Introducing a proper hydrogen liquefaction concept for using wasted heat of thermal power plants-case study:Parand gas power plant

Hamed Rezaie Azizabadi *,Masoud Ziabasharhagh ,Mostafa Mafi

1 Faculty of Mechanical Engineering,K.N.Toosi University of Technology,Tehran,Iran

2 Department of Mechanical Engineering,Imam Khomeini International University,Qazvin,Iran

Keywords:Hydrogen Liquefaction Absorption refrigeration Waste treatment Mixed-refrigerant Ortho-para conversion

ABSTRACT A hydrogen liquefaction concept with an innovative configuration and a capacity of 4 kg·s-1(345.6 t·d-1)is developed.The concept involves an ammonia absorption refrigeration system for the pre-cooling of hydrogen and MR streams from 25 °C to -30 °C.The ammonia absorption refrigeration system is fed by exhaust gases of the Parand gas power plant that are normally dissipated to the environment with a temperature of 546°C.The simulation is performed by Aspen HYSYS V9.0,using two separate equations of state for simulating hydrogen and MR streams to gain more accurate results especially for ortho-para conversion.Results show that conversion enthalpy estimated by Aspen HYSYS,fits very well to the experimental data.Determining the important independent variables and composition of MRs are done using trial and error procedure,a functional and straightforward method for complicated systems.The minimum temperature limit in the cooling section is lowered,and an ortho-para converter is implemented in this section.The proposed concept performs well from energy aspects and leads to COP and SEC equal to 0.271 and 4.54 kW·h·kg-1,respectively.The main advantage of this study is in the low SEC,eliminating the losses of the distribution network,and improving the ability of the hydrogen liquefaction for energy storage in off-peak times.

1.Introduction

Energy has always been an essential issue that its demand experiences a continuous increase year by year as a result of industrial development and lifestyle changes.Global energy markets are expected to be doubled compared to today by the year 2040 [1].Due to the increasing demand for energy and environmental problems created by the use of non-renewable energy sources such as fossil fuels,finding a clean fuel source that could meet the requirements of a clean and sustainable energy source seems necessary.Hydrogen combustion,along with heat,leads to the production of water and small amounts of NOx;therefore,hydrogen could be a promising alternative to the common fuels [2].Hydrogen is the lightest element in the periodic table and the most abundant in the world[3].Hydrogen could be easily extracted from sustainable sources,and it is expected to play an essential role in the future of energy [4].Besides all the positive features,hydrogen storage and transportation are associated with many challenges thanks to the low energy density [5].Even though hydrogen energy density could be increased through various methods,liquefaction could perform more efficiently and provide a safer solution for transportation and storage applications[6].Hydrogen liquefaction could also be used as an energy carrier and an energy storage medium[3,7],especially when it is built next to a power plant.Furthermore,the liquid form is superior to gas in the transportation of hydrogen.Liquid hydrogen has unique characteristics and many particular usages [8],so it is expected that the demand for liquid hydrogen will increase in the future.The liquid hydrogen’s energy density is three times gasoline and has the highest energy density among fuels except nuclear [9].

The first successful attempt for liquefaction of permanent gases is attributed to Cailletet L.’s in 1877[10].In 1885,Michael Faraday published an article on the liquefaction of gases.At that time,Faraday’s method was able to reach-101°C by using an ether bath and solid carbon dioxide.Sir James Dewar Scottish/British scientist carried out hydrogen liquefaction first in 1898 with a 4 ml·min-1flow rate by use of the Joule-Thompson effect[11,12].A few years later,a Linde-Hampson system was used as the first simple laboratory system for hydrogen liquefaction.Around 1900,laboratory systems were invented with higher efficiencies.In 1957,when the US Nuclear Program and NASA Apollo missions created the need for large quantities of liquid hydrogen,the first large-scale hydrogen liquefaction plant was built in US,based on the pre-cooling Claude cycle [13,14].

