A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production

Energy Conversion and Management 205 (2020) 112182

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A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production

T

Farid Safari , Ibrahim Dincer ⁎

Clean Energy Research Laboratory, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1G 0C5, Canada

ARTICLE INFO

ABSTRACT

Keywords: Energy Efficiency Hydrogen production Thermochemical cycles Environmental impact Sustainability

Thermochemical cycles are considered as promising technologies for long-term, large-scale hydrogen production in a clean and sustainable manner. In this study, various hydrogen production methods through thermochemical and hybrid cycles, including two step (Zinc oxide), Three step (Sulfur-Iodine), four-step (Iron-Chlorine, Magnesium-Chlorine and Copper-Chlorine) and hybrid types (Hybrid Sulfur) are discussed and comparatively evaluated. In this regard, major challenges, recent advancements and future directions of these methods are presented and discussed for design, analysis and assessment purposes. Moreover, a comparative evaluation of the selected thermochemical cycles is extensively performed based on the cycle’s energy and exergy efficiencies, hydrogen production cost and global warming potential (GWP). A comparative study shows that vanadiumchlorine offers the highest exergy efficiency of 77% while in terms of GWP, Sulfur-Iodine and hybrid sulfur cycles become the most promising with GWP of 0.48 and 0.50 kg CO2·eq/kg H2, respectively. The hybrid Cu-Cl cycle offers a great potential in terms of integration with nuclear heat or industrial process heat or renewable or waste heats.

1. Introduction Nowadays, 80% of the world’s primary energy is supplied from fossil fuels [1]. Among this, 32% belongs to oil which is still the largest primary fuel for transportation. According to 2015 energy technology prospective report of International energy agency (IEA), in order to limit the increase of global temperature within 2 °C, CO2 emission related to energy and industrial processes should be decreased by approximately 60% [2]. There are some solutions for reducing environmental impacts of energy related processes such as carbon capture and storage (CCS) chemical looping carbon capture, heat decarbonization and so on [3]. However, although carbon capture can save environmental cost, it cannot be a long-term solution for sustainable development in the context of energy. Hence, alternative carbon-free fuels, such as hydrogen and ammonia are considered as long-term carbon-free solutions for energy sustainability and combating climate change [4]. Hydrogen is regarded as the alternative energy carrier of the future due to the higher energy density on a mass basis, less environmental problems, its abundant presence in different forms in the universe, and its convertibility into electricity or useful chemicals. It is the lightest element in the universe which is without taste, color, odor, and non-

⁎

toxic under normal conditions and has heating values of 2.4, 2.8 and 4 times higher than those of methane, gasoline and coal, respectively [5]. As shown in Fig. 1, hydrogen is a key chain between hydrogen consuming industries such as ammonia and ethanol production plants and some important sectors such as electricity grid, gas grid, transportation, residential, agriculture and energy storage [6]. Hydrogen has an integration role between these sectors while improves the performance of electricity grid. Hydrogen can be efficiently converted into electricity, and vice versa [7]. It can be produced from renewable materials such as biomass and water, and most importantly, it is environmentally friendly at all processes utilizing hydrogen [8]. Hydrogen council has investigated hydrogen’s long-term potential for energy supply and different pathways of its deployment. A recent study by the Hydrogen Council [9] presents a comprehensive assessment of hydrogen’s long-term potential and a roadmap for its deployment. The study envisages that 18% of the global energy demand (equal to 78 EJ) can be supplied by hydrogen in 2050. In 2019, current demand for pure hydrogen is 70 million tons (Mt). Another 45 Mt of demand is related to the gas mixture products such as syngas where hydrogen is a part of a mixture (not in a pure form). Fig. 2 exhibits the current statistics for hydrogen production

Corresponding author.

https://doi.org/10.1016/j.enconman.2019.112182 Received 8 August 2019; Received in revised form 10 October 2019; Accepted 11 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature AD AECL ANL CCS CEA CERL CNRS CSIRO Co-Cl Cu-Cl DLR DOE GA GWP IAEA IEA INET Fe-Cl FSEC HyS

HySA HTE LCA LHV LTE Mg-Cl MOx NAEC NREL ORNL PEM PEWS PTG S-I SCWG SNG SNL SRNL SMR STCH T TCC TWSC V-Cl

Anaerobic Digestion Atomic Energy of Canada Limited Argonne National Laboratory Carbon Capture and Storage French Alternative Energies and Atomic Energy Commission Clean Energy Research Laboratory French National Center for Scientific Research Commonwealth Scientific and Industrial Research Organisation Cobalt-chlorine Copper-chlorine German Aerospace Center Unites States Department of Energy General Atomic Global Warming Potential International Atomic Energy Association International Energy Agency Institute of Nuclear and New Energy Technology Iron-Chlorine Florida Solar Energy Center Hybrid Sulfur

Hybrid Sulfur Ammonia High Temperature Electrolysis Life Cycle Assessment Low Heating Value Low Temperature Electrolysis Magnesium-Chlorine Metal oxide National Atomic Energy Commission National Renewable Energy Laboratory Oak Ridge National Laboratory Proton Exchange Membrane Photoelectrochemical Water Splitting Power to Gas Sulfur-Iodine Supercritical Water Gasification Synthetic Natural Gas Sandia National Laboratories Savnnah River National Laboratory Steam Methane Reforming Solar Thermochemical Hydrogen Temperature Thermochemical Cycle Thermochemical Water Splitting Cycle Vanadium-Chlorine

Fig. 1. Importance of hydrogen in integrating different energy sectors. Reproduced from [6]

worldwide. As shown, an overwhelming majority of produced hydrogen comes from the conversion of fossil fuels while only less than 1% of hydrogen is produced from renewable sources [10]. Noting that 85% of hydrogen is produced in dedicated hydrogen production facilities where hydrogen is the primary product and the 15% rest is produced in the processes where hydrogen is a by-product. Only 0.3% of by-product hydrogen is coming from renewable sources [10].

