Development and evaluation of a solar-based integrated ammonia synthesis and fuel cell system

Journal Pre-proof Development and evaluation of a solar-based integrated ammonia synthesis and fuel cell system

O. Siddiqui, I. Dincer PII:

S0959-6526(20)30440-6

DOI:

https://doi.org/10.1016/j.jclepro.2020.120393

Reference:

JCLP 120393

To appear in:

Journal of Cleaner Production

Received Date:

01 January 2020

Accepted Date:

01 February 2020

Please cite this article as: O. Siddiqui, I. Dincer, Development and evaluation of a solar-based integrated ammonia synthesis and fuel cell system, Journal of Cleaner Production (2020), https://doi.org/10.1016/j.jclepro.2020.120393

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof

Development and evaluation of a solar-based integrated ammonia synthesis and fuel cell system O. Siddiqui and I. Dincer Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada Emails: [email protected], [email protected] Abstract In the present study, a new ammonia synthesis and fuel cell system integrated with solar energy is developed. The excess energy from a solar photo-voltaic power plant is utilized for ammonia synthesis as an energy storage medium. Two different locations in Canada are considered for system modelling and simulation. The thermodynamic performance of the developed system is analyzed on a transient basis taking into account the variations in solar intensities across the year. The maximum energetic performance evaluated in terms of energy efficiency is 15.72%. Also, the peak exergetic efficiency of the solar-based system is determined as 16.55%. In addition, the daily discharge time capacity is found to reach 8.9 hours at the peak. Further, the ammonia synthesis rate obtains a value of 64.8 mol/s at the peak synthesis rate. The daily capacity of energy discharge entails the maximum value of 8502.4 kWh. The dynamic simulation results of each subsystem are discussed comprehensively presenting the applicability of the developed system for intermittency mitigation. Keywords: Ammonia fuel cell; ammonia synthesis; integrated system; exergy; efficiency 1. Introduction Clean energy production and utilization practices have become a key target for many developed and developing countries. Various types of clean energy resources, primarily with renewables, exist that include solar, wind, geothermal, hydropower etc. The usage of a given energy resource is dependent on the available technology, resources and locations. Locations with sufficient wind velocities can use wind turbines for power generation. Similarly, countries with high solar potentials can use solar energy based power generation. Commercial photovoltaic (PV) solar panels are available that allow the production of electrical power from solar energy. However, solar intensities fluctuate throughout the day and across the year. For instance, solar intensities normally increase from sunrise until noontime and then start decreasing until sunset. In addition, summer months entail higher solar intensities and winter months are associated with lower solar 1

Journal Pre-proof

availability. Considering these variations, solar-based power generation requires energy storage techniques to make it viable on a large scale. Currently, solar PV power generation facilities use batteries for energy storage. Batteries have various disadvantages such as thermal runaway and short storage times. Also, the capacity of storing energy reduces as the battery is charged and discharged in a cyclic manner. Moreover, thermal runaway causes safety issues with their usage. These have led to the investigation of new techniques of energy storage. The chemical storage is treated as one such methodology that includes energy storage in the form of a chemical. When excess energy is available, it can be used for synthesis of carbon-free chemical compounds such as hydrogen and ammonia. Hydrogen, entailing a high energy density by mass could be a possible candidate. Nevertheless, it entails low volumetric density that makes it difficult to store and transport. In addition, it has a high flammability range, which raises safety issues with its usage. However, ammonia has comparatively higher volumetric density and is easier to liquefy. It also entails a low flammability range, which makes it a suitable candidate as compared to hydrogen. In the open literature, few studies are found which have investigated the chemical energy storage option utilizing ammonia. Sanchez and Martin (2018) studied a renewable energy based system for the production of methanol and ammonia. The inputs to the system comprised of electrical power, water and air. Also, Linde’s column was utilized for generating nitrogen required for ammonia synthesis. Further, the hydrogen input requirement was met through the operation of a water electrolyser. Both solar and wind energy resources were employed for clean power generation that was utilized for reactant production and compression. Indirect as well as direct cooling were used for the developed system. The system performance was analyzed under varying operating parameters and conditions. However, the performance of the developed system was not investigated thermodynamically and the changes in the solar radiation intensities were not considered. Hasan and Dincer (2019) proposed an integrated system for producing ammonia and electrical power. The renewable energy resources of wind and solar were employed. The system included a proton exchange membrane (PEM) water electrolysis method for producing hydrogen. Also, several stages of ammonia synthesis were considered. Further, waste heat emitted from the exothermic ammonia synthesis reaction was used for generating useful power. The energetic efficiency was reported to be 75.8% for the overall system and the exergetic efficiency was found as 73.6%. Ikaheimo et al. (2018) investigated clean synthesis of ammonia utilizing renewable heat and power. The concept of converting electrical power to ammonia was considered. The 2

Journal Pre-proof

methodology of producing useful commodities from ammonia such as fertilizers was described to be advantageous. The system entailed power balancing through the synthesis of ammonia as well as district heating. This provided both environmentally benign ammonia synthesis as well as heating. The primary process of the developed system that was reported to be associated with the highest energy consumption was water dissociation. In addition, this process also entailed the highest cost comparatively. They found the developed system with ammonia as the working fluid, required the prices of carbon emissions as well as natural gas to be higher than conventional rates. Wang et al. (2017) investigated an energy storage system utilizing the chemical storage option through ammonia synthesis. Ammonia dissociation and synthesis units were integrated to develop a reversible system that entailed energy input as well as output through cyclic synthesis and dissociation. The designed system had an output capacity of 100 MW. Moreover, the integration of oxygen production during water electrolysis with the fuel cell subsystem was incorporated. An efficiency of 72% was found for the developed integrated system considering both phases of discharging and charging. Giddey et al. (2017) studied an ammonia-based transportation system that included the consumption of ammonia fuel for avoiding harmful environmental emissions. The system included clean production of hydrogen from renewable resources. Also, several methods of utilizing ammonia were considered. These comprised of ammonia combustion as well as decomposition. An efficiency of 39% was reported for the ammonia decomposition route considering both energy storage phases of charging and discharging. Also, an efficiency of 21% was found when power generation was obtained through internal combustion engines. Zhou et al. (2019) investigated a clean energy storage option considering ammonia as a promising candidate. Air and water were used as input resources for producing hydrogen through electrolysis of water and generating nitrogen by separation of air. Transportation and storage of ammonia were described to entail several advantages as compared to hydrogen and was suggested as a suitable candidate for chemical energy storage. The system included solar and wind energy inputs to the system, which were employed for synthesizing ammonia. Further, different applications were suggested for utilizing clean ammonia that included production of fertilizers, generation of electricity as well as heat. Energy input to the system was considered from the excess renewable electricity. The system performance, however, was not studied thermodynamically for varying solar intensities or wind velocities. Also, detailed analyses of the proposed infrastructure was not performed. Xue et al. (2019) studied ammonia as an energy storage medium focusing on carbon 3

Journal Pre-proof

footprints. The major impacts entailing the hotspots in the bio-geological cycles of carbon and nitrogen associated with ammonia were investigated. Different countries across the globe were considered as case studies and Japan was reported to entail high performance with the lowest footprints of carbon and nitrogen. The life cycle stages of utilization, transportation, storage and production of ammonia were studied. Major factors effecting the footprint results included the ammonia synthesis route as well as energy input source. However, only the life cycle footprints of ammonia were studied. Chen et al. (2019) studied a thermochemical ammonia-based method of energy storage. The system incorporated the usage of waste heat for generating electrical power. This was obtained from the thermal energy released during the synthesis of ammonia. Moreover, the design of applicable solar collectors was conducted for such systems. The designed collector entailed a conical configuration and was testes with varying geometrical parameters. However, the study was focused on the design of collectors and the thermodynamic performance of the overall ammonia-based system was not considered. Lovegrove et al. (2004) also investigated the cyclic ammonia dissociation and synthesis based energy storage methodology. A concentrated solar collector was tested for the developed system that entailed an area of 20 m2. Also, the collector was integrated with 20 reactors for ammonia synthesis. An overall energy efficiency of 53% was reported. Excess thermal energy was used for dissociating ammonia. The produced hydrogen and nitrogen gases were used to synthesize ammonia, which is an exothermic reaction that releases thermal energy. This was used as the output energy from the system. Different methods of energy and exergy recovery were investigated, however, the rates of exergy destruction or overall exergetic efficiencies were not considered. Also, the transient variations in the solar intensities and their effects on the system performance were not studied. Although numerous studies in the open literature have been carried out to investigate the production options of ammonia from renewable energy resources, a comprehensive thermodynamic analysis and assessment of integrated clean ammonia synthesis with direct ammonia fuel cell systems was not performed dynamically under varying solar radiation intensities. This is important to better understand the performance of these systems under the given application of solar power plants. In order, therefore, to address this gap in the open literature, the present study performs the thermodynamic assessment of a new type of integrated ammonia synthesis and fuel cell system for a solar PV power plant. The system proposed here aids in addressing the intermittency problem in clean power generation that is associated with such power 4

