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Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production
international journal of hydrogen energy xxx (xxxx) xxx
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Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production* Aras Karapekmez a,*, Ibrahim Dincer b,a a
Faculty of Mechanical Engineering, Yildiz Technical University, Besiktas, Istanbul, Turkey Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, Canada
b
article info
abstract
Article history:
The present study develops a new solar and geothermal based integrated system,
Received 2 September 2018
comprising absorption cooling system, organic Rankine cycle (ORC), a solar-driven system
Received in revised form
and hydrogen production units. The system is designed to generate six outputs namely,
30 November 2018
power, cooling, heating, drying air, hydrogen and domestic hot water. Geothermal power
Accepted 5 December 2018
plants emit high amount of hydrogen sulfide (H2S). The presence of H2S in the air, water,
Available online xxx
soils and vegetation is one of the main environmental concerns for geothermal fields. In this paper, AMIS(AMIS® - acronym for “Abatement of Mercury and Hydrogen Sulphide” in
Keywords:
Italian language) technology is used for abatement of mercury and producing of hydrogen
Hydrogen production
from H2S. The present system is assessed both energetically and exergetically. In addition,
Solar energy
the energetic and exergetic efficiencies and exergy destruction rates for the whole system
Geothermal energy
and its parts are defined. The highest overall energy and exergy efficiencies are calculated
Hydrogen sulphide
to be 78.37% and 58.40% in the storing period, respectively. Furthermore, the effects of
Exergy
changing various system parameters on the energy and exergy efficiencies of the overall
Efficiency
system and its subsystems are examined accordingly. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the last years, public attention has increasingly been shifted by the media and world governments to the concepts of saving energy, reducing pollution, protecting the environment, and developing long-term energy supply solutions. In parallel, research funding relating to alternative fuels and energy carriers is increasing on both national and international levels. The main concerns caused by the world's current
energy supply system are mainly based on fossil fuels. In fact, the energy stored in hydrocarbon-based solid, liquid, and gaseous fuel was, is, and will be widely consumed for internal combustion engine-based transportation, for electricity and heat generation in residential and industrial sectors, as it is convenient, abundant and cheap. However, such a widespread use of fossil fuels gives rise to the two problems of oil supply and environmental degradation [1]. Currently, more than 80% of the world's energy supply comes from fossil fuels. The ongoing growth in fossil fuel
*
This study was supported by the Scientific Research Projects Council of Yildiz Technical University (YTU-BAP), Grant number: 3349. * Corresponding author. E-mail address: [email protected] (A. Karapekmez). https://doi.org/10.1016/j.ijhydene.2018.12.046 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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consumption suggests that global carbon dioxide emissions are still rising [2]. One solution for these problems is aimed to find new, environmentally benign, economical, reliable, and secure energy sources and technologies. The natural sources of energy, such as solar, wind, geothermal, biomass, and municipal waste are defined as renewable such that these energy sources are replenished naturally after use. In this respect, the use of renewable energy sources plays a key role for a sustainable energy future. Geothermal energy is considered as one of the promising alternative energy resources, which can be utilized for the purpose of electricity, heating, cooling or hydrogen production goals. The utilization of geothermal source either for power generation, cooling or heating applications depends mainly on the source temperature. High-temperature geothermal resources above 150 C are generally used for power generation. Moderate-temperature (between 90 and 150 C) and low-temperature (below 90 C) geothermal resources are best suited for direct applications such as space and process heating, cooling, aquaculture, and fish farming [3,4]. It is obviously better to use high-temperature geothermal source for multi-generation purposes. Generating multiple outputs from one source will not only increase the efficiency of the system but will also make more cost effective. The multi-generation capacity of geothermal has recently attracted attention from many researchers around the world [5,6]. On the other hand, one of the most important enviromental issues related to the using geothermal operating fluids to generate electricity is non-condensable gases emission. The vent stacks in geothermal plants emit carbon dioxide (CO2) and methane (CH4) gases which consequently raise serious concerns in terms of greenhouse gases. The amount of these emissions are quite small compared to carbon and fossil fuel plants, which indicates that the contribution of these sources is practically negligible. Geothermal power plants also emit higher amount of hydrogen sulfide due to the employment of H2S as a main constituent of the geothermal fluids. The presence of H2S in the air, water, soils and vegetation is one of the main enviromental concerns [7]. H2S is normally in gas phase and can be absorbed in lungs through inhalation. Health effects include respiratory, ocular, neurological, and metabolic effects and the death after single exposures to concentrations higher than 700 mg/m3 [8]. A summary of these effects is presented in Table 1.
Table 1 e Human health effects resulting from exposure to H2S. Effect/Observation Odor threshold Bronchial constriction in asthmatic individuals Increased eye complaints Increased blood lactate concentration, decreased oxygen uptake Eye irritation Oldfactory paralysis Respiratory distress Death Source: Complied from Ref. [8].
Exposure (mg/m3) 0.011 2.8 5.0 7.0e14.0 5.0e29.0 >140 >560 700
On the other side, by virtue of its physical and chemical properties, it is clear that hydrogen has the potential to occupy a unique position in the future world energy scene. Not only it could become ultimately a universal means of conveying and storing energy but also an entirely novel fuel with properties that are distinct from those of other fuels. Hydrogen becomes an apparent obvious choice for a low-carbon economy in that it would liberate no pollutants to the atmosphere and, when coupled to carbon sequestration or derived from non-fossil primary energy sources, little or no carbon dioxide to contribute to climate change [9]. When unfavourable effects of hydrogen sulphide and on the other hand superiorities of hydrogen take into consideration, decomposition of hydrogen sulphide to hydrogen can be characterized as a reasonable and profitable process. Renewable-based energy systems should be designed in a more effective way so as to attain higher efficiencies. One way to achieve this is to plan the system by using the approaches of cogeneration and especially multigeneration. Since, multigenerational systems can provide better efficiency, costeffectiveness, environment and hence better sustainability [10]. There has been a lot of research [11e15] on renewable energy based multigeneration systems. Al-Ali et al. [11] proposed a solar and geothermal integrated based system for multigeneration for electrical power, cooling, space heating, hot water and heat for industrial use. Detailed thermodynamic analyses were conducted for single generation, cogeneration, trigeneration and multigeneration systems through energy and exergy approaches in order to show the performance of the each system and compare the results. The energy efficiencies for single generation and multigeneration systems were found to be 16.4% and 78%, respectively, while the exergy efficiencies were 26.2% and 36.6%, respectively. Ezzat et al. [12] proposed a newly developed solar and geothermal based multigenerational energy system. Their system includes a single flash geothermal cycle, heat pump system, single-effect absorption cooling system, thermal energy storage, hot water and drying system. The main goal of the study is to obtain five outputs, namely cooling, heating, domestic hot water, drying and electricity. The overall energy and exergy efficiencies are found to be 69.6% and 42.8%, respectively. Bicer et al. [13] suggested a novel solar and geothermal based combined system for hydrogen production. The system comprises PV/T modules for the purpose of heating, water heating and hydrogen production and geothermal energy sources for electricity, cooling and again hydrogen production. Energy and exergy analyses were carried out to assess the performance of the cycle, and the effects of various system parameters on energy and exergy efficiencies of the overall system. The results show that overall energy and exergy efficiencies of the combined system can reach up to 10.8% and 46.3%, respectively for a geothermal water temperature of 210 C. Akrami et al. [14] designed a geothermal based multigeneration energy system in order to generate electricity, heating, cooling and hydrogen, simultaneously. The system consists of organic Rankine cycle, domestic water heater, absorption refrigeration cycle and proton exchange membrane electrolyzer. Detailed energy and exergy analyses of the
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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system were made as well as exergoeconomic analyses. Also the effects of some important variables, i.e. geothermal water temperature, turbine inlet temperature and pressure, generator temperature, geothermal water mass flow rate and electrolyzer current density on the several parameters such as energy and exergy efficiencies of the proposed system were investigated. The overall energy and exergy efficiencies of the integrated system were calculated about 34.98% and 49.17%, respectively. Ozturk et al. [15] developed a geothermal based integrated system in order to obtain electricity, cooling, heating, domestic hot water as well as hydrogen fuel. Energy and exergy efficiencies and exergy destruction rates for the whole system and its subsytems were defined. The results show that overall energy and exergy efficiencies of the designed system are 39.46% and 44.27%, respectively. Nikolaidis et al. [16] have performed a review study for comparison of hydrogen production from renewable sources. According to their study, the most cost effective thermochemical process for hydrogen production is supercritical gasification of biomass. There is remarked that producing hydrogen from geothermal resources is less expensive than solar and wind energy sources. Mehrpooya et al. [17] investigated a combined system including solid oxide fuel cell e gas turbine power plant, Rankine steam cycle and ammonia-water absorption cooling subsystem. The proposed system is investigated through the energy and exergy approaches as well as economic perspectives. Moreover, the developed electrochemical model of the fuel cell is validated with the experimental data. Their results show that electrical efficiency of the combined system is 62.4%. In another study, Mohammadi et al. [18]. Conducted a techno-economic analysis of a solid oxide electrolyzer for hydrogen production. In their study, a dishtype solar collector system is used to provide the required energy of a solid oxide electrolyzer cell (SOEC) to produce hydrogen. Since dish collectors operate at high-temperature, they would be an ideal match for high temperature electrolyzers. Their results show that efficiency of the power cycle and the electrolyzer cell is equal to 72.69% and 61.70%, respectively. In the present study, a novel multigeneration system based on solar and geothermal energy for electricity, hydrogen production, cooling, heating, drying, and hot domestic water is developed and analyzed thermodynamically. The designed system comprises already proven and mature technologies which increases the feasibility of the system. The system differs from many multigeneration systems in terms of zero fossil resources usage and zero carbon emissions by utilizing solar and geothermal energy sources. For the first time, unlike previous studies hydrogen production from hydrogen sulfide is taken into consideration along with electrolysis of water. The underlying motivation of this process is not only to produce hydrogen from hydrogen sulfide but also to reduce the hydrogen sulfide emissions and thereby its adverse effects on the environment in geothermal power plants. In the proposed system unlike previous studies, the intermittent nature of the solar energy is taken into consideration dynamically, and a thermal energy storage system is also integrated into the solar driven cycle. Moreover, the system is designed to operate at three
3
different scenarios, namely, charging, storing, and discharging periods and the results are presented in detail for each period. Furthermore, the particular objectives of this paper are to carry out a detailed thermodynamic analyses of the designed system through energy and exergy approaches, including determination of the overall energy and exergy efficiencies of the multigeneration system and its subsystems; and to conduct a parametric study to determine the effects of various parameters on the overall energy and exergy efficiencies of the proposed system.
System description Fig. 1 shows a comprehensive schematic view of the designed integrated system, and how the subsystems interact with each other. According to the figure, the power plant comprises four main subsystems, i.e., absorption cooling system, organic Rankine cycle, the solar-driven cycle and hydrogen production units. The proposed system produces six useful outputs through the solar and geothermal energy which are electrical power, cooling, heating, drying, domestic hot water and hydrogen. High-temperature geothermal sources range between 150 and 350 C for many regions in the world [19]. In this paper, hot geothermal fluid is extracted from the production well above 250 C.
Single-effect absorption cooling system A single-effect absorption cooling system (SEACS) is integrated into the main integrated system in order to supply cold air to cold store at around 18 C for the purpose of food freezing. The ammonia-water solution is chosen as the refrigerant of the absorption cooling system, since lithium bromide (LieBr) can not operate at this low-temperature range without crystallization. In the single-effect absorption cooling system, a strong ammonia-water solution enters the generator at state (38), and is heated by the geothermal steam. As the strong ammonia-water warms, it starts to vaporize and consequently a weak solution of ammoniawater at a higher temperature than state (38) leaves the generator at state (39). The weak solution flows through the heat exchanger 1, where its thermal energy transfers to the incoming strong solution at state (37). The weak solution leaving at state (40) enters the expansion valve 1, where its pressure decreases before it enters the absorber at state (41). The concentrated ammonia-water vapor at state (31) flows into the condenser 1, where the ammonia-water vapor transfers its heat to the domestic water before leaving at state (32). The ammonia-water vapor at state (32) enters the expansion valve 2, where its pressure and temperature decrease. This cooled ammonia-water vapor at state (33) enters the evaporator, where the ammonia-water mixture absorbs heat from the air which is provided from the cold store and leaves at state (34). The weak solution at state (41) and ammonia-water mixture at state (35) flow into the absorber, where they transfer their thermal energy to the domestic water and exit as a strong solution in liquid form at state (36) and passes through the pump 1.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 1 e A schematic diagram of the present solar and geothermal based multigeneration system equipped with AMIS units.
Organic Rankine cycle To have an efficient ORC, the working fluid in the ORC should have a high critical temperature. One of the typical organic fluid types used to operate the ORC is n-octane, which has a relatively high critical temperature, 296 C [20]. Therefore it is selected as the working fluid of ORC. Hot geothermal steam at state (2) enters the boiler of the ORC and transfers its heat to the working fluid. Working fluid which is heated and vaporized in boiler goes into the turbine 1 at state (52) to generate power, and leaves at state (53) then enters the condenser 2 to be condensed. During the condensation process, air is extracted from buildings at state (54) and receives the thermal energy of the working fluid and then it is supplied to buildings at state (55) with the aim of space heating. Afterwards, working fluid passes through the pump 2.
The solar driven cycle The solar driven system including numerous huge parabolic trough collectors are supposed to transfer the solar energy to the heat transfer fluid of the solar driven cycle. Main advantages of using Syltherm 800 are environmentally-friendly characteristics, no troublesome phase change, higher working temperatures, and easy operation and hence Syltherm 800 is chosen as the heat transfer fluid of the solar driven system. Due to the intermittency of the solar energy, a thermal energy storage (TES) tank is integrated to the solar driven system. The system operates in three different scenarios: a) charging period, b) storing period and c) discharging period. During the charging period, the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated is
divided into two streams. Some part of the heat transfer fluid flows into the heat exchanger 3 at state (62) and where its heat transfers to the incoming stream at state (67) which circulates inside the TES tank. The heat transfer fluid which leaves the heat exchanger 3 at state (66) as heated passes through the TES tank and transfers its thermal energy to the phase change material (PCM) which is located inside the tank. Both sensible and latent heat transfer occur in this period. The latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas, and liquid-gas phase changes. Liquid-gas transitions have a higher heat of transformation than solid-liquid transitions. However, liquid-gas phase changes are not practical because of the large volumes or high pressures required to store the materials when they are in their gas phase states. Solid-solid phase change process is typically very slow and has a rather low heat of transformation [21]. Therefore, in the present study, the proposed system is designed for the solidliquid phase change. Afterwards, the heat transfer fluid leaves the TES tank as cooled at state (70). The rest part of the heat transfer fluid at state (59), after circulating inside the pipe line connects with the incoming flow at state (65) and then enters the heat exchanger 2. The main objective of the solar driven system is to increase temperature of the geothermal steam which flows into the heat exchanger 2 at state (3) and leaves it at higher temperatures so that more electrical power can be obtained from turbine 2. During the storing period, valve 1 and valve 3 are closed. Therefore, heat transfer fluid is not divided into two streams. The whole part of the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated circulates through the pipe line and enters the heat exchanger 2 for the purpose of heating the incoming geothermal steam.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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During the discharging period, only valve 2 is closed. The whole part of the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated flows into the heat exchanger 3 at state (62). Opposite to the charging period, during the discharging period, cold Syltherm 800 enters to the TES tank and by circulating around the PCM receives its heat and leaves the TES tank at state (66) as heated. In addition, at the end of the discharging period, PCM returns to its initial conditions through freezing.
In the selecting membrane, it is important to choose an electrolyte in which proton conductivity rises rapidly with temperature such as recently developed solid acid electrolyte (SAE). Conductivity of Cesium hydrogen sulfate (CsHSO4) is shown in Fig. 2. The conductivity scale is logarithmic. In temperatures between 120 C and 150 C, the conductivity rises up to about 4 orders of magnitude, reaching a highconductivity at 150 C which is the interested zone in the study [24].
