Likely near-term solar-thermal water splitting technologies

International Journal of Hydrogen Energy 29 (2004) 1587–1599www.elsevier.com/locate/ijhydene

Likely near-term solar-thermal water splitting technologies

Christopher Perkins, AlanW.Weimer∗

Department of Chemical and Biological Engineering, Engineering Center, ECCH 111, University of Colorado at Boulder,Campus Box 424, Boulder, CO 80309-0424, USA

Accepted 26 February 2004

Abstract

Thermodynamic and materials considerations were made for some two- and three-step thermochemical cycles to splitwater using solar-thermal processing. The direct thermolysis of water to produce H2 using solar-thermal processing is unlikelyin the near term due to ultra-high-temperature requirements exceeding 3000 K and the need to separate H2 from O2 at thesetemperatures. However, several lower temperature (¡ 2500 K) thermochemical cycles including ZnO/Zn, Mn2O3=MnO, sub-stituted iron oxide, and the sulfur–iodine route (S–I) provide an opportunity for high-temperature solar-thermal development.Although zirconia-based materials are well suited for metal oxide routes in terms of chemical compatibility at these temper-atures, thermal shock issues are a major concern for solar-thermal applications. Hence, e<orts need to be directed towardsmethods for designing reactors to eliminate thermal shock (ZrO2 based) or that use graphite (very compatible in terms oftemperature and thermal shock) with designs that prevent contact of chemical species with graphite materials at high tem-peratures. Fluid-wall reactor con=gurations where inert gases provide a blanket to protect the graphite wall appear promisingin this regard, but their use will impact process e>ciency. For the case of S–I up to 1800 K, silicon carbide appears to be asuitable material for the high-temperature H2SO4 dissociation. There is a need for a signi=cant amount of work to be done inthe area of high-temperature solar-thermal reactor engineering to develop thermochemical water splitting processes.? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Thermochemical; Water splitting; Solar; Thermal; Hydrogen

1. Introduction

Interest in hydrogen as a clean fuel has surged in therecent past as concerns over the costs of fossil fuels to theeconomy, environment, and national security have becomeparamount. As hydrogen burns only to produce water andcan be used in e>cient fuel cells, it has a great opportunity tobe the replacement for carbon-based fuels in the twenty-=rstcentury. The anticipation of this transition has been so greatthat the future vision for hydrogen power has been labeledthe “hydrogen economy.” With this in mind, a number ofissues must be taken into account.

The =rst and foremost of these is that hydrogen in andof itself is simply an energy carrier. Quantities of hydrogen

∗ Corresponding author. Tel.: +1-303-492-3759; fax: +1-303-492-4341.

E-mail address: [email protected] (A.W. Weimer).

gas on earth are limited, so it must be chemically derivedfrom some other source. Eighty-six percent of the industrialhydrogen produced today is from the steam reforming ofhydrocarbons in the production of syngas [1]. Clearly,choosing this route for the hydrogen economy does notrelease the world from fossil fuel dependence. In the fu-ture, it is desired that hydrogen be produced from a clean,renewable chemical source with a clean, sustainable energysource [1].

This paper explores the possibility of obtaining hydrogenby the splitting of water. The net reaction

H2O → H2 + 12 O2 (1)

produces only hydrogen and oxygen. When these are re-combined for the production of energy (e.g., electricity froma fuel cell), the only product is water, and so the cycle isinherently renewable and pollution-free. To provide theenergy required for this endothermic reaction to proceed,

0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2004.02.019

mailto:[email protected]

1588 C. Perkins, A.W. Weimer / International Journal of Hydrogen Energy 29 (2004) 1587–1599

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Fig. 1. Gibbs energy change of reaction for decomposition reactions.

the use of heat from concentrated solar energy is examined,which, if properly utilized, would be a sustainable and cleanenergy source.

It has been shown that solar thermal reactors can eas-ily achieve temperatures between approximately 1500 and2500 K. Temperatures even higher than this are achievable,but in those regimes materials and reradiation loss issuesbecome major concerns. Hence, likely solar-thermal processscenarios to split water should be operated at temperatures¡ 2500 K.

A number of cycles for the splitting of water are exam-ined below. As with any industrial process, both economicand e>ciency aspects of the processes must be evaluatedto see if the process would be both viable and pro=table.The processes outlined below are the ultra-high-temperaturedirect thermal splitting of water, high-temperature two-stepmetal oxide oxidation–reduction reactions to split water, andmoderate temperature three-step water splitting cycles.

