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Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system
international journal of hydrogen energy xxx (xxxx) xxx
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Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system Canan Acar a,*, Ibrahim Dincer b an Caddesi No: 4 e 6 34353 Bes‚iktas‚, Faculty of Engineering and Natural Sciences, Bahcesehir University, C¸ırag Istanbul, Turkey b Faculty of Engineering and Applied Science, Ontario Tech University, 2000 Simcoe Street North, Oshawa, Ontario, L1G 0C5, Canada a
highlights A solar-based integrated hybrid multigeneration system is introduced. The system is capable of producing H2, Cl2, heat, and power simultaneously. Performance parameters are production rates, efficiencies, and exergy destruction rate. The effects of operating and environmental temperatures, inlet mass flow rate, and total photoactive area are investigated. Optimum operating conditions for the highest possible performance are determined for different case studies.
article info
abstract
Article history:
Hydrogen is an essential component of power-to-gas technologies that are needed for a
Received 12 November 2019
complete transition to renewable energy systems. Although hydrogen has zero GHG
Received in revised form
emissions at the end-use point, its production could become an issue if non-renewable,
9 January 2020
and pollutant energy and material resources are used in this step. Therefore, a crucial
Accepted 12 January 2020
step for the fully developed hydrogen economy is to find alternative hydrogen production
Available online xxx
methods that are clean, efficient, affordable, and reliable. With this motivation, in this study, an integrated and continuous type of hydrogen production system is designed,
Keywords:
developed, and investigated. This system has three components. There is a solar spectral
Hydrogen
splitting device (Unit I), which splits the incoming solar energy into two parts. Photons with
Solar energy
longer wavelength is sent to the photovoltaic thermal hybrid solar collector, PV/T, (Unit II)
Trigeneration
and used for combined heat and power generation. Then the remaining part is transferred
Exergy
to the novel hybrid photoelectrochemical-chloralkali reactor (Unit III) for simultaneous H2,
Efficiency
Cl2, and NaOH production. This system has only one energy input, which is the solar
Sustainability
irradiation and five outputs, namely H2, Cl2, NaOH, heat, and electricity. Unlike most of the studies in the literature, this system does not use only PV or only a photoelectrochemical reactor. With this approach, solar energy utilization is maximized, and the wasted portion is minimized. By selecting PV/T rather than PV, the performance of the panels is maximized because recovering the by-product heat as a system output in addition to electricity, and the PV/T has less waste and higher efficiency. The present reactor does not use any additional electron donors, so the wastewater discharge is only depleted NaCl solution, which makes the system significantly cleaner than the ones available in the literature. The
* Corresponding author. E-mail address: [email protected] (C. Acar). https://doi.org/10.1016/j.ijhydene.2020.01.075 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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specific aim of this study is to demonstrate the optimum operating parameters to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. In this study, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. Within the selected operating parameter range, all performance criteria (except exergy destruction) are normalized and ranked for proper comparison. The maximum production rates and efficiencies with the least possible exergy destruction are observed at the operating temperature of 30 C. At 30 C, 4.18 g/h H2, 127.55 g/h Cl2, 151 W electricity, and 716 W heat are produced with an exergy destruction rate of 95.74 W and 78% and 30% energy and exergy efficiencies, respectively. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has very well-known advantages in clean energy systems as an alternative fuel and energy storage medium to minimize and eventually eliminate fossil fuel dependence. Hydrogen is a carbon-free fuel, which makes it a cleaner fuel compared to the fossil alternatives. Hydrogen is also known as a highly efficient fuel that can be used in many applications such as cooling, heating, power generation, transportation, and industrial feedstock. Besides, in many cases, it is more practical and cheaper to distribute energy in hydrogen form rather than electricity [e.g., [1]]. There are many advantages that come along with switching to hydrogen energy systems. Hydrogen can play essential roles in the energy transition. Some of the significant roles can be summarized as (i) enabling large-scale renewables integration and power generation; (ii) effective distribution of energy across different sectors and regions; (iii) acting as buffer to increase energy systems’ resilience; (iv) decarbonizing transportation and industrial sectors; (v) decarbonizing residential energy use including heating, cooling, and power; and (vi) serving as a renewable industrial feedstock such as renewable methane, ammonia, and so on. Further details are available elsewhere [2]. Since hydrogen does not exist on Earth as a gas, it must be separated from other compounds. Therefore, hydrogen production is the first step in hydrogen energy systems, and this step is essential for ensuring the sustainability of the entire hydrogen infrastructure. Among the conventional and novel hydrogen production options, solar-based water-splitting such as photoelectrochemical (PEC) hydrogen production is an up-and-coming option, especially in future energy systems, as discussed elsewhere [3]. Hydrogen production in a PEC requires two resources: water as the material resource and sunlight as the energy source. In a PEC, photoactive materials such as semiconductors are used to harvest solar energy to split water into hydrogen and oxygen. PEC technology is still in the R&D phase with no large-scale commercial applications. Nevertheless, in the long term, with improvements in materials sciences and system integration technologies, PECs have the potential to
produce hydrogen in both large and small scales with high efficiencies and minimal or zero negative impact on the environment. PEC systems generally use thin-films and/or semiconductor materials, and they have high solar-tohydrogen efficiencies at lower operating temperatures, which reduces the system costs significantly. PEC systems can be used in both small and large scales, which make them easily applicable to central and distributed production. PECs can also provide reliable energy to locations where the grid connection does not exist or is limited. Due to its simple and modular design and low operating temperatures, PEC has a wide variety of application areas for sustainable hydrogen production, as discussed further elsewhere [4]. PECs have many advantages; however, in terms of efficiency, reliability, and cost, further enhancements are needed to become competitive with traditional energy systems. Even though some difficulties stand in the way of the clean, affordable, and efficient use of PEC systems, it is a promising hydrogen production pathway for a more sustainable future [5]. Therefore, there are significant research and development activities regarding PEC design and operation, including innovative materials (such as nanomaterials), durable membranes, enhanced heat and electron transfer pathways, and so on. Some of these activities can be grouped into the following three categories [e.g., [6]]: Materials science: for better charge transfer rates and increased solar energy absorption to increase system efficiencies; System design: for enhanced durability and lifetime in portable and stationary applications; Innovative technologies: for reducing hydrogen production rates In the open literature, there have been numerous attempts to address the challenges of PEC. For instance, Ahmed and Dincer [7] have reviewed state-of-the-art materials and designs for different PEC configurations. The authors have reviewed single, dual/tandem photoelectrodes, tandem PECPV, and multi-junction models. In addition, the authors have reviewed different semiconductors in photoelectrodes with
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
international journal of hydrogen energy xxx (xxxx) xxx
their engineering and design aspects as well as solar-tohydrogen efficiencies. The authors have comparatively assessed different semiconductors for enhanced system efficiencies. The authors have also discussed effective strategies to minimize or eliminate electron-hole recombination and photocorrosion, enhance system stability and photocurrent density, and optimize photocatalysts’ energy bandgap. Moreover, the authors have presented critical challenges, key opportunities, and some vital research directions for the successful commercialization of PEC and their full integration into the energy market. Iqbal and Siddique [8] have focused on the brief description of the PEC water splitting principle, primary working mechanism, and different materials for photoanodes such as carbon and its composites, transition metals, nanomaterials, and nanostructures to overcome the challenges to an extent. The authors have pointed out that photoanodic materials are good contenders in future water splitting applications and enhance the overall photocurrent generation and hydrogen evolution efficiency. Jian et al. [9] have surveyed the most recent developments in the engineering of multinary semiconductors for improved PEC performance discussed the progress on semiconductor-liquid junctions rather than photovoltaicelectrolysis. The authors have analyzed five standard engineering protocols that have been effectively adapted for improved PEC performance, including nanostructuring, doping, surface modification, heterostructuring, and photonic management. Sahai et al. [10] have reviewed the operation and principle of quantum dot-sensitized PEC cells along with the status of the worldwide research in this domain. The authors have critically examined some critical parameters, including sensitization method, quantum dot nature, quantity, and size, and photoelectrode nature and morphology. Ding et al. [11] have focused on molybdenum disulfide (MoS2) and related compounds as inexpensive alternatives for hydrogen evolution reaction catalysis and PEC water splitting. The authors have discussed key approaches to improving the intrinsic catalytic activity and overall catalytic performance and the developments in combining MoS2 with semiconductors to enhance solar-to-fuel conversion. Rosman et al. [12] have thoroughly examined two-dimensional graphene and layered transition metal dichalcogenides based photocatalysts for enhanced PEC performance. Queyriaux et al. [13] have focused on the construction of electrodes and photoelectrodes, achieving H2 evolution, as components of PEC. The authors have particularly reviewed the various molecular-based materials developed in this context, with emphasis on those specifically exploiting the properties of Earth-abundant elements. Joy et al. [14] have focused on the recent developments in water splitting techniques using PEC based nanomaterials as well as different strategies to improve hydrogen evolution. Saraswat et al. [15] have reviewed the latest developments in various photocatalyst synthesis schemes for PEC, which are capable of achieving suitable water splitting compositions and architectures while highlighting the challenges being faced when designing visible light-active water splitting photocatalysts. Acar and Dincer [16] have reviewed and evaluated the economic, environmental, social, and