The AMIS unit
Solid oxide fuel cell
The AMIS technology (for abatement of mercury and hydrogen sulfide) is an efficient, safe and enviromentally friendly technology capable to eliminate the related issues of the gaseous emissions from geothermal power plants and in particular the unpleasant smell of hydrogen sulfide. Geothermal fluid extracted from the reservoir by production wells is sent to the power plants by a steel pipe network. The fluid mainly consists of steam with some percentage (from less than 1% up to 15%) of non-condensable gases (NCG) [22]. Instead of being released to the atmosphere, NCG can be sent to the AMIS system for mercury and hydrogen sulfide abatement. This system consists of three fundamental steps:
In the solid oxide fuel cell (SOFC), chemical energy of the fuel is converted into the electricity. In the cathode, oxygen ions are produced and follow to the anode through a membrane. Produced electrons in the anode enter the cathode after passing through an external circuit and hereby electrical current is created [25]. Hydrogen is extracted from the storage tank at state (81) and enters the fuel cell system. Along with hydrogen, oxygen gas is supplied to the fuel cell system at state (82) so as to produce electrical power. The chemical reactions taking place in fuel cell system at the anode and cathode are as follows:
Removal of mercury by chemical absorption. Selective catalytic oxidation of hydrogen sulfide to SO2. SO2 scrubbing by geothermal water. It is worth mentioning that the AMIS system has been modified for hydrogen production from hydrogen sulfide in the current study. The hydrogen sulfide, which is trapped inside the AMIS unit, is sent to an electrolyzer system in order to decompose it into hydrogen and sulfur dimer instead of sending it to the catalytic oxidation process [23].
Hydrogen sulfide electrolyzer The hydrogen sulfide is sent to an electrolyzer unit at state (24) in order to decompose it into hydrogen and sulfur dimer through the electrolysis process. The principle of hydrogen sulfide electrolysis is to pass a direct current between two electrodes in order to decompose hydrogen sulfide into hydrogen and sulfur dimer. The process consists of passing a flow with H2S gas through the anode chamber to contact a catalytic anode. Then, it reacts to produce sulfur dimer, protons and electrons. The protons pass through the membrane from the anode to the cathode chambers where they combine with electrons to form diatomic hydrogen gas at the cathode. During the process both the anode and cathode are maintained at a temperature of 150 C. Gaseous sulfur dimer is collected from the anode compartment and hydrogen is removed from the cathode compartment [23]. The chemical reactions taking place in the electrolysis process at the anode and cathode are written as follows: H2S / 12S2 þ 2Hþ þ 2e
(1)
2Hþ þ 2e / H2
(2)
H2 / 2Hþ þ 2e
(3)
2Hþ þ 12O2 þ 2e / H2O
(4)
Water electrolyzer The Polymer Electrolyte Membrane (PEM) systems owes specific technical advantages compared to alkaline systems, especially once integrated with renewable energies. In comparison to alkaline systems, PEM electrolyzer shows better efficiency and higher production rates. In addition, it has low operating temperature and a simple structure and hence PEM is chosen as electrolyzer type. A PEM electrolyzer is operated in which a solid proton conducting membrane separates an anode chamber from a cathode chamber. The principle of water electrolysis is to pass
Fig. 2 e Conductivity of CsHSO4 versus temperature [24].
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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a direct current between two electrodes in order to decompose the water into hydrogen and oxygen as shown in Fig. 3. The reactions for the anode and cathode of the PEM electrolyzer can be expressed respectively as follows: H2O (l) / 12O2 (g) þ 2Hþ (aq) þ 2e
(5)
2Hþ (aq) þ 2e / H2 (g)
(6)
In the proposed system, water is supplied from a water tank and the water primarily enters the water preheater at state (77). On the other side, the water which leaves the cooling tower at higher temperatures flows into the preheater at state (74) and so before it is sent to the injection well, its heat is transferred to the water which will in the subsequent step enter the electrolyzer unit at state (78) at the appropriate temperature level for the electrolysis process.
System analyses In order to carry out the thermodynamic analyses and evaluation of the integrated energy system, specific assumptions are made in a reasonable manner, and the calculations are performed for each component; enthalpies, exergies, mass flow rates, pressures and temperatures are taken in consideration. Exergy destructions and entropy generations are also calculated to analyze the system. The Engineering Equation Solver (EES) software is used to perform for all calculations. The entropy generations and exergy destructions are determined for each component by writing the exergy and entropy balances. Both energy and exergy efficiencies are calculated for comparative evaluations. The assumptions made for the analysis of the multigeneration system are listed as follows: The changes in kinetic and potential energy and exergy terms are negligible.
The processes taking place are steady state and steady flow. The expansion valves, compressors, pumps and turbine are adiabatic. There is no chemical reaction occurs between the refrigerant and absorbent. Therefore, chemical exergy is neglected and only physical exergy is taken into account. The working fluid of organic Rankine cycle is n-octane. The refrigerant of absorption cooling system is ammoniawater. The heat transfer fluid of the solar driven system is Syltherm 800. The phase change material is chosen as LiNO3. The ambient temperature and pressure are 25 C and 101.3 kPa, respectively. The general mass balance equation for the control volume can be expressed as follows: X
m_ in ¼
X
m_ out
(7)
The first law of thermodynamics is considered to perform energy analyses for all components of the integrated energy system. The general energy balance equation for the control volume can be expressed as follows: X
_ in ¼ m_ in hin þ Q_ in þ W
X
_ out m_ out hout þ Q_ out þ W
(8)
where Q_ and W_ represent the heat transfer and work crossing _ and h denote the mass flow the component boundaries and m rate and the specific enthalpy, respectively. The steady flow energy balance relation can be expressed for a chemically reacting steady flow system as follows: X, nr h0 f þ h h0 r X, _ out þ ¼ Q_ out þ W np h0 f þ h h0 p
_ in þ Q_ in þ W
(9)
where hºf denotes the enthalpy of formation and (h - hº) represents the sensible enthalpy relative to the standard reference state, which is the difference between h (sensible enthalpy at the specified state) and hº (sensible enthalpy at the standard reference state). Analysing only in terms of first law of thermodynamics prevents designing accurate systems. Thereby, exergy analysis is one of the most significant aspects for the design and analysis of thermal systems. It is based on the second law of thermodynamics. Exergy is a measure of the system state departure from the environment state and is considered also as a measure of the available energy [26]. The flow exergy terms for each state point are defined as following equation: exi ¼ hi h0 T0 ðsi s0 Þ
(10)
Applying the exergy balance equation on the system components at steady state, the exergy destruction in each component can be calculated depending on the following formula: X Fig. 3 e Illustration of the operating principle of the PEM electrolyzer.
_ Qin þ W _ in ¼ m_ in exin þ Ex
X
_ Qout þ W _ out þ Ex _ d m_ out exout þ Ex (11)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 2 e Balance equations of the system components. Component Generator
Condenser 1
Evaporator
Absorber
Heat Exchanger 1
Boiler
Balance equations _1 ¼ m _ 2; m _ 38 ¼ m _ 39 þ m _ 42 m _ 1 h1 þ m _ 38 h38 ¼ m _ 2 h2 þ m _ 39 h39 þ m _ 42 h42 m _ 1 s1 þ m _ 38 s38 þ S_ gen ¼ m _ 2 s2 þ m _ 39 s39 þ m _ 42 s42 m _ d _ 1 ex1 þ m _ 38 ex38 ¼ m _ 2 ex2 þ m _ 39 ex39 þ m _ 42 ex42 þ Ex m _ 31 ¼ m _ 32 ; m _ 48 ¼ m _ 49 m _ 31 h31 þ m _ 48 h48 ¼ m _ 32 h32 þ m _ 49 h49 m _ 31 s31 þ m _ 48 s48 þ S_ gen ¼ m _ 32 s32 þ m _ 49 s49 m _ d _ 31 ex31 þ m _ 48 ex48 ¼ m _ 32 ex32 þ m _ 49 ex49 þ Ex m _ 33 ¼ m _ 34 ; m _ 46 ¼ m _ 47 m _ 33 h33 þ m _ 46 h46 ¼ m _ 34 h34 þ m _ 47 h47 m _ 33 s33 þ m _ 46 s46 þ S_ gen ¼ m _ 34 s34 þ m _ 47 s47 m _ d _ 34 ex34 þ m _ 47 ex47 þ Ex ¼ m _ 35 þ m _ 41 m _ 44 h44 ¼ m _ 36 h36 þ m _ 45 h45 m _ 35 s35 þ m _ 41 s41 þ m _ 44 s44 þ S_ gen ¼ m _ 36 s36 þ m _ 45 s45 m
_ 33 ex33 þ m _ 46 ex46 m _ 44 ¼ m _ 45 ; m _ 36 ¼ m _ 35 h35 þ m _ 41 h41 þ m
_ d _ 35 ex35 þ m _ 41 ex41 þ m _ 44 ex44 ¼ m _ 36 ex36 þ m _ 45 ex45 þ Ex m _ 37 ¼ m _ 38 ; m _ 39 ¼ m _ 40 m _ 37 h37 þ m _ 39 h39 ¼ m _ 38 h38 þ m _ 40 h40 m _ 37 s37 þ m _ 39 s39 þ S_ gen ¼ m _ 38 s38 þ m _ 40 s40 m _ d _ 37 ex37 þ m _ 39 ex39 ¼ m _ 38 ex38 þ m _ 40 ex40 þ Ex m _2 ¼ m _ 3; m _ 51 ¼ m _ 52 m _ 2 h2 þ m _ 51 h51 ¼ m _ 3 h3 þ m _ 52 h52 m _ 2 s2 þ m _ 51 s51 þ S_ gen ¼ m _ 3 s3 þ m _ 52 s52 m
Turbine 1
_ d _ 2 ex2 þ m _ 51 ex51 ¼ m _ 3 ex3 þ m _ 52 ex52 þ Ex m _ 52 ¼ m _ 53 m _ Turbine _ 52 h52 ¼ m _ 53 h53 þ W m
Condenser 2
_ 52 s52 þ S_ gen ¼ m _ 53 s53 m _ Turbine þ Ex _ d _ 52 ex52 ¼ m _ 53 ex53 þ W m _ 50 ¼ m _ 53 ; m _ 54 ¼ m _ 55 m _ 53 h53 þ m _ 54 h54 ¼ m _ 50 h50 þ m _ 55 h55 m _ 53 s53 þ m _ 54 s54 þ S_ gen ¼ m _ 50 s50 þ m _ 55 s55 m
Heat Exchanger 2
Heat Exchanger 3
Parabolic Trough Collector
_ d _ 53 ex53 þ m _ 54 ex54 ¼ m _ 50 ex50 þ m _ 55 ex55 þ Ex m _3 ¼ m _ 4; m _ 56 ¼ m _ 65 m _ 3 h3 þ m _ 65 h65 ¼ m _ 4 h4 þ m _ 56 h56 m _ 3 s3 þ m _ 65 s65 þ S_ gen ¼ m _ 4 s4 þ m _ 56 s56 m _ d _ 3 ex3 þ m _ 65 ex65 ¼ m _ 4 ex4 þ m _ 56 ex56 þ Ex m _ 62 ¼ m _ 63 ; m _ 66 ¼ m _ 67 m _ 62 h62 þ m _ 67 h67 ¼ m _ 63 h63 þ m _ 66 h66 m _ 62 s62 þ m _ 67 s67 þ S_ gen ¼ m _ 63 s63 þ m _ 66 s66 m _ d _ 62 ex62 þ m _ 67 ex67 ¼ m _ 63 ex63 þ m _ 66 ex66 þ Ex m _ 57 ¼ m _ 58 m _ 57 h57 þ Q_ solar ¼ m _ 58 h58 m Q_ solar _ 58 s58 þ S_ gen ¼ m Tsun Q _ solar ¼ m _ d _ 57 ex57 þ Ex _ 58 ex58 þ Ex m _ 5þ m _ 16 þ m _ 19 þ m _ 20 ¼ m _ 6þm _ 21 m _ 5 h5 þ m _ 16 h16 þ m _ 19 h19 þ m _ 20 h20 ¼ m _ 6 h6 þ m _ 21 h21 m _ 5 s5 þ m _ 16 s16 þ m _ 19 s19 þ m _ 20 s20 þ S_ gen ¼ m _ 6 s6 þ m _ 21 s21 m _ 57 s57 þ m
Direct-Contact Condenser
Gas Cooler
Compressor
_ d _ 5 ex5 þ m _ 16 ex16 þ m _ 19 ex19 þ m _ 20 ex20 ¼ m _ 6 ex6 þ m _ 21 ex21 þ Ex m _ 15 ¼ m _ 16 ; m _ 21 ¼ m _ 22 m _ 15 h15 þ m _ 21 h21 ¼ m _ 16 h16 þ m _ 22 h22 m _ 15 s15 þ m _ 21 s21 þ S_ gen ¼ m _ 16 s16 þ m _ 22 s22 m _ d _ 15 ex15 þ m _ 21 ex21 ¼ m _ 16 ex16 þ m _ 22 ex22 þ Ex m _ 22 ¼ m _ 23 m _ Compressor ¼ m _ 22 h22 þ W _ 23 h23 m _ _ 22 s22 þ Sgen ¼ m _ 23 s23 m _ Compressor ¼ m _ d _ 22 ex22 þ W _ 23 ex23 þ Ex m (continued on next page)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 2 e (continued ) Component Turbine 2
Balance equations _4 ¼ m _5 m _ Turbine _ 4 h4 ¼ m _ 5 h5 þ W m _ _ 4 s4 þ Sgen ¼ m _ 5 s5 m
Water Preheater
Pump 1
Pump 2
Cooling Water Pump
Expansion Valve 1
Expansion Valve 2
_ Turbine þ Ex _ d _ 4 ex4 ¼ m _ 5 ex5 þ W m _ 77 ¼ m _ 78 ; m _ 74 ¼ m _ 75 m _ 74 h74 þ m _ 77 h77 ¼ m _ 75 h75 þ m _ 78 h78 m _ 74 s74 þ m _ 77 s77 þ S_ gen ¼ m _ 75 s75 þ m _ 78 s78 m _ d _ 74 ex74 þ m _ 77 ex77 ¼ m _ 75 ex75 þ m _ 78 ex78 þ Ex m _ 36 ¼ m _ 37 m _ Pump ¼ m _ 36 h36 þ W _ 37 h37 m _ 36 s36 þ S_ gen ¼ m _ 37 s37 m _ Pump ¼ m _ d _ 36 ex36 þ W _ 37 ex37 þ Ex m _ 50 ¼ m _ 51 m _ Pump ¼ m _ 50 h50 þ W _ 51 h51 m _ 50 s50 þ S_ gen ¼ m _ 51 s51 m _ Pump ¼ m _ d _ 50 ex50 þ W _ 51 ex51 þ Ex m _6 ¼ m _7 m _ Pump ¼ m _ 6 h6 þ W _ 7 h7 m _ 6 s6 þ S_ gen ¼ m _ 7 s7 m _ Pump ¼ m _ d _ 6 ex6 þ W _ 7 ex7 þ Ex m _ 40 ¼ m _ 41 m _ 40 h40 ¼ m _ 41 h41 m _ 40 s40 þ S_ gen ¼ m _ 41 s41 m _ d _ 40 ex40 ¼ m _ 41 ex41 þ Ex m _ 32 ¼ m _ 33 m _ 32 h32 ¼ m _ 33 h33 m _ 32 s32 þ S_ gen ¼ m _ 33 s33 m _ d _ 32 ex32 ¼ m _ 33 ex33 þ Ex m
Table 3 e Balance equations of cooling tower. Cooling Tower
_ 28 ¼ m _ 29 ; m _ 27 ¼ m _ 30 ; m _ 8þ m _ 9þm _ 10 ¼ m _ 11 þ m _ 15 þ m _ 17 m _ 8 h8 þ m _ 9 h9 þ m _ 10 h10 þ m _ 27 h27 þ m _ 28 h28 ¼ m _ 11 h11 þ m _ 15 h15 þ m _ 17 h17 þ m _ 29 h29 þ m _ 30 h30 m _ 8 s8 þ m _ 9 s9 þ m _ 10 s10 þ m _ 27 s27 þ m _ 28 s28 þ S_ gen ¼ m _ 11 s11 þ m _ 15 s15 þ m _ 29 s29 þ m _ 30 s30 _ 17 s17 þ m m _ 8 ex8 þ m _ 9 ex9 þ m _ 10 ex10 þ m _ 27 ex27 þ m _ 28 ex28 ¼ m _ 11 ex11 þ m _ 15 ex15 þ m _ d _ 17 ex17 þ m _ 29 ex29 þ m _ 30 ex30 þ Ex m
Table 4 e The design parameters of the parabolic trough collector system. Parameter
Fig. 4 e Schematic illustration of the parabolic trough collector system.
