Dincer_Potential Thermochemical and Hybrid Cycles for Nuclear-based Hydrogen Production

INTERNATIONAL JOURNAL OF ENERGY RESEARCH

Int. J. Energy Res. 2011; 35:123–137

Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1769

Potential thermochemical and hybrid cycles fornuclear-based hydrogen production

Ibrahim Dincer1,�,y and M. Tolga Balta2

1Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe Street North,

Oshawa, ON, Canada L1H 7K42Department of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey

SUMMARY

This paper discusses some potential low-temperature thermochemical and hybrid cycles for nuclear-basedhydrogen production and considers them as a sustainable option for hydrogen production using nuclear process/waste heat and off-peak electricity. We also assess their thermodynamic performance through energy and exergyefficiencies. The results show that these cycles have good potential and become attractive due to their high overallefficiencies 50% based on a complete reaction approach. The copper–chlorine cycle appears to be a highlypromising cycle for nuclear-based hydrogen production in this regard. Copyright r 2010 John Wiley & Sons, Ltd.

KEY WORDS

energy; exergy; efficiency; nuclear; hydrogen; thermochemical cycles

Correspondence

*Ibrahim Dincer, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe

Street North, Oshawa, ON, Canada L1H 7K4.yE-mail: [email protected]

Contract/grant sponsor: Ontario Research Excellence Fund

Contract/grant sponsor: Natural Sciences and Engineering Research Council of Canada

Received 23 February 2009; Revised 15 April 2010; Accepted 8 June 2010

1. INTRODUCTION

Energy is a mainstay of an industrial society. It is,

therefore, not surprising that many important organi-zations have attempted to analyze the future need forenergy and the availability of various energy sources.

Energy consumption growth is closely linked topopulation growth, although changes in life stylesand efficiency improvement have a substantial influ-ence on the per capita annual consumption. The

structure of population and the share between urbanand rural populations also affect energy demand [1].As a result of the worldwide increasing consumption

of energy due to an increasing population and risingliving standards in less industrialized countries, theworld faces the problem of depleting energy resources

and the impairing impact of present energy consump-tion patterns on the global climate as well as onhumanity and the environment [2].

The risk of global climate change is of great concernto policymakers and to the public. The relation

between the energy generation sector and environmentalpollution is being carefully considered in industrializedcountries. Before executing any power generation pro-

ject, extensive and comprehensive studies are preformedconcerning the impact of such a project on the en-vironment. Measures for decreasing climate change and

environmental pollution are considered.Beyond fossil fuels, the mismatch between energy

consumption and energy production becomes moreobvious. Nuclear facilities produce energy at a con-

stant rate, whereas renewable energy facilities produceenergy at a variable rate. Neither type of productionmatches demand. Because of day–night and seasonal

variations of sunlight, the typical capacity factor ofsolar devices is 18%. (The capacity factor is the actualenergy output in a year divided by the potential energy

output if the device were operated at full capacity forthe entire period.) The capacity factor for wind isabout 35%. For renewable energy sources, the mis-

match between generation and demand is so large thatit has been estimated that if as little as 15% of the

Copyright r 2010 John Wiley & Sons, Ltd.

electricity were produced by solar or wind, there wouldbe limited economic incentive to obtain more energyfrom such sources, even if they are free. This is because

backup power production facilities must be built tomeet demand when these renewable energy sources arenot available [3].

Research and development of clean, economic,stable, safe, and abundant energy should be promotedfrom the viewpoint of technology as a potential mea-sure to mitigate global warming as well as for devel-

oping large and stable energy supplies. We have variousalternative energy options to fossil fuels: solar, geo-thermal, hydropower, nuclear energy, etc. Although

available natural energy is limited due to its stability,quality, quantity, and density, it is certain that nuclearenergy has the potential to contribute a significant

share of energy supply and utilization. Nuclear energyhas been almost exclusively utilized for electric powergeneration, but the direct utilization of nuclear thermalenergy can be used to increase energy efficiency and

thereby facilitate energy savings in the near future.Hydrogen production is one of the key technologies fordirect utilization of nuclear thermal energy [4].

Hydrogen has ideal characteristics as an energy car-rier. It can be stored, transported with lower loss com-pared with electricity, and used as fuel. If necessary, the

chemical energy of hydrogen can be converted to elec-trical energy by means of fuel cells and other devices. Asit can be produced from water and, after oxidation, it

returns to water, hydrogen is ‘clean’ from the viewpointof environmental effects. Therefore, the realization of a‘hydrogen energy system’ where hydrogen and elec-tricity serve as complementary secondary energy car-

riers has been considered for a long time [5].The need for industrial hydrogen production has

increased drastically and will continue increasing even

further in the next decade. The present economic valueof all hydrogen produced worldwide is about $300billion/year. The growth rate is about 10% per year and

expected to be doubling to 20% per year by 2010 orbeyond [4]. A key challenge facing this rapid growth isa sustainable route to hydrogen production. Hydrogen

production for sustainable development has been stu-died by several investigators (e.g. [6–12]). Dincer [11]has outlined the key technical and environmental issuesof current hydrogen production technologies. The

technology for producing hydrogen from a variety ofresources, including renewables, is improving. Hydro-gen can be produced as a clean fuel from the world’s

sustainable energy sources such as nuclear, solar, wind,hydropower, biomass, and geothermal.Thermochemical cycles were proposed in the 1970s

as an alternative and potentially more efficient methodto produce hydrogen from water. Hydrogen produc-tion by thermochemical water splitting is a chemicalprocess that accomplishes the decomposition of

water into hydrogen and oxygen using only heat or, inthe case of a hybrid thermochemical process, by a

combination of heat and electricity. Thermochemicalcycles consist of series reactions, in which water isthermally decomposed and all other chemicals are

recycled. Only heat and water are consumed. Researchin this area was prominent from the 1970s through theearly 1980s [13]. Some proof-of-principle experiments

to demonstrate the feasibility of the hydrogen pro-duction process through such cycles have been per-formed [13,14], and no problems associated withtechnical feasibility and potential use are however

reported. Of course, there is still much to do to scale-up these applications to a commercial level.Recently, there have been numerous studies [15–24]

reported on several technical aspects of thermo-chemical cycles driven by the use of process heat fromnuclear and other sources. This paper aims to discuss

various potential options for nuclear-based hydrogenproduction through thermochemical and hybrid cyclesIn this regard; this study covers six low-temperaturethermochemical cycles driven by nuclear-based process

and waste heat at temperatures ranging from 450 to700K, respectively, and presents a comparative studyof these options.