3
technical performance and reliability of several hydrogen production options, including PEC. Kuang et al. [17] have examined new methods to enhance photoanode activity in PEC from the surface and interface engineering perspectives. Adnan et al. [18] have presented the progress over the last ten years in titanate perovskite modification for enhanced PEC performance and hydrogen production rates. Rai et al. [19] have given various perspectives and challenges, and a comprehensive assessment of the worldwide research on the use of carbon nanotube semiconductor hybrids in PECs for sustainable hydrogen production. Ding et al. [20] the recent developments of photoanodes based dye-sensitized PECs with a focus on their design, assembly, and performance. Jaafar et al. [21] have presented an overview of conventional pigments found in natural dyes, extraction methods, the general efficiency of natural dyes, the types of natural dyes used in water splitting, and membranes used as protective layers in PECs. Zhao et al. [22] have presented recent progress on hydrogen evolution through water splitting by using 1-D titanium dioxide nanotubes as photoelectrode materials in PECs. Wang et al. [23] have reviewed the recent progress of black TiO2 for photocatalytic H2 evolution and PEC water splitting, along with a detailed introduction to its unique structural features, optical property, charge carrier transfer property, and related theoretical calculations. Eftekhari et al. [24] have investigated the effect of the architecture of nanomaterials on photon absorption and reaction efficiency in PEC. Shen et al. [25] have comprehensively summarized the signs of progress in the design and modification of titanium dioxide nanostructures as photoelectrode materials for PEC applications, mainly in hydrogen production. Gannouni et al. [26] have developed a novel PEC using indium tin oxide as the substrate of the CuIn5S8 photoanode and Pt as the cathode for more sustainable hydrogen production. Ariffin et al. [27] have reviewed the benefits of PEC water splitting is a promising method for the production of hydrogen. The authors have investigated aerosol-assisted chemical vapor deposition as a promising photoelectrode fabrication method for the materialization of thin films in terms of their homogeneity and uniformity. All these studies have pointed out the importance of enhancing system efficiency in PEC. Furthermore, one way of improving the PEC performance is integrating PEC into multigeneration systems for multiple valuable products, increased efficiencies, and more sustainable hydrogen generation. This study aims to find the optimum working conditions of the integrated hybrid hydrogen production system by conducting parametric research on the critical operating parameters. This system has three components. There is a solar spectral splitting device (Unit I), which splits the incoming solar energy into two parts. The photons with longer wavelength is sent to the photovoltaic thermal hybrid solar collector, PV/T, (Unit II) and used for combined heat and power generation. Then, the remaining part is transferred to the novel hybrid photoelectrochemical-chloralkali reactor (Unit III) for simultaneous H2, Cl2, and NaOH production. This system has only one energy input, which is the solar irradiation and five outputs, namely H2, Cl2, NaOH, heat, and electricity. Unlike most of the studies in the literature, this system does
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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not use only PV or only a photoelectrochemical reactor. By combining these two units, the solar energy utilization of the system is maximized. With this approach, solar energy utilization is maximized, and the wasted portion is minimized. By selecting PV/T rather than PV, the performance of the panels is maximized because recovering the by-product heat as a system output in addition to electricity, and the PV/T has less waste and higher efficiency. The hybrid reactor does not use any additional electron donors, so the wastewater discharge is only depleted NaCl solution, which makes the system significantly cleaner than the ones available in the literature. The specific aim of this study is to demonstrate the optimum operating parameters to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature. In this study, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. In the end, all performance criteria (except exergy destruction) are normalized and ranked for a comprehensive comparative assessment to find the optimum operating and environmental parameters within the selected intervals.
System description Fig. 1 shows a simple block diagram and the primary energy and material flows of the integrated hybrid hydrogen production system. In addition to the three core units, there are two auxiliary units, which are electricity storage and heat storage. The solar (and only) energy input of the system (Stream 1) is split in to high and low energy portions based on their wavelength in Unit I. The photons that have wavelengths higher than 500 nm (Stream 3) follow the path toward the PV/ T. The remaining part, i.e., the photons that have wavelengths that are shorter than 500 nm (Stream 2), is sent to the hybrid reactor. By using PV/T, the performance of the solar panels is
enhanced via cooling and recovering this heat as a useful product. Unit II has two outputs: heat and electricity. Some part of this electricity (Stream 6) and Heat (Stream 4) are sent to Unit III if there is the electricity of heat requirement. The remaining electricity (Stream 7) and heat (Stream 5) are sent to the electricity and heat storage units, respectively. Unit III, the reactor, has four outputs; depleted NaCl solution (Stream 10), NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13). Overall, the only input is Stream 1, and there are five outputs, which are Stream 5, Stream 7, Stream 11, Stream 12, and Stream 13. The descriptions of each stream, along with their mass, energetic, entropic, and exergetic flow rates are given in Table 1. The primary and novel element of the present system is the reactor. The reactor uses solar energy directly via the photoelectrodes and indirectly via the electricity coming from the PV/T (Stream 6). This way, the solar energy conversion performance of the system is maximized. In most of the studies in the literature, the entire solar spectrum is sent to the PEC, but it is very well known that the photoelectrodes can only absorb photons that are above a certain energy level. Therefore, in this study, PV/T is used to utilize the lower energy portion of the spectrum rather than leaving it unused. In the present reactor, there are no additional electron donors or sacrificial agents are used because the photoelectrodes perform this duty. Hence, there are no additional chemicals used in this system, which substantially lowers the amount of waste in the output streams. Another advantage of this system is its versatility since it can operate in two modes: Dark operation: During nighttime or when the solar energy input is not strong enough, the system runs on the electrical and thermal energy stored in the associated units. In this mode, the products are NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13). Regular operation: When the solar energy input is strong enough to run the system, the PV/T produces heat (stream
Fig. 1 e Simple block diagram and the primary energy and material flows of the integrated hybrid hydrogen production system. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Table 1 e Stream descriptions along with their mass, energetic, entropic, and exergetic flow rates. Description
Mass flow rate _ (m)
1 Solar energy input
e
2 Higher energy spectrum sent to the reactor
e
3 Lower energy spectrum sent to the PV/T
e
4 Recovered heat from the PV/T (sent to the reactor) 5 Excess heat as the system output 6 PV/T electricity output (sent to the reactor)
e
7 Excess electricity as the system output
e
8 The reactor's anode side input (saturated NaCl solution)
m_ 8
9 The reactor's cathode side input (water)
m_ 9
10 The reactor's wastewater discharge (depleted NaCl solution)
m_ 10
11 The reactor's cathode side product (NaOH solution)
m_ 11
12 The reactor's anode side product (Cl2 gas)
m_ 12
13 The reactor's main product (H2 gas)
m_ 13
7) and electricity (Stream 5). In the reactor, photoelectrochemical water splitting, PV/R-based electrolysis, and chloralkali reactions take place simultaneously. Its products are NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13).
Energetic flow _ rate (E)
Entropic flow _ rate (S)
Exergetic flow _ rate (Ex)
E_1 E_2
S_1 S_2
_ 1 Ex _ 2 Ex
E_3 E_4
S_3 S_4
_ 3 Ex _ 4 Ex
E_5 E_6
S_5 S_6
_ 5 Ex _ 6 Ex
E_7 E_8 E_9
S_7 S_8
_ 7 Ex _ 8 Ex
S_9 S_10
_ 9 Ex _ 10 Ex
S_11 S_12
_ 11 Ex _ 12 Ex
S_13
_ 13 Ex
E_10 E_11 E_12 E_13
The hybrid reactor, which is shown in Fig. 2, contains a membrane photoelectrode assembly (MPEA), which is a threelayer structure: anode side, a cation exchange membrane (CEM), and the photocathode side. The reactor has direct solar energy, electricity, and heat as the energy input and saturated
Fig. 2 e Schematic illustration of the hybrid reactor. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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NaCl solution and freshwater as the material input. It should be noted that there is no heat input to the reactor when its temperature is equal to the surroundings during operation. The reactor has four outputs, three of which are considered as useful products. Among these outputs, two of them are solutions, and two in the gas state. The membrane's primary function is to separate these products from each other. In addition, since the membrane is selective (cation exchange), it only allows Naþ ions to transfer from the anode side to the cathode side. In the cathode side, these Naþ ions are then used to neutralize OH ions. As a result, avoiding the accumulation of OH ions keeps the reactor operates at optimum performance because avoiding the accumulation of OH ions keeps the water-splitting reaction going. Moreover, as a result, another industrially valuable commodity, NaOH, is produced. In order to allow the incoming irradiation (i.e., photonic energy) to reach the photocathode side, the reactor has transparent walls. The anode and cathode reactions taking place at the reactor are as follows: Anode: 2Cl / Cl2 þ 2e
E
ox
¼ 1:3578 V vs: NHE
Cathode: 2H2 O þ 2e / H2 þ 2OH E
red
(1)
¼ 0:8280 V vs: NHE (2)
Here, the negative voltages mean that the reverse reactions are spontaneous. This means that in the anode, the spontaneous reaction is the reduction of Cl2 to chlorine ion, and in the cathode, the spontaneous reaction is the oxidization of OH ions into the water. As a result, an external energy source is needed to generate the necessary voltage difference for the reactions. In this study, this energy support has two sources; one is the electricity coming from the PV/T, and the other one is the photocatalysis taking place on the photocathode side. In Section System analysis, the general equations that are used to evaluate the system performance are provided in detail. By performing a comprehensive thermodynamic analysis on the integrated hybrid system, production rates, efficiencies, and exergy destruction rates of the system are calculated at selected operating and environmental settings (parameters) are calculated. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and environmental temperature. In the end, multiobjective optimization is conducted to find the optimum parameter set for maximum possible rates of valuable product generation with maximum possible efficiencies and minimum possible exergy destruction rates.
