Waste heat recovery from a 1180 kW proton exchange membrane fuel cell (PEMFC) system by Recuperative organic Rankine cycle (RORC)

Accepted Manuscript Waste heat recovery from a 1180�kW proton exchange membrane fuel cell (PEMFC) system by Recuperative organic Rankine cycle (RORC) Mohamad Alijanpour sheshpoli, Seyed Soheil Mousavi Ajarostaghi, Mojtaba Aghajani Delavar PII:

S0360-5442(18)30966-6

DOI:

10.1016/j.energy.2018.05.132

Reference:

EGY 12967

To appear in:

Energy

Received Date: 8 January 2018 Revised Date:

6 April 2018

Accepted Date: 20 May 2018

Please cite this article as: Alijanpour sheshpoli M, Mousavi Ajarostaghi SS, Delavar MA, Waste heat recovery from a 1180�kW proton exchange membrane fuel cell (PEMFC) system by Recuperative organic Rankine cycle (RORC), Energy (2018), doi: 10.1016/j.energy.2018.05.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Waste Heat Recovery from a 1180 kW Proton Exchange Membrane Fuel Cell (PEMFC) System by Recuperative Organic Rankine Cycle (RORC) 1

Mohamad Alijanpour sheshpoli, 2 Seyed Soheil Mousavi Ajarostaghi, 3, * Mojtaba Aghajani Delavar 1

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Babol Noshirvani University of Technology, Faculty of Mechanical Engineering, Babol, Iran, [email protected] 2 Babol Noshirvani University of Technology, Faculty of Mechanical Engineering, Babol, Iran, [email protected] 3, * Babol Noshirvani University of Technology, Faculty of Mechanical Engineering, Babol, Iran, [email protected]

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Abstract Nowadays due to the permanent increase in energy consumption and its high cost and also nonrenewable energies’ destroying effects, clean technologies such as fuel cells should be supported and used. In present study, waste heat recovery of a low temperature proton exchange membrane fuel cell is surveyed using a recuperative organic Rankine cycle. The PEMFC system is equipped with a metal hydride storage. The key point of present study is that the heat absorbed from stack is utilized for other components in addition to RORC. Thus, different possible cycle configurations according to how cooling water circulates among RORC, pre-heater and metal hydride are thermodynamically analyzed. Furthermore, effects of RORC working fluid mass flow rate and pressure ratio are investigated. Moreover, three RORC working fluids are surveyed including: R245fa, R245ca and R123. Results indicate thermal efficiency rise by increase in RORC working fluid mass flow rate with different gradients. Additionally, pressure ratio increase leads to rise in net output power and thermal efficiency. Models 8 has highest efficiency with 44.3 % and R123 is the best working fluid.

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Keywords: Heat Recovery, Recuperative Organic Rankine Cycle (RORC), Proton Exchange Membrane Fuel Cell (PEMFC), Metal Hydride (MH)

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1. Introduction Nowadays, it is extensively accepted that providing work from equipment without losing some heat as energy waste is not possible. Furthermore, energy consumption has enhanced significantly in last years which makes some economic limitations and environmental problems. The large quantity of 30-40% of fuel energy is wasted and mere 12-25% is converted to usable power [1-3]. All production processes and machine operations also dissipate some waste heat and release it in different ways such as: radiation, exhaust gas or by a cooling fluid. There are many available technologies to recover waste heat. It’s worth mentioning that recovering waste heat not only decreases the demand of fossil fuels, but also reduces the greenhouse gases and as a consequence, this way can keep environment safer for future. Low grade wasted heat is a suitable resource because of its affluence. Among different technologies of heat recovery such as Stirling engines, Thermo-Electric, Micro Rakine Cycle and Inverted Brayton Cycle, organic Rankine cycle (ORC) is the most suitable technology for heat recovery specially in heat recovery of low grade waste heat sources due to some parameters such as progresses in ORC cycle (recuperative organic Rankine cycle (RORC)), fuel and energy rising price, more environmental preservation and also a better energy efficiency course. Consequently, many researches have been focused on this issue nowadays and also many companies have become interested in utilizing this efficient cycle with low grade heat recovery (200-400ºc) [4-7]. The basic reason makes the RORC flexible is that the 50 working fluid can be chosen according to different temperature of heat sink(s) or source(s). Plainly, RORC conversion efficiency is defined as the proportion of consumed heat to generated electricity. Conversion efficiency of recuperated RORC cycles can increase up to the value of 20% in comparison with conventional and professional power generation processes [8,9]. The

