Direct Expansion Cycle Research Articles

4E analysis of novel direct expansion cycle-organic Rankine cycle-integrated hydrogen refueling station system using liquid hydrogen cold energy

The global supply of fuel cell vehicles (FCEVs) is expected to increase to reduce carbon emissions. The number of hydrogen refueling stations (HRSs) is increasing annually, and liquid hydrogen (LH2) is attracting attention as an alternative to overcome the transport and storage limitations of gaseous hydrogen (GH2). If LH2 is supplied to the market, the existing GH2-HRSs can potentially become obsolete because they are not as economically feasible compared with LH2-HRSs. Therefore, in this study, we proposed a new HRS system that integrates direct expansion cycle (DEC) and organic Rankine cycle (ORC) using LH2 cold energy to increase the utilization and economic feasibility of GH2-HRSs. The proposed design was validated through 4E (energy, exergy, economy, and environmental impact) analysis. Compared to the basic design, which did not include LH2 as cold energy, the specific energy consumption (SEC) and exergy efficiency of the proposed design improved by 38.6% and 9.8% to 0.81 kWh/kg H2 and 89.1%, respectively. The minimum selling price improved by 8.3% to $ 1.02/kg H2. In addition, greenhouse gas emissions decreased by 11% from 2.9 kg CO2-eq/kg H2 to 2.6 kg CO2-eq/kg H2, and the life cycle cost (LCC) reduced by 10.5%. Finally, the net present value of the proposed design increased by $535,000, and the payback period decreased by 1 year, indicating that the proposed design was more economically feasible than the basic design. In conclusion, the proposed ORC-DEC-integrated model is practical and feasible in terms of exergy, energy, economy, and environmental impact, allowing the current GH2-HRSs to be used if LH2 becomes the main hydrogen energy.

Sensitivity and sustainability analysis of combined cycle plant with integrated LNG pressure energy recovery

Although combined cycle power plants (CCPPs) and liquefied natural gas (LNG) have arisen as solutions to the intermittent nature of renewable energy sources, they still need creative methods for performance enhancement and sustainability of the energy systems. Moreover, the energy efficiency of CCPPs is negatively impacted by environmental conditions, higher specific fuel consumption and a large quantity of waste heat released into the ambient. On the other hand, using seawater as a heating medium in LNG regasification circuits has a detrimental effect on the marine ecosystem. In this context, the LNG pressure energy recovery system through a direct expansion cycle (DEC) integrated with CCPP is proposed to improve sustainability, enhance exergetic performance and to reduce the heat recovery steam generator (HRSG) exhaust loss. To improve the cycle performance, low grade heat harnessed from CCPP is used as the heat source in the DEC rather than seawater. The system performance is analyzed by the energy and exergy method through a mathematical model. Moreover, the sensitivity analysis of the combined system is accomplished to examine the influence of LNG flow rate and ambient temperature on various performance indicators of the total system. It is observed that the integration of DEC boosts the HRSG performance by reducing the stack exergy loss and the efficiency is sensitive to LNG flow rate. The total integrated system energy efficiency ηtot, enhanced to 1.55% compared to the base case. Whereas, for ambient temperature in range of 15 – 45 °C, the system minimum and maximum exergetic efficiency ηex,tot is found to be 54.52 % and 55.94 %. Furthermore, the comparison of exergetic sustainability indicators is calculated and presented for three different simulation cases. The results demonstrate that increase in ηex,tot, affects the environment impact factor, ref and the sustainability index, θsi and improved 2.09 % compared to base case combined cycle.

Thermodynamic analysis of power generation thermal management system for heat and cold exergy utilization from liquid hydrogen-fueled turbojet engine

