The bundled TES is designed to demonstrate the performance of sulfur based thermal energy storage system operating in the temperature range of 200-600 C to store at least 5kWh thermal energy. The bundled TES design is based on the shell and tube heat exchanger, with sulfur tubes enclosed in a rectangular shell. 10 sulfur tubes are installed that results in a total storage capacity up to 8 kWh. Total 21 baffles are installed that provide tortuous path for air and enhance heat transfer between air and sulfur. A cartridge heater is installed in each sulfur container so that the TES system can be thermally charged in a hybrid mode. The system is insulated using high temperature microporous insulation to minimize the heat loss. The experimental measurements from the full cycle testing will also be utilized to validate the numerical model that is developed to investigate the influence of various design and operating parameters on the performance of thermal battery.

The single element test is an important step towards design and fabrication of a test facility for the demonstration of solar thermal energy storage technology using sulfur as a thermal storage material. It provides a controlled environment to investigate the performance of thermal storage fluid, based on the thermal storage capacity and the charging-discharging rate. The single element test serves to achieve two important objectives.

1. The first objective is to understand the thermal behavior of thermal storage fluid by analyzing temperature field within a thermal storage material during charging-discharging cycles, for various operating conditions. These experimental measurements are essential for the validation of a numerical model being developed to predict the thermo-fluidic behavior of thermal storage material.
2. The second objective is to design and fabricate a ‘shell and tube’ type thermal storage system to investigate the performance of sulfur as a thermal storage material. This system would provide a crucial design perspective, necessary to develop an effective system for on-sun demonstration of the proposed solar thermal energy storage technology. In order to achieve these objectives, two independent experimental facilities are developed.

Pressure-temperature characteristics of sulfur are investigated in sealed containers up to 600 degrees C. The analysis provides crucial information regarding the maximum system pressure during thermal charging and discharging of the system. The sulfur container is designed based on the maximum system pressure for safe and effective containment sulfur over a long period of operation. During this analysis, effect of sulfur loading fraction on the system pressure is studied. The loading fraction of the sulfur is defined in Figure 4 below. The experiments were conducted for three different loading fraction, 20%, 50%, and 80%. Experimental determination of sulfur pressure temperature characteristics have shown that sulfur has a desirably low vapor pressure (< 200 PSIg at 600 ℃). Ultimately, such a modest vapor pressure results in being able to design a system with tubes of small pipe thickness (schedule 10) that leads to low containment cost. In addition, effect of sulfur container material (SS316L, SS316 and Inconel) on the pressure-temperature characteristics were studied. No significant effect of container material was found on the pressure-temperature characteristics.

Sulfur promises low storage cost, high temperature stability, and high charge and discharge performance since it operates in the liquid-vapor regime at the temperatures of interest relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The present work investigates the thermal performance of elemental sulfur stored isochorically inside the pipes of a shell-and-tube thermal battery configuration with heat transfer fluid flowing over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using a detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena.
The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. Complementary computational, theoretical, and experimental efforts lead to strong understanding of sulfur heat transfer physics for wide range of desirable storage temperatures. We observed excellent thermal charge/discharge rate — much faster than competing PCM and solid based TES — due to the strong assistance of buoyancy driven convection currents was observed. The heat transfer characterization results will be used for system level design and analysis of sulfur based thermal energy storage system for various applications such as high temperature CHP, and CSP.

The goal of this task is to develop a tool for monitoring the performance of the developed TES system in operation with micro-CSP, parabolic troughs, and solar power towers. An in-house thermodynamic system/cost model informed by theoretical and numerical analysis of sulfur species equilibria is developed to estimate the cost and exergy efficiency of MacroTES for different CSP applications. The system model is designed to simulate the overall thermal energy system using sub-models for the solar field, thermal storage, and generation. Exergy analysis of the overall system illustrates opportunities for performance enhancement to maximize efficiency. This simulation tool optimizes the multi-variable equations of system specifications to meet the required cost and performance targets of Department of Energy (DOE).

