Yong-Ju Hong

A study on the linear compressor characteristics of the Stirling cryocooler

Abstract The purpose of this study is to analyze the characteristics of the linear compressor for Stirling cryocooler. To ensure high performance and long life of the free piston, free displacer Stirling cryocooler, the operating parameters of the linear compressor should be optimized. The experimental results show the operating frequency of the linear compressor has significant effect to the input power characteristics, and the compressor with higher charging pressure of working fluid has high pressure ratio. For the high performance of the linear compressor, compressor has uniform magnetic force with different stroke, and the optimal operating frequency with respect to the charging pressure.

An Experimental Study of the Conduction Cooling System for the 600 kJ HTS SMES

The characteristic of the superconducting magnetic energy storage (SMES) system is more fast response, long life time, more economical, and environment-friendly than other uninterruptible power supply (UPS) using the battery. So, the SMES system can be used to develop methods for improving power quality where a short interruption of power could lead to a long and costly shutdown. Recently, cryogen free SMES is developed briskly based on BSCCO wire. We fabricated and tested the conduction cooling system for the 600 kJ class HTS SMES. The experiment was accomplished for simulation coils. The simulation coils were made of aluminum, and have inner and outer coil diameters of 460 mm and 728 mm and coil height of 237 mm. The mass of coil is 171.6 kg, it is equivalent to thermal mass of 600 kJ HTS SMES coil. The coil is cooled with two GM coolers through the copper conduction bar. In this paper, we report the test results of cool-down and heat loads characteristics of the simulation coils.

The effect of operating parameters in the Stirling cryocooler

This paper presents design, manufacturing process and test results in the optimum operating condition for the free piston free displacer (FPFD) type Stirling cryocooler designed and manufactured by Korea Institute of Machinery and Materials. FPFD Stirling cryocooler is currently under development for cooling infrared detector. The compressor in the Stirling cryocooler uses opposed linear motors to drive opposed pistons. The performance of FPFD Stirling cryocooler is evaluated as a function of charging pressure and operating frequency. In general, as the charging pressure is increased, optimum-operating frequency of the compressor is increased but resonant frequency of the displacer is almost constant. The prototype has achieved no load temperature of 49 K and cooling power of 0.5 W at 72 K.

Performance tests of high temperature superconducting power cable cooling system

A high temperature superconducting power cable requires forced flow cooling. Liquid nitrogen is circulated by a pump and cooled down by a subcooling system. Typical operating temperature range is expected to be between 65 K and 80 K. The circulating liquid nitrogen is subcooled by liquid nitrogen boiling on the shell side of a subcooler heat exchanger, and then it cools the HTS cable. The HTS power cable needs sufficient cooling to overcome its low temperature heat loading. This loading typically comes in two forms. One is heat leak from the surroundings and the other is internal heat generation, i.e., AC loss. This paper describes performance tests of the cooling system. Heat exchanging performance of the subcooler, pressure drop between supply and return lines, and heat transfer coefficient of a former are examined for the performance test.

Study on Thermal Response to Instantaneous Heat Generation in LN2 Chamber for HTS-FCL

This paper describes the thermal response of a high-temperature superconducting (HTS) wire model to instantaneous heat generation in a pressurized liquid nitrogen chamber for an HTS fault current limiter. A dc impulse power input to a stainless steel strip is adopted to simulate the quench state of the HTS wire. The test sample is submerged in the liquid nitrogen, which maintains a 77 K temperature with operating pressures of 101, 250, 400, and 600 kPa. Three different levels of dc current are supplied to the test sample during 50 ms for each operating condition. The boiling phenomena are captured with a high-speed camera and the surface temperature of the sample strip is measured to investigate the recovery process. From the captured video, the suppression of bubble generation is clearly observed as increasing operating pressure, especially for the lower heat flux condition. From the measurement of temperature, temperature rise of sample strip during heat generation decreases with increasing operating pressure except for the higher heat flux. For the recovery process, increasing operating pressure delays the recovery of the sample strip, but recovery time is within a few seconds for all cases.

