Akio Kukita

Double-Switch Equalizer Using Parallel- or Series-Parallel-Resonant Inverter and Voltage Multiplier for Series-Connected Supercapacitors

A double-switch cell voltage equalizer using a parallel-resonant-inverter (PRI) or series-parallel-resonant inverter (SPRI) and voltage multiplier is proposed for series-connected supercapacitors (SCs), such as electric double-layer capacitors (EDLCs) and lithium-ion capacitors. The double-switch operation without the need for a multiwinding transformer offers simpler circuitry as well as better modularity than conventional equalizers requiring multiple switches and/or a multiwinding transformer. Furthermore, the inherent constant current characteristic of the PRI/SPRI at a fixed frequency, not only removes the need for feedback control to limit currents under desired levels but the proposed equalizer can also operate safely, even when some cell voltage is 0 V. Detailed operation analyses were separately performed for the voltage multiplier and PRI/SPRI, and a dc equivalent circuit for the proposed equalizer was mathematically derived. A 10-W prototype for 12 cells connected in series was built, and an experimental equalization test was performed for EDLCs from an initially voltage imbalanced condition. Voltage imbalance of the series-connected EDLCs was successfully eliminated by the equalizer, demonstrating the equalization performance of the proposed equalizer.

Bidirectional PWM Converter Integrating Cell Voltage Equalizer Using Series-Resonant Voltage Multiplier for Series-Connected Energy Storage Cells

In conventional energy storage systems using series-connected energy storage cells such as lithium-ion battery cells and supercapacitors (SCs), an interface bidirectional converter and cell voltage equalizer are separately required to manage charging/discharging and ensure years of safe operation. In this paper, a bidirectional PWM converter integrating cell voltage equalizer is proposed. This proposed integrated converter can be derived by combining a traditional bidirectional PWM converter and series-resonant voltage multiplier (SRVM) that functionally operates as an equalizer and is driven by asymmetric square wave voltage generated at the switching node of the converter. The converter and equalizer can be integrated into a single unit without increasing the switch count, achieving not only system-level but also circuit-level simplifications. Open-loop control is feasible for the SRVM when operated in discontinuous conduction mode, meaning the proposed integrated converter can operate similarly to conventional bidirectional converters. An experimental charge-discharge cycling test for six SCs connected in series was performed using the proposed integrated converter. The cell voltage imbalance was gradually eliminated by the SRVM while series-connected SCs were cycled by the bidirectional converter. All the cell voltages were eventually unified, demonstrating the integrated functions of the proposed converter.

Single-Switch Voltage Equalizer Using Multistacked Buck–Boost Converters for Partially Shaded Photovoltaic Modules

Partial shading on a photovoltaic (PV) string comprising multiple modules/substrings triggers issues such as a significant reduction in power generation and the occurrence of multiple maximum power points (MPPs), including a global and local MPPs, that encumber MPP tracking algorithms. Single-switch voltage equalizers using multistacked buck-boost converters are proposed to settle the partial shading issues. The single-switch topology can considerably simplify the circuitry compared with conventional equalizers requiring multiple switches in proportion to the number of PV modules/substrings. The proposed voltage equalizers can be derived by stacking capacitor-inductor-diode filters on traditional buck-boost converters, such as SEPIC, Zeta, and Ćuk converters. The optimum equalization strategy is also proposed and discussed for the equalizers to compensate the partially shaded PV modules efficiently. Operational analysis based on a simplified equivalent circuit is performed for a SEPIC-based topology. Experimental equalization tests using the SEPIC-based voltage equalizer were performed emulating partially shaded conditions for a PV panel comprising of three substrings. Local MPPs were eliminated and extractable maximum powers increased by the equalizer, demonstrating the efficacy of the proposed voltage equalizer.