Here, 20 C is the environmental temperature (T0), and 1 bar is the environmental pressure (P0). All streams, units, and auxiliary elements are at operating conditions from the beginning to the end of the process. The steady-state and steady-flow process takes place in all units throughout the operation. There are no incomplete processes and reactions. The potential and kinetic energy changes are disregarded. All units and auxiliary components are rigid (no change in their volumes). All gases are ideal. There are negligible heat losses to the surroundings. The auxiliary elements transfer thermal and electrical energy with no loss. In this study, a theoretical site with sufficient solar irradiation is selected. The solar irradiation in that site is presumed to be 1 kW/m2 with the AM1.5G solar spectrum. The number of annual operational sunlight hours is taken as 2000 h [28].
Unit I: solar spectral splitter Unit I is the first component in the integrated system, which plays a vital role because this is where the only energy input of the system is appropriately split and sent to the appropriate units to generate multiple valuable products at once. The only input to the entire system is the incoming solar energy (E_1 ). The photons with wavelengths higher than 500 nm (E_3 ) is directed to the PV/T and the remaining (E_2 ) is to the hybrid reactor. Since there is no material input or output flow in Unit I, there is no mass balance equation here. The energetic, exergetic, and entropy contents of each stream associated with this unit are calculated based on Zamfirescu and Dincer [28]'s procedure. Unit I's energy balance equation is: E_1 ¼ E_2 þ E_3
(3)
Furthermore, the energy contents of the streams associated with Unit I are calculated based on the following equations: E_1 ¼ As
Z∞
I_l dl
(4)
0
E_2 ¼ Ar
Z500
I_l dl
(5)
0
System analysis The following are the list of assumptions taken into account while evaluating the present system's performance. The system performance is examined based on the first and second laws of thermodynamics. The environmental temperature and pressure are the temperature and pressure of the surroundings, and there is no temperature or pressure gradient in the surroundings.
E_3 ¼ Ap
Z∞
I_l dl
(6)
500
Here, As, Ar, and Ap indicate the photoactive area sizes of Unit I, Unit III, and Unit II, respectively I_l is the energy content of the direct normal radiation of photons that have wavelength of l. The wavelength is in nm. After forming the energy balance equations with the first law of thermodynamics, the second law is used to form the entropy balance equation of Unit I as follows:
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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S_1 þ S_gen ¼ S_2 þ S_3
(7)
In this study, S_gen is the entropy generation rate. The entropy contents of the streams associated with Unit I are calculated based on the following equations: E_i S_i ¼ Ti
(8)
Here, i can be Stream 1, 2, or 3. Ei is the energy content of the corresponding stream, and Ti indicates each stream's temperature. Following the entropy balance equation, the exergy balance equation is formed as _ 2 þ Ex _ 3 þ Ex _ dest _ 1 ¼ Ex Ex
(9)
_ dest indicates the exergy destruction rate. From now on, Ex And the exergy contents of the streams associated with Unit I are calculated as _ i ¼ E_i 1 T0 Ex Ti
(10)
Similar to Equation (8), here, i can be Stream 1, 2, or 3, and Ti is indicating the associated stream's temperature. And after this point, T0 is the environmental temperature, which means the temperature of the surrounding environment.
S_3 þ S_gen ¼ S_4 þ S_5 þ S_6 þ S_7
(12)
_ 3 ¼ Ex _ 4 þ Ex _ 5 þ Ex _ 6 þ Ex _ 7 þ Ex _ dest Ex
(13)
Here, electrical and thermal energies are represented by _ Unit II and Unit III are specified by p and r. Up to this _ and Q. W point, the electrical, thermal, and photonic energy input to the reactor are quantified. By using the balance equations, these streams entropic and exergetic values are defined as well. In the next part, the reactor's balance equations are listed.
Unit III: hybrid reactor Unit III is the most critical constituent of the entire system. Here, H2, Cl2, and NaOH are produced simultaneously. The detailed explanations of each stream are provided in Equations (1)e(14) and also in Tables 1 and 2 The corresponding thermodynamic balance equations of this unit are calculated by using the following equations: Mass Balance Equation: m_ 8 þ m_ 9 ¼ m_ 10 þ m_ 11 þ m_ 12 þ m_ 13
(14)
Energy Balance Equation: _ r ¼ m_ 10 h10 þ m_ 11 h11 þ m_ 12 h12 þ m_ 13 h13 m_ 8 h8 þ m_ 9 h9 þ Q_ r þ W (15)
Unit II: PV/T Combined heat (E_4 and E_5 ) and power (E_6 and E_7 ) generation takes place in Unit II. The heat by-product of the PV/T is recovered and can be used in different processes (E_4 and E_5 ). Stream 6 is directed to the reactor to back up the photoelectrochemical water splitting with PV-based electrolysis. Stream 4 is sent to the reactor if the operating temperature is higher than the environmental temperature. Streams 5 and 7 are the recovered heat and leading product electricity, respectively. These streams are directed to the related storage units and considered as useful products. If the reactor is operating at environmental temperature, Stream 6 is zero, and the recovered heat is sent directly to the storage unit. There is no material input or output flow in Unit II. Hence there is no mass balance equation here. The energetic, exergetic, and entropy contents of each stream associated with this unit are given in Table 2. E_3 ¼ E_4 þ E_5 þ E_6 þ E_7
(11)
Entropy Balance Equation: m_ 8 s8 þ m_ 9 s9 þ
Q_ r þ S_gen ¼ m_ 10 s10 þ m_ 11 s11 þ m_ 12 s12 þ m_ 13 s13 T
(16)
Exergy Balance Equation: T0 _ r ¼ m_ 10 ex10 þ m_ 11 ex11 þW m_ 8 ex8 þ m_ 9 ex9 þ Q_ r 1 T _ dest þ m_ 12 ex12 þ m_ 13 ex13 þ Ex
(17)
Here, it should be noted that Stream 8 is the saturated NaCl solution input to the anode side, and Stream 9 is the freshwater input to the cathode side. These are the only material input streams. The energy input streams are shown in the earlier equations. The output streams are as follows: Stream 10 is the depleted NaCl solution (anode product), Stream 11 is NaOH solution (cathode product), Stream 12 is Cl2 (anode product), and Stream 13 is H2 (cathode product). m_ is the mass flow rate of the associated streams.
Table 2 e The energetic, entropic, and exergetic flow rates of the streams associated with the PV/T unit. Description
_ Energetic flow rate (E)
_ Entropic flow rate (S)
_ Exergetic flow rate (Ex)
4
Heat input of the reactor
E_4 ¼ Q_ r
5
Heat product
E_6 ¼ Q_ p
6
Electricity to the reactor
_ 4 ¼ Q_ r 1 T0 Ex T _ 6 ¼ Q_ p 1 T0 Ex T _r _ 6 ¼W Ex
7
Electricity product
_r E_6 ¼ W _p E_7 ¼ W
Q_ S_4 ¼ r T Q_ p _ S6 ¼ T e e
_p _ 7 ¼W Ex
Stream
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Table 3 e Energy and exergy efficiency equations of the integrated system based on the number of valuable products. Valuable products
Energy efficiency
1
H2
2
H2, Cl2
3
H2, Cl2, Electricity
4
H2, Cl2, Electricity, Heat
Exergy efficiency
E_13 h1 ¼ E_1
j1 ¼
h2 ¼
j2 ¼
h3 ¼ h4 ¼
E_12 þ E_13 E_1
E_7 þ E_12 þ E_13 E_1
j3 ¼
E_5 þ E_7 þ E_12 þ E_13 E_1
j4 ¼
10
_ 13 Ex _ 1 Ex
_ 12 þ Ex _ 13 Ex _ 1 Ex
_ 12 þ Ex _ 13 _ 7 þ Ex Ex _ 1 Ex
_ 7 þ Ex _ 12 þ Ex _ 13 _ 5 þ Ex Ex _ 1 Ex
Hydrogen
Normalized Ranking
9 Chlorine
8 7
Electricity
6 Heat
5 4
Exergy destruc on
3 2
η-4
1 ψ-4
0 20
30
40
50
60
70
80 Average
Opera ng Temperature (°C)
Fig. 3 e The effect of the operating temperature on the selected performance criteria of the integrated system.
Efficiencies
Results and discussion
The present systems’ energetic and exergetic performance is assessed based on its number of valuable products. Table 3 shows the efficiency equations of the integrated system, depending on the number of valuable products.
The hybrid multigeneration system's thermodynamic performance is evaluated by changing three operating parameters and one environmental parameter. These parameters are varied between the lower and upper limits as follows:
10
Hydrogen
Normalized Ranking
9 8
Chlorine
7 Electricity
6 5
Exergy destruc on
4 3
η-4
2 1
ψ-4
0 0
2
4
6
8
9
11
Inlet Mass Flow Rate (g/s)
13
15 Average
Fig. 4 e The effect of the inlet mass flow rate on the selected performance criteria of the integrated system. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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10
Hydrogen
Normalized Ranking
9 Chlorine
8 7
Electricity
6 Heat
5 4
Exergy destruc on
3 η-4
2 1
ψ-4
0 2.7
3.1
3.5
3.9
4.3
4.7
5.2
5.6
6.0
Average
Photoac ve Area (m2) Fig. 5 e The photoactive area size impact on the selected performance criteria of the integrated system.
