Thermal Vacuum Cycling Test Facility for the Qualification of Near Sun Missions Solar Panels

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Volume 118 No. 16 2018, 1291-1308 ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version) url: http://www.ijpam.eu ijpam.

1 Volume 118 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu Thermal Vacuum Cycling Test Facility for the Qualification of Near Sun Missions Solar Panels S.Sathish Kumara 1, Ramachandrab 2, G.V. Hegdec 3, P.Govindan d 4, S.N.Prakashe 5 and Dr.N.K.Misraf ISRO Satellite Centre, Bangalore , India. sathishs@isac.gov.in January 2, 2018 Abstract A 0.7m dia. 0.9m length thermal vacuum cycling test facility is completely built in-house and mainly designed & realized for testing solar panel coupons of size 0.5m (W) x 0.5m (L) at the temperature extremes of -180 o C and +250 o C with vacuum level of 10-5 mbar to verify its space worthiness and functional performance for the near sun missions like Aditya & other interplanetary space missions, etc. This facility is built by augmenting the existing old vacuum chamber with suitable multiport chamber collar to interface & incorporate thermal system, vacuum pumping system & electrical instrumentations. This facility can also be used for testing of other small size satellite packages which requires these extremes of temperature simulation, unlike existing thermal vacuum test facility where high temperature can be achieved only upto +150 o C due to inherent design limitations. A cuboidal box shaped plate-coil type thermal shroud made out of SS AISI 304L grade material is placed horizontally inside the chamber. Vacuum system is designed to evacuate the chamber down to 10-5 mbar and is provided

2 with clean & contaminationfree multiple chains of primary and high vacuum pumping stations. Thermal system is designed to attain & maintain the extremes of temperatures with ramp rate of 3 o C/min and temperature uniformity of ±5 o C. Thermal system consists of thermal shroud, IR lamps, Liquid Nitrogen (LN2) circulation systems and hot Gaseous Nitrogen (GN2) circulation system. The inner and outer surface of the thermal shroud is painted with space grade silicon optical black paint for higher emissivity (eir= 0.91) and also, to withstand the extremely high temperatures upto +300 o C. PID based temperature controller is provided for accurate temperature control of test specimen with control accuracy of ±2 o C with Over Temperature Protection (OTP). Instrumentation and Control system consists of electrical control panel and instruments for remote operation, control, measuring and display of all the facility operational & health parameters. This system is provided with all safety features, visual indication for current status, and audible alarms for any fault indication. This paper provides insight into the design and development towards realisation of cost effective thermal vacuum cycling test facility. The facility performance tests are conducted; its results are found to be satisfactory, and successfully demonstrated the facility operation and realisation for the intended applications. Key Words : Vacuum; Thermal shroud; solar panels; vacuum chamber; sun mission. 1 Chamber physical description The facility consists of a horizontally oriented cylindrical vacuum chamber made of Stainless Steel (SS) of grade AISI 304L with 700 mm ID and 900mm straight length. The chamber has a clear internal working volume of 620 mm (W) x 650mm (L) x 120mm (H). The chamber is made of 3 parts. Viz. (1). Central cylindrical shell, (2). Front end torispherical hinged door, and (3). Fixed torispherical Rear end with vacuum chamber collar. The vacuum chamber collar is designed, fabricated and mated with the main chamber at the rear end of the chamber for interfacing vacuum system, thermal system and electrical instrumentations. The internal surface of the

3 chamber is electro-polished to the mirror finish to minimize outgassing and thermal radiation. The vacuum chamber is lined with shrouds through which nitrogen is circulated to attain temperature in the range of -196 o C to +275 o C. An optically tight shroud covering the test volume ensures the test article to reach the required temperatures by radiation exchange. Shroud is mounted on the rails provided in the chamber using suitable support structure. Shroud is properly insulated at the supporting points with thermal insulators to avoid conduction. Holding fixtures are provided inside the shroud for solar coupon mounting. The chamber front end with Shroud mounted inside is shown in the Fig 1. Fig 1: 0.7m TVAC facility 2 Design of vacuum chamber collar Fig 2(a): Vacuum chamber collar top view

Fig 2(b): Fabricated Vacuum chamber collar The multi-port vacuum chamber collar was designed in accordance to ASME Section VIII, Division 1[1] & ASME Section IX [2] and fabricated in-house.

