Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation)

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Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation) Shoaib Khanmohammadi a, Morteza Saadat-Targhi b, Faraedoon Waly Ahmed c, Masoud Afrand d,e,* a

Department of Mechanical Engineering, Kermanshah University of Technology, Kermanshah, Iran Esfarayen University of Technology, Esfarayen, North Khorasan, Iran c Department of Physics, College of Science, University of Halabja, Iraq d Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam e Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam b

highlights
Thermodynamic and economic feasibility of integrating thermoelectric generators with geothermal and fuel cell system.
R123 as working fluid has the best performance for the conventional and proposed systems.
Employing TEG modules increase the first and second law efficiency of system 2.7% and 2.8%, respectively.
The payback time of investment can change from 1.25 to 25 years regarding TEG module cost.

article info

abstract

Article history:

The present work aim is performance improvement of an integrated geothermal system by

Received 9 September 2019

proposing the integration of organic Rankine flash cycle (ORFC) with the Proton exchange

Received in revised form

membrane fuel cell (PEMFC) and waste heat recovery from condensers using thermoelec-

14 December 2019

tric generator (TEG) modules. To achieve this goal, a novel integrated system is proposed,

Accepted 17 December 2019

thermodynamically modeled, investigated, and compared with the conventional system.

Available online xxx

To assess the performance of proposed system, thermodynamic and economic evaluations are performed. The results indicate that R123 as working fluid, has the best performance

Keywords:

for the conventional and proposed systems. The findings demonstrate that with employing

Organic Rankine flash cycle

TEG modules an increase of 2.7% and 2.8%, for the first and second law efficiencies can be

Proton exchange membrane fuel cell

obtained respectively. Additionally, the results of parametric analysis indicate that how-

Thermoelectric generator

ever the geothermal fluid temperature increment decreases the first and second law effi-

Payback rate

ciencies of the system, it leads to the net output power enhancement. Also, enhancement of the flash vessel pressure ratio increases the first and second law efficiency as well. Additionally, the simple payback method showed that a payback time between 1.25 years and 25 years according to the TEG modules cost can be achieved. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail address: [email protected] (M. Afrand). https://doi.org/10.1016/j.ijhydene.2019.12.113 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Introduction Given the limitations of fossil fuels like environmental pollution, the use of alternative energy sources (renewable energies) is crucial. One of these renewable energy sources is geothermal energy. There are a lot of low-temperature geothermal energy sources in the world. These kinds of energy are accessible, cheap and clean. The advantages of utilizing these sources of energy are continuous electricity, flexibility in contrast to wind and solar energy, clean energy production, increased energy security, reduction of air and water pollutants and the lower consumption of freshwater [1]. In this work, the performance improvement of the geothermal integrated system has been studied by proposing the integration of organic Rankine flash cycle (ORFC) with the Proton exchange membrane fuel cell (PEMFC) and waste heat recovery from condensers using thermoelectric generator (TEG) modules. A novel integrated system is introduced, thermodynamically modeled, investigated, and compared with the conventional system in terms of performance optimizations and economic evaluations. The present work is organized as follows. Firstly, past works in the proposed area is given in the literature section. Secondly, the system description to introduce the studied system is presented. In addition, the main governing equations are given. Finally, the results with related analyses in tables and graph are represented and discussed just before the conclusion to summarize the research achievements.

Literature review Geothermal/organic Rankine flash cycle Research survey shows that the organic Rankine cycle (ORC) is the best cycle for utilizing geothermal energy [2]. Khosravi et al. [3], developed a smart approach for modeling of geothermal organic Rankine cycle. Their study depicted how artificial intelligence methods can meticulously simulate the operation of a complicated renewable energy system. A thermodynamic modeling and optimization analysis of the ORC with a two-phase expander, organic Rankine flash cycle (ORFC), and ORC activated by low-temperature sensible energy is presented by Lee et al. [4]. The results showed that the cycle type or working fluid for the maximum exergy efficiency depended on the source temperature. Also, ORFC showed lower exergy efficiency than ORC, but ORC with two-phase expander showed higher efficiency than ORC, and the organic flash cycle had the potential for the efficient recovery of low-temperature energy source. Kolahi et al. A new approach for optimizing and improving a flash-binary geothermal plant which is using various combinations of zeotropic mixtures as working fluid presented in Ref. [5]. They showed that Pentane containing combinations had better performances. For instance, when the ORC unit is using the mix of Pentane (0.45)/Butane (0.55), the highest output power is gaining: 1376.87 (kW) from the ORC unit. The work for different basic and enhanced integrated geothermal flash and Kalina cycles from the thermoeconomic point of view had been presented by Hassani-Mokarram and Mosaffa [6]. The

considered cycles included different combinations of basic and modified Kalina cycles, basic geothermal single flash cycle, double flash cycle and its enhanced modes. Their results showed that when enhanced double flash/modified Kalina cycle is used, generated power increases by 6% compare to the basic cycle under the optimum operating condition. The enhanced ORFC employing an internal heat exchanger had proposed to be used to further recover the thermal energy of geofluid from a flash cycle for geothermal resources by Mosaffa and Zareei [7]. Thermodynamic modeling had been done to investigate the effects of four key thermodynamic parameters on system performance. The results illustrated that under the given condition, the net power output increases by 36.7% when two-phase expanders are employed in a flash-binary geothermal cycle with single flash ORFC. The results of optimization showed that the maximum system energy could reach 8.87%, and exergy efficiencies could reach 47.32%.

