- Email: [email protected]
SnO based thermochemical water splitting cycle
Energy Conversion and Management 135 (2017) 226–235
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle Rahul R. Bhosale ⇑, Anand Kumar, Parag Sutar Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar
a r t i c l e
i n f o
Article history: Received 16 October 2016 Received in revised form 23 December 2016 Accepted 24 December 2016
Keywords: Hydrogen production SnO2/SnO based thermochemical cycle Water splitting reaction Thermodynamic analysis Solar energy
a b s t r a c t There are many studies related on the SnO2/SnO based solar thermochemical water splitting cycle, however there are still no studies addressing on the detailed thermodynamic analysis of this process using HSC Chemistry software and its thermodynamic database. In this cycle, the first step belongs to the endothermic solar thermal reduction of SnO2 producing gaseous SnO and O2. The second step corresponds to the exothermic production of H2 via water splitting reaction using SnO produced in the first cycle, thereby regenerating SnO2 which can be recycled back to step 1. Thermodynamic equilibrium compositions associated with step 1 and 2 are identified as a function of reaction temperatures and partial pressures of O2 in the inert carrier gas. Furthermore, the thermodynamic efficiency analysis is performed by following the second law of thermodynamics to determine the cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle. Effects of thermal reduction and water splitting temperatures on various thermodynamic parameters are also investigated in detail. Obtained results indicate that the higher values of cycle efficiency (41.17%) and solar-to-fuel energy conversion efficiency (49.61%) are achievable by operating this cycle at a thermal reduction temperature of 1780 K and water splitting temperature equal to 800 K with 50% heat recuperation. This work gives a detailed thermodynamic and efficiency analysis of SnO2/SnO based two-step solar thermochemical water splitting cycle for hydrogen production. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction H2 is considered as one of the promising future energy sources due to its high energy density. To be a clean energy vector, it is essential to produce H2 via CO2-free processes [1–4]. To achieve this, a metal oxide based two-step solar thermochemical water splitting cycle has been investigated heavily by several researchers [5–14]. In this cycle, step 1 corresponds to the solar thermal reduction of the metal oxide releasing O2, whereas; step 2 yields into exothermic production of H2 via water splitting reaction (reoxidizing the metal oxide). The metal oxide is not consumed and hence can be re-used for multiple cycles. Various metal oxides were investigated towards thermochemical water splitting application e.g., zinc oxide [15–18], iron oxide [16,19–23], mixed iron oxide [24–35], tin oxide [36–40], samarium oxide [41], terbium oxide [42], erbium oxide [43], undoped and doped ceria [5,6,44–52], and perovskites [53–58]. Among these, the volatile zinc oxide based water splitting cycle is considered ⇑ Corresponding author. E-mail addresses: [email protected], solar.chemical.engineering@gmail. com (R.R. Bhosale). http://dx.doi.org/10.1016/j.enconman.2016.12.067 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.
as one of the promising one and studied extensively. Steinfeld [15] studied the second law thermodynamic analysis of the ZnO/ Zn water splitting cycle and reported cycle efficiency equal to 29%. Similar to this study, Loutzenhiser and Steinfeld [59] performed the thermodynamic analysis of the water and CO2 splitting ZnO/Zn cycle for the production of syngas and reported cycle efficiency equal to 32%. By conducting dynamic thermogravimetric experiments, the kinetics of ZnO/Zn water splitting cycle was investigated by Stamatiou et al. [16]. Recently, the pilot scale demonstration of the thermal reduction of ZnO using a 100 kWth solar reactor was reported by Villasmil et al. [18]. The working principle for the SnO2/SnO based water splitting cycle is similar to that of the ZnO/Zn water splitting cycle. Abanades et al. [36] reported that the thermal reduction of SnO2 into SnO(g) can be carried out at or above 1600 °C at 1 bar, while the H2 production via water splitting reaction can be achieved with 90% conversion in the temperature range of 500 –600 °C. Charvin et al. [40] studied the kinetics of the thermal reduction of SnO2 using a thermogravimetric analyzer and reported the activation energy and pre-exponential factor equal to 394.8 kJ/mol and 8.32 108 g/s, correspondingly. Chambon et al. [38] explored the kinetics of hydrolysis of SnO nanoparticles synthesized via solar
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
227
Nomenclature C HHV I MO n_ TH TL HR P O2
solar flux concentration ratio, suns higher heating value normal beam solar insolation, W m2 metal oxide molar flow rate, mol/s thermal reduction temperature, K water splitting temperature, K heat recuperation partial pressure of the O2, bar r Stefan – Boltzmann constant, 5.670 108 (W/ m2 K4) heat energy required for thermal reduction of Q_ SnO2 -red SnO2, kW Q_ H2 O-heating heat energy required for heating of water, kW net energy required run the cycle, kW Q_ cycle-net Q_ solar-reactor solar energy required to run the solar reactor, kW Q_ solar-heater solar energy required to run the solar heater, kW solar energy required to run the cycle, kW Q_ solar-cycle gabs-solar-reactor solar energy absorption efficiency of the solar reactor gabs-solar-heater solar energy absorption efficiency of the solar heater Q_ re-rad-solarreactor re-radiation losses from the solar reactor, kW Q_ re-rad-solar-heater re-radiation losses from the solar heater, kW Q_ re-rad-cycle re-radiation losses from the cycle, kW
_ solar-reactor Irr _ solar-heater Irr _ splitting-reactor Irr _ cooler-1 Irr _ cooler-2 Irr _ cooler-3 Irr Q_ cooler-1 Q_ cooler-2 Q_ cooler-3 Q_
splitting-reactor
Q_ FC-Ideal _ FC-Ideal W
gcycle gcycle-HR gsolar-to-fuel-HR gsolar-to-fuel
Q_ recuperable Q_
recupaerable-HR
irreversibility associated with the solar reactor, kW/K irreversibility associated with the solar heater, kW/K irreversibility associated with the water splitting reactor, kW/K irreversibility associated with cooler – 1, kW/K irreversibility associated with cooler – 2, kW/K irreversibility associated with cooler – 3, kW/K heat energy libe2rated from cooler – 1, kW heat energy liberated from cooler – 2, kW heat energy liberated from cooler – 3, kW heat energy liberated from water splitting reactor, kW heat energy liberated from an ideal fuel cell, kW work output of an ideal fuel cell, kW cycle efficiency cycle efficiency (with heat recuperation) Solar-to-fuel energy conversion efficiency (with heat recuperation) solar-to-fuel energy conversion efficiency heat energy that can be recuperated (total), kW heat energy that can be recuperated (with % HR), kW
thermal reduction and reported the order of the reaction and the activation energy equal to 2 and 122 kJ/mol, respectively. The reaction order and the activation energy for the recombination of SnO and O2 during the quenching step was also investigated [39] and observed to be equal to 1.4 and 42 kJ/mol, accordingly. Similar to the individual water splitting reaction, the SnO2/SnO redox system was also studied towards production of syngas via combined H2O and CO2 splitting using a solar reactor and a thermogravimetric analyzer [60]. Although, multiple studies are reported, the detailed thermodynamic and efficiency analysis of SnO2/SnO based solar thermochemical water splitting cycle is missing from the published literature. Therefore, in this investigation, the thermodynamic analysis of the SnO2/SnO based solar thermochemical water splitting cycle is carried out using HSC Chemistry software and its thermodynamic database. Effects of various operating parameters such as partial pressure of O2 (PO2 ) in the inert carrier gas, thermal reduction temperature (T H ), water splitting temperature (T L ) and heat recuperation (HR) on thermodynamic equilibrium composition, the net solar energy required, re-radiation losses, heat energy released, irreversibility associated and efficiency of the SnO2/SnO based thermochemical water splitting cycle are studied and reported. 2. Thermodynamic equilibrium analysis Two redox reactions involved in the SnO2/SnO based thermochemical water splitting cycle are:
SnO2 ! SnO þ 0:5O2
ð1Þ
SnO þ H2 O ! SnO2 þ H2
ð2Þ
As mentioned earlier, the thermodynamic equilibrium compositions associated with the SnO2/SnO based thermochemical water splitting cycle are determined by using HSC Chemistry software
Fig. 1. Effect of (a) T H and (b)T L on delta G associated with the thermal reduction and water splitting step of SnO2/SnO based thermochemical cycle.
228
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
and its thermodynamic database by assuming a continuous operation of the solar thermal reactor with inlet molar concentration of SnO2 equal to 1 mol/s. Alteration in the Gibbs free energy (delta G) with respect to the increase in the T H (in case of thermal reduction of SnO2) and T L (in case of re-oxidation of SnO via water splitting reaction) is shown in Fig. 1a and b. As per the data reported, delta G for the thermal reduction step decreases by 121.78% due the increase in the T H from 300 to 2800 K. Contrasting to this, the delta G associated with the water splitting step increases by 147.90% as the T L rises from 300 to 1200 K. Reported data also indicates that the thermal reduction of SnO2 is possible at or above 2300 K (at 1 bar) and the reoxidation of SnO via water splitting reaction is feasible below 875 K (at 1 bar). The equilibrium compositions associated with the water splitting step (reported in Fig. 2, at 1 bar) also confirms that this reaction is more feasible at temperatures lower than 875 K. According to the published literature, temperature required for the thermal reduction of metal oxide can be decreased with the reduction in the P O2 in the inert carrier gas [41–43,51,61]. In this regard, the effect of P O2 in the inert carrier gas on the equilibrium compositions associated and T H required for the thermal reduction of SnO2 is analyzed and the obtained results are reported in Fig. 3a and b. It is evident from the plots that the equilibrium compositions associated with the thermal reduction of SnO2 shifts to the left i.e. towards lower T H as the P O2 in the inert carrier gas reduces. For instance, at 1 bar, the thermal reduction of SnO2 starts at 2080 K and undergoes 100% completion at 2380 K. By reducing the PO2 in the inert carrier gas to 101 bar, the reduction of SnO2 can be initiated at 1650 K and complete conversion can be realized at 2090 K. Further reduction in the P O2 in the inert carrier gas to
Fig. 3. Effect of P O2 in the inert carrier gas on the equilibrium compositions of (a) SnO2 and (b) SnO(g) and T H required for the thermal reduction of SnO2.
102 and 103 bar yields into a significant decrease in the T H to 1940 and 1780 K, respectively (to achieve complete reduction of SnO2). 3. Thermodynamic efficiency analysis 3.1. Theory
Fig. 2. Equilibrium compositions associated with the re-oxidation of SnO via water splitting reaction at 1 bar, (a) Sn-based components and (b) H2-based components.
To conduct the thermodynamic efficiency analysis of the SnO2/ SnO based thermochemical water splitting cycle driven by solar power; a process flow configuration has been developed (Fig. 4). According to the process flow diagram, the SnO2/SnO based thermochemical water splitting cycle requires a solar reactor, a water splitting reactor, three coolers, one solar heater, and an ideal H2/ O2 fuel cell (assumed 100% efficiency). The operation of the SnO2/ SnO based thermochemical water splitting process can be carried out at 1 bar. Steady state conditions are assumed and viscous losses or variations in kinetic or potential energies are deserted. The solar reactor is considered as a perfectly insulated cavity reactor with negligible conductive and convective losses. It is also assumed that all the reactions involved in the SnO2/SnO based thermochemical water splitting cycle yields into 100% conversion. Calculations associated with the heat exchangers are not included in this analysis. The procedure used to perform the thermodynamic efficiency analysis is similar to the one used by other investigators, however; modifications are done to improve the accuracy of the final results. Based on the thermodynamic equilibrium analysis reported in Section 2, the effect of T H is explored by varying it in the range of 1780 to 2380 K. Likewise, the effect of T L is also investigated by altering it in the gauge of 400–800 K, respectively.
229
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
After performing the thermal reduction of SnO2, the gaseous products exiting the solar reactor are quenched to T L using cooler – 1. In this step, the gaseous SnO gets converted into the solid SnO and naturally separates from the gaseous O2 without expending any work. The rate of heat energy released and the entropy generated in cooler – 1 are estimated as:
Q_ cooler-1 ¼ n_ DHjSnOðgÞþ0:5O2 ðgÞ@T H !SnOþ0:5O2 ðgÞ@T L _ cooler-1 ¼ Irr
Q_ cooler-1 TL
ð8Þ
! þ n_ DSjSnOðgÞþ0:5O2 ðgÞ@T H !SnOþ0:5O2 ðgÞ@T L
ð9Þ
Solid SnO exiting the cooler – 1 is transferred to the water splitting reactor to produce H2 via re-oxidizing itself via water splitting reaction. The water splitting reactor is operated at T L , which is higher than 298 K, hence the H2O entering this reactor needs to be pre-heated up to T L . This can be achieved by using a solar heater as shown in the process flow diagram (Fig. 4). The solar energy absorption efficiency of the solar heater is determined as:
gabs-solar-heater ¼ 1 Fig. 4. Process flow diagram for solar driven SnO2/SnO based thermochemical water splitting cycle.