Hydrogen propulsion is generally treated as the ultimate net zero-emission option for aviation, therein liquid hydrogen storage appears to provide the only feasible solution for aircraft with the constraints imposed by aircraft weight and volume. Hydrogen liquefaction consumes a great amount of energy, which significantly boosts the cost of hydrogen as an aviation fuel. Besides the H2 chemical exergy for combustion to provide thrust, the H2 physical exergy in the form of cold energy is also a kind of high-quality clean energy to produce power output, which offers great opportunities to achieve cost-effective use of hydrogen fuel. Thus, this paper presents a Rankine cycle and H2 direct expansion cycle (RC-DEC) combined power generation thermal management system for the simultaneous utilization of exhaust gas heat exergy and hydrogen fuel cold exergy from the liquid hydrogen-fueled turbojet engine. In the Rankine cycle, the transcritical mode is applied on the hot side while the liquid separation condensation concept is adopted on the cold side to achieve better thermal matchings with heat and cold sources. The hydrocarbon/inert gas binary mixture is proposed as the working fluid of the Rankine cycle. The effects of key thermodynamic parameters and the influence of Para-Ortho hydrogen conversion on system performance are discussed, then a comparison of the mixture working fluid is conducted. Results show that there exists an optimal separation temperature ratio and condensation pressure of RC to obtain maximum total net power output. As the DEC expander inlet pressure increases, the total net power output first increases and then levels off. The introduction of the Para-Ortho hydrogen conversion has a negligible impact on net power output but leads to a significant reduction in the exergy efficiency, which is attributed to the cold energy release nature of Para-Ortho hydrogen conversion. The results of the working fluid comparison indicate that the optimal inert gas retardant for CH4 is Kr, while those for C2H6 and C3H8 are Xe to achieve the maximum net power output and exergy efficiency of the proposed power generation system. Among them, C2H6/Xe(0.6/0.4) obtains the maximum net power output of 2717.9 kW and maximum exergy efficiency of 33.1%, which suggests an opportunity to provide megawatts of electrical power for the aircraft in place of the APU.

Evaluations of energy, exergy, and economic (3E) on a liquefied natural gas (LNG) cold energy utilization system

The LNG cold energy utilization system primarily integrates additional power cycles during the gasification process to harness the cold energy released in this process efficiently. This study introduces a combined cooling and power (CCP) system, which comprises a cascade organic Rankine cycle (ORC), an air conditioning refrigeration (ACR), and a direct expansion cycle (DEC). Furthermore, the thermodynamic and economic analyses of the CCP system were evaluated, and the optimization of each influence parameters was performed by the genetic algorithms, such as the composition of the quaternary mixed working fluid (Methane: Ethane: Propane: i-Butane), condensation pressure, evaporation pressure, and the first-stage expansion pressure within the system. According to the simulation results, the suggested overall exergy destruction of the CCP system proposed by this work is reduced by 13.18% under optimized conditions, with a maximum net power generation of 230.38 kW and an exergy efficiency of 45.58%. Economic analysis reveals that the system's investment cost, annual economic benefits, and levelized energy cost are 2.02 × 106 $, 2.75 × 105 $, and 0.10 $/kWh, respectively. With the system's official operation, the investment cost will be recovered within 8.61 years.

Exergy destruction analysis of a power generation system utilizing the cold energy of LNG

The purpose of this research is in-depth understanding of the internal causes of exergy destruction in various parts of the system and to identify potential improvements for the components. The focus is on a combined cycle power generation system that utilizes the organic Rankine cycle (ORC) and direct expansion cycle (DEC). To investigate the primary sources of exergy destruction in each component, advanced exergy analysis (AEA) is utilized. The result demonstrates that the net out power of the proposed system can reach 106.64 kW with energy efficiency of 11.22%, and exergy efficiency of 21.40%. The heat exchanger is identified as the primary contributor to exergy destruction, constituting 81.70% of the total ratio. Specifically, the condenser exhibits the highest exergy destruction ratio at 59.82%, indicating a need for prioritized optimization efforts. The findings of AEA reveal that the primary source of component irreversibility stems from the endogenous part. This shows that, while most exergy destruction is unavoidable, there remains room for system improvement. Regarding the turbine, its exergy destruction is primarily attributed to inefficiencies, leading to irreversibility. Nevertheless, there is exergy destruction that may be avoidable and can be reduced by 25.93 kW, which is 2.5 times greater than that of the heat exchanger. This finding underscores the high potential for improvement in ORC and DEC turbines, making them a priority for optimization efforts.