2. Direct splitting of water

The most direct method for using solar energy to derivehydrogen from water is the one-step thermolysis of the wa-ter molecule. Concentrated solar energy is focused onto areactor through which steam passes, and the heat transferredto the steam brings it to a temperature at which it can dis-sociate. At =rst glance, this appears to be a very attractiveprocess. Its simplicity seems to make the reactor engineer-ing less involved, and as it occurs in a single step, very hightheoretical maximum e>ciencies can be obtained [2].

After closer examination, however, this system is rife withproblems. The JGf for the water dissociation reaction doesnot yield tenable values of the equilibrium constant until thereaction temperature exceeds 2500 K [3]. In fact, simulat-ing the reaction with the FACT thermodynamics software

(Facility for the Analysis of Chemical Thermodynamics;McGill University), it can be seen that the JGf for thisreaction is not zero until it exceeds a temperature of 4300 K;see Fig. 1. Even at temperatures as high as 2500 K, Koganet al. [4] showed that only a 25% dissociation of the watercan be achieved. Temperatures this high can be achievedin modern solar collector systems, but, as radiation lossesincrease as the fourth power of temperature, e>ciencies ofthe water collection array become a concern [5]. However,inherent advantages include extremely fast reaction ratesthat increase exponentially with temperature, and subsequentcompact reactor sizes that are more easily insulated.

In addition to increases in reradiation losses, very hightemperatures also bring concerns about the materials of reac-tor construction. Most nonoxide refractory materials becomeunstable at temperatures near or above 2500 K, and the ef-fect of H2 on oxides at these extremely high temperaturesis not known [2]. As more exotic and advanced materialsare employed in the construction of such a solar reactor, thecost of the reactor will increase due to higher raw materialand material processing costs. Therefore, the search for andutilization of materials capable of withstanding the ultra-high temperatures required for direct water thermolysis maymake this process economically prohibitive.

One other major problem exists with direct water splitting.This is the need to separate high temperature (T ¿ 2500 K)O2 and H2 from each other. If they are not separated at hightemperature, they have a tendency to recombine at lowertemperature or, if the temperature is successfully reducedwithout recombination, form an explosive mixture at mod-erate temperatures.

Much research has been done to discover technologiesable to achieve this separation. Kogan [4] has exploredthe use of porous ceramic membranes, primarily thoseconstructed of zirconia. Ohya et al. [6] simulated thesemembranes and Fan et al. [7] were able to successfully

C. Perkins, A.W. Weimer / International Journal of Hydrogen Energy 29 (2004) 1587–1599 1589

fabricate very high zirconia content membranes, giving hopethat high-temperature membrane separators for this reactionmight exist. Zirconia has a high enough melting temperatureto withstand the harsh conditions in a direct water splittingreactor, and it has been shown to have excellent hydrogenpermeability. However, Kogan found that at temperaturesabove the sintering temperature of zirconia (2073 K), thesintering process continues. In relatively short time, this sin-tering process causes pore blockage and a severe reductionin hydrogen permeability of the membrane [4].

Pyle et al. [8] explored using a sonic nozzle/splitter con-=guration in conjunction with a glow discharge plasma toachieve dissociation and separation of the gas mixture. How-ever, they were unable to achieve more than a 2.1% H2 sep-aration from the bulk product stream. There is projectionbut little experimental evidence to show that this separatione>ciency can be increased. With such a small overall con-version e>ciency of H2, it is unlikely that this method canbe economically viable [2].

In summary, as the temperature, material, and separationrequirements for the direct water thermolysis reaction areso severe, there is little hope that this can be an econom-ically viable process solution in the near future. This factmotivates the search for cycles that operate at lower tem-peratures (¡ 2500 K), relaxing many of the constraints feltby the direct water splitting system.

3. Metal-oxide cycles

The infeasibility of direct water splitting with current ornear-future technology has led to research in the develop-ment of multi-step cycles with the same net e<ect, i.e. thedissociation of water into its constituent elements. The ad-vantages of this process approach are many and varied. First,the temperature of the decomposition reaction in the cycleis much lower than the temperature required to carry outthe direct thermolysis of water. As high temperature wasone of the two major limiting factors in direct water split-ting in terms of e>ciency, materials, and economic bene-=ts, the reduction of this temperature can lead to signi=-cant gains on each of the aforementioned fronts. Second,as all of these multi-step processes remove hydrogen andoxygen gas in separate steps, there is no need to perform ahigh-temperature gas separation of these elements, and thereis no chance of the formation of an explosive (and thus dan-gerous) gas mixture [9–11].