Although there have been a lot of development in the past few decades in advancing hydrogen economy, there are still some challenges such as scarcity of hydrogen in its pure form in the atmosphere, complexity of production and purification, highly cost and reliability of its storage. Some of the key strategies, in addition to what is recommended by IEA [10], for scaling up works about hydrogen technologies are summarized as follows:

2

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production methods can be classified either based on the type of material which hydrogen is produced from, or the type of the process driving energy source [11]. As seen in Fig. 3a, hydrogen containing sources are renewable and non-renewables. Although the fossil material-based hydrogen production is necessary to be investigated and improved, the main approach is going to be green methods for clean hydrogen production from biomass and water using renewable electricity or heat from nuclear and solar energy. As shown in Fig. 3b, in terms of process driving energy sources, the type of energy can be thermal, biological, mechanical, electrical or photonic. Life cycle assessment is necessary for evaluating the routes of hydrogen production and selection of the best possible option. Technologies for hydrogen production are listed in Table 1 and categorized by the type of material source of hydrogen, driving energy of the process and the estimated hydrogen production capacity whether it is small scale (more than 50 t/day), medium scale (more than 100 t/day) and large scale (more than 500 t/day) [12]. Hydrogen production methods can be categorized by the maturity of the technology and their importance for a long-term and sustainable hydrogen production as well. Steam methane reforming (SMR) which contributes to the majority of world’s hydrogen production, has high global warming potential However, this method among with coal gasification are the current industrially established processes for hydrogen production due to the lower cost (less than 2$/kg H2) among other methods. A comparison between SMR technology as a near term hydrogen production technology with biomass gasification and windbased electrolysis as mid-term technologies and thermochemical cycles and PEC as long-term technologies is presented in Figs. 4 and 5 based on the recently published LCA reports and estimated central values by [13–17]. As shown, although nuclear-based TCC and photoelectrochemical water splitting (PEWC) methods have significantly lower GWP compared to SMR and biomass gasification, however, the cost of hydrogen produced by these methods is not yet competitive with SMR. Moreover, as the efficiency of solar technologies are relatively low, improvement in materials and development of catalysis are necessary for efficiency improvement. However, Since it is important to produce hydrogen with minimum GWP and environmental impact, the emerging low emission technologies such as wind-based electrolysis, photonic water splitting, along with nuclear and solar based thermochemical water splitting cycles are promising technologies which can be the ultimate near zero emission technologies for hydrogen production in the long term (Fig. 5) [17]. Nuclear-based thermochemical cycle can be a promising option for low emission and efficient hydrogen production where thermal energy of nuclear reactors is available. On the other hand, solar pathways are not cost-effective yet due to the difficult heat transfer from the sunlight to the hydrogen source. However, due to the very low environmental impact and reliability of the energy source, solar pathways are regarded as long-term pathways for hydrogen production. Fig. 6 illustrates the pathway toward green hydrogen. In this

Fig. 2. Current status of worldwide hydrogen production by source (data from [10]).

• The role of hydrogen in the long-term energy scenarios should be considered. • Adequate solutions are required to overcome investment risks of the new customers. • Further developmental and commercialization studies are required for better cost effectiveness. • The technical and commercial barriers should be eliminated. • The codes and standards should be developed. • Full international collaboration is necessary for better practices and implementations.

Development and scaling up hydrogen production technologies such as thermochemical water splitting cycles (TWSCs) in an integrated and sustainable manner for large-scale hydrogen production can foster hydrogen economy and sustainable energy development. In this paper, after an overview of hydrogen production methods, a perspective of hydrogen economy and innovative technologies are discussed. Next, thermochemical cycles (TCCs) as promising routes for large scale hydrogen production are reviewed and categorized in terms of the number of steps and energy sources. Moreover, some of the most promising TCCs are further elaborated and the state of the art, challenges, and limitations are discussed. A comparative study and efficiency evaluation are also performed for the abovementioned cycles based on energy and exergy efficiencies, cost and environmental impact. Finally, the opportunities for further development of thermochemical cycles are available for sectoral applications. 2. Hydrogen production methods Hydrogen production systems are to be designed and developed for production of hydrogen from hydrogen containing resources. Hydrogen

Fig. 3. Hydrogen production methods: a) material source based b) process driving energy source based. 3

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Table 1 Overview of hydrogen production methods with respect to material source, energy source and estimated production scale. Hydrogen production method

Electrolysis (Grid) Electrolysis (Wind) High temperature electrolysis Thermolysis Thernaochemical water splitting Photoelectochemical water splitting Sonochemical water splitting Bio-photolysis PYriliess Gasification (Biomass) Supercritical water gasification Dark fermentation Microbial fermentation Bio hydrolysis-oxidation Steam methane reforming Gasification (Coal) HAS decomposition

Material Source Water

Biomass

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓

✓

Process driving energy Fossil fuel

Mechanical

Electrical

Estimated production scale (Capacity) Biological

Photonic

✓

✓

✓

Small ✓

✓

✓ ✓ ✓ ✓ ✓

Thermal

✓ ✓ ✓

✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓

✓ ✓ ✓ ✓

✓ ✓ ✓ ✓

Fig. 4. Energy and exergy efficiencies of selected hydrogen production methods.

Fig. 5. GWP and final cost of hydrogen in different hydrogen production technologies. 4

Median ✓

Large

✓ ✓ ✓

✓ ✓ ✓

✓ ✓

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Fig. 6. Pathway toward green and sustainable hydrogen production.

figure, it is clearly shown that in the transition to green hydrogen economy, technologies with lower environmental impacts such as PEWS or photobiological hydrogen production are the ultimate solutions. However, since these technologies are yet to become cost effective and industrially established, they are categorized as long-term hydrogen production pathways. As a matter of fact, production of hydrogen from renewable sources faces many challenges the high temperature of biomass decomposition and water splitting and costly materials for the processes [18]. Since providing enough thermal energy itself has high GWP, It is important to integrate the hydrogen production units to the systems where energy or is wasted. One of the recent advances in this area is power to gas technology (PTG). PTG is an integrated system where hydrogen is produced from excessive electricity of an intermittent renewable energy system (i.g, Wind turbine, Solar PV) as an intermediate storage option for electricity to be used or to be converted to SNG and utilized in gas grid [19,20]. PTG is reported to be a promising technology for heat decarbonization and making hydrogen infrastructure more reliable by H2-electricity interconversion [21]. In a recent published study, Glenk

and Reichelstein [22] claimed that the production of hydrogen from electricity so called PTG can be a promising method for the industrially advanced countries and is cost competitive with other hydrogen production methods accordingly. 2.1. Hydrogen from biomass As indicated in Fig. 7, biomass-based hydrogen production (where biomass is the material source) can be categorized into thermochemical (where driving energy is thermal) and biological (where driving energy is biological) pathways [23]. Biological pathways such as fermentation take place in lower temperatures in presence of microorganisms or sunlight. Although biological conversion is less energy intensive and is treated as more environmentally friendly, however, hydrogen production by this method is highly sensitive to the reaction’s condition such as light and temperature (need to be sustained in a certain condition) and the yield of H2 is low compared to the thermochemical processes. Among thermochemical methods, supercritical water gasification (SCWG) has the advantage of lower required temperature and the