Journal Pre-proof

plants through chemical energy storage. The specific objectives of this study include (i) development of an integrated ammonia synthesis and direct ammonia fuel cell system for solar PV power plant applications, (ii) performing a transient system simulation across the year for two selected locations in Canada, (iii) assessing the performance of the developed system via exergetic and energetic analyses, (iv) determining both exergetic and energetic efficiencies of the overall system considering the variations in solar intensities across the year. 2. System description Figure 1 shows the developed solar PV based system with integrated ammonia synthesis and direct ammonia fuel cell. The PV power plant produces electrical power in the presence of sunlight that varies across the day and year. When excess energy is generated from the power plant, it is utilized to operate the integrated ammonia-based system. This energy is used for three subsystems, which include the hydrogen production through water electrolysis subsystem; nitrogen generation though pressure swing adsorption (PSA) subsystem; and gaseous compression subsystems. The synthesized ammonia is stored and can be utilized when low solar energy is available to generate electrical power through the direct ammonia fuel cell (AFC). At state 1, water is input to the water electrolyser (WES) subsystem that is dissociated into hydrogen and oxygen through electrical input from the PV plant when excess energy is available. At state 2, the hydrogen gas produced exits the WES and at state 3 oxygen exits the subsystem. Moreover, at state 4, air enters the PSA unit that separates air into constituent nitrogen (state 5) and oxygen (state 6). Both the hydrogen and nitrogen storage units entail pressures of 400 kPa. During ammonia synthesis, these reactants are supplied to compressor C-1 at state 7, where a mole ratio of 3 N2:1 H2 is supplied. The reactant mixture is compressed to a high pressure of 100 bar before entering the ammonia synthesis reactor (ASR). The ASR operates at a temperature of 250oC and state 9 comprises of the reactor exit stream that includes both the ammonia produced as well as the unreacted hydrogen and nitrogen. To separate and recycle the unreacted reactants, the ASR exit stream is passed through heat exchanger HX-1 to condense the formed ammonia. Further, the condensed ammonia is separated in separator (SEP) from unreacted gases that are sent back to the ASR at state 14. The ammonia produced exits the SEP at state 11. In HX-1, the working fluid comprises of refrigerant R-134a that is operated in the refrigeration cycle between states 15-18. At state 15, it enters the compressor C-2 which compresses it to a higher pressure and temperature. Further, condenser (CON) intakes state 16, which rejects heat and exits at state 17. Next, the throttle valve again drops the pressure 5

Journal Pre-proof

and thus the temperature of the refrigerant before it enters HX-1 at state 18 to cool the incoming gases. The AFC subsystem intakes ammonia at state 12 and oxygen at state 13 to produce electrical power. The AFC subsystem includes alkaline membrane electrolytes and the associated electrochemical interactions and parameters are discussed in the proceeding sections. 3. Analyses The locations of Regina and Manitoba in Canada are the focus of the present study. The solar radiation intensities are evaluated for the chosen locations for average monthly days across the year and a transient simulation of the developed system is performed. The average days are recommended days to be utilized when performing an analysis of the solar radiation intensities according to the equations that will be discussed in Eqs. (1-7). These denote the middle days of each month depending on the number of days occurring in that month. These denote the month denotes the middleday the middle of the month considering the first day of year The criterion of 90% of maximum power during a given day is used to identify the periods of excess power output. When the solar power plant output is higher than this criterion, the excess power generated used for operating the developed system. Also, the remaining power is supplied to the grid. The Engineering Equation Solver (Klein, 2019) as well as ASPEN Plus software (AspenTech, 2015) are employed for modeling and simulation. ASPEN Plus simulation software allows the analyses of various types of chemical reactions as well as physical interactions. The modeling of chemical reactions can be performed through different types of reactors embedded in the software that allow the simulation to be conducted according to the analysis method required. The Gibbs reactor is used in the present study that utilizes the Gibbs energy minimization method to simulate a given reaction under the set operating conditions. For exergy analyses, the atmospheric temperatures at the chosen locations are used from RETScreen on the respective days considered for each month (National Resources Canada, 2019). Compressors and heat exchangers are assumed to be adiabatic. Further, potential and kinetic changes in energy are assumed to entail negligible significance. The atmospheric temperatures used for exergy analyses for each location are listed in Table 1. 3.1 Analysis of solar PV power plant The power outputs from the solar PV plant are evaluated for each hour on the monthly average days. The total power output is calculated as 6

Journal Pre-proof

(1)

𝑃𝑇,𝑃𝑉 = 𝜂𝑃𝑉𝑀𝑄𝑇,𝑠,𝑃𝑉,𝑖

where 𝑃𝑇,𝑃𝑉 denotes the total power output from the PV plant, 𝜂𝑃𝑉𝑀 is the PV cell efficiency and 𝑄𝑇,𝑠,𝑃𝑉,𝑖 represents the total solar input to the PV plant. The total solar input to the PV plant is found as (2)

𝑄𝑇,𝑠,𝑖,𝑃𝑉 = 𝐴𝑇,𝑃𝑉𝐼𝑚𝑏

where 𝐴𝑃𝑉,𝑡𝑜𝑡 denotes total cell area and 𝐼𝑚𝑏 is the incoming beam radiation intensity. The values of 𝐼𝑚𝑏 are found from normal solar intensities as (3)

𝐼𝑚𝑏 = 𝐼𝑛𝑙𝐶𝑜𝑠𝜃𝑧ℎ where the normal radiation can be found from (Duffie and Beckman, 2006): 𝐼𝑛𝑙 = 0.9715𝐸𝑓𝑟𝐼𝑠𝑐𝑠𝜏𝑎𝑟𝜏𝑔𝑎𝜏𝑟𝑦𝜏𝑤𝑟𝜏𝑜𝑛

(4)

where 𝐼𝑛𝑙 is the normal radiation intensity, 𝐸𝑓𝑟 is the factor for eccentricity, 𝐼𝑠𝑐𝑠 is the solar constant, 𝜏 is the transmittance and the subscripts 𝑎𝑟, 𝑔𝑎, 𝑟𝑦, 𝑤𝑟 and 𝑜𝑛 denote aerosol, gas, Rayleigh, water and ozone respectively. Further, the eccentricity factor 𝐸𝑓𝑟 is calculated from 𝐸𝑓𝑟 = 1.00011 + 0.034221𝐶𝑜𝑠(𝐷𝑙) + 0.00128𝑆𝑖𝑛(𝐷𝑙) + 0.000719𝐶𝑜𝑠(2𝐷𝑙) + 0.000077𝑆𝑖𝑛(2𝐷𝑙)

(5)

where the day angle is represented as 𝐷𝑙 and can be evaluated as follows: 𝐷𝑙 = 15(12 ― 𝑆𝑀)

(6)

where solar time is denoted with SM Moreover, the cosine of zenith angle (𝐶𝑜𝑠𝜃𝑧ℎ) can be written as 𝐶𝑜𝑠𝜃𝑧ℎ = (𝐶𝑜𝑠𝛿𝑑𝑙)(𝐶𝑜𝑠𝜙𝑙𝑡)(𝐶𝑜𝑠𝐷𝑙) + (𝑆𝑖𝑛𝛿𝑑𝑙)(𝑆𝑖𝑛𝜙𝑙𝑡)

