Refrigeration system is regarded as the significant factor to influence the utilization efficiency of liquefied natural gas (LNG) cold energy, and proper selection of refrigerant can decrease exergy loss and increase the cold energy utilization. As refrigerant played a key role for thermal transport procedure, various refrigerant design methods were reviewed, and the refrigerant selection principle was determined. Furthermore, a new molecular design method for newly developed refrigerants and existing refrigerants modified was proposed. Based on this method, refrigerants with optimal thermodynamic properties were generated, which could work efficiently for LNG cold energy power generation system. For the molecular design method verification, a LNG cold energy power generation system with two circulation loops was designed, seawater worked as high temperature heat source, combined with the low temperature LNG cold energy.
As the energy consumption increases, utilization of the low grade thermal energy becomes more and more important because of cost effectiveness of operation. The performances of the fluid are directly connected with the effective using and transferring of thermal energy, especially in the field of thermal conversions. In particular, the organic fluids have many different characteristics from water (Stine and Harrigan, 1985). Therefore, working fluids used in organic Rankine cycles have been studied. However, performance of the organic fluids varied depending on the working conditions (Musthafah and Yamada, 2010). The thermal and physical properties of the fluid are influenced by temperature (Yuan et al., 2016), and this lead to various thermal coefficients. Fredy et al. simulated a Rankine cycle operating at low temperature (2012), with the pressure ratio of 2.5 and the temperature ranging approximately from 70 to 85, and eventually R152a offers the best performance. While for temperature between 85°C and 97°C, the fluid with best performance turned out the R290. This result is demonstrated by the research (Bertrand et al., 2009), which shows that R152a, R600a, R600 and R290 offer attractive performances. In addition, R407C appears high evaporator pressure and low efficiency, R12, R113, R114 and R500 result high GWP and high ODP, RC318 results high GWP, R141b shows high turbine outlet volume flow rate and high ODP. R134a followed by R152a, R290, R600 and R600a emerges as most suitable fluid for low-temperature applications with heat source temperature below 90°C. However, the disadvantages of R290 are also obviously: it is combustible and the size of the compressor would be bigger than that of using R22 in the assigned refrigeration machine (Hadya et al., 2012). Furthermore, R227ea gives the highest power for heat source temperature range of 80–160°C in another study (Amlaku and Bolland, 2010), and R245fa produces the highest power in the range of 160–200°C. The least heat exchanger area required at constant power rating is found when the working fluid is npentane, the turbine size factor of R134a is smaller compared to other working fluids. However, R134a requires more heat exchanger surface area. According to these results, working fluids with high vapor pressure has lower turbine size factor. Finally, 35 organic working fluids were reviewed for the conversion of low grade heat (Chen et al., 2010). The results showed that working fluids with high density and high latent heat provide high unit turbine work output, also, isentropic and dry fluids are preferred in organic Rankine cycles. In conclusion, various researches have been done on the working fluids for low temperature Organic Rankine cycles. On the other hand, there is no fluid which can fit for one certain cycle. Various fluids working for the thermal cycles have their own characteristics with the production issues, and for this reason, numerous efforts have been made to modify the thermodynamic characteristics of working fluid.
