Low-Cost Flywheel Energy Storage for Mitigating the Variability of Renewable Power Generation

In the past year, the researchers at the Center for Electromechanics at The University of Texas at Austin (UT-CEM) and the Nanotech Institute at The University of Texas at Dallas (UTD) concluded research efforts on improved flywheel designs and flywheel materials to meet energy storage requirements for the grid. UT-CEM’s efforts focused on developing design codes and methods for incorporating high temperature superconducting (HTSC) materials into flywheel bearing designs. The driving principle behind HTSC bearings is the stable levitation of permanent magnet material due to induced pinning currents in bulk superconductors. Such bearings can provide low loss rates of up to 0.1% stored energy per hour, which would make diurnal energy storage applications with flywheels practical. This development has resulted in lumped parameter models that were proven and verified against laboratory experiments and finite element models. The lumped parameter methodology will aid in the design and characterization of HTSC bearings for high speed flywheel rotors. In this past year, UT-CEM used their design code and material results from UTDallas to study the potential impact of these advanced technologies for flywheel energy storage. These results showed that significant improvements could be made to energy storage density utilizing magnetically filled nano-composites in the flywheel bearing design. Introduction The purpose of the research and development program was to investigate gamechanging technologies and materials to advanced flywheel kinetic energy storage for utility grid applications. The goal was to store 50% of the grid capacity within 50 years. Since flywheels are mechanical energy storage devices, very high cycle lives, up to and above 1 million cycles with proper design, can be achieved with little to no loss in performance degradation. Key advances have been made which should keep us on the path to reaching that goal. Previous work at the University of Texas at Austin evaluated flywheel sizing requirements for different locations within the utility grid. Typically, flywheels have been used for high power low energy storage applications such as frequency and voltage regulation where energy storage devices with high cycle life are required [1]. For longer term energy storage applications, flywheels suffer frictional losses which are due to windage and bearings. Windage losses can be significantly reduced by operation at vacuum levels below 1 mTorr, which are achievable for stationary grid utility applications. Currently, most high performance flywheels operate with contactless active magnetic bearings. Although there is no mechanical contact with these bearings, magnetic hysteresis and eddy current generation in the laminations can still produce loss rates of up to 5% stored energy per hour [2], [3]. For the technology to be viable to assist in the incorporation of large scale storage in the electrical grid, those losses must be reduced by more than a factor of ten. High temperature superconducting (HTSC) bearings show promise for making it possible to achieve this efficiency goal. The driving principle behind HTSC bearings is the stable levitation of permanent magnet material due to induced pinning currents in bulk superconductors. Such bearings can provide low loss rates of up to 0.1% stored energy per hour, [4], [5], which would make diurnal energy storage applications with flywheels practical. Although stable, the force-displacement interaction between a permanent magnet and sub-critical superconductor is highly nonlinear and has a well known hysteretic behavior. The University of Texas at Austin has developed lumped parameter modeling techniques for the design and characterization of HTSC bearings for high speed flywheel rotors. Results Over the course of the GCEP program, UT-Austin has developed techniques for designing high temperature superconducting bearings for flywheel energy storage systems. These modeling techniques significantly reduce computational time over high level finite element methods without sacrificing nonlinear hysteresis and losses that occur between a levitated permanent magnet-bulk HTSC system. Bearing designs can be faster with these developed techniques, and investigations can be performed to determine material requirements for specific applications. Dynamic Validation of Lumped Parameter Model Further testing has been performed to validate the 3D lumped parameter model, that predicts the translational and vertical forces of a levitated permanent magnet over a bulk HTSC. This model is useful for estimating the radial bearing stiffness of a HTSC bearing design, along with coupled axial forces. Bearing stiffness is a critical parameter for stable system operation. To validate the model, a frequency