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important issue must be noted is that the cycle should be flexible enough because the waste heat is not continuous. Many studies have been carried out to analyze ORC thermodynamically, in which cycle’s configurations, working fluid selection [10-13] and also economic parameters [14-21] were investigated. Some researchers focused on the ORC cycle net power output that can be utilized in current industrial plants existing in the countries of European Union (EU27). Researchers in many countries are trying to remove the limitation for combined heat and power (CHP) processes and some heat recovery remedies have been suggested [22]. ORC systems has been applied to different types of systems including solar thermal power [2326], geothermal power [27-29], industrial waste heat [30-32] and engine exhaust gases [33-35]) to recover waste heat, but few surveys focused on the waste heat recovery from PEMFC power units. PEMFC systems are considered to be very attractive power generation systems, providing efficient electricity generation with quite low environmental impact. Although fuel cell systems can apply high electricity efficiency, they lose pretty much waste heat to the cooling system or exhaust gas. On the other hand, chemical energy in fuel cell system is converted into electricity, the waste energy from fuel cell system is lost as heat (fuel cell cooling system and exhaust gas heat). Some amount of the waste heat can be recovered, thus, the energy efficiency of fuel cell system will significantly improve. Borello et al. [36] utilized a numerical model in which a PEMFC and ORC was used simultaneously and the purpose was to optimize the electric output of the system so it would have a higher system efficiency. The model was tested and both electrical and thermal balance was assessed. Zhao et al. [37] proposed and evaluate a steady-state hybrid power system consisting of a (PEM) fuel cell stack and an organic Rankine cycle (ORC) using thermodynamic laws and mathematical models. It was noticed that the electrical efficiency of the hybrid system was increased by about 5% in comparison with the single PEM fuel cell stack without ORC. Moreover, a dynamic model was proposed for using a 6 kWel ORC and solar thermal collectors together, producing electric energy and heat simultaneously by Calise et al. [38]. A new multigeneration system with an ORC was investigated by Ahmadi et al. [39]. The system included a PEM electrolyzer to produce hydrogen, in addition to combined cooling, heating, and power (CCHP) structure. A parametric analysis was done to show the effects of many impressive design parameters on the exergy and energy efficiencies of the system. As further studies, a fuel cell system for recovering the waste heat through dynamic operations was simulated by Yu et al. [40], including a turbo blower, a membrane humidifier, two cooling circuits, and a PEMFC stack. The study was proposed to evaluate the dynamic response of individual components during the dynamic change of current density. The potential for both heat and power extraction from a PEM fuel cell was investigated experimentally and also by using computer simulation by Shabani et al. [41] to improve the economics of a solar-hydrogen system supplying energy to a remote household. The overall average energy efficiency of the fuel cell was measured to be about 70% by using the heat generated for domestic water heating, compared to only 35–50% for electricity generation alone. Colmenar-Santos [42] also proposed a system that used the waste heat of 12 kW PEMFC (operating temperature around 50°C) into the heating system of the car. The new developed heating system designed integrates the heat generated by the fuel cell into the heating system of the vehicle, reducing the global energy consumption and improving the global efficiency as well. He concluded that a maximum heat of 9.27 kW can be gained by the radiator. Furthermore, Tianqi He et al. [43] investigated a PEMFC waste heat recovery by using ORC. In their survey, two systems were brought to recover waste heat from the PEM stack with a constant operating temperature of 60 C. They determined the best working fluid and the maximum work can be gained and maximum thermal efficiency of recovery system for both proposed configurations. Forde et al. [44] implemented a metal hydride (MH) storage unit in a PEM fuel cell stack. Waste heat generated from fuel cell stack helped hydrogen to release from metal hydride storage.

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2. System Description

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2.1. Recuperative Organic Rankine Cycle (RORC)

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In this study, different configurations of PEMFC heat recovery system using RORC were explored. The Considered PEMFC system is equipped with metal hydride. Also, the effect of some parameters such as RORC working fluid flow rate, RORC turbine pressure ratio and the three types of RORC working fluid is evaluated for different cycle configurations and the results are exhibited. The performance of the hybrid system is measured and depicted by evaluating net output power, power consumption and system efficiency for each configuration. The main purpose of the study is to find the optimal waste heat recovery so it can be used to release hydrogen from metal hydride storage, pre-heat hydrogen and to gain the maximum work from the turbine used in RORC. It should be noticed that there is an air compressor to elevate the air pressure to the PEMFC operating pressure and the waste heat generated in compressor is absorbed by the cooling fluid (water) from the output air.