Proton exchange membrane fuel cell A fuel cell (FC) is an electrochemical device that converts the chemical energy of hydrogen from a fuel into electrical energy directly. The fuel cell has the capability in modern energy conversion tools to produce the electric power efficiently in a single step with no moving parts. In a research work, Ravindranath Tagore Y. et al. [8] focused on modeling, simulation, and control of a fuel cell powered laptop computer. They developed an empirical model to include the phenomena of activation and ohmic polarization as well as mass transport effects to simulate the air berthing fuel cell. The proton exchange membrane fuel cell (PEMFC) for enhancing performance was investigated by Attuluri R. Vijay Babu [9]. A parametric study of PEMFC to enhance the performance used in fuel cell vehicle was done. The results showed that cell temperature has important effect on the PEMFC performance, whereas other parameters produce variation only in the activation polarization region and in the concentration polarization region. In another study was carried out by Attuluri R. An experimental work conducted on the planar and ducted cathode structures of an air-breathing PEMFC [10]. It was reported the cell orientation has significant effect on the performance of air breathing fuel cell. Generally, the classification of fuel cells is categorized according to the use of electrolytes and fuel is being used [11]. Proton exchange membrane fuel cells in the particular is one of the most promising types already in the early commercialization stage [12]. Energy and exergy analyses of a PEMFC have been investigated by Ozgur and Yakaryilmaz [13]. In their mechanism pure hydrogen had been directly used as a fuel in compressed hydrogen gas (CHG) formation. They illustrated that the energy efficiency of PEMFC is 41.27%. Also, 4.2% and 2.25% performance improvements were achieved by changing the operating pressure ratio (P/P0) from 1 to 2 and operating temperature ratio (T/T0) from 1 to 1.2, respectively. Lee et al. [14] proposed a combined PEMFC/ORC system. They presented a thermo-economic analysis to find the best working fluid. An economic analysis was also performed for different waste heat sources, types of system operation and working fluids.

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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The results illustrated that the installation cost of the ORC can be recovered within the FC lifetime in all design cases. Hybridization of a fuel cell with the energy storage system is commonly used to obtain simultaneously high power and energy density along with the fast transient response. Latha et al. [15] proposed a new reconfigurable structure in which energy storage system (ESS) can be interchangeably used both in direct or isolated modes by using a fuel cell. The new mixed refrigerant for ORC integrated with PEM electrolyzer had been proposed by Ganjehsarabi [2]. He presented that utilizing the mixed refrigerant decreases the performance of the system and results in a higher hydrogen production rate because of a glide match of temperature profiles in the heat exchangers. Bendaikha et al. [16] performed a hybrid FC and geothermal heat pump used for air conditioning in Algeria. Their results showed that using the geothermal sources located in Northern Algeria and low-temperature PEMFC for air conditioning is a promising solution. A thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production has been illustrated by Karapekmez and Dincer [17]. The results of their study showed that the highest overall energy and exergy efficiencies are 78.37% and 58.40% in the storing period, respectively. The Similar work has been done by Yuksel and Ozturk [18]. The produced energy from geothermal resources can be used to drive an ORC and an absorption cooling system, and further used to produce hydrogen by a PEMFC.

Thermoelectric waste heat recovery The waste heat is the thermal energy that is not used in a process and is rejected to the environment Uselessly. Recovering waste heat can be conducted through various waste heat recovery systems (WHRS). The WHRS provides valuable energy sources and reduces the energy consumption of an integrated system. WHRSs are considered such as direct contact, indirect contact, transport membrane and the use of units such as heat pumps, heat recovery steam generators (HRSGs), heat pipe systems and ORCs. There are new technologies for converting heat to power directly such as thermoelectric generator (TEG), piezoelectric and thermionic [19]. TEG devices are made out of semiconductor materials that generate power when they face a temperature differential between two sides. Their simple system, compact size, lack of moving parts and solid-state design make them ideal for WHRS applications. The review of cars WHRSs utilizing TEG and heat pipes have been done by Orr et al. [20]. They showed that TEGs had restriction such as relatively low efficiency and low temperature limits, heat pipes had limitations such as temperature limits and maximum rates of heat transfer. Cui et al. [21] illustrated the power output evaluation of a porous annular TEG for waste heat harvesting. A numerical model for the effects of the velocity of gas, the external resistance, porosity and the electrical/thermal contact resistances on the electricity generation has been proposed. The relation between the optimized porosity and pore diameter of a sample was presented. A lot of research has been done on the applied TEGs to vehicle WHRS. A dynamic model for TEG applied to a vehicle WHRS has been presented by Lan et al. [22]. The results showed that around 20% of the average output power increase could be expected by optimizing the thermal contact

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conductance and the heat transfer coefficient of the hot side of the heat exchanger. The similar study has been done by Demir and Dincer [23]. To improve the performance of the automobile exhaust TEGs, an intermediate fluid TEG system has been proposed by Zhao et al. [24]. In their study, the waste exhaust heat is transferred through boiling and condensation of the intermediate fluid. Based on the simulation results the peak output power increased by 32.6% and TEG module area is reduced by 73.8%. An integrated system consisting of an Alkaline Fuel Cell (AFC), a TEG and a regenerator has been simulated by Yang et al. [25]. Based on the presented results employing such an integrated system, the equivalent maximum power density of the AFC can be increased by up to 23%. Demir and Dincer [26] presented thermodynamic modeling of a hybrid solar thermal system with TEG and PEM electrolyzer for hydrogen and power production. They calculated the first and second law efficiencies of each system component and H2, freshwater, power generation capacity of the system. Dong et al. [27] presented an efficient coupling system using a thermophotovoltaic cell to harvest the waste heat from a reforming solid oxide FC. The optimal regions of the coupling system were determined, and the parametric selection criteria were provided. A numerical model of an exhaust WHRS for a high temperature PEMFC stack has been illustrated by Gao et al. [28]. Their model was based on a numerical approach. The numerical results showed that the optimized system configuration of the WHRS is six TEG modules crossing the exhaust gas flow and seven modules along the flow. A similar study has been experimentally investigated by Hasani and Rahbar [29].

Motivation Due to the waste heat in the FC and the condensers, it can be converted into the power using ORFC and TEGWHRSs. Using TEGWHRSs, the first and second law efficiencies increase. Therefore, in the present study, the use of TEGWHRSs to generate more electricity from a combined geothermal, PEMFC and ORFC has been investigated and analyzed for improving system performance.