(7)

where 𝜙𝑙𝑡 denotes the latitude and 𝛿𝑑𝑙 represents the declination angle. Table 1 lists the modelling parameters used for analyses. 3.2 Analysis of water electrolysis The water electrolysis (WES) subsystem dissociates water molecules into oxygen and hydrogen through electrical power input from the PV plan. The overall water splitting reaction is denoted as 7

Journal Pre-proof

(8)

𝐻2𝑂→𝐻2 + 0.5𝑂2 The power input to the WES subsystem is evaluated as 𝑃𝑖𝑛,𝐻2,𝑊𝐸𝑆 = 𝐽𝑐,𝐻2,𝑊𝐸𝑆𝑉𝐻2,𝑊𝐸𝑆𝐴𝐻2,𝑊𝐸𝑆

(9)

where 𝑃𝑖𝑛,𝐻2,𝑊𝐸𝑆 is the power input, 𝐽𝑐,𝐻2,𝑊𝐸𝑆 is the current density, 𝑉𝐻2,𝑊𝐸𝑆 is the operating voltage and 𝐴𝐻2,𝑊𝐸𝑆 denotes the total area of the electrodes in the WES. The operating cell voltage for WES subsystem is found as (10)

𝑉𝐻2,𝑊𝐸𝑆 = 𝑉𝑜𝑟,𝐻2,𝑊𝐸𝑆 + 𝑉𝑎𝑐𝑡,𝐻2,𝑊𝐸𝑆 + 𝑉𝑐𝑜𝑛𝑐,𝐻2,𝑊𝐸𝑆 + 𝑉Ω,𝐻2,𝑊𝐸𝑆

where 𝑉𝑜𝑟,𝐻2,𝑊𝐸𝑆 is the open circuit voltage, 𝑉𝑎𝑐𝑡,𝐻2,𝑊𝐸𝑆 is the activation polarization voltage loss, 𝑉𝑐𝑜𝑛𝑐,𝐻2,𝑊𝐸𝑆 denotes concentration polarization voltage loss and 𝑉Ω,𝐻2,𝑊𝐸𝑆 is Ohmic loss is voltage. The reversible voltage under open circuit operation is evaluated from 𝑉𝑜𝑟,𝐻2,𝑊𝐸𝑆 =

Δ𝐺𝐻2,𝑊𝐸𝑆 𝑛𝐹

(11)

where Δ𝐺𝐻2,𝑊𝐸𝑆 is the change in Gibbs energy that can be calculated as Δ𝐺𝐻2,𝑊𝐸𝑆 = ∆𝐻𝐻2,𝑊𝐸𝑆 ―𝑇∆𝑆𝐻2,𝑊𝐸𝑆

(12)

The change in enthalpies (∆𝐻𝐻2,𝑊𝐸𝑆) and entropies (∆𝑆𝐻2,𝑊𝐸𝑆) are evaluated as ∆𝐻𝐻2,𝑊𝐸𝑆 = ∑𝑝𝑟𝑜𝑑𝑁𝑖,𝑝ℎ𝑖,𝑝 ― ∑𝑟𝑒𝑎𝑐𝑁𝑖,𝑟ℎ𝑖,𝑟

(13)

∆𝑆𝐻2,𝑊𝐸𝑆 = ∑𝑝𝑟𝑜𝑑𝑁𝑖,𝑝𝑠𝑖,𝑝 ― ∑𝑟𝑒𝑎𝑐𝑁𝑖,𝑟𝑠𝑖,𝑟

(14)

where 𝑁 is the number of moles, ℎ is the specific molar enthalpy, 𝑠 is the specific molar entropy, the subscript 𝑟 represents reactants and 𝑝 represents products. The polarization voltage loss due to Ohmic resistances is evaluated as 𝑉Ω,𝐻2,𝑊𝐸𝑆 = 𝐽𝐻2,𝑊𝐸𝑆Ω𝑊𝐸𝑆

(15)

where 𝑉Ω,𝐻2,𝑊𝐸𝑆 denotes the voltage loss, 𝐽𝐻2,𝑊𝐸𝑆 is the current density, and Ω𝑊𝐸𝑆 is the Ohmic resistance. This is expressed in terms of the ionic conductivity (𝑐𝑛𝑦(𝑥)) as

8

Journal Pre-proof

1

𝐿

(16)

Ω𝑊𝐸𝑆 = ∫0𝑐𝑛𝑦(𝑀𝑡(𝑥))𝑑𝑥 where 𝑀𝑡(𝑥) is the amount of moisture in the membrane, which is expressed as 𝑀𝑡(𝑥) =

𝑀𝑡𝑎 ― 𝑀𝑡𝑐 𝐿

(17)

+𝑀𝑡𝑐

where 𝑀𝑡𝑎 is the amount of moisture at the conjunction of membrane and anode, and 𝑀𝑡𝑐 represents the amount of moisture at the membrane-cathode interface. Further, 𝑐𝑛𝑦(𝑥) can be written as

(

𝑐𝑛𝑦(𝑀𝑡(𝑥)) = (0.5139𝑀𝑡(𝑥) ―0.326)exp (1268

1 303

1

)

(18)

―𝑇 )

The loss in voltage due to activation polarization is denoted as 𝑉𝑎𝑐𝑡,𝐻2,𝑊𝐸𝑆 = sinh ―1 (

𝐽𝐻2,𝑊𝐸𝑆 2𝐽𝑒𝑥,𝑖

)

(19)

where 𝑉𝑎𝑐𝑡,𝐻2,𝑊𝐸𝑆 is the activation voltage loss, 𝐽𝐻2,𝑊𝐸𝑆 is the operating current density and 𝐽𝑒𝑥,𝑖 denotes the exchange current density, which can be expressed for cathode (𝑖 = 𝑐) or anode (𝑖 = 𝑎). Also, the exchange current density can be evaluated from 𝐽𝑒𝑥,𝑖 = 𝐽𝑓𝑝exp ( ―

𝐸𝑎𝑐𝑡,𝑊𝐸𝑆,𝑖 𝑅𝑇

)

(20)

where the factor 𝐽𝑓𝑝 denotes the pre-exponential factor, 𝑅 is the ideal gas constant, the temperature is denoted by 𝑇 and the electrode activation energy is represented by 𝐸𝑎𝑐𝑡,𝑊𝐸𝑆,𝑖. The WES subsystem includes electrochemical splitting of water molecules and the hydrogen production can be expressed as 𝑁𝑝,𝐻2,𝑊𝐸𝑆 =

𝐽𝐻2,𝑊𝐸𝑆 𝑛𝐹

(21)

where 𝑁𝑝,𝐻2,𝑊𝐸𝑆 is the molar rate of hydrogen production, the number of moles of electrons is represented by 𝑛 and 𝐹 is the Faraday’s constant. The modelling WES parameters used are provided in Table 2. Also, the thermodynamic energy and exergy balance equations applied on the water electrolysis subsystem are listed in Table 3. 3.3 Analysis of nitrogen production

9

Journal Pre-proof

The PSA subsystem produces nitrogen through air separation. This consists of two adsorbent beds which operate cyclically with pressurizing and de-pressurizing processes. Initially, the first bed is pressurized to a pressure that results in selective gaseous adsorption of nitrogen and simultaneously another bed releases gases from the previous cycle. Next, the pressure contained in the first bed of adsorbent is released and the second low-pressure bed is pressurized. These processes are undertaken cyclically to generate a continuous stream of nitrogen. The ASPEN Adsorption modelling software is used to simulate this process in accordance with the Skarstrom process (Skarstrom, 1960). In that particular model of adsorption, the ASPEN utilizes the cyclic operation through a cycle manager that models the adsorption process and determines the amount of nitrogen separated from air in each run. As one bed undergoes adsorption, the second bed goes through desorption and the amount of the nitrogen obtained from the desorption of a single bed is obtained as a result, which is utilized as the input to the ammonia synthesis reactor. The power input needed to operate the PSA process is evaluated from the compressor power input required to pressurize atmospheric air to the required PSA pressure. Further, the detailed modelling equations implemented to simulate the adsorption process and the simulation flowsheet can be obtained from the software developers (AspenTech, 2015). The energy and exergy balances applied on the PSA subsystem are tabulated in Table 3. The number of hours of operation (𝑛𝑃𝑆𝐴) depends on the number of hours of excess power from the solar PV power plant. 3.4 Analyses of ammonia synthesis subsystem The mixture of hydrogen and nitrogen is pressurized by compressor C-1 to a suitable pressure for synthesizing ammonia. The balance equations applied on C-1 are listed in Table 3. At state 9, the mixture of produced ammonia and unreacted gases exits the ASR. The reversible synthesis reaction of ammonia can be expressed as 𝑁2 +3𝐻2↔2𝑁𝐻3