Fig. 7. Biomass-based hydrogen production methods. 5

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capability to convert wet biomasses such as wastewater [24]. This process takes place in the presence of water in its supercritical condition (T greater than 374 °C, P greater than 22.1 MPa) where both biomass and water contribute in hydrogen production [25,26]. Recently, Zhang et al [27] made a breakthrough in biomass to hydrogen technologies by proposing a novel two-step hydrolysis-oxidation. In this method, hydrolysis takes place at 160 °C and oxidation occurs at 85 °C in the presence of molecularly defined iridium catalyst. H2 yield was observed as high as 95% with very low amount of CO and CH4 as byproducts. The conversion of biomass to H2 is highly dependent on catalysts and the advances in catalysis technology has a significant impact on hydrogen production technologies. The other challenge of biomass to H2 technologies is the feedstock selection where algal biomass and genetically modified organisms are figured out to be a promising solution as third and fourth generation of feedstock. Algal biomass has no competition with human food cycle and can be artificially grown with very low amount of water [28,29]. Nevertheless, it is important to study the applicability and cost-effectiveness of the produced hydrogen from biomass for energy applications such as fuel cells and hydrogen and power co-generation plants. In a recent study, Archer and Steinberger-Wilckens [30] assessed the different pathways of biomass-based hydrogen production for fuel cell application. They concluded that anaerobic digestion and metabolic processes are the best candidates due to the low feedstock and fuel gas demand and high fuel cell output. In the case of hydrogen and power cogeneration, since SCWG produces high-pressure syngas, integration of a high-temperature renewable energy sources such as concentrated solar energy with SCWG is recommended for efficient and environmentally benign cogeneration. This technology is being developed in pilot scale by the State Key Laboratory of Multiphase Flow in Power Engineering in China [31].

photonic energy in the presence of microorganisms in a hybrid process where hydrogen is produced from water by photonic energy and biochemical energy [33]. Electrolysis is one of the simplest ways to produce hydrogen from water. It can simply be summarized as conversion of electric power to chemical energy in the form of hydrogen and oxygen as a by-product with two reactions in each electrode; anode and cathode [34]. There is a separator between anode and cathode electrodes which ensure products to remain isolated. The low-temperature electrolysis (LTE) occurs in temperatures of 70–90 °C while the high temperature electrolysis (HTE) takes place in 700–1000 °C with less electricity consumption [35]. The advantage of HTE is that near zero GHG emission can be achieved if an external clean heat source is employed [7]. However, the high temperature electrolysis is not yet industrially established. Reduction in cost and increase in efficiency need to be done to meet the defined cost targets (2.30 $/kg for electrolysis) [36]. In a recently published study by National Renewable Energy Laboratory (NREL), it is mentioned that in many locations in the United States the electrolytic hydrogen production is now cost competitive with gasoline [37]. The photochemical and photoelectrochemical water splitting are solar-based technologies for hydrogen production from water using photons of sunlight in the presence of suitable catalysts. Photoelectrochemical Catalysts based on titanium oxides (TiO2), cadmium sulfides (CdS) and zinc oxide/sulfides (ZnO/ZnS) are reported to have better performance during photocatalytic water splitting for hydrogen production. This method is reported to have the minimum global warming potential (GWP) among hydrogen production methods while the exergy efficiency and the cost are not comparatively favorable [38]. The reported efficiencies for PECWS are around 10% while the maximum efficiency of 18% can be obtained in certain conditions. In a recent breakthrough, NREL reported a new record of 16% solar to H2 efficiency However, due to the high cost of production and limited durability of material, it is under development and is considered as a long-term scenario for hydrogen production [39]. Finally, sonochemical water splitting in which driving energy is mechanical in the form of ultrasound, occurs when high frequency waves of sound pass through water which vibrates water consequently (water Sonolysis) and make cavitation bulbs. The typical ultrasonic wave in a certain range of frequency accumulates a lot of energy in the form of pressure inside the cavitation bulbs. The energy accumulates this way can split the molecule of water in specific conditions and produce hydrogen. However, Sonochemical hydrogen production method is not mature enough and at the early stages of research and development. Further information can be found in [40,41].

2.2. Hydrogen from water Water, is the most abundant resource for hydrogen production which consists of hydrogen and oxygen. Hence, the water molecule can be split into hydrogen and oxygen if enough energy is provided. Water splitting process can be performed through different technologies. Water splitting for hydrogen production can be performed based on any of these sources and also some hybrid types (combined two or many energy sources. As indicated in Fig. 8, water splitting hydrogen production methods can be categorized into five major types based on the energy used for water splitting and a hybrid form where two or more types of energy are employed in a system for hydrogen production. Hydrogen from water by electricity based (electrolysis), mechanical based (from ultra sound through sonochemical method), photonic based (photolysis or photo electrochemical water splitting), thermal energy based (thermochemical cycles and thermolysis) [32]. Noting that biochemical water splitting is possible with the assistance of

3. Thermochemical cycles for water splitting Water-splitting thermochemical cycles are based on water decomposition through repetitive series of chemical reactions using intermediate reactions and substances which are all recycled during the process so that the overall reactions is equivalent to the dissociation of the water molecule into hydrogen and oxygen [42]. Theoretically, thermal energy is the only requirement for this process. Thermochemical water splitting cycle (TWSCs) are for hydrogen production using thermal energy and recycling of materials for reuse. TWSCs are not very catalyst dependent and the only consumed substance in the cycle is water, which is the source of hydrogen and all other materials can be recycled [43]. There are some advantages of TWSCs listed as follows:

• Membrane is not required for O -H separation • Temperature range of 500–1800 °C (in most cases) • No input electricity in pure TWSCs, and low electricity requirement 2

2

in hybrid TWSCs

Fig. 8. Water splitting methods for hydrogen production.

The thermochemical cycles are driven either by only thermal energy 6

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Fig. 9. General schematic of pure and hybrid thermochemical cycles.

(Fig. 9a) which are called pure thermochemical cycles, or by thermal and another form of energy (e.g. electrical, photonic) which are called hybrid thermochemical cycles (Fig. 9b). In hybrid TWSCs, water, high temperature heat from concentrated solar plants or nuclear reactors, and electricity or photonic energy are the inputs while hydrogen and oxygen are the outputs [44]. Water can be decomposed to H2 and O2 just in one step. However, due to the undesirable thermodynamics and the very high temperature required for the single-step, thermochemical cycles are proposed as a repeating set of reactions in which water is split using thermal energy at the temperatures below 2000 °C and usually in two or multi steps [45]. Research and development activities on thermochemical watersplitting cycles was begun with some studies by Funk and Reinstrom [46] during 1960 s where thermochemical splitting of water was recommended for hydrogen production instead of conventional electrolysis. The first convention on hydrogen production through TWSCs was held in European Community Joint Research Center located in Ispra, Italy, in 1969 where 24 cycles (So-called Mark cycles) were identified to be investigated in the period of 1970 to 1983 [47]. Since then, many active studies have been carried out and number of different cycles have been proposed in US, Japan and European countries in 1970s and 1980s [48–50]. Even though more than 200 thermochemical cycles have been identified for the water splitting by General Atomic [51], very few of them are proved to have the ability for large-scale hydrogen production and proceed to experimental demonstrations. The Sulfuriodine (S-I) cycle is the most famous and well-studied version of these cycles which was proposed by General Atomics as a promising processes regarding the utilization of nuclear heat sources which can supply heat at temperatures close to 900 °C [51,52]. In 1972, De Beni and Marchetti [53], proposed the first multi-step process so-called