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Fig. 1 illustrates the RORC cycle schematic and T-S diagram in which stream (1) in a liquid state absorbs the heat from the PEM fuel cell cooling water through an evaporator and the outlet is stream (2) (vapor). Stream (2) enters the turbine (expander) and leaves it in superheated state as (3) then enters the recuperator to exchange heat with stream (6) and leaves it as (4) to enter the condenser. In condenser, stream (4) loses heat to the glycol water and leaves it as (5) in saturated liquid state. Stream (5) gets in the liquid pump to reach the subcritical pressure and leaves it as (6) to enter the recuperator absorbing the (3) stream’s heat to get preheated –to increase the inlet temperature of the evaporator which caused rise in efficiency- before entering the evaporator.

Fig. 1 Cycle schematic and T-S diagram of recuperative organic Rankine cycle (RORC)

2.2. Proposed Heat Recovery Systems Fig. 2 shows the schematics of PEMFC system utilizing RORC to recover the heat wasted in PEM stack. This system includes a PEM fuel cell stack, metal hydride tank, an air compressor, metal hydride heat exchanger (to release the hydrogen from the tank), hydrogen pre-heater heat exchanger (to pre-heat hydrogen to the stack temperature), PEMFC cooling cycle heat exchanger (heat exchanger 1), air heat recovery heat exchanger (heat exchanger 2) and RORC consisting of a pump, a condenser, a recuperator and a turbine to provide work from and some other related components. Waste heat in PEMFC is employed to release hydrogen from metal hydride storage, pre-heat hydrogen and also to provide heat for RORC.

Cycle 3

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Cycle 1

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Fig. 2 cycle schematics of PEMFC heat recovery system using RORC

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According to Fig. 2, the air should be compressed to PEM stack operating pressure by a compressor and then is fed to the cathode side of the stack. Hydrogen in metal hydride tank absorbs some heat to release and then should get pre-heated to the temperature close to the PEM fuel cell stack operating temperature, and then is supplied to the anode side of the fuel cell stack. It should be noted that the closer the temperature is to the operating temperature; the greater net output power can be reached in the fuel cell stack. The DC current is produced by the reaction occurred in the PEM fuel cell stack. In addition to this current, a large amount of heat also gets released simultaneously. This released heat has a good potential to get recovered. Generally, 3 cycles are taken into consideration and according to different proportion of cooling water mass flow rate for each component, nine possible models are considered and evaluated and system performance was accounted to notice which has the best system efficiency. Specifications of different investigated models are listed in Table (1). As an example, models 4 and 5 study have been carried out for cycle 2 in which the difference is water mass flow rate of RORC and MH is equal in model 4, while in model 5 the water mass flow rate dedicated to RORC is two times bigger than the one specified for MH.

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Table (1). Specifications of different investigated models Models Cycle Value m  = m = m  Model 1 1  m = 4 m = 4 m  Model 2 1 m = m  = 2.5 m Model 3 1 m = m Model 4 2 m = 2 m Model 5 2 m = 2 m Model 6 2 m = m Model 7 3 m = 2 m Model 8 3 m = 2 m Model 9 3 Model 10 PEMFC system without RORC

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Table (2) contains parameters explaining the simulation conditions for the PEM fuel system. Performance of the hybrid System can be affected by many several parameters, such as PEM fuel cell operating temperature and pressure, hydrogen flow rate as fuel, air flow rate, turbine inlet and outlet pressure and some of others. Table (2). Simulation parameters for the proposed PEM fuel system

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Parameters

Number of electrons Faraday constant Universal gas constant Ambient temperature Ambient pressure Number of cells in stack Active surface area Limiting current density Membrane thickness Operating pressure of PEM fuel cell Operating temperature of PEM fuel cell stack Stack operating current density Higher heating value of hydrogen Turbine isentropic efficiency

Symbol

Value

ne F R Tamb Pamb Ncell Acell iL L P Tk i HHV ƞturb

2 96,485 C mol-1 8.314 J mol-1 K-1 293.15 K 101.325 kPa 13,000 232 cm2 1.5 A cm-2 0.00254 cm 303.96 kPa 358.15 K 0.6 A cm-2 285.55 kJ mol-1 80%

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Fig. 3. cycle schematics of PEMFC heat recovery system using RORC for different cycle

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The cycle schematics of PEMFC heat recovery system using RORC in Aspen HYSYS are illustrated in Fig. 3. The details of the main component are inserted in the Fig. 3 for different models. As depicted, the heat recovery system has two main cycle including PEMFC system and RORC cycle which are shown in the Fig. 3 with dashed line. The considered cycles in this study include just three cycles (as shown in Figs. 2,3) however the differences between the models are based on the cycle schematics and the inlet and outlets of the mixer and separator.