The goals of the Material Compatibility Analysis task are to: (1) test the compatibility of primary tank materials with elemental sulfur; and (2) identify potential linings (if necessary) for tank materials. Three different alloys with potential application for containment of sulfur as an energy storage fluid, namely Stainless Steel 304, Stainless Steel 316, and Inconel 600, were studied. All alloys were thermally cycled to maximum temperatures of 500 °C and 600 °C, and kept at that temperatures for a prolonged period of time. Wall thickness measurements, SEM and EDS results for all the three different alloys were performed before and after sulfur exposure. Overall for the typical range of impurities in commercially available elemental sulfur, corrosion rates for high temperature solar thermal energy applications remained acceptable for SS 316 and Inconel 600 alloys.

Large wind turbine blades are being developed at lengths of 85-125 meters, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The “biplane blade” design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This work uses a “structures-first” approach with aero-structural analyses to (1) examine the feasibility of the biplane blade, (2) determine how the dimensions of the biplane inboard region affect performance, and (3) compare the aero-structural performance of a 100-meter biplane blade to the Sandia SNL100-00 reference blade. Two-dimensional CFD simulations were used to compare the aerodynamic performance of a biplane with a thick monoplane. The lift-to-drag ratio and the maximum lift coefficient is significantly greater for the biplane than the thick monoplane for angles of attack of 0-15 degrees. Analytical methods and beam finite elements with cross-sectional analysis were both used to examine the performance of biplane blade structures. These structures varied in complexity from isotropic spars to composite spars to composite full blades. In each case, biplane blade structures were compared to monoplane blade structures of the same length, mass, and complexity. Simple load cases were applied to each structure and their displacements, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. Parametric analyses show that gap-to-chord ratios bewteen 1.0-1.2 and joint length-to-span ratios of about 0.5 give good aero-structural performance. At the tip, the biplane blade increases flapwise structural efficiency by 20-40%, depending on the load. Edgewise structural efficiency was decreased by 27-35% at the tip. The benefits for the inboard region could lead to mass reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade is an attractive design for large (100-meter) blades.

Magnetic cusp confinement of plasma at conducting surfaces involves interactions between a highly divergent magnetic field, ions, electrons, neutral particles, and the pre-sheath and sheath conditions that develop along the surface boundary. Large plasma devices have benefitted greatly by using permanent magnet cusps for bulk plasma confinement for both terrestrial and space applications; however, the magnetic cusp confinement mechanisms and the associated plasma dynamics very near the conducting surface are poorly understood. This lack of understanding has prevented researchers of micro-scale discharges from fully realizing the benefits of cusp confinement and control at smaller scales, and has also prevented the optimization of larger discharges.

We develop a computational model that properly treats the behavior of plasma very near the wall for a cusp confined plasma. The model will also serve as a tool to reveal important plasma mechanisms in the cusp region and will aid in the development of analytical description of particle motion in this region due to particle drifts, collisions, and electric field. The computational model employs the iterative Monte-Carlo (MC) method, tracking super-particles of different species (high energy primary electrons, ions, and plasma electrons) separately. In the model, particles are moved in electric and magnetic fields while they are weighted to the computational grid at every time-step. Once all the particles are tracked, the electric potential and field are updated using the densities computed from the last particle tracking subroutine. These calculations are repeated until the electric potential converges to a solution. This model can be considered as a simplified version of a particle-in-cell (PIC) model commonly used for kinetic simulation of plasma, as the electric field is computed asynchronously with particle tracking instead of being updated while the particles are tracked.

This model will be extended to a hybrid model treating ions and plasma electrons as fluids, and will be used as a tool to design an efficient micro-scale discharge. The model will use a magnetic field aligned mesh to minimize the numerical diffusion that takes place when the mesh is not aligned with the magnetic field.

Degradation of plasma-facing materials is a significant problem in electric propulsion, pulsed power technologies, and fusion energy experiments. To reduce material degradation, new micro-engineered materials are being developed that reduce sputtering and thermal stress through an increased surface volume and area exposed to the heat flux, along with the use of micron-sized dendrites, nodules, or fibers that can deform independently.