Aims and novel contributions According to the literature review, the use of ORFCs in geothermal power plants have been studied, thermodynamically. Also, researches have been conducted on the use of ORC to recover waste heat from a PEMFC. There are also researches on the use of TEGWHRSs to recover waste heat from a PEMFC, too. It seems that combining the geothermal system, PEMFC, ORFC and TEGWHRSs in the condensers of ORFC can improve the performance of the geothermal power plant. The current study aim is to improve the performance of the integrated geothermal system by proposing the integration of ORFC with the PEMFC and waste heat recovery from the condensers. A steady state model is developed in Engineering Equation Solver (EES) to thermodynamic and economic evaluation. The thermodynamic simulations of the ORFC, TEGWHRS and PEMFC have been validated to ensure the accuracy of the simulations. Besides, some parametric studies are conducted to discuss the effects of operating parameters

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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and key design variables on the performance of proposed system. The main contributions of this work include the following points:
A new integrated system include geothermal energy, ORFC, PEMFC and TEGWHRSs for power generation is introduced.
A new scheme for WHRS to recover waste heat from both condensers of ORFC is proposed
Thermodynamic model of the conventional system (GeoPEMFC-ORFC) and proposed system (Geo-PEMFC-ORFC with TEG WHRSs) is carried out.
A parametric study to examine the influences of the significant design parameters on the performance of the proposed system is presented.
The economic evaluation of the proposed system (GeoPEMFC-ORFC with TEG WHRSs) is presented. To achieve the main goals of the current research, in the following sections firstly a system description to introduce the studied system is presented then the main governing equations are given and finally the results with related analyses in tables and graph are represented.

Systems description This section provides information about the conventional and proposed integrated systems and cycle assumptions. The schematic of the conventional cycle is discussed and finally, the proposed cycle is introduced and discussed.

Description of the conventional and proposed integrated systems The detailed configuration diagram of the conventional combined ORFC and PEM FC are shown in Fig. 1. Geothermal energy is the heat source of ORFC. In the conventional cycle, geothermal fluid heat is supplied to the evaporator as sensible energy (state 1) and exhausted to the reinjection well (state 2). The pump 1 raises working fluid pressure (state 3 to state 4) and feeds it to the evaporator where it is heated by using geothermal energy. The working fluid at state 5 is throttled to the turbine inlet pressure (state 6) with the isenthalpic process and allowed to enter the flash vessel instead of entering the turbine 1 directly. Saturated vapor flowing out the flash vessel expands from state 6 to 7 in the turbine 1 where electrical energy is generated and coming out from state 8 to Grid power. The liquid phase of the flash vessel is sent to the PEM FC where it is heated by using PEM FC energy. The working fluid flowing out the PEM FC expands from state 11 to 12 in the turbine 2 to produce electrical energy and coming out from state 12 to the Grid power. Both working fluids enter the condenser 1 and 2 (state 9) and feed them to the pump 1 and 2. The cool water enters the condensers at state 15 and 17, and leaves them from state 16 and 18, while the working fluids leave the condensers at state 9 and 13. Both working fluids (state 9 and 14) are mixed, and the mixed fluid enters the pump 1 (state 3) and the cycle is completed. The cooling water

systems are also shown in the detailed configuration diagram. The proposed system (TG WHRSs for combined geothermal, organic Rankine flash cycle and fuel cell) shown in Fig. 2 is same as the conventional system (combined geothermal, organic Rankine flash cycle and fuel cell) except the cooling water systems, where the TEG WHRSs have installed to retrieve waste cooling towers heat. It is assumed that the proposed system uses the same heat source and coolant with the conventional system (Fig. 2).

Cycle assumptions In this work, to investigate the conventional and proposed systems under different operational conditions, the cycles are modeled by the Engineering Equation Solver (EES). The various components of the cycles are modeled based on mass and energy balance and to simplify the system analysis, some assumptions are made as follows:
The flow is steady state.
The heat losses in this cycle and pressure drop in pipelines are neglected.
The isentropic efficiency of the turbines and pumps are 87% and 85% respectively.
The kinetic and potential energy is ignored.
The working fluids at the inlet of turbines are at the saturated vapor state and the exit of the condensers are at the saturated liquid state.
The ambient temperature and pressure are 20 C and 101 kPa respectively.
Freshwater properties are used for the geothermal fluid. Input assumed data and operating conditions of the conventional and proposed system components are illustrated in Table 1. It should be noted that all this information has been extracted from the practical system. In this research, R245fa, R123 and Isobutene are considered as the working fluids. As previously mentioned, selecting the best working fluids for the proposed system is one of the goals of this research.

Thermo-economic analysis In this section, a detailed thermos-economic analysis, including energy, exergy and economic models of the conventional and proposed cycles is presented. The EES software is used to modeling the conventional and proposed cycles from viewpoints of the first and second laws of thermodynamics.

ORFC thermodynamic analysis Thermodynamic analysis using the mass, energy and exergy conservation equations were developed for the thermal investigation of the conventional and proposed systems as follows while neglecting the change of kinetic and potential terms:

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Fig. 1 e Schematic diagram of conventional system (Geo-PEMFC-ORFC).

Fig. 2 e Schematic diagram of proposed system (Geo-PEMFC-ORFC with TEG WHRSs).

