Hydrogen from solar thermal energy

CHAPTER 10

Hydrogen from solar thermal energy Heidi I. Villafán-Vidalesa, Camilo A. Arancibia-Bulnesa, Patricio J. Valades-Pelayoa, Hernando Romero-Paredesb, Ana K. Cuentas-Gallegosa, Carlos E. Arreola-Ramosa a Renewable Energy Institute (Instituto de Energı´as Renovables, UNAM), Temixco, Morelos, Mexico Metropolitan Autonomus University-Iztapalapa (Universidad Auto´noma Metropolitana-Iztapalapa), Mexico city, Mexico

b

Chapter Outline Nomenclature 10.1 Introduction 10.2 Thermochemical solar hydrogen production 10.2.1 Thermodynamics of thermochemical processes 10.3 Solar thermolysis 10.3.1 Separation techniques 10.4 Thermochemical solar cycles 10.4.1 Thermodynamics of two-step thermochemical cycles 10.4.2 ZnO/Zn 10.4.3 Fe3 O4 /FeO and ferrites (Ax Fe3
x O4 ) 10.4.4 Ceria 10.4.5 Perovskites 10.5 Solar reactors 10.5.1 Energy integration 10.5.2 Metal oxide loading 10.5.3 Reactor efficiency 10.6 Scale-up of solar thermochemical hydrogen production 10.6.1 Solar concentrator configurations 10.6.2 Solar towers 10.6.3 Design and modeling 10.6.4 Implementation 10.6.5 Control strategies 10.7 Economic analysis References Further reading

319 320 320 321 323 324 325 326 327 330 333 335 336 337 339 342 342 343 344 346 348 349 349 355 363

Nomenclature α Δg Δh Δs e ηcarnot

Cavity absorptance, dimensionless Gibbs free energy of the reaction [J mol
1] Enthalpy of the reaction [J mol
1] Entropy of the reaction [J mol
1 K
1] Cavity emittance, dimensionless Carnot efficiency, dimensionless

Solar Hydrogen Production https://doi.org/10.1016/B978-0-12-814853-2.00010-2

© 2019 Elsevier Inc. All rights reserved.

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ηsolar ηmax, energy ηA σ A a C I p O2 Qsolar R TH TL Tc

Solar energy absorption efficiency, dimensionless Maximum exergy efficiency, dimensionless Optical efficiency of the solar concentrator, dimensionless Stefan-Boltzmann constant, 5.67  10
8 [J K
4 m
2 s
1] Collector area [m2] Solar cavity receptor area [m2] Concentration ratio, dimensionless Direct normal irradiance [W m
2] Oxygen partial pressure [bar] Solar energy coming from solar concentrator [W] Universal gas constant, 8.3145 [J mol
1 K
1] Upper operating temperature [K] Lower operating temperature [K] Cavity temperature [K]

10.1 Introduction Hydrogen is presented as a promising renewable energy carrier [1] with potential use in the transport sector and domestic applications [2]. Currently, hydrogen production is carried out, mainly, through catalytic reforming of natural gas, where the high temperatures needed are attained by natural gas combustion [3]; hence, contributing to fossil fuels depletion and the increase of greenhouse gas emissions [1]. Migrating toward a sustainable hydrogen economy requires developing production routes based on the carbonneutral energy sources such as concentrated solar energy [3]. This chapter devotes toward the discussion of solar-driven thermochemical cycles for hydrogen production through water splitting, a process that has the advantage of using both carbon-neutral energy sources and abundant materials.

10.2 Thermochemical solar hydrogen production In the recent years, technological advances in solar concentrating systems have driven the development of novel processes to produce hydrogen from solar thermal energy with high efficiencies [1]. Some of the most promising techniques for hydrogen production are the solar thermochemical processes. The main advantage of this approach is that it utilizes the entire solar spectrum, and as such, provides a favorable thermodynamic path to solar fuels production with potentially high solar-to-fuel efficiencies and without the use of precious metal catalysts [4]. Thermochemical processes make use of highly concentrated solar energy provided by concentrating systems to carry out high-temperature endothermic chemical reactions. The fundamental structure of thermochemical processes is the following: first, solar energy passes through optical concentrating devices that enable obtaining high temperatures. These systems consist of highly reflective structures that follow the trajectory of the sun, concentrating it in a finite spot. The main

Hydrogen from solar thermal energy

concentrating technologies used in thermochemical processes are parabolic dishes, solar furnaces, or central receiver systems (also known as power towers) [5]. Afterward, concentrated solar energy is absorbed in a solar reactor where high operating temperatures are taken advantage to produce hydrogen [6]. Thermochemical methods include several routes based on either hydrocarbon conversion, such as reforming, cracking, or gasification of hydrocarbons; or H2O splitting, such as thermochemical cycles or direct thermolysis [7]. The first three routes produce syngas (a mixture of H2 and CO in different proportions that depend on the process), while thermochemical cycles and direct thermolysis of water produce pure hydrogen.

10.2.1 Thermodynamics of thermochemical processes In general terms, in solar thermochemical processes, concentrated solar radiation provides the energy required for hydrogen production by dissociation of water molecules or hydrocarbons [8]. The most straightforward process for hydrogen production is by water thermolysis; however, this process shows some disadvantages that are explained in detail in the following sections: 1 H2 O ! H2 + O2 Δh ¼ 285:83kJ mol
1 ð1bar,298KÞ 2

(10.1)

The energy necessary to carry out this process is 285.83 kJ for 1 mol of water (Δh). One portion of this energy is the Gibbs free energy of the reaction, (Δg), and must be supplied as high-quality energy in the form of work, for example, electrical energy. The difference between Δg and Δh is TΔs, the amount of energy that can be provided as thermal energy: Δg ¼ Δh
T Δs

(10.2)

For example, let us go back to water-splitting reaction (Eq. 10.1). If we want to perform this reaction in an ideal electrolyzer that decomposes 1 mol of water at room conditions (298 K and 1 bar), it is necessary to supply 237.14 kJ of electrical energy (Δg) to accomplish this process; however, the system also needs 48.69 kJ from its surroundings to bring up the total amount of energy to 285.83 kJ [9]. The variation of the Gibbs free energy and TΔs as a function of temperature indicate that Δg decreases, whereas TΔs increases with temperature. According to the above mentioned, the relation Δg/TΔs decreases as augmenting temperature, that is, the ratio of work to thermal energy is much lower at high temperatures. For this reason, when using concentrated solar energy, it is possible to reach high temperatures that allow to perform reactions only with thermal energy [8]. The level of temperature needed to carry out the process with thermal energy depends on the reaction, but for all cases, such level must guarantee that reaction proceeds spontaneous to the right (i.e., Δg  0).

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Performing thermochemical solar processes with concentrated solar energy requires the development of a special kind of chemical reactors, known as solar reactors. This kind of reactors must efficiently absorbs concentrated solar radiation, minimizing material sintering, and wear, while using solar energy as a means to drive chemical reactions [10]. Solar reactors usually feature a cavity, consisting of a well-insulated enclosure with a small aperture to trap incoming solar radiation [11]. In this respect, the solar energy absorption efficiency is the fraction of incident solar power from the concentrator that is absorbed by the cavity [12]. For a perfectly insulated solar cavity (i.e., negligible conduction and convection losses), it is estimated by applying the first law of thermodynamics: ηsolar ¼

IAηA α
EaσTc4 IA

(10.3)

where I is the direct normal irradiance, A is the solar concentrator or collector area, and ηA corresponds to the solar concentrator optical efficiency. On the second term of the numerator, a is the solar cavity receptor area, σ is the Stefan-Boltzmann constant, and Tc is the cavity temperature. Finally, α and E are the absorptance and emittance of the solar cavity, respectively. The first term of Eq. (10.3) represents the incident energy, coming from the concentrator, and absorbed by the cavity. The second term is the energy emitted by the cavity receptor at the temperature Tc, and IA is the energy from the concentrator. According to Eq. (10.3), increasing the temperature of the cavity results in high radiative losses. Thus, it is desirable to have a cavity receptor with high absorptance and low emittance [12]. Assuming a perfect optics of the concentrator, the concentration ratio of the system C ¼ Aa and a blackbody cavity (absorptance and emittance equal to 1) Eq. (10.3) reduces to ηsolar ¼

IC
σTc4 IC

(10.4)

The absorbed energy by the cavity is used to drive chemical reactions, while the solar to fuel efficiency (ηsolar-to-fuel) gives us the amount of solar energy stored as chemical energy (energy vector bonds) in a thermochemical process [5]. This efficiency is usually defined as ηsolar
to
fuel ¼


Δg Qsolar

(10.5)

where Qsolar is the solar energy coming from solar concentrator. Some studies reported (ηsolar-to-fuel) based on the Δh considering the high heating value and the amount of produced fuel. The maximum solar-to-fuel efficiency for an ideal cycling process is determined by the exergy efficiency (maximum exergy efficiency, ηmax, energy). This efficiency can be estimated by applying the second law of

Hydrogen from solar thermal energy

Fig. 10.1 Maximum system efficiency as a function of the cavity upper operating temperature for several concentrations ratios of the furnace (for a TL ¼ 300 K).

thermodynamics and is defined as the product of the solar energy absorption efficiency of a cavity and the Carnot efficiency [5, 8]:    σTH4 TL (10.6) ηmax, energy ¼ ηsolar ηcarnot ¼ 1
1
IC TH where ηcarnot is the Carnot efficiency, TH and TL are the upper and lower operating temperatures of the cavity. Maximum system efficiency as a function of temperature is depicted in Fig. 10.1, where it can be appreciated that ηmax, energy increases as cavity upper temperature rises; reaching a maximum at a given concentration ratio and decreasing to zero. In addition, it can be observed that higher concentration ratios result in higher system efficiencies, which is why thermochemical processes are intended to be carried out at high temperatures; nonetheless, higher temperatures also involve higher radiation losses. Hence, it is crucial to find optimal cavity conditions that allow reaching high upper operating temperatures, while keeping radiation heat losses low.