Temperature of the sun, Tsun Average hourly solar radiation during the charging period, Iav,c Average hourly solar radiation during the storing period, Iav,s Average hourly solar radiation during the discharging period, Iav,d Charging period, tcharging Storing period, tstoring Discharging period, tdischarging Width of the collector Length of the collector Reflection rate
Value 6000 K 0.60 kJ/m2s 0.35 kJ/m2s 0.10 kJ/m2s 7h 10 h 7h 4m 50 m 0.8
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 5 e Thermal properties of the phase change material [28]. Phase change material
Table 7 e Enthalpy of formation of the system compounds (Taken from Ref. [30]).
Freezing/Melting point (K)
Cps,av (kJ/kgK)
Cpl,av (kJ/kgK)
DHm (kJ/kg)
528
1.20
1.84
372
LiNO3
Table 6 e The average chemical compositions of noncondensable gases [29]. Compound Nitrogen Oxygen Methane Carbon dioxide Hydrogen sulfide
hºf (kJ/mol) 20.5 128.60 0 285.83 0 0
H2S (g) S2 (g) H2 (g) H2O (l) H2 (g) O2 (g)
Composition (% mole) 26.76 18.55 15.77 31.14 7.78
Table 8 e The data used for proposed system. Parameter
_ d denotes the exergy destruction rate, Ex _ Q represents where Ex the exergy rate due to heat transfer across the system boundaries. The exergy transfer due to heat can be expressed as follows: _ Q ¼ Q_ 1 T0 Ex Ts
Compound
(12)
where T0 is the ambient temperature that describes the state at which the system is in unrestricted equilibrium with the environment and it cannot undergo any state change through any kind of interaction with the environment [27] and Ts is the temperature of source in case there is a heat penetration and temperature of sink in case there is a heat loss. The exergy destruction rate is calculated by multiplication of ambient temperature with entropy generation in each component:
_1 Mass flow rate of the geothermal steam; m Inlet temperature of geothermal steam; T1 Inlet pressure of geothermal steam; P1 Ambient temperature, T0 Ambient pressure, P0
_ d ¼ T0 S_gen Ex
Value 60 kg/s 515 K 2000 kPa 298 K 101.325 kPa
(13)
where S_ gen denotes the entropy generation rate and it is determined by applying the entropy balance equation for a steady state operation on each component of the system as follows: X
m_ in sin þ
X Q_ in Q_ þ S_gen ¼ m_ out sout þ out Ts Tb
(14)
The mass, energy, entropy and exergy balance equations for each component of the integrated system are shown in Table 2. In addition, the mass, energy, entropy and exergy balance equations for the cooling tower are listed in Table 3.
Fig. 5 e Schematic illustration of the designed system. Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 9 e Thermodynamic properties of the charging period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
Geothermal steam Geothermal steam Geothermal steam Geothermal steam Geothermal steam Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water NCG mixture NCG mixture NCG mixture Hydrogen sulfide Hydrogen Sulfur NCG mixture Air Air NCG mixture Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Water Water Air Air Water Water n_octane n_octane n-octane n_octane Air Air Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800
60 60 60 60 60 3871 3871 1290 1290 1290 70 48 22 19 30 30 3771 3790 2527 1263 9 9 9 0.7388 0.0433 0.6933 8.2612 1738 1738 8.2612 7.667 7.667 7.667 7.667 5.75 50 50 50 44.25 44.25 44.25 5.75 1.917 16.66 16.66 30.23 30.23 7.186 7.186 6.02 6.02 6.02 6.02 187.7 187.7 60 60 60 42 42 18 18 18 18
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600 600 600 600 600 600 600
515 510 490 598 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 510 520 660 660 660 660 660 590 590
2882 2869 2812 3081 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 483.2 503.7 799.6 799.6 799.6 799.6 799.6 647 647
6.507 6.481 6.368 6.866 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.341 1.38 1.883 1.883 1.883 1.883 1.883 1.639 1.639
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 1040 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 95.38 104.4 250.4 250.4 250.4 250.4 250.4 170.4 170.4
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 9 e (continued ) State 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Fluid type
_ i (kg/s) m
Syltherm 800 60 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 No flow during the charging period No flow during the charging period Water 19 Water 3 Water 3 Water 51 Water 0.2 Water 0.2 Hydrogen 0.022 Oxygen 0.1778 Hydrogen 0.0433 Oxygen 0.3467 Water 0.39
Pi (kPa)
Ti (K)
224.9 237.7 159.9 159.9 149.7 149.7
200 200 200 200 200 200 200 200 200 101.3 150
388 388 355 386.1 288 350 370 370 500 298 350
481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
(15)
(16)
where wc and lc are the aperture width and length of the collector, respectively. Schematic illustration of the parabolic trough collector system is shown in Fig. 4. The respective design parameters of the parabolic trough collector system are listed in Table 4. Due to the intermittency of the solar energy, a thermal energy storage tank is integrated to the solar-driven system. Inside the hot storage tank filled with phase change material and heat transfer fluid flows through the hot storage tank via pipe coils. Both latent and sensible heat storage take place into the storage tank. Thermal properties of the phase change material is given in Table 5. The total heat stored by a solid to liquid phase changing material between initial and final temperatures would be estimated by Ref. [21]: qstored
exph,i (kJ/kg)
1.812 1.849 1.603 1.603 1.567 1.567
Here, Iav, Aa, gc are the average hourly solar radiation, collector aperture area and reflection rate of the collector, respectively. Average hourly solar radiation value varies with the time and location. Aperture area of the collector can be calculated from the following equation.
¼ mCps;av ðTm Ti Þ þ mDHm þ mCpl;av Tf Tm Tf > Tm > Ti
x (kg/kg)
752.9 776.9 625.7 625.7 604.7 604.7
The solar driven system comprises parabolic trough collectors and a thermal energy storage tank. The solar driven system including several huge parabolic trough collectors (PTCs) can transfer the solar energy to the heat transfer fluid. A parabolic collector can operate with solar beam and also diffusing of radiation because of its large acceptance angle. Power received by PTC can be calculated as follows:
Aa ¼ wc lc
si (kJ/kgK)
639 650 580 580 570 570
The solar driven cycle
Q_ ptc ¼ Iav Aa gc
hi (kJ/kg)
600 300 300 300 200 200
(17)
where m is the mass of the phase changing material, Cps,av and Cpl,av are the average heat capacities for solid and liquid
phases, respectively, Tm is the melting temperature, Ti and Tf are the initial and final temperatures, respectively and DHm is the heat of fusion. The energy and exergy balances of the overall process of TES tank can be defined as follows [27]: ðHa Hb Þ ½ðHd Hc Þ þ Qlosses ¼ DE
(18)
where Ha and Hb are the total inlet and outlet enthalpies of the heat transfer fluid during the charging period, respectively. In addition, Hc and Hd are the total inlet and outlet enthalpies of the heat transfer fluid during the discharging period, respectively. Also, Qlosses denotes the heat losses during the process and DE represents the accumulation of energy in TES tank. ðExa Exb Þ ½ðExd Exc Þ þ Xl I ¼ DEx
(19)
here, Exa and Exb are the inlet and outlet exergies of the heat transfer fluid during the charging period, respectively. In addition, Exc and Exd are the inlet and outlet exergies of the heat transfer fluid during the discharging period, respectively. Also, Xl denotes the exergy loss associated with Qlosses and I stands for the exergy consumption; and DEx is the exergy accumulation. The overall energy and exergy efficiencies of the TES tank can be defined as follows: h¼
Hd Hc Ha Hb
(20)
Exd Exc Exa Exb
(21)
and j¼
Hydrogen sulfide electrolyzer In this part electrolysis process of hydrogen sulfide will be analyzed through energy and exergy approaches along with emission rates in order to give a detailed information about the process. Table 6 shows the average chemical compositions
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 10 e Thermodynamic properties of the storing period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Water 3871 Water 3871 Water 1290 Water 1290 Water 1290 Water 70 Water 48 Water 22 Water 19 Water 30 Water 30 Water 3771 Water 3790 Water 2527 Water 1263 NCG mixture 9 NCG mixture 9 NCG mixture 9 Hydrogen sulfide 0.7388 Hydrogen 0.0433 Sulfur 0.6933 NCG mixture 8.2612 Air 1738 Air 1738 NCG mixture 8.2612 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 5.75 Ammonia-water 50 Ammonia-water 50 Ammonia-water 50 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 5.75 Ammonia-water 1.917 Water 16.66 Water 16.66 Air 30.23 Air 30.23 Water 7.186 Water 7.186 n_octane 6.02 n_octane 6.02 n-octane 6.02 n_octane 6.02 Air 187.7 Air 187.7 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 No flow during the storing period No flow during the storing period No flow during the storing period No flow during the storing period
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600 600 600
515 510 490 598 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 510 520 604 604 604
2882 2869 2812 3081 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 483.2 503.7 677 677 677
6.507 6.481 6.368 6.866 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.341 1.38 1.689 1.689 1.689
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 1040 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 95.38 104.4 185.6 185.6 185.6
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 10 e (continued ) State
Fluid type
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Syltherm 800 No flow during the No flow during the No flow during the No flow during the No flow during the No flow during the No flow during the Water Water Water Water Water Water Hydrogen Oxygen Hydrogen Oxygen Water
_ i (kg/s) m 60 storing period storing period storing period storing period storing period storing period storing period 19 3 3 51 0.2 0.2 0.022 0.1778 0.0433 0.3467 0.39
Pi (kPa)
Ti (K)
x (kg/kg)
exph,i (kJ/kg)
677
1.689
185.6
200 200 200 200 200 200 200 200 200 101.3 150
388 388 355 386.1 288 350 370 370 500 298 350
481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
The heat transfer between the electrolyzer and the environment is negligible. The auxiliary components are well insulated and capable of conducting electricity without losses. The changes in both potential and kinetic energies and exergies are negligible. The processes occur in steady-state and steady-flow. The temperature and pressure of the hydrogen sulfide are 423 K and 200 kPa, respectively. The temperature and pressure of the products of electrolysis process are 500 K and 200 kPa, respectively. The ambient temperature (T0) and pressure (Po) are 298 K and 101.325 kPa, respectively. The related equations are given for the electrolysis process. In the electrolysis process, hydrogen sulfide is decomposed to pure hydrogen and sulfur dimer by electrical power. Therefore, electrolysis is a chemically reacting process. The required electrical work of the electrolysis process by neglecting the heat transfer from/to ambient can be defined as follows [26]: X, X, np h0 f þ h h0 p nr h0 f þ h h0 r
(22)
Here, p and r subscripts represent the products and reactants of the electrolysis process. Products of the process are hydrogen and sulfur dimer and hydrogen sulfide is the only reactant for the electrochemical process. Moreover, reversible work of the electrolysis process can be expressed as follows:
jdesigned ¼
si (kJ/kgK)
604
of non-condensable gases in mole percent and the following calculations are based on these data. In addition, the following assumptions are made respectively for the analysis:
_ electrolysis ¼ W
hi (kJ/kg)
600
_ rev ¼ W
X, X, nr h0 f þ h h0 T0 s r np h0 f þ h h0 T0 s p (23)
For the electrolysis process the steady-state and steadyflow entropy balance equation can be written as follows: m_ H2 S sH2 S þ S_gen ¼ m_ H2 sH2 þ m_ S2 sS2
(24)
Solid oxide fuel cell For the electrochemical process which occurs in the fuel cell the produced electricity can be expressed as follows: _ produced ¼ W
X, X, nr h0 f þ h h0 r np h0 f þ h h0 p
(25)
The products of the fuel cell conversion process is water and reactants are hydrogen and oxygen. For the fuel cell conversion process the steady-state and steady-flow entropy balance equation can be written as follows: m_ H2 sH2 þ m_ O2 sO2 þ S_gen ¼ m_ H2 0 sH2 0
(26)
The designed system which consists of AMIS unit, hydrogen sulfide electrolyzer and solid oxide fuel cell system is shown in Fig. 5. The overall energy efficiency of the designed system which includes an electrolyzer and a fuel cell system can be defined as follows: hdesigned ¼
_ produced þ m_ H 0 hH 0 W 2 2 _ electrolysis þ m_ H S hH S þ m_ O hO þ m_ H LHVH W 2 2 2 2 2 2
(27)
Also, the overall exergy efficiency of the designed system can be written as
_ produced þ m_ H O exH O W 2 2 _ electrolysis þ m_ H S exH S þ m_ O exO þ m_ H exph þ exch W 2 2 2 2 2 H2
(28)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 11 e Thermodynamic properties of the discharging period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Water 3871 Water 3871 Water 1290 Water 1290 Water 1290 Water 70 Water 48 Water 22 Water 19 Water 30 Water 30 Water 3771 Water 3790 Water 2527 Water 1263 NCG mixture 9 NCG mixture 9 NCG mixture 9 Hydrogen sulfide 0.7388 Hydrogen 0.0433 Sulfur 0.6933 NCG mixture 8.2612 Air 1738 Air 1738 NCG mixture 8.2612 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 5.75 Ammonia-water 50 Ammonia-water 50 Ammonia-water 50 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 5.75 Ammonia-water 1.917 Water 16.66 Water 16.66 Air 30.23 Air 30.23 Water 7.186 Water 7.186 n_octane 6.02 n_octane 6.02 n-octane 6.02 n_octane 6.02 Air 187.7 Air 187.7 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 No flow during the discharging period No flow during the discharging period Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600
515 510 490 544 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 495 520 544.7
2882 2869 2812 2955 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 453.9 503.7 553
6.507 6.481 6.368 6.645 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.282 1.38 1.474
600 600 600 600
544.7 544.7 567 567
553 553 599 599
1.474 1.474 1.556 1.556
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 979.5 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 83.84 104.4 125.8
125.8 125.8 147.2 147.2
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 11 e (continued ) State 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Fluid type
_ i (kg/s) m
Syltherm 800 60 Syltherm 800 18.17 Syltherm 800 18.17 No flow during the discharging period No flow during the discharging period Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Water 19 Water 3 Water 3 Water 51 Water 0.2 Water 0.2 Hydrogen 0.022 Oxygen 0.1778 Hydrogen 0.0433 Oxygen 0.3467 Water 0.39
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
x (kg/kg)
exph,i (kJ/kg)
600 200 200
567 622 548
599 715.2 559.1
1.556 1.753 1.486
147.2 204.9 128.1
300 300 200 200 200 200 200 200 200 200 200 200 101.3 150
560 560 548 388 388 355 386.1 288 350 370 370 500 298 350
583.9 583.9 559.1 481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.531 1.531 1.486 1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
139.7 139.7 128.1 47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
Fig. 6 e Entropy generation rates of main components of the integrated system.
m_ M
Water electrolyzer
n ¼
In electrolysis process, the water is decomposed to pure hydrogen and oxygen by electrical power. Therefore electrolysis is a chemically reacting process. The required electrical work of the electrolysis process by neglecting the heat transfer to/from ambient can be defined as follows:
_ where mand M denote the mass flow rate and molecular weight, respectively. The enthalpy of formations of the system compounds are given in Table 7. The energy efficiency of the electrolysis process can be defined as follows:
_ in ¼ W _ electrolyzer ¼ W
X, X, np h0 f þ h h0 p nr h0 f þ h h0 r (29)
where n_ represents the molar flow rate and can be written as follows:
hPEM ¼
(30)
m_ H2 LHVH2 _ electrolyzer W
(31)
where LHVH2 represents the lower heating value of hydrogen. In addition, the exergy efficiency of the electrolysis process can be also written as:
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 9 e Effect of the inlet mass flow rate of geothermal steam on the overall energy efficiency of the system.
Fig. 7 e The highest entropy generation rates of main components of the integrated system.
jPEM ¼
m_ H2 exph þ exch _ electrolyzer W
H2
(32)
(35)
where exch;H2 denotes the chemical exergy of the hydrogen and can be calculated as [31]: exch;H2 ¼
236:1 1000 MWH2
(33)
where 236.1 kJ/g mole is taken to be exergy content of hydrogen andMWH2 is the molar mass of hydrogen in kg/kmol.
Main assumptions
where Q_ Heating is the sum total of domestic hot water and space heating outputs of the designed system. Also, Q_ Drying denotes the total drying heat which is provided through the heated air which leaves the cooling tower at state (29). At the end of this section, the main assumptions of the overall system are given in Table 8.
Results and discussion
By considering all useful outputs and total inputs, overall system energy and exergy efficiencies can be written as follows: hoverall ¼
_ QHeating þ Ex _ QDrying þ Ex _ QCooling þ m_ H exph þ exch _ net þ Ex W 2 H2 joverall ¼ _ QSolar _ QGeo þ Ex Ex
_ net þ Q_ Heating þ Q_ Drying þ Q_ Cooling þ m_ H LHVH W 2 2 Geo Solar E_ þ E_
(34)
Fig. 8 e Entropy generation rates of some components of the integrated system for three different periods.
