Development and Assessment of a Novel Integrated System Using an

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Development and assessment of a novel integrated system using ammonia internal combustion engine and fuel cells for cogeneration purposes Osamah Siddiqui, and Ibrahim Dincer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04323 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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Development and assessment of a novel integrated system using ammonia internal combustion engine and fuel cells for cogeneration purposes 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 this study, a novel ammonia based integrated system, incorporating both ammonia and hydrogen fueled internal combustion engine and ammonia fuel cell system, is developed for cogeneration of power and cooling. The integrated system recovers the waste heat and uses it efficiently to achieve cooling and producing power. The performance of the present system is assessed and evaluated by using thermodynamic energy and exergy approaches. The energy efficiency of the overall cogeneration system is determined to be 59.9% while the exergy efficiency is found to be 51.9%, respectively. Also, the energy efficiency of the ammonia fuel cell is evaluated as 44.4% and the exergy efficiency is found to be 41.7%. Further, the energy and exergy efficiencies of the internal combustion engine are determined to be 45.7% and 43.8%, respectively. Some parametric studies under actual operating conditions are conducted to assess the system performance at varying operating conditions and parameters. The developed system provides a new direction towards improving the performances of ammonia fuel cell systems through system integration, cogeneration and waste heat recovery. Keywords: Ammonia; hydrogen; fuel; fuel cell; cogeneration; power; cooling 1. Introduction Fossil fuel consumption in colossal amounts in the recent past has led to serious concerns about the environmental sustainability of the planet. The usage of carbon based fossils to obtain energy has been proved to be highly detrimental to the environment. For instance, the CO2 emissions arising from energy related activities globally have risen from 23.01 Gt in 2000 to 32.53 Gt in 20171. In order to overcome such detriments, alternative fuels are extensively investigated. Ammonia is considered as a potential fuel which entails promising properties that make it suitable to be utilized in a variety of applications2. Ammonia has a high volumetric density at larger pressures allowing it to be stored in greater amounts space effectively. At a pressure of 8.7 bar, for example, ammonia can be liquefied and stored at a temperature of 20oC. Furthermore, it has a 1 ACS Paragon Plus Environment

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volumetric energy density of 13.6 GJ m-3. In addition, ammonia is considered to be a comparatively cost effective fuel that has three times lower cost of energy as compared to hydrogen3. Moreover, owing to its pungent smell, it is easy to detect a leakage of ammonia. The most common method to produce ammonia is the well-known Haber-Bosh process. However, new ammonia synthesis techniques are being developed4. To produce power by utilizing ammonia fuel, two possible routes comprise of fuel combustion and fuel cell technologies. The combustion of ammonia in an internal combustion engine (ICE) was first utilized several decades earlier when there was a shortage of fossil fuels in Belgium5. Several investigations followed since then to assess the viability of using ammonia in an ICE. Granell et al.6 investigated a spark ignition engine that operates with ammonia. They found that the engine performance with ammonia would be lower than with the usage of gasoline. Nevertheless, ammonia and gasoline blend was found to provide satisfactory improvement in the performance. Ryu et al.7 also investigated an SI engine that was provided with an ammonia injector. The engine performance was investigated and a comparison was made between utilizing ammonia and gasoline blend as fuel and utilizing only gasoline. They investigated the most appropriate time as well as duration for injecting ammonia. They concluded that the brake specific energy consumptions were similar for both types of fuels. However, ammonia was found to effect the combustion temperatures as well as the peak pressure in the engine. These as well as other studies on ammonia based combustion engines determined that ammonia would require a promoter for combustion owing to its comparatively lower flame velocity as well as chemical stability that prevents ignition8. Moreover, dual-fuel engines utilizing ammonia have also been investigated. Boretti9 studied a dual-fuel TDI engine with ammonia-diesel mixture. The engine performance was described to be satisfactory in the view that the engine was capable of achieving nearly as high efficiencies as well as power outputs as an only diesel fueled engine. However, one of the major challenge with the proposed system was the requirement of high pressures in order to achieve ammonia ignition. Also, corrosion of system components, generation of untreated NOx emissions and the gradual reaction rates of ammonia molecules were identified as major challenges. Reiter and Kong10 investigated the duel-fuel compression-ignition type engine with ammonia and diesel fuels. They also studied the associated emissions. Various compositions were tested for the engine. The appropriate fuel mixture range was found to be 4060% diesel. Nevertheless, they also classified the formation of NOx emissions as a challenge of such type of engines. Yapicioglu and Dincer11 assessed the performance of ammonia based dual2 ACS Paragon Plus Environment

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fueled engines. Also, several other fuel blends involving ammonia were investigated. The ammonia-hydrogen blend was reported to be the option resulting in the least amount of greenhouse emissions. An increase in the ammonia content of dual-fuels resulted in a decrease in the power outputs as well as efficiencies. Hence, the combustion reaction of ammonia that is not very favorable was described to be a key challenge in ammonia fueled combustion engines. Various other studies investigating ammonia fuel blends in internal combustion engines have also been conducted12-14. Addition of hydrogen to ammonia in an ICE was found to enhance the ammonia combustion characteristics considerably. Usage of ammonia in internal combustion engines has been found to be advantageous for reducing lubricant loss in engines, corrosion and losses in power. Another way to obtain power by utilizing ammonia as the fuel entails the usage of fuel cells. Electrochemical anodic and cathodic reactions occurring in fuel cell technologies allow them to be utilized for power generation with ammonia. Various types of ammonia fuel cell technologies have been investigated. Shy et al.15 recently investigated an ammonia fueled solid oxide fuel cell (SOFC). The cell was tested with high gas pressures and temperatures. They concluded that elevating operating pressures and temperatures result in higher power densities. In addition, the SOFC was found to have similar power densities when tested with hydrogen fuel. Thus, signifying that ammonia dissociates satisfactorily at temperatures above 750oC. Further, several studies on SOFC based ammonia fuel cells were conducted in the recent past that investigated and confirmed the viability of its usage in high temperature fuel cells16-20. Moreover, another promising type of direct ammonia fuel cell (DAFC) technology includes alkaline electrolyte based fuel cells. In this type of DAFC, ammonia is oxidized electrochemically at the anode in the presence of catalysts to generate power, and ammonia dissociation to hydrogen and nitrogen is not required. In our previous studies, we investigated solid anion exchange membrane as well as molten alkaline electrolytes for DAFC21-23. These type of DAFC systems are specifically attractive because they allow the operation at low temperatures and do not necessitate high temperatures for ammonia dissociation. However, in our previous studies we found that operating DAFC at low temperatures results in comparatively higher amounts of unreacted ammonia fuel at the exit and hence recommended the development of new integrated ammonia fuel cell based systems that can better utilize the unreacted fuel.

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Furthermore, having comparatively better thermophysical properties than other refrigerants and entailing no global warming potentials, ammonia is also considered to be a promising refrigerant for cooling applications. In addition, favorable solubility property of ammonia in water is utilized in absorption cooling systems, which operate by taking in heat input and provide cooling. Various studies have been conducted on ammonia refrigerant based cooling systems. Ezzat and Dincer24 developed and investigated an integrated solar and geothermal based multigeneration system. They utilized an ammonia-water absorption cooling system that operated by using the waste heat in the power generation cycle. The energy efficiency of the system was reported as 69.6% and the exergy efficiency was found to be 42.8%. Higa et al.25 investigated an integrated system entailing an ammonia-water power generation system and absorption cooling system. The study was conducted particularly for an industrial plant. The performance of the system was assessed through energy and exergy analyses. The exergy destruction rate was found to decrease by 43.3% by the usage of the integrated system. Also, the coefficient of performance was reported to increase from 0.256 to 0.376. Further, energy efficiency was found to be 36% and the exergy efficiency was observed to be 46.8%. The integrated system was found to provide better outputs and results as compared to the conventional system. Yin et al.26 developed a new ammonia-water based cogeneration system that provides cooling and power. The system was designed to be driven through low grade waste heat. The system was assessed through energy and exergy analyses. The exergy efficiency of the developed system was reported to be 26.2%. In addition, the developed system was found to have an energy efficiency of 17.5%. Also, the exergy destruction rates were evaluated and were found to be highest in the waste heat recovery heat exchanger. Zhang et al.27 investigated a modified Kalina cycle that was designed for providing both cooling and power. The boiler heat was utilized to provide the required cooling effect. Further, the output power as well as the cooling effect were designed to be variable by varying the split fractions. The system performance was assessed in terms of the power recovery efficiency, which was evaluated to be 20.24% greater than the conventional Kalina cycle. Although studies have been conducted on the development of ammonia based power generation and refrigeration systems, no efforts were directed towards the development of ammonia based cogeneration systems utilizing direct ammonia fuel cells. Specifically, in order to overcome the challenges associated with anion exchange membrane based direct ammonia fuel cells that we reported in our previous studies21-23, it is essential to develop new integrated systems that allow to 4 ACS Paragon Plus Environment