These advantages come at a price, though. First and fore-most is that as the number of process steps increases, themaximum theoretical process e>ciency decreases due tothe irreversibility of each stage and of the transfer betweenstages. With decreased e>ciency comes poorer theoreticaleconomics, overall conversion concerns, and bottomlinereductions in overall energy production. In addition todecreased e>ciency, multi-stage processes require theseparation of reaction products at moderate temperatures,

transportation of products/reagents between reaction stages,and, for high-temperature reactions, the problem of re-combination of dissociated compounds as temperature isreduced [10,12,13].

A =nal disadvantage of multi-step water splitting cyclesis that they involve chemical reactants other than water. Forthese processes to be entirely sustainable and renewable, notto mention economically viable, these other reagents mustbe completely regenerated and recycled within the reactioncycle. Otherwise, the process would require a net inputof process materials other than water, many requiring moreenergy to produce than the product hydrogen wouldeventually yield. All of these disadvantages must be min-imized for these multi-stage processes to have a practicaladvantage over their single-stage water splitting counterpart.

One major class of two-step water splitting cycles wheregreat amounts of research have been concentrated is thatof metal-oxide oxidation/reduction steps. The general cycleconsists of two steps:

Dissociation : MxOy → MxOy−1 + 12 O2; (2)

Water reduction : MxOy−1 + H2O → H2 +MxOy: (3)

The net reaction is the splitting of one mole of water intoone-half mole of O2 and one mole of H2.

A number of these reaction cycles have been examinedin the literature. The JGf for some of these cycles has beengenerated as a function of T , and can be seen in Fig. 1. Ascan be seen, most of the metal-oxide cycles have negativeJGf values (corresponding to equilibrium constants greaterthan one) for reaction temperatures greater than 2500 K. Ascan be recalled, at this temperature direct water thermolysisis possible, and many of the problems associated with thatcycle were directly related to its high temperature. There-fore, cycles with negative JGf at temperatures greater than2500 K will not be considered [10]. In addition, even thoughthe metal/metal-oxide pairs Mn3O4=MnO and Co3O4=CoOhave low-enough JGf for the dissociation reaction to pro-ceed, their yields in the water splitting step of the processare too low to be considered [10]. Steinfeld et al. [10] sug-gest that the only feasible two-cycle metal-oxide pairs areZnO/Zn and Fe3O4=FeO. As a result, these are the two-stepcycles to be reviewed in detail.

Metal-oxide cycles are relatively new in consideration,mainly as previous thermochemical water dissociation re-search focused on cycles that would operate below 1573 K.This is due to the fact that till the recent past, the main sourceof thermal energy considered for use in the dissociation re-action was nuclear waste heat, and 1573 K was consideredto be the maximum safe operating temperature of such a nu-clear reactor. With the advent of high-power solar collectorsystems, much higher temperatures have become possible,with some estimates as high as 3000 K [14].

The metal-oxide cycle that has been most researched inthe technical literature is the ZnO/Zn cycle. As can be seenfrom Fig. 1, it has a JGf of zero at 2255 K, making it


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O (Vapor)O2 (Vapor)O3 (Vapor)Zn (Vapor)ZnO (Solid)

Fig. 2. Temperature dependence of equilibrium composition for decomposition of ZnO at 1 atm, 1 mol Ar inerts in feed.

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feasible for modern solar reactor systems [11]. If the Zn isfully recovered in the decomposition step, and ZnO fullyrecovered in the water splitting step, it is possible to makethe only reaction input H2O and the only products O2 andH2, thus completing a renewable, sustainable cycle.

The ZnO decomposition reaction was simulated usingthe computer program FACT, and the equilibrium com-position results of this simulation are shown in Fig. 2. Ascan be seen, FACT predicts the start of Zn vapor forma-tion around 1800 K, with complete conversion occurringaround 2150 K. At this point, only, Zn, O2, and some

elemental oxygen are formed. Due to the unstable nature ofelemental oxygen, this will most likely form the diatomic gaswhen temperatures are reduced in the post-reaction quench.The addition of more inert gas to carry the ZnO particles(argon or helium is the most likely candidate; helium iseasier to separate from oxygen) reduces the necessary tem-perature for complete conversion of the ZnO predicted byFACT. This is a result of the reduced partial pressure ofthe product gases, shifting the reaction equilibrium towardthe products. This temperature dependency is shown inFig. 3. The necessary temperature for complete conversion


dips below 2100 K for a molar feed ratio of 3:1 (Ar toZnO). The decreased temperature will reduce reradiationlosses, but it introduces an oxygen separation if one wishesto recycle the inert gas back to the system. Likewise, theenthalpy associated with heating inert gases needs to beconsidered.