Baker and Shaner[15]carried out a parametric study on a largescale liquefaction plant with a capacity of 250 t·d-1.They reported specific energy consumption (SEC) and exergy efficiency are equal 10.85 kW·h·kg-1and 36%,respectively.Matsuda and Nagami [16]proposed four different liquefaction concepts at a capacity of 300 t·d-1.Quack [17] presented a conceptual plant with 170 TPD capacity.He divided hydrogen liquefaction cycle into four subprocesses including,pre-compression,pre-cooling,cryo-cooling,and liquefaction.Belyakovet al.[18] suggested a high-efficiency hydrogen liquefaction cycle with six helium-hydrogen heat exchangers (HEs) and six ortho-para converters.Kuendiget al.[19] studied the combination of liquid natural gas (LNG) precooling with a large hydrogen liquefaction plant.They found it useful for decreasing power input and overall construction costs.Shimko and Gardiner[20]developed a small-scale pilot plant with a capacity of 20 kg·h-1at an estimated cost of 2.6 million USD as a model for larger plants with 50 t·d-1capacity.Staats[21]proposed a supercritical liquefaction cycle with helium cooling and highpressure hydrogen feed.Valenti and Macchi [22] offered a largescale hydrogen liquefier that utilizes four Joule-Brayton helium cycles and continuous ortho-para conversion on the hydrogen side of the HEs.Krasae-Inet al.[23] simulated a hydrogen liquefaction concept with a capacity of 100 t·d-1.They used mixed refrigerant(MR)and cascade Joule-Brayton refrigeration as cooling and liquefaction,respectively.Ozcan and Dincer [24] presented an integrated hydrogen production and liquefaction concept with a four-step Magnesium-Chlorine cycle which utilizes nuclear energy.They reported the total energy and exergy efficiencies in turn equal to 18.6% and 31.35%.

Asadniaet al.[25] proposed an innovative configuration for a 100 t·d-1hydrogen liquefaction plant.They used the Joule-Brayton refrigeration cycle to cool feed hydrogen down from 25°C to the temperature-198.2°C and six Linde-Hampson cycles for cooling and liquefying hydrogen to -252.2 °C.Cardellaet al.[26] studied an innovative procedure for the development of large-scale hydrogen liquefaction plants.It was found that specific costs for large-scale plants could be reduced by 67%compared to a 5 t·d-1liquefaction plant.Sadaghiani and Mehrpooya[27]investigated a novel configuration for hydrogen liquefaction with MRrefrigeration by exergy analysis.The coefficient of performance(COP) and SEC were reported as 0.1797 and 7.35 kW·h·kg-1,respectively.Aasadniaet al.[28] reviewed hydrogen liquefaction and production methods comprehensively.They presented an innovative classification for hydrogen liquefaction and studied dual liquefaction cycles that utilize renewable energy sources.Ansarinasabet al.[29]investigated different analysis methods such as exergy,exergoeconomic,and exergy-environmental.

In the last two decades,increasing attention to the renewable energy sources has led many researchers to use these energy sources in different fields.Aasadnia and Mehrpooya[30]proposed an innovative conceptual design for hydrogen liquefaction with the help of an absorption refrigeration system (ARS).They used solar energy for feeding the ARS,and reported COP and SEC as 0.2034 and 6.47 kW·h·kg-1,respectively.A new configuration for hydrogen liquefaction integrated with solar ARS and organic Rankin cycle was proposed [31].The COP and SEC were reported as 0.202 and 4.02 kW·h·kg-1,respectively.Asadniaet al.[32] investigated a hydrogen liquefaction cycle with two ammonia ARSs from exergy,exergoeconomic,and exergoenvironmental aspects.The ARSs were fed by geothermal energy,and the capacity was 260 TPD.Kaskaet al.[33] utilized a geothermal energy source with a temperature of 200 °C for hydrogen liquefaction.Hightemperatue geothermal water was used for pre-cooling hydrogen down to -30 °C for decreasing electricity work consumption in the hydrogen liquefcaction cycle [34].A novel geothermal-based multigeneration system was suggested for producing multiple commodities [35].