Marc-1 based on calcium, bromine, and mercury with almost 50% efficiency. By the end of 1976, University of Tokyo in Japan proposed a cycle so called UT-3 with 49% efficiency where Ca, Br and Fe were the main components [54]. After completion of Mark cycles investigations, JAEA research along with some other Japanese institutions continued within 1990 s [52,55]. As shown in Fig. 10, since 2003, research on TWSCs increased mainly by beginning of DOE’s solar thermochemical hydrogen production research program (STCH) [56,57] which was followed by some other research projects in Canada [58], European union [59-62] and China [63,64] with continuation of Japanese research [65]. In 2006, Argonne national laboratory (ANL) published a report where more than 280 known thermochemical cycles were investigated for hydrogen production and some recommended for further investigation [66]. German aerospace research center (DLR) and Paul Scherrer institute in Switzerland have being developing solar reactor technologies in the last two decades for hydrogen production via high temperature two-step metal oxide redox pairs and sulfur based thermochemical cycles [67,68]. In 2007, Canada’s nuclear-based hydrogen production program begun by Atomic Energy of Canada Limited (AECL) and University of Ontario Institute of Technology (Ontario Tech) in partnership with ANL. They started a project for scaling up Cu-Cl thermochemical cycle as a suitable candidate for hydrogen production and integration with nuclear reactors [58,70–74]. Ontario Tech has also developed and enhanced magnesium-chlorine (Mg-Cl) thermochemical cycle in 2016, where Ozcan and Dincer [75,76] enhanced the efficiency of the process by adding a step to the cycle and investigating the hybrid type of the cycle. Numerous research on thermochemical water splitting technologies are performed in many research centers around the world within the last 50 years. Table 2summarizes projects on TWSCs and their corresponding research institutions. 3.1. Two-step thermochemical cycles In two-step thermochemical cycles a low valance metal oxide is produced via reduction of a higher valance metal oxide or a metal oxide with a low valence is reduced to metal itself in which oxygen is also produced [102]. The heat source here, should provide a temperature of 1700–3000 K which can be achieved through concentrated solar irradiance by using solar collectors [103]. To date, the relatively low efficiency of two-step thermochemical cycles compared to electrolysis, and the implementation of materials working under very high temperatures are the major draw-backs of this type of thermochemical cycles [104,105]. As illustrated in Fig. 11, in a two-step thermochemical cycle, a metal oxide is first reduced in the solar-assisted endothermic step (so-called activation), where O2 is released, and then reacts with water (so-called hydrolysis) in an exothermic step to produce H2 and the pristine oxide, which is subsequently recycled to the first step [69,70]. Several twostep water-splitting cycles based on volatile and non-volatile metal

Fig. 10. Publications statistics on thermochemical water splitting cycles between 1970 and 2018 (data from [69]). 7

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Table 2 Summary of worldwide thermochemical cycles research projects and their corresponding institutions. Research institutions Argentina National Atomic Energy Commission (NAEC) [77,78] Canada University of Ontario Institute of Technology [73,75,76] Atomic Energy of Canada Limited (AECL) [58] Cu-Cl project contributors [70–72]

Cycle

Date

Short description of the project

Fe-Cl, Co-Cl

2015

Experimental study on chlorine cycles and evaluation of their efficiencies.

Hybrid Cu-Cl, Mg-Cl

2007

Hybrid Cu-Cl Hybrid Cu-Cl

2007 2007

Scaling up Cu-Cl cycle and its integrated cycle and development and enhancement of Mg-Cl cycle in Clean Energy Research Laboratory (CERL). Supporting the Cu-Cl project. Supported by AECL in collaboration with Five Universities in Ontario, Canada and ANL in US.

S-I

2005

Promising achievements in optimizing Bunsen reaction.

China Institute of Nuclear and New Energy Technology (INET) [63] Chinese Academy of Science [64] European Union Joint research center in Ispra, Italy [47] Julich RWTH Achen, Germany [79] HYTHEC project contributors [60]

UTC (Three step Uranium)

2018

Uranium TCC to be coupled with Thorium Molten Salt Reactor (TMSR)

Mark version of cycles Fe-Cl, V-Cl, S-I S-I, HyS

1970 1984 2004

HYDROSOL project contributors [62]

Doped ferrite cycles

2014

HYCYCLES project contributors [59]

S-I, HyS

2008

SOL2H2 project contributors [61] German Aerospace Center (DLR) [68]

Modified HyS S-I, HyS, MOx

2013 2002

CEA France [80] CRNS France [81–83]

S-I, CeO/Ce Two step MOx, Perovskite materials ZnO/Zn, Fe3O4/FeO Ceria based MOx Perovskite materials for twostem cycles.

1976 2007

Proposing and developing 24 Mark type cycles within 1970-1983. Preliminary investigation of mark cycles. Investigation of the effective potential for massive hydrogen production of the S–I thermochemical cycle, and to compare it with the HyS cycle. Test operation of a 100 kW pilot plant for solar-based hydrogen production through two-step TWSC. High temperature reduction of sulphur trioxide (SO3) as one of the crucial steps for HyS TWSC. Solving materials research and development challenges of HyS cycle. Developing a honeycomb monolith solar reactor for high temperature thermochemical cycles Collaborating with GA in S-I cycle and development of MOx cycles. Extensive studies on two-step and three step solar-based MOx cycles.

S-I

1978

Recently reached 150 hours of continuous hydrogen production [85]

UT-3 (Calcium Bromine) CeO2-MOx (M= Fe, Ni, Cu) nonstoichiometric cerium oxide

1980 1994 2012

Perovskite materials

2019

Development of a four step UT-3 or Calcium bromine cycle. Experimental research on Ferrite and ceria based MOx cycles. Demonstration of a two-step cerium oxide cycle using a particle fluidized bed reactor working with a novel type of 100kWth beam-down solar concentrating system with a secondly elliptical reflector Experimental examination of solar-based thermochemical two-step water splitting cycle using perovskite oxides based on LaSrMnAlO3

Ferrite cycles

2006

Massive production of hydrogen using a very high temperature reactor (VHTR) by the early 2020s

S-I HyS S-I, Hy Cu-Cl, S-I HyS

1972 1975 2003 2003 2003

UTC (Three step Uranium)

2009

Florida Solar Energy Center (FSEC) [98,99]

HySA

2013

Sandia National Laboratories (SNL) -EETHP project [100] National Renewable Energy Laboratory (NREL) and University of Colorado Boulder [101]

(simple binary oxide (ferrite) cycle) Co doped-hercynite (CoFe2O4)

