Bernd Goebel

A Compact HTS 5 kWh/250 kW Flywheel Energy Storage System

Flywheel energy storage systems (FESS) are expected to contribute to uninterruptible power supplies (UPS) and power quality tasks significantly. We present design and the component results of a compact 5 kWh/250 kW HTS flywheel whereby the rotor will be totally magnetically stabilized. The design is optimized for highly integrated functionality of rotor body, generator/motor and bearings. The heart of the FESS is an ironless high power PM generator/motor delivering 250 kW. An advanced flywheel body manufactured from graphite fiber is stabilized by two magnetic bearings. A prototype 200 mm HTS bearing has been constructed and tested up to a load of 1 ton axially and 0.47 ton radially capable to carry the rotor weight of about 0.5 t. With a coefficient of friction (COF) of about 10-6 the rotational drag was low. A passive PM bearing giving 80 N/mm radial stiffness/per pole was studied. HTS-PM damping response dependent on the temperature shows Lehr factors of 5-10% damping between 50 and 80 K. Eddy current dampers could support rotor dynamic stability.

HTS Magnetic Bearings in Prototype Application

ATZ Company has successfully developed high temperature superconducting (HTS) magnetic bearings for power energy application and high-speed machinery. Journal-type design and improved HTS magnetic properties increasingly fulfill industrial prototype requirements. Maximum load up to 1.1 ton, stiffnesses in 3-4 kN/mm level, simultaneous self-stabilization in axial and radial directions, largegap operation of 5-6 mm and reliable machine cooling in the 50-60 K region characterize the present progress of HTS magnetic bearings. A 200 mm HTS bearing of 10 kN load capacity is fabricated and integrated in a 5 kWh/250 kW flywheel energy storage system. We report about a new large-gap HTS magnetic coupling system ensuring 300 mm wafer treatment inside a closed processing chamber in semiconductor industry. A linear MAGLEV transport system consisting of four modular cryostat units have been recently fabricated in a prototyping process. The four HTS cryostats can carry almost 1 ton at 10 mm magnetic gap above a magnetic guideway with a force density of about 5 N/cm2. Due to perfect thermal insulation each cryostat operates more than 24 hours without refilling LN2.

Fabrication of HTS Bearings With Ton Load Performance

The considerable development effort of HTS magnetic bearings with respect to heavy loads is demonstrated. Radial type 200 mm bearing in an optimized design was developed for implementation in a new 5 kWh superconducting flywheel energy storage system. A prototype bearing, consisting of a modular assembled melt textured YBCO bulk 200 mm hollow cylinder with permanent magnet rotor is presented. Axial rotor displacement causes pinning forces of 8 and 10 kN axially at a temperature of 78 K and 71 K, respectively. Maximum radial forces are 4.7 kN at sub- cooled LN2. Fabrication technology incorporates the construction and engineering of a low-weight, nonmagnetic G-10 bearing cryostat. Ultra-low heat transfer structural elements were designed and constructed to reduce the thermal loss through the support to less than 2 Watt under 1 ton load.

Towards High-Capacity HTS Flywheel Systems

Adelwitz Technologiezentrum (ATZ) and L-3 Communications Magnet Motor (L-3 MM) are currently mounting a compact-designed flywheel energy storage system (FESS) with total magnetic bearing support. Final assembly and test operation were performed during 2008-2009. After calculations and experiments, we decided to improve rotor stabilization by stiffer geometry. In addition, two dynamical emergency bearings contribute to robust and safe flywheel operation in critical revolution-per-minute situations. A planned energy capacity of 5 kWh is now obtained at about 8000 r/min, whereas an increased capacity of 10 kWh will be stored at a speed of 10 000 r/min. The total weight of the flywheel unit is about 1200 kg plus power electronics and cooling system. The heavier 600-kg rotor causes new design and construction work in mechanical elements, magnetic support bearings, cooling, and power electronics. Due to the here reported construction changes and increased rotor speed, scaling to even larger energy storage performance of 15-20 kWh seems achievable. ATZ and L-3 MM obtained a corresponding order to develop and deliver a 15-kWh/400-kW high-temperature-superconducting FESS for a Korean local grid UPS application.

Progress toward 500 kg HTS bearings

We have tested a heavy load HTS radial bearing dedicated for use in motors, generators, and flywheels. The HTS bearing is of radial type design providing both radial and axial support. The stator and rotor are modular assemblies which allows up-scaling in size. The superconducting stator is made of polycrystalline melt-textured (pmt) YBCO 200 mm inner diameter hollow cylinder grown in cylinder-like a-b texture. The stator assembly is composed of axially stacked YBCO rings in a Cu tube and cooled from the outside by LN/sub 2/. The rotor consists of stacked annular PM rings in compression on a shaft. This results in a high radial magnetic field gradient. Three such YBCO stators have been fabricated, assembled, and tested for use with a 380 kW superconducting motor developed by SIEMENS. Maximum radial forces of more than 3.7 kN were measured (at a temperature of 68 K) with the shaft at center radial position. The goal is to demonstrate safe operation of heavy rotors in a desired frequency range by providing sufficient forces, stiffness, and damping at critical modes.

