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Sub-Cooled Liquid Nitrogen Test System for Cooling HTS Synchronous Motor Anbin Chen, Fengyu Xu, Xiaokun Liu, Yubao He, Zonglin Wu, Yingshun Zhu, Zhengnan Han, and Liyi Li
Abstract—A 400 kW radial-axial flux type experimental HTS synchronous motor is designed. There are twelve armature coils using HTS wires in the motor. They are accommodated in the cooling vessels made of FRP material, and twelve cooling vessels are enclosed in the vacuum vessel. In order to cool the HTS coils of the motor, a sub-cooled liquid nitrogen cryogenic system is presented. The operation temperature is below 70 K. This system consists of vacuum system, liquid nitrogen dewar, data acquisition system and a cold box with liquid nitrogen pump, G-M cryorefrigerator, cryogenic valve and a heater inside. The heater can present the heat of the HTS coils. In this paper, the design and the composition of this test cooling system are presented. When liquid nitrogen is injected into the vessel and the cryorefrigerator is operating, the cool down curve of the test system is obtained. After adding heat to the system, the capacity curve of the system is presented in this paper. The results proved that this sub-cooled liquid nitrogen system can be used for cooling the actual armature windings in the HTS motor. Index Terms—Cryogenic, high temperature superconductivity, sub-cooled liquid nitrogen, superconducting motor.
I. INTRODUCTION
H
TS motor technology is now in a position to deliver highly compact, lightweight, high efficiency, inherently quiet propulsion motors to meet the needs of the customers. An axial and radial flux type HTS synchronous motor is designed and constructed at Harbin Institute of Technology. The designed rating capacity of the HTS experiment motor is 400 kW. The armature terminal voltage is 690 V and the armature rating current is 240 A. The rating speed is 250 rpm [1]–[3]. Liquid nitrogen cooling systems have been developed for HTS power devices over the past several years since liquid nitrogen is a cheap and excellent cooling medium and also a good electrical insulator. In recent research of high temperature superconducting electric power application, such as HTS power line, HTS motor and HTS power transformer, sub-cooled liquid nitrogen is considered as an important coolant for cooling the device. Compared to saturated liquid nitrogen, sub-cooled Manuscript received September 06, 2011; accepted October 30, 2011. Date of publication November 03, 2011; date of current version May 24, 2012. This work was supported in part by the P.R. China Department of The Ministry of Science and Technology under Grant 2008DFR70120. A. Chen, F. Xu, X. Liu, Y. He, Z. Wu, Z. Han, and L. Li are with the Institute of Cryogenics and Superconductive Technology, Harbin Institute of Technology, Harbin 150080, Heilongjiang, P. R. China (e-mail:
[email protected]). Y. Zhu is with the Institute of High Energy Physics, Beijing 100049, China (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2011.2174558
Fig. 1. Schematic for the sub-cooled liquid nitrogen cooling system.
liquid nitrogen could obtain higher efficiency and better electrical performances for HTS system [4]–[10]. In order to cool HTS synchronous motor, a sub-cooled liquid nitrogen cooling system which operating temperature is below 70 K is presented in this paper. A cryorefrigerator is used in the system to make the liquid nitrogen achieve subcooled state. The outlet lowest temperature of the subcooled system will be kept in the range of 66–69 K. Using this cooling system, we can provide sub-cooled liquid nitrogen for cooling the actual armature windings in the HTS motor. II. DESIGN OF THE CRYOGENIC SYSTEM A. Flow Chart of the System The sub-cooled liquid nitrogen cooling system is shown in Fig. 1. The system consists of two cryorefrigerators, heat exchanger in the sub-cooler, a cryogenic pump immersed in liquid nitrogen, a helium gas tank, a liquid nitrogen dewar, vacuum system and data acquisition system. The cryorefrigerators, heat exchanger, cryogenic pump and cryogenic valve are in the same cold box. In order to produce sub-cooled liquid nitrogen, GM cryorefrigerator is needed and the temperature of the heat exchanger connected to the cryorefrigerator is controlled to maintain the temperature a little above freezing temperature of nitrogen. Passing through the heat exchanger in the sub-cooler, sub-cooled liquid nitrogen is obtained. Then the sub-cooled liquid nitrogen is fed into the HTS coils cryostat to cool the coils and comes back to the cryogenic pump vessel. To maintain the pressure of the circulated sub-cooled liquid nitrogen, helium gas is fed into the cryogenic pump vessel. Liquid nitrogen pump is used for the circulation of the sub-cooled liquid nitrogen. Based on the head load of the HTS coils, the flow rate of the cryogenic pump can be adjusted.
1051-8223/$26.00 © 2011 IEEE
CHEN et al.: SUB-COOLED LIQUID NITROGEN TEST SYSTEM FOR COOLING HTS SYNCHRONOUS MOTOR
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Fig. 4. Photograph of the test cooling system. Fig. 2. Layout of the sub-cooled liquid nitrogen system.
Fig. 5. The equipments on the top flange of the cold box. Fig. 3. Schematic for the sub-cooled liquid nitrogen test cooling system.