10.3 Solar thermolysis The simplest solar thermochemical process for hydrogen production is the splitting of water. This process takes place at temperatures above 3000 K. The overall reaction can be described as follows [13]: H2 O ! x1 H2 O + x2 OH + x3 O + x4 H + x5 O2 + x6 H2

(10.7)

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The direct solar-driven splitting of water was widely studied in the period of 1975–85. The main research during this period consisted in thermodynamic analysis and studies that demonstrated the feasibility of using concentrated solar energy to carry out the process. Thermodynamic studies indicate that at 2000 K and 1 bar of pressure, around 96% of water remains unreacted, whereas at 2500 K and 0.05 bar, only the 25% of water is dissociated [14]. The above studies exhibit that high temperatures and low pressures favor the dissociation of water [12]. Although it is possible to reach high temperatures with high-flux solar concentration systems, the thermolysis of water has been scarcely studied because there are some major withdraws that need to be solved. The first limitation is related with the construction materials of the reactor. There are a limited number of materials capable to withstand temperatures above 2500 K and hightemperature gradients. The second problem consists in the separation of the reaction products. In this case, it is necessary an effective in situ separation system to avoid recombination of H2 and O2 [12, 14]. Despite the above-mentioned limitations, a few reactors’ prototypes had been designed to perform solar thermolysis of water [13–17]. In such studies, authors had proposed the use of high-temperature refractory materials, such as zirconia, which have thermal stability at temperatures up to 2000 K [14], but low resistance to temperature gradients. This material has been used for both, the housing and insulation of solar reactors in the form of board or felt, and as porous membrane or crucible that are directly irradiated with concentrated solar energy [12–14]. However, in these works, the separation of hydrogen from mixture is still a critical issue not only to avoid forming an explosive mixture, but also to avoid efficiency losses due to recombination [18].

10.3.1 Separation techniques In general, the thermolysis process can be divided in two categories according to the gas separation process [13]. The first category includes approaches where the gas separation is carried out at the reaction temperature, and the second group where the separation of gases is performed by rapid cooling or “quenching” of the reactor gases outlet. The gas separation at the reaction temperature can be performed by using selective membranes to separate H2 or O2 of the mixture [15] or using new techniques like supersonic jets or centrifugation. In the case of using selective membranes, these can be microporous refractory membranes or membranes semipermeable to oxygen. The high-temperature in situ separation using membranes has been used in some solar experiments [13, 14, 17]. On the other hand, the separation by centrifugation and supersonic jets has been only proposed as a promising technique for the separation of hydrogen from the gases mixture, but to our best acknowledge none of these last techniques has been experimentally demonstrated [13]. The separation of hydrogen and oxygen by rapid cooling or quenching technique consists in a rapid decrease of temperature within various milliseconds. The time necessary for quenching the

Hydrogen from solar thermal energy

exit gases must be much shorter than the frequency factor of the reaction in order to stabilize the gas composition in the mixture. After quenching, hydrogen is separated from the mixture with some conventional methods [13].

10.4 Thermochemical solar cycles The main disadvantages in solar thermolysis of water motivate the possibility of lowering temperatures. One potential option consists in using metal oxides through thermochemical cycles. In this type of process, water is the principal input and oxygen and hydrogen and unreacted water are the main products [19] (Fig. 10.2). The study of thermochemical processes begun in the late 1960s with the results of the project Energy Depot carried out in the beginning of the 1960s. The main objective of this project was to produce fuels, such as hydrogen, ammonia, and hydrazine, from simple materials like earth, water, and air by using waste heat from nuclear reactors as energy source of reactions. The results of this project were not satisfactory; however, they motivated continuing the study of hydrogen production using other energy sources such as solar [20]. At the moment, over 350 thermochemical cycles are recognized to have the potential to produce hydrogen with high efficiencies [21]. These cycles are classified into two subcategories: multistep and two-step cycles. Multistep solar thermochemical processes require three or more steps to obtain hydrogen and usually the maximum temperature is around 900°C. This type of processes can be purely thermal or hybrid when including electrochemical step. Multistep cycles have been

Concentrated solar energy

MxOy

MxOy–1+ y/2 O2

O2

H2O H2 M xOy–1+ H2O

MxOy + H2 Temperature level

Fig. 10.2 Two-step thermochemical solar cycles scheme.

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scarcely studied with solar energy due to complexity of the process, which include large number of separation steps, difficulties of recover materials, and thermal losses [19]. On the other hand, two-step metal oxide thermochemical cycles require higher temperatures, but the simplicity of the process, high-efficiency, and several applications of the products reinforce the idea of using these cycles as a promising option for hydrogen production. The first step of these cycles consists in the reduction of a metal oxide at high temperatures where oxygen is released and the metal oxide is reduced to a lower valence state. The subsequent step comprises hydrogen production at low temperature, where the reduced metal is oxidized back by taking oxygen from water. In this last step, the metal oxide is regenerated establishing a cyclic process, where the metal oxide is used again in the first step [22]: y Mx Oy ! Mx Oy
1 + O2 (10.8) 2 Mx Oy
1 + H2 O ! Mx Oy + H2 (10.9) Since the metal oxide is regenerated, the net reaction in this type of process is the splitting of water: H2 O ! H2 + 12 O2 . According to the literature, two-step thermochemical cycles can be divided in two categories: nonvolatile and volatile cycles. In a nonvolatile cycle, the reduced metal oxide remains in solid phase, and the reduction of the materials is stoichiometric or nonstoichiometric. In the first category, there is a change in the crystal structure of the metal oxide, whereas in nonstoichiometric cycles, there is a partial reduction of the metal oxide. Otherwise, volatile cycles exhibit a solid to gas-phase transition of the reduced material because reduction temperature is greater than the vaporization temperature of the metal oxide [4]. In the last years, a considerable number of these cycles have been investigated and the majority of research efforts has been focused on the study of the following redox pairs: ZnO/Zn, Fe3O4/FeO, and CeO2/ Ce2O3. Other materials, like perovskites, have been recently proposed as attractive materials for hydrogen production [23].

10.4.1 Thermodynamics of two-step thermochemical cycles The selection of optimal conditions for performing thermochemical cycles depends on thermodynamics of these cycles [24]. Thermodynamics is conceived using the standard change of Gibbs energy at nonstandard pressures:   1 pO2 Δg ¼ Δh
T Δs + RTIn (10.10) p 2 where the pO2 is the oxygen partial pressure, the enthalpy and entropy change are the difference of enthalpy or entropy between reactants and products Δh ¼ hreactants
hproducts and Δs ¼ sreactants
sproducts. Nowadays, there are several softwares that contain extensive databases of several metal oxides that allow to calculate chemical thermodynamic aspects of various thermochemical

Hydrogen from solar thermal energy

cycles, some examples are FactSage and HSC Chemistry. Two-step thermochemical cycles involve two different temperatures: reduction and oxidation temperatures, which are usually defined when the standard change in Gibbs energy of Eq. (10.10) is zero. A plot of the reduction temperature versus the pressure for several redox pairs [24] show us that the reduction temperature decreases at lower partial pressures, which can be performed by using inert sweeping gas or operating in vacuum conditions; however, both cases involve energy penalties obtaining an impact in the efficiency of the process [4]. This type of graphs is also useful to determine if the reactant or product undergoes a phase change, which can be observed when the curve has a slope change [24]. Oxidation temperature is also an important element that should be considered when selecting a suitable thermochemical cycle for hydrogen production. In this case, the oxidation reaction proceeds spontaneously when Δgox  0, which results for [24]: Δhred > Δhws, where Δhws is the enthalpy change of the direct water-splitting reaction and Δhred is the enthalpy change of the reduction reaction. According to the above mentioned, higher conversions of H2O to H2 are obtained for larger values of Δhred [24]. For example, the enthalpy change of the reduction of ZnO is around 350 kJ mol
1 at 25°C, whereas for the water splitting is 250 kJ mol
1 at 25°C. In this case, Δhred, ZnO > Δhws which means that this redox pair has a satisfactory oxygen affinity to split water. The Fe3O4/FeO, CeO2/CeO2
δ, and perovskite cycles have also similar characteristics that ZnO/Zn cycle with satisfactory perspectives for hydrogen production. Thermodynamic analysis is a useful tool to determine upper efficiency and the limits of the process; however, this type of analysis is not an exclusive way for analyzing the viability of a redox material. Other studies, such as reaction kinetics, are necessary because they give important information related to reaction extent and efficiencies that affect the performance of a reactor operated under real conditions. This type of information in addition with thermodynamics are necessary for the design of novel prototypes solar reactors [25, 26]. Aside from the design of solar reactors, the materials physicochemical properties can be obtained by following the general principles of high-temperature material science, and are of great importance to optimize solar-driven thermochemical cycles [25]. In the following sections, a review of the recent advances in the synthesis and kinetic analysis of ZnO/Zn, Fe3O4/FeO, ferrites of the type (AxFe3
xO4), CeO2/CeO2
δ, and perovskites redox pairs is analyzed.

10.4.2 ZnO/Zn ZnO is a white powder insoluble in water, has an n-type semiconductor behavior due to oxygen vacancies or Zn interstitials. Based on its optical properties, it is considered as a transparent material since it has the greatest UV absorption of all commercial pigments.