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2.3. Description of the Investigated Models

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According to Fig. (1) the air gets in compressor and becomes compressed to the operating pressure of the stack. But to cool it down to the operating temperature, it loses its heat to the cooling water streaming in cooling loop system through a heat exchanger (Heat Exchanger 2). On the other hand, the hydrogen as fuel needs some heat to get released from metal hydride tank. As a consequence, it needs some heat provided by hot water in the cooling loop through a heat exchanger. Then, it should be preheated to the closest temperature to the PEM fuel cell stack operating temperature so it has the higher performance (H2 Preheater). After power generation in the stack, the cooling loop cold water will absorb the heat lost in the stack to recover it to work in RORC turbine, and also to release the hydrogen in the metal hydride tank to preheat hydrogen. After heat absorption, hot water exchanges heat with hot air and then divides into two or three stream with different mass flow proportions to carry out three tasks: (1) to release the hydrogen in the tank, (2) to preheat it to a temperature close to the operating temperature, (3) to exchange heat with RORC so it recovers the heat loss to work by RORC turbine. Accordingly, three cycle configurations are considered to recover heat from PEMFC systems. For cycle 1, hot water is divided into three streams so that one of them goes through metal hydride to set hydrogen free, another one is used in H2 preheater and the last one is utilized in RORC. Unlike cycle 1, there is another designed cycle, in which hot water is divided into two streams. One of these streams goes into RORC through an evaporator, then another one enters metal hydride and lastly has to go through H2 preheater. Furthermore, there is one cycle left in which hot water is divided into two streams, one firstly is utilized in RORC, then is conducted to the H2 preheater and the other one directly goes and enters metal hydride.

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- The recovered heat from PEMFC split into three parts: a) ORC cycle to produce power from expander. b) Metal hydride tank to release hydrogen. c) H2 Preheater to set the H2 inlet temperature at the PEMFC temperature. - The ORC cycle used for heat recovery is equipped with a recuperator (called Recuperative Organic Rankine Cycle (RORC)). - The required hydrogen for PEMFC system is provided by metal hydride (MH) so the hydrogen as fuel needs some heat to get released from metal hydride tank. Some of the recovered heat is used to release H2 from MH. - Part of the recovered heat is used to preheat H2 to the PEMFC temperature in H2 preheater. - In addition to heat which is recovered from the PEMFC, the waste heat of inlet air compressor is recovered too. 2.5. Assumption

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Some assumptions were made to ease the investigation of RORC. • Thermodynamically analyzed is performed in steady state system. • There’s no heat loss to the surrounding. • Pressure drops through pipes, condenser and recuperator are negligible. • Pump and turbine have specific given isentropic efficiency. • Mechanical conversion efficiency of Turbine and pump are 100%. There are also some assumptions that were employed to comfort the analysis. • Steady state condition is considered for system. • N2 percentage in the air is 79 vol% and O2 percentage is 21 vol%. • Reaction occurred in the stack reaches equilibrium state. • Pressures at fuel cell stack inlet channels are equal and constant. • Fuel cell stack outlet flows’ temperature have the operating temperature of the fuel cell and it remains constant. • No pressure changes happen in the stack. • An insulation layer was used to prevent heat loss to the environment. • Mechanical conversion efficiency of the using compressor is 100%.

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3. Equations 3.1. PEM fuel cell model The power generated by PEM fuel cell stack is calculated as [45].  =  .  . 

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Where N is the number of stacks and V is the voltage of stack and I is the current density. By calculating the amount of hydrogen in a fuel cell, the theoretical power of fuel cell stack can be specified directly. The consumption rate of hydrogen as well as consumption rate of air is calculated by knowing the stack current density and cell numbers as:  2  =  4

. = 

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(3)

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However, Hydrogen and air flow rate at the inlet should be equal to or a little higher than the using rate of them so as to prevent the fuel cell from membrane degradation and failure [45]. The stoichiometric rate is utilized to solve this problem.

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3.2. Recuperative organic Rankine cycle For PEM fuel cell stack, the net heat energy must be absorbed by a coolant or a cycle to prevent the stack from overheating. In the suggested system, the net heat energy is removed by organic working fluid at constant pressure situation in order to change it to vapor phase. Having the thermodynamic energy balance in PEMFC, net heat energy can be determined as [46]: Q '( = Q )* − Q ,) − Q -..

