Parthasarathi Ghosh

In this article, by using exergoeconomic approach, an economic evaluation of reverse Brayton cycle-based refrigerator has been performed for 10 kW range cooling capacity at 65 K. One of the typical application domains of this refrigerator at the considered heat load is cooling of high-temperature superconductor for future power transmission lines that are in the developmental phase in different parts of the world. Multi-objective optimization and sensitivity analysis have been performed to investigate the sources of cost attached to the exergy destruction. Based on exergoeconomic evaluation parameters, recommendations have been provided for deciding the cost-effective design parameters for refrigerators. The significant finding of this analysis is that, for the basic reverse Brayton cryocooler, component performance, the turbine being the major one, needs to be improved to meet the economic criteria of 25 $/W as enumerated in the cryogenic road map for high-temperature superconductor cooling. The exergoeconomic design applied on the reverse Brayton refrigerator for high-temperature superconductor cables cooling can be adapted for other applications such as recondensation and liquefied natural gas cold energy utilization.

Cryogenic turboexpanders are an essential part of liquefaction and refrigeration plants. The thermodynamic efficiency of these plants depends upon the efficiency of the turboexpander, which is the main cold generating component of these plants, and therefore, they should be designed for high thermodynamic efficiencies. Balje's [1] n s d s chart, which is a contour of isentropic efficiencies plotted against specific speed and specific diameter, is commonly used for the preliminary design of cryogenic turboexpanders. But, these charts were developed based on calculations for a specific heat ratio (γ) of 1.4, and studies show that care should be taken while implementing the same for gases which have a higher γ of 1.67. Hence there is a need to investigate the extent of applicability of n s d s diagram in designing expansion turbines for higher specific heat ratios. In this paper, Computational Fluid Dynamics (CFD) analysis of cryogenic turboexpanders was carried out using Ansys CFX ®. The turboexpanders were designed based on the methodologies prescribed by Kun and Sentz [2] following the n s d s diagram of Balje and Hasselgruber's technique for generating blade profile. The computational results of the two cases were analysed to investigate the applicability of Balje's n s d s diagram for the design of turboexpanders for refrigeration and liquefaction cycles.

Cold end in helium liquefiers, where finally the gas is converted to liquid, may have different alternatives in configuration to achieve maximum exergy efficiency. Apart from high exergy efficiency, which also means low specific power consumption, reliability of operation and complexity of equipment design are also some of the concerns for designers. In this work, various cold end configurations are compared at different operating conditions to help arrive at appropriate choices during design and operation. When single Joule–Thomson valve is replaced by more efficient cold ends with expanders, it allows reduction of the number of Brayton stages in precooling section as well as the total heat exchanger size. Cold end with expander and Joule–Thomson valve combination has been found to be a good compromise between reliability, liquid production, specific power consumption, and exergy efficiency. However, for such a configuration, it is important to fix the intermediate pressure at appropriate level so as to avoid liquid formation at the exit of the expander.