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ZnO is an interesting material to produce thermochemical solar fuels due to its ability to give two electrons for conversion per formula unit. This characteristic among others, such as high efficiency at moderate temperatures and high-specific fuel capacity [26], situates the ZnO/Zn cycle as a promising candidate for solar hydrogen production. The ZnO/Zn cycle was proposed in 1977 by Bilgen et al. [16] as an attractive option for solar H2 production. In such study, the authors demonstrated the technical feasibility of the use of solar energy as a heat of source of the chemical reaction. The first step of the ZnO/Zn cycle involves the thermal reduction of ZnO to Zn at temperatures above 2000 K: 1 ZnOðsÞ ! ZnðgÞ + O2 2

(10.11)

Followed by an exothermic reaction of metallic Zn with water to obtain H2 and ZnO at temperatures below 1300 K: Zns + H2 O ! ZnOðsÞ + H2

(10.12)

In the last three decades, the ZnO cycle has been extensively studied from thermodynamics to large-scale solar reactor development. However, the process has still some problems that hinder its development to a commercial stage. One of the main problems is related with the products of the reduction reaction. The Zn is in gas phase and a rapid quenching is necessary to recover metallic Zn and to avoid recombination of Zn(g) with O2, followed by a separation process to separate O2 from sweep inert gas (usually argon) [26]. The above-mentioned causes big challenges in solar reactors design, and increases the production costs of hydrogen: a techno-economic study performed in 2016 for a Zn/ZnO solar hydrogen production facility of 110 MW, found a hydrogen production cost of $53 kg
1 [26], which is not competitive with the actual production cost using nonrenewable sources. 10.4.2.1 Synthesis ZnO is obtained by different technologies, such as (1) oxidation of pure zinc under vapor phase, known as the French process; or (2) roasting ZnO in its franklinite structure or other ores with coal followed by oxidation in air [27]. The progress made when using this material is not related with its synthesis procedure itself since it evaporates, but has more relation with the physical form of ZnO. That is, it can be used as blocks, bulk, and powders with different purities; and the main issue is to design strategies that allow to have high surface areas [23]. For example, the ZnO powders exhibit higher surface area compared to ZnO blocks obtaining larger amounts of H2 and CO during the second step of solar thermochemical processes [24, 28, 29].

Hydrogen from solar thermal energy

10.4.2.2 Kinetics The analysis of the kinetic performance of both, reduction and WS reactions (Eqs. 10.11, 10.12) is a critical issue that allow to optimize the size of solar reactors, and thus obtaining higher efficiencies and competitive hydrogen costs [30]. The kinetic studies for the ZnO/Zn cycle usually are carried out in thermogravimetric (TG) balances, where it is possible to obtain in well-controlled conditions a precise measurement of the reaction progress and to determine kinetic parameters related with the microscopic chemistry of the reaction. Alternatively, global reaction kinetic parameters can be obtained by using a global inverse method, which is more convenient for general purpose studies. In this last case, larger amounts of material are used compared to a TG, which promotes some physical phenomena such as heat and mass transfer limitations [31]. In addition, the kinetics of the ZnO/Zn cycle has been also studied with a solar TG balance, which consists in a well-insulated cavity equipped with a balance to measure the mass loss of a sample subject to concentrated solar radiation. The main objective of this equipment is to obtain kinetic parameters under more real conditions, that is, high radiative fluxes and high heating rates [32]. TG studies performed in conventional TG equipment had demonstrated that the reduction reaction (Eq. 10.11) starts at temperatures around 1500 K, but requires temperatures above 1824 K to reach an acceptable dissociation degree. The reduction reaction follows a temperature dependence described by the Arrhenius law, that is, k ¼ k0 expðEa =RT Þ, where k is the reaction rate constant, k0 is the preexponential factor, Ea is the activation energy, and T is the temperature. Several studies had been performed to obtain Ea, for example, Hieschwald and Stolze obtained an activation energy of 319 kJ mol
1 in the temperature range of 1130–1385 K under a reduced pressure of 1.33 mbar [33]. Other works have analyzed the effect of the ZnO particle size and the initial loaded mass in the kinetic parameters determining that these variables have no significant effect [34]. In addition, it was found that the rate of reduction reaction decreases as increasing the oxygen concentration; therefore, this step requires low oxygen partial pressures, which can be obtained by using a constant flow of inert gas or vacuum conditions; however, both impact in the efficiency [26, 35]. The purity of the material also impact in the reaction rate. Weindekaff et al. [35] found that the presence of impurities in the solar regenerated ZnO obtained with the water-splitting reaction (Eq. 10.12) enhance the dissociation rate. On the other hand, Schunk et al. performed isothermal runs in a solar-driven TG balance to obtain Ea and k0. Their experiments were carried out in a temperature range of 1834–2109 K, obtaining that the reduction reaction is fitted to zero-order Arrhenius law with an apparent activation energy Ea ¼ 361  53 kJ mol
1 K
1 and a preexponential factor of k0 ¼ 14.03  106  2.73  106 kg m
2 s
1. Finally, Lev^eque and Abanades obtained the intrinsic kinetic parameters of the ZnO reduction via an inverse method by using an iterative method where the resulting O2 concentration is taken into account.

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Solar hydrogen production

With this method, an activation energy of 288 kJ mol
1 and k0 ¼ 1.6  108 s
1 were obtained. The water-splitting reaction kinetics has been reported in a few works. Berman and Epstein investigated the WS reaction of liquid zinc in a temperature range of 723–773 K obtaining that the specific reaction rate increases with the partial pressure of water, which is represented with the following expression: kP

H2 O Wsp ¼ ð1 + bP , H OÞ 2


10

where

k ¼ 1:86  103 expð
40, 376=RTÞ mol cm
2 s
1

and


1

b ¼ 1:55  10 expð146,330=RTÞ bar . The authors concluded that the WS step with liquid zinc is viable, and that the decisive step of WS reaction is the diffusion of reactant through the zinc oxide layer [36]. On the other hand, Wegner and Melchor analyzed the oxidation of nanoparticles of Zn at temperatures ranges between 1023 and 1073 K establishing that nanoparticles enhance reaction kinetics, heat, and mass transfer obtaining nearly complete oxidation [37]. The particle size and purity of Zn was also studied by Lv et al. [38]. In their analysis, they found that the water partial pressure scarcely impacts the zinc conversion, while the impurities of Zn highly impact the WS reaction.

10.4.3 Fe3O4/FeO and ferrites (AxFe32xO4) The thermochemical cycle based on the Fe3O4/FeO redox pair was the first cycle proposed to produce hydrogen with solar energy originally presented by Nakamura in 1977 [39]. The cycle involves the reduction of magnetite to FeO at temperatures above 2500 K at 1 bar [40]: 1 (10.13) Fe3 O4 ! 3FeOðlÞ + O2 2 In a subsequent step, FeO is reacted with water to obtain H2 at temperatures below 1000 K [40]: FeO + H2 O ! Fe3 O4 + H2

(10.14)

The ferrite cycle is one of the most studied cycles because its potential for deep reduction and oxygen affinity for hydrogen production [25]. However, the reduction of Fe3O4 occurs at temperatures above the melting points of FeO (1650 K) and Fe3O4 (1870 K), which produces a rapid deactivation due to sintering obtaining serious challenges for its practical implementation [24, 41]. An alternative to avoid this problem consists in incorporating different divalent metals in the ferrite structure, which increases the melting point and lowers the reduction temperature. Some examples of such materials are Ni, Zn, Mn, and Co [42]. However, some redox materials exhibit a poor H2 conversion and stability [24, 25], for example, the ZnFe2O4 partially decomposes forming Zn gas during reduction reaction [43].

Hydrogen from solar thermal energy

10.4.3.1 Synthesis Ferrites are prepared by dissolving ferric oxide using concentrated alkali solutions, by melting ferric oxide with alkali metal hydroxide, carbonate or chloride, or just heating ferric oxides with some metal oxides [27]. Ferric oxides have been supported on hightemperature ceramic materials, either by synthesizing via a wet chemistry method to alumina [44], zirconia (ZrO2) [44, 45], or yttria-stabilized zirconia (YSZ) nanoparticles [46] by a precipitation method from their nitrates precursors. The main objective of using these supports is to increase the interactions ferric oxides-support and have higher hydrogen [46], oxygen, or CO production [44, 45] yields. Specifically, ferric oxide supported on YSZ nanoparticles resulted in higher yield of hydrogen production compared to ZrO2 support due to the incorporation of iron to the unit cell lattice of YSZ, which inhibited the iron oxide sintering at higher temperatures during thermochemical water-splitting reaction [46]. Atomic layer deposition (ALD) is another technique to deposit iron oxide and cobalt ferrites onto a high surface zirconia support. Ferrocene and cobacene materials were deposited with this technique onto a ZrO2 support, showing rapidly and repeatedly cycling with no sintering, and thus no deactivation [47]. The results of this work demonstrated that the ALD technique is a promising route for the deposition of metal oxides onto porous ceramic surfaces that could be applied also for other metal oxides. As mentioned earlier, different doped ferrites have been synthesized and used in solar thermochemical cycling processes to produce hydrogen or carbon monoxide. Ni and Co ferrites pellets prepared by a simple and cheap handmade technique have been tested in thermochemical cycling processes [48], as well as their casting techniques [49], where their shape and appearance was kept after 10 cycles with no sintering [48], and hydrogen concentrations were higher when the material is in powder form [49]. Ni-doped ferrites have been the best material used so far for thermochemical water and CO2 splitting, but the limited surface area after several cycles has been the main issue. In order to increase the surface area and porosity, Ni-doped ferrite has been prepared by conventional ceramic processing route using zirconia and sacrificial templates to improve performance [50]. Another strategy to improve surface area has been the synthesis of Ni-doped iron oxide nanoparticles using a sol-gel synthesis technique. This approach has led to higher surface area material, where Ni substitution lower that 60% increasing CO production, although only one thermochemical cycle was performed with this type of materials [51]. Zr-doped cobalt ferrite supported on silica has been prepared by a simple sol-gel auto-combustion synthesis and used for two-step thermochemical cycles for CO2 splitting, showing higher surface area that resulted in higher CO yields and recyclability compared to undoped and unsupported cobalt ferrite. This is related to the reduced sintering effect observed when using silica as the support [52]. Mn-doped ferrites synthesized by a hydrothermal method has resulted in different microstructures of the material, and has shown that smaller particle size and fine crystallinity showed higher H2 production due to a more intimate contact between particles and better ionic transport [53, 54]. On the other hand, aerosol spray