As suggested by the thermodynamics, ZnO completelydissociates at temperatures around 2300 K. However, themajor challenge of this cycle is the separation and concen-tration of the decomposition reaction products, Zn and O2.At 2300 K, both species are gaseous (see Fig. 2). As theproducts cool, the reaction equilibrium shifts to ZnO and thedesired products have a tendency to recombine. Recombi-nation gives low yields of Zn, leading to low overall yieldsof H2 and poor process economics. Palumbo et al. [12] sug-gest that a fast quench be used to cool the products to under1200 K, where recombination kinetics are so slow as to beprohibitive. Using the kinetic data of Kashiraninov [12] pre-dicts that a quench rate of 2 × 107 K=s would be requiredto achieve 80% zinc recovery. However, their own exper-iments showed higher Zn yields with slower quench ratesthan this, suggesting a di<erent kinetic mechanism than thehomogenous elementary gas reaction proposed by early re-searchers in the =eld. Steinfeld et al. [11] showed that Znand O2 recombination is a heterogeneous process. In the ab-sence of nucleation sites, it will not proceed. Little testinghas been done on controlling the kinetics of this recombi-nation reaction by engineering the surface chemistry. If thistechnology is to be viable, Zn production in the =rst cyclestep must be maximized, and so much more research mustbe done on mitigating zinc oxide formation in the quenchstep.

The water splitting reaction for the Zn/ZnO cycle presentsfewer challenges. Steinfeld [11] has shown that it has areasonable rate at temperatures greater than 700 K, and as itis exothermic, it is possible to run this process autothermally.In addition, if the water splitting reaction is run in-line withthe decomposition reaction, the inlet preheating of the steamand Zn can come from the solar reactor waste heat [15].

Steinfeld et al. [11] have run an energy analysis on theZnO/Zn process, calculating the process e>ciency for he-liostat solar concentrations of 5000 and 10,000 suns. Theauthors assumed equilibrium conditions and focused on thegreatest sources of irreversibility: reradiation losses in thesolar reactor and the inherent irreversibility of the Zn=O2

quench. From this analysis, maximum overall e>cienciesof 36% and 25% were obtained for solar concentrations of10,000 and 5000 suns, respectively [11,12].

Other metal-oxide cycles have been studied in some de-tail, with most of the attention focused on Fe-based cycles.For example, Steinfeld [16] performed a cost and e>ciencyanalysis on the Fe3O4=FeO cycle and obtained theoretical ef-=ciencies of 61% at 1900 K and 42% at 2500 K. However,these only included reradiation losses, and no considerationwas given to the separation and quench steps, which werethe highest source of irreversibility in the ZnO/Zn cycle [12].

Partial substitution for Fe in Fe oxides has also been sug-gested, with the hopes of striking a good balance betweenlow decomposition temperatures and high H2 yields in thewater splitting reaction. However, there is little informationin the literature on simulation or experimentation with thesecycles.

As the e>ciencies of these cycles are inextricably linkedto their operating temperatures and their need for prod-uct separation, the use of reducing agents has been highlyconsidered. The reducing agent lowers the temperature ofthe decomposition reaction while also scavenging the oxy-gen, leaving the metal already separated. Good results havebeen obtained using carbon black or methane as the reducingagents. In particular, Steinfeld et al. have calculated e>cien-cies of up to 68% in using these systems, and have success-fully demonstrated these systems experimentally [17–22].The use of reducing agents, though, comes at a very high

cost. The cycle no longer becomes simply a water-splittingcycle, but requires a net inPux of the reducing agent and hasa net eQux of waste products (such as CO2). As one of themajor goals of the use of water-splitting is the achievementof a sustainable, greenhouse gas free hydrogen economy, thisoption is not compatible with the long-term goal. Moreover,if the reducing agent is strong enough to compete with themetal to be reduced for that metal’s oxygen, then one mightas well use it directly in a water splitting reaction.