The SnO2 enters the solar reactor at T L (molar flow rate = 1 mol/ s). After entering the solar reactor, it is heated up to T H and the thermal reduction is carried out by using the concentrated solar energy. The solar energy absorption efficiency is determined as:
gabs-solar-reactor ¼ 1
rT 4H
!
ð3Þ
IC
The baseline parameters used for this calculation are reported in Table 1. The heat energy needed for the thermal reduction of SnO2 can be calculated as:
Q_ SnO2 -red ¼ n_ DHjSnO2 @T L !SnOðgÞþ0:5O2 ðgÞ@T H
Q_ solar-reactor ¼
Q_ SnO2 -red
ð5Þ
gabs-solar-reactor
Re-radiation losses and the irreversibility associated with the solar reactor are calculated via following equations:
Q_ re-rad-solarreactor ¼ Q_ solar-reactor Q_ SnO2 -red _ solar-reactor ¼ Irr
Q_ solar-reactor TH
!
Q_ re-rad-solarreactor þ TL
ð10Þ
IC
Q_ H2 O-heating ¼ n_ DHjH2 OðlÞ@298 K!H2 OðlÞ@T L
ð11Þ
By using the gabs-solar-heater and Q_ H2 O-heating , the total solar energy needed to drive the solar heater is calculated as:
Q_ H2 O-heating Q_ solar-heater ¼
ð12Þ
gabs-solar-heater
Similar to the solar reactor, the solar heater also shows some reradiation losses and entropy generation which can be determined as:
Q_ re-rad-solar-heater ¼ Q_ solar-heater Q_ H2 O-heating _ solar-heater ¼ Irr
Q_ solar-heater TL
! þ
Q_ re-rad-solar-heater 298 K
þ n_ DSjH2 OðlÞ@298 K!H2 OðlÞ@T L
ð13Þ !
ð14Þ
Based on Eqs. (4) and (11), the net heat energy required to run the SnO2/SnO based thermochemical water splitting cycle is estimated as:
Q_ cycle-net ¼ Q_ SnO2 -red þ Q_ H2 O-heating
ð15Þ
ð6Þ
Likewise, as the SnO2/SnO based thermochemical water splitting cycle requires solar energy input for solar reactor as well as for solar heater, the total solar energy needed to run this cycle can be computed as:
ð7Þ
Q_ solar-cycle ¼ Q_ solar-reactor þ Q_ solar-heater
!
þ n_ DSjSnO2 @T L !SnOðgÞþ0:5O2 ðgÞ@T H
!
Furthermore, the heat energy needed to convert the liquid water into steam and then heating the steam up to T L can be estimated as:
ð4Þ
Total amount of solar energy required to heat the SnO2 from T L to T H , and to perform the thermal reduction reaction at T H is estimated according to Eq. (5).
rT 4L
ð16Þ
The total re-radiation losses from solar reactor and solar heater are computed as: Table 1 Baseline parameters for the thermodynamic analysis.
Q_ re-rad-cycle ¼ Q_ re-rad-solarreactor þ Q_ re-rad-solar-heater
Parameter
Value
I C
1 kW/m2 5000 suns 5.670 108 W/m2 K4 298 K 1 bar 100%
r Ambient temperature Total system pressure Fuel cell efficiency
ð17Þ
Water splitting is an exothermic reaction, hence when SnO and H2O(g) react, heat energy is released, which can be estimated as:
Q_ splitting-reactor ¼ n_ DHjSnOþH2 OðgÞ@T L !SnO2 þH2 ðgÞ@T L
ð18Þ
The irreversibility associated with the water splitting reactor is also estimated by using Eq. (19).
230
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
Q_ splitting-reactor TL
_ splitting-reactor ¼ Irr
!
þ n_ DSjSnOþH2 OðgÞ@T L !SnO2 þH2 ðgÞ@T L
Q_ recupaerable-HR ¼ ð%HRÞ Q_ recuperable ð19Þ
After performing the water splitting reaction, the SnO2 produced is recycled back to the solar reactor and the H2 produced forwarded to the ideal H2/O2 fuel cell (passing through cooler – 3). The operation of the ideal H2/O2 fuel cell is carried out at 298 K and hence it is essential to reduce the temperatures of H2 and O2 from T L to 298 K. To achieve this, cooler – 2 and cooler – 3 are installed in the SnO2/SnO based thermochemical water splitting cycle (Fig. 4). As the temperatures of H2 and O2 reduces from T L to 298 K, the heat energy released from both coolers is calculated individually as per Eqs. (20) and (21).
Q_ cooler-2 ¼ n_ DHj0:5O2 ðgÞ@T L !0:5O2 ðgÞ@298 K
ð20Þ
Q_ cooler-3 ¼ n_ DHjH2 ðgÞ@T L !H2 ðgÞ@298 K
ð21Þ
In addition to the energy liberated, the entropy generated during cooling of H2 and O2 is also computed as per following equations.
_ cooler-2 ¼ Irr
Q_ cooler-2 298
_ cooler-3 ¼ Irr
Q_ cooler-3 298
!
þ n_ DSj0:5O2 ðgÞ@T L !0:5O2 ðgÞ@298 K
ð22Þ
þ n_ DSjH2 ðgÞ@T L !H2 ðgÞ@298 K
ð23Þ
!
The ideal H2/O2 fuel cell is added to the SnO2/SnO based thermochemical water splitting cycle to determine the maximum work that can be extracted from the H2. The maximum work extracted and the heat energy released by the ideal H2/O2 fuel cell is calculated as:
_ FC-Ideal ¼ n_ DGj W H2 ðgÞþ0:5O2 ðgÞ@298 K!H2 OðlÞ@298 K
ð24Þ
ð30Þ
By using Q_ solar-HR , the higher values for cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting can be achieved:
_ W
gcycle-HR ¼ _ FC-Ideal Q solar-HR gsolar-to-fuel-HR ¼
ð31Þ
HHV H2 ðmoles of H2 producedÞ Q_ solar-HR
ð32Þ
The verification of the thermodynamic efficiency analysis performed can be carried out by comparing the maximum work that can be extracted from the H2 produced via water splitting reaction is calculated using Eqs. (24) and (33).
_ FC-Ideal ¼ Q_ solar-cycle -ðQ_ re-rad-cycle þ Q_ cooler-1 þ Q_ cooler-2 þ Q_ cooler-3 W þ Q_ splitting-reactor þ Q_ FC-Ideal Þ
ð33Þ
3.2. Effect of thermal reduction temperature The effect of T H on thermodynamic parameters is investigated by keeping T L constant at 600 K. The variations in gabs-solar-reactor as a function of T H estimated as per Eq. (3) are presented in Fig. 5. Results indicate that the gabs-solar-reactor decreases with the increase in the T H . This is because the radiative heat losses increase at higher operating temperatures. For instance, at T H = 1780 K, the gabs-solar-reactor is equal to 88.61%. As the T H increases up to 2380 K, the gabs-solar-reactor decreases to 63.61%. In contrast to this, the gabs-solar-heater remains constant (Table 2) as the water splitting reaction is carried out at constant temperature (T L = 600 K). Fig. 5 also shows the effect of T H on Q_ SnO -red . As per the trends 2
reported, not surprisingly, the Q_ SnO2 -red increases with the rise in the T H . At T H = 1780 K, the Q_ SnO -red is equal to 651.59 kW. The value 2
Q_ FC-Ideal ¼ ð298Þ n_ DSjH2 ðgÞþ0:5O2 ðgÞ!H2 OðlÞ@298 K
ð25Þ
Based on the maximum work extracted from the H2, the efficiency of the SnO2/SnO based thermochemical water splitting cycle is calculated by using Eq. (26).
_ W
gcycle ¼ _ FC-Ideal Q solar-cycle
ð26Þ
Similarly, the solar-to-fuel energy conversion efficiency is determined as:
gsolar-to-fuel ¼
HHV H2 ðmoles of H2 producedÞ Q_ solar-cycle
ð27Þ
of Q_ SnO2 -red increases up to 685.66 kW as the T H surges up to 2380 K. As the temperature difference between T H and T L increases due to the rise in the T H , more amount of heat energy is needed for the reduction of SnO2 and hence Q_ SnO -red increases. At fixed T L = 2
600 K, the amount of energy needed to heat water(Q_ H2 O-heating ) from 298 K up to 600 K is calculated and reported in Table 2. Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle are determined as per Eqs. (5), (15), and (16) and the variations in these thermodynamic parameters with respect to T H are presented in Fig. 6a. Since the thermal energy needed for the reduction of SnO2 increases with T H , values for all these parameters enhances with the rise in the
Both, the cycle and the solar-to-fuel energy conversion efficiencies can be improved by employing heat recuperation in the SnO2/ SnO based thermochemical water splitting cycle. The total amount of heat energy that can be recuperated in this cycle is determined as follows:
Q_ recuperable ¼ Q_ cooler-1 þ Q_ cooler-2 þ Q_ cooler-3 þ Q_ splitting-reactor
ð28Þ
As the heat energy liberated by cooler – 1, cooler – 2, cooler – 3, and water splitting reactor are recycled and re-used to run the SnO2/SnO based thermochemical water splitting cycle, the total solar energy input needed can be decreased as:
Q_ solar-HR ¼ Q_ solar-cycle Q_ recupaerable-HR Here, the Q_ recupaerable-HR can be calculated as:
ð29Þ Fig. 5. Variations in gabs-solar-reactor and Q_ SnO2 -red as a function of T H .
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
231
Table 2 Effect of T H on various thermodynamic parameters involved in the design of solar driven SnO2/SnO based thermochemical water splitting cycle. Parameter
Value
gabs-solar-heater
99.85% 54.52 kW
Q_ H2 O-heating Q_ solar-heater
54.60 kW
Q_ re-rad-solar-heater Q_
0.08 kW
Q_ cooler-3 Q_
8.70 kW
Q_ FC-Ideal _ FC-Ideal W
48.66 kW
cooler-2
splitting-reactor
_ solar-heater Irr _ cooler-2 Irr
_ cooler-3 Irr _ splitting-reactor Irr
4.62 kW 51.50 kW 237.17 kW 0.052 kW/K 0.005 kW/K 0.009 kW/K 0.027 kW/K
_ solar-reactor and Irr _ cooler-1 . Fig. 7. Effect of T H on Irr
The irreversibility associated with the solar reactor is computed using Eq. (7). The results reported in Fig. 7 indicate increase in the _ solar-reactor as a function of rise in the T H . The Irr _ solar-reactor increases Irr by a factor of 9.32 due to the upsurge in the T H from 1780 to 2380 K. This observation is similar to that of the re-radiation losses and hence it can be concluded that the entropy associated with the solar reactor is proportional to the re-radiative losses. Similar to _ solar-reactor , the irreversibility associated with the solar heater is Irr also determined as per Eq. (14) and observed to be 0.052 kW/K at all T H . The heat energy liberated and the entropy associated with the cooler – 1 is estimated as per Eqs. (8) and (9) and are reported in Figs. 6b and 7 as a function of increase in the T H . The Q_ cooler-1 and
_ cooler-1 at T H = 1780 K are equal to 355.44 kW and 0.369 kW/K, Irr accordingly. As the T H increases up to 2380 K, Q_ cooler-1 is escalated
_ cooler-1 upsurges by a factor of 1.11. Obtained results by 9.47% and Irr indicate that more energy is liberated from cooler – 1 with higher irreversibility due to the increase in the T H . The heat energy liberated and the entropy associated with cooler – 2 and cooler – 3 are also calculated as per Eqs. (20)–(23) and the values are reported in Table 2. As the water splitting reaction is carried out at constant temperature (600 K), the exothermic heat released and the irreversibility associated with this reactor are calculated by using Eqs. (18) and (19) and the respective values are reported in Table 2. Likewise, the thermodynamic calculations associated with the ideal _ FC-Ideal and H2/O2 fuel cell are also performed and the values of W Q_ FC-Ideal are also reported in Table 2.
Fig. 6. Effect of T H on (a) Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle and (b) Q_ re-rad-cycle and Q_ cooler-1 .
