David B. Scharfe

Molten Boron Phase-Change Thermal Energy Storage to Augment Solar Thermal Propulsion Systems

Abstract : Solar thermal propulsion offers a unique combination of high thrust and high specific impulse levels that can provide competitive advantages relative to traditional satellite propulsion systems. In order to enhance the functionality of this technology, thermal storage combined with a means of thermal-to-electric conversion is suggested, with the idea of providing a dual-mode power and propulsion system based on thermal energy. A system including boron phase change material for storing energy, an insulating containment system consisting of boron nitride, carbon bonded carbon fiber, and vacuum gap insulation is proposed, with thermophotovoltaics used for electrical conversion. A laboratory solar concentration system has been constructed and experiments to directly heat small quantities of boron have begun, so that the nature and challenges of this system can be evaluated.

Phase-Change Thermal Energy Storage and Conversion: Development and Analysis for Solar Thermal Propulsion

Abstract : Solar thermal propulsion offers a unique combination of high thrust and high specific impulse that can provide competitive advantages relative to traditional satellite propulsion systems. Enhancing the functionality of this technology will require a robust thermal energy storage method that can be combined with thermophotovoltaic thermal-electric conversion. This combination creates a high performance dual mode power and propulsion system that can eliminate the traditional photovoltaic-battery combination on existing satellites. A thermal energy storage system based on the phase change of molten elemental materials is proposed as the enabling technology. Molten boron is identified as the optimal phase change material (PCM), but presents significant engineering challenges. Thus, molten silicon is proposed as a near term, moderate performance storage option. A systems level comparison against existing technologies shows that both materials present a performance benefit with current technological benchmarks, and with optimistic future assumptions, it appears that a more than 40 % _V improvement over chemical system is possible from boron based STP while maintaining high satellite maneuverability. An ongoing experimental effort is focused on producing a proof of concept thermal energy storage system. Materials testing has determined the stability of boron nitride in the presence of molten silicon in the short term, and solar furnace testing has resulted in silicon melting for the first time. Testing of the solar furnace using copper as a surrogate PCM has revealed experimental concerns with PCM heat transfer rates and has resulted in a design for a new full scale solar furnace. This furnace will operate at scales that are relevant to spacecraft development.

Molten Boron Phase-Change Thermal Energy Storage: Containment and Applicability to Microsatellites

Abstract : Latent heat thermal energy storage systems promise nearly constant temperature operation and greater energy storage densities than sensible heat energy storage systems, but they are not yet commonly used in practice due to limitations in material degradation and heat transfer rates. Systems employing particular elemental materials with high melting temperatures appear to overcome these limitations, yielding significant performance increases, particularly in bimodal (thermal and electric) solar thermal power systems. A review of candidate materials has concluded that silicon is an excellent candidate for near term, moderate performance systems, while boron, the primary focus of this paper, is an excellent candidate material for future high-temperature, high performance systems suitable for advanced microsatellite solar thermal propulsion and power systems. General considerations for systems employing such materials have been identified, the required support technologies, including high temperature thermal insulation and thermal to electric power conversion, have been evaluated, and a preliminary design for a general system has been completed. Several potential applications have been identified for this technology; one of them, a solar thermal power and propulsion unit for a 100kg microsatellite, will be described in this paper. The preliminary analysis indicates that such a bimodal system would enable large delta V maneuvers for 100kg microsatellites while also producing the required electrical power. A solar thermal test facility for further evaluating such systems is described along with initial results from the build-up phase of the facility.

Experimental Investigation of Latent Heat Thermal Energy Storage for Bi-Modal Solar Thermal Propulsion

Abstract : A bi-modal solar thermal system capable of providing propulsive and electric power to a spacecraft has been identified as a promising architecture for microsatellites requiring a substantial _V . The use of a high performance thermal energy storage medium is the enabling technology for such a configuration and previous solar thermal studies have suggested the use of high temperature phase change materials (PCMs) such as silicon and boron. To date, developmental constraints and a lack of knowledge have prevented the inclusion of these materials in solar thermal designs and analysis has remained at the conceptual stage. It is the focus of this ongoing research effort to experimentally investigate using both silicon and boron as high temperature PCMs and enable a bi-modal system design which can dramatically increase the operating envelope for microsatellites. This paper discusses the current progress of a continued experimental investigation into a molten silicon based thermal energy storage system. Using a newly operational solar furnace facility, silicon samples have been melted and results indicate that volumetric expansion during freezing will be the primary difficulty in using silicon as a PCM. Further experimental studies using different materials and test section fill factors have identified potentially reliable experimental conditions at the expense of energy storage density. In addition to conducting experiments, a concurrent computational effort is underway to produce representative models of the experimental system. The current models generally follow experimental results; however, difficulties still remain in determining high temperature material properties and material interactions. This paper also discusses the future direction of this research effort including modeling improvements, analysis of convective coupling with phase change energy storage and potential facility improvements.

Preheating Cold Gas Thruster Flow Through a Thermal Energy Storage Conversion System

Abstract : A thermal energy storage system capable of receiving, absorbing, and collecting solar energy, and storing it within a phase change material, has been designed as part of a power and propulsion system for use in low Earth orbit. The design includes thermophotovoltaic cells for the conversion of stored heat to electrical energy for various satellite systems, as well as a heat exchanger imbedded in the phase change material for propellant heating during thrust maneuvers. Through computational analysis, the likely thrust was evaluated as well as the impact of the propellant flow on the thermal to electric conversion. For the scenario analyzed, a propellant exit temperature of 1500 K, translating to an Isp of 300 seconds, was achieved. Passing the propellant through the phase change material decreases the temperature of the emitter used to power the thermophotovoltaics and resulted in a 20% decrease in electric power output.

Computational Evaluation of Latent Heat Energy Storage Using a High Temperature Phase Change Material

Latent heat energy storage systems have higher energy density than their sensible heat counterparts and have the added benefit of constant temperature operation. This work computationally evaluates a thermal energy storage system using molten silicon as a phase change material. A cylindrical receiver, absorber, converter system was evaluated using the heat transfer in solids with surface-to surface radiation physics module of the commercially available COMSOL Multiphysics simulation software. The progression of the solidification and melting fronts through the phase change material was modeled for two different methods of concentrated solar irradiation delivery. Heating the core of the PCM rather than the top of the PCM decreased the required solar input by 17%, decreasing the solar collector area required as well as lowering overall system weight.Copyright

Computational Evaluation of the Effects of Voids on a Thermal Energy Storage System Using Molten Silicon as the Phase Change Material

Latent heat energy storage is one of the most efficient ways to store solar thermal energy. A system capable of receiving, absorbing, and collecting solar energy and storing it within a high temperature phase change material has been designed as part of a power system to be used on a low Earth orbit satellite. The system employs silicon as the phase change material and thermophotovoltaic cells for the conversion of stored heat energy into electrical energy. The effect of a void, in the phase change material, on system temperature and the associated thermophotovoltaic power production is determined through computational evaluation.Copyright