2. ENERGY OPTIONS FOR NUCLEAR-BASED HYDROGEN PRODUCTION

In this paper, a summary of four key reactor types isbriefly discussed (see Reference [25] for details).

2.1. High-temperature gas-cooled reactor(HTGR)

Hydrogen production using thermochemical cycles hasbeen studied during the past four decades. The thermalenergy source for these processes has been HTGRs,

which are still under development. The maximumoperating temperature of these cycles has been limitedto about 9001C, which is the available temperaturelevel of gas-cooled nuclear reactors. The HTGR is one

of the most suitable nuclear reactors for producingthe secondary energy carrier hydrogen owing to itscapability of producing high-temperature heat at close

to 10001C in order to provide an efficient energyconversion.

2.2. Advanced gas reactor (AGR)

The AGR is a commercial thermal reactor that has

been built in the U.K. for electricity production in1550MWth units, with 14 units still in operation. TheAGR core consists of uranium oxide fuel pellets in

stainless-steel cladding within graphite blocks. Thegraphite acts as a moderator and carbon dioxide is thecoolant. The achievable temperature of the coolant atthe reactor exit during normal operation is around

6501C. The CO2 circulates through the core at4.3MPa. For future design and implementation, there

Potential thermochemical and hybrid cyclesI. Dincer and M. T. Balta

124 Int. J. Energy Res. 2011; 35:123–137 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

is a potential to increase the operating pressure of theAGR in order to couple it to a direct cycle supercriticalCO2 power conversion system. The temperature of the

reactor coolant for a future design can be driven up to7501C with new designs and analysis. This combina-tion can enable high-efficiency, economic hydrogen

production through steam electrolysis at mediumtemperatures.

2.3. Advanced high-temperature reactor(AHTR)

The molten-salt–cooled AHTR is a new reactor

concept designed to provide very high-temperature(750–10001C) heat to enable efficient low-cost thermo-chemical production of H2 or production of electricity

[26]. The AHTR uses the solid-coated particle fuel ina graphite-matrix like the modular helium reactor(MHR), but a molten-fluoride-salt as coolant. It

combines the high-temperature fuel from the HTGRwith a denser coolant for the molten salt reactor. Theproposed design temperature of the coolant at the

reactor exit is 10001C. The graphite blocks arecompatible with fluoride salts as coolant. The reactorconcept is designed for atmospheric pressure opera-tion. This design uses Ni-based high-temperature

alloys that have been similarly adopted for moltensalts. The reactor is proposed to be built in large sizes(2000MWth) with passive safety systems for decay heat

removal.

2.4. Modular helium reactor (MHR)

The MHR is a thermal reactor that can be used bothfor hydrogen and electricity production in modules of

600MWth. Its core consists of prismatic blocks ofgraphite that allow coolant flow and contains ceramicfuel. The temperature of the coolant, essentially He gas,

at the reactor exit is currently designed to achievetemperatures around 8501C. It is proposed to achieve10001C in the future within a new design, based on thesame reactor concepts, called the very high-temperature

reactor. The operating pressure of the MHR is 7MPa.The core design can provide passive safety by achievinghigh temperatures during transients and by large

thermal inertia.

3. NUCLEAR-BASED HYDROGENPRODUCTION METHODS

To produce hydrogen, the hydrogen bonds in hydro-carbons or water must be broken and hydrogen must

be separated from the reaction mixture. The mostefficient method for meeting increasing energy needscould be to convert nuclear power into electricity andhydrogen, thus providing effective and universal

energy carriers. Nuclear power plants produce heatthat can be used directly or converted to electricity for

the production of hydrogen. Four classes of H2

production options are under development (e.g. [7–9]):

� electrolysis:

� conventional electrolysis (electricity1H2O

[liquid]-H21O2)� high-temperature electrolysis (electricity1H2O

[steam]-H21O2)

� hybrid cycles (electricity1heat1H2O-[cyclicchemical reactions]-H21O2), and

� thermochemical cycles (heat1H2O-[cyclic

chemical reactions]-H21O2).

The near-term option is electrolysis. The long-termoptions involve using heat to convert water to hydro-gen and oxygen. Because heat is less expensive than

electricity (because the cost is avoided of convertingheat to electricity with associated losses), these ad-vanced processes have the long-term potential of lower

production costs.Estimates from Japan are that the cost of nuclear

thermochemical H2 production could be as low as 60%of that for nuclear H2 production by the electrolysis of

water [7]. At the most fundamental level, thermo-chemical H2 production involves the conversion ofthermal energy to chemical energy (H2), whereas elec-

trolysis involves the conversion of thermal energy toelectricity and subsequent conversion of electricity tochemical energy.

Nuclear energy provides a source of heat to produceH2. Multiple processes are being investigated to pro-duce H2 from water and heat. If nuclear energy is to be

used for H2 production, the nuclear reactor must de-liver the heat at conditions that match the require-ments imposed by the H2 production process. At thisstage of development, it is unclear which chemical

processes will be the most economic; thus, the majorcandidate technologies were examined to determine ifthey impose similar requirements on the reactor [9].

Methods for obtaining hydrogen using carbon com-pounds as the raw material will probably be the mainmethods in the near future. However, the raw materials

and ecological limitations of steam conversion ofmethane are stimulating the development of processes toproduce hydrogen from water. The most interesting ofthese methods in the context of nuclear power are elec-

trolysis and thermochemical and thermoelectrochemicalcycles. Figure 1 show a brief overview of nuclear-basedhydrogen production technologies. The main processes

for hydrogen production include steam reforming ofnatural gas, catalytic decomposition of natural gas,partial oxidation of heavy oil, coal gasification, water

electrolysis, thermochemical cycles, and photo-chemical,electrochemical and biological processes. The first fourprocesses are based on fossil fuels.