Single-Switch Single-Transformer Cell Voltage Equalizer Based on Forward–Flyback Resonant Inverter and Voltage Multiplier for Series-Connected Energy Storage Cells

Cell voltage equalization is inevitable to ensure years of safe operation of series-connected energy storage cells, such as lithium-ion batteries and supercapacitors (SCs). Although various kinds of cell voltage equalizers have been proposed, most equalizer topologies require multiple switches and/or a multiwinding transformer, resulting in complex circuitry and poor modularity. In this paper, a single-switch single-transformer equalizer using a forward-flyback resonant inverter (FFRI) with a voltage multiplier is proposed. The required switch count of the proposed equalizer is the minimum without impairing modularity due to the single-switch circuitry with no need for a multiwinding transformer. An experimental equalization test performed for eight SCs connected in series successfully demonstrated the equalization performance of the proposed equalizer. The FFRI can be extended as a “resonant input cell,” and by combining one of resonant input cells and a voltage multiplier, a single-switch resonant equalization charger that is basically a charger with an equalization function is also derived. An experimental charging test using the resonant equalization charger was also performed, and its equalization-charging performance was demonstrated.

Two-Switch Voltage Equalizer Using an LLC Resonant Inverter and Voltage Multiplier for Partially Shaded Series-Connected Photovoltaic Modules

Various kinds of differential power processing converters and voltage equalizers have been proposed for series-connected photovoltaic (PV) modules to prevent negative influences of partial shading, such as significant reduction in power generation and the occurrence of multiple maximum power points (MPPs), including local and global MPPs, that hinders and confuses MPP tracking algorithms to operate properly. However, since conventional topologies are based on multiple individual dc-dc converters, the required switch count increases proportionally to the number of modules connected in series, increasing the complexity. A two-switch voltage equalizer using an LLC resonant inverter and voltage multiplier is proposed in this paper. The circuitry can be dramatically simplified compared with conventional topologies due to the two-switch configuration. Detailed operation analyses for the LLC resonant inverter and voltage multiplier are separately performed. Experimental equalization tests emulating partially shaded conditions were performed for four PV modules connected in series. With the proposed voltage equalizer, local MPPs successfully disappeared, and extractable maximum power increased compared with those without equalization, demonstrating the effectiveness and performance of the proposed voltage equalizer.

Current Sensorless Equalization Strategy for a Single-Switch Voltage Equalizer Using Multistacked Buck–Boost Converters for Photovoltaic Modules Under Partial Shading

Differential power processing converters and voltage equalizers have been proposed and used for photovoltaic strings comprising multiple modules/substrings connected in series in order to preclude negative influences of partial shading. The single-switch voltage equalizer using multistacked buck–boost converters can significantly reduce the necessary switch count compared to that of conventional topologies, achieving simplified circuitry. However, multiple current sensors are necessary for this single-switch equalizer to effectively perform equalization. In this paper, a current sensorless equalization technique (ΔV-controlled equalization) is presented. The equalization strategy using the ΔV-controlled equalization is explained and discussed on the basis of comparison with current-controlled equalization strategies. Experimental equalization tests emulating partial-shading conditions were performed using the single-switch equalizer employing the ΔV -controlled equalization. Negative impacts of partial shading were successfully precluded, demonstrating the efficacy of the proposed ΔV-controlled equalization strategy.

Cycle Life Evaluation Based on Accelerated Aging Testing for Lithium-Ion Capacitors as Alternative to Rechargeable Batteries

Lithium-ion capacitors (LICs) are a hybrid energy storage device combining the energy storage mechanisms of lithium-ion batteries (LIBs) and electric double-layer capacitors (EDLCs), and are considered attractive not only in high-power applications but also as an alternative to rechargeable batteries due to their inherent long cycle life and relatively high energy density. The cycle life testing was performed for commercial-off-the-shelf (COTS) LIC cells procured from three different manufactures, and the cycle life prediction model developed for EDLCs in the previous work was applied to LICs. Based on the resultant capacitance retention trends, the activation energies of degradation ratios were calculated using an Arrhenius equation, whereupon aging acceleration factors were determined. The calculated acceleration factors varied depending on manufacturers, suggesting that a proper aging acceleration factor should be determined for each manufacture cell based on cycle life testing rather than simply applying a rule of thumb which had been accepted for LIBs and EDLCs. The resulting and predicted capacitance retention trends correlated well, verifying that the cycle life prediction model established for EDLCs in the previous work would also be usable for LICs as an alternative to rechargeable batteries.