10
Normalized Ranking
Heat
Exergy destruc on
5 η-4
ψ-4
0 0
10
20
30
40
Average
Environmental Temperature (°C) Fig. 6 e The effect of the environmental temperature on the chosen performance criteria of the integrated system. The operating temperature is increased incrementally from 20 C to 80 C The inlet mass flow rate is increased incrementally from 0.1 g/s to 15 g/s The total photoactive area is increased incrementally from 2.7 to 6 m2 The environmental temperature is increased incrementally from 0 C to 40 C
Four different case studies are comparatively assessed based on their production rates, efficiencies, and exergy destruction rates. These case studies are (i) optimum
Table 4 e Performance comparison of selected case studies. Case 1 Case 2 Case 3 Case 4
The operating temperature is the temperature that the hybrid reactor operates. The inlet mass flow rate is Stream 8 and Stream 9's mass flow rates (Table 1). It should be noted that the anode and cathode inputs have the same inlet mass flow rate. The total photoactive area is the area covered by the photoactive surface of Unit II and the membrane photoelectrode assembly of Unit III. Unit II and Unit III are assumed to have equal photoactive areas. Hence they each have an area that is equal to half of the total photoactive area. And the environmental temperature is the temperature of the surroundings.
Temperature ( C) Flow rate (g/s) Area (m2) Environmental temperature ( C) Hydrogen (g/h) Chlorine (g/h) Electricity (W) Heat (W) Rate of exergy destruction (W) Energy efficiency (%) Exergy efficiency (%)
30 1 4 20 4.18 127.45 151 716 95.74 78.03 29.72
20 0.1 4 20 0.49 12.4 377 800 1.25 89.55 31.56
20 1 6 20 8.86 243.98 361.1 1200 21.29 86.46 29.42
20 1 4 20 4.06 123.98 159 799.6 28.03 84.51 28.01
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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operating temperature, (ii) optimum inlet mass flow rate, (iii) optimum photoactive area, and (iv) optimum environmental temperature. Each optimum case is designed to deliver the highest possible generation rates and efficiencies, and lowest possible exergy destruction rate. At the end, these case studies are compared to select the most suitable parameters for the integrated system. The aim of the case studies is to demonstrate the optimum operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature to reach the maximum achievable production rates and efficiencies while keeping the rate of exergy destruction as little as possible. In this section, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. All performance criteria are normalized and ranked by using one of the following equations:
temperatures and enhances their performance and second, recovering heat from the PV and using it as a valuable product reduces system waste. Further studies are to be conducted on the overall environmental impact and cost to investigate the role of the heat recovery further.
Case study 2: selection of optimum inlet mass flow rate In Case Study 2, a ranking is done to find the inlet mass flow rate giving the highest possible production rates, efficiencies, and lowest possible exergy destruction. For that reason, the following criteria are normalized and ranked based on a 0e10 scale: the production rates, overall efficiencies, and rate of exergy destruction. Then the average ranking at each inlet mass flow rate is calculated. The inlet mass flow rate giving the highest possible average ranking is the optimum point.
The criteria which are desired to be increased (such as efficiencies and production rates):
i minimum 10 maximum minimum
(18)
The criteria which are desired to be reduced (exergy destruction rate):
rank ¼
maximum i 10 maximum minimum
9 8 7 6 5 4 3 2 1 0 Case 1
Case 2
Case 3
Case 4
(19)
In these equations, i is a specific performance parameter (i.e., hydrogen production rate) at a specific operating condition (which are operating temperature, inlet mass flow rate, photoactive area size, or environmental temperature) and maximum and minimum are used for the maximum and minimum values of a specific performance parameter (which are efficiencies, production rates, and exergy destruction rate) within the chosen operating condition range.
b
Electricity
Case study 1: selection of optimum operating temperature
1000 800 600 400 200 0 Case 1
Case 2
c
Case 3 Energy
Case 4 Exergy
80
Efficiency (%)
In Case Study 1, it is aimed to observe the optimum operating temperature to reach the highest possible production rates, efficiencies, and the lowest reasonable rate of exergy destruction. For that reason, the following criteria are normalized and ranked based on a 0e10 scale: H2, Cl2, electricity, heat production rates, overall efficiencies, and rate of exergy destruction. Then the average ranking at each operating temperature is calculated. The operating temperature giving the highest possible average ranking is selected as the optimum operating temperature. Fig. 3 shows that the optimum operating temperature is 30 C where the most top average ranking of 5.75/10 is reached. Although Fig. 3 shows that the average ranking tends to decrease with operating temperature, it should be noted that the recovered heat from the thermal collectors is still necessary for two reasons: first, recovering heat from the PV keeps the panels at lower
Heat
1200
Genera on rate (W)
rank ¼
Hydrogen produc on rate (g/h)
a
60 40 20 0 Case 1
Case 2
Case 3
Case 4
Fig. 7 e Comparison of vase studies based on their (a) hydrogen production rates, (b) heat and electricity generation rates, and (c) energy and exergy efficiencies.
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Hydrogen 10 8 Exergy efficiency
Chlorine
6
Case 1 Case 2
4 2
Case 3
0 Energy efficiency
Electricity
Case 4 Ideal
Exergy destruc on
Heat
Fig. 8 e Overall performance comparison of the case studies based on the selected performance criteria.
Fig. 4 illustrates that the optimum point is 0.1 g/s, where the highest average ranking of 6.67/10 is reached.
Case study 3: selection of optimum photoactive area In Case Study 3, both ranking and normalization are performed to determine the photoactive area giving the highest possible H2, Cl2, electricity, and heat production rates, overall efficiencies, and lowest reasonable rate of exergy destruction. For that reason, these criteria are normalized and ranked based on a 0e10 scale. Then the average ranking at each point is calculated. The total photoactive area giving the highest possible average ranking is selected as the optimum total photoactive area. Fig. 5 presents that optimum photoactive area is 6 m2 g/s, where the most top average ranking of 10/10 is reached.
Case study 4: selection of optimum environmental temperature In Case Study 4, the following criteria are normalized and ranked based on a 0e10 scale: heat production rate, overall efficiencies, and rate of exergy destruction. Then the average ranking at each environmental temperature is calculated. Fig. 6 shows that the optimum environmental temperature is 20 C, where the highest average ranking of 8.45/10 is reached.
Overall comparison results of the case studies The overall comparison of the selected case studies is made by comparatively assessing the performance of these case studies which is summarized in Table 4. The hydrogen, heat, and electricity generation rates of these case studies, along with their energy and exergy efficiencies are provided in Fig. 7. Case Study 1 has the lowest heat and power generation rates and energy efficiency and the highest exergy destruction rate among the selected case studies. Case Study 2 has the smallest H2 and Cl2 production rates. On the other hand, this
case has the highest electricity generation and efficiencies and lowest rate of exergy destruction among the given case studies. Case Study 3 has the most top H2, Cl2, and heat production rates. Case Study 4 has the lowest exergy efficiency among the chosen case studies. The performance criteria of these case studies are normalized and ranked based on Equations (18) and (19), and the results are presented in Fig. 8. From Fig. 8, it can be seen that Case Study 3 is the most desirable case since it has the closest to ideal performance among selected options. The disadvantage of Case Study 3 is the low exergy efficiency compared to other situations. Case Study 3 has an average normalized ranking of 8.35/10 followed by Case Study 2, which is 5.96/10. The reason why Case Study 2 has low ranking is that it gives the smallest H2 and Cl2 production rates. The third highest average ranking is Case Study 4, 3.42/10. Case Study 4 has the lowest exergy efficiency and relatively small H2, Cl2, electricity, and heat production. Overall, the lowest average ranking belongs to Case Study 1, which is 2.03/10. The reason for this poor performance is because it has the lowest heat and power generation and energy efficiency and the highest exergy destruction rate among the selected case studies.
Conclusions The main objective of the present study is to demonstrate the optimum operating parameters of the innovative multigeneration system to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature. The optimum operating temperature, inlet mass flow rate, photoactive area size, and environmental temperature are found in different case studies for highest possible production rates and efficiencies and lowest possible exergy destruction rates. Furthermore, the results show that:
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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The optimum operating temperature is 30 C within the investigated range of 20 Ce80 C. At 30 C, o 4.18 g/h H2 is generated. o 127.55 g/h Cl2 is generated. o 151 W electricity is generated. o 716 W heat is generated. o The exergy destruction rate is 95.74 W. o Overall efficiencies are 78% and 30% for energy and exergy, respectively. A 0.1 g/s is the optimum inlet mass flow rate within the selected range of 0.1e15 g/s. At 0.1 g/s; o 0.41 g/h H2 is generated. o 12.40 g/h Cl2 is generated. o 377 W electricity is generated. o 800 W heat is generated. o The exergy destruction rate is 1.25 W. o Overall energy and exergy efficiencies are 90% and 32%, respectively. Within the selected, overall photoactive area interval of 2.7e6 m2, the optimum size is 6 m2. When this optimum size is used in the present integrated system, o 8.86 g/h H2 is generated. o 243.98 g/h Cl2 is generated. o 361 W electricity is generated. o 1200 W heat is generated. o The exergy destruction rate is 21.29 W. o Overall energy and exergy efficiencies are 86% and 29%, respectively. The optimum environmental temperature is 20 C within the investigated range of 0 Ce40 C. At 20 C, o 4.06 g/h H2 is generated. o 123.98 g/h Cl2 is generated. o 159 W electricity is generated. o 800 W heat is generated. o The exergy destruction rate is 23 W. o Overall energy and exergy efficiencies are 85% and 28%, respectively. This study shows that hydrogen has excellent potential as a way to reduce reliance on imported energy sources such as oil. Nevertheless, before hydrogen can play a more prominent energy role and become a widely used alternative to gasoline, many new facilities and systems must be built to ensure safe, reliable, efficient, and clean hydrogen production and distribution.