A =nal metal oxide cycle to consider is the Mn2O3=MnOcycle. It has three steps, so its maximum theoretical e>-ciency will be less than that of a two-step cycle. However,the feasibility of achieving maximum e>ciency in each ofthe steps must also be evaluated, and it might be possiblethat this three-step cycle is more easily achieved than anyof the two-step cycles. Speci=cally, this cycle is

(1) MnO + NaOH → 12 H2 + NaMnO2; (4)

(2) NaMnO2 + 12 H2O → 1

2 Mn2O3 + NaOH; (5)

(3) 12 Mn2O3 → MnO + 1

4 O2: (6)

This cycle is better than the comparative Mn3O4 cycle, as ithas a higher yield of H2 per mass Pow of oxide. Sturzeneg-ger and Nuesch [23] calculated e>ciencies of 26–51%whenignoring separation steps, and those of 16–21% whentaking all steps, including separation, into consideration.

Thermodynamic simulation of the decomposition step ofthis reaction (Eq. (6)) was conducted using the FACT soft-ware. The equilibrium composition results of this simulationcan be seen in Fig. 4. From these equilibrium data, a fewinteresting and desirable qualities of this system come tolight. First, complete decomposition to solid phase MnO andgaseous O2 occurs around 1800 K with an equimolar inertfeed. This is at a lower temperature than required for the ZnOsystem, allowing for higher reactor e>ciency due to lowerreradiation losses. In addition, the separation of gaseous O2

from solid MnO upon cooling seems straightforward. Thefeasibility of this phase separation needs to be investigatedin order to determine if the recombination of MnO (solid)


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Mn (Vapor)O2 (Vapor)O (Vapor)O3 (Vapor)Mn2O3 (Solid)Mn3O4 (Solid)MnO (Solid)MnO (Liquid)

Fig. 4. Temperature dependence of equilibrium composition for decomposition of Mn2O3 at 1 atm, 1 mol Ar inerts in feed.

and O2 can be prevented in the quench step. If so, then thisis a much simpler separation than that of Zn (vapor) andO2, potentially favoring the three-step Mn2O3=MnO processover the two-step ZnO/Zn alternative.

Finally, this cycle has only been simulated, and as experi-mental results have yet to be published in the literature, thereis no information available on its actual performance. How-ever, it should be investigated, as its lower operating tem-perature makes it favorable in terms of radiation losses andoverall solar collection =eld economics, and the two-phasenature of the decomposition products suggests fewer sep-aration problems than in the aforementioned metal oxidesystems [23].

One last consideration that must be made for metal-oxidecycles is whether they can be performed in air. This wouldeliminate the need for separation of an inert gas fromO2 and could increase process e>ciency and economicsgreatly. The aforementioned decomposition of Mn2O3 wassimulated using FACT for a typical feed rate of both airand metal oxide, and the results can be seen in Fig. 5. Ascan be seen, the process will produce MnO at temperaturesabove 1850 K, not much higher than if run under inertgas. However, at temperatures this high, the nitrogen in aircombines with oxygen to form signi=cant amounts of NO,a serious industrial pollutant. As this would be the case forany high-temperature air cycle, clean operation could neverbe achieved when running under the presence of air.

4. Low-temperature cycles

During the energy crisis of the 1970s, much research wasdirected toward the thermochemical production of hydro-

gen using the waste heat of nuclear reactors. As the maxi-mum temperature in these reactors was restricted to 1573 Kfor safety reasons, all of these thermochemical cycles wereforced to operate under this temperature limit. Of these cy-cles, the most promising is the sulfur-iodine, or SI, cycle,investigated by General Atomics in the mid-1970s [1]. Thecycle is

2H2O + xI2 + SO2 → H2SO4 + 2HIx(aq); (7)

H2SO4 → H2O + SO2 + 12 O2; (8)

2HIx → H2 + xI2: (9)

The attractiveness of this system comes from its low op-erating temperature. At low temperature, the e>ciency ofa solar array is quite high due to minimal radiation losses.In addition, if the decomposition reaction is run at highertemperatures than originally planned, available with currentsolar technology, faster kinetics and better equilibrium con-versions can be achieved [1].

The SI cycle, however, has many of the same problems asthe metal-oxide cycles, especially when considering inter-stage separation. H2SO4 must be completely removed fromthe HIx phase to avoid sulfur formation in the HI separationand HI decomposition steps [24]. Shimizu et al. [24] wereable to remove 99% of this H2SO4, but at the temperaturerequired for this separation, much of the HI vaporized, lim-iting the net yield of H2. In addition, there is an H2=HI=I2separation that must be carried out to give pure H2, and thisis yet to be successfully demonstrated. A third separationwould require the separation of HI from I2 before feeding tothe H2SO4 decomposition step. However, researchers at the


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O2O (g)NO (g)MnO (liq)Mn3O4 (s)Mn2O3 (s)MnO (s)

Fig. 5. Mn2O3 dissociation in air (5 g=min Mn2O3 feed, 15 SLPM air feed).