(22)

This reaction entails an exothermic nature such that the forward synthesis reaction is favored at lower temperatures and the backward dissociation reaction is favored at higher temperatures. Also, high operating pressures entail the synthesis reaction to be more favorable and vice versa. ASPEN Plus modelling software is employed for simulating the ASR. The Gibbs reactor is used for this 10

Journal Pre-proof

purpose, which is based upon the method of Gibbs energy minimization. Furthermore, the thermodynamic balances applied are tabulated in Table 3. To condense and separate the ammonia produced, the R134-a based refrigeration cycle is operated between states 15 and 18. This is needed in the proposed system owing to the production capacities for ammonia synthesis. High synthesis rates will not be entailed with the system at all times and the rate of ammonia produced will depend on the amount of excess power available. Thus, a refrigerant-based cooling system is required to condense and separate ammonia, especially, during periods of low ammonia synthesis rates. At state 9, the mixture of ammonia as well as unreacted hydrogen and nitrogen enter HX-1 and the cooled stream exits at state 10. Also, the refrigerant enters HX-1 at state 18 and leaves at state 15. The thermodynamic balance equations applied on heat exchanger HX-1 are listed in Table 3. Once the refrigerant exits HX-1, the compressor C-2 raises its pressure and thus temperature as it enters condenser CON. The balance equations for C-2 are also listed in Table 3. Next, CON allows the refrigerant to reject heat before entering the throttle valve, which drops the pressure and thus the temperature. The balance equations applied on the condenser and throttle valve can be obtained from Table 3. The cold stream exits HX-1 at state 9 and enters the separator (SEP). Here, the ammonia produced is separated from unreacted reactants. At state 11, ammonia leaves SEP and is stored for later usage. At state 14, the unreacted reactants are sent back to the ASR. The energetic and exergetic balance equations applied on SEP can be obtained from Table 3. Also, the modelling parameters utilized in the analyses of the above subsystems are summarized in Table 2. 3.5 Analysis of direct ammonia fuel cell After production and storage of ammonia during the availability of excess power, it can be used for generating electrical power when needed. The type of direct ammonia fuel cell utilized in the analysis entails an alkaline membrane electrolyte. Also, the oxygen required during fuel cell operation is acquired from the PSA subsystem. For the AFC subsystem, the overall reaction is written as 3

1

3

𝑁𝐻3 + 4𝑂2→2𝑁 + 2𝐻2𝑂 2

(23)

11

Journal Pre-proof

However, there are several other side electrochemical interactions that occur within the cell. The overall cathodic interaction of oxygen and water molecules is expressed as 3 4𝑂2

3

+ 2𝐻2𝑂 + 3𝑒→3𝑂𝐻 ―

(24)

Further, the overall anodic interaction can be written as 1

𝑁𝐻3 +3𝑂𝐻 ― →2𝑁 +3𝐻2𝑂 + 3𝑒 2

(25)

The AFC power output is calculated as (26)

𝑃𝑁𝐻3,𝐴𝐹𝐶 = 𝑉𝑁𝐻3,𝐴𝐹𝐶𝐽𝑁𝐻3,𝐴𝐹𝐶𝐴𝑁𝐻3,𝐴𝐹𝐶

where 𝑃𝑁𝐻3,𝐴𝐹𝐶 is the power output, 𝑉𝑁𝐻3,𝐴𝐹𝐶 is the operating voltage of AFC, 𝐽𝑁𝐻3,𝐴𝐹𝐶 is the current density and 𝐴𝑁𝐻3,𝐴𝐹𝐶 represents the total AFC electrode area. The AFC voltage is calculated as (27)

𝑉𝑁𝐻3,𝐴𝐹𝐶 = 𝑉𝑜𝑐,𝑁𝐻3,𝐴𝐹𝐶 ― 𝑉𝑖𝑟,𝑁𝐻3𝐴𝐹𝐶 ― 𝑉Ω,𝑁𝐻3,𝐴𝐹𝐶 ― 𝑉𝑎𝑐𝑡,𝑁𝐻3,𝐴𝐹𝐶 ― 𝑉𝑐𝑜𝑛𝑐,𝑁𝐻3,𝐴𝐹𝐶 where 𝑉𝑜𝑐,𝑁𝐻3,𝐴𝐹𝐶 is the reversible voltage under open circuit condition that is evaluated as 𝑉𝑜𝑐,𝑁𝐻3,𝐴𝐹𝐶 =

Δ𝐺𝑁𝐻3,𝐴𝐹𝐶

(28)

𝑛𝐹

where Eq. (9) depicts the method of evaluating Δ𝐺. Next, 𝑉𝑖𝑟,𝑁𝐻3𝐴𝐹𝐶 denotes the irreversible voltage loss occurring in low temperature direct ammonia fuel cells (Siddiqui and Dincer, 2019). The Ohmic voltage loss (𝑉Ω,𝑁𝐻3,𝐴𝐹𝐶) in AFC is evaluated as (29)

𝑉Ω,𝑁𝐻3,𝐴𝐹𝐶 = Ω𝐴𝐹𝐶𝐽𝑁𝐻3,𝐴𝐹𝐶 where the total Ohmic resistance is Ω𝐴𝐹𝐶 and the current density is 𝐽𝑁𝐻3,𝐴𝐹𝐶. Further, the voltage loss because of activation polarization is found as 𝐽𝑁𝐻3,𝐴𝐹𝐶

𝑅𝑇𝐴𝐹𝐶

𝑉𝑎𝑐𝑡,𝑁𝐻3,𝐴𝐹𝐶 = ln ( 𝐽0,𝑁𝐻 ,𝐴𝐹𝐶 ) 𝛼𝑛𝐹

(30)

3

where 𝑇𝐴𝐹𝐶 is the operating temperature, 𝐽0,𝑁𝐻3,𝐴𝐹𝐶 is the AFC exchange current density, 𝐽𝑁𝐻3,𝐴𝐹𝐶 is AFC operating current density, 𝛼 is coefficient for transfer of charge. The voltage loss because of concentration polarization is determined as

12

Journal Pre-proof

𝑉𝑐𝑜𝑛𝑐,𝑁𝐻3,𝐴𝐹𝐶 = ln

(

𝐽𝐿,𝑁𝐻3,𝐴𝐹𝐶

)

𝑅𝑇𝐴𝐹𝐶

𝐽𝐿,𝑁𝐻3,𝐴𝐹𝐶 ― 𝐽𝑁𝐻3,𝐴𝐹𝐶 𝛼𝑛𝐹

(31)

where 𝐽𝐿,𝑁𝐻3,𝐴𝐹𝐶 is the AFC limiting current density. Moreover, the thermodynamic balances applied on the AFC subsystem are tabulated in Table 3. In addition, the operating parameters utilized for analysis are summarized in Table 2. 3.6 Efficiency evaluation The performance of the proposed system is evaluated via energetic and exergetic efficiencies. The energetic efficiency is expressed as 𝑛

𝜂𝑒𝑛,𝑜𝑣 =

𝑛

𝑃 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑃 𝑑𝑡 ∫𝑡 𝑔𝑟 = 1 𝑃𝑉,𝑔𝑟 = 1 𝑁𝐻3,𝐴𝐹𝐶 𝑛