4 Fig 2(b): Fabricated Vacuum chamber collar The multi-port vacuum chamber collar was designed in accordance to ASME Section VIII, Division 1[1] & ASME Section IX [2] and fabricated in-house. Various ports for user applications as well as for interfacing of chamber subsystems have been provided at appropriate positions. All ports are terminated with standard ISO- K/ISO-B/ISO-KF type vacuum flanges with Viton O-rings. All the sealing surfaces are properly finished to eliminate leak risk. All sealing surfaces, connections and fittings are constructed of stainless steel. The fabricated vacuum chamber collar is shown in Fig 2(a) & (b). 3 Vacuum system The vacuum pumping system comprises of one Rotary Vane Pump (RVP) of 40m 3 /h pumping speed to achieve the rough vacuum (10 2 mbar) from atmospheric pressure, one turbo molecular pump (TMP) of 685 l/s backed by rotary vane pump (RVP) of 24m 3 /h to achieve the high vacuum of 10 5 mbar with LN 2 traps and flexible bellows. Cryo pump of capacity 12,500l/s is also provided to have very high and effective pumping speed for water vapours as well as to get clean oil-free high Vacuum. Electro-pneumatically operated gate valves are used for pump/chamber isolation. Compact Full Range Gauge (CFRG) is used to monitor the vacuum level down to 10 6 torr. The schematic diagram of the complete vacuum system has been shown in the Fig 3&4. The chamber can be repressurised using GN 2 as well as clean & filtered air to reach atmospheric pressure from high vacuum. The

5 Fig 3: Vacuum and Thermal system schematic Fig 4: Vacuum pumps and isolation valves fixed at the rear end of 0.7m TVAC total repressurisation time can be varied from 5 to 15 minutes by a regulated venting system. The system guarantees that the GN 2 gets into the chamber at ambient temperature. 4 Thermal system Thermal system consists of thermal shroud, IR lamps, Liquid Nitrogen (LN 2 ) header, flow control valves, safety relief valves, heater chamber and thermally insulated pipe lines, is provided to achieve hot and cold temperatures. Liquid nitrogen is circulated inside the

6 shroud to achieve the cold temperature of -196 o C. A Liquid/Gaseous Nitrogen (LN2/GN2) supply along with IR lamps was used to obtain the high temperature upto +275 o C. Heater chamber of 2kW capacity is used to heat the gaseous nitrogen. Hot gaseous nitrogen is used to push the liquid nitrogen present inside the shroud during transition from cold to hot cycle. The shroud is designed in such a way that the coolant fluid remains within the shroud for the required amount of time with optimal cooling cycles. Thermal line schematic diagram for solar coupon Thermal vacuum cycling test facility is shown in the Fig Thermal shroud The shroud type chosen is plate and coil. For achieving better uniformity and thermal exchange, the shroud is designed so the maximum possible surface is in contact with the cooling media. The base sheet is nibbled SS of grade AISI 304L of thickness 0.6mm in which the SS 304L tubes of 19mm dia are placed to provide the pathway for liquid nitrogen to achieve the desired cold temperature with minimum pressure loss. The inlet and outlet of each side of shroud panels is brought to the same side of the shroud for convenience in interconnection of them. The contact between the SS304L sheet and SS tubes is designed to make intact to ensure the thermal contact even at the extremely low temperatures for better temperature exchange. The inner and outer surfaces of the shroud is painted with low degassing space grade silicon optical black paint (e = 0.91, =0.98) to withstand the high temperature Thermal shroud fabrication Tungsten Inert Gas (TIG) welding was used for shroud fabrication and chamber augmentation. Prior to welding, all the joints were degreased and cleaned properly. All the welds were performed from inside in order to give oxide free, smooth and regular surface as per standard vacuum welding practices. Surface finish of the order of 1.6 micron Ra value was achieved on entire surface of the components exposed to vacuum. Surface finish of all the O ring grooves is also maintained better than 1.6 micron Ra value with no scratches and dents. Fabricated thermal shroud is shown in the Fig 5(a)&(b)

Fig 5(a): Top thermal shroud panel Fig 5(b): In-house fabricated thermal shroud 4.1.

7 Fig 5(a): Top thermal shroud panel Fig 5(b): In-house fabricated thermal shroud Interconnection of LN2/GN2 inlets and outlets Separate Inlet and outlet header of DN25 mm SCH 40 is provided along with the shroud to distribute liquid nitrogen to all sides of the shroud panel. SS equal angle is used to bind all the shrouds and it is bolted to the edges of the shroud using M6 bolts at twelve locations for each shroud. Proper sliding fixture is provided at the centre of the cuboid box for the ease of insertion. The connections of the piping with thermal feedthroughs at the internal and external of the chamber are SS flexible bellows to obtain easier assembly & disassembly. All the interface joints are conflat flanges