A comprehensive thermodynamic analysis, by considering all stream points, is carried out for the integrated system with hydrogen production units. In the analysis section, thermodynamic models presented and the entire system is analyzed using the Engineering Equation Solver (EES) software which can evaluate the thermodynamic state properties of different substances through built-in internal
Fig. 10 e Effect of the inlet mass flow rate of geothermal steam on the overall exergy efficiency of the system.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 11 e Effect of ambient temperature on the overall exergy efficiency of the designed system.
functions. Additionally, both energy and exergy evaluations are carried out in various operating conditions and design parameters such as inlet mass flow rate of geothermal steam and ambient temperature. Moreover, the entropy generations and exergy destructions are determined for each component of the system. The proposed multigeneration system operates in three different scenarios. In the first scenario, average hourly solar radiation has the highest value and both TES tank and heat exchanger 2 are fed by the heat transfer fluid. In other words, valves 1,2 and 3 open during this period. Thermodynamic properties of the all streams of the first working principle are listed in Table 9. In the second scenario, the average hourly solar radiation is sufficient only to feed the heat exchanger 2. During this period, valve 1 and 3 are shut-downed. Thermodynamic properties of the all stream points of this mechanism are given in Table 10. In the last scenario, the average hourly solar radiation is very low. Heat transfer fluid which is circulated in the solar driven system, enters the TES tank and receives the heat of the PCM and leaves the TES tank as heated at state (66). In this period, valve 2 is shut-downed and heat transfer fluid is not divided into two streams at state (58). Thermodynamic
Fig. 12 e Effect of the inlet temperature of water on the energy and exergy efficiencies of PEM electrolyzer.
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Fig. 13 e Hydrogen sulfide and hydrogen production rates depending on the inlet mass flow rate of geothermal fluid.
properties of the all stream points of this period are given in Table 11. As mentioned above, the exergy is destructed due to the entropy generation. Fig. 6 and Fig. 7 show the entropy generation rates of main components in the integrated cycle. The highest entropy generation rate is observed in the cooling tower with 31.48 kW/K. On the other hand, the all devices which are illustrated in Fig. 6, have entropy generation rates below 1.21 kW/K. Due to the different operation scenarios, some components of the multigeneration system have different entropy generation rates in the charging, storing and discharging periods. These data are illustrated in Fig. 8. Entropy generation rate of the parabolic trough collector system in the charging period is 27.24 kW/K. Additionally, its entropy generation rates in the storing and discharging periods are 16.82 kW/K and 5.138 kW/K, respectively. The inlet mass flow rate of the geothermal steam is a significant design criterion for the proposed system. Fig. 9 demonstrates for each 3 operation periods how overall energy of the designed system changes, depending on the inlet mass flow rate of geothermal steam. It is clear that, for all 3 periods overall energy efficiency of the system increases, while inlet mass flow rate rises. The highest overall energy efficiency takes place in the storing period at around 82% while the inlet mass flow rate of geothermal steam is about 200 kg/s.
Fig. 14 e Effect of the mole percent of hydrogen sulfide on the amounts of produced hydrogen sulfide and hydrogen.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 10 illustrates how overall exergy efficiency of the integrated system changes, depending on the inlet mass flow rate of the geothermal steam. Similarly, overall exergy efficiency of the system rises, while inlet mass flow rate increases. The highest overall exergy efficiency of the system is calculated in the storing period as approximately 68%, while inlet mass flow rate of the geothermal steam is around 200 kg/ s. Fig. 11 shows the effect of ambient temperature on the overall exergy efficiency of the integrated system for the each period. It can be clearly inferenced that, overall exergy efficiency of the system decreases, while ambient temperature rises. In the charging period, the overall exergy efficiency changes between 56.99% and 48.81% when ambient temperature rises from 273 to 313 K. Fig. 12 demonstrates how energy and exergy efficiency of the PEM electrolyzer changes depending on the inlet temperature of water. There is a linear relation between the efficiencies and water inlet temperature, such that both energy and exergy efficiency of the PEM electrolyzer increases when the inlet temperature of water rises. The energy and exergy efficiency are approximately 82.9% and 83.4%, when inlet temperature of water is around 290 K. Furthermore, as the temperature of the electrolysis process products rises, both energy and exergy efficiency get lower values. The geothermal fluid mainly consists of steam with some percentage (from less than 1% and up to 15%) of noncondensable gases. For the present study, it is important to determine the amount of hydrogen sulfide which is released to atmosphere depending on the inlet mass flow rate of the geothermal fluid. Through AMIS and electrolyzer units, hydrogen sulfide is decomposed to pure hydrogen and sulfur dimer in order to reduce the adverse effects of hydrogen sulfide and produce additional power supply by means of produced hydrogen. Fig. 13 shows the obtained amounts of hydrogen sulfide and hydrogen depending on the inlet mass flow rate of the geothermal fluid for the different NCG compositions. As expected, while NCG composition gets higher values, obtained hydrogen sulfide and correspondingly hydrogen amounts increase. While inlet mass flow rate of the geothermal fluid is 60 kg/s, 0.7388 kg hydrogen sulfide
Fig. 15 e The amount of produced electrical power depending on the mass flow rate of hydrogen sulfide.
Fig. 16 e Effect of ambient temperature on the exergy efficiency of the designed system.
and 0.0433 kg hydrogen emerge per second for 15% NCG composition. The geothermal fluid is present with some percentages of non-condensable gases. In the present study noncondensable gases of the geothermal fluid are assumed as nitrogen, oxygen, methane, carbon dioxide and hydrogen sulfide. As expected, while mole percent of hydrogen sulfide increases among the non-condensable gases, more hydrogen sulfide is emitted and correspondingly more hydrogen can be generated through the electrolysis process as shown in Fig. 14. While the mole percent of hydrogen sulfide rises from 3% to 21%, the amount of produced hydrogen increases from 26.75 g/s to 156 g/s, considering the inlet mass flow rate of the geothermal fluid as 120 kg/s. In the designed system, hydrogen which is produced from hydrogen sulfide through the electrolysis process is sent to the fuel cell system in order to generate power by interacting with incoming oxygen stream. Fig. 15 shows that how the amount of produced power changes depending on the mass flow rate of hydrogen sulfide. According to the figure, produced power rises from 0.852 MW to 12.79 MW, while the mass flow rate of hydrogen sulfide increases from 0.1 kg/s to 1.5 kg/s.
Fig. 17 e Effect of hydrogen sulfide inlet temperature on the exergy destruction rate of electrolyzer unit.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
international journal of hydrogen energy xxx (xxxx) xxx
Table 12 e Calculated parameters used to evaluate the system. Parameters
Charging Period
Storing Period
Discharging Period
Turbine 1 power output Turbine 2 power output Solar heat input, Q_solar The number of collectors Mass of the phase change material, mPCM Overall energy efficiency Overall exergy efficiency
264.9 kW 21554 kW 17756 kW 185 70571 kg
264.9 kW 21554 kW 10399 kW 185 70571 kg
264.9 kW 13994 kW 2959 kW 185 70571 kg
75.14% 52.35%
78.37% 58.40%
76.39% 48.96%
Fig. 16 illustrates how exergy efficiency of the designed system (Fig. 5) changes depending on the ambient temperature. It is clear that, it has a little impact on the efficiency. While ambient temperature rises from 280 K to 310 K, the exergy efficiency value changes between 84.92% and 85.37%, by assuming the inlet temperature of hydrogen sulfide as 483 K. The exergy destruction rate of the electrolysis process decreases when the inlet temperature of hydrogen sulfide increases as shown in Fig. 17. By contrast, the exergy destruction rate reaches the higher values with rising ambient temperatures. The calculated parameters of the integrated system are given in Table 12 for each period, considering the assumptions.
Conclusions In this paper, a novel renewable energy based multigeneration system equipped with hydrogen production units is designed and analyzed for heating, drying, cooling, domestic hot water, power and hydrogen production. The electricity production is achieved by geothermal based organic Rankine cycle and solar driven system which includes numerous huge parabolic trough collectors. Due to the intermittent nature of the solar energy, a thermal energy storage tank is integrated to the solar driven system in order to operate the designed system continually. On the other hand, geothermal power plants discharge relatively high amounts of H2S to their operating field which, in turn, can cause unfavourable impacts on the environment and the human body. Furthermore, it is reasonable to produce hydrogen as a promising energy carrier from hydrogen sulfide in geothermal power plants. The present study aims to establish a new model for hydrogen production from hydrogen sulfide by adding an electrolyzer to a geothermal power plant equipped with AMIS abatement system. The effects of certain operating conditions on the subsystems and overall system performance are investigated. The effects of various parameters on the efficiencies such as ambient temperature, inlet mass flow rate of geothermal steam, inlet temperature of water are investigated and results are comparatively discussed. The key points can be written as follows: The highest entropy generation rate is observed in the cooling tower with 31.48 kW/K. On the other hand, the all
19
devices which are illustrated in Fig. 6, have entropy generation rates below 1.21 kW/K. For all 3 periods overall energy efficiency of the system increases, while inlet mass flow rate of geothermal steam rises. The highest overall energy efficiency takes place in the storing period at around 82% while the inlet mass flow rate of geothermal steam is about 200 kg/s. The overall exergy efficiency of the proposed system rises, while inlet mass flow rate of geothermal steam increases. The highest overall exergy efficiency of the system is calculated in the storing period as approximately 68%, while inlet mass flow rate of the geothermal steam is around 200 kg/s. The overall exergy efficiency of the integrated system decreases, while ambient temperature rises. In the charging period, the overall exergy efficiency changes between 56.99% and 48.81% when ambient temperature rises from 273 to 313 K. There is a linear relation between the efficiencies and water inlet temperature, such that both energy and exergy efficiency of the PEM electrolyzer increases when the inlet temperature of water rises. The energy and exergy efficiencies are approximately 82.9% and 83.4%, when inlet temperature of water is around 290 K. Furthermore, as the temperature of the electrolysis process products rises, both energy and exergy efficiencies get lower values. While NCG composition gets higher values, obtained hydrogen sulfide and correspondingly hydrogen amounts increase. While inlet mass flow rate of the geothermal fluid is 60 kg/s, 0.7388 kg hydrogen sulfide and 0.0433 kg hydrogen emerge per second for 15% NCG composition. As expected, while mole percent of hydrogen sulfide increases among the non-condensable gases, more hydrogen sulfide is emitted and correspondingly more hydrogen can be generated through the electrolysis process. While the mole percent of hydrogen sulfide rises from 3% to 21%, the amount of produced hydrogen increases from 26.75 g/s to 156 g/s, considering the inlet mass flow rate of the geothermal fluid as 120 kg/s. The exergy destruction rate of the electrolysis process decreases when the inlet temperature of hydrogen sulfide increases. By contrast, exergy destruction rate reaches the higher values with rising ambient temperatures.
Acknowledgment This study was supported by the Scientific Research Projects Council of Yildiz Technical University [grant number 3349].
Nomenclature Aa Cp Cpl,av Cps,av ex exch
Collector aperture area (m2) Specific heat (kJ/kgK) Average specific heat of the liquid phase (kJ/kgK) Average specific heat of the solid phase (kJ/kgK) Specific exergy (kJ/kg) Chemical exergy (kJ/kg)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
20
exph E_ _ d Ex h hºf DHm Iav lc LHV m _ m MW n_ s S_ gen T Q_ wc _ W
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Physical exergy (kJ/kg) Energy transfer rate (kW) Exergy destruction rate (kW) Specific enthalpy (kJ/kg) Enthalpy of formation (kJ/mol) Heat of fusion (kJ/kg) Average hourly solar radiation (kJ/m2s) Aperture length (m) Lower heating value (kJ/kg) Mass (kg) Mass flow rate (kg/s) Molecular weight (kg/kmol) Molar flow rate (kmol/s) Specific entropy (kJ/kgK) Entropy generation (kW/K) Temperature (K) Heat transfer rate (kW) Width of the collector (m) Work rate (kW)
Greek letters h Energy efficiency j Exergy efficiency ɣ Reflection rate Subscripts av Average ch Chemical in Inlet out Outlet p Product ph Physical r Reactant Superscripts Geo Geothermal Acronyms EES Engineering equation solver NCG Non-condensable gases ORC Organic Rankine cycle PCM Phase change material PEM Polymer electrolyte membrane PTC Parabolic trough collector SAE Solid acid electrolyte SEACS Single effect absorption cooling system SOEC Solid oxide electrolyzer cell SOFC Solid oxide fuel cell TES Thermal energy storage
[5]
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[7]
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[9]
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[21]
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Rankine cycle using low-temperature geothermal source. Int J Energy Res 2011;35:530e44. Kilkis B. A lignite- geothermal hybrid power and hydrogen production plant for green cities and sustainable buildings. Int J Energy Res 2011;35:138e45. Ghafghazi S, Sowlati T, Sokhansanj S, Melin S. Technoeconomic analysis of renewable energy source options for a district heating project. Int J Energy Res 2009;34:1109e20. Renato S, Domenico G, Claudia T, Carlo T. Modelling of hydrogen sulfide dispersion from the geothermal power plants of Tuscany (Italy). Sci Total Environ 2017;583:408e20. World Health Organization (WHO). Hydrogen sulfide: human health aspects, vol. 53. Geneva: Concise International Chemical Assessment Document; 2003. Rand D, Dell R. Hydrogen energy: chalenges and prospects. The royal society of chemistry. 2008. p. 33. Chapter 1: Why hydrogen energy. Dincer I, Zamfirescu C. Renewable energy-based multigeneration systems. Int J Energy Res 2012;36:1403e15. Al-Ali M, Dincer I. Energetic and exergetic studies of a multigenerational solar-geothermal system. Appl Therm Eng 2014;71:16e23. Ezzat M, Dincer I. Energy and exergy analyses of a new geothermal-solar energy based system. Sol Energy 2016;134:95e106. Bicer Y, Dincer I. Development of a new solar and geothermal based combined system for hydrogen production. Sol Energy 2016;127:269e84. Akrami E, Chitsaz A, Nami H, Mahmoudi S. Energetic and exergoeconomic assessment of a multi- generation energy system based on indirect use of geothermal energy. Energy 2017;124:625e39. Yuksel Y, Ozturk M, Dincer I. Energetic and exergetic performance evaluations of a geothermal power plant based integrated system for hydrogen production. Int J Hydrogen Energy 2018;43:78e90. Nikolaidis P, Poullikkas A. A comparative overview of hydrogen production processes. Renew Sustain Energy Rev 2017;67:597e611. Mehrpooya M, Dehghani H, Moosavian A. Optimal design of solid oxide fuel cell, ammnonia-water single effect absorption cycle and Rankine steam cycle hybrid system. J Power Sources 2016;306:107e23. Mohammadi A, Mehrpooya M. Techno-economic analysis of hydrogen production by solid oxide electrolyzer coupled with dish collector. Energy Convers Manag 2018;173:167e78. Chamorro C, Mondejar M, Ramos R, Villamanan M. World geothermal power production status: energy, environmental and economic study of high enthalpy technologies. Energy 2012;42:10e8. Yaws C. Chemical properties handbook: physical, thermodynamic, environmental, transport, safety, and health related properties for organic and inorganic chemicals. McGraw-Hill; 1999. Demirel Y. Energy: production, concersion, storage, conservation, and coupling. 2nd ed. Springer; 2015. Baldacci A, Mannari M, Sansone F. Greening of geothermal power: an innovative technology for abatement of hydrogen sulphide and mercury emission. Proc World Geoth Congress 2005. Karapekmez A, Dincer I. Modelling of hydrogen production from hydrogen sulfide in geothermal power plants. Int J Hydrogen Energy 2018;43:10569e79. Krakow B, Weaver E, Ecklund-Mitchell L, Stefanakos E, Moore G, Smith M. Hydrogen production by electrochemical dissociation of hydrogen sulfide in IGCC plants. In: The 22nd annual international coal conference, United States; 2005.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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[25] Shekhawat D, Spivey J, Berry D. Fuel cell technologies for fuel processing. Elsevier; 2011. [26] Cengel Y, Boles M. Thermodynamics an engineering approach. McGraw-Hill Education; 2015. [27] Dincer I, Rosen M. Exergy: energy, environment and sustainable development. 2nd ed. Elsevier; 2013. [28] Coscia K, Nelle S, Elliot T, Neti S. Thermophysical properties of LiNO3-NaNO3-KNO3 mixtures for use in concentrated solar power. J Sol Energy Eng 2013;135.