∫𝑡 𝑠= 1𝑄𝑠,𝑖,𝑃𝑉𝑑𝑡

(32)

Moreover, the exergetic performance in terms of efficiency is also determined. While the energy efficiency assesses the system performance as a ratio of the useful energy output and required energy input, the exergy efficiency evaluates the system performance through the amount of exergy output obtained from the system and the total exergy input supplied to system. The exergy efficiency value provides an idea of the actual useful work potential that is lost in the operation of the system. 𝑛

𝜂𝑒𝑥,𝑜𝑣 =

𝑛

𝑃 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑃 𝑑𝑡 ∫𝑡 𝑔𝑟 = 1 𝑃𝑉,𝑔𝑟 = 1 𝑁𝐻3,𝐴𝐹𝐶 𝑛

(

𝑇0

𝑄 ∫𝑡 𝑠𝑛 1―𝑇 = 1 𝑠,𝑖,𝑃𝑉

)

𝑠𝑛

(33)

𝑑𝑡

where the power transferred to the grid is represented by 𝑃𝑃𝑉,𝑔𝑟, AFC power output is denoted by 𝑃𝑁𝐻3,𝐴𝐹𝐶, solar thermal input energy is denoted with 𝑄𝑠,𝑖,𝑃𝑉, number of hours of power supply to the grid is represented by 𝑛𝑔𝑟, number of hours the AFC can be operated is denoted by 𝑛𝐴𝐹𝐶, number of hours of daylight hours is represented by 𝑛𝑠𝑛, ambient temperature is represented 𝑇0 and sun temperature is represented with 𝑇𝑠𝑛 4. Results and discussion The developed system is simulated and the performance is assessed for the average day of each month considering the locations of Regina and Manitoba in Canada. The modelling results of each subsystem is discussed and the overall energetic and exergetic performances are evaluated as 13

Journal Pre-proof

presented in the proceeding sections. The system proposed provides a new way of storing excess electrical energy that if generated from a solar PV plant in the form of ammonia. Previous studies have not considered the usage of direct ammonia fuel cells for the application of energy storage in solar power plants. Thus, the present study investigates this route and performs energetic as well as exergetic analyses, which have not been performed in the previous studies. The primary advantages of the developed system include the mitigation of solar intermittency as well as energy storage flexibility. As solar intensities vary across the year, the excess power outputs from solar PV plants can be utilized to synthesize clean ammonia, which can be used through direct ammonia fuel cells to generate clean power during low solar intensities. Moreover, this energy storage method provides flexibility in energy storage where the amount of ammonia synthesized and stored can be varied according to the amount of excess energy available. The stored ammonia does not entail any degradation in energy capacity, which is a major problem in the conventional battery energy storage method. However, the proposed system may entail higher initial installation costs than other energy storage methods. Although the initial cost of the developed system may be higher than other energy storage methods, the operational costs can be lower due to no degradation of storage medium. It is recommended to investigate the techno-economic performance of the system considering different life cycle stages. 4.1 Results of solar PV power plant The dynamic simulation results of the power outputs from the solar PV plant are shown for Regina and Manitoba in Figures 2 (a) and (b) respectively. The variation in power outputs for the monthly average days are presented for both location. For Regina, the maximum power outputs during the year are found as 327.5 MW and 320.9 MW in the months of June and July respectively. Further, the minimum power outputs are found for the months of January and December, where the peak power outputs are 125.8 MW and 109.7 MW respectively. In addition to this, for Manitoba the maximum power outputs of the year are 317.1 MW and 308.8 MW. Also, the peak power outputs during the low solar intensity months reach 102.9 MW and 88.7 MW respectively. In addition to this, the number of daylight hours are also found to show large variations between the summer and winter months. In winter months for instance, 8 daylight hours are observed. On the other hand, 16 daylight hours are obtained for the summer month of June. Thus, the PV plants need to be designed considering the minimum solar radiation intensities throughout the year as well as the number of daylight hours. In the present study, the cut off criteria of 0.9 is implemented, where 14

Journal Pre-proof

the power supplied to the grid by the PV farm is stalled when the power outputs exceed this threshold and the excess power is sent to the developed integrated system for later usage. Hence, it is recommended to design and investigate solar-based power plants with such type of integrated systems with chemical energy storage techniques to investigate their overall life cycle and technoeconomic performance. 4.2 Results of hydrogen and nitrogen production The rates of hydrogen and nitrogen production for the chosen locations are shown in Figures 3 and 4 respectively. The hydrogen production rate for Regina as shown in Figure 3 (a), reaches a maximum of 91.9 mol/s in the month of June. Also, the minimum production rate during excess power availability was observed to be 3.6 mol/s in January. Moreover, the maximum production rate during January reaches 35.3 mol/s. Similar trends are observed for Manitoba as shown in Figure 3 (b). The peak hydrogen production rate for this location is found to reach 89 mol/s. In addition, the results of nitrogen production rates for Regina and Manitoba are presented in Figures 4 (a) and (b) respectively. The maximum nitrogen production rates are found to reach 30.7 and 29.7 for Regina and Manitoba respectively. Further, hours 10-13 are the synthesis hours that coincide with the excess energy hours for these locations. The production rates entail considerable variation between the maximum and minimum values. This is due to large fluctuations in solar intensities in these locations. Hence, the developed system provides an effective method of energy storage that is flexible according to the available excess power. The presented integrated system should be investigated for hybrid solar and wind power plants such that the high variability in wind velocities and solar intensities is mitigated by ammonia synthesis and utilization. 4.3 Results of ammonia synthesis The rates of ammonia synthesis evaluated for the chosen locations for each month are presented in Figures 5 (a) and (b). The rate of ammonia synthesis reaches 61.3 and 59.4 mol/s for Regina and Manitoba respectively in June. Also, the maximum synthesis rates in January, which has low solar potential are 23.5 and 19.3 mol/s for Regina and Manitoba. Moreover, for Manitoba, comparatively lower number of ammonia production hours are obtained than Regina in this month. This can be attributed to the lower number of hours of excess power availability. The ammonia synthesized on a given day depends upon the variations in the synthesis rates as well as number of operating hours. Thus, the application of the presented system can be extended to both the storage 15

Journal Pre-proof

of energy as well as synthesis of ammonia. Current conventional ammonia synthesis relies heavily upon fossil fuels and such renewable energy based systems can be implemented in mitigating the considerable emissions that are released from ammonia synthesis plants. In addition to this, the developed system can also be integrated with a CO2 capturing subsystem. The captured CO2 and the excess ammonia can be utilized for production of fertilizers. Analysis of such systems should be performed and their thermodynamic performance should be studied. 4.3 Results of electrochemical modelling of ammonia fuel cell Figure 6 shows the results of electrochemical modelling of the direct ammonia fuel cell. The electrochemical parameters needed to perform the simulation are acquired from our earlier experimental investigations on these type of fuel cells (Siddiqui and Dincer, 2019). Theoretically, a voltage of 1.17 V is obtained for an ammonia fuel cell under open circuit (OCV) conditions. However, owing to various irreversibilities, an OCV of nearly 0.35 V is obtained experimentally for ambient temperature anion exchange membrane electrolyte based ammonia fuel cells (Suzuki et al., 2012). A maximum power density of 6.4 W/m2 is obtained as can be observed from Figure 6. Adsorbed nitrogen atom-based catalyst active site blockage is one of the major reasons that results in lower power outputs for ammonia fuel cells. Thus, new electro-catalysts need to be developed, which are more compatible in operation with ammonia molecules. Alloys or composites with the favorable ammonia compatibility should be investigated. Further, hightemperature direct type cells can provide higher power outputs, however, they require additional energy inputs for achieving high operating temperatures. Nevertheless, different types of ammonia fuel cells should be studied when integrated with the proposed system. 4.5 Results of discharge time and AFC energy capacity Figures 7 (a) and (b) show the discharge time capacities for Regina and Manitoba respectively. The operation of the fuel cell is set at the optimal power density of 6.4 W/m2. With this density of power output, an operating current density of 61.9 A/m2 is entailed. The discharge times are evaluated from the rate of consumption of ammonia at this current that is obtained from the relation between current density and molar consumption presented in Eq. (12). The discharge time capacities reach 8.9 and 8.8 hours for Regina and Manitoba respectively in June. Also, the lowest discharge time capacities are found for December, which are 1.9 and 1.3 hours for the above locations respectively. Moreover, flexibility in storage time capacities is provided by the 16

