Design of high-efficiency Joule-Thomson cycles for high-temperature superconductor power cable cooling

Highlights

• Joule-Thomson cycles for high-temperature superconductor power cable are proposed.

• The highest COP of 0.115 is obtained in the JT cycle with a vacuum pump at 78 K.

• An integrated HTS cooling cycle is designed by unifying the working fluid.

• Cycle efficiency of each JT cycle is examined by exergy destruction analysis.

Abstract

Liquid nitrogen (LN₂) is commonly used as the coolant of a high temperature superconductor (HTS) power cable. The LN₂ is continuously cooled by a subcooler to maintain an appropriate operating temperature of the cable. This paper proposes two Joule-Thomson (JT) refrigeration cycles for subcooling the LN₂ coolant by using nitrogen itself as the working fluid. Additionally, an innovative HTS cooling cycle, of which the cable coolant and the refrigerant are unified and supplied from the same source, is suggested and analyzed in detail. Among these cycles, the highest COP is obtained in the JT cycle with a vacuum pump (Cycle A) which is 0.115 at 78 K, and the Carnot efficiency is 32.8%. The integrated HTS cooling cycle (Cycle C) can reach the maximum COP of 0.087, and the Carnot efficiency of 24.8%. Although Cycle C has a relatively low cycle efficiency when compared to that of the separated refrigeration cycle, it can be a good alternative in engineering applications, because the assembled hardware has few machinery components in a more compact configuration than the other cycles.

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