X _ m_ e ðhe Þ þ W

(2)

_ are net input heat transfer and work output, where Q_ and W the denotations i and e indices represent inlet and exit states, m_ is mass flow rate, h is specific enthalpy, f is the rate of availability of transferred heat and j is specific flow availability and presented as follow:

_ þ Ex _ D m_ i ðji Þ þ 4Q_ ¼ m_ e ðje Þ þ W

(3)

4Q_ ¼

X

m_ i ¼

Q_ þ

X

X

m_ e

m_ i ðhi Þ ¼

(1)

 X  T0 Q_ j 1  Ts;j j

(4)

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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j ¼ ðh  h0 Þ  T0 ðs  s0 Þ

(5)

where s, T0 and Ts are the specific entropy, temperature of ambient and heat source, respectively. For an illustration of detailed thermodynamic analysis of this work, mass, energy and exergy balances are provided along with the schematic of that particular component based on the states in Figs. 1 and 2 are provided in Table 2. The first-law efficiency of the integrated cycle may be expressed as: hI ¼

_ net W Q_ in

(6)

_ net ) and input thermal energy to The net output power (W the cycle (Q_ in ) are expressed for the conventional system (GeoPEMFC-ORFC) as: _ T1 þ W _ T2 þ W _ PEM FC  W _ P1  W _ P2  W _ CWP1  W _ CWP2 _ net ¼ W W

(7)

Q_ in ¼ m_ 1 h1  m_ 2 h2

(8)

These values are expressed for the proposed system (GeoPEMFC-ORFC with TEG WHRSs) as: _ T1 þ W _ T2 þ W _ PEM FC þ W _ TEG WHRS1 þ W _ TEG WHRS2  W _ P1 _ net ¼ W W _ P2  W _ CWP1  W _ CWP2  W _ WHRSP1  W _ WHRSP2 W (9) Q_ in ¼ m_ 1 h1  m_ 2 h2

(10)

The second-law efficiency of a power cycle can be defined as: hII ¼

_ net _ net W W ¼ _jin _j1  j_ 2

(11)

For the conventional and proposed systems: j_ 1  j_ 2 ¼ m_ 1 ½ðh1  h0 Þ  T0 ðs1  s0 Þ  m_ 2 ½ðh2  h0 Þ  T0 ðs2  s0 Þ (12)

PEM FC thermodynamic analysis In this section, the thermodynamic simulation of PEM FC is presented. To simplify the analysis of PEM FC, some assumption are made as following [30]:

Evaporator Ex. valve Flash vessel Turbine 1 Turbine 2 Condenser 1 Condenser 2 Pump 1 Pump 2 CWP 1 CWP 2 TEG WHRS 1 TEG WHRS 2 WHRSP1 WHRSP2

101 20 R245fa - R123 - Isobutene 87 85 8 130 300 600 400 300

Exergy relations

kPa C e % % kg/s  C kPa kPa kPa kPa 

_ D;Evap: m_ 1 j1 þ m_ 4 j4 ¼ m_ 2 j2 þ m_ 5 j5 þ Ex _ D;Ex:valve m_ 5 j5 ¼ m_ 6 j6 þ Ex _ D;Flash vessel m_ 6 j6 ¼ m_ 10 j10 þ m_ 7 j7 þ Ex _ T1 þ Ex _ D;T1 m_ 7 j7 ¼ m_ 8 j8 þ W _ T2 þ Ex _ D;T2 m_ 11 j11 ¼ m_ 12 j12 þ W _ D;cond: 1 m_ 8 j8 þ m_ 15 j15 ¼ m_ 9 j9 þ m_ 16 j16 þ Ex _ D;cond: 2 m_ 12 j12 þ m_ 17 j17 ¼ m_ 13 j13 þ m_ 18 j18 þ Ex _ P1 þ Ex _ D;P1 m_ 3 j3 ¼ m_ 4 j4 þ W _ P2 þ Ex _ D;P2 m_ 13 j13 ¼ m_ 14 j14 þ W _ CWP1 þ Ex _ D;CWP1 m_ 22 j22 ¼ m_ 20 j20 þ W _ CWP2 þ Ex _ D;CWP2 m_ 26 j26 ¼ m_ 24 j24 þ W _ TEG WHRS 1 þ Ex _ D;TEG WHRS 1 m_ 16 j16 þ m_ 20 j20 ¼ m_ 19 j19 þ m_ 21 j21 þ W _ TEG WHRS 2 þ Ex _ D;TEG WHRS 2 m_ 18 j18 þ m_ 24 j24 ¼ m_ 23 j23 þ m_ 25 j25 þ W _ WHRSP1 þ Ex _ D;WHRSP1 m_ 19 j19 ¼ m_ 15 j15 þ W _ WHRSP2 þ Ex _ D;WHRSP2 m_ 23 j23 ¼ m_ 17 j17 þ W

Value

Energy relations

Ambient pressure Ambient temperature Working fluid Turbines isentropic efficiency Pumps isentropic efficiency Mass flow rate of production well Temperature of production well Pressure of production well Turbines inlet pressure Turbine 1 outlet pressure Turbine 2 outlet pressure

Unit

m_ 1 h1  m_ 2 h2 ¼ m_ 5 h5  m_ 4 h4 m_ 5 h5 ¼ m_ 6 h6 m_ 6 h6 ¼ m_ 10 h10 þ m_ 7 h7 _ T1 ¼ m_ 7 h7  m_ 8 h8 W _ T2 ¼ m_ 11 h11  m_ 12 h12 W m_ 8 h8  m_ 9 h9 ¼ m_ 16 h16  m_ 15 h15 m_ 12 h12  m_ 13 h13 ¼ m_ 18 h18  m_ 17 h17 _ P1 ¼ m_ 3 h3  m_ 4 h4 W _ P2 ¼ m_ 13 h13  m_ 14 h14 W _ CWP1 ¼ m_ 22 h22  m_ 20 h20 W _ CWP2 ¼ m_ 26 h26  m_ 24 h24 W _ TEG WHRS 1 m_ 16 h16 þ m_ 20 h20 ¼ m_ 19 h19 þ m_ 21 h21 þ W _ TEG WHRS 2 m_ 18 h18 þ m_ 24 h24 ¼ m_ 23 h23 þ m_ 25 h25 þ W _ WHRSP1 ¼ m_ 19 h19  m_ 15 h15 W _ WHRSP2 ¼ m_ 23 h23  m_ 17 h17 W

Parameter

Table 2 e Energy and exergy relations for the components of the proposed system (TG WHRSs for combined geothermal, organic Rankine flash cycle and fuel cell) [30,31].