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achieve the maximum utilization and thus higher overall energy and exergy efficiencies. Hence, in this study we develop and investigate a novel ammonia based cogeneration system for power and cooling that uses ammonia fuel cell and an internal combustion engine. The specific objectives of this study are (i) developing a novel ammonia based cogeneration system for power and cooling utilizing ammonia fuel cell and internal combustion engine, (ii) investigating the system performance through thermodynamic energy and exergy tools, (iii) conducting a parametric study to investigate the effects of varying system parameters and operating conditions on the system efficiencies. 2. System description The developed ammonia fueled cogeneration system is depicted in Figure 1. The system outputs comprise of cooling and power. Ammonia enters the system at a pressure of 870 kPa and temperature of 20oC (State 1). The input gas is passed to the ammonia electrolysis cell (AEC) and the ammonia fuel cell (FC). The AEC is utilized to dissociate ammonia (NH3) into its constituents comprising of hydrogen and nitrogen gas. The hydrogen gas is formed via ammonia dissociation to be used in the internal combustion engine (ICE). In the ICE, the reactant fuels comprise of ammonia and hydrogen with a molar ratio of 10:1. Hydrogen is used in the ICE to promote the ammonia combustion process. An ammonia-hydrogen blend of this composition has been proved to provide better combustion speeds as well as ICE performance28-30. The ICE power output is one of the useful outputs of the system. Furthermore, the FC is also fed with ammonia (state 25) for power generation. The FC system comprises of an anion exchange membrane based direct ammonia fuel cell (DAFC) technology. In our previous studies on DAFC, we found that one of the key challenges associated with ammonia fuel cells comprises of unreacted fuel21, because ammonia is a relatively stable molecule. Thus, in the present study we develop a novel integrated system that utilizes the unreacted ammonia fuel and its properties to provide cooling as well as power generation. The unreacted fuel leaving the FC at state 13 enters a storage reservoir that stores the unreacted ammonia in water. The formed ammonia-water mixture is pumped to the desorber via a heat exchanger. It enters the heat exchanger at state 15 and is heated to a higher temperature (state 16) before it enters the desorber. The hot stream in the heat exchanger comprises of the weak solution that is at a higher temperature and lower ammonia content. It enters the heat exchanger at state 17, heats the colder incoming stream and exits the heat exchanger at state 18. Further, before returning to the storage reservoir, the pressure P18 is dropped to P19. The heat 5 ACS Paragon Plus Environment

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required to separate the ammonia-water mixture in the desorber is obtained from the ICE exhaust gases. Once the mixture is separated, the strong ammonia solution at state 7 exits the desorber and is passed through the regenerator (REG) to separate remaining water contents. Moreover, the strong solution enters the condenser at state 9 where it rejects heat to reach a lower temperature state 10. After passing through the condenser, a throttle valve is utilized to drop the pressure significantly from 1555.8 kPa (state 10) to 244.9 kPa (state 11), thus achieving the low temperature required to provide the cooling effect in the evaporator. After leaving the evaporator, the strong solution entailing an ammonia to water mass ratio of 99.96% is allowed to enter the ICE at state 12. The exhaust gases of the ICE exit at state 5 and are allowed to transfer heat to the desorber. After leaving the desorber at state 6, the high temperature and thermal content of the exhaust gas is utilized to operate a steam Rankine cycle. Water enters the pump P2 as saturated liquid at a pressure of 75 kPa at state 21 where its pressure is raised to 15 MPa (state 22). The water is then passed through heat exchanger 2 (HX2) where it absorbs the incoming heat from the ICE exhaust gases to reach the superheated state 23. This superheated steam is utilized to generate power through a turbine generator (TR). Once the steam leaves the turbine at state 24, it is allowed to pass through the condenser where it rejects heat to reach the initial state (state 21) of the steam Rankine cycle. Hence, the novel ammonia based integrated system is designed to produce power via internal combustion engine, direct ammonia fuel cell as well as steam Rankine cycle and achieve the required cooling effect by employing an ammonia based refrigeration system. The significance of the developed system lies in multi-fold advantages. Firstly, the developed system utilizes unreacted ammonia fuel exiting a DAFC, hence providing better fuel utilization. Further, the waste heat entailed with the exhaust gases of the ICE is utilized effectively, thus increasing the overall efficiencies. In addition, the absorption and desorption properties of ammonia-water mixture are also used to achieve cooling, thus further increasing the overall system efficiencies. 3. Thermodynamic analyses The developed system, its subsystems and each of their components are analysed thermodynamically. Energy as well as exergy analysis tools are utilized for this purpose and to determine the overall efficiencies of the system. In this analysis, the ambient pressure is considered as 101 kPa and the ambient temperature is considered as 25oC. Furthermore, adiabatic operation is considered for all pumps and turbines. Moreover, steady state system operation is assumed. 6 ACS Paragon Plus Environment

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Also, potential and kinetic energy changes are considered to be negligible. In addition, the isentropic efficiencies of the pumps and turbines are taken to be 80% and negligible pressure losses in the heat exchangers are considered. For a control volume system, the conservation of mass principle can be expressed as ∑𝑖𝑚𝑖 = ∑𝑒𝑚𝑒 +

𝑑𝑚𝑐𝑣

(1)

𝑑𝑡

Furthermore, the energy rate balance for a control volume can be obtained from the first law of thermodynamics:

(

𝑄 + ∑𝑖𝑚𝑖 ℎ𝑖 +

𝑉2𝑖 2

)

(

+ 𝑔𝑍𝑖 = 𝑊 + ∑𝑒𝑚𝑒 ℎ𝑒 +

𝑉2𝑒 2

)

+ 𝑔𝑍𝑒 +

𝑑𝐸𝑐𝑣 𝑑𝑡

(2)

Any real process entails irreversibilities that result in entropy generation. Hence, it is essential to assess the entropy and exergy aspects of energy systems. For a control volume system, the entropy generation and the exergy destruction rates can be evaluated respectively from: 𝑄𝑘

∑𝑖𝑚𝑖𝑠𝑖 + ∑𝑘𝑇 + 𝑆 𝑘

𝑔𝑒𝑛

= ∑𝑒𝑚𝑒𝑠𝑒 +

𝑑𝑆𝑐𝑣

(3)

𝑑𝑡

∑𝑖𝑚𝑖𝑒𝑥𝑖 + 𝐸𝑥𝑄 = ∑𝑒𝑚𝑒𝑒𝑥𝑒 + 𝐸𝑥𝑤 + 𝐸𝑥𝑑

(4)

The detailed analysis of each subsystem is described in the proceeding sections. 3.1 Internal combustion engine (ICE) In the ICE, the fuel reactants comprise of ammonia and hydrogen with an ammonia to hydrogen mole ratio 10:1. These enter the ICE at states 12 and 3 respectively. Further, air enters the ICE at state 4 at ambient conditions. The combustion reaction for ammonia is expressed as 3

3

𝑁𝐻3 + 4(𝑂2 + 3.76𝑁2)→2𝐻2𝑂 +

6.64 2

(5)

𝑁2

There are other combustion equations proposed in the literature for ammonia. However, the above equation is utilized in the present study as complete combustion is considered with stoichiometric air. This type of ammonia combustion does not result in any unreacted products and only nitrogen and water vapor are formed28, 35. Nevertheless, other combustion equations can also be investigated to study their effects on the overall system performance. 7 ACS Paragon Plus Environment

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Moreover, the combustion reaction for hydrogen is written as 1

𝐻2 + 2(𝑂2 + 3.76𝑁2)→𝐻2𝑂 +

3.76 2

(6)

𝑁2

The first step in a thermodynamic analysis comprises of the mass balance. For the ICE, the mass rate balance is expressed as (7)

𝑚12 + 𝑚3 + 𝑚4 = 𝑚5

Secondly, the energy balance is significant to quantify the energy inputs and outputs. On a rate balance, this can be expressed for the ICE as (8)