University of Aachen in Germany showed that this separa-tion is not necessary, giving higher e>ciencies due to re-moval of a heavy phase separation step.

As S and I are very reactive elements, the reactions mak-ing up this cycle can be accompanied by unwanted side reac-tions and incomplete decomposition. The =rst of these is thesulfur formation mentioned above; the second is the equilib-rium formation of intermediate SO3 instead of =nal productSO2 in the H2SO4 decomposition step. If SO3 is formed,it must be cooled and recombined with water in order toseparate it from the O2 and SO2 reaction products. ThisH2SO4=H2O mixture must then be recycled and the sulfu-ric acid decomposed once again. This would require inputof the sensible and reaction heats once again. Due to 2ndLaw considerations, the original heat input is not fully re-coverable, and this recycle presents e>ciency challenges.The H2SO4 decomposition (reaction (8)) was simulated us-ing the FACT software, and the ratio of vapor mole frac-tions of SO2 and SO3 as a function of temperature can beseen in Fig. 6. As can be seen, to signi=cantly mitigate theequilibrium formation of SO3, the reaction should be run attemperatures in excess of 1400 K. This takes away some ofthe advantage the SI process has in the form of lower op-erating temperatures; however, the operating temperature isstill signi=cantly lower than that needed for the metal-oxidecycles, so the advantage, although decreased, remains.

All of the equilibrium decomposition products from theH2SO4 decomposition reaction simulation can be seen inFig. 7. As can be seen in the =gure, a variety of productsare produced from this reaction. However, there does exista temperature range for which the desired products (H2O,SO2, and O2) are the only products produced in signi=cantquantities. This range appears to be centered at 1900 K andto be between ∼ 1650 and 2150 K. Outside of this range,

unwanted side reactions begin to have a greater e<ect. Also,this temperature range exceeds the minimum temperature forSO3 mitigation detailed above. Still, it should be noted thatsince all products are in the vapor phase, separation issuesexist, and these can be a marked disadvantage for this cycle.

The temperature range for ideal operation of the SIcycle highlights the advantages of the solar-thermal ap-proach. Because temperatures as high as 2500 K can bereached, the range chosen above is easily within the oper-ating limits of a solar thermal reactor. In contrast, a designrestricted to lower temperatures for chemical or safety rea-sons, such as the nuclear waste heat method mentionedabove, will be relegated to reaction regimes on the left sideof Fig. 7. As can be seen, unwanted side reactions aresigni=cant, requiring separation of the desired products,disposal of the undesired products, and more of the reactant(H2SO4) to achieve the desired throughput of decompositionproducts. To avoid the separation, environmental, and pro-cess feed costs associated with these additional problems,it is highly advantageous to run in the temperature regimedetailed above, giving the high-temperature solar-thermalprocess design a leg up on comparative designs for the SIwater splitting process.

Another consequence of the reactivity of S and I is the re-quirement for materials that are highly resistant to corrosionby these elements. These materials need not only withstandthe temperatures involved in these reactions, but must re-sist attacks from the sulfur and iodine. Reaction with reactormaterials would not only weaken the structure, but woulddecrease the overall yield of hydrogen and causes a “leak”in the cycle, as other materials than just water would needto be fed to the reactor to make up for side reactions [24].

E>ciencies for the SI cycle have been calculated tobe between 47% and 52% by Brown et al. [1]. These


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Fig. 6. Sulfuric acid decomposition to SO2 and SO3 in S–I cycle.

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H (Vapor)H2 (Vapor)O (Vapor)O2 (Vapor)OH (Vapor)H2O (Vapor)SO (Vapor)SO2 (Vapor)SO3 (Vapor)H2SO4 (Vapor)H2SO4 (Liquid)

Fig. 7. Temperature dependence of equilibrium composition for decomposition of H2SO4 at 1 atm.

calculations were performed with adiabatic process equip-ment and a lower-temperature, nuclear heat source. Asimulation involving solar processing equipment has yet tobe performed; to successfully evaluate the applicability ofthis process to solar thermal processing, these calculationswill need to be performed.