Table 1 e Design parameters and assumed data of the conventional and proposed systems.

Components

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Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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The system operates under the steady states conditions.
The pressure drop in PEM FC is negligible.
The composition of air in the fuel cell is considered 79% N2 and 21% O2.
There is no heat losses to the environment.
Parameter of a single cell can be lumped together to form a stack. In a PEM FC stack, hydrogen and oxygen (air) are supplied to the electrodes to generate water, heat, and electricity. The chemical reaction in such a system can be expressed as: Anode reaction: H2 / 2Hþ þ 2e

(13)

1 Cathode reaction: 2Hþ þ 2e þ O2 /H2 O þ thermal energy 2

(15)

From the well-known Nernst relation, the reversible open voltage form hydrogen-oxygen reaction can be obtained as follow: 

ENernst ¼

DG RTf lnðQÞ nel F Nel F

(16)



where DG denotes the Gibbs free energy for reaction, nel is number of electrons which transfer in the chemical reaction, F represents the Faraday constant, R is universal constant of gas, and Tf is the operating temperature of PEM FC in Kelvin and Q is the quotient of equation and it can be calculated as: pffiffiffiffiffiffiffi PH2 PO2 Q¼ Psat H2 O

(17)

where all P-value in the above equation denotes the effective partial pressure of each reactant. The water vapor saturation pressure can be computed from the following empirical relation:       5 log10 Psat H2 O ¼  2:1794 þ 0:02253 Tf  273:15  9:183  10  2  3   Tf  273:15 þ 1:4554  107 Tf  273:15 (18) The effective partial pressure of PH2 and PO 2 at anode and cathode are similar all over the cell and can be computed as: 2  6 1 6 PH2 ¼ Psat 6 2 H2 O 4 exp

!

1:653i T1:334 fc

: xsat H2 O

7 7  17 5

1 1:653i T1:334 fc

! :

 1  xsat H2 O xsat H2 O

Psat H2 O P

xchannel ¼ N2

(21)

  xN2 ;in  xN2 ;out   x ;in ln xNN2;out

  xN2 ;in ¼ 0:79 1  xsat H2 O

xN2 ;out ¼

1  xsat H2 O

  1 þ ððlair  1Þ=lair Þ 0:21 0:79

Vf ¼ ENernst  xactive  xohmic  xconcen

 xchannel N2

where the above equation parameters are:

(23)

(24)

(25)

herexactive , xohmic and xconcen are the activation, Ohmic and concentration over-potential, respectively. A semi-empirical relation for the activation over-potential can be expressed as [32]:   xactive ¼ a1 þ a1 T þ a3 Tln CO2 þ a4 TlnðiScell Þ

(26)

here Scell is cell surface area and aj are coefficients that can be expressed as: a1 ¼ 0:948; a2 ¼0:00286þ0:0002 lnðScell Þ   þ 0:000043ln CH2 ; a3 ¼0:000076; a4 ¼  0:000193 The hydrogen in anode-membrane surface (CO2 ) and oxygen concentration in cathode-membrane (CH2 ) can be specified by Henry's law as follows:

(19)

3 ! 0:291i 7 7 exp 7 5 T0:832 f

(22)

here xN2 ;in and xN2 ;out represent the molar fraction of nitrogen in inlet and outlet, respectively. lair shows the stoichiometric rate of air. Generally, the real measured voltage in PEM FC is smaller than the theoretical (the reversible voltage) form Nernst relation because of some losses that occur such as the activation over-potential, ohmic over-potential, and concentration potential. All these effects should be considered to calculate the actual voltage, which can be written as follow:

CO2 ¼ 1:97  107

2 6 6 PO2 ¼ P6 4 exp

xsat H2 O ¼

  77 CH2 ¼ 9:174  107 PH2 exp  T

3 1

xsat H2 O : The molar fraction of the water in a gas flow at saturation for the given temperature. xchannel other gases : The molar fraction of other gases in airflow except oxygen. i: Current density. : Molar fraction of nitrogen. xchannel N2 The abovementioned parameters can be calculated as the following [31]:

2

(14) 1 Total reaction: H2 þ O2 /H2 O 2

7

  498 T

(27)

(28)

The Ohmic over-potential based on Ohm's Law is as: (20)

xohmic ¼ iRint

(29)

int

where, R is internal resistance which can be calculated from the following equation including all membrane variables:

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Rint ¼

rm lm Scell

(30)

Sm shows the cell area, lm represents the membrane thickness, and rm is membrane resistivity which can be calculated from an empirical relation as: "

!

181:6 1 þ 0:03 i rm ¼

  RTf iL ne F iL  i

(41)

(31)

TEG WHRS thermodynamic analysis

The concentration over-potential is because of the concentration gradient between reactants and products at the electrode surface and the bulk solution. It can be calculated as: xconcen ¼

þ n_H2 O;produce Hv

where KH2 , KO2 , KN2 and KH2 O are specific heat capacity of hydrogen, nitrogen, and water, respectively. Hv is the vaporization heat capacity of water.

 2 !2:5 # cell i þ 0:062 i:S 303

   

11:866 þ 3 i exp 4:18 T303 T

    Q_ sens;laten ¼ KH2 n_H2 ;out Tf  n_H2 ;in Tamb þ KO2 n_O2 ;out Tf  n_O2 ;in Tout;comp     þ KN2 n_N2 ;out Tf  n_N2 ;in Tout;comp þ n_H2 O;produce Tf  Tamb KH2 O

(32)

here, iL is the limiting current density. In addition, the generated power by the PEM FC stack can be calculated as:

The following equations have been reported for the TEGs:   pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZTave  1 TC hTEG ¼ 1  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TC TH 1 þ ZTave þ TH ZT ¼

a2 sT k

(42)