PWM Converter Integrating Switched Capacitor Converter and Series-Resonant Voltage Multiplier as Equalizers for Photovoltaic Modules and Series-Connected Energy Storage Cells for Exploration Rovers

Power systems for exploration rovers tend to be complex as three separate converters are necessary; in addition to a main dc–dc converter and cell equalizer for rechargeable energy storage cells, an equalizer for photovoltaic (PV) modules is desirably equipped in order to preclude negative impacts of partial shading. This paper proposes the pulse width modulation (PWM) converter integrating voltage equalizers for PV modules and energy storage cells. The proposed integrated converter comprises a switched capacitor converter, PWM buck converter, and series-resonant voltage multiplier that perform PV equalization, power conversion from the PV modules to the load, and cell equalization, respectively. Three converters can be integrated into a single unit with reducing the total switch counts, achieving not only system-level but also circuit-level simplifications. The derivation procedure of the integrated converter is explained, followed by the operation analysis. Experimental tests were performed using series-connected supercapacitor (SC) modules and solar array simulators to emulate a partial shading condition. With the integrated converter, the extractable maximum power from the PV modules significantly increased while voltage imbalance of SC modules was adequately eliminated, demonstrating the integrated performance of the proposed converter.

Durability evaluation of InGaP/GaAs/Ge triple-junction solar cells in HIHT environments for Mercury exploration mission

The Japan Aerospace Exploration Agency has been developing the Mercury Magnetospheric Orbiter (MMO), which is Japanese part of the BepiColombo mission. During its mission around Mercury, the spacecraft will be exposed to high solar irradiance of up to 11 suns, with an estimated maximum solar panel temperature of 230°C. In such an environment, solar cells are required to operate under high intensity and high temperature (HIHT) conditions. Therefore, it is necessary to evaluate the durability of solar cells to meet the power requirements throughout the mission life. We conducted a continuous operation test under HIHT conditions to examine the validity of the solar array configuration, using the interior planetary thermal vacuum chamber. Our HIHT tests clarified the following facts: (i) Transparency of the coverglass and the performance of the solar cells do not degrade and (ii) transparency of the DC93-500 adhesive in the top cell response region degrades mainly due to ultraviolet exposure at high temperatures. We decided to use AR0213 coverglass (from JDSU) with a thickness of 300 µm, which have a longer cut-on wavelength in ultraviolet region. With this configuration, the predicted decrease in Pmax due to the HIHT environment is 17.3% and that due to radiation effects is 11.0% Our new design will offer the available power at EOL of 394.2 W, which is 46.7 W greater than the required power.

Single-Switch Single-Magnetic PWM Converter Integrating Voltage Equalizer for Partially Shaded Photovoltaic Modules in Standalone Applications

To prevent partial-shading issues in photovoltaic (PV) systems, various kinds of voltage equalizers that virtually unify characteristics of shaded and unshaded modules have been proposed. Although PV string utilization can be dramatically improved, PV systems tend to be complex and costly because, in addition to the main converter for string control, voltage equalizers are separately necessary. This paper proposes the single-switch single-magnetic pulse width modulation (PWM) converter integrating the voltage equalizer using the series-resonant voltage multiplier (SRVM) for standalone PV systems. By utilizing a square wave voltage generated across a filter inductor in a PWM buck converter for driving the SRVM, the main PWM converter and voltage equalizer can be integrated into a single unit with reducing the total switch and magnetic component counts, achieving not only system-level but also circuit-level simplifications. The experimental test using the prototype for three PV modules connected in series was performed emulating a partial-shading condition. The integrated converter effectively precluded the partial-shading issues and significantly improved the power available at a load, demonstrating its efficacy.