In this paper, the main focus is given to the potentialthermochemical cycles for nuclear-based hydrogenproduction as summarized below [27–30].

Potential thermochemical and hybrid cycles I. Dincer and M. T. Balta

Int. J. Energy Res. 2011; 35:123–137 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

125

3.1. Thermochemical and hybrid cycles

Several review articles and reports on thermochemical

hydrogen production have been published [14–20],including a comprehensive listing of past and presentactivities [31]. Specific categories of thermochemical

production of hydrogen have also been reviewed,including solar [23–25] and nuclear [32].Thermochemical processes include a thermal dis-

association of water into hydrogen and oxygen througha series of thermally driven chemical reactions. Overallreaction becomes

H2O! H21 12 O2 ð1Þ

The purpose is to produce hydrogen at lower tem-peratures than that for thermolysis of water, which takesplace at temperatures greater than 2500–30001C [33].

Figure 2 illustrates the reaction temperature levels(from 100 to 30001C, respectively) of thermochemicalcycles which are potentially driven by various heat

source options as some examples. This figure showsthat the number of reaction steps decreases with anincrease in the temperature. The number of reaction

steps also indicates which cycles may be coupled withsolar, nuclear or geothermal options. It is also clearthat the higher the temperature level the lower reduce

the number of steps in a cycle. One thing is also clearthat thermochemical cycles offer a great opportunity torealize hydrogen economy in a more efficient and moreenvironmentally benign manner.

In the literature, hundreds of individual cycleshave been proposed for hydrogen production duringthe past 40 years. At present, the most promising

high-temperature cycles appear to be the sulfur–iodine(S-I) cycle and the Br–Ca–Fe cycle. Also, one of themost important low-temperature cycles is copper–

chlorine (Cu–Cl) [e.g. [34]]. All these cycles were exa-mined and analyzed in terms of their thermodynamicfeasibilities. In a recent study, only 25 cycles were

found feasible by Brown et al. [13]. The specific aspectsof thermochemical water decomposition processeshave also been investigated recently. For the sulfur–iodine (S-I) cycle utilizing all fluid reagents, a higher

temperature (825–9001C) is employed for the oxygen-evolving reaction, while higher efficiencies are possible.The thermochemical S–I cycle has been studied in

more detail elsewhere [35,36]. This cycle has in factbeen fully demonstrated in both Japan and the UnitedStates and has been shown to be technically viable.

However, the commercial viability of any of thesecycles has yet to be demonstrated [37].The copper–chlorine (Cu–Cl) cycle has recently been

proven in the laboratory, and several commercially

appealing variants are being evaluated. Recently,Atomic Energy of Canada Limited (AECL) and theArgonne National Laboratory in the United States

have developed a low-temperature cycle, to accom-modate heat sources around 350–5501C. For thistemperature range, the Cu–Cl cycle appears to be one

of the most promising. This cycle has an estimatedefficiency of 40–60% at an envisioned operatingtemperature of 350–5501C. Development of the low-

temperature Cu–Cl cycle was proposed during thisearly research period. Although numerous studies havebeen conducted on Cu–Cl thermochemical cycle, onlya few recent ones are given in References [27–30,34,38].

Some landmark type studies on energy, exergy, andcost analyses of Cu–Cl thermochemical cycle drivenby nuclear process heat were performed by Orhan

et al. [27–29].Research on thermochemical hydrogen production

is undertaken in many countries [39–42]. Rosen [39]

reported that technologies for thermochemical hydrogenproduction are being investigated by several countries(Canada, Japan, United States, and France). In Canada,

technologies for thermochemical hydrogen productionare being investigated by AECL that couple with nuclearreactor. Sandia National Laboratory in the UnitedStates and the Atomic Energy Commission in France

Nuclear-basedhydrogenproduction

options

Electrolysis Thermochemical

Processes Hybrid Processes

Figure 1. Some potential options of nuclear-based hydrogen

production.

Figure 2. Number of thermochemical reaction steps, depending on cycle temperature.



DOI: 10.1002/er

are developing a hydrogen pilot plant with a sulphur–-iodine (S-I) cycle. The Korea Atomic Energy ResearchInstitute (KAERI) is collaborating with China to pro-

duce hydrogen with the 10-MW HTGR (HTR-10). TheJapan Atomic Energy Agency (JAEA) plans to completea large S-I plant to produce 60 000m3h�1 of hydrogen

by 2020, an amount sufficient for about 1 million fuelcell vehicles. Research in the United States on processesfor hydrogen production at temperatures below 5501C,including thermochemical cycles, has been reported re-

cently [33]. Balta et al. [30] investigated various geo-thermal-based hydrogen production methods, and theydefined four potential methods including hydrogen

production through thermochemical cycles.In this study, we also investigate some potential low-

temperature thermochemical and hybrid cycles for

nuclear-based hydrogen production at temperaturesbelow 5501C, due to their wider practicality, and ex-amine them as a sustainable option for hydrogenproduction using nuclear resources. Zamfirescu and

Dincer [17] and Granowskii et al. [18] have studied theuse of mechanical compression heat pumps and che-mical heat pumps as one of the potential options to

upgrade the heat to reach higher temperatures for suchthermochemical cycles.Such low-temperature thermochemical and hybrid

cycles, which can be coupled with process or waste heat

source, are listed in more detail in Table I. The simpleconcepts of low-temperature cycles are illustrated inFigure 3(a–d), respectively, which are drawn based on

some literature data and information, as reportedelsewhere [30,41,43–46].

3.1.1. The copper–chlorine cycle. As mentioned, thecopper–chlorine (Cu–Cl) cycle was originally proposedin the 1970s and has recently been proven at laboratorylevel. The Cu–Cl cycle is thus considered the most

promising low-temperature cycle and offers a numberof potential advantages over other cycles, such as [45]:

� The maximum cycle temperature (5001C) allowsthe use of a wider range of heat sources such asgeothermal, nuclear, or solar.

� The recycling chemicals are relatively safe,inexpensive, and abundant.