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pyrolysis synthesis has been used to obtain oxygen deficient mixed doped metal ferrites from metal nitrate precursors (Fe, Mn, Zn, and Ni), resulting in good hydrogen production for the Ni-doped ferrite [55]. Also, core-shell NiFe2O4/Y2O3 structures from sol-gel synthesis have been prepared, where grain growth or sintering through multiple thermochemical cycles was prevented showing a stable H2 volume generation. Nevertheless, it was shown that Y2O3 acted as a diffusion barrier for the thermochemical reactions of NiFe2O4 resulting in lower H2 generation volumes [56]. 10.4.3.2 Kinetics The kinetics and reaction mechanism of ferrites and doped ferrites cycles has been poorly studied and research efforts have been focused on analyzing other chemistry aspects, such as synthesis methods, stability and cyclability, support material, etc. [57] The reduction of Fe(III) to FeO occurs in two successive steps, in which Fe2O3 is first reduced to Fe3O4 at temperatures around 1500 K under air atmosphere. In a second step, magnetite is reduced to FeO. In this step, the formation of FeO begins after Fe3O4 melting point [58]. The final product of the reduction phase is a mixture of FeO and nonstoichiometric wustite. According to the previous studies, the kinetics of the first reduction step is very slow compared to the second reduction step, which is rapid [58]. Regarding hydrogen production with Fe(II), Charvin et al. [59] performed a quantitative analysis at temperatures below 873 K to determine the kinetics of reaction and the influence of the particle size and temperature on the conversion of the WS reaction (Eq. 10.13). They found that hydrogen production highly depends on temperature, especially at the beginning of the reaction. In addition, they observed that the WS reaction forms a layer of Fe3O4 in the particle, which decreases the reaction rate due to mass transfer limitations by diffusion. Abanades and Villafan-Vidales [60] also studied the reactivity of the FeO powder to produce CO and H2 at temperatures above 873 K. At this temperature range, it was found that the maximum hydrogen production was 89 NL kg
1 FeO at 1073 K after 95 min. They also found that solar nonstoichiometric Fe1
γ O enhances the reaction due large amount of defect clusters. Loutzenhiser et al. [61] carried out a TG analysis for the CO2-splitting reaction with FeO. They perform isothermal experiments at temperature ranges from 923 to 1473 K to obtain kinetic parameters. They found that the reaction order is near 0.8 and can be described with a shell-core kinetic model. As mentioned in the previous section, for doped ferrites research efforts had concentrated in the synthesis of more stable materials capable to decrease the reduction temperatures. Only a few works are devoted to study the reaction rate and mechanism of reaction of different doped ferrites. For example, Go et al. [62] obtained kinetic parameters of Mn and Zn iron oxides, finding that chemical conversion in both materials increases at temperatures above 1073 K. Regarding the reaction mechanism, authors observed that the reduction reaction of Mn iron oxide is described by the diffusion controlling mechanism, whereas Zn iron oxide follows a first-order reaction. The calculated

Hydrogen from solar thermal energy

activation energy fluctuates between 139 and 572 kJ mol
1. In the WS reaction, authors observed that the incorporation of Mn and Zn cations in the iron lattice structure lowers the oxidation temperature and increases the reaction rate. This reaction is limited by diffusion in the product layer. In the case of doped ferrites supported on high-temperature ceramics, the comprehension of the kinetics in such conditions has also barely analyzed. Neises et al. [42] investigated the reaction kinetics of zinc ferrite supported on a SiC honeycomb structure. In their experiments, they observed that the WS reaction follows a zero order and is controlled by internal diffusion of the gas through the product layer. They also conclude that the effect of water concentration in the reaction rate is negligible and that the impact of temperature in the reaction rate is described by Arrhenius law, obtaining a Ea ¼ 110 kJ mol
1. Finally, Scheffe et al. [57] analyzed the chemical behavior of cobalt ferrite/ZrO2 composite using an ALD method. They observed that WS reaction is better described when combining multiple reaction mechanisms: diffusion and second order.

10.4.4 Ceria Ceria is a pale yellow heavy powder with a melting point of 2600°C that is obtained by decomposition of cerium oxalate with heating [27]. The ceria cycle was proposed in 2006 by Abanades and Flamant [63] as a promising system for solar hydrogen production by reducing CeO2 to Ce2O3 at temperatures above 2223 K. The main drawback of this cycle is the reduction at high temperatures, which produces partial sublimation of the material, obtaining material losses and low efficiencies [64]. One option to prevent the above, consists in lowering the reduction temperature by obtaining the partial nonstoichiometric reduction and oxidation [65]: 1 CeO2 ¼)CeO2
δ + δO2 2 CeO2
δ + δH2 O ! CeO2

(10.15) (10.16)

In this cycle, the oxygen exchange capacity of nonstoichiometric ceria is lower than iron oxide; however, the process is attractive because fast splitting kinetics and sintering problems are much lower due to higher melting point of ceria. The earlier last point also simplifies the process because supporting this material on more stable structures is not necessary; therefore, fixed bed or fluidized solar reactors are suitable to perform the cycle [21, 24]. The nonstoichiometric ceria cycle has been extensively studied from thermodynamics to a laboratory scale solar experiments, where the cyclability and stability of the cycle was demonstrated [26, 66–68]. Nevertheless, the main issue with this cycle is related with the low oxygen partial pressure that is needed in the reduction step which strongly impacts in the efficiency of the process [25, 26]. In order to make this process economically viable is necessary to incorporate in the reactor an efficient recuperation of heat

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[21]. Alternatively, other option that has been widely studied consists in improving ceria by the insertion of dopants, such as Zr, Hf, or tantalum, to reduce the reduction temperature and improving the reduction yield of H2 [64]. However, not all dopants maintain its cycling capability and in a few cases H2 production decreases compared to undoped ceria [25, 64, 69]. 10.4.4.1 Synthesis As mentioned earlier, doped ceria has been used to perform thermochemical hydrogen generation with improved performance, specifically doped with zirconium and later with lanthanum or gadolinium, improving the thermal stability [70]. These doping process has been achieved following a coprecipitation of ceria hydroxide and nitrate or chloride metallic precursors [69], or via the citrate nitrate auto combustion route [71]. Specifically, zirconia-doped ceria has been obtained using wet chemical routes to ensure deposition on porous ceramic supports used in solar chemical reactors. These wet chemical routes include the coprecipitation of hydroxides already mentioned, hydrothermal synthesis, the Pechini synthesis, and/or sol-gel synthesis using alkoxide precursors. The used synthesis methodology determines the powder morphology, and has been observed that materials obtained with the Pechini method results in a zirconia-doped ceria with a porous morphology resistant to temperature avoiding deactivation during thermochemical cycling for water and carbon dioxide splitting [64]. In addition to Zr-doped ceria, Hf-doped ceria, both synthesized via a sol-gel method, have shown to be promising candidates for CO2 splitting [72]. Specially Hf-doped ceria have shown improved performance compared to pure ceria, since Hf shifts the crystallization process to higher temperatures. Nevertheless, these doped ceria were not capable of CO2 reoxidation, while the pristine ceria reoxidation was facilitated [72]. On the other hand, pure ceria has been used in the form of powder and in monolithic and reticulated foam structures made from ceria. These structures can reach more homogeneous temperatures. However, in these structures, the specific surface area is low, obtaining slow oxidation reaction [26, 73]. With the aim to improve the characteristic of foam structures, Furler et al. [73] designed structures with dual-scale porosity, which were obtained by using templates of carbon particles for obtaining micropores in the ceria foam structure. 10.4.4.2 Kinetics The reduction step of the ceria cycle is carried out at 1773 K and oxygen partial pressures between 10
6 and 10
3 atm, and the WS reaction is performed between 873 and 1273 K with water partial pressures of 0.25–0.27 atm [66]. Experimental test of 500 cycles demonstrated that above 100 cycles, the hydrogen production rate decreases around 50% of initial production, due to sintering of material [65]. Other studies used macroporous structures with millimeter pore sizes made of CeO2 obtaining better volumetric

Hydrogen from solar thermal energy

absorption of incident solar radiation and more homogeneous temperatures. The above increases the solar to fuel conversion efficiency about four more times than previous studies [68]; however, the oxidation rate is slower due to low specific area. As mentioned in the previous section, these structures can be optimized by implementing a dual-scale porosity. TG analysis concluded that micropores increase the specific surface area and enhance the oxidation reaction rate by an order of magnitude. The morphological stability of the samples was also studied obtaining stable structures after 120 h; however, the specific surface area is affected by the temperature. The kinetic performance of these structures follow an Arrhenius-type temperature dependence [73]. In the case of doped ceria, it was found that kinetic parameters depend on the synthesis method and that the morphology of the obtained powder impacts in the cyclability of the material [69].

