MINIATURE ENGINEERING SYSTEMS GROUP ( mmae.ucf/~kmkv/mini )

1. MINIATURE ENGINEERING SYSTEMS GROUP(http://www.mmae.ucf.edu/~kmkv/mini) Reverse Turbo Brayton Cycle CryoCooler Development for Liquid Hydrogen Systems

2. OBJECTIVEANDRELEVANCE • All of the previous attempts of flight cryocoolers have cooling capacities less than 2 W at liquid hydrogen temperature. There are commercially available cryocoolers that have higher cooling powers but their weight restricts their possible usage for in-space applications. • The objective of this project is to design and build a reliable, efficient, compact and light-weight reverse turbo Brayton cycle cryocooler,which is capable of removing 20-30 watts of heat at liquid hydrogen temperatureand thus significantly contribute to NASA efforts on densification and ZBO storage of cryogenic propellants for missions to Mars.

3. CURRENT APPROACH Thermodynamic Schematic - showing the two working cycle steps Mechanical Schematic - current focus is on the development of lower step of the thermodynamic cycle (highlighted in yellow)

4. COMMENT FROM MARCH 04 NASA PANEL • Too many tasks with insufficient resources to meet them ! • RESPONSE • The project tasks have been narrowed to the following significant goals – • Compressor development, • Motor development, and • Integration of Compressor and Motor. • The integrated compressor/motor is key to RTBC, and is useful for many other NASA and non-NASA applications. • The development of foil bearings and heat recuperator have been de-scoped from this project. These areas are being targeted through other funding agencies/projects.

5. DESIGN AND TEST HELIUM COMPRESSOR WITH SIMILARITY PRINCIPLE Compressor similarity function: Performance Variables Similarity Principle: When we scale up/down an existing compressor or change its rotating speed or inlet conditions, the performance variables of the compressor remain the same if we keep the similarity variables unchanged. Equivalent air test using similarity principle Single-stage compact helium compressor Open-loop air test

6. SINGLE-STAGE COMPACT CENTRIFUGAL HELIUM COMPRESSOR Impeller/diffuser assembly

7. COMPARISON OF THE COMPRESSOR PERFORMANCE – WITH OD AND ND OLD DIFFUSER NEW DIFFUSER

8. PMSM DESIGN AND FABRICATION • Criteria for selection of materials - high speed application and efficient cryogenic temperature operation. • Design of motor structure and optimization of dimensions were done to minimize losses. • Both dynamic and static analyses of mechanical stresses including rotordynamics were done. • Thermal analysis including thermal stress due to temperature gradients/transients and shaft expansion/contraction was performed. Stator with winding Hollow shaft and permanent magnet Shaft and casing

9. OPEN-LOOP CONTROLLER • Using low impedance MOSFET and high drive current chip to increase efficiency. • Optimizing v/f drive scheme. • Adjusting modulation index and switching frequency for best performance. Control block diagram Schematic diagram

10. PMSM TESTING • Free spin/no-load test. Testing - Animation • The data for speed above 130,000 rpm are the estimated results. • The projected efficiency at 200,000 rpm with 2000 W output is around 90% (meets Year 2 goal).

11. PUBLICATIONS and PATENTS (pending) on Cryocoolers (since September 03) • “Two-Stage Cryocooler Development for Liquid Hydrogen Systems”, 2003 Space Cryogenics Workshop. • “Design of a Super-High Speed PMSM for Cryocooler Application”, 2003 Space Cryogenics Workshop. • “Mesoscopic Energy Systems”, accepted to be published in Annual Review of Heat Transfer, 2004. • “Development of a Super-High Speed PMSM Controller and Analysis of the Experimental Results”, The Eighth World Multi-Conference on Systemics, Cybernetics and Informatics, 2004. • “Development of a new V/f Control for a Super-High Speed PMSM”, The Eighth World Multi-Conference on Systemics, Cybernetics and Informatics, 2004. • “Design and Simulation of a Cryogenic Electrical Motor”, AP-S International Symposium and USNC/URSI National Radio Science Meeting, 2004. • “Design of a Super-High Speed Cryogenic Permanent Magnet Synchronous Motor”, EPE-PEMC 2004, (Invited paper: Special section about high and super-high speed motor). • “Design of a Super-High Speed Axial Flux PMSM”, submitted to 2004 IEE Proceedings on Electric Power Applications. • “Miniature Joule-Thomson (JT) Cryocoolers for Propellant Management”, The 2004 ASME International Mechanical Engineering Congress and Exposition. • “Numerical Simulation of a Single-Stage Centrifugal Compressor”, abstract submitted to IGTI 05. • “Mechanical Analysis of a High-Speed PMSM”, abstract submitted to IGTI 05. • “A New Design Approach of a Super High-Speed Permanent Magnet Synchronous Motor”, submitted to Journal of Applied Physics. • “A DSP-Based Super High Speed PMSM Controller Development and Optimization”, accepted by IEEE DSP2004 (11th Digital Signal Processing Workshop & 3rd Signal Processing Education Workshop). • “Design of An Optimal V/f Control for a Super High Speed Permanent Magnet Synchronous Motor”, accepted by IEEE IECON 2004 (The 30th Annual Conference of the IEEE Industrial Electronics Society). • Compact, High Speed Centrifugal Compressor with High Efficiency. • Compact, Recuperative Heat Exchanger with High Effectiveness. • Compact, High Speed Permanent Magnet Synchronous Motor with High Energy Density and High Efficiency. • Cryogenic High Speed Motor.

