Low-voltage Storage Research Articles

DC Cascaded Energy Storage System Based on DC Collector With Gradient Descent Method

With the continuous development of distributed energy, the energy storage system (ESS) is indispensable in improving power quality. Aiming at the application of large-capacity storage battery access to medium voltage dc power grid, a dc cascaded energy storage system based on the dc collector is proposed, and the characteristic, topology and control are presented in detail. In this scheme, the low-voltage storage batteries are accessed to medium voltage dc bus directly with dc collectors, which not only has higher efficiency, but also improves power density. Especially, the diversity of state of charge (SOC) is only reflected in the duty cycle of submodules (SMs), rather than the voltage deviation. Further, a variable angle carrier phase-shifted modulation with gradient descent method is discussed to eliminate the dominant current harmonics. By calculating the harmonic amplitude and solving the Jacobian matrix, the gradient that reduces harmonic current is obtained, and the optimal combination of the carrier phases is deduced by iterative calculation. Meanwhile, the method can be extended in the dc collector with any number of SMs to eliminate the dominant harmonics. Finally, the experimental prototype is built and the benefit of proposed solution is verified.

(Invited) Safety and Performance Testing of Commercial Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged on the global market and are poised to complement the ubiquitous li-ion battery (LIB). SIBs deliver a lower energy density than LIBs but utilize more globally abundant materials and boasts a higher degree of safety. These batteries possess similarities to LIBs but contain a wider range of materials due to the different size and reactivity of the alkali ion. Herein we utilize accelerating rate calorimetry (ARC) and electrochemical analysis to characterize the safety and performance of two cell types. The Cell A contains an aqueous electrolyte and has power capability (up to 80 C or 200 A) but low energy density (~30 Wh/kg), while the Cell B cell is non-aqueous and delivers higher energy density (~80 Wh/kg). ARC reveals benign failure modes compared to LIBs for single cells and 3 cells touching. Calorimetry also reveals minimal self-heating during charge/discharge, mitigating need for pack thermal management. Electrochemical analysis identifies impressive rate capability (tested up to 20 A) and a wide range of ambient temperatures (-20 to 45 °C). Both cells utilize aluminum for both positive and negative current collectors, since sodium and aluminum do not alloy. This is unique in comparison to LIBs, that require a copper current collector at the anode, preventing low voltage storage, where the copper reduces and dissolves. Therefore we investigated “de-energized” or 0 V storage of both SIBs. Cell A proved robust to these excursions, while Cell B exhibited capacity loss but maintained a high degree of safety.

Application Analysis of Low-voltage Storage and Charging Integration Cabinets in the Pilot Program of Guesthouse in Zhangjiajie City

Based on a guesthouse in Zhangjiajie as an example, this paper carried out the installation, design, and pilot construction of low-voltage storage and charging integration cabinets to construct the new courts-level power system. In doing so, it also analyzed the regulating effect and efficiency measurement of integrated energy storage systems in the new low-voltage and courts-level power system during the sharp, peak, shoulder, and off-peak periods of the guesthouse, so as to provide a reference for more innovative practices and application of the storage and charging cabinets, and put forward the commercial promotion mode of these cabinets suitable for guesthouses in Zhangjiajie.