T H . The Q_ cycle-net increases from 706.11 up to 740.18 kW due to the escalation in T H from 1780 to 2380 K. With the similar rise in the T H value, the Q_ solar-reactor rises by a factor of 1.47 and the Q_ solar-cycle upsurges by 30.25%, respectively. Although the re-radiation losses from the solar heater are very low (Table 2), the heat energy dissipated from the solar reactor due to the re-radiation is significant and hence the cumulative reradiation losses from the SnO2/SnO based thermochemical water splitting cycle are of increasing order as the T H varies (Fig. 6b). At T H = 1780 K, the Q_ re-rad-cycle is equal to 83.80 kW; however, as the T H increases up to 2380 K, the Q_ re-rad-cycle upsurges by 308.53 kW.
Fig. 8. Variations in gcycle and gsolar-to-fuel as a function of T H .
232
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
The effect of T H on gcycle and gsolar-to-fuel is presented in Fig. 8. From the data reported, it is realized that both gcycle and gsolar-to-fuel decreases with the increase in T H . At T H ¼ 1780 K, the gcycle ¼ 30:02% and gsolar-to-fuel ¼ 36:19%. As the T H increases up to 2380 K, both gcycle and gsolar-to-fuel decreases to 20.94 and 25.24%, respectively. As the T H increases, the Q_ solar-cycle increases and hence
both gcycle and gsolar-to-fuel reduces considerably. The increase in the gcycle and gsolar-to-fuel can be achieved by recuperating the heat energy liberated by the coolers and water splitting reactor installed. As per Eq. (28), the total amount of recuperable heat energy is estimated and at T H ¼ 1780 K it is observed to be equal to 420.28 kW. Heat recuperation is increases up to 454.35 kW if the thermal reduction of SnO2 is carried out at a higher temperature of 2380 K. Because of heat recuperation, the total solar energy input needed to run the SnO2/SnO based thermochemical water splitting cycle is decreases which can result into increase in the gcycle and gsolar-to-fuel , respectively. The increase in the efficiency values as a function of percentage heat recuperation are reported in Table 3 (at T H = 1780 K). For instance, at 10% heat recuperation, the total solar energy needed decreases to 747.88 kW which results into increase in the gcycle and gsolar-to-fuel up to 31.71 and 38.22%, respectively. As the heat recuperation increases to 50%, the Q_ solar-HR
Table 4 Effect of heat recuperation on efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K. HR (%)
Q_ recupaerable-HR (kW)
Q_ solar-HR (kW)
gcycle-HR (%)
gSolar-to-fuel-HR (%)
5 10 15 20 25 30 35 40 45 50
20.98 41.95 62.93 83.91 104.88 125.86 146.83 167.81 188.79 209.76
764.93 743.95 722.97 701.99 681.02 660.05 639.07 618.09 597.12 576.14
31.01 31.88 32.80 33.78 34.83 35.93 37.11 38.37 39.72 41.17
37.37 38.42 39.54 40.72 41.97 43.31 44.73 46.25 47.87 49.61
reduces to 579.99 kW which further enhances the gcycle and gsolar-to-fuel up to 40.91 and 49.30%, respectively. Thermodynamic calculations performed to study the effect of _ FC-Ideal as per Eq. (33) and comT H are verified by calculating the W
_ FC-Ideal determined as per Eq. (24). Based on the paring it with W _ FC-Ideal determined as per Eq. (33) equals with data calculated, the W _ FC-Ideal calculated by using Eq. (24), which confirms that there the W are no errors involved in the thermodynamic calculations performed in this study.
Fig. 9. Effect of T L on Q_ H2 O-heating and Q_ SnO2 -red .
is needed to heat the water molecule up to this temperature and hence the Q_ H O-heating is enhances. However, with the increase in 2
3.3. Effect of water splitting temperature In this section, the effect of T L (400–800 K) on various thermodynamic parameters associated with the SnO2/SnO based thermochemical water splitting cycle is studied by keeping the T H constant at 1780 K. As the thermal reduction is carried out at constant temperature, the gabs-solar-reactor remain unchanged at 88.62% for the entire study (Table 4). By using Eq. (10), the gabs-solar-heater is calculated by varying T L in the range of 400–800 K and observed to be in the range of 99.41–99.97%. The effect of T L on Q_ SnO -red and Q_ H O-heating is determined and the 2
2
obtained results are reported in Fig. 9. With the increase in T L from 400 to 800 K, the Q_ H O-heating increases from 47.47 up to 62.04 kW. 2
Contrasting to this, the Q_ SnO2 -red decreases by 4.37% due to the similar increase in the T L . As the T L increases, more amount of energy Table 3 Effect of heat recuperation on efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle at T H ¼ 1780 K and T L ¼ 600 K. HR (%)
Q_ recupaerable-HR (kW)
Q_ solar-HR (kW)
gcycle-HR (%)
gSolar-to-fuel-HR (%)
5 10 15 20 25 30 35 40 45 50
21.01 42.03 63.04 84.06 105.07 126.08 147.10 168.11 189.12 210.14
768.90 747.88 726.87 705.85 684.84 663.83 642.81 621.80 600.78 579.77
30.85 31.71 32.63 33.60 34.63 35.73 36.90 38.14 39.48 40.91
37.18 38.22 39.33 40.50 41.74 43.06 44.47 45.97 47.58 49.30
the T L , the difference between T H and T L decreases and hence less energy is needed to heat the SnO2 from T L up to T H resulting into decrease in the Q_ SnO -red . For example, at 800 K, since the SnO2 2
entering the reactor is already at an elevated temperature as compared to 400 K, less amount of solar energy is needed to heat the reactant up to the thermal reduction temperature (1780 K). Variations in the Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle as a function of T L are shown in Fig. 10a. The Q_ cycle-net decreases from 720.20 to 705.36 kW due to the increase in the T L (400–800 K). With the similar increase in T L , the Q_ solar-reactor and Q_ solar-cycle reduces also by 4.37 and 2.27%, respectively. As the T L increases from 400 to 800 K, the rate of increase in the Q_ H O-heating (14.57 kW) is less as compared to 2
the rate of decreases in the Q_ SnO2 -red (29.41 kW), thereby resulting in an overall decrease in the above parameters. Contrasting to this, as shown in Fig. 10b, the re-radiation losses from the SnO2/SnO based thermochemical water splitting cycle decreases by 3.5 kW with the increase in the T L from 400 to 800 K. This indicates that the re-radiation losses form the solar reactor are inversely proportional to T L . Effect of T L on the heat energy released by the different coolers and water splitting reactor is shown in Fig. 11a and b. As per Fig. 11a, the Q_ cooler-1 decreases from 375.47 to 348.31 kW as the T L increases from 400 to 800 K. This is again due to the fact that as the T L increases up to 800 K, the difference between the T H and T L decreases by 50% and hence less amount of heat energy is released by cooler – 1. Similar to this, the Q_ splitting-reactor also reduces by 5.73 kW due to the likewise increase in the T L . In contrast to this observation, the Q_ cooler-2 and Q_ cooler-3 increases with the rise in T L . At T L = 400 K, Q_ cooler-2 and Q_ cooler-3 are equal to
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
233
Fig. 11. Effect of T L on (a) Q_ cooler-1 and Q_ splitting-reactor and (b) Q_ cooler-2 and Q_ cooler-3 . Fig. 10. Effect of T L on (a)Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle and (b) Q_ re-rad-cycle .
1.52 and 2.54 kW, respectively. As the T L increases to 600 K, the Q_ cooler-2 and Q_ cooler-3 increases up to 4.62 and 8.70 kW, Further increase in the Q_ cooler-2 and Q_ cooler-3 up to 7.92 and 14.58 kW is seen when the T L upsurges to 800 K. Both Q_ cooler-2 and Q_ cooler-3 are
employed for the cooling of gaseous H2 and O2 produced during the thermal reduction and water splitting reaction from T L to 298 K (operating temperature for the fuel cell). Therefore, as T L increases from 400 to 800 K, the difference between the T L and operating temperature of the fuel cell increases from 25.5 to 62.8% and hence the energy released by Q_ cooler-2 and Q_ cooler-3 increases. After cooling, the H2 and O2 are conveyed to the ideal H2/O2 fuel cell for the conversion of H2 energy into work. The _ FC-Ideal and Q_ FC-Ideal associated with the fuel cell are calculated W and observed to be equal to 48.66 and 237.17 kW, respectively. The influence of T L on irreversibilities associated with the various devices installed in the SnO2/SnO based thermochemical water splitting cycle is also explored and the results are shown in Fig. 12a and b. In case of solar reactor, water splitting reactor, and the cooler – 1, the irreversibility’s decreases by 95.80, 91.48, and 66.24% respectively with the increase in the T L from 400 to 800 K. This is because the amount of heat energy needed by the solar reactor and the rate of heat energy dissipated by the water splitting reactor and cooler – 1 decreases with the reduction in the difference between T H and T L . However, due to the similar increase in the T L , the irreversibility’s associated with cooler – 2, cooler – 3, and the solar heater increases by a factor of 7.55, 15.64, and 14.82, individually as the difference between T L and room temperature increases. The gcycle and gsolar-to-fuel of the SnO2/SnO based thermochemical water splitting cycle are positively affected due to the increase in
the T L . At T L = 400 K, the gcycle and gsolar-to-fuel are equal to 29.40 and 35.44%, respectively. As the T L increases up to 800 K, the gcycle and gsolar-to-fuel also increases up to 30.08 and 36.26%, correspondingly. As mentioned in Section 3.2, further increase in the efficiencies can be achieved by employing heat recuperation. The total amount of energy that can be recuperated in this cycle decreases with the increase in the T L as less energy is dissipated by cooler – 1 and water splitting reactor. For instance, as the T L increases from 400 to 800 K, the Q_ recuperable decreases by 3.42%. The heat energy recuperated can be used to run the SnO2/SnO based thermochemical water splitting cycle, which reduces the requirement of solar energy input and increases the efficiency values. Table 4 reports that effect of heat recuperation on gcycle and gsolar-to-fuel at constant T L ¼ 800 K. As per the data reported, both efficiencies significantly upsurge by applying heat recuperation. The thermodynamic calculations performed in this section are also verified by following the same procedure used in Section 3.2. The results obtained in this study clearly indicate that to achieve gcycle and gsolar-to-fuel higher than the previously reported ZnO/Zn cycle (29%) [15], it is essential to operate the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K. 4. Summary and conclusions By using HSC Chemistry software and its thermodynamic database, the thermodynamics associated with the solar driven SnO2/ SnO based thermochemical water splitting cycle is investigated in detail. The thermodynamic equilibrium analysis performed at pressure equal to 1 bar indicates that the thermal reduction of SnO2 with complete conversion can be achieved at or above 2380 K. This analysis further indicates that the temperatures
234
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
References
Fig. 12. Effect of T L on irreversibilities associated with the various devices installed in the SnO2/SnO based thermochemical water splitting cycle [(a) and (b)].
required for the complete thermal reduction of SnO2 decreases to 1780 K by reducing the partial pressure of O2 in the inert carrier carried gas to 103 bar. Similar to the thermal reduction temperature, this analysis also shows that the water splitting reaction via re-oxidation of SnO is feasible below 875 K. The effects of T H and T L on various thermodynamic parameters associated with the SnO2/SnO based thermochemical water splitting cycle are also explored. It is realized that the solar energy needed to run this cycle increases by 342.62 kW due to the increase in the T H from 1780 to 2380 K. Contrasting to this, if the T L increases from 400 to 800 K, the solar energy requirement decreases by 18.32 kW. The gcycle and gsolar-to-fuel decreases by 10 to 12% with the increase in the T H from 1780 to 2380 K. However, both efficiencies increase by 0.5 to 1% when the T L increases from 400 to 800 K. Higher gcycle (30.08%) and gsolar-to-fuel (36.26%) can be achieved by conducting the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K, respectively. These efficiency values increase further up togcycle = 41.17% and gsolar-to-fuel ¼ 49:61% when 50% heat energy is recuperated. In the ongoing work, the SnO2/ SnO based solar thermochemical water splitting cycle with tin oxide and zinc oxide are conducted in laboratory scale. Hence, for future work, comparative studies using other metal oxides will be suggested to be used in the existing system. Acknowledgement This publication was made possible by the NPRP grant (NPRP8370-2-154) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of author(s).