Acknowledgements The authors would like to thank the kind support of the Young _ of the Turkish Academy Scientists Award Programme (GEBIP) ¨ BA). of Sciences (TU
references
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Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system Canan Acar a,*, Ibrahim Dincer b an Caddesi No: 4 e 6 34353 Bes‚iktas‚, Faculty of Engineering and Natural Sciences, Bahcesehir University, C¸ırag Istanbul, Turkey b Faculty of Engineering and Applied Science, Ontario Tech University, 2000 Simcoe Street North, Oshawa, Ontario, L1G 0C5, Canada a
highlights A solar-based integrated hybrid multigeneration system is introduced. The system is capable of producing H2, Cl2, heat, and power simultaneously. Performance parameters are production rates, efficiencies, and exergy destruction rate. The effects of operating and environmental temperatures, inlet mass flow rate, and total photoactive area are investigated. Optimum operating conditions for the highest possible performance are determined for different case studies.
article info
abstract
Article history:
Hydrogen is an essential component of power-to-gas technologies that are needed for a
Received 12 November 2019
complete transition to renewable energy systems. Although hydrogen has zero GHG
Received in revised form
emissions at the end-use point, its production could become an issue if non-renewable,
9 January 2020
and pollutant energy and material resources are used in this step. Therefore, a crucial
Accepted 12 January 2020
step for the fully developed hydrogen economy is to find alternative hydrogen production
Available online xxx
methods that are clean, efficient, affordable, and reliable. With this motivation, in this study, an integrated and continuous type of hydrogen production system is designed,
Keywords:
developed, and investigated. This system has three components. There is a solar spectral
Hydrogen
splitting device (Unit I), which splits the incoming solar energy into two parts. Photons with
Solar energy
longer wavelength is sent to the photovoltaic thermal hybrid solar collector, PV/T, (Unit II)
Trigeneration
and used for combined heat and power generation. Then the remaining part is transferred
Exergy
to the novel hybrid photoelectrochemical-chloralkali reactor (Unit III) for simultaneous H2,
Efficiency
Cl2, and NaOH production. This system has only one energy input, which is the solar
Sustainability
irradiation and five outputs, namely H2, Cl2, NaOH, heat, and electricity. Unlike most of the studies in the literature, this system does not use only PV or only a photoelectrochemical reactor. With this approach, solar energy utilization is maximized, and the wasted portion is minimized. By selecting PV/T rather than PV, the performance of the panels is maximized because recovering the by-product heat as a system output in addition to electricity, and the PV/T has less waste and higher efficiency. The present reactor does not use any additional electron donors, so the wastewater discharge is only depleted NaCl solution, which makes the system significantly cleaner than the ones available in the literature. The
* Corresponding author. E-mail address: [email protected] (C. Acar). https://doi.org/10.1016/j.ijhydene.2020.01.075 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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specific aim of this study is to demonstrate the optimum operating parameters to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. In this study, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. Within the selected operating parameter range, all performance criteria (except exergy destruction) are normalized and ranked for proper comparison. The maximum production rates and efficiencies with the least possible exergy destruction are observed at the operating temperature of 30 C. At 30 C, 4.18 g/h H2, 127.55 g/h Cl2, 151 W electricity, and 716 W heat are produced with an exergy destruction rate of 95.74 W and 78% and 30% energy and exergy efficiencies, respectively. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has very well-known advantages in clean energy systems as an alternative fuel and energy storage medium to minimize and eventually eliminate fossil fuel dependence. Hydrogen is a carbon-free fuel, which makes it a cleaner fuel compared to the fossil alternatives. Hydrogen is also known as a highly efficient fuel that can be used in many applications such as cooling, heating, power generation, transportation, and industrial feedstock. Besides, in many cases, it is more practical and cheaper to distribute energy in hydrogen form rather than electricity [e.g., [1]]. There are many advantages that come along with switching to hydrogen energy systems. Hydrogen can play essential roles in the energy transition. Some of the significant roles can be summarized as (i) enabling large-scale renewables integration and power generation; (ii) effective distribution of energy across different sectors and regions; (iii) acting as buffer to increase energy systems’ resilience; (iv) decarbonizing transportation and industrial sectors; (v) decarbonizing residential energy use including heating, cooling, and power; and (vi) serving as a renewable industrial feedstock such as renewable methane, ammonia, and so on. Further details are available elsewhere [2]. Since hydrogen does not exist on Earth as a gas, it must be separated from other compounds. Therefore, hydrogen production is the first step in hydrogen energy systems, and this step is essential for ensuring the sustainability of the entire hydrogen infrastructure. Among the conventional and novel hydrogen production options, solar-based water-splitting such as photoelectrochemical (PEC) hydrogen production is an up-and-coming option, especially in future energy systems, as discussed elsewhere [3]. Hydrogen production in a PEC requires two resources: water as the material resource and sunlight as the energy source. In a PEC, photoactive materials such as semiconductors are used to harvest solar energy to split water into hydrogen and oxygen. PEC technology is still in the R&D phase with no large-scale commercial applications. Nevertheless, in the long term, with improvements in materials sciences and system integration technologies, PECs have the potential to
produce hydrogen in both large and small scales with high efficiencies and minimal or zero negative impact on the environment. PEC systems generally use thin-films and/or semiconductor materials, and they have high solar-tohydrogen efficiencies at lower operating temperatures, which reduces the system costs significantly. PEC systems can be used in both small and large scales, which make them easily applicable to central and distributed production. PECs can also provide reliable energy to locations where the grid connection does not exist or is limited. Due to its simple and modular design and low operating temperatures, PEC has a wide variety of application areas for sustainable hydrogen production, as discussed further elsewhere [4]. PECs have many advantages; however, in terms of efficiency, reliability, and cost, further enhancements are needed to become competitive with traditional energy systems. Even though some difficulties stand in the way of the clean, affordable, and efficient use of PEC systems, it is a promising hydrogen production pathway for a more sustainable future [5]. Therefore, there are significant research and development activities regarding PEC design and operation, including innovative materials (such as nanomaterials), durable membranes, enhanced heat and electron transfer pathways, and so on. Some of these activities can be grouped into the following three categories [e.g., [6]]: Materials science: for better charge transfer rates and increased solar energy absorption to increase system efficiencies; System design: for enhanced durability and lifetime in portable and stationary applications; Innovative technologies: for reducing hydrogen production rates In the open literature, there have been numerous attempts to address the challenges of PEC. For instance, Ahmed and Dincer [7] have reviewed state-of-the-art materials and designs for different PEC configurations. The authors have reviewed single, dual/tandem photoelectrodes, tandem PECPV, and multi-junction models. In addition, the authors have reviewed different semiconductors in photoelectrodes with
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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their engineering and design aspects as well as solar-tohydrogen efficiencies. The authors have comparatively assessed different semiconductors for enhanced system efficiencies. The authors have also discussed effective strategies to minimize or eliminate electron-hole recombination and photocorrosion, enhance system stability and photocurrent density, and optimize photocatalysts’ energy bandgap. Moreover, the authors have presented critical challenges, key opportunities, and some vital research directions for the successful commercialization of PEC and their full integration into the energy market. Iqbal and Siddique [8] have focused on the brief description of the PEC water splitting principle, primary working mechanism, and different materials for photoanodes such as carbon and its composites, transition metals, nanomaterials, and nanostructures to overcome the challenges to an extent. The authors have pointed out that photoanodic materials are good contenders in future water splitting applications and enhance the overall photocurrent generation and hydrogen evolution efficiency. Jian et al. [9] have surveyed the most recent developments in the engineering of multinary semiconductors for improved PEC performance discussed the progress on semiconductor-liquid junctions rather than photovoltaicelectrolysis. The authors have analyzed five standard engineering protocols that have been effectively adapted for improved PEC performance, including nanostructuring, doping, surface modification, heterostructuring, and photonic management. Sahai et al. [10] have reviewed the operation and principle of quantum dot-sensitized PEC cells along with the status of the worldwide research in this domain. The authors have critically examined some critical parameters, including sensitization method, quantum dot nature, quantity, and size, and photoelectrode nature and morphology. Ding et al. [11] have focused on molybdenum disulfide (MoS2) and related compounds as inexpensive alternatives for hydrogen evolution reaction catalysis and PEC water splitting. The authors have discussed key approaches to improving the intrinsic catalytic activity and overall catalytic performance and the developments in combining MoS2 with semiconductors to enhance solar-to-fuel conversion. Rosman et al. [12] have thoroughly examined two-dimensional graphene and layered transition metal dichalcogenides based photocatalysts for enhanced PEC performance. Queyriaux et al. [13] have focused on the construction of electrodes and photoelectrodes, achieving H2 evolution, as components of PEC. The authors have particularly reviewed the various molecular-based materials developed in this context, with emphasis on those specifically exploiting the properties of Earth-abundant elements. Joy et al. [14] have focused on the recent developments in water splitting techniques using PEC based nanomaterials as well as different strategies to improve hydrogen evolution. Saraswat et al. [15] have reviewed the latest developments in various photocatalyst synthesis schemes for PEC, which are capable of achieving suitable water splitting compositions and architectures while highlighting the challenges being faced when designing visible light-active water splitting photocatalysts. Acar and Dincer [16] have reviewed and evaluated the economic, environmental, social, and