Cool-Down and Thermal Performance Test for Toroidal Configuration HTS SMES

The high-temperature superconducting (HTS) magnetic energy storage system working temperature is around 20 ~ 30 K because the HTS coils must be cooled below the critical temperature. In this paper, we have designed a system with an operating temperature of 15 K or below for the purpose of full utilization of the HTS wire material, minimization of the hardware dimension, and stable operation. For the cooling, four of 2-stage Gifford-McMahon coolers were used. The cooling system has a short and uniform thermal passage from the cryocooler head to each HTS coil for an effective cooling of HTS coils. The cold head first stage cools the thermal shield and current lead, and the second stage cools the HTS coils. Our full system has a total of 28 double pancake coils forming a toroidal configuration. For this study, we assembled a test set consisting of six dummy double pancake coils for cooling and load tests without charging current. This paper describes the cool-down and the thermal characteristics of the conduction cooling system. The modified cooling method is optimized for cool-down time reduction, and thermal tests were performed with heat loads up to 300 W.

Experimental Study on Vaporization of Subcooled Liquid Nitrogen by Instantaneous Heat Generation in LN2 Chamber for HTS-FCL

It is one of important design issues in the high-temperature superconducting fault current limiter (HTS-FCL) to estimate how much liquid nitrogen vaporizes during its quench process. This paper describes the small scale experiment about the vaporization of subcooled liquid nitrogen by instantaneous heat generation corresponding to the quench in HTS-FCL. In the experiment, the heat is given liquid nitrogen by Joule heating of the stainless steel strip for the short time of about 100 ms. The time variations of temperature and pressure in the liquid nitrogen test chamber are measured for different subcooling conditions of liquid nitrogen and various heat flux conditions. In addition, the amount of vaporization of liquid nitrogen is calculated from experimental results according to the subcooling of liquid nitrogen. The experimental results show that the vaporization of liquid nitrogen strongly depends on heat flux and subcooling conditions. The amount of vaporization linearly decreases with the increase in the subcooling of liquid nitrogen. The calculation results about the amount of vaporization suggest that the subcooling of more than 20 K is necessary to suppress bubble generation for the heat flux condition of 46 W/cm2 or more.

The Radiation Heat Treansfer Analysis of the Cryochamber

The cooling load, which should be removed by the cooling system to maintain the nominal operating temperature of a cryochamber, depends on the thermal insulation efficiency of the cryochamber. Therefore, to decrease and minimize the heat loss of the cryochamber has become a big problem in the design stage, in considering the very low thermal efficiency and small cooling capacity of the cryocooler. Radiation and conduction heat transfer occur simultaneously in a cryochamber. The radiation heat transfer from the hot surface to the cold surface depends on the emissivity, temperature, area and radiation exchange factor.In the present work, the energy equation, which includes the rarefied gas conduction, radiation between the boundary surfaces and solid conduction, is solved to evaluate the steady cooling loads at the cold finger of a small cryocooler. The surface to surface radiation model is used for the calculation of the radiation exchange factor. This paper presents the effects of the emissivity of the ma...

Design and test of the Stirling-type pulse tube cryocooler

Stirling type pulse tube cryocoolers are very attractive for cooling of diverse application because it has it has several inherent advantages such as no moving part in the cold end, low manufacturing cost and long operation life. To develop the Stirling-type pulse tube cryocooler, we need to design a linear compressor to drive the pulse tube cryocooler. A moving magnet type linear motor of dual piston configuration is designed and fabricated, and this compressor could be operated with the electric power of 100 W and the frequency up to 60 Hz. A single stage coaxial type pulse tube cold finger aiming at over 1.5 W at 80K is built and tested with the linear compressor. Experimental investigations have been conducted to evaluate their performance characteristics with respect to several parameters such as the phase shifter, the charging pressure and the operating frequency of the linear compressor.

A Study on Generating efficiency of the Double Acting Stirling Engine/Generator

>> This paper describes generating efficiency characteristics of the double acting Stirling engine/generator for domestic small-scale CHP (Combined Heat and Power) system. In small distributed generation applications, Stirling engine has competition from fuel cell, microturbine and etc. In order to be economical in the applications, a long life with minimum maintenance is generally required. Free piston Stirling engine (FPSE) has no crank and rotating parts to generate lateral forces and require lubrication. Double acting Stirling engine/generator has one displacer and two power piston which are supported by flexure springs. Two power pistons oscillate with symmetric displacement and are connected with moving magnet type linear generators for power generation from PV work. In experiments, 1 kW class double acting free piston Stirling engine/generator is fabricated and tested. Heat is supplied to hot end of engine by the combustion of natural gas and converted to electric power by linear generators which are assembled with power pistons. The electric parameters such as voltage, current and phase are measured with for variable flow rate of fuel gas. Especially, generating efficiency of FPSE is measured with three different measurement methods. Generating efficiency of the double acting Stirling engine/alternator is about 24%.