Hot water temperature supplied by the geothermal energy is in the range of 130°C to 200°C[36-39].Only 25%of the geothermal resources are capable of delivering temperatures above 150 °C[36],and few ones could provide hot water at temperatures around 200 °C,while the higher temperature of the supplied hot water leads to the higher efficiency [7,38].However,in Iran,in gas and combined cycle power plants,a large amount of thermal energy with higher quality is wasted to the environment.In 2016,gas and combined cycle power plants accounted for 36% and 25% of the Iran’s power generation network capacity,respectively [40].Most of the gas and combined power plants in Iran are equipped with V94.2 gas turbine,in which the exhaust gases leave the stack with a temperature of around 515-580 °C [41-43].Due to the enormous amount of heat in the exhaust gases,it has been utilized in ARSs,for different applications such as turbine inlet air cooling[44-46],but so far,it has not been applied to the hydrogen liquefaction processes.

Due to the low boiling temperature of hydrogen (-252.9 °C)and low efficiency of the current liquefaction plants,nearly onethird of its substantial energy is used in the liquefaction processes[1],while this is 10%for liquefaction of natural gas[47].Up to now,most of the large-scale liquefaction plants are similar to the first cases established in the US and Germany [48].Modern hydrogen liquefaction plants are currently located in Luena,Germany,and Tokyo,Japan,which were launched in 2008 and have been improved only slightly in comparison to the older ones [49].The minimum theoretical work for hydrogen liquefaction is 3.9 kW·h·kg-1,provided actual plants have a SEC in the range of 12-15 kW·h·kg-1[50].It seems that there is a great potential for improving new liquefaction processes with higher efficiencies.

In this study,an innovative configuration of hydrogen liquefaction integrated with an ARS for pre-cooling of hydrogen and MR streams is developed.The ARS is fed by the exhaust gases of the gas power plants,which is free and normally dissipated to the environment.Presenting a novel configuration with lower SEC and higher COP,using two separate equations of state for MR and hydrogen streams to yield more accurate results,are novelties of the proposed concept.The lowest temperature limit in cooling section is decreased to be able to implement an ortho-para converter in this section.Furthermore,new compositions for MR in cooling and liquefaction sections are gained through trial and error method.Reducing power transmission losses thanks to the construction of the plant next to a power plant and its capability for energy storage are beneficiaries of the presented concept.Power transmission networks always have tolerated significant losses that could be reduced or eliminated by approaching the power generator source (such as power plant)to the consumer[51].Parand gas power plant,which is equipped with six V94.2 gas turbines and is located in Parand city near Tehran,has been selected as a case study.It has a total capacity of 954 MW and is selected as is near Tehran,the capital city of Iran,with more than 4,000,000 cars,which will has a vast demand for liquid hydrogen in the future.

The following sections contain assumptions applied to the simulation and a detailed description of the process configuration.Then thermodynamic modeling and ortho-para conversion are presented,respectively.Finally,the validation of the simulation is discussed and the results are presented.

2.Simulation Assumptions and Process Configuration

Simulation of the process is performed by Aspen HYSYS V.9,which has a rich databank and provides different EOSs and models for calculating fluid properties.Aspen HYSYS has been used in many similar studies[52-54],and its accuracy in simulating cryogenic processes has been proved [55,56].Modified-Benedict-Webb-Rubin (MBWR) and Peng-Robinson (PR) equations of state are applied to hydrogen,and MR streams,respectively,to increase the accuracy of the simulation.The MBWR equation of state has been suggested by Jacobsen and Stewart [57] for nitrogen,but it was later applied to hydrogen by Younglove [58].PR is the most widely used cubic EOS,developed in 1976.It has since been widely used for industrial and academic applications thanks to its accuracy and simplicity [59].It deserves to note that in many similar works [30,60],only a unit equation of state has been used for the whole cycle.The liquefying capacity of the developed cycle is assumed to be 345.6 TPD (equivalent to 4 kg·s-1),which is sufficient for about 345,000 to 371,000 cars[30,61].Other assumptions are as follows:

? Reference conditions are set to 25 °C and 1.013 bar (1 bar=0.1 MPa).