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Q ,) = Q '( = m(h1 − h23 )

Power generated in RORC turbine is calculated as: W(6 7 = m(h1 − h8 )

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The heat ejected by RORC condenser is:

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Where Qnet is net heat energy, Qch shows chemical energy, Qs,l is for sensible and latent heat. In the stack, the heat transferred to the RORC working fluid is calculated (RORC evaporator):

Q ) = m(h9 − h: )

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The work used in RORC pump to increase the pressure is: W;6<; = m(h= − h: )

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Where > is RORC working fluid mass flow rate and h shows the stream’s enthalpy.

W(6 7 − W;6<; Q @'

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η(*< =

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3.3. Performance criteria The performance of the suggested hybrid power system can be investigated by overall electrical efficiency, the thermal efficiency of RORC cycle, and PEM fuel cell electrical efficiency. On the basis of the first law of thermodynamic, the thermal efficiency in RORC cycle is determined as the ratio of the net power output to the absorbed heat wasted form the PEM stack can be written as: (11)

PEM fuel cell electrical efficiency is calculated as: W,) − W)A<; n.)A'- × HHV

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Overall electrical efficiency is determined as: η.) =

W,) + W(6 7 − W)A<; − W;6<; n.)A'- × HHV

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4. Result and discussion A PEM fuel cell system –containing metal hydride tank, fuel cell stack, cooling loop (to cool down the stack) - integrated with an RORC was taken into consideration to investigate effect of some key parameters on the performance of this hybrid system. The system is under steady state condition and Aspen Hysys software (having thermodynamic properties of many materials data) was used to ease this simulation. 4.1. Validation the thermodynamic modeling of the PEMFC and RORC cycles

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(a) (b) Fig (4). (a) Expander output power (Wexpander) as a function of the expander pressure ratio (rp) (b) Power consumption of pump as a function of the working fluid mass flow rate for R245fa compared with experimental study (Desideri et al [47])

Then the PEM fuel cell stack in which fuel (hydrogen) gets in with floating temperature –as what was utilized in this survey- was taken into consideration and it was noticed that present model can show the fuel inlet temperature effect on the total power generated in stack as it’s shown in Fig. (4a). 0.72

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(c) Fig. (5). (a) Total power generated in PEMFC as a function of hydrogen inlet temperature (b) Simulated PEMFC Polarization curve in comparison with Zhao et al. [37] (c) Comparison of the single cell power output versus current density of present study with Zhao et al. [37]

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According to Fig. (5a), it has been observed in constant air inlet temperature state, the more the hydrogen inlet temperature, the more total power will be generated in the stack. Fig. (5a) shows the dynamic behavior of the PEMFC cycle simulated. The polarization curve and also power output curve was depicted to check whether the simulated PEM fuel cell stack is correct logically and was compared with a parametric analysis of a hybrid system done by Zhao et al. [37]. According to Figs. (5b and 5c), It can be easily noticed that there’s good corresponding between the reference and simulation results. In this study, firstly, impact of different configurations of hybrid cycle is explored and results are depicted. In addition, RORC working fluid mass flow rate is studied in different configurations and results are compared. Furthermore, the effect of pressure ratio in RORC is investigated. Rp changes between 5.5 and 8. Lastly, three different RORC working fluids are used and their performances for different configurations are compared and shown.

4.2. Effects of RORC working fluid mass flow rate

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Fig. (6) shows the efficiency and net output power changes according to RORC different mass flow rates. As it can be observed, power and efficiency increase by mass flow rate increasing with different gradient for each model. For instance, efficiency and net output power of models 1, 4 and 7 are shown in Fig. (6). The trends for other models are the same. As it can be seen, the mass flowrate of RORC working fluid has significant effects on the performance of the system (net output power, consumption power and efficiency) and it is not related to cycle schematics of the proposed system.