2017

Developing S-I, and participating in STCH project. Inventing a hybrid thermochemical cycle (HyS) Shifting TCC research from nuclear to solar in US (supported by DOE). Prioritizing TCCs, Scaling up Cu-Cl in partnership with AECL Completing the flowsheet of HyS cycle and enhancement in efficiency of the process particulary electrolysis step Patenting a novel carbonate thermochemical cycle (CTC) for the production of hydrogen using uranium Five step Hybrid process using both photonic and thermal energy of sun in photochemical and thermochemical steps Coupling a proton-conducing membrane (PCM) to the thermochemical cycle for pure H2 production. Isothermal on-sun H2 production using active iron aluminate (hercynite) particles contained in dual fluidized bed reactors

Paul Scherrer Institute and ETH Zurich, Switzerland [67,82,83]] Superior School of Experimental Science and Technology, (ESCET), Spain [84] Japan Japan Atomic Energy Agency (JAEA) [49,52,55,65] University of Tokyo [54] Tokyo Institute of Technology [86] Joint project of University of Miyazaki, Niigata University, and Mitaka Kohki Co., LTD [87] Niigata University [88] South Korea Korea Atomic Energy Research Institute (KAERI) [89] United States General Atomics (GA) [90] Westinghouse corporation [91] STCH project contributors [57] Argonne National Laboratories (ANL) [92] Savannah River National Laboratories (SRNL) [93–96] Oak ridge National Laboratory (ORNL) [97]

2002 2017

2017

oxides redox pair reactions including ZnO/Zn, Fe3O4/Fe, SnO2/SnO, CeO2/Ce2O3, Mn2O3/MnO, Co3O4/CoO, CdO/Cd, GeO2/GeO have been proposed [106-108]. Among them, TWSCs based on ZnO/Zn redox pairs are known to have thermodynamic superiority among other metal oxide redox pairs and their processes are examined thermodynamically and tested practically in solar reactors [109]. Bilgen et al [110] proposed the first demonstration of ZnO/Zn. Paul Scherrer Institute in Switzerland has begun a program since early 90s on zinc oxide thermochemical cycle using solar-driven high-temperature reactors [111]. However, the ZnO/Zn cycle is not yet industrially competitive. Separation of zinc from oxygen, reactor window, slow kinetic, and back

27 years of research on reactor technologies for high-flux solar-driven TCCs based on MOx redox pairs and Ceria. Evaluation of commercial perovskite materials as La1-xSrxMeO3 (M= Mn, Co and Fe) for thermochemical water splitting.

reactions are mentioned as major challenges of Zinc oxide two-step thermochemical cycle [111,112]. A techno-economic analysis by Steinfeld et al. [109] reported the unit cost of 5 $/kg H2 which is not competitive with other hydrogen production technologies. Due to the abovementioned challenges, other metal oxides were examined for twostep TCWSs and CeO2 was found out to be a promising candidate and remains to be the most pragmatic one [113]. Overall, although two-step cycles are an attractive option for water splitting due to their unique advantage of simplicity of having only two chemical reactors and intrinsic hydrogen–oxygen separation features, the required working temperature is too high. If the cycle comprises 8

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nuclear reactors for water splitting. Schematic diagram of HyS cycle is presented by Fig. 12. The process is comprised of following steps [119]. 1) Bunsen section which is an exothermic Sulfuric acid production in presence of water 2) Endothermic sulfuric acid decomposition reaction which requires high temperature. 3) hydriodic acid (HI) decomposition which is slightly endothermic. The key solution in this cycle is reported to be the optimization of the Bunsen reaction. The following issues are reported as the main challenges of this cycle [120].

• High temperatures (above 800 °C) is required. • Reactants are too corrosive and harmful to be used in industrial applications. • Separation of HI from H SO takes place in presence of excessive

Fig. 11. Schematic diagram for a two-step metal oxide thermochemical cycle.

2

three or more steps, then the temperature level for the heat sources can be reduced below 1500 K. Besides, improvement in solar reactor technology is a key factor for scaling up this technology.

The excessive iodine and the high temperature sulfuric acid decomposition as well as corrosive environment are reported as some of the main drawbacks of this cycle. There have been some efforts done by various researchers to overcome such challenges and possibly to provide more commercially viable S-I cycle based systems. For example, Zhou et al. [121] and Liberatore et al. [122] proposed an open-loop SI thermochemical cycle for the production of hydrogen, sulfuric acid and electric power is one of the new markets. In this process, the H2SO4 decomposition step is avoided and SO2 is supplied from a flue gas, which saves the cost in the related industrial sector, and H2SO4 and H2 are produced as marketable products, which consequently reduces the cost of the S-I process and accelerates the commercialization of this cycle. Moreover, another improvement in S-I process is reported in a recent work by Park et al. [123] where a novel reactor-separator network is proposed for avoiding oxygen contact in Bunsen reaction which decreases the efficiency of sulfuric acid production. Studies on the S-I process have mainly focused on the use of nuclear energy as the hightemperature heat source for sulfuric acid decomposition. Besides S-I which is the most established three step cycle, experimental demonstrations of some metal oxide three step cycles such as GeO2/GeO by Bhosale [124], and CeO2/Ce2O3, Fe3O4/FeO, Mn3O4/ MnO, Co3O4/CoO, by Charvin et al. [82] are investigated to be coupled with a high temperature solar reactors.

3.2. Three step thermochemical cycles Three-step processes can be constructed from two-step processes in which the highest temperature reaction is replaced by a two-step reaction process. The effect of this change will be a reduction of the maximum temperature requirement for the cycle. For a three-step thermochemical cycle, the set of reactions repeating in the cycle is written as follows [114]: H2O + A → 0.5 O2 (or 0.5 H2) + AH2(or AO)

(1)

AH2 (or AO) + B → 0.5 H2(or 0.5 O2) + AB

(2)

AB → A + B

(3)

4

iodine where significant amount of Iodine is lost.

The Sulphur-Iodine (S-I) which is known as general electric cycle is the most well-known cycle in this category. It was originally proposed in 1970′s by general electric [113]. Research and development on S-I cycle continued by many institutions. In Japan, S-I thermochemical cycle has been investigated to be scaled up by the Japan Atomic Energy Agency (JAEA) [52]. This cycle was also investigated by GA, Sandia National Laboratory (SNL) in USA, and the CEA in France [115,116]. It was shown that efficiency of 52% is possible when a high temperature nuclear reactor is integrated with this cycle [117]. Also in China, 10 litr per hour of hydrogen production rate was achieved in 2009 in a total closed S-I cycle operated for 7 h [118]. The S-I cycle has is considered as a high temperature TCC while it offers a high yield of hydrogen through implementing the concentrated solar energy or high temperature

3.3. Four or more steps thermochemical cycles Although there are many thermochemical cycles proposed in the literature, most of them require very high temperatures. However,

