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Existence of huge reserves of natural gas in the country and also the extent of its distribution lines has caused the use of natural gas as the main energy carrier. Seasonal fluctuations in gas consumption in domestic sector and giving priority to this sector has led that the gas supply to other sectors such as thermal power plants is faced with many problems in the cold season. One way to deal with this issue (shortage of natural gas) is the liquefaction and storage of surplus natural gas in the summer, using peak-shaving gas liquefaction plants. In this study, SMR and N2-expander processes have been evaluated. Changing in operational and environmental parameters (such as changes in flow rate, pressure, temperature and composition of the feed gas and working fluid of the cycle) are the main problems that peak-shaving plants will be permanently encountered with them, thus low sensitivity to changing conditions is the one of the important criteria in the selection of suitable process for peak-shaving. In this study, the sensitivity of liquefaction processes has been investigated using normalized sensitivity analysis. The results indicate that SMR process, despite lower power consumption is more sensitive to changes of the environmental and operational parameters and even, in some cases, the applied perturbation in the probable error range of measurement devices (such as 20 kPa uncertainty or fluctuation in compressor suction pressure), causes malfunction of the liquefaction process (wet entering the compressor).

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Supercritical carbon dioxide refrigeration is a proposed system to provide extremely low temperatures. The waste heat from the gas-cooler is noticeable. So, it can be used as a promising heat source in other systems like multi-effect-desalination system (MED), in order to provide cooling and fresh water, simultaneously; as well as reduction of power consumption. In this paper, the energy analysis and comparison of two novel combined systems are carried out. The combined systems consist of CO2 refrigeration system and two MED's models, the Boosted model and the water preheaters (PH) model. The effect of operating parameters such as evaporator temperature, ambient temperature and compressor outlet pressure on system performances are studied. Results showed that for both combined systems, by decreasing the evaporator temperature or increasing the ambient temperature, the coefficient of performance (COP) and the distilled water flow rate, decreases and increases, respectively. On the other hand, increasing the compressor outlet pressure would increase COP and decrease distilled water flow rate up to an optimum point. Also, MED-Boosted could produce more fresh water compared to MED-PH. In order to decrease the power consumption of the combined system two methods are presented. In two compressors method the COP enhances 6.2% compared to the base system (consists of one compressor and an expansion valve). However, the produced fresh water would be reduced by 60%. On the other hand, the expander method could improve the COP by 23.4%, compared to the base system, while the amount of distillated water decreases less than 8%.

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In this paper, the effect of expander and the internal heat exchanger is investigated in transcritical carbon dioxide direct-expansion geothermal heat pump. In this regard, a comparison is performed between four cycles. The four cycles are: (1) the cycle with expansion valve, (2) the cycle with expander, (3) the cycle with expansion valve and internal heat exchanger, (4) the cycle with expander and internal heat exchanger. The present numerical model has been investigated performance analysis of the four cycles under study in different operating conditions in two district cases. The first study includes a specific heating load, and the second study is a constant evaporator loop length (ELL). Then model evaluates characteristics including coefficient of performance (COP), evaporator loop length and heating capacity of the four cycles under study. To examine the performance of the four cycles, a parametric study is performed to investigate the effect of different parameters such as difference between soil temperature and evaporator outlet temperature, water inlet temperature, water mass flow rate, gas cooler length. The results indicate that COP associated with the expander cycle is always higher than the values related to the expansion valve cycle. The use of internal heat exchanger in a cycle including an expansion valve always leads to a slight increase in COP. An internal heat exchanger has negligible effect on COP in the cycle with an expander but reduces the evaporator loop length.