𝑁12ℎ12 + 𝑁3ℎ3 + 𝑁4ℎ4 = 𝑁5ℎ5 + 𝑊𝐼𝐶𝐸 + 𝑄𝑐𝑙 + 𝑄𝑙𝑢𝑏

where 𝑁 represents the molar flow rate, ℎ represents the specific molar enthalpy, 𝑊𝐼𝐶𝐸 is the ICE power output and 𝑄𝑐𝑙 and 𝑄𝑙𝑢𝑏 denote the heat transfer and losses to the cooling system and lubrication respectively. The heat losses to the lubrication system are considered to be 10% of the ICE power output31. The entropy and exergy balances are necessary to quantify the irreversibilities in the system and locate their hot-spots. For the ICE, the entropy rate balance is expressed as 𝑄𝑐𝑙

𝑄𝑙𝑢𝑏

(9)

𝑁12𝑠12 + 𝑁3𝑠3 + 𝑁4𝑠4 + 𝑆𝑔𝑒𝑛,𝐼𝐶𝐸 = 𝑁5𝑠5 + 𝑇𝑐𝑙 + 𝑇𝑙𝑢𝑏

where 𝑠 represents the specific molar entropy, 𝑆𝑔𝑒𝑛,𝐼𝐶𝐸 denotes the entropy generation rate in the ICE, 𝑇𝑐𝑙 represents the temperature of the engine cooling system and 𝑇𝑙𝑢𝑏 represents the lubricant temperature. The exergy rate balance equation for the ICE is expressed as

(

𝑇0

)

(

𝑇0

)

𝑁12𝑒𝑥12 + 𝑁3𝑒𝑥3 + 𝑁4𝑒𝑥4 = 𝑁5𝑒𝑥5 + 𝑊𝐼𝐶𝐸 + 𝑄𝑐𝑙 1 ― 𝑇𝑐𝑙 + 𝑄𝑙𝑢𝑏 1 ― 𝑇𝑙𝑢𝑏 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝐼𝐶𝐸 (10) where 𝑒𝑥 represents the specific molar exergy and 𝐸𝑥𝑑𝑒𝑠𝑡,𝐼𝐶𝐸 denotes the exergy destruction rate in the ICE. The system parameters utilized in the analysis of the ammonia-hydrogen fueled ICE are summarized in Table 1. 3.2 Ammonia electrolysis cell (AEC) The ammonia gas entering the AEC is dissociated into hydrogen and nitrogen gas: 8 ACS Paragon Plus Environment

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(11)

𝑁𝐻3→0.5𝑁2 +1.2𝐻2

In the AEC, the rate of hydrogen produced can be expressed as a function of the current density ( 𝐼𝐴𝐸𝐶) and cell area (𝐴) as 𝑁𝐻2,𝐴𝐸𝐶 = 𝑁3 =

𝐼𝐴𝐸𝐶𝐴

(12)

𝑛𝐹

where 𝑛 denotes the number of electrons (3) participating in the electrochemical reaction of ammonia dissociation. In this regard, the Nernst voltage for the ammonia electrolysis can be evaluated as32 𝑉𝐴𝐸𝐶,𝑁 =

(

𝑅𝑇𝐴𝐸𝐶 𝑛𝐹

3

1

)

2 ln 𝑃𝐻 𝑃2 + 2 𝑁2

∆𝐺0

(13)

𝑛𝐹

where 𝑇𝐴𝐸𝐶 represents the AEC temperature, 𝑃 denotes the partial pressure, 𝑅 is the gas constant and ∆𝐺 is the change in Gibbs free energy that can be evaluated as (14)

∆𝐺 = ∆𝐻 ― 𝑇∆𝑆

where ∆𝐻 and ∆𝑆 are the change in the enthalpy and entropy respectively between the reactants and products. When the electrochemical cell is operated at a given current density, the electrolysis voltage needs to be evaluated by adding the activation, concentration and Ohmic losses in the cell: 𝑉𝐴𝐸𝐶 = 𝑉𝐴𝐸𝐶,𝑁 + 𝑉𝑎𝑐𝑡,𝐴𝐸𝐶 + 𝑉𝑐𝑜𝑛𝑐,𝐴𝐸𝐶 + 𝑉𝑂ℎ𝑚,𝐴𝐸𝐶

(15)

The total activation loss in the cell is evaluated from the cathodic and anodic activation losses: (16)

𝑉𝑎𝑐𝑡,𝐴𝐸𝐶 = 𝑉𝑎𝑐𝑡,𝑎𝑛,𝐴𝐸𝐶 + 𝑉𝑎𝑐𝑡,𝑐𝑎,𝐴𝐸𝐶 where the activation loss at each electrode is determined as 𝐼𝐴𝐸𝐶

𝑅𝑇

𝑉𝑎𝑐𝑡,𝑖,𝐴𝐸𝐶 = 𝑛𝐹sinh ―1 (2𝐼0,𝑖)

(17)

where 𝑖 denotes anode or cathode, 𝐼𝐴𝐸𝐶 represents the current density and 𝐼0,𝑖 denote the anodic or cathodic exchange current density. Further, the total concentration loss also includes the cathodic and anodic concentration losses that can be evaluated as31 𝑅𝑇

(

𝐼𝐴𝐸𝐶𝑅𝑇𝛿𝑎𝑛

𝑉𝑐𝑜𝑛𝑐,𝑎𝑛 = 𝑛𝐹ln [ 1 + 𝑛𝐹𝐷

1 2

)]

(18)

0 𝑒𝑓𝑓𝑃𝑁2


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1+ 𝑅𝑇

𝑉𝑐𝑜𝑛𝑐,𝑐𝑎 = 𝑛𝐹ln [

1―

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𝐼𝐴𝐸𝐶𝑅𝑇𝛿𝑐𝑎 𝑛𝐹𝐷𝑒𝑓𝑓𝑃0 𝐻2

(19)

]

𝐼𝐴𝐸𝐶𝑅𝑇𝛿𝑐𝑎 𝑛𝐹𝐷𝑒𝑓𝑓𝑃0 𝑁𝐻3

where 𝐷𝑒𝑓𝑓 denotes the effective coefficient of gas diffusion. Equations (18) and (19) are dependent on the limiting current density that is a function of the effective diffusion coefficient as36 𝐶𝑏

(20)

𝐽𝐿 = 𝑛𝐹𝐷 𝛿

where 𝐹 denotes the Faraday’s constant, 𝐷 represents the diffusion coefficient and 𝐶𝑏 is the specie concentration that can also be represented through partial pressures as depicted in equations (18) and (19). In the present study, a limiting current density of 21000 A m-2 is considered31. The Ohmic losses in the AEC are evaluated from: 𝑉𝑂ℎ𝑚,𝐴𝐸𝐶 = (𝜌𝑒𝑙𝛿𝑒𝑙 + 𝜌𝑐𝑎𝛿𝑐𝑎 + 𝜌𝑎𝑛𝛿𝑎𝑛)𝐼𝐴𝐸𝐶

(21)

where 𝛿 represents the thickness, 𝜌 denotes the resistivity and 𝑒𝑙 is denoting electrolyte. Thus, the power input to the AEC can be evaluated as (22)

𝑊𝐴𝐸𝐶 = 𝑉𝐴𝐸𝐶𝐼𝐴𝐸𝐶𝐴𝐴𝐸𝐶

where 𝐴 denotes the cell area utilized. Important parameters of the AEC system are listed in Table 2. A liquid electrolyte-based AEC is considered in the present study, further details can be found in References33-34. The required power input to the AEC is provided by an external battery source (BT). 3.3 Ammonia fuel cell (FC) The direct ammonia fuel cell comprises of an alkaline electrolyte based system. Ammonia enters the anodic side of the fuel cell at state 27. Furthermore, oxygen (O2) and water (H2O) molecules are input at the cathodic side of the cell. The half-cell electrochemical reaction that takes place at the fuel cell anode can be expressed as 2𝑁𝐻3 +6𝑂𝐻 ― →𝑁2 +6𝐻2𝑂 + 6𝑒 Further, the H2O and O2 molecules are utilized at the cathode to react electrochemically as 10 ACS Paragon Plus Environment

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Energy & Fuels

1.5𝑂2 +3𝐻2𝑂 + 6𝑒→6𝑂𝐻 ― The overall reaction for the FC can be expressed as 3 2𝑁𝐻3 + 𝑂2→𝑁2 + 3𝐻2𝑂 2 For the direct ammonia fuel cell, the open circuit theoretical voltage can be evaluated from 3

(𝑃𝑁𝐻3)2(𝑃𝑂2)2 𝑅𝑇 𝑉𝐹𝐶,𝑁 = 𝑉0 + 𝑛𝐹ln [ ] (𝑃𝐻2𝑂)3(𝑃𝑁2)