[1] Villasmil W, Steinfeld A. Hydrogen production by hydrogen sulfide splitting using concentrated solar energy–thermodynamics and economic evaluation. Energy Convers Manage 2010;51:2353–61. [2] Charvin P, Abanades S, Beche E, Lemont F, Flamant G. Hydrogen production from mixed cerium oxides via three-step water-splitting cycles. Solid State Ionics 2009;180:1003–10. [3] Lin X, Zhang Y, Wang Z, Wang R, Zhou J, Cen K. Hydrogen production by HI decomposition over nickel–ceria–zirconia catalysts via the sulfur–iodine thermochemical water-splitting cycle. Energy Convers Manage 2014;84:664–70. [4] Ghandehariun S, Naterer G, Rosen M, Wang Z. Indirect contact heat recovery with solidification in thermochemical hydrogen production. Energy Convers Manage 2014;82:212–8. [5] Chueh WC, Falter C, Abbott M, Scipio D, Furler P, Haile SM, et al. High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 2010;330:1797–801. [6] Chueh WC, Haile SM. A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation. Philos Trans A Math Phys Eng Sci 2010;368:3269–94. [7] Wei B, Fakhrai R, Saadatfar B. The thermodynamic analysis of two-step conversions of CO2/H2O for syngas production by ceria. Int J Hydrogen Energy 2014;39:12353–60. [8] Weibel D, Jovanovic ZR, Gálvez E, Steinfeld A. Mechanism of Zn Particle Oxidation by H2O and CO2 in the Presence of ZnO. Chem Mater 2014;26:6486–95. [9] Villasmil W, Meier A, Steinfeld A. Dynamic modeling of a solar reactor for zinc oxide thermal dissociation and experimental validation using IR thermography. J Sol Energy Eng 2014;136:010901. [10] Scheffe JR, Steinfeld A. Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: a review. Mater Today 2014;17:341–8. [11] Roeb M, Neises M, Monnerie N, Call F, Simon H, Sattler C, et al. Materialsrelated aspects of thermochemical water and carbon dioxide splitting: a review. Materials 2012;5:2015–54. [12] Bhosale RR, Kumar A, AlMomani F, Ghosh U, Dardor D, Bouabidi Z, et al. Solar co-production of samarium and syngas via methanothermal reduction of samarium sesquioxide. Energy Convers Manage 2016;112:413–22. [13] Huang X, Chen X, Shuai Y, Yuan Y, Zhang T, Li B, et al. Heat transfer analysis of solar-thermal dissociation of NiFe2O4 by coupling MCRTM and FVM method. Energy Convers Manage 2015;106:676–86. [14] Sarwar J, Georgakis G, Kouloulias K, Kakosimos K. Experimental and numerical investigation of the aperture size effect on the efficient solar energy harvesting for solar thermochemical applications. Energy Convers Manage 2015;92:331–41. [15] Steinfeld A. Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. Int J Hydrogen Energy 2002;27:611–9. [16] Stamatiou A, Loutzenhiser P, Steinfeld A. Solar syngas production from H2O and CO2 via two-step thermochemical cycles based on Zn/ZnO and FeO/Fe3O4 redox reactions: kinetic analysis. Energy Fuels 2010;24:2716–22. [17] Levêque G, Abanades S. Investigation of thermal and carbothermal reduction of volatile oxides (ZnO, SnO2, GeO2, and MgO) via solar-driven vacuum thermogravimetry for thermochemical production of solar fuels. Thermochim Acta 2015;605:86–94. [18] Villasmil W, Brkic M, Wuillemin D, Meier A, Steinfeld A. Pilot scale demonstration of a 100-kWth solar thermochemical plant for the thermal dissociation of ZnO. J Sol Energy Eng 2014;136:011016. [19] Abanades S, Villafan-Vidales I. CO2 valorisation based on Fe3O4/FeO thermochemical redox reactions using concentrated solar energy. Int J Energy Res 2013;37:598–608. [20] Bhosale RR, Kumar A, van den Broeke Leo JP, Gharbia S, Dardor D, Jilani M, et al. Solar hydrogen production via thermochemical iron oxide–iron sulfate water splitting cycle. Int J Hydrogen Energy 2015;40:1639–50. [21] Loutzenhiser P, Gálvez M, Hischier I, Stamatiou A, Frei A, Steinfeld A. CO2 splitting via two-step solar thermochemical cycles with Zn/ZnO and FeO/Fe3O4 redox reactions II: Kinetic analysis. Energy Fuels 2009;23:2832–9. [22] Scheffe JR, Francés A, King DM, Liang X, Branch BA, Cavanagh AS, et al. Atomic layer deposition of iron (III) oxide on zirconia nanoparticles in a fluidized bed reactor using ferrocene and oxygen. Thin Solid Films 2009;517:1874–9. [23] Bhosale RR, Kumar A, AlMomani F, Alxneit I. Sol–gel derived CeO2–Fe2O3 nanoparticles: synthesis, characterization and solar thermochemical application. Ceram Int 2016;42:6728–37. [24] Scheffe JR, McDaniel AH, Allendorf MD, Weimer AW. Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite– zirconia composite. Energy Environ Sci 2013;6:963–73. [25] Scheffe JR, Li J, Weimer AW. A spinel ferrite/hercynite water-splitting redox cycle. Int J Hydrogen Energy 2010;35:3333–40. [26] Roeb M, Gathmann N, Neises M, Sattler C, Pitz-Paal R. Thermodynamic analysis of two-step solar water splitting with mixed iron oxides. Int J Energy Res 2009;33:893–902. [27] Agrafiotis CC, Pagkoura C, Zygogianni A, Karagiannakis G, Kostoglou M, Konstandopoulos AG. Hydrogen production via solar-aided water splitting thermochemical cycles: combustion synthesis and preliminary evaluation of spinel redox-pair materials. Int J Hydrogen Energy 2012;37:8964–80.
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235 [28] Agrafiotis C, Zygogianni A, Pagkoura C, Kostoglou M, Konstandopoulos AG. Hydrogen production via solar-aided water splitting thermochemical cycles with nickel ferrite: experiments and modeling. AIChE J 2013;59: 1213–25. [29] Bhosale RR, Kumar A, AlMomani F, Alxneit I. Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production. Ceram Int 2016;42:2431–8. [30] Bhosale RR, Shende RV, Puszynski JA. H2 generation from thermochemical water-splitting using sol-gel synthesized Zn/Sn/Mn-doped Ni-Ferrite. Int Rev Chem Eng 2010;2:852. [31] Bhosale RR, Alxneit I, van den Broeke Leo LP, Kumar A, Jilani M, Gharbia SS et al. Sol-gel synthesis of nanocrystalline Ni-Ferrite and Co-Ferrite redox materials for thermochemical production of solar fuels, vol. 1675; 2014. p. mrss14-1675-rr06-10. [32] Bhosale RR, Shende RV, Puszynski JA. Thermochemical water-splitting for H2 generation using sol-gel derived Mn-ferrite in a packed bed reactor. Int J Hydrogen Energy 2012;37:2924–34. [33] R.R. Bhosale, R.V. Shende, J.A. Puszynski, Sol-Gel Derived NiFe2O4 Modified with ZrO2 for Hydrogen Generation from Solar Thermochemical WaterSplitting Reaction 1387 (2012) mrsf11-1387-e09-07. [34] Amar V, Puszynski J, Shende R. H2 generation from thermochemical watersplitting using yttria stabilized NiFe2O4 core-shell nanoparticles. J Renew Sustain Energy 2015;7:023113. [35] Muhich CL, Evanko BW, Weston KC, Lichty P, Liang X, Martinek J, et al. Efficient generation of H2 by splitting water with an isothermal redox cycle. Science 2013;341:540–2. [36] Abanades S, Charvin P, Lemont F, Flamant G. Novel two-step SnO2/SnO watersplitting cycle for solar thermochemical production of hydrogen. Int J Hydrogen Energy 2008;33:6021–30. [37] Chambon M, Abanades S, Flamant G. Thermal dissociation of compressed ZnO and SnO2 powders in a moving-front solar thermochemical reactor. AIChE J 2011;57:2264–73. [38] Chambon M, Abanades S, Flamant G. Kinetic investigation of hydrogen generation from hydrolysis of SnO and Zn solar nanopowders. Int J Hydrogen Energy 2009;34:5326–36. [39] Chambon M, Abanades S, Flamant G. Solar thermal reduction of ZnO and SnO2: characterization of the recombination reaction with O2. Chem Eng Sci 2010;65:3671–80. [40] Charvin P, Abanades S, Lemont F, Flamant G. Experimental study of SnO2/SnO/ Sn thermochemical systems for solar production of hydrogen. AIChE J 2008;54:2759–67. [41] Bhosale R, Kumar A, AlMomani F, Ghosh U, Saad Anis M, Kakosimos K, et al. Solar hydrogen production via a samarium oxide-based thermochemical water splitting cycle. Energies 2016;9:316. [42] Bhosale R, Kumar A, AlMomani F. Solar thermochemical hydrogen production via terbium oxide based redox reactions. Int J Photoenergy 2016. 2016. [43] Bhosale RR, Sutar P, Kumar A, AlMomani F, Ali MH, Ghosh U, et al. Solar hydrogen production via erbium oxide based thermochemical water splitting cycle. J Renew Sustain Energy 2016;8:034702. [44] Abanades S, Le Gal A. CO2 splitting by thermo-chemical looping based on ZrxCe1xO2 oxygen carriers for synthetic fuel generation. Fuel 2012;102:180–6.
235
[45] Bhosale RR, Kumar A, AlMomani F, Ghosh U, Al-Muhtaseb S, Gupta R, et al. Assessment of CexZryHfzO2 based oxides as potential solar thermochemical CO2 splitting materials. Ceram Int 2016;42:9354–62. [46] Hao Y, Yang C, Haile SM. Ceria-zirconia solid solutions (Ce1–xZrxO2 d, x 6 0.2) for solar thermochemical water splitting: a thermodynamic study. Chem Mater 2014;26:6073–82. [47] Haussener S, Steinfeld A. Effective heat and mass transport properties of anisotropic porous ceria for solar thermochemical fuel generation. Materials 2012;5:192–209. [48] Kaneko H, Ishihara H, Taku S, Naganuma Y, Hasegawa N, Tamaura Y. Cerium ion redox system in CeO2–xFe2O3 solid solution at high temperatures (1,273– 1,673 K) in the two-step water-splitting reaction for solar H2 generation. J Mater Sci 2008;43:3153–61. [49] Kaneko H, Miura T, Ishihara H, Taku S, Yokoyama T, Nakajima H, et al. Reactive ceramics of CeO2–MOx (M = Mn, Fe, Ni, Cu) for H2 generation by two-step water splitting using concentrated solar thermal energy. Energy 2007;32:656–63. [50] Krenzke PT, Davidson JH. On the efficiency of solar H2 and CO production via the thermochemical cerium oxide redox cycle: the option of inert-swept reduction. Energy Fuels 2015;29:1045–54. [51] Scheffe JR, Steinfeld A. Thermodynamic analysis of cerium-based oxides for solar thermochemical fuel production. Energy Fuels 2012;26:1928–36. [52] Furler P, Scheffe JR, Steinfeld A. Syngas production by simultaneous splitting of H2O and CO2 via ceria redox reactions in a high-temperature solar reactor. Energy Environ Sci 2012;5:6098–103. [53] Demont A, Abanades S. High redox activity of Sr-substituted lanthanum manganite perovskites for two-step thermochemical dissociation of CO2. RSC Adv 2014;4:54885–91. [54] Evdou A, Zaspalis V, Nalbandian L. La1xSrxFeO3d perovskites as redox materials for application in a membrane reactor for simultaneous production of pure hydrogen and synthesis gas. Fuel 2010;89:1265–73. [55] Gálvez M, Jacot R, Scheffe J, Cooper T, Patzke G, Steinfeld A. Physico-chemical changes in Ca, Sr and Al-doped La–Mn–O perovskites upon thermochemical splitting of CO2 via redox cycling. Phys Chem Chem Phys 2015;17:6629–34. [56] Jiang Q, Tong J, Zhou G, Jiang Z, Li Z, Li C. Thermochemical CO2 splitting reaction with supported LaxA1 xFeyB1 yO3 (A = Sr, Ce, B = Co, Mn; 0 6 x, y 6 1) perovskite oxides. Sol Energy 2014;103:425–37. [57] Nalbandian L, Evdou A, Zaspalis V. La1xSrxMO3 (M = Mn, Fe) perovskites as materials for thermochemical hydrogen production in conventional and membrane reactors. Int J Hydrogen Energy 2009;34:7162–72. [58] Scheffe JR, Weibel D, Steinfeld A. Lanthanum–strontium–manganese perovskites as redox materials for solar thermochemical splitting of H2O and CO2. Energy Fuels 2013;27:4250–7. [59] Loutzenhiser PG, Steinfeld A. Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: thermodynamic cycle analysis. Int J Hydrogen Energy 2011;36:12141–7. [60] Abanades S. CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: thermogravimetric analysis. Int J Hydrogen Energy 2012;37:8223–31. [61] Bader R, Venstrom LJ, Davidson JH, Lipin´ski W. Thermodynamic analysis of isothermal redox cycling of ceria for solar fuel production. Energy Fuels 2013;27:5533–44.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle Rahul R. Bhosale ⇑, Anand Kumar, Parag Sutar Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar
a r t i c l e
i n f o
Article history: Received 16 October 2016 Received in revised form 23 December 2016 Accepted 24 December 2016
Keywords: Hydrogen production SnO2/SnO based thermochemical cycle Water splitting reaction Thermodynamic analysis Solar energy
a b s t r a c t There are many studies related on the SnO2/SnO based solar thermochemical water splitting cycle, however there are still no studies addressing on the detailed thermodynamic analysis of this process using HSC Chemistry software and its thermodynamic database. In this cycle, the first step belongs to the endothermic solar thermal reduction of SnO2 producing gaseous SnO and O2. The second step corresponds to the exothermic production of H2 via water splitting reaction using SnO produced in the first cycle, thereby regenerating SnO2 which can be recycled back to step 1. Thermodynamic equilibrium compositions associated with step 1 and 2 are identified as a function of reaction temperatures and partial pressures of O2 in the inert carrier gas. Furthermore, the thermodynamic efficiency analysis is performed by following the second law of thermodynamics to determine the cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle. Effects of thermal reduction and water splitting temperatures on various thermodynamic parameters are also investigated in detail. Obtained results indicate that the higher values of cycle efficiency (41.17%) and solar-to-fuel energy conversion efficiency (49.61%) are achievable by operating this cycle at a thermal reduction temperature of 1780 K and water splitting temperature equal to 800 K with 50% heat recuperation. This work gives a detailed thermodynamic and efficiency analysis of SnO2/SnO based two-step solar thermochemical water splitting cycle for hydrogen production. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction H2 is considered as one of the promising future energy sources due to its high energy density. To be a clean energy vector, it is essential to produce H2 via CO2-free processes [1–4]. To achieve this, a metal oxide based two-step solar thermochemical water splitting cycle has been investigated heavily by several researchers [5–14]. In this cycle, step 1 corresponds to the solar thermal reduction of the metal oxide releasing O2, whereas; step 2 yields into exothermic production of H2 via water splitting reaction (reoxidizing the metal oxide). The metal oxide is not consumed and hence can be re-used for multiple cycles. Various metal oxides were investigated towards thermochemical water splitting application e.g., zinc oxide [15–18], iron oxide [16,19–23], mixed iron oxide [24–35], tin oxide [36–40], samarium oxide [41], terbium oxide [42], erbium oxide [43], undoped and doped ceria [5,6,44–52], and perovskites [53–58]. Among these, the volatile zinc oxide based water splitting cycle is considered ⇑ Corresponding author. E-mail addresses: [email protected], solar.chemical.engineering@gmail. com (R.R. Bhosale). http://dx.doi.org/10.1016/j.enconman.2016.12.067 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.