A systematic investigation is underway at UCLA to characterize the plasma-material interactions (PMI) of these materials under plasma exposure through experiments within the Plasma Interactions (Pi) facility. Pi consists of a high-power lanthanum hexaboride hollow cathode DC plasma discharge, several transport coils to the target region, a target manipulator arm, and a high-speed, high-voltage circuit for biasing the target for simulating the conditions relevant to pulsed power technologies. Material samples are exposed to a range of plasma temperature, flux, and fluence levels. Existing diagnostics include Langmuir and emissive probes for obtaining plasma temperature, density, and potential, scanning electron microscopy for qualitative erosion assessment, and energy-dispersive x-ray diffraction spectroscopy for material composition. Several diagnostic upgrades are also being implemented to improve the quality of information obtained: a set of laser confocal microscopes are being installed for in situ, quantitative evaluation of sample morphology changes during plasma exposure; optical emission spectroscopy will provide information on the content and temperature of the plasma species (including sputtered material) in the near surface region; and vacuum ultra-violet spectroscopy will provide information of the neutral gas density (typically xenon).

To complement the experimental effort, a series of plasma models are being developed. A hybrid fluid/particle-in-cell (PIC) code DC-ION, developed initially for ion thrusters, is being modified to simulate the bulk plasma of the Pi facility to allow for optimization of the plasma discharge and magnetic field design. A fluid-based pre-sheath model will use the results from the bulk model and will couple to a full PIC sheath model in the near-surface region, which will determine the species currents to the wall and the sheath potential. The plasma models, coupled with thermo-kinetic material models being developed by professors Ghoniem and Kodambaka at UCLA, will self-consistently predict sputtering rates and changes to material and near-surface plasma properties.

References

[1] T S Matlock, D M Goebel, R Conversano and R E Wirz (2014) A dc plasma source for plasma–material interaction experiments

The interaction of charge-exchange collisions ions created in the plume of ion and Hall-effect thrusters interacting with background electric fields contribute to unwanted spacecraft surface sputtering and grid erosion, thereby limiting the thruster and spacecraft operation and lifetime. Improving our understanding of heavy species collisions (e.g. momentum-exchange and charge exchange collisions between ions and neutrals) in intermediately-ionized plasma is necessary for furthering the development and use of electric propulsion devices for deep space missions.

This effort acquires detailed measurements from a simple, well-characterized plasma experiment that can precisely capture the behavior of heavy species collisions. This is accomplished by accelerating and steering an ion beam into an axisymmeteric test cell populated with neutrals, and taking precise low-current measurements of the test cell walls (from scattered ions that experience collisions) and downstream of the test cell (from unscattered beam ions). The simplified setup reveals ion transport mechanisms in a collisional environment that are otherwise hidden by complicated conditions of most electric propulsion devices. By providing full disclosure, these canonical experiments supply benchmark data for the development and validation of analytical techniques and computational codes that model plasma behavior, including the 3-D Improved Concurrent Electromagnetic Particle –In-Cell (ICEPIC) code developed by AFRL. Additionally, semi-analytical and computational models aid in the interpretation of laboratory measurements and to provide insight into future experimental modifications and design.

Current work is focused on characterization of the ion beam (with various facility diagnostics and electrostatic modeling) throughout the facility and at the entrance of the Test Cell. Electron emission from plasma-confining materials due to ion impact are investigated by modifying the test cell to hold the target material in the beam’s line of sight.

The development of permanent magnet microdischarge on the scale of 1 cm requires an improved understanding of magnetic cusp confinement physics very near the anode. Larger magnetically confined discharges benefit from relatively low surface-to-volume ratios, which can provide favorable electron confinement and high ionization efficiency. As a result, designers of many larger cusp confined devices, such as ring-cusp ion thrusters, have been able to yield favorable performance for large discharges by focusing on the design of the macroscopic magnetic field structure far from the magnet surfaces.,, However, these permanent magnets also result in strong microscopic cusp structures that dominate a large region in smaller discharges, a region that involves interactions between a divergent magnetic field, multiple plasma species, and the sheath conditions near the surface. The experimental effort aims to better understand cusp physics by examining plasma behavior for both a single and multi-array of permanent magnet cusps. Previous work measuring primary electron loss have shown that it is strongly influenced by the upstream magnetic field structure. The current experiments using a simplified cusp confinement geometry will determine if this behavior extends to a weakly ionized plasma. Results can then also also be used to validate computational and analytical models.