� All reactions have been proven in the laboratory

and no significant side reactions have beenobserved.

A simple conceptual drawing of the Cu–Cl cycle isshown in Figure 3(a). It basically consists of five mainsteps:

(a) HCl(g) production step with equipment like afluidized bed,

(b) O2 production step,

Table I. Low-temperature thermochemical and hybrid cycles.

Cycle Name

Number of

steps Name of steps Reactions

Reaction

temperatures (1C) References

1 Copper–chlorine 5 HCl production 2CuCl21H2O-CuO�CuCl212HCl 400 [41]

O2 production CuO�CuCl2-2CuCl11/2O2 500

Cu production 4CuCl1H2O-2CuCl212Cu 25–80

Drying CuCl2 (aq)-CuCl2(s) 4100

H2 production 2Cu12HCl-2CuCl1H2 430–475

2 Lithium–nitrite 3 HI production LiNO21I21H2O-LiNO312HI 25 [43]

H2 production 2HI-I21H2 425

O2 production LiNO3-LiNO211/2O2 475

3 Magnesium

Chloride

3 HCl production MgCl21H2O-2HCl1MgO 450 [44]

O2 production MgO1Cl2-MgCl211/2O2 500

H2 production 2HCl-Cl21H2 80

4 Heavy-element

halide

4 Decomposition of

UO2Br2 � 3H2O

2(UO2Br2�3H2O)-2UO3�H2O1

4HBr12H2O

300 [45]

H2 production 4EuBr214HBr-4EuBr312H2 Exothermic

Decomposition of

EuBr3

4EuBr3-4EuBr212Br2 300

O2 production 2UO3�H2O12Br214H2O-

2(UO2Br2� 3H2O)1O2

Exothermic

5 Sulphuric acid 3 H2 production 2H2O1SO2-H2SO41H2 80 [45]

SO3 production H2SO4-H2O1SO3 450

O2 production SO3-SO211/2O2 550

6 Cesium (Astrojet) 4 H2 production 2H2O12Cs-2CsOH1H2 450 [46]

CsO2 production 2CsOH13/2O2-2CsO21H2O 250

O2 production 2CsO2-Cs2O13/2O2 450

O2 and Cs production Cs2O-2Cs11/2O2 —



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127

(c) Cu production step,

(d) drying step, and finally(e) hydrogen production step.

These chemical reactions form a closed internal loop

that recycles all of the copper–chlorine compounds ona continuous basis, without emitting any greenhousegases externally to the atmosphere [41]. One of thereactions in this Cu–Cl cycle is electrochemical, which

affects a considerable amount of energy cost. However,the electrolytic step requires voltages significantlylower than needed for direct water electrolysis. Petri

et al. [33] mentioned that copper–chlorine cycle hasbeen examined at Argonne National Laboratory,consisting of three thermal reactions and one electro-

lytic reaction [38]. Experimental work has been per-formed at Argonne National Laboratory to study thereaction kinetics for the hydrogen and oxygen pro-

duction reactions. The experiments were conducted inbeds of solid material with a continuous flow of excessgaseous reactants. The individual steps in the Cu–Clcycle have been confirmed, the kinetics of the hydrogen

and oxygen generation reactions have been studied,and the temperatures of the reaction steps have beenverified (e.g. [33]).

3.1.2. The lithium–nitrite cycle. The Argonne NationalLaboratory has initially proposed this cycle, which

consists of three main steps: (i) hydrogen iodide (HI)production step, (ii) hydrogen production step, and(iii) oxygen production step. A schematic representa-tion of this cycle is given in Figure 3(b). As can be seen

in this figure, lithium nitrite is oxidized by iodine atabout 251C to produce lithium nitrate and HI, so that

HI produces through thermal dissociation and lithiumnitrate produces oxygen through thermal decomposi-tion, respectively. The oxidation of lithium nitrite by

iodine involved in this cycle, however, can hardly bedescribed as proceeding smoothly [42]. The first stepproceeds slowly and similar to the well-studiedoxidation of sulphurous acid by iodine to form the

sulphate ion and HI. There is no reason to believe thereaction will not go as suggested by Abraham andSchreiner [43]. The second step in the cycle, the thermal

decomposition of HI, is well known. At the tempera-ture indicated 23% dissociation will occur. Separationof HI from the mixture can easily be accomplished by

thermal means such as distillation [43]. The third stepof the reaction may possibly produce LiO2, which isvery corrosive compound, under certain reaction

conditions [47]. This cycle has not yet been developedto the extent of commercial application yet. Furtherdetails and references to the chemistry of the reactionson this cycle can be found elsewhere (e.g. [48]).

3.1.3. The magnesium–chloride cycle. The magne-sium–chloride (Reverse Deacon Cycle) has been

developed by Simpson et al. [44] at Idaho NationalLaboratory and Argonne National Laboratory. Themagnesium-chloride cycle basically consists of three

reactions: two thermochemical and one electrolytic. Theconcept of this cycle is illustrated in Figure 3(c). Thethermochemical reactions sum to the reverse Deaconreaction. The electrolytic step involves the electrolysis of

Figure 3. Illustrations of four potential cycles (see details in Reference [30]).



DOI: 10.1002/er

anhydrous HCl instead of aqueous HCl. Motupally

et al. [49] specified that anhydrous HCl electrolysisrequires significantly less electrical energy than aqueousHCl electrolysis. This cycle steps are defined by Petriet al. [33]. They mentioned that HCl was successfully

generated through step 1, while step 2 has not beentested, but is thermodynamically possible. Highertemperatures, though, may be required to eliminate

undesirable side steps. Step 3 has been demonstratedand optimized by Weidner’s team at the University ofSouth Carolina. Further proof-of-principle tests must be

run to establish the viability of this process. Further-more, Simpson et al. [44] analyzed preliminarily theefficiency aspects. The proof-of-concept and lab-scale

experiments have been performed. Based on their resultsand temperature range of the reactions, one mayindicate that the cycle is a promising route to low-temperature hydrogen production although there is no

any ongoing work in this regard.