3
technical performance and reliability of several hydrogen production options, including PEC. Kuang et al. [17] have examined new methods to enhance photoanode activity in PEC from the surface and interface engineering perspectives. Adnan et al. [18] have presented the progress over the last ten years in titanate perovskite modification for enhanced PEC performance and hydrogen production rates. Rai et al. [19] have given various perspectives and challenges, and a comprehensive assessment of the worldwide research on the use of carbon nanotube semiconductor hybrids in PECs for sustainable hydrogen production. Ding et al. [20] the recent developments of photoanodes based dye-sensitized PECs with a focus on their design, assembly, and performance. Jaafar et al. [21] have presented an overview of conventional pigments found in natural dyes, extraction methods, the general efficiency of natural dyes, the types of natural dyes used in water splitting, and membranes used as protective layers in PECs. Zhao et al. [22] have presented recent progress on hydrogen evolution through water splitting by using 1-D titanium dioxide nanotubes as photoelectrode materials in PECs. Wang et al. [23] have reviewed the recent progress of black TiO2 for photocatalytic H2 evolution and PEC water splitting, along with a detailed introduction to its unique structural features, optical property, charge carrier transfer property, and related theoretical calculations. Eftekhari et al. [24] have investigated the effect of the architecture of nanomaterials on photon absorption and reaction efficiency in PEC. Shen et al. [25] have comprehensively summarized the signs of progress in the design and modification of titanium dioxide nanostructures as photoelectrode materials for PEC applications, mainly in hydrogen production. Gannouni et al. [26] have developed a novel PEC using indium tin oxide as the substrate of the CuIn5S8 photoanode and Pt as the cathode for more sustainable hydrogen production. Ariffin et al. [27] have reviewed the benefits of PEC water splitting is a promising method for the production of hydrogen. The authors have investigated aerosol-assisted chemical vapor deposition as a promising photoelectrode fabrication method for the materialization of thin films in terms of their homogeneity and uniformity. All these studies have pointed out the importance of enhancing system efficiency in PEC. Furthermore, one way of improving the PEC performance is integrating PEC into multigeneration systems for multiple valuable products, increased efficiencies, and more sustainable hydrogen generation. This study aims to find the optimum working conditions of the integrated hybrid hydrogen production system by conducting parametric research on the critical operating parameters. This system has three components. There is a solar spectral splitting device (Unit I), which splits the incoming solar energy into two parts. The photons with longer wavelength is sent to the photovoltaic thermal hybrid solar collector, PV/T, (Unit II) and used for combined heat and power generation. Then, the remaining part is transferred to the novel hybrid photoelectrochemical-chloralkali reactor (Unit III) for simultaneous H2, Cl2, and NaOH production. This system has only one energy input, which is the solar irradiation and five outputs, namely H2, Cl2, NaOH, heat, and electricity. Unlike most of the studies in the literature, this system does
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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not use only PV or only a photoelectrochemical reactor. By combining these two units, the solar energy utilization of the system is maximized. With this approach, solar energy utilization is maximized, and the wasted portion is minimized. By selecting PV/T rather than PV, the performance of the panels is maximized because recovering the by-product heat as a system output in addition to electricity, and the PV/T has less waste and higher efficiency. The hybrid reactor does not use any additional electron donors, so the wastewater discharge is only depleted NaCl solution, which makes the system significantly cleaner than the ones available in the literature. The specific aim of this study is to demonstrate the optimum operating parameters to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature. In this study, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. In the end, all performance criteria (except exergy destruction) are normalized and ranked for a comprehensive comparative assessment to find the optimum operating and environmental parameters within the selected intervals.
System description Fig. 1 shows a simple block diagram and the primary energy and material flows of the integrated hybrid hydrogen production system. In addition to the three core units, there are two auxiliary units, which are electricity storage and heat storage. The solar (and only) energy input of the system (Stream 1) is split in to high and low energy portions based on their wavelength in Unit I. The photons that have wavelengths higher than 500 nm (Stream 3) follow the path toward the PV/ T. The remaining part, i.e., the photons that have wavelengths that are shorter than 500 nm (Stream 2), is sent to the hybrid reactor. By using PV/T, the performance of the solar panels is
enhanced via cooling and recovering this heat as a useful product. Unit II has two outputs: heat and electricity. Some part of this electricity (Stream 6) and Heat (Stream 4) are sent to Unit III if there is the electricity of heat requirement. The remaining electricity (Stream 7) and heat (Stream 5) are sent to the electricity and heat storage units, respectively. Unit III, the reactor, has four outputs; depleted NaCl solution (Stream 10), NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13). Overall, the only input is Stream 1, and there are five outputs, which are Stream 5, Stream 7, Stream 11, Stream 12, and Stream 13. The descriptions of each stream, along with their mass, energetic, entropic, and exergetic flow rates are given in Table 1. The primary and novel element of the present system is the reactor. The reactor uses solar energy directly via the photoelectrodes and indirectly via the electricity coming from the PV/T (Stream 6). This way, the solar energy conversion performance of the system is maximized. In most of the studies in the literature, the entire solar spectrum is sent to the PEC, but it is very well known that the photoelectrodes can only absorb photons that are above a certain energy level. Therefore, in this study, PV/T is used to utilize the lower energy portion of the spectrum rather than leaving it unused. In the present reactor, there are no additional electron donors or sacrificial agents are used because the photoelectrodes perform this duty. Hence, there are no additional chemicals used in this system, which substantially lowers the amount of waste in the output streams. Another advantage of this system is its versatility since it can operate in two modes: Dark operation: During nighttime or when the solar energy input is not strong enough, the system runs on the electrical and thermal energy stored in the associated units. In this mode, the products are NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13). Regular operation: When the solar energy input is strong enough to run the system, the PV/T produces heat (stream
Fig. 1 e Simple block diagram and the primary energy and material flows of the integrated hybrid hydrogen production system. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Table 1 e Stream descriptions along with their mass, energetic, entropic, and exergetic flow rates. Description
Mass flow rate _ (m)
1 Solar energy input
e
2 Higher energy spectrum sent to the reactor
e
3 Lower energy spectrum sent to the PV/T
e
4 Recovered heat from the PV/T (sent to the reactor) 5 Excess heat as the system output 6 PV/T electricity output (sent to the reactor)
e
7 Excess electricity as the system output
e
8 The reactor's anode side input (saturated NaCl solution)
m_ 8
9 The reactor's cathode side input (water)
m_ 9
10 The reactor's wastewater discharge (depleted NaCl solution)
m_ 10
11 The reactor's cathode side product (NaOH solution)
m_ 11
12 The reactor's anode side product (Cl2 gas)
m_ 12
13 The reactor's main product (H2 gas)
m_ 13
7) and electricity (Stream 5). In the reactor, photoelectrochemical water splitting, PV/R-based electrolysis, and chloralkali reactions take place simultaneously. Its products are NaOH solution (Stream 11), Cl2 (stream 12), and H2 (Stream 13).
Energetic flow _ rate (E)
Entropic flow _ rate (S)
Exergetic flow _ rate (Ex)
E_1 E_2
S_1 S_2
_ 1 Ex _ 2 Ex
E_3 E_4
S_3 S_4
_ 3 Ex _ 4 Ex
E_5 E_6
S_5 S_6
_ 5 Ex _ 6 Ex
E_7 E_8 E_9
S_7 S_8
_ 7 Ex _ 8 Ex
S_9 S_10
_ 9 Ex _ 10 Ex
S_11 S_12
_ 11 Ex _ 12 Ex
S_13
_ 13 Ex
E_10 E_11 E_12 E_13
The hybrid reactor, which is shown in Fig. 2, contains a membrane photoelectrode assembly (MPEA), which is a threelayer structure: anode side, a cation exchange membrane (CEM), and the photocathode side. The reactor has direct solar energy, electricity, and heat as the energy input and saturated
Fig. 2 e Schematic illustration of the hybrid reactor. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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NaCl solution and freshwater as the material input. It should be noted that there is no heat input to the reactor when its temperature is equal to the surroundings during operation. The reactor has four outputs, three of which are considered as useful products. Among these outputs, two of them are solutions, and two in the gas state. The membrane's primary function is to separate these products from each other. In addition, since the membrane is selective (cation exchange), it only allows Naþ ions to transfer from the anode side to the cathode side. In the cathode side, these Naþ ions are then used to neutralize OH ions. As a result, avoiding the accumulation of OH ions keeps the reactor operates at optimum performance because avoiding the accumulation of OH ions keeps the water-splitting reaction going. Moreover, as a result, another industrially valuable commodity, NaOH, is produced. In order to allow the incoming irradiation (i.e., photonic energy) to reach the photocathode side, the reactor has transparent walls. The anode and cathode reactions taking place at the reactor are as follows: Anode: 2Cl / Cl2 þ 2e
E
ox
¼ 1:3578 V vs: NHE
Cathode: 2H2 O þ 2e / H2 þ 2OH E
red
(1)
¼ 0:8280 V vs: NHE (2)
Here, the negative voltages mean that the reverse reactions are spontaneous. This means that in the anode, the spontaneous reaction is the reduction of Cl2 to chlorine ion, and in the cathode, the spontaneous reaction is the oxidization of OH ions into the water. As a result, an external energy source is needed to generate the necessary voltage difference for the reactions. In this study, this energy support has two sources; one is the electricity coming from the PV/T, and the other one is the photocatalysis taking place on the photocathode side. In Section System analysis, the general equations that are used to evaluate the system performance are provided in detail. By performing a comprehensive thermodynamic analysis on the integrated hybrid system, production rates, efficiencies, and exergy destruction rates of the system are calculated at selected operating and environmental settings (parameters) are calculated. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and environmental temperature. In the end, multiobjective optimization is conducted to find the optimum parameter set for maximum possible rates of valuable product generation with maximum possible efficiencies and minimum possible exergy destruction rates.