5. Material considerations

Solar-thermal processes cannot be considered kind to ma-terials of reactor construction. Their very nature is quitesevere, with strong oxidizing environments, high tempera-

tures, and extremely fast heating and quenching rates as partof their normal operation. Therefore, it is quite important to=nd materials resistant to both the harsh chemical and phys-ical environments they will encounter. Two materials areexamined below, selected for their high strength and thermalproperties. Both zirconia (ZrO2) and silicon carbide (SiC)are ceramic materials that exhibit high temperature resis-tance, making them healthy candidates for use in one of thesolar-thermal reactors proposed above.

The thermodynamics simulation package FACT wasused to simulate these materials’ resistance to the chemicalenvironments to which they would be subjected. Simula-tions were run to predict equilibrium compositions at 1 atm


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Fig. 8. Zirconia resistance to decomposition in solar decomposition reactor environment.

of pressure at temperatures ranging from 400 to 3000 K.As a strongly conservative estimate, 20 mol of ZrO2 wereallowed to react with 1 mol of the decomposition productat these conditions. This would suggest virtually all of atypical reactor would be available to react at once, a physi-cally unrealistic situation. However, this assumption pushesthe equilibrium strongly to any products that would possiblybe formed, giving a good indication of how these materialswould fare over time in the reaction environments describedabove. For SiC, the material has been shown to form a thinsilica (SiO2) =lm at moderate temperatures, protecting thematerial from further oxidation [25,26]. To account for this,simulations were run for a silicon carbide tube to determinewhether, at high temperatures, the SiO2 will react with theSiC or with the various decomposition reagents used in thethermochemical cycles described above. These simulationswere run at 1 atm of pressure, between 400 and 3000 K,and for 1 mol of SiO2 with 1 mol of SiC and 1 mol of SiO2

with 1 mol of each of the decomposition reagents, ZnO,Mn2O3, and H2SO4.Fig. 8 shows the results of the simulation for ZrO2.

Results are given in the percentage of the original 20 molreacted at equilibrium as a function of temperature. Ascan be seen from Fig. 8, ZrO2 shows excellent resistanceto the various decomposition environments, not reactingat all below temperatures of 2600 K, and even then onlyreacting at 0.002% of the total 20 mol allowed. However,it also must be considered that ZrO2 has very low thermalshock resistance. Using this material within a solar reac-tor, where temperatures increase on the order of 105 K=s,could cause failure rapidly. In light of these, ZrO2 shouldbe avoided, even though it is chemically stable in thereaction regimes.

As can be seen in Figs. 9–12, the SiC shows much lessresistance. In Fig. 9 it can be seen that the SiC–SiO2 systemis stable up to 1900 K. At this point, the silica layer melts,and system stability breaks down, with the formation of COand SiO from the starting reagents. With this information,it can be readily seen that SiC cannot be used with con=-dence above 1900 K. Examining Figs. 10–12, it can be fur-ther seen that only the H2SO4–SiO2 system shows stabilityup to the silica melting point. The metal-oxide systems ofZnO and Mn2O3 show equilibrium products with silica attemperatures as low as 500 K, forming the products ZnSiO4

and Mn2SiO4, respectively. Whether these reactions occurto completion depends on their inherent kinetics and uponthe transport of the reactive metal-oxides to the reactor wall.However, the materials are thermodynamically incompati-ble and it would be best to explore other material options to=nd a more suitable material.

6. Solar-thermal $uid-wall graphite transport tubereactor process

A graphite Puid-wall transport tube reactor process [27–31] is an option for high-temperature solar-thermal watersplitting thermochemical dissociation reaction processing.Graphite is capable of operating continuously at high tem-peratures near 3000 K in nonoxidizing environments and isinsensitive to thermal shock. A Puid-wall di<usion barrierof inert gas prevents reactants and products of reaction fromcoming in contact with the graphite reaction tube. Key to theprocess will be the ability to realize high rates of reactionwhere dissociation can be completed in fractions of a sec-ond. An inert gas, such as argon or helium, can provide the


0

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COSi (g)Si2C (g)SiO (g)Si (l)SiC (s)SiO2 (s)

Fig. 9. SiC–SiO2 equilibrium behavior (1 mol SiC, 1 mol SiO2, 1 atm).

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O (g)O2 (g)H2O (g)SiO (g)SO (g)SO2 (g)SO3 (g)H2SO4 (l)SiO2 (s)

Fig. 10. SiO2–H2SO4 equilibrium behavior (1 mol SiO2, 1 mol H2SO4, 1 atm).