Journal Pre-proof

developed system, where once the ammonia is synthesized it can be stored without any loss is energy. When energy output is required, the stored ammonia can be utilized in the direct ammonia fuel cell. In addition to this, the discharge time capacities can also be varied as required by changing the rates of energy output as well as ammonia consumption. The results of AFC energy capacities are also presented in Figures 7 (a) and (b), where the energy capacities reach 8502.4 and 8409.9 kWh for Regina and Manitoba respectively on the average day of June, which entails higher capacities as compared to other months. The minimum output energy capacities occur in December for both locations and are found as 1867.4 and 1290.0 kWh respectively. The energy capacities are a representation of the amount of energy that can be discharged during a particular day depending on the amount of ammonia synthesized. Hence, the developed system can also be implemented for other renewable resources such as wind power, where the fluctuations in energy availability can be mitigated. 4.6 Results of exergy destruction in ASR and WES subsystems Figure 8 shows the results obtained for total daily exergy destruction in the ASR and WES subsystems. The magnitude of useful potential for work that is lost because of associated irreversibilities in the system is depicted through exergy destruction. For Regina, an exergy destruction of 12998.8 kWh occurs in the ASR on the monthly average day of June and 12753.9 kWh is evaluated for Manitoba during the same month. The lowest ASR exergy destruction for Regina and Manitoba, on the other hand, entail values of 2401.3 and 1620.7 kWh, which occur in the winter months of January and December. The results of exergy destruction are observed to be proportional to the number of ASR operational hours. As in the month of June, the ASR operates for larger number of hours than the winter months, the exergy destructions are also higher. Nevertheless, to decrease the exergy destruction in ASR, ammonia synthesis reactors that include lower temperature differences between the input and output streams should be developed. These will aid in decreasing the irreveribilities and thus the entropy generated. The total daily exergy destruction in the WES hydrogen production subsystem reaches peak values of 11583.2 and 11460.0 kWh for Regina and Manitoba respectively. Also, when the WES operates for lower number of hours at lower hydrogen production rates, daily exergy destructions of 1758.3 and 2545.2 kWh are obtained for the chosen locations respectively in the month of December. The exergy destruction occurring in the WES is due to the electrochemical irreversibilities, which occur with the splitting of water molecules. Hence, electro-catalysts with higher hydrogen production 17

Journal Pre-proof

rates and lower polarization losses should be developed that can aid in overcoming these irreversibilities. Lower the exergy destruction in WES, lower is the power input needed to produce hydrogen at a given current density. Therefore, leading to better efficiencies. 4.7 Results of exergy destruction in C-1 and AFC subsystems Figure 9 depicts the daily exergy destruction results for compressor C-1 and AFC on the monthly average days. For Regina, a maximum exergy destruction of 80.5 MWh is evaluated in the AFC subsystem. For Manitoba, 82.9 MWh of exergy destruction is determined to occur in the AFC. Both peak values are observed to occur in June. The limitations in the adsorption-desorption phenomena of nitrogen atoms in the AFC lead to higher irreversibilities and thus exergy destructions. Hence, better electro-catalysts with favorable nitrogen interaction properties need to be developed to lower the exergy destruction in the AFC subsystem. Further, exergy destruction associated with the compressor C-1 is evaluated as 626.2 kWh for Regina and 619.4 kWh for Manitoba in June, which entails the maximum values. These can be decreased through the development of compressors with higher isentropic efficiencies. Compressors with higher isentropic efficiencies will lead to lower entropy generation and hence lower exergy destruction. Also, higher isentropic efficiency compressors will require lower input power that will aid in increasing the overall efficiencies. 4.8 Results of energy and exergy efficiency evaluation Figure 10 shows the results of overall energetic and exergetic efficiency evaluation. For Regina, comparable overall efficiencies are obtained in the month of January and June. The ammonia synthesized was lower in January, however, lower solar energy input was consumed. In addition, June has the highest amount of ammonia synthesis and AFC energy output, however, July is entailed with higher overall efficiencies. This can be attributed to lower exergy destruction as well as energy losses. A peak energy efficiency of 15.71% is evaluated for Regina that occurs in July and the peak exergy efficiency is found to be 16.55%. Moreover, the lowest efficiencies are found for the average day of December where the energetic efficiency is evaluated as 15.6% energetically and 16.3% exergetically. The efficiency results for Manitoba are depicted in Figure 10 (b). The maximum exergy efficiency is found to be marginally lower than Regina. However, similar energy efficiency as Regina is obtained at the peak value. The exergy efficiencies have been evaluated considering the ambient temperatures (listed in Table 1) at the chosen locations on the average 18

Journal Pre-proof

days of each month. Hence, the exergy efficiencies show larger variation than energy efficiencies. Therefore, exergy analyses should be utilized in the analysis and assessment of energy systems as it provides enhanced understanding of the overall performance, especially when ambient conditions are varying. 5. Conclusions A new ammonia synthesis and fuel cell system is proposed and its thermodynamic performance is assessed for two different locations of Regina and Manitoba. Excess energy from a solar PV power plant is used to produce ammonia as a medium for energy storage. The developed system is modelled dynamically on the monthly average days. The peak daily discharge time capacity is found to reach 8.9 hours. Also, the ammonia synthesis rate is evaluated to obtain a value of 64.8 mol/s at the maximum production point. Moreover, the daily energy discharge capacity is found to entail a maximum value of 8502.4 kWh. The overall system performance is assessed in terms of the energy efficiency that is found to be 15.72% at the maximum value. The corresponding overall maximum exergetic efficiency is evaluated as 16.55%. The present study provides a new method of intermittency mitigation for clean power generation as well as clean ammonia synthesis through solar energy. Also, a comprehensive dynamic study of the developed system is performed. Future research work can focus on performing exergo-economic studies to analyze the economic aspects. In addition, the developed system can be integrated with other variable renewable energy resources such as wind energy. Nomenclature A cny 𝐷 𝑒𝑥 𝐸𝑥 F G ℎ H I 𝐽 𝑚 𝑀𝑡 𝑁 𝑃

area (m2) conductivity (S/m) day angle (o) specific exergy (kJ/kg) exergy rate (kW) Faradays constant (96500 C/mol) Gibbs free energy (J) specific molar enthalpy (kJ/mol) enthalpy (kJ) solar intensity current density (A/m2) mass flow rate (kg/s) moisture content molar production rate (mol/s) power (kW) 19

Journal Pre-proof

thermal energy transfer (kW) 𝑄 s specific entropy (kJ/kg K) S entropy (kJ/K) ST solar time (h) temperature (oC) 𝑇 V voltage (V) work rate (kW) 𝑊 Greek letters resistance (Ohms.cm2) Ω charge transfer coefficient 𝛼 efficiency 𝜂 declination angle 𝛿 Subscripts a anode act activation ar aerosol c cathode con condenser conc concentration dest destroyed dl declination elec electric ex exchange ga gas gr grid gen generation in incoming limiting L lt latitude mb beam nl normal on ozone or open circuit PV photovoltaic Rayleigh ry s solar scs solar constant sn sun T temperature TV throttle valve wr water zh zenith Acronyms AEM anion exchange membrane AFC ammonia fuel cell ASR ammonia synthesis reactor 20

Journal Pre-proof

COMP compressor CON condenser Dl day angle EES engineering equation solver PSA pressure swing adsorption SEP separator PV photovoltaic WES water electrolysis References Aspen Tech, Aspen Plus Simulation Software, 2015. Available http://www.aspentech.com/products/aspen-plus/ (accessed 25 November 2019).

at:

