Energy, exergy, and eco-environment modeling of proton exchange membrane electrolyzer coupled with power cycles: Application in natural gas pressure reduction stations

Highlights

• A system is proposed to produce power, heat, and hydrogen in NG PRSs

• Robust energy, exergy, and eco-environment models are developed

• Optimal outputs: 20.25 MW power, 19.91 MW heat, and 11.96 kg/h hydrogen

• The payback period is approximately 6.77 years at the best conditions

Abstract

This paper focuses on introducing a new configuration of a hybrid system for producing power and hydrogen together with pre-heating natural gas in PRSs. The configuration is based on regenerative Brayton, Rankine and proton exchange membrane electrolyzer cycles. Also, a heat exchanger is embedded for supplying the required heating load of NG pre-heating to prevent hydrate formation. A robust energy, exergy, and eco-environment mathematical model with real assumptions is developed to prove feasibility of the introduced system. To make the study applicable for different PRS capacities, the analyses are done for different equipment sizes during different months of year. The parametric study showed that design variables of the Brayton cycle and pressure of inlet NG are very effective parameters in the design of this proposal.

Analyzing the results in different months illustrated that the best performance is achieved in January; so that, 20.25 MW of power, 19.91 MW of heating load, and 11.96 kg/h hydrogen are produced for the optimum equipment variables in this month. At these conditions, the first- and second-law efficiencies and the levelized total costs rate are respectively obtained as 58.91%, 34.02%, and 7.03 $/GJ. Also, the payback period is 6.77 years based on the NPV approach.

Introduction

Although the use of renewable energy resources has grown in the last decade [1], but the energy image of the world is still framed in a context characterized by the serious need for fossil fuel resources, especially natural gas (NG) [2,3]. This is while the use of such fuels leads to serious concerns such as pollutant/greenhouse gases emission [4] and global warming phenomenon [5]. Accordingly, using new efficient technologies for the utilization of fossil fuels, such as designing new energy systems for recovering the wasted energies and producing hydrogen and freshwater, are as important challenges of nowadays [6,7].

The NG pressure reduction stations (PRSs) are one of the most energy-destructive places in the NG industry [8]. These stations are utilized for reducing the pressure of the NG transmission pipelines to be useable for industrial and residential customers. During the pressure reduction process in the NG PRSs, the NG temperature is also dropped significantly. As a result, in some months of the year, this decrease in temperature may lead to the freezing of NG, which is called NG hydrate formation [9,10]. Hence, before the pressure reduction process, a water-bath heater is used for pre-heating the NG to a certain temperature in order to prevent it from freezing [11]. The exhaust gases (EG) of heaters (which have a high heating capacity) are discharged into the atmosphere in most conventional water-bath heaters. This is while the heating potential of the EG can be recovered for different purposes namely energy improvement and costs/pollution reductions. Using turbo-expanders and thermodynamics cycles are the two methods to achieve the above-mentioned benefits.

The available literature can be categorized into two groups of studies: (i) the studies around the performance improvement of the NG PRSs through pressure recovery by utilizing turbo-expanders or waste heat recovery (WHR) of conventional PRS heaters (PRSHs). (ii) the studies around designing novel hybrid energy systems (HESs) for use in PRSs instead of the conventional PRSHs.

In the first category, in one of the most recent studies, Deymi-Dashtebayaz et al. [12] derived a water desalination unit and a turbo-expander through WHR of the PRSH's flue gases for producing fresh water and some power. They developed exergo-economic model of the system and utilized the Genetic Algorithm (GA) for achieving the optimal working conditions. Rahmatpour and Shaibani [13] utilized the pressure recovery approach to produce some power in an NG PRS in Iran. They used techno-economic and environmental analyses to evaluate the system and obtained investment payback time of 7.7 and 15 years based on the simple and discounted payback approaches, respectively. and Yaxuan et al. [14] inserted a turbo-expander on the way of high-pressure inlet NG and generated some power and used a heat pump cycle for NG pre-heating process. Their results illustrated a daily energetic and exergetic of 25% and 37.02% for their proposal. Saadat-Targhi and Khanmohammadi [15] applied the first- and second-law of thermodynamics to evaluate the performance of power generation by WHR of the PRSHs. They used a flash ORC (organic Rankine cycle) and a thermo-electric generator to achieve this goal. Applying the GA approach, they reached the optimum energetic efficiency and total exergy destruction of 67.16% and 45.08 kW, respectively. In another work conducted by the same authors [16], they investigated five different layouts of combining PRSH, flash ORC, and thermo-electric generator. In the best layout, the maximum generated power of the flash ORC and thermo-electric generator modules were 132 W and 11.5 kW, respectively. Borelli et al. [17] recovered the energy of the pressure reduction process in the NG PRSs for the performance improvement purpose. They developed a dynamic model for analyzing the proposed method. Olfati et al. [18] utilized the first- and second-law of thermodynamics for finding the most exergy destructive sources in four seasons of a year. They considered a certain NG PRS (with a nominal capacity of 20000 SCMH) for their analyses and found that the best and worst exergetic efficiencies were 77% and 69%, which occurred in winter and summer, respectively. Neseli et al. [19] applied a WHR approach for producing electricity in a case study PRS in Turkey. They developed the first- and second-law of thermodynamics models and found the energetic and exergetic efficiencies of 71.96% and 78.25%, respectively.