Thermodynamic analysis and assessment of novel ORC- DEC integrated PEMFC system for liquid hydrogen fueled ship application

Proton-exchange membrane fuel cell (PEMFC) and liquid hydrogen are gaining attention as a power generation system and alternative fuel of ship. This study proposes a novel PEMFC system, integrated with the organic Rankine cycle–direct expansion cycle (ORC-DEC), which exploits cold exergy from liquid hydrogen and low temperature waste heat generated by the PEMFC for application in a liquid hydrogen fueled ship. A thermodynamic model of each subsystem was established and analyzed from the economic, energy, and exergy viewpoints. Moreover, parametric analysis was performed to identify the effects of certain key parameters, such as the working fluid in the ORC, pressure exerted by the fuel pump, cooling water temperature of the PEMFC, and the stack current density on the system performance. The results showed that the proposed system could generate 221 kW of additional power. The overall system achieved an exergy and energy efficiency of 43.52 and 40.45%, respectively. The PEMFC system had the largest exergy destruction, followed by the cryogenic heat exchanger. Propane showed the best performance among the several investigated ORC working fluids and the system performance improved with the increase in the cooling water temperature of the PEMFC. The economic analysis showed that the average payback time of ORC-DEC was 11.2 years and the average net present value (NPV) was $295,268 at liquid hydrogen costing $3 to $7, showing the potential viability of the system.

Feasibility study of a combined system of electricity generation and cooling from liquefied natural gas to reduce the electricity cost of data centres

Global data centre power demands would expand from 286 TWh in 2016 to around 321 TWh in 2030. The cooling system represents electricity consumption of approximately 40–50% of the total energy used. The global LNG trade has reached 356.1 MTPA. Cold energy equal to 89025 GW is released into the ocean. Therefore, this study focused on the technical and economic feasibility of an LNG receiving terminal combined with a data centre using a direct expansion cycle (DEC), a Rankine cycle (RC) and a combination of Rankine cycle and direct expansion cycle (RC + DEC) under different natural gas distribution pressures to produce a supply of cooling water and electricity to reduce electricity consumption and greenhouse gas emissions. According to the study, the RC + DEC produces the maximum cold water at 7 °C, with the total cold energy of 44.23 MW which is sufficient for cooling a data centre with a capacity of 5345 racks, to reduce the electricity for conventional cooling system is 13521 kWh and generated electricity form turbine is 9968 kWh. This research has the potential to reduce the operating costs of data centres by more than USD 23.87 million per annum as well as CO2 emissions by 83859 t per annum with exergy efficiency of 78.94%. In an economic study, indicated a payback period of 1.60 years with an IRR of 62%.

Thermodynamic assessment of a geothermal power and cooling cogeneration system with cryogenic energy storage

Geothermal energy is one of the main renewable energy sources for power generation and district cooling, and liquid air energy storage is an emerging technology suitable for both power and cold storages. Accordingly, a combined power and cooling cogeneration system with cryogenic energy storage is proposed in this paper, which is powered by geothermal energy and connected with the grid. The system is formed by integrating a turbine ejector cogeneration cycle with an air liquefaction cycle, a liquid air direct expansion cycle and a cryogenic organic Rankine cycle. In normal operation mode, only the turbine ejector cogeneration cycle works in the system based on the cooling load requirement. In charging operation mode, all the power produced in the system is used to liquefy air in the air liquefaction cycle owing to cheap electricity tariff. In discharging operation mode, the additional electricity is provided to meet peak time energy requirement by the direct expansion cycle and cryogenic organic Rankine cycle. A geothermal source at 180 °C with a flow rate of 100 kg/s is used as the heat source in this study, the cogeneration system has the ability to produce 15,470 kW of power and supply 4800 kW of cooling simultaneously, the system round trip efficiency is 41.07%, and the exergy efficiency of cryogenic energy storage is 60.43%. Also, the effects of geothermal energy temperature, system size, turbine mass split ratio and normalized mass flow rate on the system performance are clarified.

Advanced exergy and exergo-economic analyses of a novel combined power system using the cold energy of liquefied natural gas

In this study, a new combined power system using the cold energy of liquefied natural gas (LNG) is designed. Conventional and advanced exergy and exergo-economic analyses of the system were made. According to the research in the literature, for the first time, advanced exergo-economic analysis was applied to a combined power system using the cold energy of LNG with the Modified Productive Structure Analysis (MOPSA) method. 23% of the exergy destruction and exergy destruction cost rate of the combined system is avoidable. In both conventional and advanced exergy analysis, the highest exergy destruction and cost rate were found in the parabolic solar collector. In addition, as a result of the analyses, it has been seen that the most effective main system components with the potential to improve the performance of the combined system are the parabolic solar collector, the turbine in direct expansion cycle and the turbine in the Organic Rankine cycle, respectively. The study revealed that there is potential to improve the performance of the proposed combined power system.