3.1.4. The hybrid sulphur (sulphuric acid) cycle. The

hybrid sulphur cycle (sulphuric acid cycle) has beendeveloped by the JAEA. This cycle mainly consists ofthree steps: (i) hydrogen production step, (ii) sulphur

trioxide (SO3) production step and (iii) oxygen produc-tion step. The steps 1 and 3 are electrolytic reactions.Schematic concept of this cycle can be seen in Figure 3(d).

The system is based on a sulphuric acid (H2SO4) process,which was developed as the Westinghouse process. Thesulphur trioxide (SO3) decomposition process is facilitatedby electrolysis using a solid electrolyte that conducts

oxygen ions. By this way, the temperature of the step canbe reduced approximately to 2501C compared with theWestinghouse process. The details about the cycle may be

found elsewhere [45]. The theoretical thermal efficiency ofthe system based on chemical reactions depending on theH2SO4 concentration and heat recovery was in the range

of 35–55%. In the Argonne National Laboratory,improved SO3 electrolysis cells have been developed tolower the needed voltage and increase overall efficiencyfor step 3. There is a remarkable potential to produce

hydrogen using this cycle as the cycle operates at lowtemperature.

3.1.5. The U-Eu-Br heavy-element halide cycle. Thecycle is based on heavy-element halide chemistry with amaximum reaction temperature of 3001C. Heavy-

element halide cycle is the lowest known temperaturefor a purely thermochemical hydrogen production.The heavy-element halide cycle essentially consists of

four steps. The steps 1 and 3 are endothermic, whereassteps 2 and 4 are exothermic. This cycle was performedat Argonne National Laboratory to determine the eachstep (e.g. [45]).

� Step 1: Determine the chemical productsthat result from thermal decomposition ofUO2Br2 � 3H2O.

� Step 2: Investigate and model the factors thatinfluence the reaction of Eu21 ions with H1 ionsin aqueous hydrobromic acid to generate H2 gas.

� Step 3: Study the thermal reduction of EuBr3 toEuBr2 and establish the degree of completionat 3001C and whether a potentially interfering

EuOBr impurity is produced;� Step 4: Determine the chemical consequences of

reacting hydrated uranium trioxide (UO3 �H2O(s)) with an excess amount of ‘bromine water’ (Br2dissolved in H2O).

As a result, thermodynamic data of this systemare not well known. An engineering application of

the heavy-element halide cycle needs to improve forcorrosiveness of the chemicals. The low operatingtemperature of 3001C cycle can be more applicableto nuclear-based hydrogen production systems, rather

than for higher temperature cycles.

3.1.6. The astrojet central cycle. One of the potential

methods for thermochemical hydrogen productioninvolves the use of cesium through the astrojet centralcycle [46]. This thermochemical cycle consists of four

steps with a maximum reaction temperature of 4501C.It basically consists of four chemical reactions as theirsteps, namely;

(a) hydrogen production,(b) CsO2 production,(c) oxygen production, and(d) oxygen and cesium production.

This cycle is technically a closed cycle, in whichwater is thermally decomposed and all the otherchemicals are recycled. The ideal hydrogen production

efficiency for this cycle was estimated as 66% [50],where the efficiency was defined as the higher heatingvalue (HHV) of combustion of H2 (�286 kJmol�1)

divided by the external thermal heat supplied to theprocess per gram mole of H2 produced. A practicalefficiency of 45% was estimated by [50] for the processbased on the thermal dissociation of Cs2O. It is

important to note that if dissociation and separationwere accomplished by the high-temperature electrolysisof molten Cs2O, the efficiency would be calculated to

be 31.6% [50].

4. THERMODYNAMIC ANALYSIS

Efficient use of energy is a significant contributor to any

sustainable plan for meeting growing energy demands.Therefore, it is essential to evaluate the technologies fornuclear-based hydrogen production in terms of energy

and exergy efficiencies. The overall efficiencies of thecycles studied for utilizing nuclear process/water heat forthis purpose depend on parameters such as operating



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129

temperature, recycling ratio and heat loss. We thermo-dynamically analyze four low-temperature cycles in thisstudy for comparison purposes.

In the thermodynamic analysis, we consider 1 kmolof hydrogen produced per each cycle considered;hence, all quantities are provided in terms of per kmol

of hydrogen produced. In addition, we assume thefollowing:

� The values for the reference environment (deadstate) temperature (T0) and pressure (P0) are 251C

and 1 atm, respectively.� In all chemical reactions, reactants and products

are at the reaction temperature and a pressure of

1 atm.� All processes are considered steady-state and

steady-flow with negligible potential and kineticenergy effects.

The heat loss through the cycle is assumed as 30%.For a general steady-state, steady-flow process, the

three balance equations, namely mass, energy, and

exergy balance equations, are employed to find thework input, the rate of exergy destruction, energy andenergy efficiencies.As mass is conserved in chemical reactions, the mass

of products and reactants are equal and in general, themass balance equation can be expressed in the rateform asX

_min ¼X

_mout orX

_mR ¼X

_mP ð2Þ

where _m is the mass flow rate, and the subscript instands for inlet and out for outlet.For a steady-state process, the general energy

balance can be expressed below as the total energyinput equal to total energy output:X

_Ein ¼X

_Eout ð3Þ

The heat transfer for a chemical process involving

no work interaction W is determined from the energybalance

P _Ein ¼P _Eout applied to the system with

W5 0. For a steady-state reaction process, the energy

balance reduces to

Q ¼X

nPð �hof 1 �h� �hoÞP �X

nRð �hof 1 �h� �hoÞR ð4Þ

and the exergy balance for the steady-state process,involving chemical reactions, becomesX

_Exin ¼X

_Exout ð5Þ

where X_Exout ¼ _ExH2

1 _Exd1 _Exl ð6Þ

The exergy associated with a process at specified

state is the sum of two contributions: physical andchemical. Thus, the specific exergy of the process iscalculated by

ex ¼ ðh� h0Þ � T0ðs� s0Þ1exch ð7Þ

where h is enthalpy, s is entropy, and the subscript zeroindicates properties at the reference (dead) state of P0

and T0.