as one of the promising one and studied extensively. Steinfeld [15] studied the second law thermodynamic analysis of the ZnO/ Zn water splitting cycle and reported cycle efficiency equal to 29%. Similar to this study, Loutzenhiser and Steinfeld [59] performed the thermodynamic analysis of the water and CO2 splitting ZnO/Zn cycle for the production of syngas and reported cycle efficiency equal to 32%. By conducting dynamic thermogravimetric experiments, the kinetics of ZnO/Zn water splitting cycle was investigated by Stamatiou et al. [16]. Recently, the pilot scale demonstration of the thermal reduction of ZnO using a 100 kWth solar reactor was reported by Villasmil et al. [18]. The working principle for the SnO2/SnO based water splitting cycle is similar to that of the ZnO/Zn water splitting cycle. Abanades et al. [36] reported that the thermal reduction of SnO2 into SnO(g) can be carried out at or above 1600 °C at 1 bar, while the H2 production via water splitting reaction can be achieved with 90% conversion in the temperature range of 500 –600 °C. Charvin et al. [40] studied the kinetics of the thermal reduction of SnO2 using a thermogravimetric analyzer and reported the activation energy and pre-exponential factor equal to 394.8 kJ/mol and 8.32 108 g/s, correspondingly. Chambon et al. [38] explored the kinetics of hydrolysis of SnO nanoparticles synthesized via solar
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
227
Nomenclature C HHV I MO n_ TH TL HR P O2
solar flux concentration ratio, suns higher heating value normal beam solar insolation, W m2 metal oxide molar flow rate, mol/s thermal reduction temperature, K water splitting temperature, K heat recuperation partial pressure of the O2, bar r Stefan – Boltzmann constant, 5.670 108 (W/ m2 K4) heat energy required for thermal reduction of Q_ SnO2 -red SnO2, kW Q_ H2 O-heating heat energy required for heating of water, kW net energy required run the cycle, kW Q_ cycle-net Q_ solar-reactor solar energy required to run the solar reactor, kW Q_ solar-heater solar energy required to run the solar heater, kW solar energy required to run the cycle, kW Q_ solar-cycle gabs-solar-reactor solar energy absorption efficiency of the solar reactor gabs-solar-heater solar energy absorption efficiency of the solar heater Q_ re-rad-solarreactor re-radiation losses from the solar reactor, kW Q_ re-rad-solar-heater re-radiation losses from the solar heater, kW Q_ re-rad-cycle re-radiation losses from the cycle, kW
_ solar-reactor Irr _ solar-heater Irr _ splitting-reactor Irr _ cooler-1 Irr _ cooler-2 Irr _ cooler-3 Irr Q_ cooler-1 Q_ cooler-2 Q_ cooler-3 Q_
splitting-reactor
Q_ FC-Ideal _ FC-Ideal W
gcycle gcycle-HR gsolar-to-fuel-HR gsolar-to-fuel
Q_ recuperable Q_
recupaerable-HR
irreversibility associated with the solar reactor, kW/K irreversibility associated with the solar heater, kW/K irreversibility associated with the water splitting reactor, kW/K irreversibility associated with cooler – 1, kW/K irreversibility associated with cooler – 2, kW/K irreversibility associated with cooler – 3, kW/K heat energy libe2rated from cooler – 1, kW heat energy liberated from cooler – 2, kW heat energy liberated from cooler – 3, kW heat energy liberated from water splitting reactor, kW heat energy liberated from an ideal fuel cell, kW work output of an ideal fuel cell, kW cycle efficiency cycle efficiency (with heat recuperation) Solar-to-fuel energy conversion efficiency (with heat recuperation) solar-to-fuel energy conversion efficiency heat energy that can be recuperated (total), kW heat energy that can be recuperated (with % HR), kW
thermal reduction and reported the order of the reaction and the activation energy equal to 2 and 122 kJ/mol, respectively. The reaction order and the activation energy for the recombination of SnO and O2 during the quenching step was also investigated [39] and observed to be equal to 1.4 and 42 kJ/mol, accordingly. Similar to the individual water splitting reaction, the SnO2/SnO redox system was also studied towards production of syngas via combined H2O and CO2 splitting using a solar reactor and a thermogravimetric analyzer [60]. Although, multiple studies are reported, the detailed thermodynamic and efficiency analysis of SnO2/SnO based solar thermochemical water splitting cycle is missing from the published literature. Therefore, in this investigation, the thermodynamic analysis of the SnO2/SnO based solar thermochemical water splitting cycle is carried out using HSC Chemistry software and its thermodynamic database. Effects of various operating parameters such as partial pressure of O2 (PO2 ) in the inert carrier gas, thermal reduction temperature (T H ), water splitting temperature (T L ) and heat recuperation (HR) on thermodynamic equilibrium composition, the net solar energy required, re-radiation losses, heat energy released, irreversibility associated and efficiency of the SnO2/SnO based thermochemical water splitting cycle are studied and reported. 2. Thermodynamic equilibrium analysis Two redox reactions involved in the SnO2/SnO based thermochemical water splitting cycle are:
SnO2 ! SnO þ 0:5O2
ð1Þ
SnO þ H2 O ! SnO2 þ H2
ð2Þ
As mentioned earlier, the thermodynamic equilibrium compositions associated with the SnO2/SnO based thermochemical water splitting cycle are determined by using HSC Chemistry software
Fig. 1. Effect of (a) T H and (b)T L on delta G associated with the thermal reduction and water splitting step of SnO2/SnO based thermochemical cycle.
228
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
and its thermodynamic database by assuming a continuous operation of the solar thermal reactor with inlet molar concentration of SnO2 equal to 1 mol/s. Alteration in the Gibbs free energy (delta G) with respect to the increase in the T H (in case of thermal reduction of SnO2) and T L (in case of re-oxidation of SnO via water splitting reaction) is shown in Fig. 1a and b. As per the data reported, delta G for the thermal reduction step decreases by 121.78% due the increase in the T H from 300 to 2800 K. Contrasting to this, the delta G associated with the water splitting step increases by 147.90% as the T L rises from 300 to 1200 K. Reported data also indicates that the thermal reduction of SnO2 is possible at or above 2300 K (at 1 bar) and the reoxidation of SnO via water splitting reaction is feasible below 875 K (at 1 bar). The equilibrium compositions associated with the water splitting step (reported in Fig. 2, at 1 bar) also confirms that this reaction is more feasible at temperatures lower than 875 K. According to the published literature, temperature required for the thermal reduction of metal oxide can be decreased with the reduction in the P O2 in the inert carrier gas [41–43,51,61]. In this regard, the effect of P O2 in the inert carrier gas on the equilibrium compositions associated and T H required for the thermal reduction of SnO2 is analyzed and the obtained results are reported in Fig. 3a and b. It is evident from the plots that the equilibrium compositions associated with the thermal reduction of SnO2 shifts to the left i.e. towards lower T H as the P O2 in the inert carrier gas reduces. For instance, at 1 bar, the thermal reduction of SnO2 starts at 2080 K and undergoes 100% completion at 2380 K. By reducing the PO2 in the inert carrier gas to 101 bar, the reduction of SnO2 can be initiated at 1650 K and complete conversion can be realized at 2090 K. Further reduction in the P O2 in the inert carrier gas to
Fig. 3. Effect of P O2 in the inert carrier gas on the equilibrium compositions of (a) SnO2 and (b) SnO(g) and T H required for the thermal reduction of SnO2.
102 and 103 bar yields into a significant decrease in the T H to 1940 and 1780 K, respectively (to achieve complete reduction of SnO2). 3. Thermodynamic efficiency analysis 3.1. Theory
Fig. 2. Equilibrium compositions associated with the re-oxidation of SnO via water splitting reaction at 1 bar, (a) Sn-based components and (b) H2-based components.
To conduct the thermodynamic efficiency analysis of the SnO2/ SnO based thermochemical water splitting cycle driven by solar power; a process flow configuration has been developed (Fig. 4). According to the process flow diagram, the SnO2/SnO based thermochemical water splitting cycle requires a solar reactor, a water splitting reactor, three coolers, one solar heater, and an ideal H2/ O2 fuel cell (assumed 100% efficiency). The operation of the SnO2/ SnO based thermochemical water splitting process can be carried out at 1 bar. Steady state conditions are assumed and viscous losses or variations in kinetic or potential energies are deserted. The solar reactor is considered as a perfectly insulated cavity reactor with negligible conductive and convective losses. It is also assumed that all the reactions involved in the SnO2/SnO based thermochemical water splitting cycle yields into 100% conversion. Calculations associated with the heat exchangers are not included in this analysis. The procedure used to perform the thermodynamic efficiency analysis is similar to the one used by other investigators, however; modifications are done to improve the accuracy of the final results. Based on the thermodynamic equilibrium analysis reported in Section 2, the effect of T H is explored by varying it in the range of 1780 to 2380 K. Likewise, the effect of T L is also investigated by altering it in the gauge of 400–800 K, respectively.
229
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
After performing the thermal reduction of SnO2, the gaseous products exiting the solar reactor are quenched to T L using cooler – 1. In this step, the gaseous SnO gets converted into the solid SnO and naturally separates from the gaseous O2 without expending any work. The rate of heat energy released and the entropy generated in cooler – 1 are estimated as:
Q_ cooler-1 ¼ n_ DHjSnOðgÞþ0:5O2 ðgÞ@T H !SnOþ0:5O2 ðgÞ@T L _ cooler-1 ¼ Irr
Q_ cooler-1 TL
ð8Þ
! þ n_ DSjSnOðgÞþ0:5O2 ðgÞ@T H !SnOþ0:5O2 ðgÞ@T L
ð9Þ
Solid SnO exiting the cooler – 1 is transferred to the water splitting reactor to produce H2 via re-oxidizing itself via water splitting reaction. The water splitting reactor is operated at T L , which is higher than 298 K, hence the H2O entering this reactor needs to be pre-heated up to T L . This can be achieved by using a solar heater as shown in the process flow diagram (Fig. 4). The solar energy absorption efficiency of the solar heater is determined as:
gabs-solar-heater ¼ 1 Fig. 4. Process flow diagram for solar driven SnO2/SnO based thermochemical water splitting cycle.