8 with OFHC copper gaskets. The interconnection diagram for the shroud is shown in the Fig. 6. Fig 6: Thermal shroud interconnection schematic 4.2 Cooling of shroud To attain the cold temperature (-196 deg C), liquid nitrogen is circulated into the shroud through LN2 flash chamber and vented to the atmosphere. The LN 2 is drawn from LN2 storage dewar at a pressure of 1.5bar (g). The flash chamber and all the transfer pipe lines are stainless steel insulated with Poly Urethane Foam (PUF) to minimise evaporation loss and two-phase flow. 4.3 Heating of shroud To achieve the hot cycle, a combination of IR lamps and hot gaseous nitrogen is used. During the transition from cold cycle to hot cycle, hot GN 2 is used to push the LN 2 present inside the shroud. IR

1 IR heaters power and location details 8 Nos of IR lamps (240V, 1000W) and 2Nos of IR lamps (120V, 500 W) with parabolic reflectors are

9 lamps are suitably positioned on all sides of the shroud to have good uniformity and full coverage. Fig 7(a): IR cage with IR lamps and reflectors Fig 7(b): IR cage mounted on top of the shroud IR heaters power and location details 8 Nos of IR lamps (240V, 1000W) and 2Nos of IR lamps (120V, 500 W) with parabolic reflectors are provided for all the shroud panels to achieve the desired hot temperature. The IR lamps are distributed in such a way (as shown in Fig7 (a) & (b)) so as to have full coverage and better uniformity on the shroud panels. The inner

10 and outer surfaces of the shroud are painted with low out gassing space grade silicone optical black paint (Material designation: MLS- 85SB of M/s AZ Technology, USA) for better functional properties and also to withstand the high temperature of +300 o C. The whole unit is covered with reflective aluminium sheet to minimize the radiation loss. The heater location is as shown in the Fig 8. Heater power requirement for heating the each side of shroud panel has been worked out using standard heat transfer equations. The total time required to attain the temperature of +275 o C from cold temperature of -196 o C during transition is 60 minutes which is taken for estimating the heater power. Safety margin of 10% is considered to compensate for the additional elements of the shroud and radiation loss. For uniform coverage of the shroud, 10 IR lamps are provided with total power of 9kW. Fig 8: IR lamps & Thermocouple location schematic

11 5 MSLD Testing After assembly, the entire shroud assembly, all the joints (welded as well as demountable) and thermal feedthroughs were leak tested with Helium Mass Spectrometer Leak Detector (He-MSLD) to confirm the leak tightness of 1x 10 9 std.cc/sec. The leak rate of individual shroud panels is presented in the below Table 1. Table 1: Leak Rate Values of Individual Shroud Panel SHROUD DESCRIPTION BACK GROUND (mbar l/s) ACTUAL LEAK RATE (mbar l/s) Top panel 3x10 9 3x10 9 Bottom panel 3x10 9 3x10 9 Right side panel 3x10 9 3x10 9 Left side panel 3x10 9 3x10 9 Rear side panel 3x10 9 3x Instrumentation and Control System Instrumentation and control system consists of Yokogawa make (Model No: CX2620) data acquisition and control system (DACS), PID based temperature controller, T type temperature sensors, RTD (Pt-100) temperature sensors, vacuum gauges and Over Temperature Protection (OTP). DACS has 20 measurement channels and 6 PID control loops. The entire facility can be operated from a centrally located control console. The IR lamps are controlled through 5 PID control loops. The control system has provision of setting all PID parameters either in manual mode or Auto mode. Various displays like ON-OFF status, % of output and process values are indicated on the screen. It can precisely control the set point temperature with minimal deviation. The thermocouple sensors are fixed on internal surface of the shroud panel and exactly in line with the IR heater locations to provide accurate feedback to the system for closed loop control

12 6.1 System description All 8 Nos of IR lamps are operated at 230V AC power and are switched through SSRs (solid state relays) which are placed in separate electrical console. The cryogenic valves are controlled by set of RTD temperature sensors (Pt-100) located at inlets of the thermal line to the shroud. The LN2 flow is controlled through set point temperature by taking feedback from the inlet to the thermal shroud. Temperature controller uses ON/OFF control mode to control the solenoid valves. The schematic of enclosed looping heating circuit is shown in fig. the thermocouple location, IR heater, SSR and power lines is shown in the Fig 9. Fig 9: Control configuration diagram to control IR Heaters with Yokogawa CX Temperature control and measurement For normal operation, Eurotherm PID temperature controller is used to control and to maintain a preset temperature in the chamber. Once the temperature at the control point reaches the set point temperature, the temperature controller will cycle heaters and LN2 valves to keep the temperature steady during hot and cold cycles