10.4.5 Perovskites Perovskites materials have been studied recently for thermochemical solar hydrogen production, due to oxygen exchange ability during cycling [74], and its stability and versatility to introduce into its structure different metal ions obtaining a wide variety of material configurations [21, 74, 75]. In addition, these materials have been proposed as an alternative to ceria by improving the reduction yield at lower temperatures [4]. Perovskite has the general form ABO3 or A2BO4, where A represents large cations and B smaller cations [75]. The redox reactions for hydrogen production with perovskites are the following [4]:   δox
δred ABO3
δox ! ABO3
δred + (10.17) O2 2 ABO3
δred + ðδred
δox ÞH2 O ! ABO3
δox + ðδred
δox ÞH2

(10.18)

Although some configurations of this material improve fuel yield compared to that obtained with CeO2, it is necessary an excess of oxidant, which it is not practical and reduces efficiency [76]. Other important issue with perovskites is that is mandatory to improve its chemical stability, which also affects the efficiency [74]. 10.4.5.1 Synthesis There are only few works relating the synthesis methodology with their ability to perform thermochemical cycles. Commercial La1
xSrxMeO3 with Me ¼ Mn, Co, and Fe perovskites have been used for water splitting, resulting in higher hydrogen yields for cosubstituted perovskites [75]. Perovskites have been synthesized by solid-state reactions of stoichiometric amounts of the different oxide and carbonate precursors [77]. These perovskites show high reactivity toward oxygen vacancy formation upon heating, making them very useful for CO2 and water splitting to produce hydrogen and CO. It has been shown that several manganese perovskites have even superior behavior than ceria

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[74, 78]. For example, La0.6Sr0.4Mn1
xAlxO3 perovskites system synthesized with a modified Pechini method were used to study CO2 splitting by thermochemical redox cycles, revealing structural stability, no phase segregation, and very fast oxidation kinetics in 10 cycles as expected for Al-doped perovskites [79]. 10.4.5.2 Kinetics In the case of perovskites, to date only a few kinetics studies have been performed. However, several perovskites have been investigated to analyze its stability and capacity for hydrogen production. Studies performed on (La, Br, Sr) (Co, Fe) O3
δ, (La, Sr) FeO3
δ, and (La, Sr) CoO3
δ, (La, Sr) MnO3
δ had reported that La and Sr-based perovskites favor the reduction degree compared to ceria at temperatures above 1500 K; however, the oxidation reaction is incomplete and depends on the oxidant concentration [80]. Conversely, LaxSr1
xMnyAl1
yO3 materials exhibit a reaction kinetics similar to ceria cycle and stability of 80 redox cycles [21, 81]; however, the increase of the H2 production results in a decrease of reaction rate [82].

10.5 Solar reactors Solar thermochemical processes need to be carried out in specifically designed reactors, loosely referred to as solar reactors. Solar reactors, unlike traditional chemical reactors, need to efficiently utilize high-intensity radiation as an energy source [83]. This stems from the fact that high-intensity radiation is readily available in the form of solar energy by using concentrated solar power (CSP) technologies, allowing solar reactors to attain maximum temperatures exceeding 1500°C [40] in a sustainable and potentially affordable way. Controlling the reaction rates of two-step thermochemical cycles requires accounting for the energy being transported to determine temperature fields. Nonetheless, due to the strong role of solar radiation within the reactor, radiative transfer considerations in conjunction with other aspects such as heat, mass, and momentum transfer heavily determine reactor design [84]. Moreover, due to the nature of radiation itself, a considerably different behavior is exhibited when compared to other energy transfer mechanisms; as such, the design of solar reactors tends to be considerably different from their traditional counterparts. More specifically, thermal radiation is transported by photons, whereas heat conduction and convection are transported by phonons. Radiation transport is governed by direction-dependent phenomena, so that the geometry of a solar reactor must be carefully defined, that is, the reactor design and the metal oxide layout must redirect incident, reflected, and emitted radiation, such as to minimize thermal gradients while absorbing most of the incoming radiation. Nonetheless, while radiation is a dominant phenomenon within a solar reactor, other aspects such as mixing patterns and mass transport cannot be overlooked, hence complicating the design procedure. Different metal oxides present different challenges, for example, some require higher operating temperatures while

Hydrogen from solar thermal energy

Cavity-type solar reactor

Cavity

Concentrated solar energy

Insulation

Fig. 10.3 Scheme of a typical insulated cavity-type solar reactor.

others present phase changes during operations. Although there is no global consensus regarding how a solar reactor should look like, an insulated cavity-type geometric configuration (Fig. 10.3) is, usually, a feature that most solar reactors share, as this favors the efficient capture [85]. Since the early 1980s many solar reactor concepts have been conceived, depending on the metal oxide state, which can be suspended or supported; the contact between the solar energy and the metal oxide, which can be directly or indirectly heated; the operating mode, which can be continuous or semibatch; and the scale of the solar reactor, which defines the kind of CSP technology used to heat the reactor. In this section, different solar reactor concepts are presented along their advantages and disadvantages.

10.5.1 Energy integration As mentioned earlier, one of the main ways to classify solar reactors depends on the way solar energy is integrated into the reaction chamber and put in contact with the metal oxide. When solar radiation is absorbed directly on the metal oxide, the reactor is considered to be directly irradiated; on the other hand, when an absorbing material other than the metal oxide, is used as an intermediate to absorb radiation, the reactor is considered indirectly irradiated (Fig. 10.4) [6]. In general, direct irradiation allows reaching higher temperatures for a given energy input, which favors the thermochemical cycle efficiency, from a thermodynamic perspective [7]. Nonetheless, if care is not taken, this can lead to localized temperature rises or “hotspots” on the metal oxide surface, which can cause metal oxide sintering, or metal oxide loss, due to sublimation. As will be discussed in detail later in the following section, this effect is more pronounced in directly irradiated reactors with supported metal oxides as their high opacity causes radiation to absorb at the bulk surface while their low thermal effective diffusivity does little to alleviate this. Achieving direct contact between incoming concentrated solar energy and the metal oxide requires having a window opening,

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Fig. 10.4 Scheme of general characteristics of directly (left) and indirectly (right) irradiated solar reactors. Left bottom: Scheme of a 4-kW thermochemical solar reactor prototype. (Reprinted from P. Furler, A. Steinfeld, Heat transfer and fluid flow analysis of a 4-kW solar thermochemical reactor for ceria redox cycling, Chem. Eng. Sci. 137 (2015) 373–383, p. 11, with permission from Elsevier). Right bottom: Scheme of a 50-kW indirectly irradiated solar reactor. (Reprinted from S. Rodat, et al., A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting, Int. J. Hydrogen Energy 35 (15) (2010) 7748–7758, p. 11, with permission from Elsevier.)

Hydrogen from solar thermal energy

usually made of quartz, through which radiation can enter the reactor. Although some radiation is bound to escape through the window opening, the efficiency of these reactors tends to improve as reactor scale grows, mainly due to the favorable decrease of window area to reactor volume ratio [86]. Due to the high operating temperatures, window breakage is the main problem of directly irradiated solar reactors. When particles are considered, particle abrasion promotes dust deposition [7, 87]; when volatile metal oxide is considered, products tend to deposit over long operation times [88]; where both can lead to window breakage. Despite these disadvantages, the reactors most extensively studied in the recent years consider direct irradiation of suspended metal oxide particles. Fluidized particles directly exposed to a concentrated radiative flux provide the most efficient means to heat a metal oxide using solar energy [89–91]. On this basis, several attempts have been made to prevent window breakage; by using a screen flow under the window [92], diverting particles; a horizontal rotary cavity [7, 87], where centrifugal forces help keep particles away from the window; and even a vertical rotary cavity using beam-down optics [93–96], where centrifugal forces and gravity help keeping the window safe. On the other hand, indirectly irradiated solar reactors do not require a window, nonetheless, they tend to present heat transfer limitations and lower operating temperatures; hence, special care must be put into the design of the surfaces interacting with radiation [97]. Common concepts in this category are the two-cavity [98], tubular nozzles [99], and tubular reactors [100]. For multitubular cavity reactors, angular thermal gradients should be carefully controlled, as they can lead to tube bending and breakage. Numerical studies suggest that to improve the cavity thermal efficiency and angular thermal gradients [83] tubes should have a cavity-like distribution within the actual cavity; some should be positioned as further back as possible, behind the focal point; others should be sideways, right next to the focal point, just outside of the radiative inlet cone; they should form a “wall” from the cavity opening perspective [97].

10.5.2 Metal oxide loading Another common classification of solar reactors considers the ways in which the metal oxide is present. This decision must account for different factors, such as the possibility of phase changes during operation, whether the different stages of the thermochemical cycle take place in the same reactor or not, or the optical properties of the metal oxide, among others. The metal oxide can be supported in a structured (monoliths or walls) or unstructured support (porous foam media); in contrast, the metal oxide can also be unsupported, that is, present as particles (recirculating or continuously flowing through the reactor) or volatilized. 10.5.2.1 Supported Supported solar reactors consist of three-dimensional ceramic structures with high porosities (usually above 0.7), coated or built with the active oxides; they are also named

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volumetric solar receivers, due to the gradual attenuation of the incoming radiation (called the volumetric effect). Volumetric receivers used for high-temperature solar thermochemical cycles consist of high-porosity structures, such as wire meshes, foams, or honeycombs [101, 102]. Upon this concept was built the world’ s first closed solar-thermochemical cycle in operation, capable of large-scale continuous hydrogen production, while using renewable and abundant energy sources and raw materials [103]. The main advantage of supported solar reactors lies in the fact that the location of the metal oxides can be controlled in full detail, or fine-tuned, while allowing the usage of direct irradiance, minimizing the risk of window breakage. Supported solar reactors are robust and relatively simple to build and operate in a semibatch mode; nonetheless, when operated continuously they need to resort to complex designs involving several chambers and the use of moving parts, which are liable to fail in high-temperature environments [104]. On the downside, semibatch reactors tend to present low thermal-to-fuel efficiency, below 2%, given that the amount of redox material loaded on supported solar reaction system tends to be low relative to the overall mass of the system [105]. Indeed, low redox material loading, means a relatively small mass of hydrogen is produced in one cycle, while high system thermal mass, means relatively high sensible heat losses when alternating temperatures during cycling [68]. In this regard, lowering the metal oxide loading hinders thermal-to-fuel efficiency. On the other hand, increasing it makes the metal oxide film thickness grow and lowers the bed void fraction, limiting heat transfer and promoting hotspots formation along its aforementioned drawbacks. The severity of the localized temperature increase depends, first, on how strongly void fraction loss promotes inhomogeneous radiation absorption; second, on the effect that increasing film thickness has on the heat diffusion time within the solid; and third, on the incoming irradiance distribution. Hence, the metal oxide loading and distribution within the reactor is a delicate parameter that should also be carefully selected. As such, numerous different types of support geometries have been explored in the literature, which can be classified depending on whether the supporting surface presents regular or irregular patterns. The usage of patterned, monolithic supports was borrowed from the automobile exhaust catalytic converter systems [7]. Monolithic structures such as honeycombs and multichanneled supports have the main advantages of having the lowest pressure drops due to the patterned channel nature; thin walls that minimize diffusive heat transfer limitations; while specific surface area is still relatively high (when compared with simple supporting surfaces such as the reactor walls) [106]. On the other hand, unstructured supports offer higher specific surface areas (for a given bed porosity), although pressure drops and diffusive heat limitations can be higher. It has been reported that a dual-scale porosity, namely a combination of millimeter- and micrometer-sized pores, seems to promote heat transfer within this kind of supporting structures, at high-temperature operation [73]. The concept behind this is that millimeter-sized pores allow radiation to pass through by lowering the supported