12. SIGNIFICANT COLLABORATIONS • Partnered with Rini Technologies, Inc. (Dr. Dan Rini)-development of a miniature heat recuperator and a 77 K RTBC cryocooler. • Frequent communication with – • - AFRL (DoD cryocooler needs) • - NIST (Cryocooler needs and space mission requirements) • - NASA KSC (miniature heat recuperator and JT cryocooler) • - NASA GRC, ARC, JPL (NASA cryocooler needs and space mission requirements) • Collaborated with Heli-Cal, Inc.(Mr. Gary Boehm)-high speed flexible coupler development. • Communication with NASA GRC (Dr. Christopher Dellacorte)-foil bearing design. • Collaborated with Electrodynamics Associates, Inc. (Mr. Jay Vaidya)-development of high speed mesoscale motors. • Collaborated with UF (Dr. Nagaraj Arakere)-rotordynamic issues in the design of mesoscale high-speed rotors. RELATED WORK AND OTHER SOURCES OF FINANCIAL SUPPORT • Miniature Joule Thomson Cryocooler, funded by NASA KSC and ASRC, 2002-till date • Development of a Compact Heat Recuperator, funded by MDA and AFRL, 2002-till date • Development of a 77K Reverse Turbo Brayton Cryocooler, funded by MDA and AFRL, 2003-till date; NASA JSC, 2004-till date • Portable Vapor Compression Cooler, funded by Army (Natick, 2002-till date; Edgewood, 2001-till date); NASA JSC, 2004-till date •  The above funds include direct contracts to UCF; and SBIR/STTR funds through RTI.

13. FUTURE PLANS • To design and develop a one-step (thermodynamic) cycle cryocooler operating between room temperature and 18 K, and which would be able to remove 20-50 W of heat at liquid hydrogen temperature and thus meet the project objective. • To design and develop a two (compression) stage compressor for the above cryocooler. • To design and develop a 5.4 kW motor that could spin the above compressor to 313,000 rpm and to consider a fast DSP chip, soft switching and close-loop control to further improve its performance. • To design an integrated motor-compressor shaft, and thus eliminate the need for a difficult-to-do high-speed flexible coupler. • For Project Enhancement (by Dr. Dhere, FSEC): • To characterize the tribological coatings for the one-step cryocooler and thus reduce friction and the wear which may occur as we reduce the tip clearance in the compressor to improve efficiency. In the longer term, if any compliant surface gas bearings are used, these coatings will minimize the wear and friction and thus reduce bearing losses significantly and improve reliability. At low temperature, tribological coating is also needed for the turboexpander.

14. SINGLE-STEP RTBC VS. TWO-STEP RTBC • Pros: • Simplicity • Light weight • Compact • Cons: • Difficulty in helium compressor development due to larger pressure ratio and higher rotational speed • Pros: • System flexibility (neon in top, helium in bottom) • Ideal for liquid H2/O2 transfer line cooling down • The two steps of the cycle can be individually designed for maximum efficiency • Helium compressor working at cryogenic temperature (helps to reduce the rotational speed) • Cons: • System complexity causes extra weight and size • Coupling of the two steps needs complicated controller • Different working fluids cause mixing problems

15. TWO-STAGE INTERCOOLED MOTOR-COMPRESSOR ASSEMBLY Features: Ultra-compact Low maintenance Two stage Pr = 2.8 Light weight 10 kg High efficiency 58% High speed 313K rpm Flow rate = 4.6 g/s

16. GIFFORD McMAHON VS. RTBC CRYOCOOLER COMPARISON Cryomech G-M Cryocooler AL330 (40W @ 20K) UCF Miniature RTBC Cryocooler (20–30W @ 18K) Motor/Compressor unit Motor/Compressor unit 119-176 kg 10 kg The rest of the cryocooler The rest of the cryocooler Heat regenerator, Flexible lines, Cold head Ceramic micro-channel heat recuperator, Cold head, Expander/Alternator 24 kg 12 kg Total weight Total weight 143-200 kg 22 kg COP COP 0.005 0.005

17. PROJECT TIMELINE Task 1. Design and Fabrication of Miniature Centrifugal Compressor Task 2. Design of a High-speed, High-efficiency PMSM Task 3. Miniature Centrifugal Compressor Design Verification by Numerical Simulation and Testing (with appropriate scaling) Task 4. Fabrication and Testing of PMSM Task 5. Two-stage Centrifugal Compressor – Design and Fabrication Task 6. 5.4 kW PMSM – Design and Fabrication Task 7. Integration and Preliminary Testing of the Motor/Compressor Assembly Task 8. Overall System Optimization – Systematic Testing of the Motor/ Compressor Assembly, Evaluation, Possible Design Changes Start Date: July 01, 2002; Estimated End Date: September 30, 2006; Estimated Duration: 51 months