This research aims to demonstrate the applicability of magnetic shielding to low power Hall thrusters as a means to significantly improve operational lifetime. The key life-limiting factors of conventional Hall thrusters, including ion-bombardment sputter erosion of the discharge channel and high-energy electron power deposition to the channel walls, are well understood on all thruster scales. As thruster power is reduced to the sub-500 W nominal power regime, the increasing surface-to-volume ratio of the discharge channel and decreased thruster component sizes promotes increased plasma-wall interactions and susceptibility to overheating, thereby reducing thruster operational lifetime and performance. Although methods for compensating for these issues have been investigated, unshielded miniature Hall thrusters are generally limited to sub-45% anode efficiencies and maximum lifetimes on the order of 1,000 h. A magnetically shielded field topology aims to maintain low electron temperatures along the channel surfaces and a near-constant plasma potential near that of the discharge voltage across the discharge channel and along its full axial length. These features result in a reeducation of the kinetic energy of ions on trajectories to impact the channel surfaces to below the sputtering threshold, thereby preventing ion-bombardment erosion of the discharge channel. Improved confinement of high-energy electrons is another byproduct of the field structure, aiding in the reduction of electron power deposition to the channel. Magnetic shielding has been shown to dramatically reduce plasma-wall interactions on 4-6 kW Hall thrusters, resulting in significant increases in projected operational lifetimes with minimal effects to thruster performance.

In an effort to explore the scalability of magnetic shielding to low power devices, two magnetically shielded miniature (MaSMi) Hall thrusters were designed and fabricated. The performance of the first thruster, called the MaSMi 40, was characterized at an operating condition of 275 V and 325 W. A peak thrust of approximately 13 mN with a specific impulse of approximately 1,100 s at an anode efficiency of approximately 22% were measured at the nominal operating point. Observations of the near exit plasma discharge during operation and the discharge channel after operation suggested that the outer channel wall of the thruster was well shielded while the inner channel wall appeared to be weakly shielded. Further analysis determined that the MaSMi-40 was unsuccessful in generating a symmetric magnetically shielded field topology; however, the shortcomings of the magnetic circuit design were well understood.

The second design iteration in the development of a low power magnetically shielded Hall thruster was the MaSMi-60. Magnetic field measurements confirmed that a symmetric and fully shielded magnetic field topology was generated by this device across a wide range of possible operating conditions. At operating powers of less than 150 W to nearly 750 W, the key performance metrics of the MaSMi-60 included a measured thrust ranging from approximately 8 mN to over 33 mN with anode specific impulses ranging from under 800 s to nearly 1400 s at anode efficiencies of up to approximately 29%. Visual observations of the discharge plasma and channel walls during and after thruster operation offered strong evidence of magnetic shielding while the discharge was ignited. The plasma properties that define magnetic shielding were then confirmed using experimentally validated computational simulations of the MaSMi 60’s plasma discharge.

This project investigates the existence of high energy ions generated in high current hollow cathode plumes using Laser Induced Fluorescence (LIF) and plasma probes. Ion acoustic turbulence is generated from a high electron to ion relative drift velocity, and wave-particle interactions lead to heating of plasma in the plume. Energetic ions can cause erosion of cathode surfaces used for long duration missions using Hall thrusters. We test a 100 A-class hollow cathode at JPL and perform LIF measurements to study the formation of energetic ions and the evolution of the IVDF in the plume. Ongoing studies include analytical and computational approaches to model the wave heating mechanisms.

References
[1] Christopher Dodson, Daniel Perez-Grande, Benjamin Jorns, Dan M. Goebel, and Richard E. Wirz. “Laser-induced Fluorescence Measurements of Energetic Ions in a 100-A LaB6 Hollow Cathode Plume”, 52nd AIAA/SAE/ASEE Joint Propulsion Conference, Propulsion and Energy Forum, (AIAA 2016-4838) http://dx.doi.org/10.2514/6.2016-4838