Feasibility study for supercapacitor-based spacecraft power system and its development 大容量キャパシタを蓄電源に用いた宇宙機電源システムのフィージビリティスタディとその開発
Feasibility study for supercapacitor-based spacecraft power system and its development
The aims of this work are to study the feasibility of supercapacitor (SC) technologies to be alternatives to secondary batteries in spacecraft power systems, and to develop power system components for SC-based power systems. Mass saving, life extension, and improvement in operational flexibility are a primary research and development challenge in spacecraft power systems, where secondary battery technologies have been used as energy storage sources. In order to meet long mission life requirements of spacecrafts, secondary batteries need to be operated under precisely controlled conditions, implying that secondary battery technologies are facing obstacles to achieve longer life performance and improved operability. SC technologies offer longer life performance over wide temperature range which is attractive for spacecraft power systems. However, in order for SC technologies to be used as alternatives to secondary batteries, research efforts on system and component level are needed to address and overcome major hurdles, which originate from innate SC characteristics, including: (1) need of accelerated ageing testing method and cycle life prediction model, (2) mass increase due to low specific energy of SCs, thus needing a consideration for mass reduction at a power system level, (3) voltage imbalance among series-connected SCs, and (4) large voltage variation during cycling. Among a variety of SC technologies, electric double-layer capacitors (EDLCs) and lithium-ion capacitors (LICs), which are different types of SCs, were used to establish evaluation methods for feasibility study and to develop power system components universally applicable to other SC technologies. Since cycle life performance of SCs is inherently longer than that of conventional secondary batteries, an ageing acceleration and/or cycle life prediction model are necessary to design and evaluate spacecraft power systems properly. Although many research efforts have been done for life evaluations and ageing accelerations for SCs in terrestrial use, similar studies emulating conditions in spacecrafts are necessary because operational conditions, such as cycle profiles and current rates, are totally different. Cycle life testing emulating typical low-Earth orbit profiles was performed at various conditions to study the feasibility of accelerated ageing testing. Resultant ageing trends could be extrapolated linearly with the square root of the number of cycles as the x-axis. Degradations were mainly influenced by temperature, and were governed by the Arrhenius model, which determines a temperature dependence of chemical reaction rates. Ageing acceleration factors were obtained from activation energies determined by the Arrhenius equation. A cycle life prediction model was established combining the linear extrapolation and acceleration factor. The experimental and predicted aging trends were in good agreement, verifying that achievable cycle life at a given temperature is predictable with the established model. In order for SCs to be of benefit to a spacecraft as a whole, an SC-based power system must outperform in terms of system mass and/or operational life. Comparative analyses on power system mass were made for lithium-ion battery (LIB)-based and SC-based systems consisting of photovoltaic (PV) arrays and power conditioning system components. The gap between LIBs and SCs in terms of specific energy can be bridged to great extent by operating SCs with deep depth of discharge. In addition, the mass of the PV arrays in the SC-based system can be saved by introducing a constant-power charging scheme for SCs. The comparative analysis results implied that the LIC-based power system has a potential to be an alternative energy storage source for long life requirement, for which LIBs must be operated with very shallow DoD to mitigate degradations. Various equalization techniques for LIBs have been proposed and developed for not only terrestrial but also space applications, but further improvement is desired for SCs in space. Novel cell voltage equalizers and equalization chargers, which can simplify the power system configuration as a whole by integrating a charge regulator and equalizers into one unit, were proposed taking into account SC characteristics and general requirements in spacecrafts, such as reliability, design flexibility, and poor variety of rad-hard circuit components. Since the proposed equalizers and equalization chargers can operate with a single switch, the circuit complexity can be dramatically reduced compared with conventional ones, thus contributing to meet the requirements. Their individual equalizers' performances were experimentally demonstrated using series-connected EDLCs. Since voltage variations of SCs during charge–discharge cyclings are wider than those of secondary batteries, power conversion electronics with a wide operational voltage range are required for SCs to be used as alternatives to batteries. Traditional dc-dc power converters using magnetic components tend to be massive and inefficient for extended operational voltage range. Novel magnetic-less unregulated interface converters (UICs), which can achieve high-efficiency power conversions over wide voltage range, were also proposed for SCs. With the proposed UICs, bus voltage variations can be maintained within a desired voltage range, whereas an SC voltage varies significantly. EDLCs were experimentally cycled with the proposed UICs for the demonstrations. Finally, an experimental 28-V SC-based power system with 50 W load power requirement was considered for a 60-V EDLC. The experimental SC-based power system was designed based on the sun-regulated bus architecture using the proposed equalization charger and the UIC, and the system operation was experimentally verified emulating sunlight-eclipse cycles.