? Pressure drop through the pipes and equipment are ignored.? Work consumption in the coolers is ignored.

? All the equipment are in steady-state and steady flow condition.? Compressors,HEs,and turbo-expanders are adiabatic.

? The minimum approach temperature in the HEs is in the range of 1 °C to 2 °C as recommended by Barron [62].

? Compressor,pump,and turbo-expander efficiencies are 90%,90%,and 85%,respectively.

? The temperature of the exhaust gases fed to the ARS by the Parand gas power plant is considered as 546 °C.

As shown in Fig.1,proposed concept involves the main liquefaction cycle,the ARS for pre-cooling,and the gas power plant in which its exhaust gases supply the required heat for the ARS.The liquefaction cycle,which includes the cooling section,and the liquefaction section,is presented in detail in Fig.2.The feed hydrogen has a pressure of 21 bar and a temperature of 25 °C.A higher feed pressure for hydrogen leads to lower energy consumption[16,17].

In the cooling section,the hydrogen is cooled down from 25 °C to -210 °C using ARS and MR refrigeration,while in the most of large scale hydrogen liquefaction plants such as Inglostat,nitrogen is used for cooling hydrogen from 25 °C to -198 °C [48].Asadniaet al.[25] and Berstadet al.[63] set this temperature to -198 °C.In other studies,this temperature has been set to -193 °C[20,23].In the liquefaction section,hydrogen is cooled from-210°C to-253.4°C with the help of an MR refrigeration,including helium,neon,and hydrogen.The initial refrigerant composition in cooling and liquefaction sections are assumed as presented in Table 1.In the cooling section,hydrogen is cooled down to-30 °C,-75°C,and-131 °C through passing the heat exchangers HE4,HE1,and HE2,respectively.Then hydrogen is introduced to the ortho-para converter to reach the equilibrium state,with 28.6%of para-hydrogen correspond to-131°C.Due to the exothermic nature of the ortho-para conversion,the temperature of the hydrogen decreases to -127.7 °C in the outlet of the converter.Finally,the hydrogen is cooled down to -210 °C through the heat exchanger HE3 before entering the liquefaction section.

As mentioned previously,the required refrigeration in the cooling section is provided by ARS and MR refrigeration.The lithium ARS are not capable of providing cooling to temperatures below zero,while ammonia ARS could provide cooling to -50 °C [36].Therefore,here,the ammonia ARS is used to cool hydrogen down to -30 °C [7,34].It deserves to note that here a single-effect absorption chiller is used as a result of the lower cost and complexity,while double effect chiller could lead to a higher efficiency[45].The COP for ammonia absorption chillers generally is in the range of 0.45 to 0.60 [34].The refrigerant is compressed through two stages of compression from 1.8 bar to 16 bar,in the second stage liquid phase compression is done by a pump.At each step,the refrigerant passes through heat exchanger HE4 to be cooled down to -30 °C before introducing to the compression equipment.

The hydrogen stream H6,in the liquefaction section is introduced first to the ortho-para converter ‘‘Con 2”to reach the equilibrium concentration of para-hydrogen,i.e.,63.4%.Then the hydrogen passes through the heat exchangers HE5 and HE6 to be cooled down to -250 °C.Another ortho-para converter ‘‘Con 3”is considered to yield 95% para-hydrogen,which is suitable for storage.The outlet of the converter ‘‘Con 3”with a temperature of-237.8 °C passes through the heat exchanger HE7 to reach-253 °C.Then in the final step,liquid hydrogen enters the turboexpander to reach a storage pressure of 0.13 MPa.Four stages of compression compress MR in the liquefaction section from 1 to 10 bar.In this section,the MR stream is cooled to -30 °C before entering each compressor to reduce power consumption.

3.Methodology of Simulation

3.1.Thermodynamic modeling

Thermodynamic modeling of the processes is subjected to the thermodynamic analysis of individual components,including compressors,HEs,pumps,expansion valves,and expanders [64].The governing equations for each of these components are as follows.