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[29] Peralta O, Castro T, Duron M, Salcido A, Murillo A, Gonzalez R, Marquez C. H2S emissions from Cerro Prieto geothermal power plant, Mexico, and air pollutants measurements in the area. Int J Geoth Res Appl 2013;46:55e65. [30] http://nshs-science.net/chemistry/common/pdf/Rstandard_enthalpy_of_formation.pdf ; [Accessed 13 August 2018]. [31] The Exergoecology Portal. http://www.exergoecology.com/ excalc/ ; [Accessed 17 August 2018].
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production* Aras Karapekmez a,*, Ibrahim Dincer b,a a
Faculty of Mechanical Engineering, Yildiz Technical University, Besiktas, Istanbul, Turkey Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, Canada
b
article info
abstract
Article history:
The present study develops a new solar and geothermal based integrated system,
Received 2 September 2018
comprising absorption cooling system, organic Rankine cycle (ORC), a solar-driven system
Received in revised form
and hydrogen production units. The system is designed to generate six outputs namely,
30 November 2018
power, cooling, heating, drying air, hydrogen and domestic hot water. Geothermal power
Accepted 5 December 2018
plants emit high amount of hydrogen sulfide (H2S). The presence of H2S in the air, water,
Available online xxx
soils and vegetation is one of the main environmental concerns for geothermal fields. In this paper, AMIS(AMIS® - acronym for “Abatement of Mercury and Hydrogen Sulphide” in
Keywords:
Italian language) technology is used for abatement of mercury and producing of hydrogen
Hydrogen production
from H2S. The present system is assessed both energetically and exergetically. In addition,
Solar energy
the energetic and exergetic efficiencies and exergy destruction rates for the whole system
Geothermal energy
and its parts are defined. The highest overall energy and exergy efficiencies are calculated
Hydrogen sulphide
to be 78.37% and 58.40% in the storing period, respectively. Furthermore, the effects of
Exergy
changing various system parameters on the energy and exergy efficiencies of the overall
Efficiency
system and its subsystems are examined accordingly. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the last years, public attention has increasingly been shifted by the media and world governments to the concepts of saving energy, reducing pollution, protecting the environment, and developing long-term energy supply solutions. In parallel, research funding relating to alternative fuels and energy carriers is increasing on both national and international levels. The main concerns caused by the world's current
energy supply system are mainly based on fossil fuels. In fact, the energy stored in hydrocarbon-based solid, liquid, and gaseous fuel was, is, and will be widely consumed for internal combustion engine-based transportation, for electricity and heat generation in residential and industrial sectors, as it is convenient, abundant and cheap. However, such a widespread use of fossil fuels gives rise to the two problems of oil supply and environmental degradation [1]. Currently, more than 80% of the world's energy supply comes from fossil fuels. The ongoing growth in fossil fuel
*
This study was supported by the Scientific Research Projects Council of Yildiz Technical University (YTU-BAP), Grant number: 3349. * Corresponding author. E-mail address: [email protected] (A. Karapekmez). https://doi.org/10.1016/j.ijhydene.2018.12.046 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
2
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consumption suggests that global carbon dioxide emissions are still rising [2]. One solution for these problems is aimed to find new, environmentally benign, economical, reliable, and secure energy sources and technologies. The natural sources of energy, such as solar, wind, geothermal, biomass, and municipal waste are defined as renewable such that these energy sources are replenished naturally after use. In this respect, the use of renewable energy sources plays a key role for a sustainable energy future. Geothermal energy is considered as one of the promising alternative energy resources, which can be utilized for the purpose of electricity, heating, cooling or hydrogen production goals. The utilization of geothermal source either for power generation, cooling or heating applications depends mainly on the source temperature. High-temperature geothermal resources above 150 C are generally used for power generation. Moderate-temperature (between 90 and 150 C) and low-temperature (below 90 C) geothermal resources are best suited for direct applications such as space and process heating, cooling, aquaculture, and fish farming [3,4]. It is obviously better to use high-temperature geothermal source for multi-generation purposes. Generating multiple outputs from one source will not only increase the efficiency of the system but will also make more cost effective. The multi-generation capacity of geothermal has recently attracted attention from many researchers around the world [5,6]. On the other hand, one of the most important enviromental issues related to the using geothermal operating fluids to generate electricity is non-condensable gases emission. The vent stacks in geothermal plants emit carbon dioxide (CO2) and methane (CH4) gases which consequently raise serious concerns in terms of greenhouse gases. The amount of these emissions are quite small compared to carbon and fossil fuel plants, which indicates that the contribution of these sources is practically negligible. Geothermal power plants also emit higher amount of hydrogen sulfide due to the employment of H2S as a main constituent of the geothermal fluids. The presence of H2S in the air, water, soils and vegetation is one of the main enviromental concerns [7]. H2S is normally in gas phase and can be absorbed in lungs through inhalation. Health effects include respiratory, ocular, neurological, and metabolic effects and the death after single exposures to concentrations higher than 700 mg/m3 [8]. A summary of these effects is presented in Table 1.
Table 1 e Human health effects resulting from exposure to H2S. Effect/Observation Odor threshold Bronchial constriction in asthmatic individuals Increased eye complaints Increased blood lactate concentration, decreased oxygen uptake Eye irritation Oldfactory paralysis Respiratory distress Death Source: Complied from Ref. [8].
Exposure (mg/m3) 0.011 2.8 5.0 7.0e14.0 5.0e29.0 >140 >560 700
On the other side, by virtue of its physical and chemical properties, it is clear that hydrogen has the potential to occupy a unique position in the future world energy scene. Not only it could become ultimately a universal means of conveying and storing energy but also an entirely novel fuel with properties that are distinct from those of other fuels. Hydrogen becomes an apparent obvious choice for a low-carbon economy in that it would liberate no pollutants to the atmosphere and, when coupled to carbon sequestration or derived from non-fossil primary energy sources, little or no carbon dioxide to contribute to climate change [9]. When unfavourable effects of hydrogen sulphide and on the other hand superiorities of hydrogen take into consideration, decomposition of hydrogen sulphide to hydrogen can be characterized as a reasonable and profitable process. Renewable-based energy systems should be designed in a more effective way so as to attain higher efficiencies. One way to achieve this is to plan the system by using the approaches of cogeneration and especially multigeneration. Since, multigenerational systems can provide better efficiency, costeffectiveness, environment and hence better sustainability [10]. There has been a lot of research [11e15] on renewable energy based multigeneration systems. Al-Ali et al. [11] proposed a solar and geothermal integrated based system for multigeneration for electrical power, cooling, space heating, hot water and heat for industrial use. Detailed thermodynamic analyses were conducted for single generation, cogeneration, trigeneration and multigeneration systems through energy and exergy approaches in order to show the performance of the each system and compare the results. The energy efficiencies for single generation and multigeneration systems were found to be 16.4% and 78%, respectively, while the exergy efficiencies were 26.2% and 36.6%, respectively. Ezzat et al. [12] proposed a newly developed solar and geothermal based multigenerational energy system. Their system includes a single flash geothermal cycle, heat pump system, single-effect absorption cooling system, thermal energy storage, hot water and drying system. The main goal of the study is to obtain five outputs, namely cooling, heating, domestic hot water, drying and electricity. The overall energy and exergy efficiencies are found to be 69.6% and 42.8%, respectively. Bicer et al. [13] suggested a novel solar and geothermal based combined system for hydrogen production. The system comprises PV/T modules for the purpose of heating, water heating and hydrogen production and geothermal energy sources for electricity, cooling and again hydrogen production. Energy and exergy analyses were carried out to assess the performance of the cycle, and the effects of various system parameters on energy and exergy efficiencies of the overall system. The results show that overall energy and exergy efficiencies of the combined system can reach up to 10.8% and 46.3%, respectively for a geothermal water temperature of 210 C. Akrami et al. [14] designed a geothermal based multigeneration energy system in order to generate electricity, heating, cooling and hydrogen, simultaneously. The system consists of organic Rankine cycle, domestic water heater, absorption refrigeration cycle and proton exchange membrane electrolyzer. Detailed energy and exergy analyses of the
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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system were made as well as exergoeconomic analyses. Also the effects of some important variables, i.e. geothermal water temperature, turbine inlet temperature and pressure, generator temperature, geothermal water mass flow rate and electrolyzer current density on the several parameters such as energy and exergy efficiencies of the proposed system were investigated. The overall energy and exergy efficiencies of the integrated system were calculated about 34.98% and 49.17%, respectively. Ozturk et al. [15] developed a geothermal based integrated system in order to obtain electricity, cooling, heating, domestic hot water as well as hydrogen fuel. Energy and exergy efficiencies and exergy destruction rates for the whole system and its subsytems were defined. The results show that overall energy and exergy efficiencies of the designed system are 39.46% and 44.27%, respectively. Nikolaidis et al. [16] have performed a review study for comparison of hydrogen production from renewable sources. According to their study, the most cost effective thermochemical process for hydrogen production is supercritical gasification of biomass. There is remarked that producing hydrogen from geothermal resources is less expensive than solar and wind energy sources. Mehrpooya et al. [17] investigated a combined system including solid oxide fuel cell e gas turbine power plant, Rankine steam cycle and ammonia-water absorption cooling subsystem. The proposed system is investigated through the energy and exergy approaches as well as economic perspectives. Moreover, the developed electrochemical model of the fuel cell is validated with the experimental data. Their results show that electrical efficiency of the combined system is 62.4%. In another study, Mohammadi et al. [18]. Conducted a techno-economic analysis of a solid oxide electrolyzer for hydrogen production. In their study, a dishtype solar collector system is used to provide the required energy of a solid oxide electrolyzer cell (SOEC) to produce hydrogen. Since dish collectors operate at high-temperature, they would be an ideal match for high temperature electrolyzers. Their results show that efficiency of the power cycle and the electrolyzer cell is equal to 72.69% and 61.70%, respectively. In the present study, a novel multigeneration system based on solar and geothermal energy for electricity, hydrogen production, cooling, heating, drying, and hot domestic water is developed and analyzed thermodynamically. The designed system comprises already proven and mature technologies which increases the feasibility of the system. The system differs from many multigeneration systems in terms of zero fossil resources usage and zero carbon emissions by utilizing solar and geothermal energy sources. For the first time, unlike previous studies hydrogen production from hydrogen sulfide is taken into consideration along with electrolysis of water. The underlying motivation of this process is not only to produce hydrogen from hydrogen sulfide but also to reduce the hydrogen sulfide emissions and thereby its adverse effects on the environment in geothermal power plants. In the proposed system unlike previous studies, the intermittent nature of the solar energy is taken into consideration dynamically, and a thermal energy storage system is also integrated into the solar driven cycle. Moreover, the system is designed to operate at three
3
different scenarios, namely, charging, storing, and discharging periods and the results are presented in detail for each period. Furthermore, the particular objectives of this paper are to carry out a detailed thermodynamic analyses of the designed system through energy and exergy approaches, including determination of the overall energy and exergy efficiencies of the multigeneration system and its subsystems; and to conduct a parametric study to determine the effects of various parameters on the overall energy and exergy efficiencies of the proposed system.
System description Fig. 1 shows a comprehensive schematic view of the designed integrated system, and how the subsystems interact with each other. According to the figure, the power plant comprises four main subsystems, i.e., absorption cooling system, organic Rankine cycle, the solar-driven cycle and hydrogen production units. The proposed system produces six useful outputs through the solar and geothermal energy which are electrical power, cooling, heating, drying, domestic hot water and hydrogen. High-temperature geothermal sources range between 150 and 350 C for many regions in the world [19]. In this paper, hot geothermal fluid is extracted from the production well above 250 C.
Single-effect absorption cooling system A single-effect absorption cooling system (SEACS) is integrated into the main integrated system in order to supply cold air to cold store at around 18 C for the purpose of food freezing. The ammonia-water solution is chosen as the refrigerant of the absorption cooling system, since lithium bromide (LieBr) can not operate at this low-temperature range without crystallization. In the single-effect absorption cooling system, a strong ammonia-water solution enters the generator at state (38), and is heated by the geothermal steam. As the strong ammonia-water warms, it starts to vaporize and consequently a weak solution of ammoniawater at a higher temperature than state (38) leaves the generator at state (39). The weak solution flows through the heat exchanger 1, where its thermal energy transfers to the incoming strong solution at state (37). The weak solution leaving at state (40) enters the expansion valve 1, where its pressure decreases before it enters the absorber at state (41). The concentrated ammonia-water vapor at state (31) flows into the condenser 1, where the ammonia-water vapor transfers its heat to the domestic water before leaving at state (32). The ammonia-water vapor at state (32) enters the expansion valve 2, where its pressure and temperature decrease. This cooled ammonia-water vapor at state (33) enters the evaporator, where the ammonia-water mixture absorbs heat from the air which is provided from the cold store and leaves at state (34). The weak solution at state (41) and ammonia-water mixture at state (35) flow into the absorber, where they transfer their thermal energy to the domestic water and exit as a strong solution in liquid form at state (36) and passes through the pump 1.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 1 e A schematic diagram of the present solar and geothermal based multigeneration system equipped with AMIS units.
Organic Rankine cycle To have an efficient ORC, the working fluid in the ORC should have a high critical temperature. One of the typical organic fluid types used to operate the ORC is n-octane, which has a relatively high critical temperature, 296 C [20]. Therefore it is selected as the working fluid of ORC. Hot geothermal steam at state (2) enters the boiler of the ORC and transfers its heat to the working fluid. Working fluid which is heated and vaporized in boiler goes into the turbine 1 at state (52) to generate power, and leaves at state (53) then enters the condenser 2 to be condensed. During the condensation process, air is extracted from buildings at state (54) and receives the thermal energy of the working fluid and then it is supplied to buildings at state (55) with the aim of space heating. Afterwards, working fluid passes through the pump 2.
The solar driven cycle The solar driven system including numerous huge parabolic trough collectors are supposed to transfer the solar energy to the heat transfer fluid of the solar driven cycle. Main advantages of using Syltherm 800 are environmentally-friendly characteristics, no troublesome phase change, higher working temperatures, and easy operation and hence Syltherm 800 is chosen as the heat transfer fluid of the solar driven system. Due to the intermittency of the solar energy, a thermal energy storage (TES) tank is integrated to the solar driven system. The system operates in three different scenarios: a) charging period, b) storing period and c) discharging period. During the charging period, the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated is
divided into two streams. Some part of the heat transfer fluid flows into the heat exchanger 3 at state (62) and where its heat transfers to the incoming stream at state (67) which circulates inside the TES tank. The heat transfer fluid which leaves the heat exchanger 3 at state (66) as heated passes through the TES tank and transfers its thermal energy to the phase change material (PCM) which is located inside the tank. Both sensible and latent heat transfer occur in this period. The latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas, and liquid-gas phase changes. Liquid-gas transitions have a higher heat of transformation than solid-liquid transitions. However, liquid-gas phase changes are not practical because of the large volumes or high pressures required to store the materials when they are in their gas phase states. Solid-solid phase change process is typically very slow and has a rather low heat of transformation [21]. Therefore, in the present study, the proposed system is designed for the solidliquid phase change. Afterwards, the heat transfer fluid leaves the TES tank as cooled at state (70). The rest part of the heat transfer fluid at state (59), after circulating inside the pipe line connects with the incoming flow at state (65) and then enters the heat exchanger 2. The main objective of the solar driven system is to increase temperature of the geothermal steam which flows into the heat exchanger 2 at state (3) and leaves it at higher temperatures so that more electrical power can be obtained from turbine 2. During the storing period, valve 1 and valve 3 are closed. Therefore, heat transfer fluid is not divided into two streams. The whole part of the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated circulates through the pipe line and enters the heat exchanger 2 for the purpose of heating the incoming geothermal steam.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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During the discharging period, only valve 2 is closed. The whole part of the heat transfer fluid which leaves the parabolic trough collector at state (58) as heated flows into the heat exchanger 3 at state (62). Opposite to the charging period, during the discharging period, cold Syltherm 800 enters to the TES tank and by circulating around the PCM receives its heat and leaves the TES tank at state (66) as heated. In addition, at the end of the discharging period, PCM returns to its initial conditions through freezing.
In the selecting membrane, it is important to choose an electrolyte in which proton conductivity rises rapidly with temperature such as recently developed solid acid electrolyte (SAE). Conductivity of Cesium hydrogen sulfate (CsHSO4) is shown in Fig. 2. The conductivity scale is logarithmic. In temperatures between 120 C and 150 C, the conductivity rises up to about 4 orders of magnitude, reaching a highconductivity at 150 C which is the interested zone in the study [24].