Fig. 12. Schematic diagram for a three-step S-I thermochemical cycle. 9

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lower temperature processes gives the opportunity of coupling with variety of industrial waste heat or different sources of thermal energy. Studies on thermochemical cycle show that an increase in the number of steps in a cycle can reduce the maximum temperature required for the cycle’s heat source. The General form of four step thermochemical cycle reactions can be written as follows: H2O + AB → AH2 + BO

(4)

AH2 → H2 + A

(5)

BO → 0.5O2 + B

(6)

A + B → AB

(7)

Canavesio et al. [77,78] performed two experimental studies on four-step Fe-Cl and Co-Cl cycles. In their studies, the validity of thermochemical reactions were investigated and kinetics of the reaction and influencing parameters were studied for enhancement in the cycle efficiency and hydrogen production rate. Moreover, a comparative evaluation was performed for the original and modified TCC based on the energy requirement and the balance of enthalpy. Results indicated that an increase in efficiency of the cycle is possible from 24 to 28% to 32–37%. They reported this increase in efficiency as a promising sign for further investigation of this cycle. The Cu-Cl is another multi-step thermochemical cycle which is investigated by AECL and University of Ontario institute of technology (Ontario Tech) in Canada. It promises a good performance in hydrogen production and efficiency [129]. As of the advantage of Cu-Cl cycle among other TCCs are lower maximum operating temperature (Tmax < 550 °C) and ability to integrate with lower temperature waste heat from industries [130,131].

Thermochemical cycles with four steps comprise in general a hydrolysis reaction (reaction (4)), a hydrogen-evolving reaction (reaction (5)), an oxygen-evolving reaction (reaction (6)), and a reagent-recycling reaction (reaction (7)). In the framework of the Nuclear Hydrogen Initiative (NHI) [66] some four step metallic chloride based thermochemical cycles are identified and recommended for further investigation based on their feasibility of production of large scale hydrogen production, efficiency evaluation and cost analysis. In the 2007 report of Argonne national laboratory (ANL), regarding the evaluation of alternative thermochemical cycles for hydrogen production [66] and three other papers on evaluation of alternative thermochemical cycles for hydrogen production, the thermochemical cycles for hydrogen production were prioritized based on the following factors; chemical viability, thermodynamic feasibility, thermal efficiency, and cost competitiveness. The chloride cycles selected in that report were Ce-Cl, Cu-Cl, Fe-Cl, Mg-Cl, and V-Cl. Among them Cu-Cl, V-Cl with reported efficiencies of 43%, and 46%, respectively, considered as most promising options while FeCl was also recommended for further studies for efficiency improvement [125,126]. In the developed versions of these cycles, however, some intermediate reactions as an additional step are recommended to lower the maximum temperature and enhance the efficiency. Fe-Cl is an example of a pure four-step thermochemical cycle. This cycle was essentially considered advantageous due to the fact that the chemicals employed are relatively cheap and that the chemistry of the iron-oxides is well known. This cycle was first investigated by Funk [127] in 1976. However, Fe-Cl cycle had two critical problems as poor reactivity of thermal FeCl3 decomposition and hydrolysis of FeCl2 [128]. This four step TCC basically consists of four reactions (shown in Fig. 13) which are one low temperature, one moderate temperature and two high temperature reactions.

3.4. Hybrid thermochemical cycles Hybrid thermochemical water splitting cycles use both heat and work with one reaction driven by electricity. Electrochemical reaction is selected in such a way that work consumption of this step is lower than that of water electrolysis step. The advantage of hybrid processes is that two-step processes require much lower temperatures and can be driven by nuclear waste heat or moderate temperature heat resources [35]. Energy efficiencies up to 48–50% can be obtained from hybrid processes, Two-step hybrid sulfur (HyS) cycle known as Westinghouse cycle is the most well-known hybrid process for thermochemical hydrogen production. HyS was originally proposed by Westinghouse in the 1970 s as a combined thermochemical-electrochemical cycle for large-scale hydrogen production [91]. It is the first demonstrated thermochemical water splitting process with only two reactions, which is the combination of the thermal decomposition of sulfuric acid with the electrochemical oxidation of SO2 with water to yield sulfuric acid and hydrogen [132]. As shown in Fig. 14, the cycle consists of one step thermal decomposition of sulfuric acid where heat is consumed and oxygen is produced and one electrochemical step for SO2 electrolysis where electricity is consumed and hydrogen is produced [133]. The required voltage for electrolysis of sulfur dioxide is about 0.17 V, which is much lower than that of water electrolysis (1.23 V) [134]. From electrochemical point of view, it was concluded that high overall efficiency is achievable when the electrolyzer can produce concentrated

Fig. 13. Schematic diagram of a four-step Fe-Cl thermochemical cycle. 10

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Fig. 14. Schematic diagram of a Hybrid Sulfur thermochemical cycle.

sulfuric acid at low voltage The first evaluation of this cycle by Bilgen et al [135] showed an overall energy efficiency of 40%. Savannah River National Laboratory (SRNL) has performed extensive research on flowsheet simulation and components development of this cycle [9396]. High temperature SO3 reduction and corrosive materials are reported to be the main challenges of this process [50,136]. Iron oxide catalysts are commonly used for enhancing the reaction rate at high temperature step. Moreover, Silicon Carbide (SiC) was recognized as the suitable material for facing the corrosive material produced in the process [137]. The integration of this cycle with concentrated solar energy has been investigated mainly by SRNL and Commonwealth Scientific and Industrial Research Organization (CSIRO) and the plant efficiency of 27% and hydrogen production cost in the range of 3.19–5.57 $/kg were reported [134,137]. A practical solar thermal sulfuric acid decomposition reactor was built up and assessed by DLR in Cologne, Germany, where the efficiency of 28% was achieved for this step while the reflective radiation of collectors was recognized as the main source of energy loss [50]. In a recent advance in HyS TCCs development, SRNL has presented a conceptual design HyS hydrogen production cycle integrated and with concentrated solar thermal plants using SNL’s Falling Particle Receiver with a pressurized helium secondary heat transfer fluid. The efficiency of HyS was reported as 36% by LHV basis and the overall efficiency of solar to H2 was 17% [95].