Here, 20 C is the environmental temperature (T0), and 1 bar is the environmental pressure (P0). All streams, units, and auxiliary elements are at operating conditions from the beginning to the end of the process. The steady-state and steady-flow process takes place in all units throughout the operation. There are no incomplete processes and reactions. The potential and kinetic energy changes are disregarded. All units and auxiliary components are rigid (no change in their volumes). All gases are ideal. There are negligible heat losses to the surroundings. The auxiliary elements transfer thermal and electrical energy with no loss. In this study, a theoretical site with sufficient solar irradiation is selected. The solar irradiation in that site is presumed to be 1 kW/m2 with the AM1.5G solar spectrum. The number of annual operational sunlight hours is taken as 2000 h [28].
Unit I: solar spectral splitter Unit I is the first component in the integrated system, which plays a vital role because this is where the only energy input of the system is appropriately split and sent to the appropriate units to generate multiple valuable products at once. The only input to the entire system is the incoming solar energy (E_1 ). The photons with wavelengths higher than 500 nm (E_3 ) is directed to the PV/T and the remaining (E_2 ) is to the hybrid reactor. Since there is no material input or output flow in Unit I, there is no mass balance equation here. The energetic, exergetic, and entropy contents of each stream associated with this unit are calculated based on Zamfirescu and Dincer [28]'s procedure. Unit I's energy balance equation is: E_1 ¼ E_2 þ E_3
(3)
Furthermore, the energy contents of the streams associated with Unit I are calculated based on the following equations: E_1 ¼ As
Z∞
I_l dl
(4)
0
E_2 ¼ Ar
Z500
I_l dl
(5)
0
System analysis The following are the list of assumptions taken into account while evaluating the present system's performance. The system performance is examined based on the first and second laws of thermodynamics. The environmental temperature and pressure are the temperature and pressure of the surroundings, and there is no temperature or pressure gradient in the surroundings.
E_3 ¼ Ap
Z∞
I_l dl
(6)
500
Here, As, Ar, and Ap indicate the photoactive area sizes of Unit I, Unit III, and Unit II, respectively I_l is the energy content of the direct normal radiation of photons that have wavelength of l. The wavelength is in nm. After forming the energy balance equations with the first law of thermodynamics, the second law is used to form the entropy balance equation of Unit I as follows:
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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S_1 þ S_gen ¼ S_2 þ S_3
(7)
In this study, S_gen is the entropy generation rate. The entropy contents of the streams associated with Unit I are calculated based on the following equations: E_i S_i ¼ Ti
(8)
Here, i can be Stream 1, 2, or 3. Ei is the energy content of the corresponding stream, and Ti indicates each stream's temperature. Following the entropy balance equation, the exergy balance equation is formed as _ 2 þ Ex _ 3 þ Ex _ dest _ 1 ¼ Ex Ex
(9)
_ dest indicates the exergy destruction rate. From now on, Ex And the exergy contents of the streams associated with Unit I are calculated as _ i ¼ E_i 1 T0 Ex Ti
(10)
Similar to Equation (8), here, i can be Stream 1, 2, or 3, and Ti is indicating the associated stream's temperature. And after this point, T0 is the environmental temperature, which means the temperature of the surrounding environment.
S_3 þ S_gen ¼ S_4 þ S_5 þ S_6 þ S_7
(12)
_ 3 ¼ Ex _ 4 þ Ex _ 5 þ Ex _ 6 þ Ex _ 7 þ Ex _ dest Ex
(13)
Here, electrical and thermal energies are represented by _ Unit II and Unit III are specified by p and r. Up to this _ and Q. W point, the electrical, thermal, and photonic energy input to the reactor are quantified. By using the balance equations, these streams entropic and exergetic values are defined as well. In the next part, the reactor's balance equations are listed.
Unit III: hybrid reactor Unit III is the most critical constituent of the entire system. Here, H2, Cl2, and NaOH are produced simultaneously. The detailed explanations of each stream are provided in Equations (1)e(14) and also in Tables 1 and 2 The corresponding thermodynamic balance equations of this unit are calculated by using the following equations: Mass Balance Equation: m_ 8 þ m_ 9 ¼ m_ 10 þ m_ 11 þ m_ 12 þ m_ 13
(14)
Energy Balance Equation: _ r ¼ m_ 10 h10 þ m_ 11 h11 þ m_ 12 h12 þ m_ 13 h13 m_ 8 h8 þ m_ 9 h9 þ Q_ r þ W (15)
Unit II: PV/T Combined heat (E_4 and E_5 ) and power (E_6 and E_7 ) generation takes place in Unit II. The heat by-product of the PV/T is recovered and can be used in different processes (E_4 and E_5 ). Stream 6 is directed to the reactor to back up the photoelectrochemical water splitting with PV-based electrolysis. Stream 4 is sent to the reactor if the operating temperature is higher than the environmental temperature. Streams 5 and 7 are the recovered heat and leading product electricity, respectively. These streams are directed to the related storage units and considered as useful products. If the reactor is operating at environmental temperature, Stream 6 is zero, and the recovered heat is sent directly to the storage unit. There is no material input or output flow in Unit II. Hence there is no mass balance equation here. The energetic, exergetic, and entropy contents of each stream associated with this unit are given in Table 2. E_3 ¼ E_4 þ E_5 þ E_6 þ E_7
(11)
Entropy Balance Equation: m_ 8 s8 þ m_ 9 s9 þ
Q_ r þ S_gen ¼ m_ 10 s10 þ m_ 11 s11 þ m_ 12 s12 þ m_ 13 s13 T
(16)
Exergy Balance Equation: T0 _ r ¼ m_ 10 ex10 þ m_ 11 ex11 þW m_ 8 ex8 þ m_ 9 ex9 þ Q_ r 1 T _ dest þ m_ 12 ex12 þ m_ 13 ex13 þ Ex
(17)
Here, it should be noted that Stream 8 is the saturated NaCl solution input to the anode side, and Stream 9 is the freshwater input to the cathode side. These are the only material input streams. The energy input streams are shown in the earlier equations. The output streams are as follows: Stream 10 is the depleted NaCl solution (anode product), Stream 11 is NaOH solution (cathode product), Stream 12 is Cl2 (anode product), and Stream 13 is H2 (cathode product). m_ is the mass flow rate of the associated streams.
Table 2 e The energetic, entropic, and exergetic flow rates of the streams associated with the PV/T unit. Description
_ Energetic flow rate (E)
_ Entropic flow rate (S)
_ Exergetic flow rate (Ex)
4
Heat input of the reactor
E_4 ¼ Q_ r
5
Heat product
E_6 ¼ Q_ p
6
Electricity to the reactor
_ 4 ¼ Q_ r 1 T0 Ex T _ 6 ¼ Q_ p 1 T0 Ex T _r _ 6 ¼W Ex
7
Electricity product
_r E_6 ¼ W _p E_7 ¼ W
Q_ S_4 ¼ r T Q_ p _ S6 ¼ T e e
_p _ 7 ¼W Ex
Stream
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Table 3 e Energy and exergy efficiency equations of the integrated system based on the number of valuable products. Valuable products
Energy efficiency
1
H2
2
H2, Cl2
3
H2, Cl2, Electricity
4
H2, Cl2, Electricity, Heat
Exergy efficiency
E_13 h1 ¼ E_1
j1 ¼
h2 ¼
j2 ¼
h3 ¼ h4 ¼
E_12 þ E_13 E_1
E_7 þ E_12 þ E_13 E_1
j3 ¼
E_5 þ E_7 þ E_12 þ E_13 E_1
j4 ¼
10
_ 13 Ex _ 1 Ex
_ 12 þ Ex _ 13 Ex _ 1 Ex
_ 12 þ Ex _ 13 _ 7 þ Ex Ex _ 1 Ex
_ 7 þ Ex _ 12 þ Ex _ 13 _ 5 þ Ex Ex _ 1 Ex
Hydrogen
Normalized Ranking
9 Chlorine
8 7
Electricity
6 Heat
5 4
Exergy destruc on
3 2
η-4
1 ψ-4
0 20
30
40
50
60
70
80 Average
Opera ng Temperature (°C)
Fig. 3 e The effect of the operating temperature on the selected performance criteria of the integrated system.
Efficiencies
Results and discussion
The present systems’ energetic and exergetic performance is assessed based on its number of valuable products. Table 3 shows the efficiency equations of the integrated system, depending on the number of valuable products.
The hybrid multigeneration system's thermodynamic performance is evaluated by changing three operating parameters and one environmental parameter. These parameters are varied between the lower and upper limits as follows:
10
Hydrogen
Normalized Ranking
9 8
Chlorine
7 Electricity
6 5
Exergy destruc on
4 3
η-4
2 1
ψ-4
0 0
2
4
6
8
9
11
Inlet Mass Flow Rate (g/s)
13
15 Average
Fig. 4 e The effect of the inlet mass flow rate on the selected performance criteria of the integrated system. Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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10
Hydrogen
Normalized Ranking
9 Chlorine
8 7
Electricity
6 Heat
5 4
Exergy destruc on
3 η-4
2 1
ψ-4
0 2.7
3.1
3.5
3.9
4.3
4.7
5.2
5.6
6.0
Average
Photoac ve Area (m2) Fig. 5 e The photoactive area size impact on the selected performance criteria of the integrated system.
10
Normalized Ranking
Heat
Exergy destruc on
5 η-4
ψ-4
0 0
10
20
30
40
Average
Environmental Temperature (°C) Fig. 6 e The effect of the environmental temperature on the chosen performance criteria of the integrated system. The operating temperature is increased incrementally from 20 C to 80 C The inlet mass flow rate is increased incrementally from 0.1 g/s to 15 g/s The total photoactive area is increased incrementally from 2.7 to 6 m2 The environmental temperature is increased incrementally from 0 C to 40 C
Four different case studies are comparatively assessed based on their production rates, efficiencies, and exergy destruction rates. These case studies are (i) optimum
Table 4 e Performance comparison of selected case studies. Case 1 Case 2 Case 3 Case 4
The operating temperature is the temperature that the hybrid reactor operates. The inlet mass flow rate is Stream 8 and Stream 9's mass flow rates (Table 1). It should be noted that the anode and cathode inputs have the same inlet mass flow rate. The total photoactive area is the area covered by the photoactive surface of Unit II and the membrane photoelectrode assembly of Unit III. Unit II and Unit III are assumed to have equal photoactive areas. Hence they each have an area that is equal to half of the total photoactive area. And the environmental temperature is the temperature of the surroundings.