The exergy rate is calculated by

_Ex ¼ _mex ð8Þ

Combining Equations (6) and (7) yields

exd ¼X½ðh� h0Þ � T0ðs� s0Þ1exch�in

�X½ðh� h0Þ � T0ðs� s0Þ1exch�out

1 1�T0

Tr

� �Q ð9Þ

After writing mass, energy, and exergy balancesfor the system, enthalpy values of compounds areevaluated using the Shomate equations [51] as follows:

�h� �h0 ¼A � T1B �T2

21C �

T3

31D �

T4

4

� E �1

T1F�H ð10Þ

�s ¼A � lnðTÞ1B � T1C �T2

21D �

T3

3

� E �1

2T21G ð11Þ

where T is 1/1000 of the specified temperature (in K)

of compound and A, B, C, D, E, F,G, and H areconstants, as given in Table II for each compound.With the specific enthalpy and entropy values, we

can calculate the specific chemical exergy exch value ofeach compound. The chemical exergy based on atypical exergy reference environment exhibiting stan-

dard values of the environmental temperature T0 andpressure P0 such as 251C and 1 atm is called standardchemical exergy. The values of the standard chemicalexergies for the reactants and products are taken from

the literature [52], as listed in Table III. However, thereis no information about LiNO2 and LiNO3 in thisRef. [52]; therefore, these data are assumed as constant

and taken from [53].To determine the standard chemical exergy of any

substance not present in the environment, we consider

the reaction of the substance with other substances forwhich the standard chemical exergies are known, andwrite

exch ¼ �DG1XP

nexch �XR

nexch ð12Þ

where DG is the change in Gibbs function for the

reaction, regarding each substance as separate attemperature T0 and pressure P0. The other two termson the right side of Equation (12) are evaluated using

the known standard chemical exergies, together withvalues of n, which denotes the moles of these reactantsand products per mole of the substance whose chemi-

cal exergy is being evaluated.The energy efficiency of the overall cycle can be

defined as the total energy output of hydrogen to the



DOI: 10.1002/er

Tab

leII

.E

nth

alp

yof

form

ation,

refe

rence

entr

opy,

and

Shom

ate

consta

nts

for

chem

icalcom

pounds.

Com

pound

� ho f(k

Jkm

ol�

1)

� s 0(k

Jkm

ol�

1K�

1)

AB

CD

EF

GH

Cl 2

(g)

0223.0

833.0

506

12.2

294

�12.0

651

4.3

8533

�0.1

59494

�10.8

348

259.0

29

0

Cu

(s)

033.1

717.7

2891

28.0

987

�31.2

5289

13.9

7243

0.0

68611

�6.0

56591

47.8

9592

0

H2

(g)

0130.6

833.0

661

�11.3

634

11.4

32816

�2.7

72874

�0.1

58558

�9.9

80797

172.7

079

0

O2

(g)

0205.0

729.6

59

6.1

37261

�1.1

86521

0.0

9578

�0.2

19663

�9.8

61391

237.9

48

0

I 2(g

)62

420

260.6

937.7

9763

0.2

25453

�0.9

12556

1.0

34913

�0.0

83826

50.8

6865

305.9

199

62.4

211

I 2(l)

13

520

150.3

680.6

6919

6.8

557E�

08

�8.7

24E�

08

3.7

231E�

08

4.7

3583E�

10

�10.5

2782

247.9

798

13.5

2302

HI

(g)

26360

206.5

926.0

454

4.6

89678

4.9

11765

�2.6

54397

0.1

21419

18.7

5499

237.2

018

26.3

5903

H2O

(l)

�285

830

69.9

5�

203.6

06

1523.2

9�

3196.4

13

2474.4

55

3.8

55326

�256.5

478

�488.7

163

�285.8

304

H2O

(g)

�241

830

188.8

430.0

92

6.8

32514

6.7

93435

�2.5

3448

0.0

82139

�250.8

81

223.3

967

�241.8

264

CuC

l 2(s

)�

205

850

108.0

670.2

1882

23.3

6132

�14.8

6876

4.0

53899

�0.3

66203

�228.9

405

184.6

378

�205.8

532

CuO

(s)

�156

060

42.5

948.5

6494

7.4

98607

�0.0

5598

0.0

13851

�0.7

60082

�173.4

272

94.8

5128

�156.0

632

CuC

l(l)

�131

180

93.7

566.9

44

�3.6

9E�

10

2.1

6E�

10

�3.9

0E�

11

�9.8

1E�

12

�151.1

374

174.7

653

�131.1

78

CuC

l(s

)�

138

070

87.0

475.2

71

�26.8

3212

25.6

9156

�7.3

57982

�1.8

47747

�165.7

299

174.6

644

�138.0

72

HC

l(g

)�

92

310

186.9

32.1

2392

�13.4

5805

19.8

6852

�6.8

53936

�0.0

49672

�101.6

206

228.6

866

�92.3

1201

MgO

(s)

�601

240

26.8

547.2

5995

5.6

81621

�0.8

72665

0.1

043

�1.0

53955

�619.1

316

76.4

6176

�601.2

408

MgC

l 2(s

)�

641

620

89.6

278.3

0733

2.4

35888

6.8

58873

�1.7

28967

�0.7

29911

�667.5

823

179.2

639

�641.6

164

H2S

O4

(g)

�735

130

298.7

847.2

8924

190.3

314

�148.1

299

43.8

6631

�0.7

40016

�758.9

525

301.2

961

�735.1

288

SO

3(g

)�

395

770

256.7

724.0

2503

119.4

607

�94.3

8686

26.9

6237

�0.1

17517

�407.8

526

253.5

186

�395.7

654

SO

2(g

)�

296

840

248.2

121.4

3049

74.3

5094

�57.7

5217

16.3

5534

0.0

86731

�305.7

688

254.8

872

�296.8

422

Source:

Adaptedfrom

[51].



DOI: 10.1002/er

131

total energy input by

Zoverall ¼ð1� rÞ _mH2

LHVH2

_We;in1P _Qin

ð13Þ

where LHVH2is the lower heating value per kmol of

hydrogen and r is the total recycling ratio of thechemical reaction, defined by the number of kmols of

unreacted substances over the number of kmols ofsupplied reactants. The lower heating value of hydro-gen is taken as 239.92MJkmol�1 H2.

