Electrical and Computer Engineering

University of Idaho College of Engineering Moscow, ID 83844

December 4, 2012

Dr. Joseph Law Electrical and Computer Engineering Moscow, ID 83844

Subject: Interim Report on Flywheel Energy Storage

Team INSTAR is pleased to submit an interim report for the implementation of the flywheel energy storage system for lunar colonization.

This report contains components that describe where we picked this project up from the last two years of being a senior design project, where we have taken the project since then, and what we plan on accomplishing in the spring semester of 2013. More information regarding this project can be found on Team INSTAR’s website: http://seniordesign.engr.uidaho.edu/2012- 2013/flywheel.

Please feel free to give us any feedback you have, as well as any other comments, concerns, and or suggestions you may have. From all of us on Team INSTAR we would like to thank you for all you have done for us this Fall and we all look forward to working with you in the Spring.

Sincerely,

Team INSTAR Nick Frazey, Trace McGrady, Gregory Parker, Vincent Colson, Andy Ivy

2 Table of Contents

3 Executive Summary

The University of Idaho senior design team, Team INSTAR, is prototyping a Flywheel Energy Storage system for the purpose of lunar colonization. This team is building on research and designs completed by Phase I of the project. Solar power is the primary energy source available on the moon, and periods of darkness can continue for up to 336 hours consecutively. Therefore, it is vital to implement energy storage systems during periods when solar energy is not available. NASA requires a reliable, efficient and economically viable design. Funding for the NASA flywheel project is provided by the Ralph C. Steckler Grant. The design consists of many subsystems. Vacuum systems, field reluctance regulation machine, liquid nitrogen cooling, microcontroller correction, and magnetic levitation systems being among them. The goal of phase II is to provide a levitating and spinning version of the prototype that is operable at relatively low operational speeds (less than 2000 rpm). Proof of concept and energy density of Flywheel Energy Storage is key in this project design. Upon receiving funding for the third and final project phase, the flywheel design would need to be revisited in order to operate at speeds of nearly 40000 rpm.

Background

The National Aeronautics and Space Agency (NASA) would like energy generation as well as storage methods for future colonization on the moon. Lunar daylight cycles between 336 hours of light and darkness, and because power generation would come mainly from solar sources energy storage is of paramount importance during periods of darkness. Flywheel Energy Storage (FES) is an option the University of Idaho has been chosen to explore. The flywheel provides several design advantages, including operability in low temperature environments, high efficiency, relatively low maintenance requirements, high energy density, and long-term storage capability. Thus, it is a highly viable option for NASA’s future designs. A comparison between common energy storage devices can be seen in Figure 1 below.

Figure 1 Energy Density Comparison

4 Flywheel Energy Storage operates on the principle of kinetic energy storage – that is, energy stored in a rapidly rotating disk. This design will use magnetic-levitation bearings while operating in a vacuum chamber to both minimize friction and air drag losses while mimicking a lunar environment. This design would allow NASA to increase mission time on the moon and the technology can potentially be applied to many other applications in the future.

Problem Definition

Design Team Goals:

 Develop & test a working prototype of the Flywheel Energy Storage system

 Provide a stable and reliable design, while maintaining economic viability

 Implement microcontrollers to control flywheel positioning

 Run hub-less flywheel prototype at rotational speeds near 2000 rpm

Deliverables:

 Updated CATIA modeling of final prototype, with drawing package

 Manufactured and tested flywheel subsystems

 Microcontroller algorithm programming for future designs

 Levitating and spinning final prototype flywheel design

 Final Design Report for the University of Idaho and NASA

 Final design presentation for Engineering EXPO in May

5 Stakeholders:

Individuals with interest in the completion of the flywheel design:

 NASA Sponsors

 Professors Law, Odom, Riley, Berven and Beyerlein – University Mentoring Faculty

 Mechanical, Electrical and Physics Graduate Students

 Senior Design Team Members

Concepts Considered

Stator split

Our project being the third year of an ongoing effort, many of our designs are constrained by the work of previous teams. It was decided early on that the stator was to be redesigned to a split orientation. Having the stator divided and separated allows for the control system to generate forces that cause a transaxial torque on the rotor, and will give control over the angle of the rotor rotation axis. Inertial moments of the rotor were to be analyzed for any design change to check for rotor dynamic instability. As it is understood by senior design, the rotational velocities at which instabilities occur coincide when the ratio of the moments of inertia (MOI) of the rotor . These coincidences are likely to cause catastrophic failure, and are to be avoided at all costs.

Cooling structure

Due to the HTS passive magnetic bearing near field cooling, there is need to control the elevation of the rotor before machine operation, while the HTS array cools to liquid nitrogen

6 temperature. The elevation of the rotor must be controlled during the cooling stage, which must be done after vacuum is enabled. The vacuum chamber purchased in the previous year of the project helps to limit us on options for mechanisms to control the rotor elevation, but options remain widely varying. Elevation requirements are as of yet unknown, but we will design for an acceptable range (up to 50 mm) of elevation options. Mechanism needs to be out of the way for operation, and not obstruct magnetic field.

