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Over a decade, there have been several efforts to develop a few kW levels of Brayton coolers at liquid-nitrogen temperature for various applications. In 2007, Hirai et al. [1] developed a prototype of Brayton cooler (2 kW at 70 K) with a small turbo-expander for HTS power machines. They used neon gas as refrigerant because of their preference in using small turbo-expanders. Lately, Yoshida et al. [2] presented a new design for 2.5 kW at 65 K with improved thermodynamic performance. Earlier than these works, Saji et al. [3] designed an oil-free turbo-type of refrigerator with helium-neon mixture. It was reported that a compact refrigeration system could be designed with 6 kW at 65 K for long 500 MVA cables. In 2008, Breedlove et al. [4] developed a turbo-Brayton cryocooler (1 kW at 95 K) for onboard air-separation in large aircraft. For the ongoing HTS power cable project in Long Island Power Authority (LIPA) grid, Air Liquide designed a Brayton cryocooler with helium as refrigerant for 12 kW at 65 K [5], and they lately started to design a modular refrigeration system based on Brayton cycle for 22 kW at 72 K [6]. For the HTS cable system produced by the Tokyo Electric Power Co. [7], several units of Stirling cryocoolers with 0.8 kW at 67 K were employed for short-length tests. The KEPCO (Korea Electric Power Corporation) HTS cable systems [8] have been successfully operated with decompression-pump cooling units. Recently in Korea, a pilot plant to liquefy bio-methane was designed and constructed based on a reverse-Brayton cycle [9,10] under the New and Renewable Energy Program. The pilot system in the Metropolitan Seoul Landfill Site succeeded in producing 99% pure liquid methane at a rate of 4,000 liters per day. With this experience and for the upcoming need in long-length HTS cable system in Korea, this thermodynamic study presents the design data for 10 kW Brayton cryocooler. The design includes the selection of refrigerant and the determination of operating conditions, aiming at high efficiency and reliability. COOLING REQUIREMENT The objective of refrigeration is to continuously sub-cool a liquid-nitrogen (LN) flow from 72 K to 65 K at a rate of 10 kW. The mass flow rate of liquid nitrogen is therefore calculated as m LN = QLN =0.710kg/s (1) where CLN is the specific heat of liquid nitrogen, and the subscripts i and e denote the inlet (72 K) and exit (65 K) of nitrogen heat exchanger, respectively, as schematically shown in FIGURE 1. From the combined energy and entropy balance, there exists an absolute minimum work for this refrigeration, which is the difference of flow availability (exergy) of liquid nitrogen between the inlet and exit [11]. ⎡ Te⎤ W m i n = m L N C L N ⎢ ( T e − T i ) − T H l n T ⎥ = 3 3 . 8 k W ( 2 ) ⎣i⎦ where TH is the ambient temperature (300 K in this study), at which heat is rejected by the cryocooler. The figure of merit (FOM) is a dimensionless thermodynamic parameter to evaluate the performance of refrigeration, defined as the ratio of minimum work to actual work. CLN (Ti −Te) FOM = Wmin = Wmin (3) W Wcomp −Wexp 1665PDF Image | Thermodynamic design of 10 kW Brayton cryocooler for HTS cable
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