Chen, C., Liu, Y., Aryafar, H., Wen, T., Lavine, A.S., 2019. Performance of conical ammonia dissociation reactors for solar thermochemical energy storage. App. Energy 255, 113785. Duffie, J. A., Beckman, W.A., 2006. Solar Engineering of Thermal Processes, Third Ed., New Jersey: Wiley. Giddey, S., Badwal, S.P.S., Munnings, C., Dolan, M., 2017. Ammonia as a Renewable Energy Transportation Media. ACS Sust. Chem. Eng. 5, 10231-10239. Hasan, A., Dincer, I., 2019. Development of an integrated wind and PV system for ammonia and power production for a sustainable community. J. Clean. Prod. 231, 1515-1525. Ikaheimo, J., Kiviluoma, J., Weiss, R., Holttinen, H., 2018. Power-to-ammonia in future North European 100% renewable power and heat system. Int. J. Hydrogen Energy 43, 17295-17308. Klein, S.A., Engineering equation solver (EES), Professional V10.462, 2019 (Madison USA. http://www.fchart.com). Lovegrove, K., Luzzi, A., Soldiani, I., Kreetz, H., 2004. Developing ammonia based thermochemical energy storage for dish power plants. Solar Energy 76, 331-337. Natural Resources Canada, RETScreen Clean Energy Management Software, Available at: https://www.nrcan.gc.ca/energy/retscreen/7465 (accessed 25 November 2019). Ni, M., Leung, M.K.H., Leung, D.Y.C., 2008. Energy and exergy analysis of hydrogen production by a proton exchange membrane (PEM) electrolyzer plant, Energy 49, 2748-2756. Sanchez, A., Martin, M., 2018. Optimal renewable production of ammonia from water and air. J. Clean. Prod. 178, 325-342. Siddiqui, O., Dincer, I., 2019. Development and performance evaluation of a direct ammonia fuel cell stack. Chem. Eng. Sci. 200, 285-293. Siddiqui, O., Dincer, I., 2019. Experimental investigation and assessment of direct ammonia fuel cells utilizing alkaline molten and solid electrolytes. Energy 169, 914-923. Skarstrom, C. W., 1960. Oxygen Concentration Process, U.S Patent: US3237377A.

21

Journal Pre-proof

Suzuki, S., Muroyama, H., Matsui, T., Eguchi, K., 2012. Fundamental studies on direct ammonia fuel cell employing anion exchange membrane, J. Power Sources 208, 257-262. Wang, G., Mitsos, A., Marquardt, W., 2017. Conceptual Design of Ammonia-Based Energy Storage System: System Design and Time-Invariant Performance. AIChE. 63, 1620-1637. Xue, M., Wang, Q., Lin, B., Tsunemi, K., 2019. Assessment of Ammonia as an Energy Carrier from the Perspective of Carbon and Nitrogen Footprints. ACS Sust. Chem. Eng. 7, 12494-12500. Zhou, M., Wang, Y., Chu, Y., Tang, Y., Tian, K., Zheng, S., Chen, J., Wang, Z., 2019. Ammonia as an environmentally benign energy carrier for the fast growth of China. Energy Procedia 58, 4986-4991.

Figure 1 Schematic representing the developed solar PV-based integrated ammonia synthesis and direct ammonia fuel cell system

22

Journal Pre-proof

a

b Figure 2 Results of solar PV power output on monthly average days for (a) Regina and (b) Manitoba

23

Journal Pre-proof

a

b

Figure 3 Results of daily hydrogen production rate variation for (a) Regina and (b) Manitoba

24

Journal Pre-proof

a

b Figure 4 Results of daily variations in the nitrogen production rates for (a) Regina and (b) Manitoba

25

Journal Pre-proof

a

b Figure 5 Results of daily variations in the ammonia synthesis rates for (a) Regina and (b) Manitoba

26

Journal Pre-proof

0.4

8

Voltage

Cell voltage (V)

0.3

6

0.2

4

0.1

2

0 0

20

40

Current density (A/m2)

60

Power density (W/m2)

Power density

0 80

Figure 6 Variation of ammonia fuel cell voltage and power density with current density

27

Journal Pre-proof

a

b Figure 7 Results of discharge time and AFC energy output capacities for (a) Regina and (b) Manitoba

28

Journal Pre-proof

a

b Figure 8 Results of exergy destruction for ASR and WES subsystems for (a) Regina and (b) Manitoba

29

Journal Pre-proof

a

b Figure 9 Results of exergy destruction for AFC and COMP subsystems for (a) Regina and (b) Manitoba

30

Journal Pre-proof

a

b

Figure 10 Results of overall system energy and exergy efficiencies for (a) Regina and (b) Manitoba

31

Journal Pre-proof

Table 1 Modelling parameters used in the analysis of solar PV power pant Parameter Geographical latitudes PV cell manufacturer Cell type Unit capacity Frame area Total cell area Number of cells Efficiency

Ambient temperature (oC)

Value Toronto: 43.6532 Manitoba: 53.7609 Regina: 50.442 Canadian Solar Poly-Si-CSX-310 0.31 kW 1.918 m2 300000 m2 1564130 16.16% Regina Manitoba Jan -16.5 -21.5 Feb

-12.9

-18.3

Mar

-6

-10.4

Apr

4.1

-0.4

May 11.4

7.6

Jun

16.4

14.5

Jul

19.1

17.8

Aug

18.1

16.6

Sep

11.6

10.2

Oct

5.1

2.2

Nov

-5.1

-8.1

Dec

-13.6

-17.2

Source: (National Resources Canada, 2019)

32

Journal Pre-proof

Table 2 Modelling parameters used for the analysis of WES, ASR and AFC subsystems Parameter Total WES electrode area WES operating pressure WES pre-exponential factors

Value 64330 cm2 101 kPa Anodic: 1.7 × 105 A/m2 Cathodic: 4.6 × 103 A/m2 WES temperature 30oC Membrane thickness 100 µm Type of water splitting Proton conducting electrolyte based Faraday’s constant 96485 C/mol Gas constant 8.314 J/mol K WES limiting current density 6 A/cm2 ASR reactant inlet molar ratio 3 H2: 1 N2 ASR operating temperature 250oC ASR operating pressure 15 MP ASR reactor type Gibbs reactor Compressor isentropic efficiency 85% Minimum R134a pressure 244 kPa Minimum R134a temperature -4.89 oC AFC type Anion exchange membrane electrolyte entailing direct type cells Total AFC electrode area 150000 m2 Exchange current density 0.0028 A/m2 Limiting AFC current density 73 A/m2 Operating pressure 101 kPa Operating temperature 25oC Source: (AspenTech, 2015; Siddiqui and Dincer, 2019; Ni et al., 2008)

33

Journal Pre-proof

Table 3 Energy and exergy balance equations for each subsystem Subsystem

Energy balance

Exergy balance

Water electrolysis (WES)

∫𝑡 𝑊𝐸𝑆 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑊𝐸𝑆 𝑃 𝑑𝑡 = =1 1 1 = 1 𝐻2,𝑊𝐸𝑆

𝑛

𝑛

∫𝑡 𝑊𝐸𝑆 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑊𝐸𝑆 𝑃 𝑑𝑡 = =1 1 1 = 1 𝐻2,𝑊𝐸𝑆

𝑛

𝑛

∫𝑡 𝑊𝐸𝑆 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑊𝐸𝑆 𝑚 𝑒𝑥 𝑑𝑡 + =1 2 2 =1 3 3

∫𝑡 𝑊𝐸𝑆 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑊𝐸𝑆 𝑚 ℎ 𝑑𝑡 =1 2 2 =1 3 3

𝑛

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝑊𝐸𝑆 𝐸𝑥𝑑𝑒𝑠𝑡,𝑊𝐸𝑆𝑑𝑡 =1 Nitrogen generation (PSA)

𝑛

𝑛

∫𝑡 𝑃𝑆𝐴 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑃𝑆𝐴 𝑃 𝑑𝑡 = =1 4 4 = 1 𝑁2,𝑃𝑆𝐴

𝑛

𝑛

∫𝑡 𝑃𝑆𝐴 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑃𝑆𝐴 𝑚 𝑒𝑥 𝑑𝑡 + =1 5 5 =1 6 6