(43)

where Ncell represents the number of cell in the stack. The energy balance for the whole of PEM FC can be written as follows:

where a is Seebeck coefficient, k is total thermal conductivity, and s is electrical conductivity.where Z is the thermoelectric figure of merit, ZTave is the dimensionless thermoelectric figure of merit, Tave is the average temperature of TEG, TC and TH are the temperatures of the cool and hot TEG's plates respectively. Using the first law of thermodynamic for TEG WHRS 1 and TEG WHRS 2, the following equations are obtained:

_ f  Q_ sens;laten Q_ net ¼ Q_ chem  W

_ TEG WHRS 1 m_ 16 h16 þ m_ 20 h20 ¼ m_ 19 h19 þ m_ 21 h21 þ W

(44)

_ TEG WHRS 2 m_ 18 h18 þ m_ 24 h24 ¼ m_ 23 h23 þ m_ 25 h25 þ W

(45)

Wf ¼ Ncell Vf I

(33)

(34)

where Q_ chem and Q_ sens;laten are chemical energy and sensible and _ f presents the PEM FC power output. In a fuel latent heat and W cell, the oxygen consumption rate, the hydrogen consumption rate and water production rate can be computed using the following relationships: n_H2 ;consum ¼ Ncell

n_O2;consum ¼ Ncell

I 2F

(35)

I 4F

n_H2 O;produc ¼ Ncell

(36)

I 2F

n_O2 ¼ lH2 n_O2 ;consum ¼ lO2 Ncell

(46)

hTEG WHRS 2 ¼

_ TEG WHRS 2 W Q_ TEG WHRS 2

(47)

where Q_ TEG WHRS 1 ¼ m_ 16 h16  m_ 19 h19

(48)

Q_ TEG WHRS 2 ¼ m_ 18 h18  m_ 23 h23

(49)

Economic analysis

I 2F

(38)

I 2F

(39)

Concerning the input hydrogen rate and about the above relations, the air intake can be achieved. The theoretical work rate released from the electrochemical reaction is obtained from the following equation: Q_ chem ¼ n_H2 ;consum :HHV

_ TEG WHRS 1 W Q_ TEG WHRS 1

(37)

The reactant molar flow rate can be calculated as follows: n_H2 ¼ lH2 n_H2 ;consum ¼ lH2 Ncell

hTEG WHRS 1 ¼

(40)

here HHV represents the higher heating value of hydrogen. Additionally, to calculate the latent heat and sensible heat the following equation can be employed:

Each engineering design requires financial estimates so that its multi aspects of performance can be determined correctly. There are several techniques for evaluating the economic characteristics of energy conversion systems, each of which can be used in specific projects. Methods such as the Simple Payback Rate (SPR), Internal Rate of Return (IRR) and Net Present Value (NPV) are widely used approaches in the economic assessment of energy systems which two latter indexes account the time value of money in projects. Here, to investigate the effects of employing the thermoelectric system, the IRR and NPV indexes applied in the economic analysis of the system.

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Based on the definition the NPV is the difference between the current value of cash inflows and the current cash outflow over a period of time. NPV is used in capital budgeting to analyze the profitability of an investment or project. The NPV can be calculated from the following equation: NPV ¼

t X t¼1

Ct  C0 ð1 þ RÞt

(50)

where, Ct is the Net cash flow during the period, C0 is the total initial investment costs, r denotes discount rate, and t the represents the number of periods. To carry out an economic analysis of the system, calculation of the total cost of the component is necessary as follow: Zk ¼ cost of component þ operation and maintenance cost

(51)

To obtain the total cost rate of the component using capital recovery factor (CRF) and the number of operation years (q) the following relation can be used. Zk CRF Z_k ¼ q

(52)

The CRF is an important parameter to calculate the cost rate of components of a system. The following equation commonly uses to calculate CRF [37]. n

CRF ¼

pð1 þ iÞ ð1 þ pÞn  1

(53)

here p denotes the interest rate and n is the number of operation year of the system.

Results and discussion In this section, the results of thermodynamic and economic evaluations are presented. Before presenting the results of modeling, validation are presented to verify the results of the present work.

Validation In order to validate the obtained results of thermodynamic modeling of TEG, the results of TEGs are compared with the experimental available data in literatures. Also, the results of the PEMFC are compared with the results of a previous research.

TEG validation The HZ-20 TEG [33] has been selected to validate the results of the present simulation. The specifications of this model are presented in the datasheet. This model can withstand temperatures up to 350  C. The comparison of simulation and empirical results of TEG power generation are shown in Table 3. As the results indicate, the difference is very small, with the highest error of about 0.49% for power generation.

PEM FC validation To ensure the output of both studied systems, it is necessary to validate the main component with the experimental data. The PEMFC model is validated with Ju and wang experimental data [34] which are shown in Fig. 3.

9

Comparison of systems System performance (the first and second law efficiencies) of the three working fluids for the conventional and proposed systems are shown in Fig. 4. These results are presented for three different working fluids. The reason for choosing these three working fluids are better performance than others and environmental benefits of these fluids. As it is shown in the results, the first and second law efficiencies of the proposed system have increased compared to the conventional system. Also, R123 has the best performance for both systems. In the proposed system, if the R123 was used as the working fluid, instead of the isobutene, 3.9% increase in the efficiency of the first law can be observed. This increase is 5.7% for the second law efficiency. According to the results, the best working fluid for use in the cycle is R123. It should be noted that R123 is considered as the working fluid in the next results. The other results outlined in Fig. 4 are the improved performance of the suggested system compared to the conventional one. It can be seen, for the R123, the first law efficiency for the conventional system is 23.56% and for the proposed system is 24.21%, which an increment about 2.7% is experienced. The results also show that using the suggested system, the second law efficiency has a rise of 2.8%. It is worth noting that these increase in efficiencies have been achieved without changing the main components of the cycle. Of course, there are further discussions on economic analysis. In order to more accurately compare the proposed system with the conventional one, the results related to the power generation or consumption of different components of the cycle for both systems are presented in Fig. 5. As it can be seen in the results, the output power of the turbine 1, turbine 2, FC and power consumption of pump 1 and pump 2 are the same for both systems. That's because the different parts of the proposed system have not changed over the conventional system. The total power consumption of the CWP1, CWP2, WHRSP1 and WHRSP2 in the proposed system is more than the conventional system. This is due to the addition of the two other pumps to the cycle and an increase in the fluid flow rate of the cooling tower. According to the results of Fig. 5, the output power of TEG WHRSs is zero in the conventional system. While in the proposed system, the output power of TEG WHRS1 and TEG WHRS2 are 13.92 kW and 37.26 kW respectively. In total, the amount of power produced by TEG WHRSs is higher than the pumps power consumption of WHRSP1. Therefore, the net power is increased from 1625 kW to 1670 kW. In thermodynamic evaluation of systems from the second law perspective, determining the contribution of each component to the destruction of exergy is very important. So that the rate of exergy destruction is one of the most important characteristics of each component of the cycle. For this goal, the results of the exergy destruction rate for each element are shown in Fig. 6. It should be noted that these results are presented for both conventional and proposed systems. According to the results, FC has the highest amount of exergy destruction rate. After the FC, condenser 2 and condenser 1 have the highest amount of exergy destruction rate. According to the results of Fig. 6, the least amount of exergy destruction is related to the TEG WHRS1.