HTS geometry

The HTS array is a very large expenditure for our portion of the project. Geometry of the array is to be decided to allow for full operation of the passive magnetic bearing, while minimizing cost. Individual superconductors are available for machining in three puck diameters, 28 mm, 35 mm, and 44 mm. Since cylindrical pucks in a circular array are to be used, they must be trimmed to be adjacent for full coverage of the permanent magnets in the rotor. Geometry must be calculated to give an integer number of pucks while also calculating coverage. Coverage and number and size of pucks were analyzed to determine a cost effective solution.

Microcontroller

The microcontroller that we have been working with is an Atmel AT32UC3C0512C. It comes with a 32 bit processor which is run at 66 MHz. Our goal is to use the inductance, voltage, and current values of the stator windings to bypass distance sensors to determine the distance between the stator and the rotor. This self-sensing approach will utilize the microcontroller’s Pulse Width Modulation (PWM), Analog to Digital Converter (ADC), Digital to Analog Converter (DAC), and External Interrupt Controller (EIC) components. Distance sensors will still be required in order to test if the accuracy of the self-sensing implementation will be sufficient.

Active Magnetic Bearing Control Theory

The active magnetic bearing (AMB) control is responsible for maintaining the one millimeter air gap between the rotor and the stator. The rotor is levitated by the high temperature superconductors (HTS) and permanent magnets, but a method is needed to control the horizontal position of the rotor. The stator includes 24 coils, which will be individually controlled. For rotation, each coil will be powered to behave as a field winding or an armature winding, depending on rotor position. When the rotor begins to drift, the AMB control can temporarily adjust the power in a coil to push or pull the rotor. The AMB will have to determine the air gap. Voltage and current sensors will take measurements of each coil. When there is a change in winding voltage, four to six current values will be read at different times to obtain an accurate slope of the current in the coil. The four to six values will be stored and used in a Least Squares algorithm to acquire the most accurate slope of the current. These measurements of voltage and change in current can then

7 be used to calculate the mutual inductance between the rotor and stator. This mutual inductance can then be used to calculate the distance between the rotor and stator. Based off of the calculated air gap, the AMB can then take corrective action to adjust the power to certain coils to properly push and pull the rotor back into the proper position. The AMB will provide a desired current value to the current controller. This current value will correlate with the desired air gap. The current controller will then properly pulse width modulated signals to an H bridge to ensure the proper current gets passed through the coil. This adjusted current flowing through the coil will adjust the rotor position. A block diagram description is shown below.

Concept Selection

Stator split - Moment of inertia study

Several various configurations were considered and analyzed in CATIA for MOI ratio. Configurations with a generally more solid fill material between lamination stacks produced MOI ratios that were out of the range of possibilities. Less dense fill materials produced varying results, but were generally more acceptable. A math model was developed with help of CATIA and given to EE grad students to allow them to manipulate the lamination stacks and get MOI results in real time.

Cooling structure - Fiberglass plate platform

Determination of the mechanism to be used to hold the rotor at elevation while cooling the HTS array was difficult because so many options were available that would fit the minimum criteria. Eventually proposals were weeded out due to weaknesses in design, and a few concepts were settled on. Selecting one among the few remaining was left to the decision matrix below. Concept 1: Concept 2: Concept 3: Concept 4: Weight Rotating Auxiliary Scissor Retracting [10 scale] Wedge Plate Jack Wedge Accuracy 10 3 8 8 5 Strength 7 7 4 9 9 Ease of build 8 6 9 4 6

8 Range 5 3 6 9 4 Space 9 8 8 6 7 Requirements Expense 3 9 3 4 4 Totals 241 291 286 255

Strength and expense will be considerably improved by design recommendations by Dr. Berven and Kysen Palmer to use a reinforcing annular ring along the top surface of the plate and to use a G-10 fiberglass construction material. With reinforcement and proper actuation this material will perform well at our temperature and vacuum conditions. The orientation of the structure has the added bonus of providing protection of the HTS array during operation, as the plate will not retract, but will instead rest between the HTS and rotor. A prototype was built on Dec. 5 that displayed very good strength under the weight of the rotor.

HTS geometry - Geometry decision

Every feasible integer configuration of HTS array geometry was tabulated and considered based on a few criteria. Cost was first considered, which eliminated the 44 mm pucks from the selection field. Coverage was limited to a minimum 100%, eliminating many possibilities, and then gap spacing was considered. Thoughts on gap spacing led the group to the 35 mm pucks since they would allow for fewer gaps over the array than 28 mm pucks. The number of pucks was determined based on potential resonance issues when magnet spacing is divisible by HTS spacing. It was decided by Dr. Law that the resonance issue must be avoided, and 21 pucks gave the best coverage for numbers that complied with the resonance issue. The angle of the cuts that fit a 21 puck 35 mm array is 17.143° and the array gives 125% permanent magnet coverage.