Hydrogen from solar thermal energy

metal oxide effective extinction coefficient, impacting on the radiation heat transfer. On the other hand, micro-sized particles help increasing the specific surface area for enhanced reaction kinetics, convective heat transfer, and oxygen diffusion through the pores [107]. 10.5.2.2 Unsupported Unsupported solar reactors consider metal oxides loaded in the form of a particle bed or in the gas phase, for volatile metal oxides. This kind of reactor can be subdivided depending on whether the bed is stacked, fluidized, or entrained [86, 108]. Cavity shape and injection location needs to be carefully considered in this reactor concept as this affects the particles trajectories within the reactor, affecting metal oxide particles residence time and the degree of particle deposition on the quartz window. Entrained solar reactors proposed consider metal oxides moving along the flow, hence the metal oxide is either in the form of small particles or volatilized. This reactor type considers cyclonic reactors, offering the chance to continuously feed reactants and remove products, while having a high bed absorbance [109, 110]. However, these reactors have been used mainly for high-temperature applications other that H2 production, stemming from the fact that entrained reactors tend to have relatively short particle residence times; a limitation for hydrogen production. Fluidized solar reactors work with nonvolatile metal oxides, such as ceria and ferrites [105]. Unlike entrained solar reactors, these reactors allow having larger residence times, while offering improved solid-gas contact; hence, being a good option for reactions with slow reaction rates [86]. To avoid window damage, these have also considered rotating cavity receivers, that efficiently trap radiation due to their geometry while keeping particles from damaging the quartz window [92, 94–96]. At the laboratory scale, preliminary tests are often carried out in fluidized-bed reactors, mainly because they are easy to build, and simple to operate. Windowed fluidized-bed reactors, when irradiated from above, can reach temperatures of 1500–1800 K in the fluidized-bed top section, while in the bed bottom, temperatures can be around 1200–1500 K; hence, thermal reduction and hydrolysis can be conducted in the same reactor, by switching the feed from inert gas to steam, respectively [105], provided particles have enough thermal stability. Although theoretically these reactors are expected to be more efficient, in practice this is not yet demonstrated. Gokon et al. [95], using a NiFeO4/m-ZrO2 particles, report a thermal-to-fuel efficiency below 1%. The authors argue that this low efficiency is mainly is due to design flaws such as insufficient insulation in the reactor wall and unnecessarily long reaction times. The operation of both thermal reduction and hydrolysis steps within a cavity receiver reactor has been validated in a larger scale, using CeO2 in a circulating fluidized reactor [111]. Among important design parameters of fluidized solar rectors are the particle-size distribution and particle loading. Particle loading depends on the reactor geometry, more specifically, on the bed optical thickness, that is, bed extinction coefficient and the bed

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characteristic length, such as height for a fluidized bed irradiated from above. Desirable particle size for fluidized-bed reactors ranges 10–300 μm [112]. Indirectly irradiated particle reactors, on the other hand, bypass the need to have a quartz window, and the problems associated; nonetheless, they usually tend to be more robust while still having good heat and mass properties between the gas and the particles [113]. For the efficiency of these reactors, absorber material and geometry play a crucial role [114]. Among the most commonly used indirectly irradiated particle reactors are cavity receivers with multitubular arrays [105], where the tube array distribution is one of the key design parameters.

10.5.3 Reactor efficiency Most solar thermochemical reactors consider semibatch operation, although continuously operating reactor concepts have started to emerge in the open literature. Among the main difficulties for continuous hydrogen production in solar reactors stems from the fact that solar thermochemical cycles are comprised of, at least, two separate stages requiring very distinct operating conditions. Hence, continuous operation requires transporting a solid metal oxide from a unit operating above 1500 K (thermal reduction stage) to a unit operating bellow 900 K (water hydrolysis stage). Nonetheless, the interest in this kind of reactors is continuously growing. Studies estimate that a 30% thermal-to-fuel efficiency can be achieved, for hydrogen production [115]. Reactor efficiency is heavily affected by the operational temperature; higher reduction temperatures lead to higher overall efficiencies [116–119]. However, due to material limitations and solar collector efficiency, reactors cannot operate at arbitrarily high temperatures [104]. Another factor influencing efficiency is the temperature difference between the reduction and the watersplitting steps; as it favors the reactor efficiency thermodynamically, while it also increases the sensible heat losses. The temperature swings of about 400°C are usually encountered in the literature [25, 120], nonetheless, solar thermochemical hydrogen production reaction systems can work isothermally, that is, simply by changing reactant feed. Isothermal operating conditions present better heating efficiency, while offering better material life and faster oxidation kinetics, relative to temperature swing systems. However, the hydrogen production driving force comes from lower oxygen and water partial pressures [121], hence considerably increasing costs due to vacuum pumping at high temperature [122] or large inert gas and steam flowrates [104]. As a result, potentially the best operating mode comprises the near isothermal operating mode, where temperature swings remain below 150°C and partial pressure of oxygen can rise while that of steam is lowered. Hence, the specific temperature swing needs to consider the costs of steam heating against the reactor heating duties and recuperation [119].

10.6 Scale-up of solar thermochemical hydrogen production There has been a relative small number of experiences carried out so far on the scale-up of solar thermochemical processes. It would be extremely limited in scope to discuss in the

Hydrogen from solar thermal energy

(B)

(A)

(C) Fig. 10.5 Point focus solar concentrating technologies: parabolic dish (A), solar tower (B), and solar furnace (C).

following section only those works aimed at the production of hydrogen by using metal oxide cycles because, as has been pointed out by Koepf et al. [28], knowledge and insights gained on type of process are valuable for the development of others. Therefore, we would cover also works aimed at other thermochemical processes, like CO2 or steam reforming of hydrocarbons [123], as well as carbothermal reduction of metal oxides [124]. Due to the high temperatures required for thermochemical processes for hydrogen production (typically above 1273 K for the reduction step, in the case of metal oxide cycles), the utilization of point focus solar concentrators is required. Three kinds of systems can be used: parabolic dish concentrators, solar furnaces, and solar towers (Fig. 10.5).

10.6.1 Solar concentrator configurations In parabolic dish concentrators, the solar reactor is located at the focus of the parabola. This implies that the reactor has an important contribution to the moment of inertia of

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the system, with the associated mechanical problems for the structure of the concentrator. Moreover, the reactor is continuously rotating during the day, and should be able to operate in the different positions. These two conditions pose practical restriction on the scale of the reactor, and on the types of the reactors that can be used in this configuration. Reactors requiring very strict gas or particle flow patterns cannot be used in a parabolic dish. The problems mentioned earlier are solved by the configuration known as solar furnace [125]: the parabolic dish is put in a static position and the sun tracking role is transferred to a heliostat. Thus, the reactor does not need any more to be supported by the mirror structure, and also is located in a fixed position during the whole of the operation. This configuration also has the added advantage of facilitating the installation of a shutter between the heliostat and the concentrator, which allows a precise control of the radiative power reaching the reactor. Typically solar furnaces and concentrating lamp solar simulators are used to investigate these processes at the laboratory scale (up to 10 kW) [126]. However, both the parabolic dish and solar furnace configurations have practical limitations concerning scale. As a parabolic dish becomes larger, its area increases and so do the wind loads on the system. Also, the weight of the whole system increases. Both of these factors contribute to dynamical deformations of the concentrator structures and make very expensive to attain good enough optical accuracy for large system sizes. Solar furnaces also become very difficult to scale, because the size of the concentrating mirror requires very strong supporting structure and the costs increase rapidly. The largest example is the 1 MW solar furnace of PROMES-CNRS, in Odeillo, France. This system is operated with a large number of heliostats that have to track the sun with high accuracy to have a good overall optical efficiency. So, beyond that scale, it is not economically viable to build a solar furnace. Thus, if one wants to go to the megawatt scale and beyond, at reasonable costs, solar towers are a natural option.