Compressors:The primary duty of compressors in a process is increasing the gas pressure.Several parameters,such as inlet temperature,intermediate cooling,discharge pressure,and the molar weight of the suction stream,are important for compressor performance [27].The required brake horsepower (BHP) for a compressor could be written as follows [65]:

Fig.1. Block diagram of the liquefaction process.

Fig.2. Schematic of the proposed innovative configuration for hydrogen liquefaction.

where .qin,Ps,CR,k,Z,ηisand ML stand for inlet volumetric flow rate,suction pressure,compression ratio,specific heat ratio,gas compressibility factor,isentropic efficiency of the compressor,and mechanical losses,respectively.The adiabatic efficiencies of the compressors in the process are supposed to be 90%,and Schultz’s Polytropic method is applied to them in Aspen HYSYS.

Pumps:As like compressors,the primary duty of the pumps is increasing pressure;however,pumps are just for liquids.If the pump is considered as an ideal machine,the required power could be expressed as follows [66]:

Table 1 Initial MR composition in cooling and liquefaction sections

Expansion valve:One of the devices which are used for expansion process,i.e.,decreasing temperature by decreasing pressure is expansion valve.It provides an isenthalpic process,so the governing equation could be written as below:

wherehinandhoindicate input and output mass enthalpy of hydrogen,respectively.

Turbo-expander:The expansion process could be integrated with power generation if turbo-expander is applied instead of an expansion valve.Turbo-expanders are more efficient than expansion valves thanks to the lower discharge temperature and power generation [27].The actual power generated by a turbo-expander could be expressed as follows [67]:

where .M,TinandCpstand for molar flow rate,inlet temperature,and specific heat capacity at constant pressure,respectively.

Multi-stream heat exchangers:The heat transfer between hot and cold streams is done through multi-stream heat exchangers.The heat balance for these types of heat exchangers is as follows[68]:

whereCp,,AandUstand for specific heat capacity,molar flow rate,heat transfer area,and overall heat transfer coefficient.The multi-stream heat exchangers through the process are simulated,such that the minimum approach temperature is in the range of 1 °C to 2 °C.

3.2.Energy analysis

Using the first law of thermodynamics,one could determine the criteria for the performance of thermodynamic cycles.This method,called energy analysis,is obtained by examining the thermodynamic equations of energy and mass balance.Considering a general control volume in a steady-state,excluding the kinetic and potential terms,the energy and mass balance equations are obtained as follows:

Energy analysis is widely used to evaluate thermodynamic systems.In energy analysis,there are two critical indicators,namely specific energy consumption (SEC) and coefficient of performance(COP),which are defined as [69]:

where .mH,net,.minand .moindicate the mass flow rate of hydrogen,the net consumed work in the process and input and output mass flow rate,respectively.

As in this study,waste heat inherent in the exhasut gases is recovered and used in the ARS for pre-cooling,it worths to define separate indexes for COP and SEC so that recovered heat be included.The COPrhand SECrhare defined as follows:

whereQrhstands for recovered heat from the exhuast gases.

3.3.Ortho-para conversion

Due to the relative spin orientation of nuclear protons,molecular hydrogen occurs in two spin isomers called para-hydrogen and ortho-hydrogen [70].Ortho-hydrogen has a higher energy level,and it’s concentration increases with temperature [71].The ortho-para conversion is an exothermic reaction and takes place slowly,so it needs to be accelerated using catalysts.If the orthopara converter is not considered in the liquefaction process,it proceeds slowly.Eventually,the ortho-para conversion will occured in the storage vessel to achieve equilibrium.Ortho-para conversion releases significant amounts of heat in which leads to 50% and 65% evaporation of liquid hydrogen after 100 and 1000 hours,respectively[72].The para-hydrogen content in equilibrium condition is shown in Table 2.

The conversion (%) obeys the below equation:

whereTdenotes temperature andC0,C1andC2are constant experimental coefficients.In the ortho-para conversion simulation,the coefficients must be determined so that the output temperature and para-hydrogen fraction are consistent with the experimental data [25,31].In this cycle,three stages of ortho-para conversion at three different temperature levels are considered.