The SnO2 enters the solar reactor at T L (molar flow rate = 1 mol/ s). After entering the solar reactor, it is heated up to T H and the thermal reduction is carried out by using the concentrated solar energy. The solar energy absorption efficiency is determined as:
gabs-solar-reactor ¼ 1
rT 4H
!
ð3Þ
IC
The baseline parameters used for this calculation are reported in Table 1. The heat energy needed for the thermal reduction of SnO2 can be calculated as:
Q_ SnO2 -red ¼ n_ DHjSnO2 @T L !SnOðgÞþ0:5O2 ðgÞ@T H
Q_ solar-reactor ¼
Q_ SnO2 -red
ð5Þ
gabs-solar-reactor
Re-radiation losses and the irreversibility associated with the solar reactor are calculated via following equations:
Q_ re-rad-solarreactor ¼ Q_ solar-reactor Q_ SnO2 -red _ solar-reactor ¼ Irr
Q_ solar-reactor TH
!
Q_ re-rad-solarreactor þ TL
ð10Þ
IC
Q_ H2 O-heating ¼ n_ DHjH2 OðlÞ@298 K!H2 OðlÞ@T L
ð11Þ
By using the gabs-solar-heater and Q_ H2 O-heating , the total solar energy needed to drive the solar heater is calculated as:
Q_ H2 O-heating Q_ solar-heater ¼
ð12Þ
gabs-solar-heater
Similar to the solar reactor, the solar heater also shows some reradiation losses and entropy generation which can be determined as:
Q_ re-rad-solar-heater ¼ Q_ solar-heater Q_ H2 O-heating _ solar-heater ¼ Irr
Q_ solar-heater TL
! þ
Q_ re-rad-solar-heater 298 K
þ n_ DSjH2 OðlÞ@298 K!H2 OðlÞ@T L
ð13Þ !
ð14Þ
Based on Eqs. (4) and (11), the net heat energy required to run the SnO2/SnO based thermochemical water splitting cycle is estimated as:
Q_ cycle-net ¼ Q_ SnO2 -red þ Q_ H2 O-heating
ð15Þ
ð6Þ
Likewise, as the SnO2/SnO based thermochemical water splitting cycle requires solar energy input for solar reactor as well as for solar heater, the total solar energy needed to run this cycle can be computed as:
ð7Þ
Q_ solar-cycle ¼ Q_ solar-reactor þ Q_ solar-heater
!
þ n_ DSjSnO2 @T L !SnOðgÞþ0:5O2 ðgÞ@T H
!
Furthermore, the heat energy needed to convert the liquid water into steam and then heating the steam up to T L can be estimated as:
ð4Þ
Total amount of solar energy required to heat the SnO2 from T L to T H , and to perform the thermal reduction reaction at T H is estimated according to Eq. (5).
rT 4L
ð16Þ
The total re-radiation losses from solar reactor and solar heater are computed as: Table 1 Baseline parameters for the thermodynamic analysis.
Q_ re-rad-cycle ¼ Q_ re-rad-solarreactor þ Q_ re-rad-solar-heater
Parameter
Value
I C
1 kW/m2 5000 suns 5.670 108 W/m2 K4 298 K 1 bar 100%
r Ambient temperature Total system pressure Fuel cell efficiency
ð17Þ
Water splitting is an exothermic reaction, hence when SnO and H2O(g) react, heat energy is released, which can be estimated as:
Q_ splitting-reactor ¼ n_ DHjSnOþH2 OðgÞ@T L !SnO2 þH2 ðgÞ@T L
ð18Þ
The irreversibility associated with the water splitting reactor is also estimated by using Eq. (19).
230
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
Q_ splitting-reactor TL
_ splitting-reactor ¼ Irr
!
þ n_ DSjSnOþH2 OðgÞ@T L !SnO2 þH2 ðgÞ@T L
Q_ recupaerable-HR ¼ ð%HRÞ Q_ recuperable ð19Þ
After performing the water splitting reaction, the SnO2 produced is recycled back to the solar reactor and the H2 produced forwarded to the ideal H2/O2 fuel cell (passing through cooler – 3). The operation of the ideal H2/O2 fuel cell is carried out at 298 K and hence it is essential to reduce the temperatures of H2 and O2 from T L to 298 K. To achieve this, cooler – 2 and cooler – 3 are installed in the SnO2/SnO based thermochemical water splitting cycle (Fig. 4). As the temperatures of H2 and O2 reduces from T L to 298 K, the heat energy released from both coolers is calculated individually as per Eqs. (20) and (21).
Q_ cooler-2 ¼ n_ DHj0:5O2 ðgÞ@T L !0:5O2 ðgÞ@298 K
ð20Þ
Q_ cooler-3 ¼ n_ DHjH2 ðgÞ@T L !H2 ðgÞ@298 K
ð21Þ
In addition to the energy liberated, the entropy generated during cooling of H2 and O2 is also computed as per following equations.
_ cooler-2 ¼ Irr
Q_ cooler-2 298
_ cooler-3 ¼ Irr
Q_ cooler-3 298
!
þ n_ DSj0:5O2 ðgÞ@T L !0:5O2 ðgÞ@298 K
ð22Þ
þ n_ DSjH2 ðgÞ@T L !H2 ðgÞ@298 K
ð23Þ
!
The ideal H2/O2 fuel cell is added to the SnO2/SnO based thermochemical water splitting cycle to determine the maximum work that can be extracted from the H2. The maximum work extracted and the heat energy released by the ideal H2/O2 fuel cell is calculated as:
_ FC-Ideal ¼ n_ DGj W H2 ðgÞþ0:5O2 ðgÞ@298 K!H2 OðlÞ@298 K
ð24Þ
ð30Þ
By using Q_ solar-HR , the higher values for cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting can be achieved:
_ W
gcycle-HR ¼ _ FC-Ideal Q solar-HR gsolar-to-fuel-HR ¼
ð31Þ
HHV H2 ðmoles of H2 producedÞ Q_ solar-HR
ð32Þ
The verification of the thermodynamic efficiency analysis performed can be carried out by comparing the maximum work that can be extracted from the H2 produced via water splitting reaction is calculated using Eqs. (24) and (33).
_ FC-Ideal ¼ Q_ solar-cycle -ðQ_ re-rad-cycle þ Q_ cooler-1 þ Q_ cooler-2 þ Q_ cooler-3 W þ Q_ splitting-reactor þ Q_ FC-Ideal Þ
ð33Þ
3.2. Effect of thermal reduction temperature The effect of T H on thermodynamic parameters is investigated by keeping T L constant at 600 K. The variations in gabs-solar-reactor as a function of T H estimated as per Eq. (3) are presented in Fig. 5. Results indicate that the gabs-solar-reactor decreases with the increase in the T H . This is because the radiative heat losses increase at higher operating temperatures. For instance, at T H = 1780 K, the gabs-solar-reactor is equal to 88.61%. As the T H increases up to 2380 K, the gabs-solar-reactor decreases to 63.61%. In contrast to this, the gabs-solar-heater remains constant (Table 2) as the water splitting reaction is carried out at constant temperature (T L = 600 K). Fig. 5 also shows the effect of T H on Q_ SnO -red . As per the trends 2
reported, not surprisingly, the Q_ SnO2 -red increases with the rise in the T H . At T H = 1780 K, the Q_ SnO -red is equal to 651.59 kW. The value 2
Q_ FC-Ideal ¼ ð298Þ n_ DSjH2 ðgÞþ0:5O2 ðgÞ!H2 OðlÞ@298 K
ð25Þ
Based on the maximum work extracted from the H2, the efficiency of the SnO2/SnO based thermochemical water splitting cycle is calculated by using Eq. (26).
_ W
gcycle ¼ _ FC-Ideal Q solar-cycle
ð26Þ
Similarly, the solar-to-fuel energy conversion efficiency is determined as:
gsolar-to-fuel ¼
HHV H2 ðmoles of H2 producedÞ Q_ solar-cycle
ð27Þ
of Q_ SnO2 -red increases up to 685.66 kW as the T H surges up to 2380 K. As the temperature difference between T H and T L increases due to the rise in the T H , more amount of heat energy is needed for the reduction of SnO2 and hence Q_ SnO -red increases. At fixed T L = 2
600 K, the amount of energy needed to heat water(Q_ H2 O-heating ) from 298 K up to 600 K is calculated and reported in Table 2. Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle are determined as per Eqs. (5), (15), and (16) and the variations in these thermodynamic parameters with respect to T H are presented in Fig. 6a. Since the thermal energy needed for the reduction of SnO2 increases with T H , values for all these parameters enhances with the rise in the
Both, the cycle and the solar-to-fuel energy conversion efficiencies can be improved by employing heat recuperation in the SnO2/ SnO based thermochemical water splitting cycle. The total amount of heat energy that can be recuperated in this cycle is determined as follows:
Q_ recuperable ¼ Q_ cooler-1 þ Q_ cooler-2 þ Q_ cooler-3 þ Q_ splitting-reactor
ð28Þ
As the heat energy liberated by cooler – 1, cooler – 2, cooler – 3, and water splitting reactor are recycled and re-used to run the SnO2/SnO based thermochemical water splitting cycle, the total solar energy input needed can be decreased as:
Q_ solar-HR ¼ Q_ solar-cycle Q_ recupaerable-HR Here, the Q_ recupaerable-HR can be calculated as:
ð29Þ Fig. 5. Variations in gabs-solar-reactor and Q_ SnO2 -red as a function of T H .
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
231
Table 2 Effect of T H on various thermodynamic parameters involved in the design of solar driven SnO2/SnO based thermochemical water splitting cycle. Parameter
Value
gabs-solar-heater
99.85% 54.52 kW
Q_ H2 O-heating Q_ solar-heater
54.60 kW
Q_ re-rad-solar-heater Q_
0.08 kW
Q_ cooler-3 Q_
8.70 kW
Q_ FC-Ideal _ FC-Ideal W
48.66 kW
cooler-2
splitting-reactor
_ solar-heater Irr _ cooler-2 Irr
_ cooler-3 Irr _ splitting-reactor Irr
4.62 kW 51.50 kW 237.17 kW 0.052 kW/K 0.005 kW/K 0.009 kW/K 0.027 kW/K
_ solar-reactor and Irr _ cooler-1 . Fig. 7. Effect of T H on Irr
The irreversibility associated with the solar reactor is computed using Eq. (7). The results reported in Fig. 7 indicate increase in the _ solar-reactor as a function of rise in the T H . The Irr _ solar-reactor increases Irr by a factor of 9.32 due to the upsurge in the T H from 1780 to 2380 K. This observation is similar to that of the re-radiation losses and hence it can be concluded that the entropy associated with the solar reactor is proportional to the re-radiative losses. Similar to _ solar-reactor , the irreversibility associated with the solar heater is Irr also determined as per Eq. (14) and observed to be 0.052 kW/K at all T H . The heat energy liberated and the entropy associated with the cooler – 1 is estimated as per Eqs. (8) and (9) and are reported in Figs. 6b and 7 as a function of increase in the T H . The Q_ cooler-1 and
_ cooler-1 at T H = 1780 K are equal to 355.44 kW and 0.369 kW/K, Irr accordingly. As the T H increases up to 2380 K, Q_ cooler-1 is escalated
_ cooler-1 upsurges by a factor of 1.11. Obtained results by 9.47% and Irr indicate that more energy is liberated from cooler – 1 with higher irreversibility due to the increase in the T H . The heat energy liberated and the entropy associated with cooler – 2 and cooler – 3 are also calculated as per Eqs. (20)–(23) and the values are reported in Table 2. As the water splitting reaction is carried out at constant temperature (600 K), the exothermic heat released and the irreversibility associated with this reactor are calculated by using Eqs. (18) and (19) and the respective values are reported in Table 2. Likewise, the thermodynamic calculations associated with the ideal _ FC-Ideal and H2/O2 fuel cell are also performed and the values of W Q_ FC-Ideal are also reported in Table 2.
Fig. 6. Effect of T H on (a) Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle and (b) Q_ re-rad-cycle and Q_ cooler-1 .
T H . The Q_ cycle-net increases from 706.11 up to 740.18 kW due to the escalation in T H from 1780 to 2380 K. With the similar rise in the T H value, the Q_ solar-reactor rises by a factor of 1.47 and the Q_ solar-cycle upsurges by 30.25%, respectively. Although the re-radiation losses from the solar heater are very low (Table 2), the heat energy dissipated from the solar reactor due to the re-radiation is significant and hence the cumulative reradiation losses from the SnO2/SnO based thermochemical water splitting cycle are of increasing order as the T H varies (Fig. 6b). At T H = 1780 K, the Q_ re-rad-cycle is equal to 83.80 kW; however, as the T H increases up to 2380 K, the Q_ re-rad-cycle upsurges by 308.53 kW.