10.6.2 Solar towers A solar tower plant consists of a large field of mirrors, which track the sun in two axes. These mirrors reflect solar radiation to a common target, located at the top of a tower (Fig. 10.5B). Concentration is achieved both by overlapping of the images produced by the many individual heliostats and by slight curvature of each of these heliostats. Solar towers combine the possibility of having static reactors, as in solar furnaces, with a proven scalability, where systems of 100 MWth are normal. However, the state-of-theart commercial solar tower power plants do not comply with some of the requirements of the thermochemical processes: on the one hand, the concentration ratio of a typical solar tower is relatively low to sustain the required temperatures, as they are designed to

Hydrogen from solar thermal energy

operate around 873 K with solar concentration ratios around 500 suns. To carry out a two-step thermochemical cycle for hydrogen production, typically temperatures above 1273 K are required, with concentration ratios above 1500 suns [127]. On the other hand, locating a large chemical reactor atop a tower could be problematic because of weight considerations, which can increase significantly the costs of civil works. Also, continuous conveying of reactant particles to heights above 100 m could be energy consuming. To solve the limitation on concentration ratio, a secondary concentrator can be used. This is generally a compound parabolic concentrator (CPC), with a concentration ratio up to five suns (Fig. 10.6A). Such concentrator is generally located at the entrance of the solar reactor, and needs to be cooled down, to avoid damage due to the small fraction of concentrated radiation it absorbs. Besides the standard solar tower configuration, with the solar reactor mounted at the top of the tower, the so-called “beam-down” concept has been utilized for solar chemistry [128–130]. In the beam-down scheme, a hyperbolic secondary mirror is installed on the tower, which reflects the radiation coming from the heliostat field toward a receiver located directly below (Fig. 10.6B). This kind of arrangement corresponds to the Cassegrain geometry for telescopes, where generally either hyperbolic or elliptical secondary mirrors can be used [129], but hyperbolic has been found to be more convenient. The advantage of the beam-down concept is relieving the problem of mounting the reactor and transporting the reactive material to the top of the tower, but on the other hand implies the construction of more complicated structures to support a large secondary mirror [131]. Experimental beam-down facilities have been developed at the Weizmann

(A)

(B)

Fig. 10.6 Solar tower configurations for solar chemistry: solar tower with secondary CPC reflector at the opening of the reactor (A), and beam-down configuration with hyperbolic secondary and CPC tertiary mirror (B).

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Institute of Science in Israel [132], at the Miyazaki University [133], in Japan, and at the Masdar Institute in Abu Dhabi [134]. On the other hand, tower facilities with conventional configuration which have been used for solar chemistry research are the SSPS solar tower at Plataforma Solar de Almerı´a, Spain [135], the solar tower of CSIRO in Australia [123], and also the solar tower at the Weizmann Institute of Science, and the solar tower of IMDEA, in Madrid, Spain [136].

10.6.3 Design and modeling There are several works aimed at the design of solar plants for thermochemical processes. Some of these works are aimed at the global analysis and design of solar tower and heliostat fields. For instance, a simulation of a solar tower plant coupled to a hybrid sulfuric acid for hydrogen production was carried out by Kolb et al. [137]. They consider a solar tower with a falling particle receiver, and where sand is heated and then used as thermal storage material. The cycle produces hydrogen by a thermal decomposition of sulfuric acid, which leads to sulfur trioxide, and then to sulfur dioxide, by means of a thermocatalytic process. Afterwards, sulfur dioxide is used in an electrolyzer to decompose water, which produces hydrogen and sulfuric acid. The DELSOL code was used to design the solar tower plant, which considers a receiver of 700 MWth capacity, operation at 1273 K, and 13 h thermal storage. The thermal part of cycle was rated at 255 MWth and employed a 60 MWe SO2 electrolyzer. A 64% efficiency was considered for the cycle which, coupled to the solar equipment, gave a theoretical overall solar to hydrogen conversion efficiency of 21%. A transient model, including the different heat transfer modes and chemical kinetics, of a thermochemical reactor for the reduction of zinc oxide was developed by Schunk et al. [138]. This model was validated against experimental results obtained in a 10-kW directly irradiated rotating cavity reactor illuminated by a high flux solar simulator. They used the developed model to predict the performance of scaled-up reactors of 100 and 1000 kW, considering average solar concentration ratios of 3500 suns, and operation temperatures from 1994 to 2126 K. Efficiencies up to 50% are predicted for the larger reactor. No explicit consideration of the heliostat field design is considered in this work. A work on a similar line of thought was developed by Maag et al. [139]. They consider the cracking of methane for the coproduction of hydrogen and solid carbon, in an indirectly irradiated cavity reactor. Methane flows through several tubes. The model considers steady-state multimode heat transfer in a two-phase medium, together with chemical kinetics. The model was compared to experimental results obtained using a 10-kWth solar reactor and a solar furnace. The model was used to simulated the scale-up process to the 10-MWth solar power scale. A system with three receivers, each

Hydrogen from solar thermal energy

one facing a different part of a heliostat field were considered. Each aperture has in front an array of CPC secondary concentrators, which allow reaching a concentrated flux density of 3000 suns. The model predicts 42% solar to chemical efficiency. A critical analysis is carried out by Martinek et al. [140], about the usual assumptions made when designing solar tower heliostat fields for thermochemical processes. They carried out a simplified modeling of the optics of solar tower plants of different sizes, coupled to two different thermochemical cycles, based on zinc oxide and a nickel ferrite. The thermochemical plant part of the design was carried out by using AspenPlus. They found that designs based on using the Carnot efficiency to represent the limiting value of the chemical processes, tend to lead to oversizing of the heliostat fields, as compared to designs based purely on the combined thermal efficiency of the reactor and optical efficiency of the field. Along a similar line, Pitz-Paal et al. [127] proposed a methodology to optimize the design of a heliostat field to maximize the annual solar-to-chemical conversion efficiency. Instead of coupling a thermochemical process to a given flux density and power provided by a heliostat field, they carried out an integral process coupling the design of the field to the performance of the process. The chemical reaction rate strongly depends on the temperature, in a nonlinear fashion. Temperature in turn depends on the concentrated flux level and on reaction rate. Thus, the optimization requires to consider explicitly the coupling of the optical model of the field with a model for the reactor performance. The HFLCAL code was used for the optical modeling, combined with three different optimization algorithms. As examples, they considered two model processes: the thermal reduction of zinc oxide for hydrogen production and coal gasification for syngas production. Conversion efficiencies of 30% and 40% were estimated, although many idealizations were incorporated in the reactor model. A model for the dynamical behavior of a cavity reactor with a monolithic honeycomb absorbed coated with a ferrite has been developed by S€ack et al. [141]. This type of reactor was implemented at the 100-kW scale within the HYDROSOL-II project in the SSPS solar tower of Plataforma Solar de Almerı´a [142]. The model presented by the authors includes simulation of the solar flux at the cavity aperture, and of the temperatures and hydrogen generation within the reactor. The model has been validated against the experimental results, and is aimed to be used both for the control of the process and for the analysis of the system operation. A further model for the same system was proposed by De La Calle et al. [143]. The model was developed to test the control algorithms to automatize the hydrogen production in the HYDROSOL-II plant. The emphasis is put on a model with reduced computational effort, and thus the solar field model incorporates several simplifications. The reactor is considered a single mass block exchanging heat by convection and radiation with the ambient and the circulating gases. The model was developed in Modelica language and was calibrated by genetic algorithms.

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10.6.4 Implementation Several research projects have been aimed to scale-up thermochemical processes to pilot plant level [28]. A 300-kW pilot plant was implemented in the Weizmann Institute of Science (WIS) tower facility [124] within the project SOLZINC. The targeted process was the carbothermic reduction of ZnO to produce Zn. This was accomplished by using beam-down configuration, where a secondary CPC concentrator is used to increase concentrated irradiance levels at the reactor aperture. The reactor had a dual cavity configuration, where the first cavity served as absorber for the concentrated solar flux, and was directly below the CPC, with its entrance closed by a quartz window. The lower cavity functioned as the reactor and it was separated from the upper cavity by a SiC/graphite wall. In this lower cavity, the reactants were contained as a packed bed of mixed ZnO and C particles. The operation was in batch mode, with initial load of material of 500 kg, and operating temperatures between 1300 and 1500 K. SiC pipes were used to introduce and extract recycled gas to sweep the product gases. Electrical heating was used to avoid Zn condensation in the outlet pipe. Care was taken to protect the receiver window, by flushing with inert gas to avoid fouling, and using a water cooled mounting ring. Also, pressure was regulated within the cavity to avoid breakage of the window. An important component was the system for cooling down and recycling the off-gas. Two coolers were used, followed by a cyclone to separate the condensed Zn particles, in the range of 3–5 μm. Smaller particles were recirculated within this cooling system for further growth. The resulting filtered off-gas was recycled, by introducing it again to the reaction chamber. This facility demonstrated the capability of producing 50 kg h
1 of Zn particles with sizes between 2.5 and 5 μm. The purity of the resulting material was 95%. The thermal efficiency of the process reached 30%. As mentioned in the previous section, within the project HYDROSOL-II, a 100-kW reactor was tested in the SSPS solar tower of Plataforma Solar de Almeria [142]. This is used for water splitting by means of a ferrite or mixed oxide based on iron as main component [142]. The reactor is based on a monolithic ceramic honeycomb where the redox material is supported. This concept allows to carry out the reduction and oxidation steps in the same reactor in an periodic manner. Different temperatures are required for the reduction (1373–1473 K), and oxidation (1073 K) steps. This was achieved by periodically focusing an defocusing heliostats at the receiver. Moreover, two identical reactors were located in the tower side by side and they alternated between the two steps of the process in 20–30-min cycles. While one was carrying out the reduction step in a nitrogen atmosphere with more heliostats aiming at it, the second one performed the oxidation with steam, under a lower concentrated flux. Several cycles could be run without problems, with a conversion of steam to hydrogen of up to 30%. However, degradation of the redox material with the cycles was an issue.

Hydrogen from solar thermal energy

Gonzalez-Pardo et al. [135] discuss the development of a multitubular cavity reactor to carry out a thermochemical cycle with ferrites in the SSPS tower at PSA. In particular, they report results for the preliminary thermal test of the reactor. They aim at achieving uniform concentrated flux and temperatures inside the cavity, as well as at studying the transient effects produced by clouds during operation. The reactor is formed by a cavity semicylindrical shape, which contains 80 alumina tubes, arranged in three rows near the back wall. Each tube is filled with mixed ferrite particles. Nitrogen circulates within these packed beds during the reduction step, which may be alternated with a mixture of nitrogen and steam for the hydrogen production step.