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Fig(6). Net power output and efficiency of cycles (models 1, 4 and 7) for different RORC working fluid mass flowrate

4.3. Effects of RORC pressure ratio (rp)

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In this section, the impact of RORC pressure ratio in different models is studied. RORC pressure ratio (rP) changes between 5.5 and 8. In Fig. (7) having different pressure ratio, net output power and system performance for some configurations are surveyed and results are shown. For instance, efficiency and net output power of models 2, 5 and 8 are shown in Fig. (7). The trends for other models are the same. According to Fig. (7), it can be noticed that net output power and cycle performance increases with the rise in pressure ratio as it was expected. Net output power, power consumption and thermal efficiency of the proposed heat recovery system is depicted in Figs (8a, 8b and 8c), respectively in order to compare different configurations at different RORC pressure ratio. As it can be observed, models 1, 2 and 6 have the highest net output power than the others. Accordingly, Model 3 has the lowest quantity. As it is illustrated in Fig. (8a). Net output power increases by small slope according to pressure ratio rise. The power consumption is also depicted in Fig. (8b). According to Fig. (8b). Models 2, 6 and 8 have the highest power consumption in all investigated pressure ratios. It should be noticed that the power consumption in model 6 and 8 are equal due to the equal RORC working fluid mass flow rate, hence, the quantity of transferred thermal power to the RORC’s evaporator. Having equal transferred thermal power, the power consumption for both cycles will be equal.

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Fig. (8c) shows the effect of pressure ratio on performance of the cycles. As it’s shown, the system efficiency increases by pressure ratio increasing. According to Fig. (8c). Models 1, 2, and 6 have the highest efficiency and model 1 has the highest one among them. The difference between model 1, 2 and 6 efficiencies gets more obvious in high pressure ratios and negligible difference can be observed in low pressure ratios.

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4.4. Effects of proposed system cycle schematic

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In this section, the effect of the proposed heat recovery system is thermodynamically investigated. Ten different models of cycle schematic are compared with together. Overly, three cycle schematic are proposed but based on the different mass flowrate of the circulate water through the equipment, nine different models are studied. The different models are compared with output results including system power consumption, net output power and efficiency. In order to focus on the performance of the RORC cycle in the proposed heat recovery system, the results related to RORC cycle (including turbine output power, pump consumed power and efficiency of RORC) are listed in Table 3. Accordingly, between the all models, models 6 and 8 have the highest turbine output power of RORC and model 2 is the next one. Also, models 6 and 8 have the most pump power consumption too. Finally, models 1, 4 and 7 have the most RORC thermal efficiency (by considering the both turbine output power and pump power consumption).

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6.5

85

82

Model Model Model Model Model Model Model Model Model 1 2 3 4 5 6 7 8 9

6

RI PT

5.5

POWER CONSUMPTION (KW)

NET OUTPUT POWER (KW)

1220

43.5

M AN U

42.5 41.5 40.5 39.5

Model Model Model Model Model Model Model Model Model 1 2 3 4 5 6 7 8 9

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(c) Fig. (8). (a) Net output power of models for different pressure ration (b) Power consumption of models for different pressure ration (c) Cycle efficiency of models for different pressure ratio

WT (kW)

WP (kW)

QEvaporator (kW)

GHIHJ (%)

EP

Table (3). Produced power in RORC turbine, consumed power in RORC pump and RORC thermal efficiency for different models at rP=0.16

26.73 53.46 14.43 40.10 26.74 53.73 40.1 53.73 26.74

3.364 6.728 1.817 5.046 3.365 6.76 5.046 6.76 3.365

358.055 716.38 193.416 537.22 358.33 720 537.22 720 358.33

6.525 6.523 6.521 6.525 6.523 6.523 6.525 6.523 6.523

-

-

-

-

Models

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Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 Model 7 Model 8 Model 9 Model 10

Fig. (9a) shows the power consumption for each model. As it’s shown, all models have a higher power consumption than basic model. Among all models, model 2, 6 and 8 have highest power consumption and model 8 has the highest one between these three models by having the quantity of 85 kW. It can be noticed that model 3 has the lowest quantity among others. Net power output for each configuration is illustrated in Fig. (9b). As it’s shown, model 2,6 and 7 have the maximum power output among all models and it’s limited between 1180 to

ACCEPTED MANUSCRIPT

88

1200 1180

86

1160

82

80

1140 1120

SC

Wnet (kW)

84

Wp (kW)

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1200 kW and model 8 has the maximum power generation among all configuration with a little difference to others. On the other hand, it can be noticed that all of the models have a higher output power than the basic model (PEMFC without RORC - model 10). As a result, model 3 has the minimum power output. Both output and power consumption should be considered in exploring a power generation system. To do this, efficiency parameter is used. The efficiency of all models was depicted in Fig. (9c). As it’s shown, all models have higher efficiency than the basic one and among them model 2, 6 and 8 has the highest quantity while model 3 has the lowest one.