(23)

where 𝑛 denotes the number of electrons transferred, 𝑃 represents the partial pressure, and 𝑉0 is the reversible voltage that can be determined as ∆𝐺

𝑉0 = 𝑛𝐹

(24)

where ∆𝐺 is the change in Gibbs free energy as described in equation (14) As discussed earlier, in an electrochemical cell when a current is drawn, several losses including activation, concentration and Ohmic losses occur. The ammonia fuel cell voltage can be determined by subtracting these losses from 𝑉𝐹𝐶,𝑁 : (25)

𝑉𝐹𝐶 = 𝑉𝐹𝐶,𝑁 ― 𝑉𝑜ℎ𝑚,𝐹𝐶 ― 𝑉𝑎𝑐𝑡,𝐹𝐶 ― 𝑉𝑐𝑜𝑛𝑐,𝐹𝐶 where the activation voltage losses in the fuel cell are evaluated as 𝑅𝑇

𝐼𝐹𝐶

(26)

𝑉𝑎𝑐𝑡,𝐹𝐶 = 𝑐𝑛𝐹ln (𝐼0,𝐹𝐶)

where 𝐼𝐹𝐶 represents the current density in the fuel cell, 𝐼0,𝐹𝐶 is the exchange current density and 𝑐 denotes the transfer coefficient. Further, the concentration polarization losses in the fuel cell can be determined from 𝑅𝑇

𝐼𝐿

(27)

𝑉𝑐𝑜𝑛𝑐,𝐹𝐶 = 𝑐𝑛𝐹ln (𝐼𝐿 ― 𝐼𝐹𝐶) Moreover, the Ohmic losses in the fuel cell can be evaluated as

(28)

𝑉𝑜ℎ𝑚,𝐹𝐶 = 𝐼𝐹𝐶𝑅𝐹𝐶 11 ACS Paragon Plus Environment

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Page 12 of 34

where 𝑅𝐹𝐶 is the total Ohmic resistance in the ammonia fuel cell. Important system parameters are listed in Table 3. The fuel cell power output is evaluated as (29)

𝑊𝐹𝐶 = 𝑉𝐹𝐶𝐼𝐹𝐶𝐴𝐹𝐶 where 𝐴𝐹𝐶 represents the cell area. 3.4 Steam Rankine cycle

The steam Rankine cycle obtains the required heat input by absorbing heat from the ICE exhaust gases through heat exchanger 2 (HX2). The energy balance can be applied per unit time on the HX2 respectively as (30)

𝑚6ℎ6 + 𝑚22ℎ22 = 𝑚20ℎ20 + 𝑚23ℎ23

Further, the entropy generation and exergy destruction rates for the HX2 can be determined respectively from 𝑚6𝑠6 + 𝑚22𝑠22 + 𝑆𝑔𝑒𝑛,𝐻𝑋2 = 𝑚20𝑠20 + 𝑚23𝑠23

(31)

𝑚6𝑒𝑥6 + 𝑚22𝑒𝑥22 = 𝑚20𝑒𝑥20 + 𝑚23𝑒𝑥23 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝐻𝑋2

(32)

After absorbing heat from the HX2, steam enters the turbine (TR) to generate power. The power output of the TR can be determined by applying the energy balance: (33)

𝑚23ℎ23 = 𝑊𝑇𝑅 + 𝑚24ℎ24

Moreover, the entropy generation rate and the exergy destruction rate in the turbine are determined respectively from: 𝑚23𝑠23 + 𝑆𝑔𝑒𝑛,𝑇𝑅 = 𝑚24𝑠24

(34)

𝑚23𝑒𝑥23 = 𝑊𝑇𝑅 + 𝑚24𝑒𝑥24 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝑇𝑅

(35)

After leaving TR, water is passed through the condenser of the Rankine cycle where it rejects heat and exits at state 21 to enter the pump 2 (P2). The heat rejection, entropy generation and exergy destruction rates in the condenser can be determined respectively from: (36)

𝑚24ℎ24 = 𝑄𝐶𝑂𝑁2 + 𝑚21ℎ21 12 ACS Paragon Plus Environment

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Energy & Fuels

𝑄𝐶𝑂𝑁2

(37)

𝑚24𝑠24 + 𝑆𝑔𝑒𝑛,𝐶𝑂𝑁2 = 𝑚21𝑠21 + 𝑇𝐶𝑂𝑁2

(

𝑇0

)

𝑚24𝑒𝑥24 = 𝑚21𝑒𝑥21 + 𝑄𝐶𝑂𝑁2 1 ― 𝑇𝐶𝑂𝑁2 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝐶𝑂𝑁2

(38)

The flow rates, pressures, temperatures and all other thermophysical properties of steam in the Rankine cycle at different states can be found in Table 4. 3.5 Ammonia-water cooling system The present study employs an ammonia-water based cooling system that presents a novel way to utilize the unreacted ammonia coming from the FC system. The unreacted ammonia fuel exits the FC at state 13 and enters the ammonia-water storage tank (STR). An ammonia-water mixture with a composition of 39.8 wt% ammonia (state 14) exits the STR that is pumped to a higher pressure and preheated before entering the desorber via HX1. The energy, entropy and exergy rate balance equations for HX1 can be written as 𝑚15ℎ15 + 𝑚17ℎ17 = 𝑚16ℎ16 + 𝑚18ℎ18

(39)

𝑚15𝑠15 + 𝑚17𝑠17 + 𝑆𝑔𝑒𝑛,𝐻𝑋1 = 𝑚16𝑠16 + 𝑚18𝑠18

(40)

𝑚15𝑒𝑥15 + 𝑚17𝑒𝑥17 = 𝑚16𝑒𝑥16 + 𝑚18𝑒𝑥18 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝐻𝑋1

(41)

In the desorber, the ICE exhaust gases transfer heat to the ammonia-water mixture to vaporize the absorbed water molecules. The energy, entropy and exergy rate balance equations for the desorber can be written as 𝑚5ℎ5 + 𝑚16ℎ16 + 𝑚8ℎ8 = 𝑚6ℎ6 + 𝑚7ℎ7 + 𝑚17ℎ17

(42)

𝑚5𝑠5 + 𝑚16𝑠16 + 𝑚8𝑠8 + 𝑆𝑔𝑒𝑛,𝑑𝑒𝑠 = 𝑚6𝑠6 + 𝑚7𝑠7 + 𝑚17𝑠17

(43)

𝑚5𝑒𝑥5 + 𝑚16𝑒𝑥16 + 𝑚8𝑒𝑥8 = 𝑚6𝑒𝑥6 + 𝑚7𝑒𝑥7 + 𝑚17𝑒𝑥17 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝑑𝑒𝑠

(44)

Primarily ammonia with small percentage of water exits the desorber and is passed through the regenerator (REG) to remove the remaining water content. The rate balance equations can be applied to REG as follows: (45)

𝑚7ℎ7 = 𝑚8ℎ8 + 𝑚9ℎ9 + 𝑄𝑅𝐸𝐺 13 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

𝑄𝑅𝐸𝐺

(46)

𝑚7𝑠7 + 𝑆𝑔𝑒𝑛,𝑅𝐸𝐺 = 𝑚8𝑠8 + 𝑚9𝑠9 + 𝑇𝑅𝐸𝐺

(

Page 14 of 34

𝑇0

)

𝑚7𝑒𝑥7 = 𝑚8𝑒𝑥8 + 𝑚9𝑒𝑥9 + 𝑄𝑅𝐸𝐺 1 ― 𝑇𝑅𝐸𝐺 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝑅𝐸𝐺

(47)

Ammonia is passed through the condenser (CON 1) to extract the excess heat before entering the throttle valve (TV) where the pressure and thus the temperature is reduced considerably. The balance equations for the CON 1 are similar to Eqs. (35)-(37) described earlier. However, the throttling process is analysed through energy, entropy and exergy approaches respectively as follows: 𝑚10ℎ10 = 𝑚11ℎ11

(48)

𝑚10𝑠10 + 𝑆𝑔𝑒𝑛,𝑇𝑉 = 𝑚11𝑠11

(49)

𝑚10𝑒𝑥10 = 𝑚11𝑒𝑥11 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝑇𝑉

(50)

After leaving the TV, low temperature ammonia is passed through the evaporator (EVAP) where the required cooling effect is provided. The cooling effect provided by the evaporator as a function of the mass flow rate and enthalpies of the inlet and outlet streams can be analysed from the energy balance equation: (51)