Some hybrid cycles are driven by thermal and another forms of energy such as photonic energy. Huang et al [138] developed a novel family developed a novel family of hybrid sulfur-ammonia (HySA) photothermochemical water splitting cycles, by introducing ammonia, NH3, as working reagent which is shown in Fig. 15. The main advantage of the HySA cycle is the combined utilization of the photonic (i.e., UV, visible) portion of the solar irradiance for the H2 production step and the thermal (infra-red) portion for the O2 evolution step. Like other thermochemical cycles, hydrogen and oxygen are the only outputs. However, in this hybrid cycle, in addition to water and heat, the photonic energy is also one of the inputs of the cycle. Hydrogen is produced in reaction (9) where ammonium sulfite undergoes an oxidation reaction in a near ambient temperature. The five main reactions of HyS are listed as follows: Chemical absorption(25 °C) SO2(g) + 2NH3(g) + H2O(l)→(NH4)2SO3(aq)

(8)

Electrolytic oxidation (80–150 °C) (NH4)2SO3(aq) + H2O(l)→(NH4)2SO4(aq) + H2(g) Solar thermal conversion (400 °C)

Fig. 15. hybrid Sulphur ammonia thermochemical cycle for hydrogen production. 11

(9)

12

Metal oxide family ZnO/Zn

HyS

Sulphur family S-I

V-Cl

Fe-Cl

Mg-Cl

Chlorine family Cu-Cl

Thermochemical Cycle 450 25 90 500–550 300 450–500 70–90 450 100 300 800 25–125 727 25 727 120 850 450 900 100–120 1800 400–500

2HCl + 2CuCl → 2CuCl2 + H2 2CuCl2 → H2O + 2CuCl2 MgCl2 + H2O → MgOHCl + HCl 2MgOHCl + Cl2 → 2MgO + H2O + 1/2O2 2HCl → H2 + Cl2 3FeCl2 + 4H2O → Fe3O4 + 6HCl + H2 Fe3O4 + 8HCl → FeCl2 + 2FeCl3 + 4H2O 2FeCl3 → 2FeCl2 + Cl2 Cl2 + H2O → 2HCl + ½O2 2VCl2 + 2HCl → 2VCl3 + H2

4VCl3 → 2VCl4 + 2VCl2 2VCl4 → 2VCl3 + Cl2 Cl2 + H2O → 2HCl + O2

SO2 + 2H2O → 2HI + H2SO4 H2SO4 → SO2 + H2O + ½ O2 2HI → I2 + H2 H2SO4 → SO2 + H2O + ½O2 2H2O + SO2 → H2SO4 + H2

ZnO → Zn + 1/2O2 Zn + H2O → ZnO + H2

T (range in °C)

2CuCl2 + H2O2 → 2Cu2OCl + 2HCl 2Cu2OCl → 2CuCl + 1/2O2

Reactions

Table 3 Overview of selected thermochemical cycles.

Optimization of Hydrolysis reaction Development of electrochemical reaction MgO chlorination Rapid formation of MgCl2 hydrates

- High cost materials for high temperature solar reactor

- High temperature H2SO4 decomposition - Corrosive materials

- Inexpensive materials - Sulfur oxide as only intermediate - Hydrogen production in two step No side reaction

- Corrosive materials - High temperature solar for integration

- Separations/high temperature for the reverse Deacon reaction - Slow kinetic of chlorination reaction - Unknown thermodynamics

- Low efficiency - High temperature FeCl3 decomposition - Dimerization of FeCl3 to FE2Cl6

-

- Solids handling between processes - Corrosive working fluids

Major challenges

- High thermal efficiency - There are not any harmful by products or emissions from the process

- Capable to be coupled with high temperature solar energy

- High predicted efficiency

- Known thermodynamic - Low cost and abundance of materials

- Low temperature - Promising to be coupled with solar and nuclear

- lower operating temperatures, - Ability to use low-grade waste heat to improve energy efficiency, and potentially lower cost materials.

Distinct features

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Fig. 16. Efficiency comparison of different thermochemical cycles.

(NH4)2SO4(aq) + K2SO4(l) → K2S2O7(l) + 2NH3(g) + H2O(g)

(10)

Photolytic Sulfur Ammonia has not yet achieved sufficient maturity to settle on a conceptual design since a choice between beam-splitting mirrors or dual solar fields has not been made. Water splitting is achieved via a photochemical oxidation of ammonium sulfite with a simultaneous release of hydrogen [142].

Solar thermal conversion (550 °C) K2S2O7(l) → K2SO4(l) + SO3(g)

(11)

Solar thermal conversion (850 °C) SO3(g) → SO2(g)+½O2(g)

(12)

4. Comparative evaluation of thermochemical cycles

The other thermochemical reactions form a sub-cycle by which potassium sulfate is reacted with ammonium sulfate in the low-temperature reactor, to form potassium pyrosulfate. That is then fed to the lower temperature reactor where it is decomposed to SO3 and K2SO4, and closes the sub-cycle [139]. The K2SO4 and K2S2O7 form a liquid melt which makes the separations and the transfer of the chemicals in reactions (10, 11) possible. The oxygen production step is the high temperature step is presence of a catalyst. Separation of O2 from sulfur dioxide takes place occurs in presence of H2O in reaction (8). These reactions are all performed in Florida solar energy center by T-Raissi et al. [91] and no side reaction is reported [140]. The main advantage of HySA cycle is the combined utilization of the photonic (i.e., UV, visible) portion of the solar irradiance for the H2 production step and the thermal (infra-red) portion for the O2 evolution step [141].

TWSCs are being developed for more than half a century and it is still under development. TWSCs are considered as long-term solutions for large-scale hydrogen production. In this part, a comparative evaluation of selected thermochemical cycles from chlorine and sulfur family is presented. Fe-Cl, Mg-Cl, Cu-Cl and V-Cl from chlorine family group, and HyS and S-I from sulfur family group are compared from energetic, exergetic, environmental and economical point of view. The selected thermochemical cycles and their reaction steps, temperatures, distinct feature of each cycle and major challenge on the way of their development are mentioned in Table 3. The selection of these thermodynamic cycles are based on the priority assessment by Lewis et al [66] and Balta and Dincer [143]. There are some comparative assessment of TWSCs with each other or with other hydrogen production methods in the literature. For example, Dincer and Acar [144]

Fig. 17. Cost and GWP comparison of different thermochemical cycles. 13

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reviewed and evaluated different hydrogen production methods from sustainability, Cost and efficiency points of view. Balta et al [143] presented a comparative evaluation of chlorine family thermochemical cycles where Cu-Cl and Mg-Cl found to be more sustainable in terms of working temperature while V-Cl showed higher exergy efficiency. The V-Cl was recommended for further investigations in order to overcome the technical challenges. El-Emam and Ozcan [145] reviewed the nuclear-based large-scale hydrogen production options. Cu-Cl, Mg-Cl, Ca-Br, S-I, HyS were selected to be evaluated in terms their performance in integration with various solar or nuclear reactor technologies. In another study by ElEmam and Khamis [146] a review on International Atomic Energy Agency (IAEA) research projects on hydrogen production was presented and energy and exergy efficiencies of some chlorine family thermochemical cycles were compared. Fig. 16, indicates the comparative evaluation of selected thermochemical cycles in terms of their LHV based energy and exergy efficiencies based on data from [94,123,143–148]. As seen, in terms of energy efficiency, V-Cl has the higher exergy efficiency among the studied thermochemical cycles while Fe-Cl has the lowest. Moreover, Fig. 17 presents a comparative evaluation of selected thermochemical cycles in terms of their hydrogen production cost and GWP base on the data reported in the literature [148–152]. As shown, ZnO/Zn shows high production cost for 1 kg of hydrogen due to the highly cost materials in solar integration. Enhancement in solar heat production can significantly reduce the cost and GWP. The sulfurine family cycles has lower cost of less than 2$/kg H2 among the other selected thermochemical cycles. Beside, environmental analysis shows low GWP of 0.48 and 0.5 for HyS and S-I thermochemical cycle. However, due to the corrosion and high-temperature SO3 decomposition step, durability of the sulfur based cycles is still a challenge to overcome. Cu-Cl provides a low GWP of 0.55 kg CO2.eq/ kg H2 and the cost of 2.24 4/kg H2. Taking into account the lower maximum temperature of Cu-Cl cycle (550 °C) and its high capability in integration with the fourth generation of nuclear reactors, it is considered as a reliable and promising cycle for large-scale hydrogen production.