Thermodynamic and economic analysis of a novel hydrogen liquefaction process with LNG precooling and dual-pressure Brayton cycle

Liquid hydrogen with high energy density and cleanliness is a superior alternative to current energy carriers. However, high costs and low efficiency are barriers to liquefy hydrogen. To reduce the energy consumption and investment costs of hydrogen liquefaction, a hydrogen liquefaction process that utilizes a direct expansion cycle of liquefied natural gas for hydrogen precooling and a dual-pressure Brayton cascaded cycle for hydrogen cryo-cooling is proposed. The hydrogen liquefaction performance and economic benefits of the proposed process are assessed by comparing it with two reference processes with different cryo-cooling cycles. The results reveal the advantageous operating costs and capital costs of the proposed process, especially in terms of the costs of helium and heat exchangers. Moreover, the specific energy consumption of the proposed process is 6.60 kWh/kgH2, which is 4.0% and 4.5% lower than those of the reference processes. The exergy losses and exergy efficiency of the proposed process are 12.36 MW and 47.0%, respectively, and the exergy losses are mainly caused by the compressors and expanders. The energy consumption of the proposed process decreases at first and subsequently increases with the increasing pre-compression pressure of helium.

Thermodynamic comparison of modified Rankine cycle configurations for LNG cold energy recovery under different working conditions

In this study, we present the evaluation of two modified single-stage Rankine cycles (RC) and two two-stage RCs to provide a guidance for configuration selection under different working conditions. Moreover, a regenerative-reheat RC, which is the combination of the modified single-stage RCs, is proposed to utilize the cold energy of liquefied natural gas (LNG) and the thermal energy of low grade heat. The key parameters of these five configurations using eight working fluids are optimized with exergy efficiency as the optimization objective. The results show that the modified single-stage RCs are usually better than the two-stage RCs since they have simpler configurations with similar or higher efficiencies. The regenerative-reheat RC performs even better under most working conditions and it achieves the maximum efficiency of 24.57% by using ethane as the working fluid under the heat source temperature of 180 °C and the pipeline pressure of 3 MPa. The addition of direct expansion cycle can improve the exergy efficiencies of most fluids and configurations under lower pipeline pressures. However, the improvements decrease greatly for fluids with low normal boiling point temperatures (NBPTs) and especially under higher pipeline pressures. As to the fluid selection, high critical temperature and low NBPT are both important for reheat RC and parallel two-stage RC. NBPT weights more than critical temperature for regenerative RC and regenerative-reheat RC. Besides, wet working fluids are more suitable for reheat RC while dry and isentropic fluids are more suitable for regenerative RC and regenerative-reheat RC. The optimal regenerative ratio and reheat pressure ratio vary with different working fluids and working conditions. Lower reheat pressure ratio usually leads to higher regenerative ratio.

LNG regasification and electricity production for port energy communities: Economic profitability and thermodynamic performance

The integration of electricity and gas systems is a key priority for the energy transition. Alongside electricity, low-carbon energy vectors, such as Liquified Natural Gas (LNG) or hydrogen, are required in future energy systems. Hence, regasification systems, often located nearby port areas, will be strategic assets promoting environmental, social, and economic benefits.This study investigates and compares three integrated configurations exploiting LNG cold exergy for the efficient production of electricity in a port area . In all three configurations, electricity is produced by a gas turbine fuelled with regasified natural gas and different combinations of two bottoming cycles, consisting of a transcritical CO2 Rankine cycle and a natural gas direct expansion cycle. These configurations are compared with a traditional regasification and combined cycle power plant. The study aims at identifying the design of the system that maximizes the second law efficiency and the Net Present Value (NPV) for various relative sizes between gas turbine and regasification capacity, including analyses on Internal Rate of Return (IRR). A sensitivity analysis on the installation of turbine inlet air-cooling taking advantage of LNG cold exergy is also performed. The optimization is performed through Particle Swarm Optimization (PSO) algorithm and Aspen Hysys.Results show that the mutual size between the gas turbine and regasification unit is an important variable that requires careful analysis, as the second law efficiency can vary between 49% and 42% depending on the case. Electric efficiency of the proposed configurations can approach 65%, thus resulting greater than the traditional combined cycle in most of the test conditions. Economic optimizations confirm that the NPV of two configurations is often 10–15 M€ greater than the corresponding value of the base case system; however, only a single configuration can increase the IRR by 1–2% in comparison to the case with separate units. These results can guide designers and developers in the definition of the most cost-effective solution of any integrated energy systems including regasification, similar to a port system, which are expected to be key facilities in the future energy scenario.