𝑚11ℎ11 + 𝑄𝐶𝑜𝑜𝑙,𝐸𝑉𝐴𝑃 = 𝑚12ℎ12

Moreover, the entropy generation and exergy destruction rates in the evaporator can be evaluated from: 𝑚11𝑠11 +

𝑄𝐶𝑜𝑜𝑙,𝐸𝑉𝐴𝑃 𝑇𝐸𝑉𝐴𝑃

(52)

+ 𝑆𝑔𝑒𝑛,𝐸𝑉𝐴𝑃 = 𝑚12𝑠12

(

𝑇0

)

𝑚11𝑒𝑥11 + 𝑄𝐶𝑜𝑜𝑙,𝐸𝑉𝐴𝑃 1 ― 𝑇𝐸𝑉𝐴𝑃 = 𝑚12𝑒𝑥12 + 𝐸𝑥𝑑𝑒𝑠𝑡,𝐸𝑉𝐴𝑃

(53)

The thermophyscial property values evaluated at all state points given above can be obtained from Table 4. 3.6 Efficiencies It is essential to determine and assess the efficiencies of the overall system as well as the subsystems. The energy efficiency and exergy efficiency of the ICE can be evaluated as 14 ACS Paragon Plus Environment

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Energy & Fuels

𝜂𝑒𝑛,𝐼𝐶𝐸 =

𝑊𝐼𝐶𝐸

(54)

𝑁𝑁𝐻3,𝑖𝑛𝐿𝐻𝑉𝑁𝐻3 + 𝑁𝐻2,𝑖𝑛𝐿𝐻𝑉𝐻2

𝜂𝑒𝑥,𝐼𝐶𝐸 = 𝑁

𝑊𝐼𝐶𝐸 𝑁𝐻3,𝑖𝑛𝑒𝑥𝑁𝐻3

(55)

+ 𝑁𝐻2,𝑖𝑛𝑒𝑥𝐻2

where 𝐿𝐻𝑉 and 𝑒𝑥 denote the molar lower heating value and specific molar exergy respectively. Furthermore, the energy and exergy efficiencies of the SRC is expressed as 𝜂𝑒𝑛,𝑆𝑅𝐶 =

𝑊𝑇𝑅

(56)

𝑄𝑖𝑛 𝑊𝑇𝑅

𝜂𝑒𝑥,𝑆𝑅𝐶 =

𝑄𝑖𝑛(1 ―

(57)

𝑇0

𝑇)

where 𝑄𝑖𝑛 is the heat input to the SRC that can be evaluated from equation (29) and 𝑇 represents the average temperature of streams 6 and 20. Moreover, the energy and exergy efficiencies of the AEC is evaluated as 𝜂𝑒𝑛,𝐴𝐸𝐶 =

𝑁𝐻2𝐿𝐻𝑉𝐻2

(58)

𝑊𝐴𝐸𝐶 + 𝑁𝑁𝐻3𝐿𝐻𝑉𝑁𝐻3

𝜂𝑒𝑥,𝐴𝐸𝐶 = 𝑊

𝑁𝐻2𝑒𝑥𝐻2

𝐴𝐸𝐶

(59)

+ 𝑁𝑁𝐻3𝑒𝑥𝑁𝐻3

where 𝑁𝐻2 represents the molar outlet flow of hydrogen. Further, the efficiencies of the FC are determined as 𝜂𝑒𝑛,𝐹𝐶 =

𝑊𝐹𝐶

(60)

𝑁𝑁𝐻3,𝑖𝑛𝐿𝐻𝑉𝑁𝐻3

𝜂𝑒𝑥,𝐹𝐶 = 𝑁

𝑊𝐹𝐶

(61)

𝑁𝐻3,𝑖𝑛𝑒𝑥𝑁𝐻3

where 𝑁𝑁𝐻3,𝑖𝑛 represents the ammonia fuel input to FC. The energetic and exergetic coefficient of performance (COP) of the cooling subsystem can be evaluated as 𝑄𝐸𝑉𝐴𝑃

𝐶𝑂𝑃𝑒𝑛 = 𝑄

(62)

𝑖𝑛,𝑑𝑒𝑠


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

𝑄𝐸𝑉𝐴𝑃

𝐶𝑂𝑃𝑒𝑥 =

(

𝑇0 𝑇𝐸𝑉𝐴𝑃

(

) )

Page 16 of 34

― 1 𝑇0

𝑄𝑖𝑛,𝑑𝑒𝑠 1 ― 𝑇

(63)

𝑑𝑒𝑠

Lastly, the overall system energy efficiency and exergy efficiencies can be evaluated as the ratio of the total useful system outputs to the required input: 𝜂𝑒𝑛,𝑜𝑣 =

𝑊𝐼𝐶𝐸 + 𝑊𝐹𝐶 + 𝑊𝑇𝑅 + 𝑄𝐸𝑉𝐴𝑃

𝑊𝐼𝐶𝐸 + 𝑊𝐹𝐶 + 𝑊𝑇𝑅 + 𝑄𝐸𝑉𝐴𝑃

𝜂𝑒𝑥,𝑜𝑣 =

(64)

𝑁𝑁𝐻3𝐿𝐻𝑉𝑁𝐻3 + 𝑊𝐴𝐸𝐶

(

𝑇0 𝑇𝐸𝑉𝐴𝑃

)

―1

(65)

𝑁𝑁𝐻3𝑒𝑥𝑁𝐻3 + 𝑊𝐴𝐸𝐶

4. Results and discussion The ammonia based integrated cogeneration system developed in the present study is analysed thermodynamically. The ambient temperature and pressure are considered as 25oC and 101 kPa respectively. The evaluated specific exergy, entropy and enthalpy values along with the associated temperatures and pressures are listed in Table 4. Engineering Equation Solver (EES) is utilized to obtain the thermophysical properties and conduct the energy and exergy analyses. A summary of the system results is given in Table 5. The overall system energy efficiency is evaluated to be 59.9% and the overall system exergy efficiency is determined as 51.9%. The overall efficiencies of the present system are higher than the efficiencies reported by Ezzat and Dincer31, where an ammonia-hydrogen internal combustion engine based system was developed. They found the energy efficiency of the system to be 31.1% and the exergy efficiency to be 28.94%. The lower values can be attributed to the usage of thermoelectric generators. Although thermoelectric generators allow the generation of electricity from waste heat, they are entailed with substantially low efficiencies. Moreover, the ammonia-water based cogeneration system developed by Yin et al.26 was evaluated to have an energy efficiency of 17.5% and an exergy efficiency of 26.2%. The comparatively lower efficiencies can be attributed to the usage of low grade waste heat. In the present study, high grade waste heat is utilized, hence attaining higher overall efficiencies. Furthermore, the energy efficiency of the SRC is found as 25.5% and the exergy efficiency is evaluated as 38.2%. In addition, the AEC system is found to have an energy efficiency of 93.9% and an exergy efficiency of 90.9%. Also, for the ammonia fuel cell system the energy efficiency is evaluated as 44.4% and the exergy efficiency is determined to be 41.7%. Further, the energy 16 ACS Paragon Plus Environment

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Energy & Fuels

and exergy efficiencies of the ICE are found to be 45.7% and 43.8% respectively. The energetic COP of the cooling system is evaluated to be 0.54 and the exergetic COP is found to be 0.29. The exergy destruction rate is observed to be higher for the ICE subsystem that is found to be 1308 kW. Moreover, the HX2 is also found to have a high exergy destruction rate of 997.6 kW. The AEC and TR are determined to have exergy destruction rates of 136.1 kW and 377.5 kW respectively. Similar trend was reported Ezzat and Dincer31 where the ICE was found to have comparatively highest exergy destruction rate and the AEC was reported to have the lowest exergy destruction rates. Furthermore, a parametric study is conducted to analyse the effect of varying system parameters on the efficiencies of the overall system as well as the subsystems included. 4.1 Effect of ambient temperature on the system performance The effect of the ambient temperature on the overall system and SRC energy and exergy efficiencies is depicted in Figure 2. As the ambient temperature increases from 0 to 40oC, the energy efficiency of the overall system is observed to rise from 59.4% to 60.2% respectively. Furthermore, the overall system exergy efficiency increases from 50.9% to 52.5% respectively for the same rise in ambient temperature. This increase in energy efficiency is due to the usage of ambient air in the ICE. Further, the increase in exergy efficiency can be attributed to the decrease in entropy generation in the ICE with the increasing ambient temperature. In addition, the exergy efficiency of the SRC is observed to increase from 36.6% to 39.1% for an ambient temperature increase from 0 to 40oC. However, the energy efficiency remains constant with varying ambient temperature. These results are in conjunction with the change in exergy efficiencies reported by other studies with the change in ambient temperature. Siddiqui and Dincer37 found the exergy efficiency of the steam Rankine cycle to change nearly by about 8% for a 36oC rise in temperature. In the present study, a rise of 2.5% is observed for nearly the same temperature rise. The difference in the efficiency increases between the two studies can be attributed to the difference in the hat source temperatures of the Rankine cycle. Also, other studies31, 35 have reported similar increasing trends in exergy efficiencies with ambient temperature. Hence, this shows the significance of applying exergy analyses on energy systems. Exergy analyses allows the assessment of a given energy system and provides more valuable information. The subsystems that utilize ambient air during their operation are the ICE and the FC. The effect of ambient temperature on their efficiencies is depicted in Figure 3. The ICE energy efficiency is found to rise from 45.2% to 46% 17 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

