Thermodynamic design of 10 kW Brayton cryocooler for HTS cable

Abstract

Thermodynamic design of Brayton cryocooler is presented as part of an ongoing governmental project in Korea, aiming at 1 km HTS power cable in the transmission grid. The refrigeration requirement is 10 kW for continuously sub-cooling liquid nitrogen from 72 K to 65 K. An ideal Brayton cycle for this application is first investigated to examine the fundamental features. Then a practical cycle for a Brayton cryocooler is designed, taking into account the performance of compressor, expander, and heat exchangers. Commercial software (Aspen HYSYS) is used for simulating the refrigeration cycle with real fluid properties of refrigerant. Helium is selected as a refrigerant, as it is superior to neon in thermodynamic efficiency. The operating pressure and flow rate of refrigerant are decided with a constraint to avoid the freezing of liquid nitrogen.

Dynamic simulation of 10 kW Brayton cryocooler for HTS cable

“…The pressure ratio is assumed to be constant at 2.5 over the entire operation [1]. The exit temperature of the after-cooler (T 2 ) is more or less dependent on outdoor conditions, such as temperature and humidity of ambient air.…”

“…The efforts are directed towards a reliable long-term operation of the cryogenic refrigeration system. Figure 1 is the simplified flow diagram of a 10 kW Brayton cryocooler [1] for sub-cooling liquid nitrogen (from 78 K to 67 K) in an HTS power cable system. The cycles of refrigerant (helium: He) and coolant (liquid nitrogen: LN) are also shown on the temperature-entropy diagram and phase diagram, respectively.…”

“…For the purpose of closed-cycle refrigeration, a Brayton cryocooler with capacity of 10 kW at liquid-nitrogen temperatures is under development as well. Thermodynamic design of the Brayton-cycle refrigeration was completed [1], and the key components such as heat exchangers [2], cryogenic turbo-expander, and compressors were also selected with a basis of steady-state operation. As a next step, this work intends to investigate the dynamic behavior of a Brayton cryocooler with commercial software (Aspen HYSYS), taking into account the dimensions and specifications of the components employed for prototype construction.…”

“…The turbo-expander (Exp) produces the coldest helium and plays a leading role in the transient behavior. Helium was selected as the refrigerant, because it is superior to neon in thermodynamic efficiency [1]. Liquid nitrogen is circulated by a cryogenic pump for delivering the thermal load in the HTS cable to the cryocooler.…”

Plate-fin Heat-exchangers for a 10 kW Brayton Cryocooler and a 1 km HTS Cable

“…A 10 kW Brayton cryocooler is under development in Korea as partial effort of the governmental project for hightemperature superconductor (HTS) power cable [1] [2]. The goal of this project is to install and demonstrate the HTS cable and its cryogenic cooling system over 1 km length of transmission line (at 154 kV) in Jeju Island by 2016.…”