An exergy balance can be used in formulating exergyefficiency for the cycles. At steady state, the rate atwhich exergy enters the cycles equals to the rate atwhich exergy exits plus the rate at which exergy is

destroyed within the system. We assume a heat loss of30% occurred.The exergy efficiency of the overall cycle is defined,

as the ratio of the exergy output of the hydrogen (H2)to the total exergy input required for the cycle, asfollows:

coverall ¼ð1� rÞ _ExH2

_We;in1P _Exin

ð14Þ

Here, we also investigate some parameters aboutsustainability and system improvement potential.

These are expected to give a comparative perspectiveabout the process and application and help people tosee how they will contribute to the environment and

sustainable development.In this regard, sustainable development requires not

only that the sustainable supply of clean and afford-

able energy resources be used but also the resourcesshould be used efficiently. Exergy methods are veryuseful tools for improving efficiency, which maximize

the benefits and usage of resources and also minimizethe undesired effects (such as environmental damage).Exergy analysis can be used to improve the efficiency

and sustainability [54]. Rosen et al. [55] defined arelation between exergy efficiency (c) and the sus-tainability index (SI) as

c ¼ 1�1

SIð15Þ

SI ¼1

1� cð15aÞ

which shows how sustainability increases with theexergy efficiency of a process increases.

Furthermore, Van Gool [56] proposed that maxi-mum improvement in the exergy efficiency for a pro-cess or system is obviously achieved when the exergyloss or irreversibility rates ð _Exin � _ExoutÞ are mini-

mized. Consequently, he suggested that it is usefulto employ the concept of an exergetic ‘improvementpotential’ when analyzing different processes or sectors

of the economy. This improvement potential in the rateform, denoted by I _P, is given by

I _P ¼ ð1� cÞð _Exin � _ExoutÞ ð16Þ

5. RESULTS AND DISCUSSION

In this paper, some significant low-temperature thermo-chemical and hybrid cycles, are listed in Table I, arereviewed and examined for potential applications with

nuclear thermal energy and hence analyzed thermo-dynamically through energy and exergy. The parametricstudies with variable recycling ratios and heat lossesare performed, and some critical results are given in

Figures 4 and 5. These cycles appear to play somecrucial role in getting integrated with nuclear-basedprocess and waste heat for hydrogen production, which

is particularly aimed to help hydrogen economy. Usingenergy and exergy balances, both energy and exergyefficiencies of the process cycles are calculated. Figure 4

shows the energy (a) and exergy (b) efficiencies asso-ciated with various recycling ratios from 0 to 0.9.An important parameter is recycling ratio that

affects the cycle’s energy and exergy efficiencies.However, there is no enough information about therecommended recycling ratios of the analyzed cycles inthe open literature. In this context, we performed a

parametric study to investigate the cycle performancefor a range of practical recycling ratio. Of course, thesewill provide an application possibility for future.

Therefore, in this study, we investigate how theenergy and exergy efficiencies in the studied cyclesvary with changing recycling ratio. The heat flow for

the cycles per kmol of hydrogen is calculated fromEquation (5). The energy efficiencies of the eachcycle are determined to about 50% based on complete

reaction and lower heating value (LHV) usingEquation (13). In this calculation, the auxiliary worksare not considered.

Table III. Gibbs free energy of formation and standard chemical

energy for each compound.

Compounds

Specific Gibbs free energy

of formation (kJ kmol�1)

Standard chemical

exergy (kJ kmol�1)

H2O (g) �228 638 9437

H2O (l) �237 170 920

CuCl2 (s) �161 689 94 510

HCl (g) �95 296 84 549

CuO(s) �128 312 6273

CuCl (l) �115 994 78 406

CuCl (s) �120 884 73 516

MgO (s) �568 950 62 405

MgCl2 (s) �592 113 160 857

H2SO4 (g) �691 375 197 630

SO3 (g) 371 069 241 936

SO2 (g) �300 144 310 876

HI (g) 37 453 242 868

Source: Reference [52].



DOI: 10.1002/er

If we continue elaboration on the results, we can

indicate that, as introduced above, Figure 4 shows theinfluence of total recycling ratio on energy (a) andexergy (b) efficiencies, respectively. The followingcycles are included in the analysis presented in the

figure: Cu–Cl, LiNO2, Mg-Cl and H2SO4. The highestenergy efficiency, such as 51%, is obtained by Cu–Cland H2SO4 cycles. However, the exergy efficiency of

Cu–Cl cycle is better than that of H2SO4 cycle with65 vs 57%. Also, it can be seen in Figure 4, the cycleexergy efficiencies are slightly higher than their corres-

ponding energy efficiencies, which are calculated basedon LHVH2. This difference may be caused by the effectof inlet heat in Equation (13) and inlet exergy in

Equation (14). In these two equations, LHV of hydro-gen is very close to its exergy concept as inlet exergyincludes also the chemical exergy of compounds, andalso inlet works are the same in both equations.

However, inlet heat in Equation (13) is higher thaninlet exergy in Equation (14) as inlet exergy includes

also the exergetic temperature factor. Thus, exergy

efficiency is higher than energy efficiency.Figure 5 illustrates the effects of the heat loss on the

overall Cu–Cl cycle energy (a) and exergy (b) effi-ciencies. The energy efficiency values for the Cu–Cl

cycle vary between 4 and 60%, whereas the corres-ponding exergy efficiency values for the same tem-perature range from 5 to 77%, respectively. Also, it is

clear in this figure that both energy and exergy effi-ciencies decrease with the recycling ratio from 0 to 0.9.As can be seen in Figure 4, for the Cu–Cl cycle the

energy (a) and exergy (b) efficiency values vary bet-ween 5–51% and 6–65%, respectively, assuming 30%of input energy as heat loss from the cycle while the

recycling ratio increases from 0 to 0.9. It is importantto note that the Cu–Cl cycle efficiency values are in agood agreement with those reported by Rosen andScott [57] and Yildiz and Kazimi [58]. In the present

study, the energy efficiency of thermochemical hydro-gen production is found to be less than that of the

0

10

20

30

40

50

60

70

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

En

ergy

Eff

icie

ny

(%)

Recycling Ratio

0.1 Qloss0.2 Qloss

0.3 Qloss0.4 Qloss0.5 Qloss

0

10

20

30

40

50

60

70

80

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Exe

rgy

Eff

icie

ncy

(%

)

Recycling Ratio

0.1 Qloss

0.2 Qloss

0.3 Qloss

0.4 Qloss

0.5 Qloss

(a) (b)

Figure 5. Variation of energy (a) and exergy (b) efficiencies with recycling ratio for the Cu–Cl cycle.