as the ambient temperature increases from 0 to 40oC. For the same temperature rise, the ICE exergy efficiency is observed to increase from 43.3% to 44% respectively. The rise in the efficiencies of the overall system can also be attributed to an increase in the ICE efficiencies as the ICE work output exceeds other subsystems. The FC efficiencies are found to decrease with an increase in the ambient temperature as can be observed from Figure 3. However, as the FC system has a lower impact on the overall efficiencies compared to other subsystems, the decline in their efficiencies does not affect the overall system significantly. 4.2 Effect of ICE outlet temperature on the system performance The ICE outlet temperature is an important system parameter that effects the overall system performance. Its effect on the efficiencies of the ICE and the overall system is depicted in Figure 4. The overall system energy efficiency decreases from 65.9% at an outlet temperature of 500oC to 57.9% at an outlet temperature of 700oC. Also, the exergy efficiency decreases from 57.6% at 500oC to 50% at 700oC. This can be attributed to the decrease in the work output of the ICE with a rise in the outlet temperature. The specific enthalpies of the exhaust gases increase with temperature. When the enthalpy of the products of a combustion process increases, the work output decreases as described by Eq. (8). Hence, the overall efficiencies are observed to reduce with increasing exhaust temperatures. This is further elucidated through the ICE efficiencies depicted in Figure 4. The ICE energy efficiency is observed to decrease from a value of 51.6% at 500oC to 43.8% at 700oC. Also, the ICE exergy efficiency decreases from 49.4% to 41.9 % as the temperature increases from 500oC to 700oC. Thus, the usage of optimum operating parameters is suggested according to the required system performance. 4.3 Effect of ammonia-water mass flow rate and evaporator exit temperature on the system performance In the present study, the energetic COP of the cooling system is found to be 0.54 and the exergetic COP is determined to be 0.29. Another study24 utilizing ammonia-water based cooling system also found comparable coefficients of performance for the cooling system. They reported an energetic COP of 0.67 and an exergetic COP of 0.25. Although the energetic COP of the cooling system is lower in the present study, the exergetic COP is higher. This can be attributed to the difference in temperatures of the heat source providing the input heat to the desorber. Furthermore, other parameters also effect the performance of the cooling system as well as the overall integrated system. The mass flow rate of ammonia-water mixture (𝑚14) in the system is one such important 18 ACS Paragon Plus Environment

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Energy & Fuels

system parameter. The effect of 𝑚14 on the cooling provided by the evaporator, heat required by the desorber and the exergy destruction rate in both system components is depicted in Figure 5. At a mass flow rate of 5 kg s-1, the cooling provided by the evaporator is evaluated to be 732.9 kW. If the mass flow rate is increased to 15 kg s-1, a cooling effect of 2198.6 kW can be achieved. However, the heat input required by the desorber is 1339.6 kW for a cooling effect of 732.9 kW and this increases to 4018.8 kW for a cooling effect of 2198.6 kW. Moreover, the exergy destruction rates occurring in the evaporator and the desorber as a function of 𝑚14 are also shown in Figure 5. The exergy destruction rate in the evaporator is evaluated to be 211.9 kW at a mass flow rate of 5 kg s-1, this increases to 635.7 kW for a mass flow rate of 15 kg s-1. Furthermore, the exergy destruction rate in the desorber is found to be 538.3 kW for a cooling effect of 732.9 kW and 1615 kW for a cooling effect of 2198.6 kW. Hence, it is suggested to utilize the mass flow rates according to the cooling effect required, considering the exergy destruction rates in the system components. System parameters that provide the required useful outputs and entail low exergy destructions should be utilized accordingly. Also, the evaporator exit temperature is another important system parameter. Figure 6 depicts the effect of the evaporator exit temperature on the cooling effect, ICE efficiencies and overall system energy and exergy efficiencies. The ICE is considered in this parametric study as the evaporator exit is connected to the ICE inlet. Hence, the temperature of ammonia leaving the evaporator effects both the ICE as well as the evaporator. At an evaporator exit temperature of -5 oC, the cooling effect provided by the evaporator is 1360.2 kW. The cooling effect reduces to 1168.2 kW at a lower exit temperature of -20oC. Further, the ICE energy efficiency is found to be 45.7% at 5oC evaporator exit temperature, which decreases to 45.6% at an exit temperature of -20oC. In addition, the ICE exergy efficiency reduces from 43.8% to 43.7% for the same temperature change. Moreover, the energy and exergy efficiencies of the overall system are found to change from 60.2% to 59.3% and 51.9% to 51.8% respectively as the evaporator exit temperature changes from -5oC to -20oC. Thus, it is recommended to use evaporator temperatures that provide higher efficiencies and lower exergy destruction rates considering the cooling effect required. 4.4 Effect of SRC steam flow rate and turbine inlet pressure on the system performance In the present study, the SRC energy efficiency is evaluated to be 25.5% at the operating conditions specified in Table 4. Also, the exergy efficiency is found to be 38.2%. These values are relatable 19 ACS Paragon Plus Environment

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to other studies in the literature that have utilized steam Rankine cycles in integrated systems. For instance, the study mentioned in Ref. (37) on solar based integrated system with ammonia fuel cell reported a steam Rankine cycle energy efficiency of 20.5% and exergy efficiency of 55.9%. The higher exergy efficiency obtained in their study can be attributed to various reasons including heat source or sink temperatures as well as operating pressures and flow rates. Hence, the effect of the molar flow rate of steam in the SRC on the efficiencies is investigated which is depicted in Figure 7. As the molar flow rate increases from 0.1 kmol s-1 to 0.5 kmol s-1, the energy efficiency of the SRC reduces from 34.7% to 17.1%. Also, the SRC exergy efficiency decreases from 51.9% to 25.6% for the same change in the molar flow rate. Furthermore, the overall energy efficiency is evaluated as 62.7% for a steam molar flow rate of 0.1 kmol s-1 and is observed to reduce to 57.3% if a molar flow rate of 0.5 kmol s-1 is utilized. The overall system exergy efficiency is observed to change from 54.5% to 49.5% as the steam molar flow rate varies from 0.1 to 0.5 kmol s-1. The drop in efficiencies with increasing steam flow rate can be attributed to the decrease in the specific enthalpy of state 23. As the steam flow rate increases, ℎ23 decreases for a given heat input coming into the Rankine cycle from the ICE exhaust gases. Thus, the mass flow rate of steam in the SRC should be chosen in a way that it provides the required turbine power output as well as optimum efficiencies. Furthermore, the steam pressure at the turbine inlet is also an important system parameter that effects the system efficiencies. The effect of turbine inlet pressure on the overall efficiencies and the SRC efficiencies is shown in Figure 8. The overall system energy efficiency increases marginally from 59.5% to 59.9% as the turbine inlet pressure increases from 10000 kPa to 15000 kPa respectively. Further, the exergy efficiency of the overall system also increases from 51.5% to 51.9% for the same pressure change. In addition, the energy efficiency and exergy efficiency of the SRC are determined to be 24.2% and 36.1% respectively at an inlet pressure of 10000 kPa. However, at a higher inlet pressure of 15000 kPa, the SRC energy efficiency is observed to increase to 25.5% and the exergy efficiency is observed to increase to 38.2%. Hence, it is recommended to analyse the optimum turbine inlet pressure that would provide maximum efficiencies and would also be cost-effective. 4.5 Effect of AEC operating pressure on the system performance