Table 2 Para-hydrogen content in equilibrium condition [73]

3.4.Results validation

Due to the innovative configuration of the present study,validation of the results should be carried out through a partial validating method [30,31].In the Claude cycle,except for the hydrogen streams,data are compared to the references [25,27].For the ARS,similar published studies [32,74] are used.Besides using PR for MR streams,MBWR is used for hydrogen streams to increase the accuracy of the results.For validating the application of MBWR to the hydrogen streams,the enthalpy curve of ortho-para conversion is used.In Fig.3,the results of the Aspen HYSYS for conversion enthalpy are compared to the experimental data from reference[75].As shown in Fig.3,the results are well consistent,so that the difference between the two considered cases is always less than 2%.

4.Results and Discussion

After finalizing the initial configuration,it is necessary to determine major parameters and MR composition in the cooling and liquefaction sections to decrease the SEC.The MR composition affects plant performance and has great importance for the process.Due to the interaction of the various parameters and several linked equipment in the process,every change in one parameter of the process affects all the others.Besides,a successful run is achieved in the simulator,when all limitations are simultaneously met.Considering these points,the developed model-based optimization model would be complicated,and the convergence is not easy to achieve.However,the results can be further improved if a model-based optimization method is applied[30].Many researchers concentrate their effort on optimizing the existing liquefaction processes,while some other like this study are focused on the development of a novel configuration and systematic optimization is not in their scope.Here trial and error method is utilized for determining the MR composition and independent variables since it is a functional and simple method for analysis of the complicated systems [25,30].The variables should be changed step by step to get the minimum SEC,while minimum approach temperature met the requirement.Below variables are determined by trial and error method.

(1) The MR compositions and flow rates in the cooling and liquefaction sections are hardly determined through a trial and error procedure [30].

(2) The optimum pressure for successive compression stages.However,the initial guess for the optimum pressure could be obtained from the below equation:

(3)The outlet and inlet temperatures of HEs.It should be noted that these variables should be determined so that the limitation of minimum approach temperature is met.

(4) The lower and higher levels of pressure in the cooling and liquefaction sections.

Determining MR composition is one of the most time consuming job [25] and should be done through a repetitive procedure.The finalized composition of MR in cooling and liquefaction sections are presented in Table 3.

Table 4 shows the characteristics of different streams,including temperature,pressure,mass flow rate,enthalpy,and entropy,for the finalized case.

The specifications of the HEs in the process are presented in Table 4.This table includes the heat load,the logarithmic mean temperature difference (LMTD),and the minimum approach temperature.As it is shown in Table 5,the minimum approach temperature for all HEs are in the range of 1°C to 2°C,which is available with the current technology level.Although lower approach temperature could lead to higher efficiency and lower energy consumption,it imposes more cost and is not economical.The minimum approach temperature has been considered in this range in other similar studies [27,32].The specifications of compressors,expanders,and pump are presented in Table 6.

Table 3 Final MR composition in cooling and liquefaction sections

Fig.3. Comparison of conversion enthalpy in different temperatures between Aspen HYSYS by MBWR and Experimental data [75].

Table 4 Thermodynamic characteristics for different streams in the liquefaction cycle (1bar=0.1MPa)

Table 5 Heat exchanger specifications

Table 6 Specifications of compressors,expanders,and pump

Table 7 Comparison between the present study and the other studies

The exhaust gases are fed to the ARS with a mass flow rate of 442.2 kg·s-1and 546 °C.The gases leave the ARS with 176.7 °C,therefore 184,658 kW heat is recovered by ARS.This implies that only one unit of V94.2 gas turbine could provide the required heat for the ARS considered in this study.In other words,the Parand gas power plant could feed at least a capacity of 2000 t·d-1liquefaction plant with the current configuration.