10.6.5 Control strategies Very few studies have been carried out specifically oriented to the control strategies of solar tower plants for carrying out thermochemical processes. In particular, these center around the pilot plant tested within the HYDROSOL-II project [141, 142, 144], already described in the previous sections. The SSPS tower system consists of a 43 m high tower and 91 heliostats of 39.3 m2 area, distributed on 16 rows. This field can deliver a 2-MW thermal power to the receiver area. In this pilot plant, the need for rapid switching between temperature regimes when completing one of the steps of the process was identified [142]. This was achieved mainly by moving the aim point of groups of heliostats from one reactor to another. It was identified in particular that the increase on temperature of a reactor from 1073 to 1473 K, when switching from the hydrogen production (oxidation) to the regeneration step (reduction) was the most critical, in order to achieve rapidly the reaction temperatures. On the other hand, the inverse switching, to reduce temperature to oxidation conditions was less important, because this reaction can proceed at high temperature, although with lower efficiency. However, they warn that care must be exerted in order to avoid possible deterioration of the redox material during high-temperature oxidation (above 1273 K). Other possible control parameters to ensure stable temperature levels were analyzed. It was found little usefulness on using mass flow and preheating temperature of the feed gas to control reactor temperature. Instead, solar flux variations were compensated by the number of heliostats aimed at a given reactor, using high-flux (closer) heliostats for the coarse control and low-flux heliostats for fine tuning. Other works related to this system [141, 144] implement models to analyze these control strategies.

10.7 Economic analysis Every day the number of countries that join to implement programs for the research and technological development of hydrogen production with clean energy increases in order to guarantee its energy, environmental, and economic security. Examples of such

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countries are: the United States, the European Union, Switzerland, Japan, Australia, Canada, Iceland, Singapore, and India. The hydrogen production is also an important task of international organizations, such as the International Energy Agency which has the Hydrogen Implementing Agreement where one of its tasks is the production of this energy vector with renewable energies (task 35). In addition, global initiatives such as the Hydrogen Council has initiated in 2017 to lead energy, transport, and industrial companies with a united vision toward a hydrogen economy. The 18 companies’ members have the ambition to accelerate their significant investment in the development and commercialization of the hydrogen and fuel cells sectors. The Council’s main objective is that hydrogen technologies play an essential role in the global energy transition [145]. On the other hand, hydrogen production through solar-driven thermochemical approaches has been investigated by the program on hydrogen and fuel cells of the Department of Energy (DOE) of the United States. In its annual report, this department established, for the year 2015, a hydrogen production cost target of $6.0 kg
1 H2; and for the year 2025, a cost of $2.0–$3.0 kg
1 H2. To reach the earlier mentioned, the DOE had organized several research groups to investigate the thermochemical water dissociation processes, with the objective to develop a commercially viable technology and to study its implementation in a solar thermochemical pilot plant (STCH). Hydrogen production technology through WS thermochemical cycles still has great challenges to cover, ranging from the development of new materials for the receiver or metal oxides that can have better stability in multiple cycles, to other engineering aspects, such as the development of efficient solar reactors. All these challenges seek the same objective: to reduce the capital costs and the operating expenses in order to reach the goal of hydrogen production cost proposed by the DOE for the year 2025. To achieve these objectives, it is mandatory that the annual cost in reactive materials must be lower than $11,000.00 year
1 with an overall efficiency of the process exceeding 25% and a rate of hydrogen production above $2.1  10
6 kg s
1 m2 [146]. Several authors have carried out economic studies to evaluate the cost of hydrogen production through WS thermochemical cycles. In 1989, Aochi et al. [147] perform an economic analysis for the UT-3 cycle, based on three basic compounds: CaO, Br2, and Fe3O4. In this analysis, authors concluded that hydrogen production cost with this process has a high potential to be competitive compared to water electrolysis process. Dincer and Acar [148] make an analysis on the sustainability of various methods of hydrogen production obtaining a comparative assessment of these methods and its environmental impact taking into account the financial and social cost of carbon. In their work, they observed that there are various uncertainties since costs are strongly affected by the level of development of the technology, availability of existing infrastructure, and feedstock prices. They found that the average costs of hydrogen production vary between $1 and $10 kg
1 H2. According to the authors, the most financially advantageous methods for hydrogen production are steam methane reforming, coal and biomass gasification, and plasma arc decomposition. Thermochemical cycles and biomass

Hydrogen from solar thermal energy

conversion, as well as hybrid thermochemical cycles also seem to be competitive to fossil fuel and biomass prices (about $2 kg
1 H2). It should be noted that the average of production costs was taken from the consulted literature by the authors [1, 149, 150]. Leybros et al. [151] made an economic evaluation for the sulfur-iodine and hybrid sulfur-iodine cycle processes from advanced methods coupled to a nuclear heat source. They concluded that for its model the cost of hydrogen production is $13.8 kg
1 H2. The cost of the investment is largely dominated by the price of the equipment. Regarding the operating and maintenance cost, they represent around 30% of hydrogen production costs. The cost of energy is approximately $2.3 kg
1 of hydrogen, which is close to the cost of energy consumption of alkaline electrolysis (given similar energy costs). Therefore, the efficiency of the plant must be improved to be competitive with alkaline electrolysis. Weimer et al. [152], from the Colorado University on its 2011 Annual Progress Report of the DOE hydrogen and fuel cell program 10, conducted an analysis using ferrite deposited on a ZrO2 support to carry out the design and cost evaluation of a solar WS thermochemical process in order to produce 100,000 kg H2 per day. In their evaluation, they performed a sensitivity analysis varying the redox cycle time of 1, 5, and 15 min. In this study, they found that reducing the time of redox cycle reduces the amount of ferrite used, and thereby, reduces the size and cost of the solar reactor. From their results, they obtained the distribution of capital costs in several aspects, such as heliostat, solar reactor, metal oxide, towers, compression system, vacuum pumps, and the projected costs for the year 2015 and 2025, concluding that the resulting sale price of hydrogen is not sensitive to the purchase price of the ferrite studied. Recently, McDaniel et al. [153], from the Sandia National Laboratories, reported a study in which they demonstrated the continuous operation of a 3-kWt solar reactor prototype fitted to produce more than 3 L min
1 H2. In this report, the authors established a base model of 29 MW central tower receiver that when combined with other 82 MW it is possible to have a plant capable to produce 100,000 mt H2 per day. In the study, authors identified 14 major elements that depict mass and energy flows between several components of the plant, including the solar field, solar receiver, thermochemical reactor, heat exchangers, recuperators, condensers, and pumps. Economic evaluation of a hydrogen production process based on WS thermochemical cycles depends on the arrangement in the flow diagram, the solar concentration system, and the temperatures required in the reduction and oxidation processes. The solar reactor plays a very important role because a good part of the efficiency of the cycle falls on this device. Reactive materials are also important in their replacement costs, their reaction rate, and cycling efficiency. The duration of each cycle is fundamental in the rate of production of hydrogen. In this section, we present an economic evaluation of a hydrogen production process based on a metal oxide (MnFeO4) by using the DOE H2A Analysis software. For this purpose, a flow diagram is designed in which a central tower with a beam-down solar concentrator is involved (Fig. 10.7).

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Fig. 10.7 Flowchart of two-step thermochemical solar process.

Hydrogen from solar thermal energy

Table 10.1 Cost of investment distribution among the main equipment to be installed Capital costs

H2A total direct capital cost H2A carbon sequestration total direct capital cost Indirect depreciable capital costs Nondepreciable capital costs Total capital cost Operating costs Fixed operating costs Variable operating costs Total operating costs

$362,121,599 $5,598,862 $134,860,954 $2,378,058 $504,959,472 $18,638,364 $2,193,900 $20,832,264

A fixed-bed tubular solar reactor is used where, alternately, the processes of reduction and oxidation with water vapor are carried out. In addition, there are two plate heat exchangers for heat recovery from the output streams of the solar reactor: oxygen-inert gas and hydrogen-water. For the recovery of the inert gas and hydrogen, two membrane separation columns and their respective compressors are proposed. For the generation of low-pressure steam, a conventional steam generator with natural gas supply is suggested. Eventually, this equipment can be replaced and directly injected into the reactor with preheated liquid water, where it is evaporated before coming into contact with the powder reactive. Table 10.1 shows the detail of the cost estimate of a WS thermochemical solar plant for an annual H2 production of 100,000 kg. The estimated cost per kilogram of hydrogen is $3.69. The total cost of the investment is $504,959,472 and the fixed annual operating costs were estimated at $18,638,364 and the variable operating costs at $2,193,900. Table 10.2 describes with more detail the costs of the main equipment of the production process. Fig. 10.8 shows the distribution of costs of this equipment, where it can be seen that the heliostats use less than 50% of the costs of investment, followed by the solar reactor and the central tower with 19% and 15.8%, respectively. These results show a promising future where the price of hydrogen can fall up to $2 kg
1 H2, under certain conditions: lower the price of heliostats to values of 70–90 $m2, an increase the half-life of reactive materials, reduce the cost of solar reactors and have a more favorable cost of money. The earlier analysis assumes the following: reference year dollar of 2005, inflation rate of 1.9%, effective total tax rate of 38.9%, 1 year of half-life of reactive materials, 20 years of lifetime of equipments, and 40 years of lifetime of the central plant.

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Table 10.2 Cost of investment distribution among the main equipment to be installed Major pieces/systems of equipment

ZrO2 support Compression system Solar reactors Vacuum pumps Water pumps Turbine Heat exchangers Heliostats Secondary concentrators Towers Metal oxide (chemical reactive) Indirect depreciable capital costs Nondepreciable capital costs H2A carbon sequestration total direct capital cost Total capital investment

Fig. 10.8 Cost distribution of main equipment.

Baseline installed costs

$39,661 $28,696,832 $69,199,077 $12,543,896 $252,907 $1,168,123 $939,337 $151,525,894 $579,080 $57,206,004 $39,970,788 $134,860,954 $2,378,058 $5,598,862 $504,959,472

Hydrogen from solar thermal energy

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