Fig. 2 shows the layout of the system. B. Characters of the System There were some following characters in the sub-cooled liquid nitrogen cooling system, which could be obtained from [11]: • The heat loss of the HTS coils cryostat is 658.8 W by calculation. • The operating temperature of the liquid in the sub-cooler vessel is lower than 66 K. The operating pressure is kept at 0.02 MPa in the vessel. • The design operating flow rate of sub-cooled liquid nitrogen is 908 L/h. • The surface area of each coil heat exchanger in the subcooler is 0.85 . When the heat load is maximal, if the inlet temperature of sub-cooler is 69 K, the outlet temperature is 67 K. III. TEST AND ANALYSIS OF THE SYSTEM Because the HTS motor has not been installed, we cancel the transfer-line from the cooling system. In order to predict the performance of the cooling system, we add a heater in the cryogenic pump vessel instead of the heat load of the HTS coils. Sub-cooled liquid nitrogen from the sub-cooler is not used to cool the HTS coils through the transfer-line, but direct goes into the cryogenic pump vessel and be heated in the vessel. Then the higher temperature liquid nitrogen is feed into the sub-cooler by the cryogenic pump. The whole circulation of the Sub-cooled liquid nitrogen is finished in one cold box. Fig. 3 shows the flow chart for the test cooling system.
A. Composition of the Test Cooling System Fig. 4 shows the photograph of the test cooling system. There are cold box, dewar, helium tank, compressor and vacuum pump in the system. Fig. 5 shows the equipments on the top flange of the cold box. We only install one cryorefrigerator on top flange of the cold box in the test cooling system. Because two cryorefrigerators are the same, they are parallel connected to the cryogenic pipes. So, if we get the cooling capacity using one cryorefrigerator, we could deduce the cooling capacity using two cryorefrigerators. The temperature, the pressure and the flow rate of sub-cooled liquid nitrogen are gathered in real time. There are 17 platinum resistance temperature sensors in the system. The cold head temperature, inlet and outlet temperature of sub-cooler, the temperature of heater plate and the sub-cooled liquid nitrogen temperature in two containers are all presented. There are three pressure transducers in the system, two of them are used to measure the inlet and outlet pressure of the heat exchanger in the sub-cooler (a pressure head of the pump), the other is used to measure the pressure in the sub-cooler. There is one venture flow meter in the system for measuring the flow rate of sub-cooled liquid nitrogen. Detailed position of all these sensors is shown in Fig. 6. B. Power Determination of the Heater There is a heater in the cryogenic pump vessel instead of the heat load of the HTS coils. Before installed in the vessel, the power of the heater must be determined. The power of the heater is equal to the cooling capacity of the cryorefrigerator minus the heat load of the system. The heat load of the system comprises the heat loss generated by cryogenic valve, access ports, mechanical supports, radiation
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Fig. 8. The heater arranged in the cryogenic pump vessel.
Fig. 6. Interface of the data acquisition system.
TABLE I HEAT LOAD OF THE SYSTEM
Fig. 9. Cool-down curve of the liquid nitrogen in the sub-cooler. The pump work can be calculated by the following equation.
Fig. 10. Cool-down curve of the liquid nitrogen at inlet and outlet of the subcooler.
Fig. 7. Cooling capacity of AL600 cryorefrigerator (50 Hz).
and pump work. The heat load of the system is calculated and shown in Table I. The total heat load of the system is 53.9 W. According to Fig. 7, the cooling capacity of AL600 cryorefrigerator is 425 W at 60 K. So, the power of the heater should be higher than 369.5 W (when operating temperature more than 60 K). We use eight heaters in the vessel. Each heater is 50 W. Total amount of the heating power is 400 W. The power can be adjusted from 0 W to 400 W. Fig. 8 shows the arrangement of the heaters in the cryogenic pump vessel. They are well connected to an aluminum
plate. Four of them are on top of the plate and the others are under. C. Test Result of the Cooling System Fig. 9 shows the cool down curve of the liquid nitrogen in the sub-cooler, and Fig. 10 shows the cool down curve of the inlet and outlet temperature of the sub-cooler. From Figs. 9 and 10 we could see, it is almost 130 minutes to fill the sub-cooler vessel and pump vessel full with liquid nitrogen. The temperature of the sub-cooler is 77 K. Then turn on the cryorefrigerator and pump, the temperature and the pressure of the sub-cooler will gradually drop. It cost about 190 minutes reducing the outlet temperature of the sub-cooler to 65 K. Then turn on the heater.
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K. So, when the sub-cooled liquid nitrogen is below 70 K, this test system will provide maximum 375 W cooling capacity. If we add another cryorefrigerator in the cold box, then the system will provide maximum 750 W cooling capacity. In the future, the system will be used for cooling the HTS motor. The heat load of the HTS coils is 658.8 W and the heat load of the cold box is 47.6 W. So if the pump work in the system is lower than 43.6 W, this cooling capacity will meet the needs of cooling the actual armature windings in the HTS motor. REFERENCES
Fig. 11. The cooling capacity with respect to various temperatures.
When the outlet temperature of the sub-cooler is 66.9 K, the power of the heater is 347 W. The sub-cooled liquid nitrogen system reaches balance at this time. The pressure of the subcooler is 0.02 MPa. The inlet pressure of the heat exchanger in the sub-cooler is 0.15 MPa, and the outlet pressure is 0.1 MPa. The flow rate of the sub-cooled liquid nitrogen achieves 450 L/h according to the venture flow meter in the system. When the power of the heater is adjusted, the outlet temperature of the sub-cooler will be changed. Fig. 11 shows the cooling capacity with respect to various temperatures. Fig. 11 indicates when the outlet temperature of the sub-cooler changes from 65.4 K to 69.9 K, the cooling capacity of the system changes from 330 W to 375 W. IV. CONCLUSION A sub-cooled liquid nitrogen test system is set up for cooling HTS motor. A cryorefrigerator and a liquid nitrogen pump are used in this system to produce sub-cooled liquid nitrogen. Both of them are installed in the same test cold box. It takes about 3 hours to cool down the liquid nitrogen to sub-cooled state and the outlet temperature of the sub-cooler is 65 K. When the heating power is changed from 330 W to 375 W, the outlet temperature of the sub-cooler will be changed from 65.4 K to 69.9
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