In the second category, limited researches have been done in recent years. Golchoobian et al. [20] developed a HES based on the WHR in the PRSs. Their proposed system was comprised of some power and refrigeration cycles and a water desalination unit. They concluded that the payback period of their proposal is less than 5 years depending on different electricity prices. Razmi et al. [21] used Aspen software to analyze a new heat and power cogeneration biomass-based system. They compared three different biomass materials and concluded that municipal solid waste provides maximum efficiency of around 70%. Alirahmi et al. [22] introduced a solar-based HES for electricity and hydrogen generation which could be used in different energy production industries such as NG PRSs. Applying exergo-economic optimization, they found exergy efficiency and total cost rate of 60.4% and 117.5 $/GJ, respectively. Shokouhi Tabrizi et al. [23] utilized the heating potential of a modified Brayton cycle (BC) to supply the required heating of the NG pressure reduction process in PRSs. They presented a detailed energetic, exergetic, and economic framework to assess the performance. They applied the model to the Birjand PRS (in Iran) and reached 4.87 million m³ fuel-saving compared with the current conventional system used in the station. Ebrahimi-Moghadam et al. [24] proposed a CHP plant for pre-heating the NG in PRSs together with producing electricity and developed a robust 4E model for evaluating their proposal. Applying the GA optimization, they reached to optimal ACI¹ of 0.763. Kowsari et al. [25] sought the optimal operating condition of a tri-generation system (power, heat, and hydrogen) for use in the NG PRSs. They developed exergoeconomic and environmental models and applied them for six different NG PRSs in Iran as case studies. Among all case studies, the highest productions were 5315 kW of power and 31.062 ton/y of hydrogen, while the lowest CO₂ emission of 246.7 ton/y. Li et al. [26] combined the use of a turbo-expander and an ORC for reducing the NG pressure and producing some power in the NG PRSs. They applied a thermo-economic optimization procedure and reached to 42.23% reduction in the electricity generation cost. Ghaebi et al. [27] proposed two systems for producing hydrogen and electricity in the NG PRSs together with satisfying the required heating load of the NG pre-heating. The PEME (proton exchange membrane electrolyzer) was used to produce hydrogen in both systems. An RC was utilized for the power generation in the first system while an absorption cycle was used in the second one. They compared two systems and found that the first system had higher efficiency and lower production costs. Lo Cascio et al. [28] proposed a new HES for use in the NG PRSs which pre-heats the NG within the pressure reduction process, produces some heat (to be used in local users near PRS), and generates some electricity (to be used in city users). They concluded that their system could maximally recover 69% of waste energy.

The literature around the field of the study illustrated that the researches published in recent years on the improvement of NG PRSs have focused more on the development of HESs instead of conventional water-bath heaters. Because for a certain amount of fuel consumption, in addition to providing the required heat for the NG pre-heating, the production of various types of energy will be possible and water consumption in water-bath heaters will also be eliminated. An overview of the presented literature indicates that the design and feasibility of using different arrangements of HESs in the NG PRSs (i.e. the second category in the above-reviewed literature) is an important issue that can lead to a reduction in energy consumption, costs, and pollution. Although some researches have been conducted in this area in recent years, but there are still gaps in the development of novel HESs based on the hydrogen production in the NG PRSs and the presentation of robust and comprehensive assessment models with real assumptions for approving the feasibility of their construction. Consequently, to fulfill the mentioned gap, the objectives and novelties of the present paper can be expressed as follows:

• Introducing a new layout of a HES for use in the NG PRSs: The introduced system is used for satisfying the required heating demand of the NG pre-heating process in PRSs together with producing some electricity and hydrogen.

• Utilizing the first- and second-law of thermodynamics and economic principles for verifying the feasibility of using the introduced system: Real assumptions and robust equations are applied to achieve this objective.

• Conducting a sensitivity study for investigating the system performance in different operating months during a year: The effects of some important systematic design variables are also investigated to make the results useable for different sizes and capacities.