13 Totally, 20 Nos of T type thermocouple sensors (shielded type with fibre glass insulation to withstand extremely high temperature of +400 o C) are used for temperature control and measurement. Out of these, 5 Nos of thermocouple are used for controlling the IR lamps, which are located very near to the IR heaters and 6 Nos of thermocouple are used to monitor the shroud temperature. Remaining 8 Nos used to measure the temperature of the specimen. All the thermocouples are fixed on to the surface of the shroud panel using high vacuum compatible low temperature adhesive STYCAST 2850 FT with 24LV catalyst of M/s. Emerson & Cuming, Belgium. All these sensors are connected and configured to CX 2620 Yokogawa controller Temperature fail safe A separate OTP (Over Temperature Protection) controller is provided to prevent over shooting of temperature by cutting off power to the IR lamp heaters. A safety interlock. 6.3 Vacuum measurement Pirani gauges are used to measure the chamber rough vacuum as well as to measure all fore-line vacuum levels of RVP pumps, TMP pump and Cryo backing pump Vacuum fail safe In case, if chamber vacuum is below 5.0x10 4 mbar before starting the test, thermal cycling cannot be started. During the course of test, if the vacuum level falls below 5.0x10 4 mbar, automatically thermal cycling will be aborted by putting off all the IR heaters. Heaters will come online again when ever chamber vacuum is regained to 5.0x10 4 mbar. 6.4 Electrical equipment and safety Power cabling, power switches which required for operation of all electrical equipments, IR lamps, & electro-pneumatic valves is centralised in a power console located in proximity to the chamber. The shielded electrical cables are guided in such a way that it doesnt

14 interfere with maintenance works. Every input and output cable is properly identified & numbered. Proper care is taken to ensure all the electrical equipment of the thermal vacuum cycling chamber does not affect from electromagnetic point of view to its own functioning and to the control and data acquisition system. The system has necessary safety interlocks for equipment & human safety for failure-free operations during the test. 7 Vacuum Pump down tests and Baking Subsequent to MSLD testing, chamber was subjected to ultimate vacuum testing. Initially, the high vacuum achieved was 4.2 x 10 5 mbar after two and half hours of pumping. Subsequently, the shroud was subjected to baking at 80C for 4 hours. After baking, the ultimate vacuum of 1x 10 5 mbar is achieved. The pump down curve for chamber prior to shroud baking and after baking is shown in Fig 10. Fig 10: Pump down curve before and after baking

15 8 Performance trail tests A performance trial test was conducted on dummy solar panel coupon to verify the performance of the facility to bake/thermal cycle the solar panel coupons of near sun missions. The chamber vacuum, performance of shroud and temperature uniformity & control accuracy achieved are satisfactory as per the design requirements during both hot and cold cycles. Time required for completing one cycle with temperature extremes of -180 o C and +250 o C on test article was approximately 6 hrs including dwell time of 15 minutes. The test data and plots are presented below in Table 2. Table 2: Test article temperature values during Stabilisation Fig 11: Shroud Temperature

16 Fig 12: Specimen (Solar panel Coupon) Temperature 9 Conclusion A thermal vacuum chamber facility has been designed, fabricated, tested, and commissioned to qualify the solar panel coupons for the temperature extremes of -180 o C and +250 o C under simulated vacuum of 1x10 5 mbar for near sun missions. Performance of this facility has been successfully demonstrated by conducting three Thermal cycles (Hot & Cold) between extreme temperature limits under high vacuum of 1 x 10 5 mbar. Hence this test facility can be used for the qualification of near sun missions solar panel coupons. 10 Acknowledgement The authors wish to acknowledge contributions from all supporting staff of ETF division for their invaluable support at every stage of system development. It would not have been possible to successfully complete this project without the support and encouragement of Dr. M. Annadurai, Director, ISRO Satellite Centre (ISAC), Bangalore

17 References [1] ASME Boiler and pressure vessels code, ASME SEC-VIII DIV- 1 [2] ASME Sec-IX, Welding procedures/qualifications

18 1308

Specification Sheet. Equipment Model: BVAC-1200 Vacuum Furnace. Type. Chamber. Maximum Temperature 1250 C. Hot Zone

Specification Sheet. Equipment Model: BVAC-1200 Vacuum Furnace. Type. Chamber. Maximum Temperature 1250 C. Hot Zone Specification Sheet Equipment Model: BVAC-1200 Vacuum Furnace Type Vertical / Top Loading, High Vacuum Chamber Electropolished Stainless Steel, Coldwall Maximum Temperature 1250 C. Hot Zone 12.5 (320 mm)