The AMIS unit
Solid oxide fuel cell
The AMIS technology (for abatement of mercury and hydrogen sulfide) is an efficient, safe and enviromentally friendly technology capable to eliminate the related issues of the gaseous emissions from geothermal power plants and in particular the unpleasant smell of hydrogen sulfide. Geothermal fluid extracted from the reservoir by production wells is sent to the power plants by a steel pipe network. The fluid mainly consists of steam with some percentage (from less than 1% up to 15%) of non-condensable gases (NCG) [22]. Instead of being released to the atmosphere, NCG can be sent to the AMIS system for mercury and hydrogen sulfide abatement. This system consists of three fundamental steps:
In the solid oxide fuel cell (SOFC), chemical energy of the fuel is converted into the electricity. In the cathode, oxygen ions are produced and follow to the anode through a membrane. Produced electrons in the anode enter the cathode after passing through an external circuit and hereby electrical current is created [25]. Hydrogen is extracted from the storage tank at state (81) and enters the fuel cell system. Along with hydrogen, oxygen gas is supplied to the fuel cell system at state (82) so as to produce electrical power. The chemical reactions taking place in fuel cell system at the anode and cathode are as follows:
Removal of mercury by chemical absorption. Selective catalytic oxidation of hydrogen sulfide to SO2. SO2 scrubbing by geothermal water. It is worth mentioning that the AMIS system has been modified for hydrogen production from hydrogen sulfide in the current study. The hydrogen sulfide, which is trapped inside the AMIS unit, is sent to an electrolyzer system in order to decompose it into hydrogen and sulfur dimer instead of sending it to the catalytic oxidation process [23].
Hydrogen sulfide electrolyzer The hydrogen sulfide is sent to an electrolyzer unit at state (24) in order to decompose it into hydrogen and sulfur dimer through the electrolysis process. The principle of hydrogen sulfide electrolysis is to pass a direct current between two electrodes in order to decompose hydrogen sulfide into hydrogen and sulfur dimer. The process consists of passing a flow with H2S gas through the anode chamber to contact a catalytic anode. Then, it reacts to produce sulfur dimer, protons and electrons. The protons pass through the membrane from the anode to the cathode chambers where they combine with electrons to form diatomic hydrogen gas at the cathode. During the process both the anode and cathode are maintained at a temperature of 150 C. Gaseous sulfur dimer is collected from the anode compartment and hydrogen is removed from the cathode compartment [23]. The chemical reactions taking place in the electrolysis process at the anode and cathode are written as follows: H2S / 12S2 þ 2Hþ þ 2e
(1)
2Hþ þ 2e / H2
(2)
H2 / 2Hþ þ 2e
(3)
2Hþ þ 12O2 þ 2e / H2O
(4)
Water electrolyzer The Polymer Electrolyte Membrane (PEM) systems owes specific technical advantages compared to alkaline systems, especially once integrated with renewable energies. In comparison to alkaline systems, PEM electrolyzer shows better efficiency and higher production rates. In addition, it has low operating temperature and a simple structure and hence PEM is chosen as electrolyzer type. A PEM electrolyzer is operated in which a solid proton conducting membrane separates an anode chamber from a cathode chamber. The principle of water electrolysis is to pass
Fig. 2 e Conductivity of CsHSO4 versus temperature [24].
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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a direct current between two electrodes in order to decompose the water into hydrogen and oxygen as shown in Fig. 3. The reactions for the anode and cathode of the PEM electrolyzer can be expressed respectively as follows: H2O (l) / 12O2 (g) þ 2Hþ (aq) þ 2e
(5)
2Hþ (aq) þ 2e / H2 (g)
(6)
In the proposed system, water is supplied from a water tank and the water primarily enters the water preheater at state (77). On the other side, the water which leaves the cooling tower at higher temperatures flows into the preheater at state (74) and so before it is sent to the injection well, its heat is transferred to the water which will in the subsequent step enter the electrolyzer unit at state (78) at the appropriate temperature level for the electrolysis process.
System analyses In order to carry out the thermodynamic analyses and evaluation of the integrated energy system, specific assumptions are made in a reasonable manner, and the calculations are performed for each component; enthalpies, exergies, mass flow rates, pressures and temperatures are taken in consideration. Exergy destructions and entropy generations are also calculated to analyze the system. The Engineering Equation Solver (EES) software is used to perform for all calculations. The entropy generations and exergy destructions are determined for each component by writing the exergy and entropy balances. Both energy and exergy efficiencies are calculated for comparative evaluations. The assumptions made for the analysis of the multigeneration system are listed as follows: The changes in kinetic and potential energy and exergy terms are negligible.
The processes taking place are steady state and steady flow. The expansion valves, compressors, pumps and turbine are adiabatic. There is no chemical reaction occurs between the refrigerant and absorbent. Therefore, chemical exergy is neglected and only physical exergy is taken into account. The working fluid of organic Rankine cycle is n-octane. The refrigerant of absorption cooling system is ammoniawater. The heat transfer fluid of the solar driven system is Syltherm 800. The phase change material is chosen as LiNO3. The ambient temperature and pressure are 25 C and 101.3 kPa, respectively. The general mass balance equation for the control volume can be expressed as follows: X
m_ in ¼
X
m_ out
(7)
The first law of thermodynamics is considered to perform energy analyses for all components of the integrated energy system. The general energy balance equation for the control volume can be expressed as follows: X
_ in ¼ m_ in hin þ Q_ in þ W
X
_ out m_ out hout þ Q_ out þ W
(8)
where Q_ and W_ represent the heat transfer and work crossing _ and h denote the mass flow the component boundaries and m rate and the specific enthalpy, respectively. The steady flow energy balance relation can be expressed for a chemically reacting steady flow system as follows: X, nr h0 f þ h h0 r X, _ out þ ¼ Q_ out þ W np h0 f þ h h0 p
_ in þ Q_ in þ W
(9)
where hºf denotes the enthalpy of formation and (h - hº) represents the sensible enthalpy relative to the standard reference state, which is the difference between h (sensible enthalpy at the specified state) and hº (sensible enthalpy at the standard reference state). Analysing only in terms of first law of thermodynamics prevents designing accurate systems. Thereby, exergy analysis is one of the most significant aspects for the design and analysis of thermal systems. It is based on the second law of thermodynamics. Exergy is a measure of the system state departure from the environment state and is considered also as a measure of the available energy [26]. The flow exergy terms for each state point are defined as following equation: exi ¼ hi h0 T0 ðsi s0 Þ
(10)
Applying the exergy balance equation on the system components at steady state, the exergy destruction in each component can be calculated depending on the following formula: X Fig. 3 e Illustration of the operating principle of the PEM electrolyzer.
_ Qin þ W _ in ¼ m_ in exin þ Ex
X
_ Qout þ W _ out þ Ex _ d m_ out exout þ Ex (11)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 2 e Balance equations of the system components. Component Generator
Condenser 1
Evaporator
Absorber
Heat Exchanger 1
Boiler
Balance equations _1 ¼ m _ 2; m _ 38 ¼ m _ 39 þ m _ 42 m _ 1 h1 þ m _ 38 h38 ¼ m _ 2 h2 þ m _ 39 h39 þ m _ 42 h42 m _ 1 s1 þ m _ 38 s38 þ S_ gen ¼ m _ 2 s2 þ m _ 39 s39 þ m _ 42 s42 m _ d _ 1 ex1 þ m _ 38 ex38 ¼ m _ 2 ex2 þ m _ 39 ex39 þ m _ 42 ex42 þ Ex m _ 31 ¼ m _ 32 ; m _ 48 ¼ m _ 49 m _ 31 h31 þ m _ 48 h48 ¼ m _ 32 h32 þ m _ 49 h49 m _ 31 s31 þ m _ 48 s48 þ S_ gen ¼ m _ 32 s32 þ m _ 49 s49 m _ d _ 31 ex31 þ m _ 48 ex48 ¼ m _ 32 ex32 þ m _ 49 ex49 þ Ex m _ 33 ¼ m _ 34 ; m _ 46 ¼ m _ 47 m _ 33 h33 þ m _ 46 h46 ¼ m _ 34 h34 þ m _ 47 h47 m _ 33 s33 þ m _ 46 s46 þ S_ gen ¼ m _ 34 s34 þ m _ 47 s47 m _ d _ 34 ex34 þ m _ 47 ex47 þ Ex ¼ m _ 35 þ m _ 41 m _ 44 h44 ¼ m _ 36 h36 þ m _ 45 h45 m _ 35 s35 þ m _ 41 s41 þ m _ 44 s44 þ S_ gen ¼ m _ 36 s36 þ m _ 45 s45 m
_ 33 ex33 þ m _ 46 ex46 m _ 44 ¼ m _ 45 ; m _ 36 ¼ m _ 35 h35 þ m _ 41 h41 þ m
_ d _ 35 ex35 þ m _ 41 ex41 þ m _ 44 ex44 ¼ m _ 36 ex36 þ m _ 45 ex45 þ Ex m _ 37 ¼ m _ 38 ; m _ 39 ¼ m _ 40 m _ 37 h37 þ m _ 39 h39 ¼ m _ 38 h38 þ m _ 40 h40 m _ 37 s37 þ m _ 39 s39 þ S_ gen ¼ m _ 38 s38 þ m _ 40 s40 m _ d _ 37 ex37 þ m _ 39 ex39 ¼ m _ 38 ex38 þ m _ 40 ex40 þ Ex m _2 ¼ m _ 3; m _ 51 ¼ m _ 52 m _ 2 h2 þ m _ 51 h51 ¼ m _ 3 h3 þ m _ 52 h52 m _ 2 s2 þ m _ 51 s51 þ S_ gen ¼ m _ 3 s3 þ m _ 52 s52 m
Turbine 1
_ d _ 2 ex2 þ m _ 51 ex51 ¼ m _ 3 ex3 þ m _ 52 ex52 þ Ex m _ 52 ¼ m _ 53 m _ Turbine _ 52 h52 ¼ m _ 53 h53 þ W m
Condenser 2
_ 52 s52 þ S_ gen ¼ m _ 53 s53 m _ Turbine þ Ex _ d _ 52 ex52 ¼ m _ 53 ex53 þ W m _ 50 ¼ m _ 53 ; m _ 54 ¼ m _ 55 m _ 53 h53 þ m _ 54 h54 ¼ m _ 50 h50 þ m _ 55 h55 m _ 53 s53 þ m _ 54 s54 þ S_ gen ¼ m _ 50 s50 þ m _ 55 s55 m
Heat Exchanger 2
Heat Exchanger 3
Parabolic Trough Collector
_ d _ 53 ex53 þ m _ 54 ex54 ¼ m _ 50 ex50 þ m _ 55 ex55 þ Ex m _3 ¼ m _ 4; m _ 56 ¼ m _ 65 m _ 3 h3 þ m _ 65 h65 ¼ m _ 4 h4 þ m _ 56 h56 m _ 3 s3 þ m _ 65 s65 þ S_ gen ¼ m _ 4 s4 þ m _ 56 s56 m _ d _ 3 ex3 þ m _ 65 ex65 ¼ m _ 4 ex4 þ m _ 56 ex56 þ Ex m _ 62 ¼ m _ 63 ; m _ 66 ¼ m _ 67 m _ 62 h62 þ m _ 67 h67 ¼ m _ 63 h63 þ m _ 66 h66 m _ 62 s62 þ m _ 67 s67 þ S_ gen ¼ m _ 63 s63 þ m _ 66 s66 m _ d _ 62 ex62 þ m _ 67 ex67 ¼ m _ 63 ex63 þ m _ 66 ex66 þ Ex m _ 57 ¼ m _ 58 m _ 57 h57 þ Q_ solar ¼ m _ 58 h58 m Q_ solar _ 58 s58 þ S_ gen ¼ m Tsun Q _ solar ¼ m _ d _ 57 ex57 þ Ex _ 58 ex58 þ Ex m _ 5þ m _ 16 þ m _ 19 þ m _ 20 ¼ m _ 6þm _ 21 m _ 5 h5 þ m _ 16 h16 þ m _ 19 h19 þ m _ 20 h20 ¼ m _ 6 h6 þ m _ 21 h21 m _ 5 s5 þ m _ 16 s16 þ m _ 19 s19 þ m _ 20 s20 þ S_ gen ¼ m _ 6 s6 þ m _ 21 s21 m _ 57 s57 þ m
Direct-Contact Condenser
Gas Cooler
Compressor
_ d _ 5 ex5 þ m _ 16 ex16 þ m _ 19 ex19 þ m _ 20 ex20 ¼ m _ 6 ex6 þ m _ 21 ex21 þ Ex m _ 15 ¼ m _ 16 ; m _ 21 ¼ m _ 22 m _ 15 h15 þ m _ 21 h21 ¼ m _ 16 h16 þ m _ 22 h22 m _ 15 s15 þ m _ 21 s21 þ S_ gen ¼ m _ 16 s16 þ m _ 22 s22 m _ d _ 15 ex15 þ m _ 21 ex21 ¼ m _ 16 ex16 þ m _ 22 ex22 þ Ex m _ 22 ¼ m _ 23 m _ Compressor ¼ m _ 22 h22 þ W _ 23 h23 m _ _ 22 s22 þ Sgen ¼ m _ 23 s23 m _ Compressor ¼ m _ d _ 22 ex22 þ W _ 23 ex23 þ Ex m (continued on next page)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 2 e (continued ) Component Turbine 2
Balance equations _4 ¼ m _5 m _ Turbine _ 4 h4 ¼ m _ 5 h5 þ W m _ _ 4 s4 þ Sgen ¼ m _ 5 s5 m
Water Preheater
Pump 1
Pump 2
Cooling Water Pump
Expansion Valve 1
Expansion Valve 2
_ Turbine þ Ex _ d _ 4 ex4 ¼ m _ 5 ex5 þ W m _ 77 ¼ m _ 78 ; m _ 74 ¼ m _ 75 m _ 74 h74 þ m _ 77 h77 ¼ m _ 75 h75 þ m _ 78 h78 m _ 74 s74 þ m _ 77 s77 þ S_ gen ¼ m _ 75 s75 þ m _ 78 s78 m _ d _ 74 ex74 þ m _ 77 ex77 ¼ m _ 75 ex75 þ m _ 78 ex78 þ Ex m _ 36 ¼ m _ 37 m _ Pump ¼ m _ 36 h36 þ W _ 37 h37 m _ 36 s36 þ S_ gen ¼ m _ 37 s37 m _ Pump ¼ m _ d _ 36 ex36 þ W _ 37 ex37 þ Ex m _ 50 ¼ m _ 51 m _ Pump ¼ m _ 50 h50 þ W _ 51 h51 m _ 50 s50 þ S_ gen ¼ m _ 51 s51 m _ Pump ¼ m _ d _ 50 ex50 þ W _ 51 ex51 þ Ex m _6 ¼ m _7 m _ Pump ¼ m _ 6 h6 þ W _ 7 h7 m _ 6 s6 þ S_ gen ¼ m _ 7 s7 m _ Pump ¼ m _ d _ 6 ex6 þ W _ 7 ex7 þ Ex m _ 40 ¼ m _ 41 m _ 40 h40 ¼ m _ 41 h41 m _ 40 s40 þ S_ gen ¼ m _ 41 s41 m _ d _ 40 ex40 ¼ m _ 41 ex41 þ Ex m _ 32 ¼ m _ 33 m _ 32 h32 ¼ m _ 33 h33 m _ 32 s32 þ S_ gen ¼ m _ 33 s33 m _ d _ 32 ex32 ¼ m _ 33 ex33 þ Ex m
Table 3 e Balance equations of cooling tower. Cooling Tower
_ 28 ¼ m _ 29 ; m _ 27 ¼ m _ 30 ; m _ 8þ m _ 9þm _ 10 ¼ m _ 11 þ m _ 15 þ m _ 17 m _ 8 h8 þ m _ 9 h9 þ m _ 10 h10 þ m _ 27 h27 þ m _ 28 h28 ¼ m _ 11 h11 þ m _ 15 h15 þ m _ 17 h17 þ m _ 29 h29 þ m _ 30 h30 m _ 8 s8 þ m _ 9 s9 þ m _ 10 s10 þ m _ 27 s27 þ m _ 28 s28 þ S_ gen ¼ m _ 11 s11 þ m _ 15 s15 þ m _ 29 s29 þ m _ 30 s30 _ 17 s17 þ m m _ 8 ex8 þ m _ 9 ex9 þ m _ 10 ex10 þ m _ 27 ex27 þ m _ 28 ex28 ¼ m _ 11 ex11 þ m _ 15 ex15 þ m _ d _ 17 ex17 þ m _ 29 ex29 þ m _ 30 ex30 þ Ex m
Table 4 e The design parameters of the parabolic trough collector system. Parameter
Fig. 4 e Schematic illustration of the parabolic trough collector system.
