Research paperDesign of high-efficiency Joule-Thomson cycles for high-temperature superconductor power cable cooling
Introduction
As newer high-temperature superconductors (HTS) with critical temperatures higher than liquid nitrogen (LN2) temperature are discovered, HTS materials continuously show great potential in engineering and commercial applications. Nowadays, a HTS power cable is considered as a good alternative of a conventional power cable for high electric demand areas. Though the demand of electric power becomes drastically higher than that in the past decades in metropolitan areas, the space to construct new transmission cables is limited. HTS power cables are, therefore, often suggested as a viable option to solve those critical transmission expansion issues [1], [2].
HTS power cables are more complex than the conventional transmission lines in a technical point of view [2]. Bi-2223 (Bi2Sr2Ca2Cu3O10) or YBCO (YBa2Cu3O7) is commonly utilized materials in HTS power cables. LN2 coolant is usually circulated through the cables to maintain the superconductivity of the HTS power cables [1], [3]. The operating temperature of HTS power cables should be carefully controlled according to the current density, electrical insulation and electromagnetic interference of the cable itself [4]. A cryogenic refrigeration system is indispensable in an HTS power cable application for the continuous supply of LN2 coolant. In general, LN2 coolant is subcooled by a refrigeration system to lower than 70 K to guarantee the reliable operating conditions of the HTS power cables [5]. A highly efficient large-scale cryogenic refrigeration plant is needed in order to scale up and increase the efficiency of a HTS power cable system. The state-of-the-art HTS power cable operating at about 70 K has a heat inleak of approximately 1 W per unit length (1 m) from the environment [2] This means that several kilowatts of cooling capacity at 70 K is required for a few kilometers of HTS power cable length. In this circumstance, many researchers focus on the demonstrations of the HTS power cables in conjunction with the actual electric transmission grid.
There are two kinds of cryogenic refrigeration systems to cool down the LN2 coolant with a few kilowatts cooling capacity: (i) a direct decompression system and (ii) a large capacity refrigerator (Fig. 1). A decompression system is simply constructed in an open-loop configuration, and mainly consists of a LN2 storage tank and a vacuum pump, as seen in Fig. 1(a). The LN2 temperature drops to the corresponding saturation temperature when LN2 pressure is lowered by a vacuum pump. This kind of refrigeration system is utilized in many HTS power cable facilities [6], [7] because of its simple construction. The periodical replenishment of LN2, however, should be carried out due to the continuous loss of boil-off gas, which is a critical drawback. To solve this problem, a closed-loop cooling system was devised by using a large capacity cryocooler and the circulation of the coolant (Fig. 1(b)). The closed loop system is also shown in the recent demonstration project of HTS power cable [8], [9] which benefits from the thermodynamic efficiencies increase of cryocoolers, such as Joule-Thomson (JT), Brayton, and Stirling systems. Furthermore, there were some efforts to consider the application of mixed refrigerant (MR) JT refrigerators to cool down a HTS power cable [10], [11] to remove the moving parts at the cold end and increase the reliability of refrigeration system, and 6.5% of Carnot efficiency was achieved at the cooling temperature of 63.6 K.
This paper proposes two JT refrigeration cycles for subcooling LN2 in the HTS cable cooling cycle by using N2 as the working fluid. Additionally, a high-efficiency integrated HTS cooling cycle with a phase separator is suggested. The efficiencies of various refrigeration systems, including an open-loop decompression system, a JT refrigeration cycle with a vacuum pump (Cycle A), a JT refrigeration cycle with cold compressors (Cycle B) and an integrated HTS cooling cycle (Cycle C), are analysed and compared with each other in the same operating conditions in detail. A commercial software, ASPEN HYSYS version 8.0, with Peng-Robinson equation of state (EOS) is used to calculate the thermodynamic states. The exergy destruction analysis is conducted to evaluate the irreversibility of each cycle
Section snippets
Configuration and operating conditions of HTS cable cooling cycle
The target temperature of the refrigeration cycle for HTS cable is assigned to approximately 67 K to maintain a sufficient subcooling degree of the HTS cable coolant. The schematic diagram and the operating mechanism of a basic JT cycle are shown in Fig. 2. The cooling temperature of a JT cycle is determined as the saturation temperature at the pressure after the JT expansion process. To achieve the saturation temperature of 67 K, the pressure after the JT expansion process should maintain as
Decompression system
Work consumption in the decompression system mainly occurs in the vacuum pump and the LN2 pump as well as in the LN2 production and transportation process. With the presumed efficiency values, the work requirements of the vacuum pump and LN2 pump are calculated as 8.2 kW and 0.17 kW, respectively. As the required mass flow rate of LN2 coolant is 0.051 kg/s in this system, the work consumption for LN2 production is 88.3 kW. If LN2 is transported by a vehicle for the distance of 130 km, the work
Concluding remarks
Various cooling cycles for HTS power cable are investigated by using nitrogen JT refrigeration cycles. A decompression system, a JT refrigeration cycle with a vacuum pump, a JT refrigeration cycle with cold compressors, and an integrated cooling cycle for HTS power cable are specifically considered to calculate their refrigeration efficiencies. The decompression system has the COP between 0.100 and 0.103 when the transportation distance of LN2 is shorter than 130 km. The JT refrigeration cycle
Acknowledgement
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF-2017R1A2B3003152) funded by the Ministry of Science, ICT & Future Planning (MSIP).
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