Figure 4. Energy (a) and exergy (b) efficiencies of cycles versus recycling ratio.



DOI: 10.1002/er

133

SMR technology [58]. But nuclear-based hydrogenproduction using thermochemical cycles appears to bea promising solution for the future hydrogen economy

to be run on renewable, and it will help to reduceenvironmental impact (greenhouse gas emissions) andincrease sustainable development. Therefore, copper–

chlorine cycle has been identified as a highly promisingoption among all the cycles considered for thermo-chemical hydrogen production.Figure 6 shows the effects of recycling ratio on the

sustainability index of the four considered cycles. It isclearly seen in this figure that the sustainability indexof the analyzed cycles decreases exponentially with the

recycling ratios. In this regard, at lower recyclingratios, the corresponding sustainability of cyclesincreases. The highest sustainability index, such as

2.85, is obtained by Cu–Cl cycle, which is based oncomplete reaction. The sustainability index for theanalyzed cycles varies from 1.0 to 2.85 with therecycling ratios varying between 0 and 0.9.

Van Gool’s improvement potential on the ratebasis (I _P) given in Equation (16) is calculated for the

analyzed low-temperature thermochemical cycles andgiven in Figure 7. It is found that the H2SO4 cycle hasthe highest I _P value with about 78 000 kW, followed by

Li–NO3, Mg–Cl and Cu–Cl with about 60 500, 60 400and 42 000 kW, respectively.In summary, some key advantages about the Cu–Cl

cycle can be listed as follows:

� completed proof-of-principle experiments for all

of the steps,� maximum temperature requirement of the cycle

about 5001C,� thermodynamic data of this cycle which are well

known,� relatively safe, inexpensive, and abundant recycling

chemicals,

� A relatively high-efficiency operation (about40–50%) at an envisioned operating temperatureof 350–5001C.

� conceptual process design that uses commerciallypracticed processes.

Other cycles, e.g. the lithium–nitrite cycle, themagnesium–chloride cycle, the heavy-element halide

cycle and the sulphuric acid cycle have some value andpromise for future. Although no more information oncycles 4 and 6 as listed in Table I is available, we still

feel that they have some potential for hydrogen pro-duction using nuclear-based process/waste heat, butrequire further studies and experiments.

5. CONCLUSIONS

In this paper, some potential thermochemical andhybrid cycles for nuclear-based hydrogen production

have been discussed, with some key focus on theiroperating conditions, temperature ranges, cycle pheno-mena and performance aspects, and a brief perfor-mance analysis through energy and exergy efficiencies

has been presented for comparison purposes. Somefindings may be summarized as follows:

� The energetic and exergetic efficiencies of theCu–Cl cycle are obtained to be 51 and 65%,respectively, based on the complete reactions.

� The recycling ratio has an important effect onthe energy and exergy efficiencies of the cycles.As it increases, energy and exergy efficiency values

decrease.� Low-temperature thermochemical and hybrid

cycles for hydrogen production, which can be

coupled with nuclear option, should be developedand comprehensively analyzed in terms of theirthermodynamic feasibilities.

� The sustainability index for the low-temperature

thermochemical cycles varies from 1.0 to 2.85with the recycling ratios varying between 0 and

0

20000

40000

60000

80000

100000

120000

140000

0

Imp

rove

men

t P

ote

nti

al R

ate

(kW

)

Recycling Ratio

0.1 0.2 0.3 0.4 0.5

Figure 7. Variation of improvement potential with recycling

ratio for various thermochemical cycles.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

Su

stai

nab

ility

Ind

ex

Recycling Ratio

LiNO

CuCl

MgCl

H SO

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Figure 6. Variation of sustainability index with recycling ratio for

various thermochemical cycles.



DOI: 10.1002/er

0.9. The highest sustainability index, such as 2.85,is obtained by Cu–Cl cycle, which is based oncomplete reaction.

� The Cu–Cl cycle is one of the most pro-mising ways to produce hydrogen efficiently,without emitting any greenhouse gases to the

atmosphere.

NOMENCLATURE

_E 5 energy rate (kW)ex 5 specific exergy (kJ kg�1)

exch 5 specific chemical exergy (kJ kg�1)_Ex 5 exergy rate (kW)G 5Gibbs function (kJ)

h 5 specific enthalpy (kJ kg�1)h 5 specific enthalpy (kJ kmol�1)ho

5 specific enthalpy at reference state

(kJ kmol�1)ho

f 5 specific enthalpy of formation(kJ kmol�1)

I _P 5 improvement potential rate (kW)LHVH2

5 lower heating value of H2 (MJkg�1)_m 5mass flow rate (kg s�1)n 5 number of moles (kmol)

Q 5 heat (kJ)r 5 recycling ratio(–)s 5 specific entropy (kJ kg�1K�1)

SI 5 sustainability index(–)T 5 temperature (K or 1C)W 5work (kJ)

Greek symbols

Z 5 energy efficiency

c 5 exergy efficiencyDG 5 change in Gibbs function for a

reaction (kJ)

Subscripts

d 5 destruction

e 5 electricityin 5 input, inletl 5 loss

out 5 output, outletP 5 productR 5 reactants 5 system

0 5 reference or dead state

ACKNOWLEDGEMENTS

The author gratefully acknowledge the support pro-vided by the Ontario Research Excellence Fund andthe Natural Sciences and Engineering Research Coun-cil of Canada.

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DOI: 10.1002/er

137

Dincer_Potential Thermochemical and Hybrid Cycles for Nuclear-based Hydrogen Production

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