response test was constructed, which permitted the determination of local stiffness coefficients, that are critical for characterizing rotor dynamic behavior. The test setup is shown in Figure 1, in which a linear stepper motor was used to give either a step displacement to the superconducting base, or a random input signal to shake the superconducting base. Laser displacement sensors, LDS's, were used to measure the displacement of the base and the displacement of the levitated magnet, which was embedded in a G10 carrier. Figure 1: Test setup for dynamic transverse model validation For the dynamic transverse test, a larger HTSC test bearing setup was constructed. The HTSC base consisted of 6 bulk YBCO superconductors provided by Adelwitz Technologiezentrum that have an expected critical current density of 10 to 100 kA/cm2 [6]. The rectangular bulk superconductors each measured on average 67 mm L x 35 mm W x 13 mm H and were assembled to produce a base which was approximately 134 mm L x 105 mm W. These bulk superconductors were produced by using three different seed locations, which means each block has three separate crystals, and grain boundaries in between. The grain boundaries can impede bulk current flow in the superconducting material and constrain current paths [7]. The superconductors were bonded to a sheet of G10 plate and placed in a cryostat, Figure 2. Figure 2: Base of bulk YBCO superconductors for dynamic transverse test A larger permanent magnet was also used for the dynamic test setup. A high strength, N42, neodymium magnet was used, that measured 76.2 mm OD x 50.8 mm ID x 25.4 mm H. The field strength of the magnet at the surface was measured at 0.51 T, which correlates to a coercive strength of 1030 kA/m. The magnet was bonded into a G10 carrier, that measured 114.3 mm OD x 50.8 mm H, and provided the system with some additional mass, Figure 3. The magnet and G10 carrier had a net mass of 1.292 kg. Figure 3 Permanent magnet bonded in G10 carrier for dynamic test. For the test, the permanent magnet was initially centered and positioned 2 mm above the surface of the bulk YBCOs in the cryostat. Measured spacers were used to maintain this height during field cooling. Once the magnet was positioned, liquid nitrogen was then added to the cryostat to bring the temperature down below critical, at which time, the spacers were removed to allow the permanent magnet assembly to freely levitate. The first tests evaluated system response to a step input. For these tests, the stepper motor quickly moved the YBCO base forward 1 mm, and the LDS probes measured the displacement of both the base and G10 carrier. A sample rate of 1000 Hz was used for data collection, and data was passed through a 100 Hz, 2nd order, Butterworth low pass filter to reduce noise. The time domain response to the step input shows significant damping in the system. This high level of damping was not expected to occur between the permanent magnet and bulk superconductor. This damping could be a product of viscous friction between the permanent magnet and the surrounding liquid nitrogen. An FFT (fast Fourier transform) of the damped response shows a peak response at 19.6 Hz. For the lumped parameter model, the YBCO blocks of the HTSC bearing plate are modeled by discrete, overlapping superconducting discs that couple permanent magnet displacement to induced voltages per Faraday's law of induction. The disc mesh of the HTSC plate was modeled as 18 discrete bulk superconducting regions, which considers the 6 individual YBCO blocks and 3 grain crystals per block, as shown in Figure 4. This type of mesh prevents bulk current flow between blocks and grains within the bulk YBCO to better replicate current flow. Figure 4: Updated mesh of bulk superconductors based on meshing individual grains Input base measurements from the step input and shaker tests were used with the lumped parameter model to assess model performance. The following set of dynamic equations (1) were used to simulate the motion of the magnet, where and _ are state variables of the translational magnet position and translational velocity. Motion of the permanent magnet in the vertical direction is also captured by the state variables and _ .. The inputs and refer to the position and velocity of the YBCO base, which comes from the recorded data. Although the base position is only recorded, the base velocity was estimated by the position differential between time steps. The gyrator moduli for the model, , are calculated based on the magnet field strength and the differential distance between the magnet and base, , and the vertical position of the magnet. An additional damping term, , is added to the set of dynamic equations to add additional losses which may come from viscous friction between the liquid nitrogen and magnet, as observed by the damp step response from the data.