Temperature of the sun, Tsun Average hourly solar radiation during the charging period, Iav,c Average hourly solar radiation during the storing period, Iav,s Average hourly solar radiation during the discharging period, Iav,d Charging period, tcharging Storing period, tstoring Discharging period, tdischarging Width of the collector Length of the collector Reflection rate
Value 6000 K 0.60 kJ/m2s 0.35 kJ/m2s 0.10 kJ/m2s 7h 10 h 7h 4m 50 m 0.8
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 5 e Thermal properties of the phase change material [28]. Phase change material
Table 7 e Enthalpy of formation of the system compounds (Taken from Ref. [30]).
Freezing/Melting point (K)
Cps,av (kJ/kgK)
Cpl,av (kJ/kgK)
DHm (kJ/kg)
528
1.20
1.84
372
LiNO3
Table 6 e The average chemical compositions of noncondensable gases [29]. Compound Nitrogen Oxygen Methane Carbon dioxide Hydrogen sulfide
hºf (kJ/mol) 20.5 128.60 0 285.83 0 0
H2S (g) S2 (g) H2 (g) H2O (l) H2 (g) O2 (g)
Composition (% mole) 26.76 18.55 15.77 31.14 7.78
Table 8 e The data used for proposed system. Parameter
_ d denotes the exergy destruction rate, Ex _ Q represents where Ex the exergy rate due to heat transfer across the system boundaries. The exergy transfer due to heat can be expressed as follows: _ Q ¼ Q_ 1 T0 Ex Ts
Compound
(12)
where T0 is the ambient temperature that describes the state at which the system is in unrestricted equilibrium with the environment and it cannot undergo any state change through any kind of interaction with the environment [27] and Ts is the temperature of source in case there is a heat penetration and temperature of sink in case there is a heat loss. The exergy destruction rate is calculated by multiplication of ambient temperature with entropy generation in each component:
_1 Mass flow rate of the geothermal steam; m Inlet temperature of geothermal steam; T1 Inlet pressure of geothermal steam; P1 Ambient temperature, T0 Ambient pressure, P0
_ d ¼ T0 S_gen Ex
Value 60 kg/s 515 K 2000 kPa 298 K 101.325 kPa
(13)
where S_ gen denotes the entropy generation rate and it is determined by applying the entropy balance equation for a steady state operation on each component of the system as follows: X
m_ in sin þ
X Q_ in Q_ þ S_gen ¼ m_ out sout þ out Ts Tb
(14)
The mass, energy, entropy and exergy balance equations for each component of the integrated system are shown in Table 2. In addition, the mass, energy, entropy and exergy balance equations for the cooling tower are listed in Table 3.
Fig. 5 e Schematic illustration of the designed system. Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 9 e Thermodynamic properties of the charging period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
Geothermal steam Geothermal steam Geothermal steam Geothermal steam Geothermal steam Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water NCG mixture NCG mixture NCG mixture Hydrogen sulfide Hydrogen Sulfur NCG mixture Air Air NCG mixture Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Ammonia-water Water Water Air Air Water Water n_octane n_octane n-octane n_octane Air Air Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800 Syltherm 800
60 60 60 60 60 3871 3871 1290 1290 1290 70 48 22 19 30 30 3771 3790 2527 1263 9 9 9 0.7388 0.0433 0.6933 8.2612 1738 1738 8.2612 7.667 7.667 7.667 7.667 5.75 50 50 50 44.25 44.25 44.25 5.75 1.917 16.66 16.66 30.23 30.23 7.186 7.186 6.02 6.02 6.02 6.02 187.7 187.7 60 60 60 42 42 18 18 18 18
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600 600 600 600 600 600 600
515 510 490 598 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 510 520 660 660 660 660 660 590 590
2882 2869 2812 3081 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 483.2 503.7 799.6 799.6 799.6 799.6 799.6 647 647
6.507 6.481 6.368 6.866 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.341 1.38 1.883 1.883 1.883 1.883 1.883 1.639 1.639
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 1040 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 95.38 104.4 250.4 250.4 250.4 250.4 250.4 170.4 170.4
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 9 e (continued ) State 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Fluid type
_ i (kg/s) m
Syltherm 800 60 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 No flow during the charging period No flow during the charging period Water 19 Water 3 Water 3 Water 51 Water 0.2 Water 0.2 Hydrogen 0.022 Oxygen 0.1778 Hydrogen 0.0433 Oxygen 0.3467 Water 0.39
Pi (kPa)
Ti (K)
224.9 237.7 159.9 159.9 149.7 149.7
200 200 200 200 200 200 200 200 200 101.3 150
388 388 355 386.1 288 350 370 370 500 298 350
481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
(15)
(16)
where wc and lc are the aperture width and length of the collector, respectively. Schematic illustration of the parabolic trough collector system is shown in Fig. 4. The respective design parameters of the parabolic trough collector system are listed in Table 4. Due to the intermittency of the solar energy, a thermal energy storage tank is integrated to the solar-driven system. Inside the hot storage tank filled with phase change material and heat transfer fluid flows through the hot storage tank via pipe coils. Both latent and sensible heat storage take place into the storage tank. Thermal properties of the phase change material is given in Table 5. The total heat stored by a solid to liquid phase changing material between initial and final temperatures would be estimated by Ref. [21]: qstored
exph,i (kJ/kg)
1.812 1.849 1.603 1.603 1.567 1.567
Here, Iav, Aa, gc are the average hourly solar radiation, collector aperture area and reflection rate of the collector, respectively. Average hourly solar radiation value varies with the time and location. Aperture area of the collector can be calculated from the following equation.
¼ mCps;av ðTm Ti Þ þ mDHm þ mCpl;av Tf Tm Tf > Tm > Ti
x (kg/kg)
752.9 776.9 625.7 625.7 604.7 604.7
The solar driven system comprises parabolic trough collectors and a thermal energy storage tank. The solar driven system including several huge parabolic trough collectors (PTCs) can transfer the solar energy to the heat transfer fluid. A parabolic collector can operate with solar beam and also diffusing of radiation because of its large acceptance angle. Power received by PTC can be calculated as follows:
Aa ¼ wc lc
si (kJ/kgK)
639 650 580 580 570 570
The solar driven cycle
Q_ ptc ¼ Iav Aa gc
hi (kJ/kg)
600 300 300 300 200 200
(17)
where m is the mass of the phase changing material, Cps,av and Cpl,av are the average heat capacities for solid and liquid
phases, respectively, Tm is the melting temperature, Ti and Tf are the initial and final temperatures, respectively and DHm is the heat of fusion. The energy and exergy balances of the overall process of TES tank can be defined as follows [27]: ðHa Hb Þ ½ðHd Hc Þ þ Qlosses ¼ DE
(18)
where Ha and Hb are the total inlet and outlet enthalpies of the heat transfer fluid during the charging period, respectively. In addition, Hc and Hd are the total inlet and outlet enthalpies of the heat transfer fluid during the discharging period, respectively. Also, Qlosses denotes the heat losses during the process and DE represents the accumulation of energy in TES tank. ðExa Exb Þ ½ðExd Exc Þ þ Xl I ¼ DEx
(19)
here, Exa and Exb are the inlet and outlet exergies of the heat transfer fluid during the charging period, respectively. In addition, Exc and Exd are the inlet and outlet exergies of the heat transfer fluid during the discharging period, respectively. Also, Xl denotes the exergy loss associated with Qlosses and I stands for the exergy consumption; and DEx is the exergy accumulation. The overall energy and exergy efficiencies of the TES tank can be defined as follows: h¼
Hd Hc Ha Hb
(20)
Exd Exc Exa Exb
(21)
and j¼
Hydrogen sulfide electrolyzer In this part electrolysis process of hydrogen sulfide will be analyzed through energy and exergy approaches along with emission rates in order to give a detailed information about the process. Table 6 shows the average chemical compositions
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 10 e Thermodynamic properties of the storing period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Water 3871 Water 3871 Water 1290 Water 1290 Water 1290 Water 70 Water 48 Water 22 Water 19 Water 30 Water 30 Water 3771 Water 3790 Water 2527 Water 1263 NCG mixture 9 NCG mixture 9 NCG mixture 9 Hydrogen sulfide 0.7388 Hydrogen 0.0433 Sulfur 0.6933 NCG mixture 8.2612 Air 1738 Air 1738 NCG mixture 8.2612 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 5.75 Ammonia-water 50 Ammonia-water 50 Ammonia-water 50 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 5.75 Ammonia-water 1.917 Water 16.66 Water 16.66 Air 30.23 Air 30.23 Water 7.186 Water 7.186 n_octane 6.02 n_octane 6.02 n-octane 6.02 n_octane 6.02 Air 187.7 Air 187.7 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 No flow during the storing period No flow during the storing period No flow during the storing period No flow during the storing period
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600 600 600
515 510 490 598 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 510 520 604 604 604
2882 2869 2812 3081 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 483.2 503.7 677 677 677
6.507 6.481 6.368 6.866 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.341 1.38 1.689 1.689 1.689
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 1040 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 95.38 104.4 185.6 185.6 185.6
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 10 e (continued ) State
Fluid type
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Syltherm 800 No flow during the No flow during the No flow during the No flow during the No flow during the No flow during the No flow during the Water Water Water Water Water Water Hydrogen Oxygen Hydrogen Oxygen Water
_ i (kg/s) m 60 storing period storing period storing period storing period storing period storing period storing period 19 3 3 51 0.2 0.2 0.022 0.1778 0.0433 0.3467 0.39
Pi (kPa)
Ti (K)
x (kg/kg)
exph,i (kJ/kg)
677
1.689
185.6
200 200 200 200 200 200 200 200 200 101.3 150
388 388 355 386.1 288 350 370 370 500 298 350
481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
The heat transfer between the electrolyzer and the environment is negligible. The auxiliary components are well insulated and capable of conducting electricity without losses. The changes in both potential and kinetic energies and exergies are negligible. The processes occur in steady-state and steady-flow. The temperature and pressure of the hydrogen sulfide are 423 K and 200 kPa, respectively. The temperature and pressure of the products of electrolysis process are 500 K and 200 kPa, respectively. The ambient temperature (T0) and pressure (Po) are 298 K and 101.325 kPa, respectively. The related equations are given for the electrolysis process. In the electrolysis process, hydrogen sulfide is decomposed to pure hydrogen and sulfur dimer by electrical power. Therefore, electrolysis is a chemically reacting process. The required electrical work of the electrolysis process by neglecting the heat transfer from/to ambient can be defined as follows [26]: X, X, np h0 f þ h h0 p nr h0 f þ h h0 r
(22)
Here, p and r subscripts represent the products and reactants of the electrolysis process. Products of the process are hydrogen and sulfur dimer and hydrogen sulfide is the only reactant for the electrochemical process. Moreover, reversible work of the electrolysis process can be expressed as follows:
jdesigned ¼
si (kJ/kgK)
604
of non-condensable gases in mole percent and the following calculations are based on these data. In addition, the following assumptions are made respectively for the analysis:
_ electrolysis ¼ W
hi (kJ/kg)
600
_ rev ¼ W
X, X, nr h0 f þ h h0 T0 s r np h0 f þ h h0 T0 s p (23)
For the electrolysis process the steady-state and steadyflow entropy balance equation can be written as follows: m_ H2 S sH2 S þ S_gen ¼ m_ H2 sH2 þ m_ S2 sS2
(24)
Solid oxide fuel cell For the electrochemical process which occurs in the fuel cell the produced electricity can be expressed as follows: _ produced ¼ W
X, X, nr h0 f þ h h0 r np h0 f þ h h0 p
(25)
The products of the fuel cell conversion process is water and reactants are hydrogen and oxygen. For the fuel cell conversion process the steady-state and steady-flow entropy balance equation can be written as follows: m_ H2 sH2 þ m_ O2 sO2 þ S_gen ¼ m_ H2 0 sH2 0
(26)
The designed system which consists of AMIS unit, hydrogen sulfide electrolyzer and solid oxide fuel cell system is shown in Fig. 5. The overall energy efficiency of the designed system which includes an electrolyzer and a fuel cell system can be defined as follows: hdesigned ¼
_ produced þ m_ H 0 hH 0 W 2 2 _ electrolysis þ m_ H S hH S þ m_ O hO þ m_ H LHVH W 2 2 2 2 2 2
(27)
Also, the overall exergy efficiency of the designed system can be written as
_ produced þ m_ H O exH O W 2 2 _ electrolysis þ m_ H S exH S þ m_ O exO þ m_ H exph þ exch W 2 2 2 2 2 H2
(28)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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international journal of hydrogen energy xxx (xxxx) xxx
Table 11 e Thermodynamic properties of the discharging period. State 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Fluid type
_ i (kg/s) m
Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Geothermal steam 60 Water 3871 Water 3871 Water 1290 Water 1290 Water 1290 Water 70 Water 48 Water 22 Water 19 Water 30 Water 30 Water 3771 Water 3790 Water 2527 Water 1263 NCG mixture 9 NCG mixture 9 NCG mixture 9 Hydrogen sulfide 0.7388 Hydrogen 0.0433 Sulfur 0.6933 NCG mixture 8.2612 Air 1738 Air 1738 NCG mixture 8.2612 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 7.667 Ammonia-water 5.75 Ammonia-water 50 Ammonia-water 50 Ammonia-water 50 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 44.25 Ammonia-water 5.75 Ammonia-water 1.917 Water 16.66 Water 16.66 Air 30.23 Air 30.23 Water 7.186 Water 7.186 n_octane 6.02 n_octane 6.02 n-octane 6.02 n_octane 6.02 Air 187.7 Air 187.7 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 No flow during the discharging period No flow during the discharging period Syltherm 800 60 Syltherm 800 60 Syltherm 800 60 Syltherm 800 60
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
2000 2000 2000 2000 250 200 250 250 250 250 200 200 200 200 200 200 200 200 200 200 150 150 200 200 200 200 200 130 130 200 332.5 332.5 130 130 130 156.5 400 400 400 400 160 400 130 101.3 101.3 101.3 101.3 101.3 101.3 100 400 400 100 101.3 101.3 600 1000 600
515 510 490 544 403 392 392 392 392 392 388 388 388 385 384 384.8 385 385 385 385 398 387 423 423 500 500 350 320 385 385 318.7 282.1 249 302 302 278 299.6 305 325 302 298 340 255 276 300 308 255 278 313 385 390 505 483.5 288 305 495 520 544.7
2882 2869 2812 2955 2722 498.9 499 499 499 499 481.9 481.9 481.9 469.2 465 468.4 469.2 469.2 469.2 469.2 177 164.9 204.5 126.6 6850 107.1 131.5 320.4 386 169.3 1396 1258 1160 1370 1370 42.86 61.74 180 34.04 99.57 91.75 1442 1204 12.08 112.6 308.4 255 20.49 167 208.3 221.5 786 742 288.2 305.3 453.9 503.7 553
6.507 6.481 6.368 6.645 7.066 1.515 1.515 1.515 1.515 1.515 1.472 1.472 1.472 1.439 1.428 1.437 1.439 1.439 1.439 1.439 1.789 1.758 1.782 0.187 58.04 3.839 1.717 6.86 7.047 1.82 5.184 4.727 4.777 5.549 5.549 0.8166 0.8166 1.206 0.7233 0.3009 0.3282 5.236 4.952 0.04363 0.3928 6.893 6.703 0.07399 0.5702 0.6114 0.6441 1.899 1.905 6.825 6.883 1.282 1.38 1.474
600 600 600 600
544.7 544.7 567 567
553 553 599 599
1.474 1.474 1.556 1.556
x (kg/kg)
0.99 0.99 0.99 0.99 0.99 0.6 0.6 0.6 0.4 0.4 0.4 0.99 0.99
exph,i (kJ/kg) 947.6 942 919.2 979.5 620.8 51.83 51.89 51.89 51.89 51.89 47.83 47.83 47.83 44.92 43.96 44.73 44.92 44.92 44.92 44.92 44.77 41.87 74.33 70.46 1528 25.464 56.51 22.09 32.09 63.65 170.5 169.3 55.78 35.82 35.82 35.52 54.4 56.67 11.54 3.785 3.477 201 47.78 3.581 0.02795 0.1652 3.458 2.944 1.528 26.17 29.58 220.2 174.2 0.1727 0.08149 83.84 104.4 125.8
125.8 125.8 147.2 147.2
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Table 11 e (continued ) State 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Fluid type
_ i (kg/s) m
Syltherm 800 60 Syltherm 800 18.17 Syltherm 800 18.17 No flow during the discharging period No flow during the discharging period Syltherm 800 18.17 Syltherm 800 18.17 Syltherm 800 18.17 Water 19 Water 3 Water 3 Water 51 Water 0.2 Water 0.2 Hydrogen 0.022 Oxygen 0.1778 Hydrogen 0.0433 Oxygen 0.3467 Water 0.39
Pi (kPa)
Ti (K)
hi (kJ/kg)
si (kJ/kgK)
x (kg/kg)
exph,i (kJ/kg)
600 200 200
567 622 548
599 715.2 559.1
1.556 1.753 1.486
147.2 204.9 128.1
300 300 200 200 200 200 200 200 200 200 200 200 101.3 150
560 560 548 388 388 355 386.1 288 350 370 370 500 298 350
583.9 583.9 559.1 481.9 481.9 342.8 473.7 62.48 321.8 4967 66.03 6850 0.4316 321.8
1.531 1.531 1.486 1.472 1.472 1.097 1.45 0.222 1.038 53.68 0.02187 58.04 0.002138 1.038
139.7 139.7 128.1 47.83 47.83 20.37 46.21 0.8169 17.13 944.7 59.56 1528 0 17.08
Fig. 6 e Entropy generation rates of main components of the integrated system.