Maximization of the power production in LNG cold energy recovery plant via genetic algorithm

This paper presents an optimization model via genetic algorithm (GA) to maximize the power generation potential of a liquefied natural gas (LNG) cold energy recovery plant. LNG releases a large amount of cold energy during vaporization prior to transport for service, and this cold energy can be effectively utilized to generate power using a heat engine. We performed a thermodynamic analysis for a power generation system combining the organic rankine cycle (ORC) driven by LNG exergy and the direct expansion cycle. Both LNG and the working fluid in the combined ORC are light hydrocarbon mixtures, and their physical properties were estimated using the Peng-Robinson equation. We conducted a thorough investigation of the effects that the working fluid composition brought about on the thermal efficiency of the heat engine through an analysis using Aspen HYSYS interfaced with a GA-based Matlab solver. The results showed that optimization of the working fluid composition led to an increase of 58.4% in the performance of the combined ORC in terms of the net work.

Thermodynamic analysis of a cryogenic power generation system recovering both sensible heat and latent heat of flue gas

This paper proposed a cryogenic power generation system composed of Rankine cycles and direct expansion cycle, aiming at the efficient utilization of flue gas waste heat and liquefied natural gas (LNG) cold energy. Both the sensible heat and latent heat of the humid flue gas are recovered to meet the heat demand of a typical LNG terminal. Performance analysis was conducted to reveal the potential improvement direction and determine the key parameters. Then, parametric study was carried out to evaluate the effects of key design parameters on system performance. Four combinations of working fluids were compared, and the system performance was analyzed at different flow rates of natural gas. Results indicate that the net power output decreases as the condensing temperature of the bottoming Rankine cycle increases, and there exists a volcano-type relationship between the net power output and natural gas expansion pressure. R290/R1150 is the preferred combination of working fluids under the design conditions, and the maximum thermal efficiency, exergy efficiency, and specific power can reach 27.07%, 48.42%, and 74.15 kWh/ton-LNG, respectively. Moreover, the specific power decreases with the increase of LNG flow rate. R1270/R1150 becomes the best combination of working fluids, when the LNG flow rate exceeds 120.28 kg/s.

The study of a novel two-stage combined rankine cycle utilizing cold energy of liquefied natural gas

In order to save energy and protect the environment, the cold energy of liquefied natural gas and industrial waste heat are utilized in the Rankine cycle. In this study, a novel two-stage Rankine cycle that combines the advantages of the cascade two-stage Rankine cycle, parallel two-stage Rankine cycle, and regenerative Rankine cycle was proposed, in which the cold energy of LNG is utilized step by step. Ethane + R32, ethane + propylene, R23 + propylene, and R23 + R32 are selected as the working fluid combinations. The performance of working fluid is compared by studying the cycle performance under different parameters, and the cycle parameters are optimized through a genetic algorithm. The results show that Ethane + R32 is the best combination, with a higher exergy and thermal efficiency in most cases; further, ethane + propylene achieved the maximum exergy and thermal efficiency of 31% and 20.8%, respectively. When the temperature of the heat source is changed from 303 K to 323 K, the exergy efficiency of the cycle varies by approximately 16%. Compared with the parallel two-stage Rankine cycle + direct expansion cycle and the cascade two-stage Rankine cycle + direct expansion cycle, the exergy efficiency of the novel cycle increases by 104% and 29.76%, respectively.

Possibilities of Liquefied Natural Gas (LNG) use for power generation

Liquefied natural gas (LNG) has an important role in the global industry and energy balance. The use of this energy carrier has been increasing for last decades. The broad development of the LNG sector has been noticeable in the search for new supply directions by natural gas customers. An important option to transport the gas is to convert it into liquid natural gas (LNG) and convey it using insulated LNG tankers. At receiving terminals, the LNG is unloaded into storage tanks and then pumped for the required pressure, vaporized and compressed for final pipeline transmission to natural gas pipeline system. The LNG production process consumes a considerable amount of energy. This energy is stored in LNG as cold energy. At an unloading terminal, LNG is evaporated into gas phase at ambient temperature before pumping into the natural gas transmission system. Seawater or ambient airare commonly used for the regasification process of the LNG. In process of regasification the large part of energy stored in LNG may be recovered and used for electricity generation. In the presented paper a general analysis of the various thermodynamic schemes proposed for power production from regasification has been made. Direct expansion cycle, Rankine cycle and Brayton cycle are analyzed in presented case.