∫𝑡 𝑃𝑆𝐴 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑃𝑆𝐴 𝑃 𝑑𝑡 = =1 4 4 = 1 𝑁2,𝑃𝑆𝐴 ∫𝑡 𝑃𝑆𝐴 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑃𝑆𝐴 𝑚 ℎ 𝑑𝑡 =1 5 5 =1 6 6

𝑛

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝑃𝑆𝐴 𝐸𝑥𝑑𝑒𝑠𝑡,𝑃𝑆𝐴𝑑𝑡 =1 Compressor (C-1)

𝑛

𝑛

∫𝑡 𝐶= 1𝑚7ℎ7𝑑𝑡 + ∫𝑡 𝐶= 1𝑃𝐶 ― 1𝑑𝑡 = 𝑛

∫𝑡 𝐶= 1𝑚8ℎ8𝑑𝑡 Ammonia synthesis reactor (ASR)

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝐶= 1𝑚7𝑒𝑥7𝑑𝑡 + ∫𝑡 𝐶= 1𝑃𝐶 ― 1𝑑𝑡 = ∫𝑡 𝐶= 1𝑚8𝑒𝑥8𝑑𝑡 + ∫𝑡 𝑅𝐶 𝐸𝑥𝑑𝑒𝑠𝑡,𝐶 ― 1𝑑𝑡 =1

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝐴𝑆𝑅 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝐴𝑆𝑅 𝑚 ℎ 𝑑𝑡 = =1 8 8 = 1 14 14 ∫𝑡 𝐴𝑆𝑅 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝐴𝑆𝑅 𝑄 𝑑𝑡 =1 9 9 = 1 𝐴𝑆𝑅

∫𝑡 𝐴𝑆𝑅 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝐴𝑆𝑅 𝑚 𝑒𝑥 𝑑𝑡 = =1 8 8 = 1 14 14 𝑇0

𝑄 (1 ― 𝑇 )𝑑𝑡 + ∫𝑡 𝐴𝑆𝑅 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝐴𝑆𝑅 = 1 𝐴𝑆𝑅 =1 9 9 𝑛

∫𝑡 𝐴𝑆𝑅 𝐸𝑥𝑑𝑒𝑠𝑡,𝐴𝑆𝑅𝑑𝑡 =1 Heat exchanger (HX-1)

𝑛

𝑛

∫𝑡 𝑅𝐶 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑅𝐶 𝑚 ℎ 𝑑𝑡 = =1 9 9 = 1 18 18 𝑛

𝑛

∫𝑡 𝑅𝐶 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝑅𝐶 𝑚 ℎ 𝑑𝑡 = 1 10 10 = 1 15 15

𝑛

𝑛

∫𝑡 𝑅𝐶 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑅𝐶 𝑚 𝑒𝑥 𝑑𝑡 = =1 9 9 = 1 18 18 𝑛

𝑛

∫𝑡 𝑅𝐶 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑅𝐶 𝑚 𝑒𝑥 𝑑𝑡 + = 1 10 10 = 1 15 15 𝑛

∫𝑡 𝑅𝐶 𝐸𝑥𝑑𝑒𝑠𝑡,𝐻𝑋 ― 1𝑑𝑡 =1 Compressor (C-2)

𝑛

𝑛

∫𝑡 𝐶= 1𝑚15ℎ15𝑑𝑡 + ∫𝑡 𝐶= 1𝑃𝐶 ― 2𝑑𝑡 = 𝑛

∫𝑡 𝐶= 1𝑚16ℎ16𝑑𝑡 Condenser (CON)

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝑐𝑜𝑛 𝑚 𝑒𝑥 𝑑𝑡 = ∫𝑡 𝑐𝑜𝑛 𝑚 𝑒𝑥 𝑑𝑡 + = 1 16 16 = 1 17 17 𝑛

∫𝑡 𝑐𝑜𝑛 𝑄 𝑑𝑡 = 1 𝑐𝑜𝑛

𝑇0

𝑛

∫𝑡 𝑐𝑜𝑛 𝑄 (1 ― 𝑇 )𝑑𝑡 + ∫𝑡 𝑐𝑜𝑛 𝐸𝑥𝑑𝑒𝑠𝑡,𝑐𝑜𝑛𝑑𝑡 = 1 𝑐𝑜𝑛 =1 𝑛

Throttling valve (TV)

∫𝑡 𝑇𝑉 𝑚 ℎ 𝑑𝑡 = ∫𝑡 𝑇𝑉 𝑚 ℎ 𝑑𝑡 = 1 17 17 = 1 18 18

Separator (SEP)

∫𝑡 𝑆𝐸𝑃 𝑚 ℎ 𝑑𝑡 = ∫𝑡 𝑆𝐸𝑃 𝑚 ℎ 𝑑𝑡 + = 1 10 10 = 1 11 11

𝑛

𝑛

∫𝑡 𝑇𝑉 𝑚 𝑒𝑥 𝑑𝑡 = ∫𝑡 𝑇𝑉 𝑚 𝑒𝑥 𝑑𝑡 + = 1 17 17 = 1 18 18 𝑛

∫𝑡 𝑇𝑉 𝐸𝑥𝑑𝑒𝑠𝑡,𝑇𝑉𝑑𝑡 =1 𝑛

𝑛

𝑛

∫𝑡 𝑆𝐸𝑃 𝑚 ℎ 𝑑𝑡 = 1 14 14 Ammonia fuel cell (AFC)

𝑛

∫𝑡 𝐶= 1𝑚16𝑒𝑥16𝑑𝑡 + ∫𝑡 𝑅𝐶 𝐸𝑥𝑑𝑒𝑠𝑡,𝐶 ― 2𝑑𝑡 =1

∫𝑡 𝑐𝑜𝑛 𝑚 ℎ 𝑑𝑡 = ∫𝑡 𝑐𝑜𝑛 𝑚 ℎ 𝑑𝑡 + = 1 16 16 = 1 17 17

𝑛

𝑛

∫𝑡 𝐶= 1𝑚15𝑒𝑥15𝑑𝑡 + ∫𝑡 𝐶= 1𝑃𝐶 ― 2𝑑𝑡 =

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

𝑛

∫𝑡 𝑆𝐸𝑃 𝑚 𝑒𝑥 𝑑𝑡 = ∫𝑡 𝑆𝐸𝑃 𝑚 𝑒𝑥 𝑑𝑡 + = 1 10 10 = 1 11 11 ∫𝑡 𝑠𝑒𝑝 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝑆𝐸𝑃 𝐸𝑥𝑑𝑒𝑠𝑡,𝑆𝐸𝑃𝑑𝑡 = 1 14 14 =1

𝑛

𝑛

∫𝑡 𝐴𝐹𝐶 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑚 𝑒𝑥 𝑑𝑡 = = 1 12 12 = 1 13 13

𝑛

𝑛

∫𝑡 𝐴𝐹𝐶 𝑚 𝑒𝑥 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑃 𝑑𝑡 + = 1 19 19 = 1 𝑁𝐻3,𝐴𝐹𝐶

∫𝑡 𝐴𝐹𝐶 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑚 ℎ 𝑑𝑡 = = 1 12 12 = 1 13 13 ∫𝑡 𝐴𝐹𝐶 𝑚 ℎ 𝑑𝑡 + ∫𝑡 𝐴𝐹𝐶 𝑃 𝑑𝑡 = 1 19 19 = 1 𝑁𝐻3,𝐴𝐹𝐶

𝑛

∫𝑡 𝐴𝐹𝐶 𝐸𝑥𝑑𝑒𝑠𝑡,𝐴𝐹𝐶𝑑𝑡 =1

34

Journal Pre-proof

Author Contribution Statement Osamah Siddiqui: Conceptualization, Methodology, Writing Original Draft, Validation, Investigation, Ibrahim Dincer: Data curation, Visualization, Formal analysis, Writing- Review and Editing, Supervision, Project administration.

Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof

Highlights  A new ammonia synthesis and fuel cell system integrated with solar energy  Maximum energetic performance in terms of efficiency reaches 15.72%  The peak exergetic efficiency of the solar-based system is determined as 16.55%  Daily discharge time and energy capacity found to reach 8.9 hours and 8502.4 kWh