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Table 3 e Comparison between the simulation results and empirical results for power generation of the TEG [33].

1 2 3 6

Cold side Temperature ( C)

Hot side temperature ( C)

Power-experiment (kW)

Power- modeling (kW)

Difference (%)

40 60 80 120

340 260 382 320

0.03805 0.0201 0.03205 0.0181

0.0381 0.0202 0.03212 0.01816

0.13 0.49 0.21 0.33

Parametric study results The effects of maximum pressure on the net power of the ORFC for both conventional and proposed systems are presented in Fig. 7. The system with TEG WHRSs has a better performance than the conventional system. Particularly, the

Fig. 3 e The average polarization curves for cathode A stoichiometry of 3.0 at 0.75 cm 2.

highest value of output power can be achieved in pressure range between 1050 and 1200 kPa. The reason for this improvement is proper recovery performance in this pressure range. There is also an optimal point in which the amount of generated power is maximized. The existence of an optimum point in power cycles has also been observed in previous research [35,36].

Fig. 5 e Power production/consumption within the components of the conventional and proposed systems.

Fig. 4 e System performance (the first and second law efficiencies) of the three working fluids for the conventional and proposed systems. Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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The mass flow rates and stream thermodynamic properties of the proposed system for stream line numbers 1e26 as described in Table 4. Different points properties like temperature, pressure and working fluid are based on the design parameters and assumed data are given in Table 4. The results of variations in the first and second law efficiencies with variations of the pump 1 outlet pressure (P4 ) for the conventional and proposed systems are shown in Fig. 8. The higher efficiency of the first and second law of the proposed system than the conventional system are again evident in the results. The increase in the pump 1 outlet pressure, which is the highest ORFC pressure, increases the first and second law efficiencies. This increase in efficiency has also been observed in the previous researches (for example see Ref. [36]). The reason for the rise in both efficiencies is the reduction of energy and exergy input to the evaporator due to the increase in pump 1 outlet pressure (P4 ). Also, according to the results presented in this figure, the rate of increase in both efficiencies for both conventional and proposed systems is the

Fig. 6 e Exergy destruction rates within the components of the conventional and proposed systems.

Fig. 7 e The effects of the pump 1 outlet pressure (P4 ) on the net power of the conventional and proposed systems.

same. This equality is due to the lack of effect of increased cycle pressure on the performance of the TEG WHRSs. One of the parameters affecting the cycle's performance is the geothermal fluid temperature. The results of the variation in the production power of various components of the proposed cycle with geothermal fluid temperature are shown in Fig. 9. As can be seen, with increasing the geothermal fluid

Table 4 e Thermodynamics properties of different points for the proposed system (TG WHRSs for combined geothermal, organic Rankine flash cycle and fuel cell). stream Mass flow rate Pressure Temperature Working (kg/s) (kPa) ( C) fluid 0 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

8 8 30.41 30.41 30.41 30.41 8.137 8.137 8.137 22.27 22.27 22.27 22.27 22.27 63.9 63.9 171 171 63.9 63.29 63.29 63.29 171 169.3 169.3 169.3

100 300 300 400 1200 1200 600 600 400 400 600 600 300 300 400 120 110 120 110 100 120 110 100 100 120 110 100

20 130 70 61.82 62.3 119.9 88.27 88.27 72.15 62.15 88.27 88.27 68.38 61.65 61.7 22 27 22 27 22 20 25 20 22 20 25 20

Water Water R123 R123 R123 R123 R123 R123 R123 R123 R123 R123 R123 R123 Water Water Water Water Water Water Water Water Water Water Water Water

Fig. 8 e The effects of the pump 1 outlet pressure (P4 ) on the first and second law efficiencies of the conventional and proposed systems.

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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temperature, the power of all components of the cycle increases. The reason for this increase is absorbed more heat in the evaporator. The increase in the absorbed heat leads to an increase in the output power of both turbines (turbine 1 and 2) and FC. Also, increasing the heat absorbed in the evaporator increases the heat dissipated in both condensers. Therefore, the output power of the TEG WHRSs can also be increased by the WHRSs. Of course, in the next results, it will be seen that increased power generation will not result in increased efficiency of the cycle. As mentioned, one of the parameters that influence the system performance is geothermal fluid temperature. The effects of the geothermal fluid temperature on the first and second law efficiencies of the proposed system have been illustrated in Fig. 10. As the results show, both efficiencies decrease with the geothermal fluid temperature increases. The first law efficiency will become 24.21% (a reduction of 6%) if the geothermal fluid temperature increases by 10  C. Also, the second law efficiency will become 37.11% (a reduction of 6.1%) if the geothermal fluid temperature increases by 10  C. As it is observed in the previous results, with the geothermal fluid temperature increment, the net power production increases. But this increase in geothermal fluid temperature also leads to an increase in the heat received in the evaporator. In fact, the efficiency reduction is due to the increased heat input to the cycle and not a result of the reduced power output from the ORFC. Another parameter that influences the performance of the proposed cycle is the flash vessel pressure ratio. This flash vessel pressure ratio is regulated by the expansion valve in the ORFC. The results of the variation of power generation of various components, including turbine 1 turbine 2, FC, TEG WHRS1 and TEG WHRS2, with variations in the flash vessel pressure ratio, are shown in Fig. 11. According to the results, increasing the output power of the turbine 1 turbine 2, FC, and TEG WHRS2 increases with increasing flash vessel pressure ratio. The reason for this increase is the increase in the mass flow rate of the flash vessel fluid by increasing the flash vessel pressure ratio. Of course, increasing the power of turbine 1 is