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Energy & Fuels

The ammonia electrolysis cell (AEC) is an important system component. The energy efficiency of the AEC in the developed system is found to be 93.9% and the exergy efficiency is found to be 90.9%. These values are comparable to a previous study31, where the energy efficiency of the ammonia electrolysis system was reported to be 83.2% and the exergy efficiency was found to be 81.5%. The higher efficiencies found in the present study can be attributed to the favorable operating conditions chosen. One such key parameter includes the AEC operating pressure. Thus, the effect of AEC pressure on the exergy destruction rates of the AEC as well as the ICE is studied that is depicted in Figure 9. The exergy destruction rate occurring in the AEC is observed to decrease as the operating pressure increases. As the operating pressure increases from 200 kPa to 500 kPa, the exergy destruction rate in the AEC decreases from 141.3 kW to 119.9 kW. However, the exergy destruction in the ICE increases with increasing AEC pressures. At an operating AEC pressure of 200 kPa, the exergy destruction rate in the ICE is found to be 1304 kW. However, at a higher operating pressure of 500 kPa, the exergy destruction rate in the ICE is observed to be 1320 kW. This can be attributed to the change in specific exergy of hydrogen gas that exits the AEC and enters the ICE. Thus, it is recommended to utilize system operating parameters that minimize the exergy destructions and hence the irreversibilities in the system. 5. Conclusions In this study, we develop and investigate a novel ammonia based cogeneration system for power and cooling. The system incorporates a direct ammonia fuel cell technology as well as an internal combustion engine fueled with ammonia and hydrogen. In addition, waste heat is utilized to achieve cooling as well as operate a steam Rankine cycle. The developed system provides an efficient way to enhance the current ammonia based power generation systems. The overall energy efficiency of the cogeneration system is evaluated to be 59.9% and exergy efficiency is determined to be 51.9%. Further, the ammonia fuel cell efficiencies are determined to be 44.4% energetically and 41.7% exergetically. The ICE energy efficiency is found to be 45.7% while the corresponding exergy efficiency becomes 43.8%. Further studies on increasing the efficiencies of ammonia fuel cells should be conducted. Also, it is recommended to investigate the integration of hightemperature ammonia fuel cells such as solid oxide fuel cells with ammonia-based cooling systems. This will entail the utilization of waste heat and aid in achieving higher overall energy and exergy efficiencies. Lastly, the developed system should be studied for a specific application 21 ACS Paragon Plus Environment

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such as transportation or stationary power generation according to the required power demands and hence the corresponding fuel inputs.

Nomenclature A area (m2) BT battery COP coefficient of performance D diffusion coefficient specific exergy (kJ kg-1) 𝑒𝑥 𝐸𝑉𝐴𝑃 evaporator exergy rate (kW) 𝐸𝑥 F Faradays constant g gravitational constant G Gibbs free energy (J) specific enthalpy (kJ kg-1) ℎ molar specific enthalpy (kJ mol-1) ℎ H enthalpy (kJ) HX heat exchanger I current density (A m-2) 𝐿𝐻𝑉 molar lower heating value (kJ) mass flow rate (kg s-1) 𝑚 molar fow rate (mol s-1) 𝑁 pressure (kPa) 𝑃 heat transfer rate (kW) 𝑄 s specific entropy (kJ kg-1 K-1) S entropy (kJ K-1) 𝑆𝑔𝑒𝑛 entropy generation rate (kW K-1) SRC secondary Rankine cycle TR turbine TV throttle valve temperature (oC) 𝑇 V velocity (m s-1), voltage (V) work rate (kW) 𝑊 Z height Greek letters efficiency 𝜂 Subscript act activation an anode AEC ammonia electrolysis cell AFC ammonia fuel cell 22 ACS Paragon Plus Environment

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Energy & Fuels

ca cl con conc cv des dest e el en ex EVAP FC gen HX i ICE lub N Ohm ov SRC TR TV 0

cathode coolant condenser concentration control volume desorber destroyed exit electrolyte enery exergy evaporator fuel cell generation heat exchanger inlet internal combustion engine lubricant Nernst Ohmic overall secondary Rankine cycle turbine throttle valve dead state

References (1) Global Energy & CO2 Status Report, International Energy Agency (IEA). Available from: https://www.iea.org/geco/emissions/. (2) Zamfirescu, C; Dincer, I. Using Ammonia as a Sustainable Fuel. J. Power Sources 2008, 185, 459-465. (3) Kramer, D.A. US Geological Survey, Mineral and Commodities Summaries, January 2007. (4) Dincer, I; Zamfirescu, C. Ammonia as a potential substance. Sustain. Energy Syst. Appl., Boston, MA: Springer US; 2011, 203–232. (5) E.A. Brohi. Ammonia as fuel for internal combustion engines? An evaluation of the feasibility of using nitrogen-based fuels in ICE. Gothenburg, Sweden: Chalmers University of Technology; 2014. (6) Grannell, SM; Assanis, DN; Bohac, SV; Gillespie, DE. The fuel mix limits and efficiency of a stoichiometric, ammonia, and gasoline dual fueled spark ignition engine. J Eng Gas Turbines Power 2008, 130, 42802. 23 ACS Paragon Plus Environment

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(7) Ryu, K; Zacharakis-Jutz, G.E; Kong, S.C. Effects of gaseous ammonia direct injection on performance characteristics of a spark-ignition engine. Appl. Energy 2014, 116, 206–215. (8) Valera-Medina, A; Xiao, H; Owen-Jones, M; David, W.I.F; Bowen, P.J. Ammonia for power. Prog. Energy Comb. Sci. 2018, 69, 63-102. (9) Boretti, A. Novel dual fuel diesel-ammonia combustion system in advanced TDI engines, Int. J. Hydrogen Energy 2017, 42, 7071-7076. (10) Reiter, A. J; Kong, S.C. Combustion and emissions characteristics of compression-ignition engine using dual ammonia-diesel fuel, Fuel 2011, 90, 87-97. (11) Yapicioglu, A; Dincer, I. Performance assessment of hydrogen and ammonia combustion with various fuels for power generators, Int. J. Hydrogen Energy 2018, 43, 21037-21048. (12) Reiter, A.J; Kong, S.C. Demonstration of compression-ignition engine combustion using ammonia in reducing greenhouse gas emissions, Energy Fuels 2008, 22, 2963-2971. (13) Xiao, H; Valera-Medina, A; Bowen, P. J. Modeling Combustion of Ammonia/Hydrogen Fuel Blends under Gas Turbine Conditions. Energy Fuels 2017, 31, 8631-8642. (14) Reiter, A. J; Kong, S.C. Demonstration of Compression-Ignition Engine Combustion Using Ammonia in Reducing Greenhouse Gas Emissions. Energy Fuels 2008, 22, 2963-2971. (15) Shy, S.S; Hsieh; S.C; Change, H.Y. A pressurized ammonia-fueled anode-supported solid oxide fuel cell: Power performance and electrochemical impedance measurements. J. Power Sources 2018, 396, 80-87. (16) Ni, M; Leung, D. Y.C; Leung, M.K.H, Thermodynamic analysis of ammonia fed solid oxide fuel cells: Comparison between proton-conducting electrolyte and oxygen ion-conducting electrolyte. J. Power Sources 2008, 83, 682-686. (17) Ni, M; Leung, D.Y.C; Leung, M.K.H, Mathematical modeling of ammonia-fed solid oxide fuel cells with different electrolytes, Int. J. Hydrogen Energy 2008, 33, 5765-5772. (18) Maffei; N., Pelletier; L; Charland, J.P; McFarlan, A. A Direct Ammonia Fuel Cell Using Barium Cerate Proton Conducting Electrolyte Doped With Gadolinium and Praseodymium. Fuel Cells 2007, 7, 323-328. (19) Ni, M. Thermo-electrochemical modeling of ammonia-fueled solid oxide fuel cells considering ammonia thermal decomposition in the anode. Int. J. Hydrogen Energy 2011, 33, 3153-3166. (20) Farhad, S; F. Hamdullahpur, Conceptual design of a novel ammonia-fuelled portable solid oxide fuel cell system. J. Power Sources 2010, 195, 3084-3090. (21) Siddiqui, O; Dincer, I, Investigation of a New Anion Exchange Membrane-based Direct Ammonia Fuel Cell System. Fuel Cells 2018, 18, 379-388. (22) Siddiqui, O; Dincer, I, A novel hybrid ammonia fuel cell and thermal energy storage system. Int. J. Energy Res., 2018.