• •

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] Key world energy statistics. International Energy Agency. Paris: France; 2018. [2] World energy outlook. International Energy Agency. Paris: France; 2015. [3] Nicoletti G, Arcuri N, Nicoletti G, Bruno R. A technical and environmental comparison between hydrogen and some fossil fuels. Energy Convers Manag 2015;89:205–13. [4] Muradov N, Veziroglu TN. From hydrocarbon to hydrogen–carbon to hydrogen economy. Int J Hydrogen Energy 2005;30:225–37. [5] Pagliaro M, Konstandopoulos AG. Solar Hydrogen: Fuel of the Future. Cambridge, UK: Royal Society of Chemistry; 2012. [6] H2@Scale concept. 2017. US department of energy. http://www.energy.gov/eere/ fuelcells/h2scale. [7] Staffell I, Scamman D, Velazquez Abad A, Balcombe P, Dodds PE, Ekins P, et al. The role of hydrogen and fuel cells in the global energy system. Energy Environ Sci 2018;12:463–91. [8] Abdalla AM, Hossain S, Nisfindy OB, Azad AT, Dawood M, Azad AK. Hydrogen production, storage, transportation and key challenges with applications: a review. Energy Convers Manag 2018;165:602–27. [9] Hydrogen scaling up. A sustainable pathway for the global energy transition, Hydrogen Council; 2017. [10] The Future of hydrogen; Seizing today’s opportunities. International energy agency. Report prepared by the IEA for the G20, Japan; 2019. [11] Dincer I, Zamfirescu C. Sustainable hydrogen production options and role of IAHE. Int J Hydrog Energy 2012;37:16266–86. [12] US department of energy (DOE), hydrogen energy strategy plan; 2011. [13] Parkinson B, Balcombe P, Speirs JF, Hawkes AD, Hellgardt K. Levelized cost of CO2 mitigation from hydrogen production routes. Energy Environ Sci 2019;12:19. [14] The Development of Lifecycle Data for Hydrogen Fuel Production and Delivery. By the Institute of Transportation Studies, UC Davis. Prepared for the California Air Resources Board and the California Environmental Protection Agency; 2017. [15] Landman A, Dotan H, Shter GE, Wullenkord M, Houaijia A, Maljusch A, et al. Photoelectrochemical water splitting in separate oxygen and hydrogen cells. Nat Mater 2017;16:646–51. [16] Guerra OJ, Eichman J, Kurtz J, Hodge B. Cost Competitiveness of Electrolytic Hydrogen. Joule 2019. https://doi.org/10.1016/j.joule.2019.07.006. [17] Editorial On the right track Nature Energy. 4; 2019: 69. [18] Arregi A, Amutio M, Lopez G, Bilbao J, Olazar M. Evaluation of thermochemical routes for hydrogen production from biomass: a review. Energy Convers Manag 2018;165:696–719. [19] Safari F, Dincer I. Assessment and optimization of an integrated wind power system for hydrogen and methane production. Energy Convers Manag 2018;177:693–703. [20] Lefebvre J, Friedemann M, Manuel G, Graf F, Bajohr S, Reimert R, et al. Renewable power-to-gas: a technological and economic review. Renew Energy 2016;85:1371–90. [21] Haghi E, Raahemifar K, Fowler M. Investigating the effect of renewable energy incentives and hydrogen storage on advantages of stakeholders in a microgrid. Energy Policy 2018;113:206–22. [22] Glenk G, Reichelstein S. Economics of converting renewable power to hydrogen. Nat Energy 2019;4:216–22. [23] Kalinci Y, Hepbasli A, Dincer I. Biomass-based hydrogen production: a review and analysis‖. Int J Hydrog Energy 2009;34:8799–817. [24] Safari F, Javani N, Yumurtaci Z. Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts. Int J Hydrog Energy 2018;43:1071–80. [25] Safari F, Tavasoli A, Ataei A. Gasification of sugarcane bagasse in supercritical water media for combined hydrogen and power production: a novel approach. Int J Environ Sci Technol 2016;13:2393–400. [26] Safari F, Tavasoli A, Ataei A. Gasification of Iranian walnut shell as a bio-renewable resource for hydrogen-rich gas production using supercritical water technology. Int J Ind Chem 2017;8:29–36.

5. Conclusions Hydrogen economy is a progressive multi-decadal project which is gradually in transition to marketplace and is expected to be fully commercialized within a decade. Thermochemical water splitting cycles for hydrogen production are being investigated for more than fifty years as a long-term, large-scale hydrogen production method. Many cycles have been identified and examined by different countries and research institutions. However, only a few of them have been practically integrated with sustainable source of energy for hydrogen production. Chlorine and sulfur family of thermochemical cycles are highly regarded as promising options for integration with nuclear reactors due to the high efficiency and capability for integration. Besides, solar driven two-step cycles are being developed mostly for the cases where a high temperature solar reactor is available. A Comparison between representative thermochemical cycles is performed in this study from energetic, exergetic, economic, and environmental points of view. Concluding remarks are summarized as follows:

• V-Cl has a high efficiency while there are still technical barriers in • •

production process. However, corrosion remained as the main challenge on the way of development of these cycles. Two-step cycles based on metal oxide redox pairs (ZnO/Zn, CeO/Ce, etc), non-stoichiometric ferrites, ceria, doped-ceria, and perovskite materials are known as promising options for thermochemical hydrogen production in an integration with high-temperature solar reactors. Thermochemical cycles are not yet seen cost competitive compared to the conventional methods while a proper integration with concentrated solar or nuclear reactors is necessary for further improvement.

its integration with energy source. This cycle needs more investigation. Cu-Cl and Mg-Cl are promising in terms of hydrogen production cost and capability of integration with relatively low temperature industrial waste heat while some alternative chlorine cycles such as Fe-Cl are promising for further investigation Sulfur family thermochemical cycles (S-I and HyS) are good candidates with respect to their high capability of integration with high temperature nuclear reactors and their low GWP in hydrogen 14

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