Fig. 8. Variations in gcycle and gsolar-to-fuel as a function of T H .
232
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
The effect of T H on gcycle and gsolar-to-fuel is presented in Fig. 8. From the data reported, it is realized that both gcycle and gsolar-to-fuel decreases with the increase in T H . At T H ¼ 1780 K, the gcycle ¼ 30:02% and gsolar-to-fuel ¼ 36:19%. As the T H increases up to 2380 K, both gcycle and gsolar-to-fuel decreases to 20.94 and 25.24%, respectively. As the T H increases, the Q_ solar-cycle increases and hence
both gcycle and gsolar-to-fuel reduces considerably. The increase in the gcycle and gsolar-to-fuel can be achieved by recuperating the heat energy liberated by the coolers and water splitting reactor installed. As per Eq. (28), the total amount of recuperable heat energy is estimated and at T H ¼ 1780 K it is observed to be equal to 420.28 kW. Heat recuperation is increases up to 454.35 kW if the thermal reduction of SnO2 is carried out at a higher temperature of 2380 K. Because of heat recuperation, the total solar energy input needed to run the SnO2/SnO based thermochemical water splitting cycle is decreases which can result into increase in the gcycle and gsolar-to-fuel , respectively. The increase in the efficiency values as a function of percentage heat recuperation are reported in Table 3 (at T H = 1780 K). For instance, at 10% heat recuperation, the total solar energy needed decreases to 747.88 kW which results into increase in the gcycle and gsolar-to-fuel up to 31.71 and 38.22%, respectively. As the heat recuperation increases to 50%, the Q_ solar-HR
Table 4 Effect of heat recuperation on efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K. HR (%)
Q_ recupaerable-HR (kW)
Q_ solar-HR (kW)
gcycle-HR (%)
gSolar-to-fuel-HR (%)
5 10 15 20 25 30 35 40 45 50
20.98 41.95 62.93 83.91 104.88 125.86 146.83 167.81 188.79 209.76
764.93 743.95 722.97 701.99 681.02 660.05 639.07 618.09 597.12 576.14
31.01 31.88 32.80 33.78 34.83 35.93 37.11 38.37 39.72 41.17
37.37 38.42 39.54 40.72 41.97 43.31 44.73 46.25 47.87 49.61
reduces to 579.99 kW which further enhances the gcycle and gsolar-to-fuel up to 40.91 and 49.30%, respectively. Thermodynamic calculations performed to study the effect of _ FC-Ideal as per Eq. (33) and comT H are verified by calculating the W
_ FC-Ideal determined as per Eq. (24). Based on the paring it with W _ FC-Ideal determined as per Eq. (33) equals with data calculated, the W _ FC-Ideal calculated by using Eq. (24), which confirms that there the W are no errors involved in the thermodynamic calculations performed in this study.
Fig. 9. Effect of T L on Q_ H2 O-heating and Q_ SnO2 -red .
is needed to heat the water molecule up to this temperature and hence the Q_ H O-heating is enhances. However, with the increase in 2
3.3. Effect of water splitting temperature In this section, the effect of T L (400–800 K) on various thermodynamic parameters associated with the SnO2/SnO based thermochemical water splitting cycle is studied by keeping the T H constant at 1780 K. As the thermal reduction is carried out at constant temperature, the gabs-solar-reactor remain unchanged at 88.62% for the entire study (Table 4). By using Eq. (10), the gabs-solar-heater is calculated by varying T L in the range of 400–800 K and observed to be in the range of 99.41–99.97%. The effect of T L on Q_ SnO -red and Q_ H O-heating is determined and the 2
2
obtained results are reported in Fig. 9. With the increase in T L from 400 to 800 K, the Q_ H O-heating increases from 47.47 up to 62.04 kW. 2
Contrasting to this, the Q_ SnO2 -red decreases by 4.37% due to the similar increase in the T L . As the T L increases, more amount of energy Table 3 Effect of heat recuperation on efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle at T H ¼ 1780 K and T L ¼ 600 K. HR (%)
Q_ recupaerable-HR (kW)
Q_ solar-HR (kW)
gcycle-HR (%)
gSolar-to-fuel-HR (%)
5 10 15 20 25 30 35 40 45 50
21.01 42.03 63.04 84.06 105.07 126.08 147.10 168.11 189.12 210.14
768.90 747.88 726.87 705.85 684.84 663.83 642.81 621.80 600.78 579.77
30.85 31.71 32.63 33.60 34.63 35.73 36.90 38.14 39.48 40.91
37.18 38.22 39.33 40.50 41.74 43.06 44.47 45.97 47.58 49.30
the T L , the difference between T H and T L decreases and hence less energy is needed to heat the SnO2 from T L up to T H resulting into decrease in the Q_ SnO -red . For example, at 800 K, since the SnO2 2
entering the reactor is already at an elevated temperature as compared to 400 K, less amount of solar energy is needed to heat the reactant up to the thermal reduction temperature (1780 K). Variations in the Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle as a function of T L are shown in Fig. 10a. The Q_ cycle-net decreases from 720.20 to 705.36 kW due to the increase in the T L (400–800 K). With the similar increase in T L , the Q_ solar-reactor and Q_ solar-cycle reduces also by 4.37 and 2.27%, respectively. As the T L increases from 400 to 800 K, the rate of increase in the Q_ H O-heating (14.57 kW) is less as compared to 2
the rate of decreases in the Q_ SnO2 -red (29.41 kW), thereby resulting in an overall decrease in the above parameters. Contrasting to this, as shown in Fig. 10b, the re-radiation losses from the SnO2/SnO based thermochemical water splitting cycle decreases by 3.5 kW with the increase in the T L from 400 to 800 K. This indicates that the re-radiation losses form the solar reactor are inversely proportional to T L . Effect of T L on the heat energy released by the different coolers and water splitting reactor is shown in Fig. 11a and b. As per Fig. 11a, the Q_ cooler-1 decreases from 375.47 to 348.31 kW as the T L increases from 400 to 800 K. This is again due to the fact that as the T L increases up to 800 K, the difference between the T H and T L decreases by 50% and hence less amount of heat energy is released by cooler – 1. Similar to this, the Q_ splitting-reactor also reduces by 5.73 kW due to the likewise increase in the T L . In contrast to this observation, the Q_ cooler-2 and Q_ cooler-3 increases with the rise in T L . At T L = 400 K, Q_ cooler-2 and Q_ cooler-3 are equal to
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
233
Fig. 11. Effect of T L on (a) Q_ cooler-1 and Q_ splitting-reactor and (b) Q_ cooler-2 and Q_ cooler-3 . Fig. 10. Effect of T L on (a)Q_ solar-reactor , Q_ cycle-net , and Q_ solar-cycle and (b) Q_ re-rad-cycle .
1.52 and 2.54 kW, respectively. As the T L increases to 600 K, the Q_ cooler-2 and Q_ cooler-3 increases up to 4.62 and 8.70 kW, Further increase in the Q_ cooler-2 and Q_ cooler-3 up to 7.92 and 14.58 kW is seen when the T L upsurges to 800 K. Both Q_ cooler-2 and Q_ cooler-3 are
employed for the cooling of gaseous H2 and O2 produced during the thermal reduction and water splitting reaction from T L to 298 K (operating temperature for the fuel cell). Therefore, as T L increases from 400 to 800 K, the difference between the T L and operating temperature of the fuel cell increases from 25.5 to 62.8% and hence the energy released by Q_ cooler-2 and Q_ cooler-3 increases. After cooling, the H2 and O2 are conveyed to the ideal H2/O2 fuel cell for the conversion of H2 energy into work. The _ FC-Ideal and Q_ FC-Ideal associated with the fuel cell are calculated W and observed to be equal to 48.66 and 237.17 kW, respectively. The influence of T L on irreversibilities associated with the various devices installed in the SnO2/SnO based thermochemical water splitting cycle is also explored and the results are shown in Fig. 12a and b. In case of solar reactor, water splitting reactor, and the cooler – 1, the irreversibility’s decreases by 95.80, 91.48, and 66.24% respectively with the increase in the T L from 400 to 800 K. This is because the amount of heat energy needed by the solar reactor and the rate of heat energy dissipated by the water splitting reactor and cooler – 1 decreases with the reduction in the difference between T H and T L . However, due to the similar increase in the T L , the irreversibility’s associated with cooler – 2, cooler – 3, and the solar heater increases by a factor of 7.55, 15.64, and 14.82, individually as the difference between T L and room temperature increases. The gcycle and gsolar-to-fuel of the SnO2/SnO based thermochemical water splitting cycle are positively affected due to the increase in
the T L . At T L = 400 K, the gcycle and gsolar-to-fuel are equal to 29.40 and 35.44%, respectively. As the T L increases up to 800 K, the gcycle and gsolar-to-fuel also increases up to 30.08 and 36.26%, correspondingly. As mentioned in Section 3.2, further increase in the efficiencies can be achieved by employing heat recuperation. The total amount of energy that can be recuperated in this cycle decreases with the increase in the T L as less energy is dissipated by cooler – 1 and water splitting reactor. For instance, as the T L increases from 400 to 800 K, the Q_ recuperable decreases by 3.42%. The heat energy recuperated can be used to run the SnO2/SnO based thermochemical water splitting cycle, which reduces the requirement of solar energy input and increases the efficiency values. Table 4 reports that effect of heat recuperation on gcycle and gsolar-to-fuel at constant T L ¼ 800 K. As per the data reported, both efficiencies significantly upsurge by applying heat recuperation. The thermodynamic calculations performed in this section are also verified by following the same procedure used in Section 3.2. The results obtained in this study clearly indicate that to achieve gcycle and gsolar-to-fuel higher than the previously reported ZnO/Zn cycle (29%) [15], it is essential to operate the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K. 4. Summary and conclusions By using HSC Chemistry software and its thermodynamic database, the thermodynamics associated with the solar driven SnO2/ SnO based thermochemical water splitting cycle is investigated in detail. The thermodynamic equilibrium analysis performed at pressure equal to 1 bar indicates that the thermal reduction of SnO2 with complete conversion can be achieved at or above 2380 K. This analysis further indicates that the temperatures
234
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235
References
Fig. 12. Effect of T L on irreversibilities associated with the various devices installed in the SnO2/SnO based thermochemical water splitting cycle [(a) and (b)].
required for the complete thermal reduction of SnO2 decreases to 1780 K by reducing the partial pressure of O2 in the inert carrier carried gas to 103 bar. Similar to the thermal reduction temperature, this analysis also shows that the water splitting reaction via re-oxidation of SnO is feasible below 875 K. The effects of T H and T L on various thermodynamic parameters associated with the SnO2/SnO based thermochemical water splitting cycle are also explored. It is realized that the solar energy needed to run this cycle increases by 342.62 kW due to the increase in the T H from 1780 to 2380 K. Contrasting to this, if the T L increases from 400 to 800 K, the solar energy requirement decreases by 18.32 kW. The gcycle and gsolar-to-fuel decreases by 10 to 12% with the increase in the T H from 1780 to 2380 K. However, both efficiencies increase by 0.5 to 1% when the T L increases from 400 to 800 K. Higher gcycle (30.08%) and gsolar-to-fuel (36.26%) can be achieved by conducting the SnO2/SnO based thermochemical water splitting cycle at T H = 1780 K and T L = 800 K, respectively. These efficiency values increase further up togcycle = 41.17% and gsolar-to-fuel ¼ 49:61% when 50% heat energy is recuperated. In the ongoing work, the SnO2/ SnO based solar thermochemical water splitting cycle with tin oxide and zinc oxide are conducted in laboratory scale. Hence, for future work, comparative studies using other metal oxides will be suggested to be used in the existing system. Acknowledgement This publication was made possible by the NPRP grant (NPRP8370-2-154) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of author(s).
