorde r to cover the needs a t any t ime. Finally , when energysupply and demand are out of phase, as in (c) , the totalenergy requirement must be s tored (e.g . , nocturnal indoorheating via solar heat).

Th e poten tial significance of heat storage , is impressivelydemon strated by the fact th at suff icient solar energy could becollected on the roof of a bui ld ing dur ing the summer fo rcover ing the heating requirements of th e building dur ing thewinter (Fig. 2). In the case of a typical one-family house inGermany with a roof surface of 100 m2-either facing sou thor orien ted horizontally (flat roof)-an avera ge incident so-lar radiat ion of lo5 kW hC 5] ould be collected per year . This

energy is suff icient to produce an amount of warm waterequivalent to that requir ing approximately 10000 L of heat-ing oil , i .e. , an am oun t which is more th an a deq uate forheating a household d ur ing the winter . I f one could mak e useof this energy in all of the househo lds in the FR G, then upto ca . 40% of the currently consum ed pr imary energy co uldbe saved.

The a mo unt o f warm water whose energy con ten t oncooling from, e.g., 90 to 40C cor responds to tha t of10000L of heating oil has however, in the ideal case, i.e.without heat s torage loss, a volume of 1724 m3. This is farlarger than th e volume of an a verage one-family house. Be-cause of the unavoidab le heat losses which constan tly occurdue to insulat ion problems, the volume of the warm-waterstorage tank would have to be substantial ly larger . Thus, i tis immediately appare nt tha t the, as yet , unsolved problem ofseasonal heat storage is the main reason why solar radiat ioncan presently play only an insignificant role as a sourceof energy, par t icular ly in mo re nor ther ly latitude^.[^.^]

An important aspect in connection with the amount ofstored heat , nam ely the quali ty of the energy, or the so-calledexergy, should not be over looked. Heat energy can beconver ted into oth er forms of energy with good Ca rno t eff i-ciency, e.g. mechanical or electrical energy, but only above

Borislav BogdanoviC was born in 1934 in Navi Sad, Yugoslavia and studied chemistry at theUniversity of Belgrade. He gained his doctorate in 1962 under the supervision of G . W il ke at theTechnische Hochschule Aachen. Since 1960 he has been working at the Max-P lanck-lnstitut fu rKohlenforschung in Miilheim an der Ruhr, where he is now head of a catalysis department.BogdanoviC habilitated in 1974 at the Universitat Bochum where he was awarded a professorshipin 1976. His research interests center on organometallic chemistry and catalysis and its applica-tions in organic and inorganic synthesis; in 1986 he was awarded the Ruhrpreis fu r K unst undWissenschaft der Stadt Miilheim an der Ruhr for his more recent works on the storage of energywith the MgH,-Mg system.

Alfred Ritter was born in 1928 in Rottweil am Neckar and studied chemistry at the TechnischeHochschule Stu ttga rt (wh ere he gained his doctorate under L. Birk ofer). Thereafter he workedas a scientific assistant at the Universitat Koln and was engaged in research on organosiliconreactions in organic synthesis. In 1963 he was awarded a fellowship grant b y the Dow CorningCorporation (M idlan d, Michig an) fo r research with L. H. Sommer at the Pennsylvania StateUniversity. Since 1964 he has been in charge of a research group at the Max-Pla nck-lnstitut fG rStrahlenchemie in Miilheim an der Ruhr. His current interests center on the radiation chemistryof organosilicon compounds and reversible chemical reactions fo r the storage of solar energy.

Bernd Spliethoff was born in 1953 in Oberhausen and completed his training as a laboratorytechnician at the Ma x-Pla nck-ln stitut u r Kohlenforschung in Miilheim an der Ruhr. In 1971 hewon the Bundeskanzlers Prize f o r young scientists in the open competition Jugend fors cht .Since com pleting his studies in engineering science at the Gesamthochschule Essen in 1975 he hasbeen working at the M ax-Plan ck-lnstitut fu r Kohlenforschung. He has participated right fr omthe beginning in the development ofa ctiv e MgH ,-Mg syste ms as energy storers; he has developedmeasuring instruments and measuring techniquesfor the characterization oft hei r storage proper-ties.

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a I 3. Methods of Heat StorageLt -18=t -

6- 18 W 6-t -Fig. 1. Energy available from store a, hich balances out the mismatch be-tween energy supply (.- ) and need (-): a) constant energy supply, short peakperiod: b) constant energy requirement, variable (periodic) energy supply dur-ing a half-day cycle; c) phase shift between energy supply and need; the en tireenergy need is covered by stored energy [3a] (reproduced by kind permission ofKluwer Academic Publishers).

300-400 C. Consequently, energy storage systems must bedeveloped, not only for low-temperature heat (LT) but alsofor high-temperature heat (HT).

An example of the spatial separation between energysupply and need, is provided by electrical power plants,which produce large amounts of low-grade waste heat, butowing to the distances between supply and consumer, oftenonly part of this heat can be economically used for domesticheating (power-heat-coupling).

1 ,///

J F M A M J J A S O N Dt-Fig. 2. Utilizable solar energy and heating energy needs in summer and winter.- ooftop solar energy, -- - heating energy need.

The oldest and simplest method used in everday life, as inindustry, is to store heat in the form of sensible (perceptible)heat using materials having a high specific heat c. The mate-rials most commonly used, and their heat capacities, arelisted in Table 1.

Table 1. Frequently used liquid and solid materials for the storage of sensibleheat [3 a, b] .

M.p. Temperature e