Junseok Ko

Advanced design and experiment of a small-sized flywheel energy storage system using a high-temperature superconductor bearing

A small-sized flywheel energy storage system has been developed using a high-temperature superconductor bearing. In our previous paper, a small-sized flywheel was fabricated and successfully rotated at 38 000 rpm under a vacuum condition. However, a large drag torque was present because of the non-axisymmetric magnetic flux of the motor/bearing magnet and the eddy current loss in the planar stator, resulting in a short spin-down time of 20–30 s and a coefficient of friction of 0.15. This paper presents the design, fabrication and electromagnetic analysis of the flywheel in order to reduce the large drag torque. The advanced flywheel designed for solving the non-axisymmetric magnetic flux problem comprises eight motor magnets and a single bearing magnet, a magnetic screening disc, and an aluminium disc which is 50 mm in diameter and 5 mm in thickness. The eddy current loss in the planar stator is minimized by reducing the contact area between the planar stator and the motor magnets. The maximum rotational speed increases 1.3 times to 51 000 rpm and the spin-down time increases 600 times to 3 h 20 min. On the basis of these results, the coefficient of friction decreases 100 times to 0.001–0.002.

An Effect of HTS Wire Configuration on Quench Recovery Time in a Resistive SFCL

We experimentally investigated the correlation between the high-temperature superconducting (HTS) wires configuration in an HTS element and recovery time after quench in a resistive superconducting fault current limiter. The variables of the configuration are horizontal and vertical gap distances between HTS tapes in an element. Eight samples were made with different gap distances and tested. A SUS-stabilized YBCO tape with 4.4 mm width had been used in the experiment. It was cooled by LN2 in a cryostat under the pressure of 1 bar, saturated state. In the short-circuit test, the temperature of the wires surface was measured. Recovery time of the HTS sample increased with decreasing horizontal and vertical gap distance due to stagnation of bubbles. When the gap distance was larger than a size of a bubble, the effect of gap distance was ignorable. Considering a volume and recovery time, the sample that has narrower gap distance was favorable.

Double-Evaporator Thermosiphon for Cooling 100 kWh Class Superconductor Flywheel Energy Storage System Bearings

This paper presents an idea for a thermosiphon that uniquely implements two integrated evaporators to cool two HTS (High Temperature Superconductor) bulk sets in different locations, simultaneously. A so-called double-evaporator thermosiphon was designed, fabricated and tested using nitrogen as the working fluid under sub-atmospheric pressure conditions. The operating target temperature was approximately 65 K. To confirm the feasibility of the double-evaporator thermosiphon, experiments during the cool down process and steady state operation were extensively conducted on the double-evaporator thermosiphon (Ltot = 1075 mm, do = 160 mm). The double-evaporator thermosiphon worked successfully at steady state operation. The results showed that it had a maximum total temperature difference between the condenser and the evaporator of 1.3 K and a temperature difference between the two evaporators of 0.6 K at a heat flow of 87 W. This thermosiphon was designed for actual application to a 100 kWh SFES (Superconducting Flywheel Energy Storage) system. The potential impact of superior heat transfer characteristics of the double-evaporator thermosiphon is discussed in the paper.

Micro generator using flywheel energy storage system with high-temperature superconductor bearing

This paper presents the design and fabrication of the micro generator using flywheel energy storage system with high-temperature superconductor bearing. The micro generator is characterized by the three-phase axial flux permanent magnets. The planar stator is fabricated with 5-lines, 100 mum-width, and 50 mum-height for low eddy current loss and high current density. The flywheel rotates up to 51,000 rpm (3.4 [kHz] times 60 [sec]/4 [pairs of poles]) and stores the rotational kinetic energy of 337 J. The flywheel can rotate freely during 3 hr 20 min without input voltage and the coefficient of friction is calculated as 0.001~0.002. At maximum speed, the no-load three-phase electric output of the micro generator is 802 mVrms.

Experiment and analysis for a small-sized flywheel energy storage system with a high-temperature superconductor bearing

This paper presents a small-sized flywheel energy storage system that uses a high-temperature superconductor (HTS) bearing characterized by a non-contacting bearing with no active control. The small-sized flywheel is made up several magnets for a motor/generator as well as an HTS bearing, and they are fitted into a 34 mm diameter, 3 mm thick aluminium disc. For simplicity and miniaturization of the whole system, the small-sized flywheel takes torque directly from a planar stator, which consists of an axial flux-type brushless DC motor/generator. The small-sized flywheel successfully rotated up to 38 000 rpm in a vacuum while levitated above the stator with a gap of about 1 mm. However, there are some eddy current losses in the stator and non-axisymmetry in the magnetic field causing large drag torque. In order to solve these problems, an improved magnet array in the flywheel, including magnetic screening, is proposed and 3D electromagnetic simulations have been conducted.

An integrated micro HTS system for energy storage and attitude control for three-axis stabilized nanosatellites

This paper presents the development of an integrated micro high-temperature superconductor system for energy storage and attitude control of three-axis stabilized nano satellites. The micro HTS system consists of a flywheel/rotor, motor/generator, motor electronics, and a cooling system. The flywheel/rotor has been fabricated by using sintered NdFeB and the stator for motor/generator has been fabricated by micro fabrication technology. An alternative stator has been fabricated by cutting a 50 micron-thick copper film for comparison. A servo amplifier to drive the DC brushless motor of the integrated HTS system has been developed and successfully tested. A cooling system has been developed to test the system. It has been observed that the micro fabricated stator can make the flywheel rotate at a constant velocity of up to 12,000 rpm whereas the other stator generates a constant velocity of up to 2,500 rpm. Experiments show that the micro fabricated stator can significantly reduce the motor/generator losses.

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.

Investigation on dynamic behavior of linear compressor in stirling-type pulse tube refrigerator

This paper describes the experimental study of the dynamic behavior of a linear compressor in a Stirling-type pulse tube refrigerator (PTR). The dynamic behavior of the piston is closely coupled with the hydraulic force of gas and, therefore, directly influenced by the specific load condition of the pulse tube refrigerator. In the experiment, the frequency response of the pressure at each component, the cooling performance and the piston displacement are measured while an alternate current with fixed magnitude is supplied to the linear motor. The linear compressor in this study was originally designed for a Stirling cryocooler and its maximum input power is approximately 200 W. The pulse tube refrigerator is configured as an in-line type and an inertance tube is incorporated as the phase control device in the pulse tube refrigerator. The pressure difference between both ends of the piston imposes additional stiffness and the PV power in the compression space can be considered a damping effect in the vibra...

Micro flywheel energy storage system with axial flux machine

This paper presents the design and fabrication of the micro generator using flywheel energy storage system with high-temperature superconductor bearing. The micro generator is characterized by the three-phase axial flux permanent magnets. The axial flux permanent magnet machine has compact construction and high power density. The large diameter rotor with its high moment of inertia is proper as a flywheel. The planar stator is fabricated with 5-lines, 100 mum-width, and 50 mum-height for low eddy current loss and high current density. The flywheel rotates up to 51,000 rpm (3.4 [kHz]times60 [sec] / 4 [pairs of poles]) and stores the rotational kinetic energy of 337 J. The flywheel can rotate freely during 3 hr 20 min without input voltage and the coefficient of friction is calculated as 0.001-0.002. At maximum speed, the no-load three-phase electric output of the micro generator is 802 mVrms.

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.