[1] Villasmil W, Steinfeld A. Hydrogen production by hydrogen sulfide splitting using concentrated solar energy–thermodynamics and economic evaluation. Energy Convers Manage 2010;51:2353–61. [2] Charvin P, Abanades S, Beche E, Lemont F, Flamant G. Hydrogen production from mixed cerium oxides via three-step water-splitting cycles. Solid State Ionics 2009;180:1003–10. [3] Lin X, Zhang Y, Wang Z, Wang R, Zhou J, Cen K. Hydrogen production by HI decomposition over nickel–ceria–zirconia catalysts via the sulfur–iodine thermochemical water-splitting cycle. Energy Convers Manage 2014;84:664–70. [4] Ghandehariun S, Naterer G, Rosen M, Wang Z. Indirect contact heat recovery with solidification in thermochemical hydrogen production. Energy Convers Manage 2014;82:212–8. [5] Chueh WC, Falter C, Abbott M, Scipio D, Furler P, Haile SM, et al. High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 2010;330:1797–801. [6] Chueh WC, Haile SM. A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation. Philos Trans A Math Phys Eng Sci 2010;368:3269–94. [7] Wei B, Fakhrai R, Saadatfar B. The thermodynamic analysis of two-step conversions of CO2/H2O for syngas production by ceria. Int J Hydrogen Energy 2014;39:12353–60. [8] Weibel D, Jovanovic ZR, Gálvez E, Steinfeld A. Mechanism of Zn Particle Oxidation by H2O and CO2 in the Presence of ZnO. Chem Mater 2014;26:6486–95. [9] Villasmil W, Meier A, Steinfeld A. Dynamic modeling of a solar reactor for zinc oxide thermal dissociation and experimental validation using IR thermography. J Sol Energy Eng 2014;136:010901. [10] Scheffe JR, Steinfeld A. Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: a review. Mater Today 2014;17:341–8. [11] Roeb M, Neises M, Monnerie N, Call F, Simon H, Sattler C, et al. Materialsrelated aspects of thermochemical water and carbon dioxide splitting: a review. Materials 2012;5:2015–54. [12] Bhosale RR, Kumar A, AlMomani F, Ghosh U, Dardor D, Bouabidi Z, et al. Solar co-production of samarium and syngas via methanothermal reduction of samarium sesquioxide. Energy Convers Manage 2016;112:413–22. [13] Huang X, Chen X, Shuai Y, Yuan Y, Zhang T, Li B, et al. Heat transfer analysis of solar-thermal dissociation of NiFe2O4 by coupling MCRTM and FVM method. Energy Convers Manage 2015;106:676–86. [14] Sarwar J, Georgakis G, Kouloulias K, Kakosimos K. Experimental and numerical investigation of the aperture size effect on the efficient solar energy harvesting for solar thermochemical applications. Energy Convers Manage 2015;92:331–41. [15] Steinfeld A. Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. Int J Hydrogen Energy 2002;27:611–9. [16] Stamatiou A, Loutzenhiser P, Steinfeld A. Solar syngas production from H2O and CO2 via two-step thermochemical cycles based on Zn/ZnO and FeO/Fe3O4 redox reactions: kinetic analysis. Energy Fuels 2010;24:2716–22. [17] Levêque G, Abanades S. Investigation of thermal and carbothermal reduction of volatile oxides (ZnO, SnO2, GeO2, and MgO) via solar-driven vacuum thermogravimetry for thermochemical production of solar fuels. Thermochim Acta 2015;605:86–94. [18] Villasmil W, Brkic M, Wuillemin D, Meier A, Steinfeld A. Pilot scale demonstration of a 100-kWth solar thermochemical plant for the thermal dissociation of ZnO. J Sol Energy Eng 2014;136:011016. [19] Abanades S, Villafan-Vidales I. CO2 valorisation based on Fe3O4/FeO thermochemical redox reactions using concentrated solar energy. Int J Energy Res 2013;37:598–608. [20] Bhosale RR, Kumar A, van den Broeke Leo JP, Gharbia S, Dardor D, Jilani M, et al. Solar hydrogen production via thermochemical iron oxide–iron sulfate water splitting cycle. Int J Hydrogen Energy 2015;40:1639–50. [21] Loutzenhiser P, Gálvez M, Hischier I, Stamatiou A, Frei A, Steinfeld A. CO2 splitting via two-step solar thermochemical cycles with Zn/ZnO and FeO/Fe3O4 redox reactions II: Kinetic analysis. Energy Fuels 2009;23:2832–9. [22] Scheffe JR, Francés A, King DM, Liang X, Branch BA, Cavanagh AS, et al. Atomic layer deposition of iron (III) oxide on zirconia nanoparticles in a fluidized bed reactor using ferrocene and oxygen. Thin Solid Films 2009;517:1874–9. [23] Bhosale RR, Kumar A, AlMomani F, Alxneit I. Sol–gel derived CeO2–Fe2O3 nanoparticles: synthesis, characterization and solar thermochemical application. Ceram Int 2016;42:6728–37. [24] Scheffe JR, McDaniel AH, Allendorf MD, Weimer AW. Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite– zirconia composite. Energy Environ Sci 2013;6:963–73. [25] Scheffe JR, Li J, Weimer AW. A spinel ferrite/hercynite water-splitting redox cycle. Int J Hydrogen Energy 2010;35:3333–40. [26] Roeb M, Gathmann N, Neises M, Sattler C, Pitz-Paal R. Thermodynamic analysis of two-step solar water splitting with mixed iron oxides. Int J Energy Res 2009;33:893–902. [27] Agrafiotis CC, Pagkoura C, Zygogianni A, Karagiannakis G, Kostoglou M, Konstandopoulos AG. Hydrogen production via solar-aided water splitting thermochemical cycles: combustion synthesis and preliminary evaluation of spinel redox-pair materials. Int J Hydrogen Energy 2012;37:8964–80.
R.R. Bhosale et al. / Energy Conversion and Management 135 (2017) 226–235 [28] Agrafiotis C, Zygogianni A, Pagkoura C, Kostoglou M, Konstandopoulos AG. Hydrogen production via solar-aided water splitting thermochemical cycles with nickel ferrite: experiments and modeling. AIChE J 2013;59: 1213–25. [29] Bhosale RR, Kumar A, AlMomani F, Alxneit I. Propylene oxide assisted sol–gel synthesis of zinc ferrite nanoparticles for solar fuel production. Ceram Int 2016;42:2431–8. [30] Bhosale RR, Shende RV, Puszynski JA. H2 generation from thermochemical water-splitting using sol-gel synthesized Zn/Sn/Mn-doped Ni-Ferrite. Int Rev Chem Eng 2010;2:852. [31] Bhosale RR, Alxneit I, van den Broeke Leo LP, Kumar A, Jilani M, Gharbia SS et al. Sol-gel synthesis of nanocrystalline Ni-Ferrite and Co-Ferrite redox materials for thermochemical production of solar fuels, vol. 1675; 2014. p. mrss14-1675-rr06-10. [32] Bhosale RR, Shende RV, Puszynski JA. Thermochemical water-splitting for H2 generation using sol-gel derived Mn-ferrite in a packed bed reactor. Int J Hydrogen Energy 2012;37:2924–34. [33] R.R. Bhosale, R.V. Shende, J.A. Puszynski, Sol-Gel Derived NiFe2O4 Modified with ZrO2 for Hydrogen Generation from Solar Thermochemical WaterSplitting Reaction 1387 (2012) mrsf11-1387-e09-07. [34] Amar V, Puszynski J, Shende R. H2 generation from thermochemical watersplitting using yttria stabilized NiFe2O4 core-shell nanoparticles. J Renew Sustain Energy 2015;7:023113. [35] Muhich CL, Evanko BW, Weston KC, Lichty P, Liang X, Martinek J, et al. Efficient generation of H2 by splitting water with an isothermal redox cycle. Science 2013;341:540–2. [36] Abanades S, Charvin P, Lemont F, Flamant G. Novel two-step SnO2/SnO watersplitting cycle for solar thermochemical production of hydrogen. Int J Hydrogen Energy 2008;33:6021–30. [37] Chambon M, Abanades S, Flamant G. Thermal dissociation of compressed ZnO and SnO2 powders in a moving-front solar thermochemical reactor. AIChE J 2011;57:2264–73. [38] Chambon M, Abanades S, Flamant G. Kinetic investigation of hydrogen generation from hydrolysis of SnO and Zn solar nanopowders. Int J Hydrogen Energy 2009;34:5326–36. [39] Chambon M, Abanades S, Flamant G. Solar thermal reduction of ZnO and SnO2: characterization of the recombination reaction with O2. Chem Eng Sci 2010;65:3671–80. [40] Charvin P, Abanades S, Lemont F, Flamant G. Experimental study of SnO2/SnO/ Sn thermochemical systems for solar production of hydrogen. AIChE J 2008;54:2759–67. [41] Bhosale R, Kumar A, AlMomani F, Ghosh U, Saad Anis M, Kakosimos K, et al. Solar hydrogen production via a samarium oxide-based thermochemical water splitting cycle. Energies 2016;9:316. [42] Bhosale R, Kumar A, AlMomani F. Solar thermochemical hydrogen production via terbium oxide based redox reactions. Int J Photoenergy 2016. 2016. [43] Bhosale RR, Sutar P, Kumar A, AlMomani F, Ali MH, Ghosh U, et al. Solar hydrogen production via erbium oxide based thermochemical water splitting cycle. J Renew Sustain Energy 2016;8:034702. [44] Abanades S, Le Gal A. CO2 splitting by thermo-chemical looping based on ZrxCe1xO2 oxygen carriers for synthetic fuel generation. Fuel 2012;102:180–6.
235
[45] Bhosale RR, Kumar A, AlMomani F, Ghosh U, Al-Muhtaseb S, Gupta R, et al. Assessment of CexZryHfzO2 based oxides as potential solar thermochemical CO2 splitting materials. Ceram Int 2016;42:9354–62. [46] Hao Y, Yang C, Haile SM. Ceria-zirconia solid solutions (Ce1–xZrxO2 d, x 6 0.2) for solar thermochemical water splitting: a thermodynamic study. Chem Mater 2014;26:6073–82. [47] Haussener S, Steinfeld A. Effective heat and mass transport properties of anisotropic porous ceria for solar thermochemical fuel generation. Materials 2012;5:192–209. [48] Kaneko H, Ishihara H, Taku S, Naganuma Y, Hasegawa N, Tamaura Y. Cerium ion redox system in CeO2–xFe2O3 solid solution at high temperatures (1,273– 1,673 K) in the two-step water-splitting reaction for solar H2 generation. J Mater Sci 2008;43:3153–61. [49] Kaneko H, Miura T, Ishihara H, Taku S, Yokoyama T, Nakajima H, et al. Reactive ceramics of CeO2–MOx (M = Mn, Fe, Ni, Cu) for H2 generation by two-step water splitting using concentrated solar thermal energy. Energy 2007;32:656–63. [50] Krenzke PT, Davidson JH. On the efficiency of solar H2 and CO production via the thermochemical cerium oxide redox cycle: the option of inert-swept reduction. Energy Fuels 2015;29:1045–54. [51] Scheffe JR, Steinfeld A. Thermodynamic analysis of cerium-based oxides for solar thermochemical fuel production. Energy Fuels 2012;26:1928–36. [52] Furler P, Scheffe JR, Steinfeld A. Syngas production by simultaneous splitting of H2O and CO2 via ceria redox reactions in a high-temperature solar reactor. Energy Environ Sci 2012;5:6098–103. [53] Demont A, Abanades S. High redox activity of Sr-substituted lanthanum manganite perovskites for two-step thermochemical dissociation of CO2. RSC Adv 2014;4:54885–91. [54] Evdou A, Zaspalis V, Nalbandian L. La1xSrxFeO3d perovskites as redox materials for application in a membrane reactor for simultaneous production of pure hydrogen and synthesis gas. Fuel 2010;89:1265–73. [55] Gálvez M, Jacot R, Scheffe J, Cooper T, Patzke G, Steinfeld A. Physico-chemical changes in Ca, Sr and Al-doped La–Mn–O perovskites upon thermochemical splitting of CO2 via redox cycling. Phys Chem Chem Phys 2015;17:6629–34. [56] Jiang Q, Tong J, Zhou G, Jiang Z, Li Z, Li C. Thermochemical CO2 splitting reaction with supported LaxA1 xFeyB1 yO3 (A = Sr, Ce, B = Co, Mn; 0 6 x, y 6 1) perovskite oxides. Sol Energy 2014;103:425–37. [57] Nalbandian L, Evdou A, Zaspalis V. La1xSrxMO3 (M = Mn, Fe) perovskites as materials for thermochemical hydrogen production in conventional and membrane reactors. Int J Hydrogen Energy 2009;34:7162–72. [58] Scheffe JR, Weibel D, Steinfeld A. Lanthanum–strontium–manganese perovskites as redox materials for solar thermochemical splitting of H2O and CO2. Energy Fuels 2013;27:4250–7. [59] Loutzenhiser PG, Steinfeld A. Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: thermodynamic cycle analysis. Int J Hydrogen Energy 2011;36:12141–7. [60] Abanades S. CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: thermogravimetric analysis. Int J Hydrogen Energy 2012;37:8223–31. [61] Bader R, Venstrom LJ, Davidson JH, Lipin´ski W. Thermodynamic analysis of isothermal redox cycling of ceria for solar fuel production. Energy Fuels 2013;27:5533–44.