Temperature ( C) Flow rate (g/s) Area (m2) Environmental temperature ( C) Hydrogen (g/h) Chlorine (g/h) Electricity (W) Heat (W) Rate of exergy destruction (W) Energy efficiency (%) Exergy efficiency (%)
30 1 4 20 4.18 127.45 151 716 95.74 78.03 29.72
20 0.1 4 20 0.49 12.4 377 800 1.25 89.55 31.56
20 1 6 20 8.86 243.98 361.1 1200 21.29 86.46 29.42
20 1 4 20 4.06 123.98 159 799.6 28.03 84.51 28.01
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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operating temperature, (ii) optimum inlet mass flow rate, (iii) optimum photoactive area, and (iv) optimum environmental temperature. Each optimum case is designed to deliver the highest possible generation rates and efficiencies, and lowest possible exergy destruction rate. At the end, these case studies are compared to select the most suitable parameters for the integrated system. The aim of the case studies is to demonstrate the optimum operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature to reach the maximum achievable production rates and efficiencies while keeping the rate of exergy destruction as little as possible. In this section, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. All performance criteria are normalized and ranked by using one of the following equations:
temperatures and enhances their performance and second, recovering heat from the PV and using it as a valuable product reduces system waste. Further studies are to be conducted on the overall environmental impact and cost to investigate the role of the heat recovery further.
Case study 2: selection of optimum inlet mass flow rate In Case Study 2, a ranking is done to find the inlet mass flow rate giving the highest possible production rates, efficiencies, and lowest possible exergy destruction. For that reason, the following criteria are normalized and ranked based on a 0e10 scale: the production rates, overall efficiencies, and rate of exergy destruction. Then the average ranking at each inlet mass flow rate is calculated. The inlet mass flow rate giving the highest possible average ranking is the optimum point.
The criteria which are desired to be increased (such as efficiencies and production rates):
i minimum 10 maximum minimum
(18)
The criteria which are desired to be reduced (exergy destruction rate):
rank ¼
maximum i 10 maximum minimum
9 8 7 6 5 4 3 2 1 0 Case 1
Case 2
Case 3
Case 4
(19)
In these equations, i is a specific performance parameter (i.e., hydrogen production rate) at a specific operating condition (which are operating temperature, inlet mass flow rate, photoactive area size, or environmental temperature) and maximum and minimum are used for the maximum and minimum values of a specific performance parameter (which are efficiencies, production rates, and exergy destruction rate) within the chosen operating condition range.
b
Electricity
Case study 1: selection of optimum operating temperature
1000 800 600 400 200 0 Case 1
Case 2
c
Case 3 Energy
Case 4 Exergy
80
Efficiency (%)
In Case Study 1, it is aimed to observe the optimum operating temperature to reach the highest possible production rates, efficiencies, and the lowest reasonable rate of exergy destruction. For that reason, the following criteria are normalized and ranked based on a 0e10 scale: H2, Cl2, electricity, heat production rates, overall efficiencies, and rate of exergy destruction. Then the average ranking at each operating temperature is calculated. The operating temperature giving the highest possible average ranking is selected as the optimum operating temperature. Fig. 3 shows that the optimum operating temperature is 30 C where the most top average ranking of 5.75/10 is reached. Although Fig. 3 shows that the average ranking tends to decrease with operating temperature, it should be noted that the recovered heat from the thermal collectors is still necessary for two reasons: first, recovering heat from the PV keeps the panels at lower
Heat
1200
Genera on rate (W)
rank ¼
Hydrogen produc on rate (g/h)
a
60 40 20 0 Case 1
Case 2
Case 3
Case 4
Fig. 7 e Comparison of vase studies based on their (a) hydrogen production rates, (b) heat and electricity generation rates, and (c) energy and exergy efficiencies.
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Hydrogen 10 8 Exergy efficiency
Chlorine
6
Case 1 Case 2
4 2
Case 3
0 Energy efficiency
Electricity
Case 4 Ideal
Exergy destruc on
Heat
Fig. 8 e Overall performance comparison of the case studies based on the selected performance criteria.
Fig. 4 illustrates that the optimum point is 0.1 g/s, where the highest average ranking of 6.67/10 is reached.
Case study 3: selection of optimum photoactive area In Case Study 3, both ranking and normalization are performed to determine the photoactive area giving the highest possible H2, Cl2, electricity, and heat production rates, overall efficiencies, and lowest reasonable rate of exergy destruction. For that reason, these criteria are normalized and ranked based on a 0e10 scale. Then the average ranking at each point is calculated. The total photoactive area giving the highest possible average ranking is selected as the optimum total photoactive area. Fig. 5 presents that optimum photoactive area is 6 m2 g/s, where the most top average ranking of 10/10 is reached.
Case study 4: selection of optimum environmental temperature In Case Study 4, the following criteria are normalized and ranked based on a 0e10 scale: heat production rate, overall efficiencies, and rate of exergy destruction. Then the average ranking at each environmental temperature is calculated. Fig. 6 shows that the optimum environmental temperature is 20 C, where the highest average ranking of 8.45/10 is reached.
Overall comparison results of the case studies The overall comparison of the selected case studies is made by comparatively assessing the performance of these case studies which is summarized in Table 4. The hydrogen, heat, and electricity generation rates of these case studies, along with their energy and exergy efficiencies are provided in Fig. 7. Case Study 1 has the lowest heat and power generation rates and energy efficiency and the highest exergy destruction rate among the selected case studies. Case Study 2 has the smallest H2 and Cl2 production rates. On the other hand, this
case has the highest electricity generation and efficiencies and lowest rate of exergy destruction among the given case studies. Case Study 3 has the most top H2, Cl2, and heat production rates. Case Study 4 has the lowest exergy efficiency among the chosen case studies. The performance criteria of these case studies are normalized and ranked based on Equations (18) and (19), and the results are presented in Fig. 8. From Fig. 8, it can be seen that Case Study 3 is the most desirable case since it has the closest to ideal performance among selected options. The disadvantage of Case Study 3 is the low exergy efficiency compared to other situations. Case Study 3 has an average normalized ranking of 8.35/10 followed by Case Study 2, which is 5.96/10. The reason why Case Study 2 has low ranking is that it gives the smallest H2 and Cl2 production rates. The third highest average ranking is Case Study 4, 3.42/10. Case Study 4 has the lowest exergy efficiency and relatively small H2, Cl2, electricity, and heat production. Overall, the lowest average ranking belongs to Case Study 1, which is 2.03/10. The reason for this poor performance is because it has the lowest heat and power generation and energy efficiency and the highest exergy destruction rate among the selected case studies.
Conclusions The main objective of the present study is to demonstrate the optimum operating parameters of the innovative multigeneration system to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. The selected parameters are operating temperature, inlet mass flow rate, total photoactive area, and the environmental temperature. The optimum operating temperature, inlet mass flow rate, photoactive area size, and environmental temperature are found in different case studies for highest possible production rates and efficiencies and lowest possible exergy destruction rates. Furthermore, the results show that:
Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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The optimum operating temperature is 30 C within the investigated range of 20 Ce80 C. At 30 C, o 4.18 g/h H2 is generated. o 127.55 g/h Cl2 is generated. o 151 W electricity is generated. o 716 W heat is generated. o The exergy destruction rate is 95.74 W. o Overall efficiencies are 78% and 30% for energy and exergy, respectively. A 0.1 g/s is the optimum inlet mass flow rate within the selected range of 0.1e15 g/s. At 0.1 g/s; o 0.41 g/h H2 is generated. o 12.40 g/h Cl2 is generated. o 377 W electricity is generated. o 800 W heat is generated. o The exergy destruction rate is 1.25 W. o Overall energy and exergy efficiencies are 90% and 32%, respectively. Within the selected, overall photoactive area interval of 2.7e6 m2, the optimum size is 6 m2. When this optimum size is used in the present integrated system, o 8.86 g/h H2 is generated. o 243.98 g/h Cl2 is generated. o 361 W electricity is generated. o 1200 W heat is generated. o The exergy destruction rate is 21.29 W. o Overall energy and exergy efficiencies are 86% and 29%, respectively. The optimum environmental temperature is 20 C within the investigated range of 0 Ce40 C. At 20 C, o 4.06 g/h H2 is generated. o 123.98 g/h Cl2 is generated. o 159 W electricity is generated. o 800 W heat is generated. o The exergy destruction rate is 23 W. o Overall energy and exergy efficiencies are 85% and 28%, respectively. This study shows that hydrogen has excellent potential as a way to reduce reliance on imported energy sources such as oil. Nevertheless, before hydrogen can play a more prominent energy role and become a widely used alternative to gasoline, many new facilities and systems must be built to ensure safe, reliable, efficient, and clean hydrogen production and distribution.
Acknowledgements The authors would like to thank the kind support of the Young _ of the Turkish Academy Scientists Award Programme (GEBIP) ¨ BA). of Sciences (TU
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Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075
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Please cite this article as: Acar C, Dincer I, Enhanced generation of hydrogen, power, and heat with a novel integrated photoelectrochemical system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.075