m_ M
Water electrolyzer
n ¼
In electrolysis process, the water is decomposed to pure hydrogen and oxygen by electrical power. Therefore electrolysis is a chemically reacting process. The required electrical work of the electrolysis process by neglecting the heat transfer to/from ambient can be defined as follows:
_ where mand M denote the mass flow rate and molecular weight, respectively. The enthalpy of formations of the system compounds are given in Table 7. The energy efficiency of the electrolysis process can be defined as follows:
_ in ¼ W _ electrolyzer ¼ W
X, X, np h0 f þ h h0 p nr h0 f þ h h0 r (29)
where n_ represents the molar flow rate and can be written as follows:
hPEM ¼
(30)
m_ H2 LHVH2 _ electrolyzer W
(31)
where LHVH2 represents the lower heating value of hydrogen. In addition, the exergy efficiency of the electrolysis process can be also written as:
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 9 e Effect of the inlet mass flow rate of geothermal steam on the overall energy efficiency of the system.
Fig. 7 e The highest entropy generation rates of main components of the integrated system.
jPEM ¼
m_ H2 exph þ exch _ electrolyzer W
H2
(32)
(35)
where exch;H2 denotes the chemical exergy of the hydrogen and can be calculated as [31]: exch;H2 ¼
236:1 1000 MWH2
(33)
where 236.1 kJ/g mole is taken to be exergy content of hydrogen andMWH2 is the molar mass of hydrogen in kg/kmol.
Main assumptions
where Q_ Heating is the sum total of domestic hot water and space heating outputs of the designed system. Also, Q_ Drying denotes the total drying heat which is provided through the heated air which leaves the cooling tower at state (29). At the end of this section, the main assumptions of the overall system are given in Table 8.
Results and discussion
By considering all useful outputs and total inputs, overall system energy and exergy efficiencies can be written as follows: hoverall ¼
_ QHeating þ Ex _ QDrying þ Ex _ QCooling þ m_ H exph þ exch _ net þ Ex W 2 H2 joverall ¼ _ QSolar _ QGeo þ Ex Ex
_ net þ Q_ Heating þ Q_ Drying þ Q_ Cooling þ m_ H LHVH W 2 2 Geo Solar E_ þ E_
(34)
Fig. 8 e Entropy generation rates of some components of the integrated system for three different periods.
A comprehensive thermodynamic analysis, by considering all stream points, is carried out for the integrated system with hydrogen production units. In the analysis section, thermodynamic models presented and the entire system is analyzed using the Engineering Equation Solver (EES) software which can evaluate the thermodynamic state properties of different substances through built-in internal
Fig. 10 e Effect of the inlet mass flow rate of geothermal steam on the overall exergy efficiency of the system.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 11 e Effect of ambient temperature on the overall exergy efficiency of the designed system.
functions. Additionally, both energy and exergy evaluations are carried out in various operating conditions and design parameters such as inlet mass flow rate of geothermal steam and ambient temperature. Moreover, the entropy generations and exergy destructions are determined for each component of the system. The proposed multigeneration system operates in three different scenarios. In the first scenario, average hourly solar radiation has the highest value and both TES tank and heat exchanger 2 are fed by the heat transfer fluid. In other words, valves 1,2 and 3 open during this period. Thermodynamic properties of the all streams of the first working principle are listed in Table 9. In the second scenario, the average hourly solar radiation is sufficient only to feed the heat exchanger 2. During this period, valve 1 and 3 are shut-downed. Thermodynamic properties of the all stream points of this mechanism are given in Table 10. In the last scenario, the average hourly solar radiation is very low. Heat transfer fluid which is circulated in the solar driven system, enters the TES tank and receives the heat of the PCM and leaves the TES tank as heated at state (66). In this period, valve 2 is shut-downed and heat transfer fluid is not divided into two streams at state (58). Thermodynamic
Fig. 12 e Effect of the inlet temperature of water on the energy and exergy efficiencies of PEM electrolyzer.
17
Fig. 13 e Hydrogen sulfide and hydrogen production rates depending on the inlet mass flow rate of geothermal fluid.
properties of the all stream points of this period are given in Table 11. As mentioned above, the exergy is destructed due to the entropy generation. Fig. 6 and Fig. 7 show the entropy generation rates of main components in the integrated cycle. The highest entropy generation rate is observed in the cooling tower with 31.48 kW/K. On the other hand, the all devices which are illustrated in Fig. 6, have entropy generation rates below 1.21 kW/K. Due to the different operation scenarios, some components of the multigeneration system have different entropy generation rates in the charging, storing and discharging periods. These data are illustrated in Fig. 8. Entropy generation rate of the parabolic trough collector system in the charging period is 27.24 kW/K. Additionally, its entropy generation rates in the storing and discharging periods are 16.82 kW/K and 5.138 kW/K, respectively. The inlet mass flow rate of the geothermal steam is a significant design criterion for the proposed system. Fig. 9 demonstrates for each 3 operation periods how overall energy of the designed system changes, depending on the inlet mass flow rate of geothermal steam. It is clear that, for all 3 periods overall energy efficiency of the system increases, while inlet mass flow rate rises. The highest overall energy efficiency takes place in the storing period at around 82% while the inlet mass flow rate of geothermal steam is about 200 kg/s.
Fig. 14 e Effect of the mole percent of hydrogen sulfide on the amounts of produced hydrogen sulfide and hydrogen.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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Fig. 10 illustrates how overall exergy efficiency of the integrated system changes, depending on the inlet mass flow rate of the geothermal steam. Similarly, overall exergy efficiency of the system rises, while inlet mass flow rate increases. The highest overall exergy efficiency of the system is calculated in the storing period as approximately 68%, while inlet mass flow rate of the geothermal steam is around 200 kg/ s. Fig. 11 shows the effect of ambient temperature on the overall exergy efficiency of the integrated system for the each period. It can be clearly inferenced that, overall exergy efficiency of the system decreases, while ambient temperature rises. In the charging period, the overall exergy efficiency changes between 56.99% and 48.81% when ambient temperature rises from 273 to 313 K. Fig. 12 demonstrates how energy and exergy efficiency of the PEM electrolyzer changes depending on the inlet temperature of water. There is a linear relation between the efficiencies and water inlet temperature, such that both energy and exergy efficiency of the PEM electrolyzer increases when the inlet temperature of water rises. The energy and exergy efficiency are approximately 82.9% and 83.4%, when inlet temperature of water is around 290 K. Furthermore, as the temperature of the electrolysis process products rises, both energy and exergy efficiency get lower values. The geothermal fluid mainly consists of steam with some percentage (from less than 1% and up to 15%) of noncondensable gases. For the present study, it is important to determine the amount of hydrogen sulfide which is released to atmosphere depending on the inlet mass flow rate of the geothermal fluid. Through AMIS and electrolyzer units, hydrogen sulfide is decomposed to pure hydrogen and sulfur dimer in order to reduce the adverse effects of hydrogen sulfide and produce additional power supply by means of produced hydrogen. Fig. 13 shows the obtained amounts of hydrogen sulfide and hydrogen depending on the inlet mass flow rate of the geothermal fluid for the different NCG compositions. As expected, while NCG composition gets higher values, obtained hydrogen sulfide and correspondingly hydrogen amounts increase. While inlet mass flow rate of the geothermal fluid is 60 kg/s, 0.7388 kg hydrogen sulfide
Fig. 15 e The amount of produced electrical power depending on the mass flow rate of hydrogen sulfide.
Fig. 16 e Effect of ambient temperature on the exergy efficiency of the designed system.
and 0.0433 kg hydrogen emerge per second for 15% NCG composition. The geothermal fluid is present with some percentages of non-condensable gases. In the present study noncondensable gases of the geothermal fluid are assumed as nitrogen, oxygen, methane, carbon dioxide and hydrogen sulfide. As expected, while mole percent of hydrogen sulfide increases among the non-condensable gases, more hydrogen sulfide is emitted and correspondingly more hydrogen can be generated through the electrolysis process as shown in Fig. 14. While the mole percent of hydrogen sulfide rises from 3% to 21%, the amount of produced hydrogen increases from 26.75 g/s to 156 g/s, considering the inlet mass flow rate of the geothermal fluid as 120 kg/s. In the designed system, hydrogen which is produced from hydrogen sulfide through the electrolysis process is sent to the fuel cell system in order to generate power by interacting with incoming oxygen stream. Fig. 15 shows that how the amount of produced power changes depending on the mass flow rate of hydrogen sulfide. According to the figure, produced power rises from 0.852 MW to 12.79 MW, while the mass flow rate of hydrogen sulfide increases from 0.1 kg/s to 1.5 kg/s.
Fig. 17 e Effect of hydrogen sulfide inlet temperature on the exergy destruction rate of electrolyzer unit.
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
international journal of hydrogen energy xxx (xxxx) xxx
Table 12 e Calculated parameters used to evaluate the system. Parameters
Charging Period
Storing Period
Discharging Period
Turbine 1 power output Turbine 2 power output Solar heat input, Q_solar The number of collectors Mass of the phase change material, mPCM Overall energy efficiency Overall exergy efficiency
264.9 kW 21554 kW 17756 kW 185 70571 kg
264.9 kW 21554 kW 10399 kW 185 70571 kg
264.9 kW 13994 kW 2959 kW 185 70571 kg
75.14% 52.35%
78.37% 58.40%
76.39% 48.96%
Fig. 16 illustrates how exergy efficiency of the designed system (Fig. 5) changes depending on the ambient temperature. It is clear that, it has a little impact on the efficiency. While ambient temperature rises from 280 K to 310 K, the exergy efficiency value changes between 84.92% and 85.37%, by assuming the inlet temperature of hydrogen sulfide as 483 K. The exergy destruction rate of the electrolysis process decreases when the inlet temperature of hydrogen sulfide increases as shown in Fig. 17. By contrast, the exergy destruction rate reaches the higher values with rising ambient temperatures. The calculated parameters of the integrated system are given in Table 12 for each period, considering the assumptions.
Conclusions In this paper, a novel renewable energy based multigeneration system equipped with hydrogen production units is designed and analyzed for heating, drying, cooling, domestic hot water, power and hydrogen production. The electricity production is achieved by geothermal based organic Rankine cycle and solar driven system which includes numerous huge parabolic trough collectors. Due to the intermittent nature of the solar energy, a thermal energy storage tank is integrated to the solar driven system in order to operate the designed system continually. On the other hand, geothermal power plants discharge relatively high amounts of H2S to their operating field which, in turn, can cause unfavourable impacts on the environment and the human body. Furthermore, it is reasonable to produce hydrogen as a promising energy carrier from hydrogen sulfide in geothermal power plants. The present study aims to establish a new model for hydrogen production from hydrogen sulfide by adding an electrolyzer to a geothermal power plant equipped with AMIS abatement system. The effects of certain operating conditions on the subsystems and overall system performance are investigated. The effects of various parameters on the efficiencies such as ambient temperature, inlet mass flow rate of geothermal steam, inlet temperature of water are investigated and results are comparatively discussed. The key points can be written as follows: The highest entropy generation rate is observed in the cooling tower with 31.48 kW/K. On the other hand, the all
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devices which are illustrated in Fig. 6, have entropy generation rates below 1.21 kW/K. For all 3 periods overall energy efficiency of the system increases, while inlet mass flow rate of geothermal steam rises. The highest overall energy efficiency takes place in the storing period at around 82% while the inlet mass flow rate of geothermal steam is about 200 kg/s. The overall exergy efficiency of the proposed system rises, while inlet mass flow rate of geothermal steam increases. The highest overall exergy efficiency of the system is calculated in the storing period as approximately 68%, while inlet mass flow rate of the geothermal steam is around 200 kg/s. The overall exergy efficiency of the integrated system decreases, while ambient temperature rises. In the charging period, the overall exergy efficiency changes between 56.99% and 48.81% when ambient temperature rises from 273 to 313 K. There is a linear relation between the efficiencies and water inlet temperature, such that both energy and exergy efficiency of the PEM electrolyzer increases when the inlet temperature of water rises. The energy and exergy efficiencies are approximately 82.9% and 83.4%, when inlet temperature of water is around 290 K. Furthermore, as the temperature of the electrolysis process products rises, both energy and exergy efficiencies get lower values. While NCG composition gets higher values, obtained hydrogen sulfide and correspondingly hydrogen amounts increase. While inlet mass flow rate of the geothermal fluid is 60 kg/s, 0.7388 kg hydrogen sulfide and 0.0433 kg hydrogen emerge per second for 15% NCG composition. As expected, while mole percent of hydrogen sulfide increases among the non-condensable gases, more hydrogen sulfide is emitted and correspondingly more hydrogen can be generated through the electrolysis process. While the mole percent of hydrogen sulfide rises from 3% to 21%, the amount of produced hydrogen increases from 26.75 g/s to 156 g/s, considering the inlet mass flow rate of the geothermal fluid as 120 kg/s. The exergy destruction rate of the electrolysis process decreases when the inlet temperature of hydrogen sulfide increases. By contrast, exergy destruction rate reaches the higher values with rising ambient temperatures.
Acknowledgment This study was supported by the Scientific Research Projects Council of Yildiz Technical University [grant number 3349].
Nomenclature Aa Cp Cpl,av Cps,av ex exch
Collector aperture area (m2) Specific heat (kJ/kgK) Average specific heat of the liquid phase (kJ/kgK) Average specific heat of the solid phase (kJ/kgK) Specific exergy (kJ/kg) Chemical exergy (kJ/kg)
Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046
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exph E_ _ d Ex h hºf DHm Iav lc LHV m _ m MW n_ s S_ gen T Q_ wc _ W
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Physical exergy (kJ/kg) Energy transfer rate (kW) Exergy destruction rate (kW) Specific enthalpy (kJ/kg) Enthalpy of formation (kJ/mol) Heat of fusion (kJ/kg) Average hourly solar radiation (kJ/m2s) Aperture length (m) Lower heating value (kJ/kg) Mass (kg) Mass flow rate (kg/s) Molecular weight (kg/kmol) Molar flow rate (kmol/s) Specific entropy (kJ/kgK) Entropy generation (kW/K) Temperature (K) Heat transfer rate (kW) Width of the collector (m) Work rate (kW)
Greek letters h Energy efficiency j Exergy efficiency ɣ Reflection rate Subscripts av Average ch Chemical in Inlet out Outlet p Product ph Physical r Reactant Superscripts Geo Geothermal Acronyms EES Engineering equation solver NCG Non-condensable gases ORC Organic Rankine cycle PCM Phase change material PEM Polymer electrolyte membrane PTC Parabolic trough collector SAE Solid acid electrolyte SEACS Single effect absorption cooling system SOEC Solid oxide electrolyzer cell SOFC Solid oxide fuel cell TES Thermal energy storage
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Please cite this article as: Karapekmez A, Dincer I, Thermodynamic analysis of a novel solar and geothermal based combined energy system for hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.046