System Architecture

The flywheel energy storage system contains various systems that all contribute for perfect operation. These systems are broken down into rotor levitation, rotor rotation, control systems, maintaining operating temperature/pressure, assembly of system, or any combination thereof. Each system is individually designed for controllability and overall system performance. Much of the initial design for each system was done prior to this second phase of the flywheel project. The mechanical engineers of the project dealt with rotor levitation and rotation, operating temperature/pressure and the assembly of system. The electrical engineers dealt with the control system of the rotor operation. Team design was given by our client in order to solve certain parameters. The biggest concern to date is to achieve our rotor’s levitation and rotation. We have worked on redesigning the rotor for rotation stabilization including optimizing the moment of inertia and center of gravity concerns. The designs were used using the same geometric parameters instated by the original design. The development came to split the rotor laminations to have a smaller stack act as an active magnetic bearing for the rotor as a whole. The active magnetic

9 bearing allows the control systems to stabilize any rotor “wobble” that is unnecessary by creating corrective moments that act of the rotor opposite of the wobble. The importance of solving this concern includes knowing exact rotor geometry. Even though this may sound very mundane, the rotor geometry is affected by maintaining the proper center of gravity. But the height of the active magnetic bearing needs to be such that it doesn’t increase it. The new rotor design splits 10% of the laminations and places them 2.7” above the rest. This will produce a moment of inertia of which will prevent rotor wobble. The levitation of the rotor requires expensive superconductors and magnets that must be place in high tolerance to receive true levitation. The magnetic place on the underside of the rotor was designed and machined last year. The superconductors were not given a thorough design to optimize the entire length of the magnets’ flux field. Therefore we were able to effectively create an array of cut circular superconductors that would be fixed into our copper plate. This change in superconductor design dramatically improves the rotor levitation potential. This new design not only allows us to have a higher control of the rotor levitation but also will help center it around the stator. This array consists of having 21 superconductors each with a chord length of over 0.75”. The assembly of the flywheel system is going to be somewhat tricky due to the requirement of the superconductors to be cooled down to 77K. We want the magnetic field around the superconductors to be a larger specific distance away from the magnets while the liquid nitrogen cools them. This requires a lifting mechanism that will hold the weight of the rotor independently while the liquid nitrogen flows through the copper plate. This lifting mechanism must be operable under a vacuum and withstand low temperatures. Our initial design strategy was to incorporate a lifted wedge that would lift and lower the rotor using vertical actuation. Our second idea was to create a linkage system that can translate the lifting from a more compact direction of operation (probably horizontally) to vertically. A linear actuation device turned out to be very expensive for our needs. Another problem we faced was that these designs weren’t able to completely retract out of the way of the spinning rotor. Therefore we brainstormed to come up with a design that was to optimize the space between the rotor and the superconductors. Our final design uses a non magnetic, lightweight, strong plate that sits between the superconductors and the rotor and just rises and lowers slowly due to manual crank operation from below the vacuum chamber. The plate will lift the rotor right off the superconductors to an exact height by a rotational lifting device. The benefit of this design is not only simple but allows for protection over the expensive superconductors in case of flywheel failure. This design will help save thousands of dollars in precise linear actuation systems. Due to high temperature differences between different components of the flywheel system, there needs to be a control on how to maintain acceptable temperatures. A large concern is the temperature increase produced by the coils packed into the stator. We want to keep them below about 150oF so the individual wire wrappings don’t melt off and cause a lag in magnetic flux. Therefore a thermal model has been made to understand how much water flow is needed to run through the stator shaft. This model consists of heat transfer fundamentals including material properties. If this model is overlooked or not done correctly, then the rotor will never rotate once the wire insulators are melted causing the whole system to be useless. The amount of heat transferred out of the stator is upwards of 250 Btu/hr.

10 The control systems of the rotor are done by microcontrollers. These microcontrollers will be able to dynamically stabilize the rotor by applying specific forces given off by certain coils. Therefore these microcontrollers are programmed to be able to react to instability and correct automatically. These are tested with an h-bridge to monitor accuracy of where the current flow is traveling. Future Work For our flywheel system, much of the design was drawn in the CATIA 3D modeling software. During much of the current design process we found many of the dimensions to be inaccurate and impeding system spacial awareness. This being said, we must spend time to redraw the new designs into the model to appropriately make the system geometrically accurate. Some of the analysis for the control systems depends directly on how we found our moments of inertia and center of gravity. Initially both of these calculations we based off of the old model design giving us inaccurate numbers. This is helping us understand the importance of constantly updating work as progress is made in design. The time it takes to update the model will take several weeks if not months. This is a useful visual aid to the actual system assembly for reference and must be accurate. Designing the copper plate for the placement of the superconductors is very important and we have only just briefly discussed. We noticed that the previous design includes the whole plate with just the round superconductors spaced in an array underneath the rotor. This copper plate needs to be updated to include the new superconductor shape. Also there is no path to run the liquid nitrogen through in the old design. I believe that this update shouldn’t take too long but should incorporate a fully machined part upon completion. We also plan on incorporating eddy current sensors that will determine air gap distances to a much higher degree of accuracy which will be used initially to get the system spinning before tackling the sensorless design that is the eventual goal of the project.

11 Appendices

12 Copper Plate

13 Acrylic Deformation

14 HTS Sketch