1100

78

1080

76 0

1

2

3

4

5

6

7

8

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1060

9

10

1

2

3

4

5

6

7

8

9

10

11

Configuration Number

Configuration Number

(a) 46

44

42

(b)

40

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Efficiency (%)

0

11

38

36

1

EP

0

2

3

4

5

6

7

8

9

10

11

Configuration Number

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(c) Fig (9). (a) Power consumption, (b) Net power output, (c) Efficiency of different models

In order to realize the results, the thermodynamic properties of the different points of the proposed cycle are listed in Table (4). Four first points (1-4) are related to inlet circulate water of RORC evaporator, metal hydride and H2 preheater respectively (as it can be seen in Fig. 2). Point 4 is referring to PEMFC inlet hydrogen which has significant effect on the output power of PEMFC. Points 5-10 are placed in the RORC which can are illustrated in Fig. 2. Model 10 is just the PEMFC system without RORC. All the results, listed in Table (4), are obtained at rP=0.16 (which the maximum and minimum pressure of the RORC are 789 and 126.2 kPa respectively). According to Figs. (1 and 3) and Table (3), the difference between the models with the same cycle configuration is mass flowrate of the circulate water through different equipment which the amounts of mass flowrate for the three considered equipment (RORC evaporator, metal hydride and H2 preheater) are listed in Table (4). The mass flowrate of circulate water in evaporator is more than the mass flow rate of RORC working fluid significantly and because of considering constant pressure ratio, the temperatures of the RORC points don’t change between the models. The most important

ACCEPTED MANUSCRIPT point of the presented results in Table (4) is the temperature of the point 4. As it can be seen, the temperature of the points 4 has different values in different models. On the other hand, different temperature of point 4 means different PEMFC output power in different models which affects the efficiency of the proposed system (Equation (13)). The present simulation of the PEMFC is a dynamic modeling which by changing the temperature of the PEMFC inlet hydrogen and air, the output power and heat loss of the PEMFC change. The dynamic behavior of the present PEMFC model is shown in Fig. 5a.

P (kPa)

AC C

K (LM N OP )

3 358.23 358.23 358.23 345.82 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 22.186 50.528 50.528 0.0187 0.8156 0.8156 0.8156 0.8156 0.8156 0.8156

4 358.23 358.23 358.22 347.2 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 61.639 61.639 61.639 0.0187 2.2653 2.2653 2.2653 2.2653 2.2653 2.2653

5 358.23 358.23 358.23 349.74 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 41.083 82.167 82.167 0.0187 1.5105 1.5105 1.5105 1.5105 1.5105 1.5105

6 358.23 358.23 358.22 344.59 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 82.584 40.667 40.667 0.0187 3.0361 3.0361 3.0361 3.0361 3.0361 3.0361

7 358.23 358.23 356.15 347.19 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 61.638 61.638 61.638 0.0187 2.265 2.265 2.265 2.265 2.265 2.265

SC

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

2 358.23 358.23 358.23 342.09 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 82.167 20.542 20.542 0.0187 3.0195 3.0195 3.0195 3.0195 3.0195 3.0195

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T (K)

Models 1 358.23 358.23 358.23 344.64 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 41.084 41.084 41.084 0.0187 1.5103 1.5103 1.5103 1.5103 1.5103 1.5103

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Point

EP

Properties

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Table (4). Thermodynamic properties of the system different states for rP=0.16 8 358.23 358.23 356.15 349.79 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 82.584 40.667 82.584 0.0187 3.0361 3.0361 3.0361 3.0361 3.0361 3.0361

9 358.23 358.23 356.15 344.64 358.15 316.79 311.27 293.82 294.26 298.15 100 100 100 303 789 126.2 126.2 126.2 789 789 41.083 82.167 41.083 0.0187 1.5105 1.5105 1.5105 1.5105 1.5105 1.5105

4.5. Effects of RORC working fluid In this section the effect of RORC working fluid on performance of the considered systems is investigated and depicted. Three working fluids such as R245fa, R245ca and R123 are used to explore their effects. The thermodynamic properties of the RORC working fluids are listed in Table (5).