Exergy Analysis of Liquid Nitrogen Power Cycles

Nitrogen is by-product from cryogenic air separation processes used for oxygen production for metallurgy and oxygen-enriched combustion purposes. If the gases are delivered from air separation unit (ASU) in liquid phase, liquid nitrogen (LN2) can be used as energy accumulator for stabilization of electrical grid system with large share of renewable energy sources. When the energy demand is high and not enough electricity is generated in power plants, energy accumulated inLN2may be recovered in a cryogenic power cycle. In this research complete exergy analysis of liquid nitrogen direct expansion cycle and combined direct expansion/Brayton cycle was performed.

Analysis of different combined cycles and working fluids for LNG exergy recovery during regasification

It has been estimated that the world's consumption of Liquefied Natural Gas (LNG) will increase significantly over the next 20 years, thus making exergy recovery from the regasification process a fundamental issue. When LNG is regasified in order to distribute the fuel through a pipeline network, a large amount of exergy is released.Three combined cycle schemes for energy generation have been analysed in this paper: the first one is a direct expansion cycle, combined with a Rankine cycle, the second one presents a double expansion with reheating and a recovery heat exchanger, and the last one shows two parallel Rankine cycles working under different turbine pressures. The performance of the three cycles has been compared, and the effects of using working fluids with different characteristics have been analysed in detail. Twelve working fluids were selected, according to their thermodynamic, ambient and safety proprieties. The working pressure and temperature that maximise the specific work have been found for each cycle and fluid.

Thermodynamic optimization and comparative study of different ORC configurations utilizing the exergies of LNG and low grade heat of different temperatures

ORC is a good solution to recover the cold exergy of LNG (liquefied natural gas) and the thermal exergy of low grade heat simultaneously. The optimal working fluids and parameters change greatly with different configurations and heat source temperatures. In this paper, three different ORC configurations: SORC (single-stage ORC), PTORC (parallel two-stage ORC) and CTORC (cascade two-stage ORC), combined with DEC (direct expansion cycle) are analyzed and compared. Up to seven key parameters of sixty four fluid combinations under four heat source temperatures are optimized by the particle swarm optimization. The results show that PTORC is more suitable for lower heat source temperatures and CTORC performs better for higher heat source temperatures. PTORC+DEC with ammonia/ethane as the fluid combination yields the maximum efficiency of 17.36% for the heat source temperature of 50 °C. CTORCs without DEC, also with ammonia/ethane as the fluid combination, achieve the maximum efficiencies of 19.49%, 21.69% and 24.37% for the heat source temperatures of 100, 150 and 200 °C, respectively. The optimal turbine inlet temperatures are usually their upper bounds and the optimal condensation temperatures are the normal boiling temperatures of the fluids. There are optimum evaporation temperatures for all configurations, which increase with the heat source temperature.

Comparative study of Rankine cycle configurations utilizing LNG cold energy under different NG distribution pressures

NG (Natural gas) distribution pressure varies in a wide range in different applications and has significant effect on system exergy efficiency. RC (Rankine cycle) and DEC (direct expansion cycle) can utilize the cold and pressure exergies of LNG (liquefied natural gas), respectively. In this paper, SRC (single-stage RC), PTRC (parallel two-stage RC) and CTRC (cascade two-stage RC) with and without DEC are optimized by Particle Swarm Optimization and compared to obtain the optimal configurations for different NG distribution pressures. Propane, propylene, ethane and ethylene are adopted as the working fluids. The results show that the addition of DEC could increase the exergy efficiency by lifting the evaporation temperature of LNG. CTRC + DEC with propane/ethylene and CTRC + DEC with propane/ethane as the working fluid achieve the largest exergy efficiencies of 23.89% and 18.25% at NG distribution pressures of 1 MPa and 2 MPa, respectively. PTRC + DEC with propane/ethane is the optimal system with maximum exergy efficiencies of 15.18%, 13.24%, 11.7% and 10.4% at NG distribution pressures of 3, 4, 5 and 6 MPa, respectively. PTRC + DEC with ethane as the working fluid in both cycles is a generally suitable system for all NG distribution pressures with high efficiency and only one type of working fluid.