Fig. 9 e The effects of the geothermal fluid temperature on the power production within the main components of the proposed system.

Fig. 10 e The effects of the geothermal fluid temperature on the first and second law efficiencies of the proposed system.

negligible. It should be noted that changes in the flash vessel pressure ratio have a significant influence on the percentage of fluid inside the flash vessel. The output power of the TEG WHRS1 decreases with increasing flash vessel pressure ratio, and the reason for this is the reduction in the heat dissipated from the condenser 1 due to the reduction in mass flow rate by increasing the flash vessel pressure ratio. It should be noted that by increasing the flash vessel pressure ratio, the heat dissipated of the condenser 2 increases, which leads to an increase in the production capacity of the TEG WHRS2. Increase output power of the turbine 2 is more intense (about one and a half times), due to the increased mass flow through the turbine 2. To see the effect of increasing the flash vessel pressure ratio on the total cycle performance, the results of the changes in the efficiency of the first and second law of thermodynamics with changes in the flash vessel pressure ratio are presented in Fig. 12. As shown in the results of the previous figure, the increase in the flash vessel pressure ratio leads to an increase in the output power of the components of the

Fig. 11 e The effects of the flash vessel pressure ratio (P6 =P5 ) on the power production within the main components of the proposed system.

Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

international journal of hydrogen energy xxx (xxxx) xxx

13

cycle except for the TEG WHRS1. Due to the constant absorbed heat in the evaporator, by changing the flash vessel pressure ratio, the effect of increasing the output power is overcome on the reduction of output power of TEG WHRS1 and leads to an increase in the efficiency of the first law. The same conditions are set for the efficiency of the second law.

Economic evaluation

Fig. 12 e The effects of the flash vessel pressure ratio (P6 = P5 ) on the first and second law efficiencies of the proposed system.

Fig. 13 e The simple payback rate and annual cash flow for different TEG modules cost.

In the following section, the results of the economic analysis of the application of thermoelectric generators are discussed. Since the main difference between suggested and common systems is the added TEG modules, to assess the system economically, the cost of TEG modules is considered. The TEGs purchase cost in $/kW ranging is from 5 to 20 $/W [37]. However, it is expected to fall to 1 $/W in the coming years. In addition Chinese factories of the TEG modules have recently suggested remarkable cost reduction as low as 1500 $/kW. Fig. 13 represents the simple payback rate and annual cash flow for different cost of TEG modules. Since the annual cash flow independent of TEG module cost the value of this parameter remain constant in 40800 $/year with changing the unit cost of TEG variation. Additionally, the simple payback rate shows an enhancement from 1.25 years to 25 years. In addition, Fig. 14 shows the changing levelized cost of TEG modules with change of interest rate for different TEG modules costs. As it can be observed in the higher TEG modules cost and interest rate leads to the higher levelized cost of the system. Any feasible solution in the engineering design certainly should include the economic analysis. Based on the conducted economic investigation it can be seen in the case that the cost of TEG is less than 6 $/W the payback time is reasonable. In addition the lower interest rate of the system lead to the lower levelized cost.

Fig. 14 e The change of levelized cost for different TEG module cost and various interest rate. Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113

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Conclusion [4]

This study aims to improve the performance of the geothermal integrated system by proposing the integration of ORFC with the PEMFC and recover waste heat from the condensers. The proposed system performance is modeled and analyzed from the viewpoints of energy, exergy and economics. Through the proposed system simulation and parameters study, the following conclusions can be drawn:
The efficiency of the first and second law of thermodynamic of the proposed system has increased compared to the conventional system. Also, R123 has the best performance for both systems.
According to the results, in the proposed system, the output power of TEG WHRS1 and TEG WHRS2 are 13.92 kW and 37.26 kW, respectively.
According to the results, the FC has the highest amount of exergy destruction. After the FC, condenser 2 and condenser 1 have the highest amount of exergy destruction.
The increase in pump 1 outlet pressure, which is the highest ORFC pressure, increases the efficiency of the first and second laws. The reason for the increase in both efficiencies is the reduction of energy and exergy input to the evaporator due to the increase in pump 1 outlet pressure (P4 ).
The first law efficiency will become 24.21% (a decrease of 6%) if the geothermal fluid temperature increases by 10  C. Also, the second law efficiency will become 37.11% (a reduction of 6.1%) if the geothermal fluid temperature increases by 10  C.
Since the annual cash flow independent of the TEG module cost the value of this parameter remain constant in 40800 $/year with changing units cost of TEG variation. Additionally, the simple payback rate shows an enhancement from 1.25 years to 25 years. The results of current study can introduce new ways for researchers and energy system designers to create the new integrated energy systems with higher performance. Also, for the future work in this field, the optimization especially multiobjective optimization with different targets such as cost of products and exergy efficiency of the system should be performed to achieve the best system configuration.

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Please cite this article as: Khanmohammadi S et al., Potential of thermoelectric waste heat recovery in a combined geothermal, fuel cell and organic Rankine flash cycle (thermodynamic and economic evaluation), International Journal of Hydrogen Energy, https://doi.org/ 10.1016/j.ijhydene.2019.12.113