The net power consumption in the liquefaction cycle is calculated as 65,087 kW.Regarding the mass flow rate of liquid hydrogen,enthalpy of the feed hydrogen and outlet liquid hydrogen,and the net power consumption,SEC and COP are calculated as 4.5 k W·h·kg-1and 0.271,respectively.A comparison is made between this study and other works through COP and SEC indexes in Table 7 to yield a better understanding about the benefits of the novel concept presented here.

The results show that the COP of the currentt study is much better than similar studies,even the ones which utilize geothermal energy.The SEC of the current concept is lower than the other studies except the concept presented by Ghorbaniet al.[31].They utilized the organic Rankine cycle (ORC) for power generation while this burden high cost on the process and is not considered in the proposed concept here.

The SECrhand COPrhare calculated as 17.37 kW·h·kg-1and 0.0708,respectively.The results show that using free recovered heat from exhaust gases is of great importance in the presented concept and could improve SEC and COP intensely.Furthermore it is found that the current configuration is feasible just when enough amount of free heat is available.

5.Conclusions

In this study,an innovative concept for hydrogen liquefaction is developed that uses heat inherent in the exhaust gases of the Parand gas power plant.The concept has a capacity of 4 kg·s-1(345.6 t·d-1),which could be sufficient for about 345,000 to 371,000 cars,and involves an ammonia ARS for pre-cooling of hydrogen and MR streams to-30°C.For the simulation of the process,Aspen HYSYS V9.0 is employed,which its accuracy has been proved through similar studies.Two separate EOSs are applied to hydrogen and MR streams to increase the accuracy.Determination of the independent variables and MR composition for cooling and liquefaction sections,are done through a trial and error procedure.The proposed concept has been improved in many aspects compared to the other studies and could lead to a high-efficiency process with COP and SEC equal to 4.54 kW·h·kg-1and 0.271,respectively.The main advantage of the proposed concept is using exhaust gases of gas power plants that are abundant in Iran and are normally dissipated to the environment.The present concept,compared to the ones that use geothermal or solar energies for feeding ARS,could perform much better since the exhaust gases of the power plants have a higher temperature(about 550°C)in comparison to the other sources of heat,such as solar and geothermal(max.200 °C).Moreover,recovering heat from exhaust gases is more economical than extracting heat from geothermal or solar sources.Since the liquefaction plant uses exhaust gases,it should be constructed near gas power plants,and this could eliminate the losses of the distribution network and facilitate the use of hydrogen liquefaction for energy storage in the off-peak times.High efficiency and performance capabilities have made the suggested cycle,superior in many aspects to the other cycles,especially in Iran,with a lot of gas power plants.Therefore,it is expected to be the basis for further development in the future.It should be noted that the proposed configuration performs well when a free source of heat is available while it is not feasible for typical applications.In this work,the proposed cycle has been examined only by energy analysis.Given the potential benefits and results of the other conventional analyses,it is suggested to investigate this cycle with different methods,such as exergy,exergy-economic,or exergy-environmental analysis.Also,due to the abundance of the combined cycle power plants,the development of an innovative cycle integrated into combined cycle power plants could be subjected to future researches.

CRediT authorship contribution statement

Hamed Rezaie Azizabadi:Conceptualization,Methodology,Investigation,Software,Validation,Writing -original draft.Masoud Ziabasharhagh:Conceptualization,Methodology,Supervision,Reviewing and Editing.Mostafa Mafi:Conceptualization,Methodology,Investigation,Writing -original draft

Nomenclature

Aheat transfer area,m2

Cpspecific heat capacity at constant pressure,kJ·kg-1·°C-1

CR compression ratio

HE heat exchanger

hspecific enthalpy,kJ·kg-1

kspecific heat ratio

.Mmolar flow rate,kgmol·s-1

ML mechanical losses

.mmass flow rate,kg·s-1

Ppressure,bar

.qvolumetric flow rate,m3·s-1or L·s-1

sspecific entropy,kJ·kg-1·°C-1

Ttemperature,°C

Uheat transfer coefficient,kW·m-2·°C-1

Wpower,kW

Zgas compressibility factor

η efficiency

ρ density,kg·m-3

Subscripts

H hydrogen

in inlet

is isentropic

o outlet

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.