Recent Up-Scaling in HTS Magnetic Device Technology

High Temperature Superconducting (HTS) interaction shows a great potential in rotational bearing and linear Maglev technology. Besides the superconductor each application sets additional specific technical requirements. The challenges of use the various constructions and applications in the combination of the cold HTS material and the required technical periphery are discussed. ATZs 0.5-ton HTS production per annum has enabled increased performance in a number of magnetic applications. Engineering properties, advanced magnetic excitation systems and thermal and mechanical stability of robust high-load YBCO bearings have been demonstrated. Effort is being made in the development of mobile HTS Maglev devices. We have designed and produced advanced 40-cm-long bulk cryostats, having 2-mm magnetic distance to the outside and operating more than one day without refilling. 24 vacuum cryostats have been manufactured for a magnetic train construction capable to transport up to 60-kN total vehicle and passenger weight and dedicated to operate on a 200-m-long track.

Encapsulated HTS bearings: technical and cost considerations

Thermal encapsulation of HTS is one way to fabricate high-efficient HTS magnetic bearings for flywheels, motors and generators. We have been designed, manufactured and tested 200 mm axial and radial bearings for loads close to 300 kg. The maximum load to bearing weight is in the 10 to 1 ratio. At 77 K, a vacuum cryostat around the HTS reduces basic thermal losses to 6.4 W with very small rotational ac losses of 0.1 W/1000 rpm. The encapsulation reduces the cryogenic threshold in the application and allows a convenient adaptation of cryo-coolers. Basic elements are the melt textured YBCO single crystals, appropriately machined and glued in a copper ring in the center of the vacuum cryostat combined with an intrinsic cryo-sorption pump. Ten axially stacked 200 mm PM rings comprise the magnetic excitation system of the rotor. The coefficient of friction is about 10/sup -4/ at ambient and 10 times lower at reduced pressure. For higher loads the manufacturing effort of HTS bearings, becomes comparable with mechanical bearings, increasing the chances of replacing mechanical bearings.

Reel-to-Reel Copper Electroplating on Pulse Laser Deposition Coated Conductor

By applying high-rate copper electroplating technique the reliable fabrication of a copper stabilizer on coated conductor with an 1 μm Ag cap is investigated. The obtained mechanical, electrical and encapsulation properties due to a 20 μm thick Cu stabilizer on the surface improve the practical and technical performance. A copper sulphate CuSO4 based pulse plating technique is capable to deposit a 20 micrometer thick copper layer in a fast and continuous non-vacuum process. A surround Cu layer protects the HTS conductor surface against salt aqueous solutions, humidity, and against cryogenic liquids LN2 and liquid helium. With the copper stabilizer conductor solder joining with a contact resistance of 200 nΩcm2 becomes possible. Further effort is being made to develop and construct a high-rate and high throughput continuous reel-to-reel modular plating unit for deposition of long length ( >; 400 m). The paper describes the practical achievements within the conductor fabrication.

Long-Length Coated Conductor Copper Plating Fabrication

Most application of high-temperature superconducting (HTS) wires require conductor engineering with respect to superior electric and mechanical properties and robustness. A long-length reel-to-reel Cu plating manufacturing technology was developed, tested, and transferred to the Bruker HTS production facility. Details of the design and the structure of the plating unit as well as plating results are reported. The pulse-plating technology provides a high quality of one- or two-sided metallic Cu layers on the surface of the pulsed laser deposition synthesized HTS tape. Acidic copper sulphate CuSO4 has been tested to optimize and enhance the plating process with respect to the Cu deposition rate. Using reel-to-reel mode together with the 6-m-long plating apparatus, up to a 2000 m length conductor has been treated with our unique Cu-plating technique, yielding reproducible Cu stabilizer performance with deposition speeds of up to 50 m/h. The plating technology is considered to have a high flexibility in processing a metallic copper stabilizer of a thickness of 5-50 μm. The resulting Cu-layered conductor has allowed face-to-face standard soldered joints of 10-7 Ωcm2 resistance to become routine. This work describes the overall analysis of the constructed reel-to-reel plating unit and evaluates the final conductor properties.

Current transport and FCL properties of polycrystalline melt textured YBCO

Toward demonstrating the potential of resistive HTS fault current limiting behavior bulk polycrystalline melt textured (pmt) YBCO devices in meander-like shape were developed and tested by using 50 Hz AC pulses at 77 K. Meander fabrication is based on high-dense and temperature gradient growth textured YBCO plates, which were mechanically stabilized, diamond tooling CAD cut and prepared with low resistive Cu contacts in lengths between 1-5 m. The nonideal I-V characteristics at normal operation due to the residual grain boundary resistivity was measured to a few tens of microvolts per centimeter at 1000 A. Due to additional metallic shunt layer the conditions for quench protection of inherently inhomogeneous material have been derived, tested and substantially improved. We investigated the feasibility, technical performance and economy of employing robust pmt YBCO to take decisions about the material line of future superconducting fault current limiter (FCL) modules.