Puid-wall and be recycled to the reactor after O2 is removeddownstream.

Early investigation of a Puid-wall reactor was for anelectrically heated reactor where graphite heating elementsheated a porous graphite reaction tube to above 2500 K[27]. A number of reactions were carried out, includingcarbothermal reduction of metal oxides, coal gasi=cation,reforming reactions, and dissociation reactions, amongothers.

Recent solar-thermal Puid-wall reactor developmentdemonstrated reactor temperatures of 2200 K with a porousgraphite reaction tube heated indirectly using concentrated

sunlight. Methane dissociation reactions achieved over 90%conversion for residence times of ∼ 10 ms at near 2200 K.The reactor was very compact and the deposition of car-bon black on the tube wall was prevented. In addition, thethermal mass of the reactor is minimal, allowing for on/o<processing that is very compatible with the dynamic anddiurnal nature of sunlight.

E<orts should be directed towards developing e>cientsolar-thermal Puid-wall graphite reactor designs and pro-cesses (Fig. 13) with minimized reradiation losses that canbe used for water splitting thermochemical cycles. Theamount of inert gas required should be minimized in order to


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O (g)O2 (g)

SiO (g)Zn (g)

SiO2 (l)

SiO2 (s)ZnSiO4 (s)

Fig. 11. SiO2–ZnO equilibrium behavior (1 mol SiO2, 1 mol ZnO, 1 atm).

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O (g)O2 (g)SiO (g)Mn (g)Mn2SiO4 (l)SiO2 (s)Mn2O3 (s)Mn3O4 (s)Mn2SiO4 (s)

Fig. 12. SiO2–Mn2O3 equilibrium behavior (1 mol SiO2, 1 mol Mn2O3, 1 atm).

reduce e>ciency losses associated with the heating andseparation/recycle of this inert gas in the process. Suchreactors would overcome existing limitations regardingmaterials of construction for such processes.

7. Conclusions

Likely near-term solar-thermal water splitting thermo-chemical cycles were identi=ed, reviewed and examinedfor technical and economic viability. It was found that di-rect thermolysis of water has little chance of achieving this

viability in the near future, while certain lower-temperaturemetal-oxide cycles and the SI cycle look promising.High-temperature chemically resistant materials are still anissue regarding solar-thermal reactor design.

For metal-oxide cycles, the two-step ZnO/Zn appears tobe the most promising due to its relatively low decomposi-tion temperature. However, more work must be done on op-timizing the quench and water splitting stages in this cycle.The Mn2O3=MnO cycle has a number of desirable quali-ties, highlighted by a comparatively very low decompositiontemperature and a phase di<erence in the decomposition


MxOy Inert Gas

MxOy-1, O2, Inert Gas

O2Inert Gas

O2

Solar-thermalFluid-wallReactor

(MxOy Dissociation)

SeparatorSeparatorWaterSplitterH2O

MxOy-1

H2

Fig. 13. Solar-thermal Puid-wall thermochemical water-splittingcycle reactor process (oxide cycles).

reaction products, allowing for what appears to be easy phaseseparation of O2 gas from solid MnO. This cycle shouldbe investigated experimentally, in addition to more exoticmetal-oxide cycles, like partially substituted iron oxides.

For low-temperature cycles, if side reactions in the SI cy-cle can be mitigated and membranes developed for the sep-aration steps, this cycle could become economically viable.It was noted that solar-thermal processing was ideal for theSI cycle, as it would allow operation in the optimal tem-perature range (∼ 1900 K), not easily accessible by otherprocessing methods. Furthermore, H2SO4 dissociation in thepresence of SiC (protective SiO2 layer) is stable over thedesirable temperature range. Material concerns are still out-standing, and experimental work must be done to validatematerials used in a real process.

A graphite Puid-wall reactor heated solar-thermally [28–32] is an option at the present time for water splitting ther-mochemical processing. Such a process will provide for tem-peratures approaching 3000 K, can accommodate on/o< sunand the associated thermal shock, and can prevent contactof the reactants and reaction products with the graphite wall.Work needs to be done to increase the thermal e>ciency ofsuch a reactor and to develop an understanding of the gov-erning transport issues and the kinetics of high-temperaturedissociation reactions that will need to be carried out in frac-tions of a second to only several seconds reaction time atultra-high temperatures.

Acknowledgements

The authors wish to thank the National Science Founda-tion for their support of Chris Perkins through the GraduateResearch Fellowship program.

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Likely near-term solar-thermal water splitting technologies