10 358.15 303.96 0.0187 -

ACCEPTED MANUSCRIPT Table(5).Thermodynamic properties of the RORC working fluids Working Fluid

Molecular Wight (Kg/Kmol)

Critical Temperature (oC)

Critical Pressure (bar)

Boiling Point Temperature at 1 bar (oC)

R-123 R-245fa R-245ca

152.9 134 134.048

183.67 153.9 174.42

36.62 36.51 39.41

27.84 15.3 25

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SC

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Net output power, net power consumption and performance of the cycle are illustrated in Fig. (10,11). As it’s illustrated in Fig. (10a) all models have higher performance than the case in which RORC is not used and R-123 has the best performance, while, R245fa has the second place. According to Fig. (10a), it can be understood that models 6 and 8 are the best, due to their high performances. Furthermore, model 8 has the maximum performance using R245fa and R245ca, while model 6 has the best performance when R123 is selected as RORC working fluid. All models power consumption is depicted in Fig. (10b). As it’s illustrated in Fig. (10b), using different RORC working fluids, all models have higher power consumption than the case without using RORC. In addition, R245ca and R123 consume the lowest power, while R245fa is determined as high-consumed working fluid between them. Moreover, R123 has a better performance in contrast with R245ca by a little difference. Between all models, model 3 has the lowest power consumption, while model 2, 6 and 8 have the highest quantities. To be detailed, model 6 has the highest using power. 88

1250 R245fa R245ca R123

TE D

W n (kW)

1150

1050 (2)

(3)

EP

1100

(4)

(5)

(6)

(7)

(8)

Configuration Number

(9)

(10)

W p (kW)

86

1200

(1)

R245fa R245ca R123

84

82

80

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

Configuration Number

(a)

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(b) Fig. (10). (a) Net output power of models for different RORC working fluid (b) Power consumption of models for different RORC working fluids

The thermodynamic states of the ROCR cycle points are listed in Table (6) which the simulation results are provided for model 2. In order to have an appropriate compare, both net produced power and power consumption should be compared for each model. The model having the highest produced power, and on the other hand, uses the lowest power, is the best model, thus it has the highest performance. Due to this, efficiency factor is used and showed for each model in Fig. (11). As it’s shown in Fig. (11), the efficiency of all models is higher than the case in which PEMFC doesn’t use RORC. Among all models, model 8 has the best performance with efficiency of about 44.30%, while model 3 has the minimum quantity of 41.90%. Also, R123 is determined as the best working fluid, while R245fa has the second place.

ACCEPTED MANUSCRIPT Table (6). Simulation results for the investigated system with three working fluids for models 8 T (K)

P (kPa)

R-123

1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6

358.15 313.91 308.47 293.95 294.25 298.15 358.15 316.79 311.27 293.82 294.26 298.15 358.15 320.27 317.13 295.54 295.83 298.15

489 78.24 78.24 78.24 489 489 789 126.2 126.2 126.2 789 789 569 91.04 91.04 91.04 569 569

R-245ca

R245fa R245ca R123

48

42 40

TE D

44

EP

Efficiency (%)

46

M AN U

R-245fa

K (LM N OP ) 3.4417 3.4417 3.4417 3.4417 3.4417 3.4417 3.0194 3.0194 3.0194 3.0194 3.0194 3.0194 2.8306 2.8306 2.8306 2.8306 2.8306 2.8306

RI PT

State

SC

Working Fluid

38

AC C

36

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

Configuration Number

Fig. (11). Efficiency of models for different working fluids

Conclusion

In this survey, thermodynamic analysis of waste heat recovery of a PEM fuel cell using a recuperative RORC has been carried out. Hydrogen needed for PEM fuel cell is provided from a metal hydride tank. For this system, different configuration is devised by coupling RORC and PEM fuel cell. Impacts of RORC working fluid mass flow rate, pressure ratio and also the material used as RORC working fluid are studied. Result are given as following: • Power and efficiency increase by mass flow rate rise with different gradient for each model. • Net output power and cycle thermal efficiency increases with the rise in pressure ratio.

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• Three working fluids such as R245fa, R245ca and R123 are used to explore their effects. • All models have higher performance than the case in which RORC is not used and R-123 has the best performance, while, R245fa has the second place. • R245ca and R123 consume the lowest power, while R245fa is determined as highconsumed working fluid between them. • Model 8 has the best performance with efficiency of about 44.30%, while model 3 has the minimum quantity of 41.90%. • Among three investigated fluids for RORC, R123 is the most appropriate, while R245fa has the second place. References

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Highlights  Thermodynamic analysis of recovering waste heat from PEMFC by RORC is

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carried out.  The PEMFC is equipped with metal hydride that using waste heat to release Hydrogen.

 The recovered heat is also utilized in H2 preheater to increase H2 temperature.

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 Possible cycle configurations and different models are investigated.

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 Effects of Three RORC working fluids and operational parameters are surveyed.