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(23) Siddiqui, O; Dincer, I, Experimental investigation and assessment of direct ammonia fuel cells utilizing alkaline molten and solid electrolytes. Energy 2019,169, 914-923. (24) Ezzat, M.F; Dincer, I, Energy and exergy analyses of a new geothermal–solar energy based system. Solar Energy 2016, 134, 95-106. (25) Higa, M; Yamamoto; E.Y., Oliveira; J.C.D; Conceição, W.A.S. Evaluation of the integration of an ammonia-water power cycle in an absorption refrigeration system of an industrial plant. Energy Conv. Mgmt, 2018, 178, 265-276. (26) Yin , J; Yu, Z; Zhang, C, Tian, M; Han, J. Thermodynamic analysis of a novel combined cooling and power system driven by low-grade heat sources. Energy 2018, 156, 319-327. (27) Zhang, S; Chen, Y; Wu, J; Zhu, Z. Thermodynamic analysis on a modified Kalina cycle with parallel cogeneration of power and refrigeration. Energy Conv. Mgmnt, 2018, 163, 1-12. (28) Frigo, S; Gentili, R. Analysis of the behaviour of a 4-stroke Si engine fuelled with ammonia and hydrogen. Int. J. Hydrogen Energy, 2013, 38, 1607–1615. (29) Mørch, C. S; Bjerre, A; Gøttrup, M.P; Sorenson, S.C.; Schramm, J. Ammonia/hydrogen mixtures in an SI-engine: engine performance and analysis of a proposed fuel system. Fuel 2011, 90, 854–864. (30) Li, J; Huang, H; Kobayashi, N; He, Z; Nagai, Y. Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation. Int. J. Energy Res. 2014, 38, 1214–1223. (31) Ezzat, M.F; Dincer, I. Development and assessment of a new hybrid vehicle with ammonia and hydrogen. App. Energy 2018, 219, 226-239. (32) Dong, B.X; Tian; H; Wu, Y; Bu, F; Liu, W; Teng, Y; Diao, G. Improved electrolysis of liquid ammonia for hydrogen generation via ammonium salt electrolyte and Pt/Rh/Ir electrocatalysts. Int. J. Hydrogen Energy 2016, 41, 14507-14518. (33) Mulder, F. Electrolytic cell for the production of ammonia. US patent, 2014, Patent no. 20160194767 A1. (34) Palaniappan, R. Improving the efficiency of ammonia electrolysis for hydrogen production. Ph.D. thesis, 2013, Ohio University. (35) Ezzat, M.F; Dincer, I. Development, analysis and assessment of fuel cell and photovoltaic powered vehicles. Int. J. Hydrogen Energy 2018, 43, 968-978. (36) Siddiqui, O; Dincer, I. Development of a novel hybrid regenerative-electrode ammonia fuel cell and battery system. Energy Conv. Mgmt. 2019, 181, 476-484. (37) Siddiqui, O; Dincer, I, Analysis and performance assessment of a new solar-based multigeneration system integrated with ammonia fuel cell and solid oxide fuel cell-gas turbine combined cycle. J. Power Sources 2017, 370, 138-154.


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Energy & Fuels

Table 1 Important system parameters of ammonia-hydrogen fueled ICE System parameter Operating pressure level Ammonia input flow rate Hydrogen mass fraction Air fuel ratio (stoichiometric) Auto ignition temperature Lower heating value

Value 250 kPa 1.21 kg s-1 1% Ammonia: 6.06 (kg air kg fuel-1) Hydrogen: 34.2 (kg air kg fuel-1) Ammonia: 651 oC Hydrogen: 571 oC Ammonia: 317.56 kJ mol-1 Hydrogen: 240 kJ mol-1

Sources: [28-31] Table 2 Important system parameters of ammonia electrolysis system System parameter Input ammonia pressure (kPa) Input ammonia temperature (oC) Thickness of electrolyte (cm) Thickness of cathode (cm) Thickness of anode (cm) Operating pressure (kPa) Operating temperature (oC) Exchange current density (A m-2) Cell area (m2) Current density (A m-2) Sources: [33-34]

Value 870 20 0.004 0.002 0.002 250 25 0.37 1 2500

Table 3 Important system parameters of direct ammonia fuel cell System parameter Current density Electrolyte Limiting current density Fuel utilization Exchange current density Sources: [21-23]

Value 20.3 A m-2 Anion exchange membrane 65 A m-2 50% 0.4 A m-2


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Table 4 Evaluated and input thermophysical properties of all state points State no.

Substance

Pressure (kPa)

Temperat ure (oC)

1 2 3 4

Ammonia Nitrogen Hydrogen Nitrogen Oxygen Nitrogen Water vapor Nitrogen Water vapor Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Ammonia/water Nitrogen, Water vapor Water Water Water Water Ammonia

870 250 250 101

20 30 30 25

250

651

250

505.7

1555.8 1555.8 1555.8 1555.8 244.9 244.9 244.9 244.9 1555.8 1555.8 1555.8 1555.8 244.9 250

107.3 107.3 44.1 40.0 -14.1 -10.0 30 40 40.4 110.0 130.3 40.4 40.7 351.3

75 15000 15000 75 870

91.8 92.7 250 91.8 20

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Molar or mass flow rate (kg s-1 or *kmol s1) 0.0047* 0.0023* 0.007* 0.21* 0.056* 0.25* 0.11* 0.25* 0.11* 1.32 0.11 1.21 1.21 1.21 1.21 1.21 8.80 8.80 8.80 7.59 7.59 7.59 0.25* 0.11* 0.20* 0.20* 0.20* 0.20* 0.14*


Specific Enthalpy (kJ kg1 or *kJ kmol-1)

Specific Entropy (kJ kg-1 K-1 or *kJ mol-1 K-1)

-46115* 145.3* 144.6* 19008* -238664* 14398* -249611* 1544.1 260.2 1297.3 190.8 190.8 1258.8 1361.3 -43.3 -40.2 302.6 396.6 -0.72 -0.72 9641* -261243* 6926* 7205* 19559* 17505* -46115*

174.3* 184.5* 123.5* 191.5* 205.1* 218.0* 155.1* 212.6* 142.2* 4.88 1.34 4.18 0.66 0.75 4.86 5.22 0.472 0.477 1.46 1.64 0.53 0.54 205.8* 125.6* 21.85* 21.86* 49.85* 50.84* 174.3

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Table 5 Summary of system results System result Turbine power output (𝑊𝑇𝑅) ICE power output (𝑊𝐼𝐶𝐸) FC power output (𝑊𝐹𝐶) Cooling provided (𝑄𝐸𝑉𝐴𝑃) Heat input to SRC (𝑄𝑖𝑛) Heat input to desorber (𝑄𝑖𝑛,𝑑𝑒𝑠) AEC power input( 𝑊𝐴𝐸𝐶)

Value 1848 kW 11145 kW 19.7 kW 1290 kW 7244 kW 2389 kW 1.85 kW

Major exergy destruction rates

AEC: 136.1 kW HX 2: 997.9 kW ICE: 1308 kW Turbine: 377.5 kW

Efficiencies

Overall energy (𝜂𝑒𝑛,𝑜𝑣): 59.9% Overall exergy (𝜂𝑒𝑥,𝑜𝑣): 51.9% SRC energy (𝜂𝑒𝑛,𝑆𝑅𝐶): 25.5% SRC exergy (𝜂𝑒𝑥,𝑆𝑅𝐶): 38.2% AEC energy (𝜂𝑒𝑛,𝐴𝐸𝐶): 93.9% AEC exergy (𝜂𝑒𝑥,𝐴𝐸𝐶): 90.9% FC energy (𝜂𝑒𝑛,𝐹𝐶): 44.4% FC exergy (𝜂𝑒𝑥,𝐹𝐶): 41.7% ICE energy (𝜂𝑒𝑛,𝐼𝐶𝐸): 45.7% ICE exergy (𝜂𝑒𝑥,𝐼𝐶𝐸): 43.8%


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Figure 1 Schematic representing the developed ammonia fueled cogeneration system


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Energy & Fuels

Figure 2 Effect of ambient temperature on the overall and Rankine cycle energy and exergy efficiencies

Figure 3 Effect of ambient temperature on the ICE and FC energy and exergy efficiencies


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Figure 4 Effect of ICE outlet temperature on the ICE and overall system energy and exergy efficiencies

Figure 5 Effect of ammonia-water flow rate on the evaporator and desorber


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Energy & Fuels

Figure 6 Effect of evaporator exit temperature on the cooling effect, ICE efficiencies and the overall system efficiencies

Figure 7 Effect of steam molar flow rate on the energy and exergy efficiencies of SRC and the overall system


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Figure 8 Effect of turbine inlet pressure on the energy and exergy efficiencies of SRC and the overall system

Figure 9 Effect of AEC pressure on the exergy destruction rates occurring in the AEC and ICE


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Development and Assessment of a Novel Integrated System Using an

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