High Performance Li-Ion Battery Capable of Near Zero Volt (~0V) Storage

State-of-the-art (SOA) Li-ion batteries (e.g., with graphite anode and LiCoO2 cathode) for consumer electronic devices (e.g., cell phones, laptops) are designed to operate at a certain voltage range, e.g., 3-4.2V [1]. When an electronic device, like a cell phone, indicates “low battery”, the battery is near its low operating voltage threshold, and soon the device may automatically shut down to prevent the battery from further use (discharge). If a SOA Li-ion battery is allowed to further discharge to a very low cell voltage, e.g., near 0V, the battery will deteriorate quickly, manifested with a reduced battery capacity (electric charge, Ah), i.e., reduced battery use time. This is because, at or near 0V, the anode potential can exceed the substrate (commonly Cu) dissolution potential, which results in corrosion of the Cu substrate (pittings, holes) leading to compromised battery performance [2]. A battery capable of 0V (or near 0V) storage is desirable as it overcomes irreversible battery performance losses from self-discharge (to a very low voltage). For this reason, SOA Li-ion batteries are maintained above a certain threshold, e.g. > 20-30% capacity. For large format Li-ion batteries/packs, the energy left inside batteries at storage can cause serious concerns regarding: (i) safe battery transport, (ii) high maintenance cost (monitoring and periodically charging the batteries) and (iii) safe battery storage. Li-ion battery safety is a real issue exemplified by the recent Boeing 787 battery [3].Prior art to remedy Li-ion battery at or near 0V storage includes the use of an anode substrate with a higher dissolution potential, e.g., Ti substrate [4]. However Ti current collectors possess shortcomings including high cost of the Ti current collector and poor electrode coating adhesion on Ti substrate. Moreover, fundamentally, for a graphite-based anode, a solid-electrolyte-interface (SEI) film formed on graphite anode (due to organic electrolyte reduction) can break down at or near 0V and adversely impact battery performance and life. Here we present a novel approach to develop a high performance Li-ion battery, capable of deep discharge and/or storage near 0V. Our anode active material selection criterions include: (i) amenable to substrates with high dissolution potentials, (ii) sufficient (electrical) potential to avoid SEI film formation and (iii) high specific capacity for battery high energy density. Our cathode active material selection criterions include high specific capacity and high voltage to achieve high battery specific energy. Other criterions common to the anode and the cathode include high rate capability and robust cycle life. Figure 1 shows cell capacity retention as a function of cycle number for ADA near 0V capable Li-ion cells. The cells were stored at ~ 0V for 2 days for every 5 cycles. The control cell was cycled without low voltage storage. After accumulative storage for 30 days at 50mV and 100mV, respectively, the ADA near 0V capable cells demonstrated little or no capacity fade, comparable to the control cell. We attribute the excellent cell deep discharge/storage resilience to our near 0V tolerant electrode material selections. For comparison purposes, we applied the identical test conditions to graphite based Li-ion cells as shown in Figure 2. The graphite based Li-ion cells experienced rapid capacity deterioration (i.e., capacity drop to zero) after a short period of ~ 0V storage. This is likely due to Cu current collector dissolution and dissolution/reformation of SEI films on graphite anode.Our study showed that this battery technology is a very promising high performance and safe power source capable of long-term ~ 0V deep discharge/storage. This battery technology is being developed for applications in the transportation, consumer electronics, military and space markets. References "Lithium Ion technical handbook". Gold Peak Industries Ltd. November 2003. Rui Guo, Languang Lu, Minggao Ouyang, and Xuning Feng, Scientific Report, 2016; 6: 30248. National Transportation Safety Board Report, Case No. DCA131A037, March 7, 2013. Hisashi Tsukamoto, Clay Kishiyama, Mikito Nagata, Hiroshi Nakahara, Tiehua Piao, US6596439. Figure 1

Correlation between stress-induced leakage current and dielectric degradation in ultra-porous SiOCH low-k materials

Stress-Induced Leakage Current (SILC) behavior during the dielectric degradation of ultra-porous SiOCH low-k materials was investigated. Under high voltage stress, SILC increases to a critical value before final hard breakdown. This SILC increase rate is mainly driven by the injected charges and is negligibly influenced by temperature and voltage. SILC is found to be transient and shows a t−1 relaxation behavior, where t is the storage time at low voltages. This t−1 transient behavior, described by the tunneling front model, is caused by both electron charging of neutral defects in the dielectric close to the cathode interface and discharging of donor defects close to the anode interface. These defects have a uniform density distribution within the probed depth range, which is confirmed by the observed flat band voltage shift results collected during the low voltage storage. By applying an additional discharging step after the low voltage storage, the trap energies and spatial distributions are derived. In a highly degraded low-k dielectric, the majority of defects have a trap depth between 3.4 eV and 3.6 eV and a density level of 1 × 1018 eV−1 cm−3. The relation between the defect density N and the total amount of the injected charges Q is measured to be sub-linear, N ∼ Q0.45±0.07. The physical nature of these stress-induced defects is suggested to be caused by the degradation of the Si-O based skeleton in the low-k dielectric.

A vacuum switch trigger generator with a low-voltage storage capacitor switched by a thyristor

An improved pulse generator for triggering a vacuum switch is designed. The generator is based on the discharge of a storage capacitor to a trigger gap of the vacuum switch through a pulse step-up transformer. It differs from analogs in an additional capacitor introduced at the output of the transformer. The pulse charging of the additional capacitor allows one to increase the amplitude of the output voltage (in practice, by a factor of over 1.7). This provides a decrease in the transformer ratio for obtaining the specified output voltage and in the value and rise rate of the primary-circuit current to levels, allowing one to use cheap standard thyristors as a switch. The description of the generator, designed for triggering high-current vacuum switches, which are used in switching systems of capacitor banks with an energy storage of 30–100 kJ and discharge-current rise times of 100–250 μs, is given. The generator provides voltage pulses with amplitudes of >5 kV and, after the breakdown of the trigger gap of the vacuum switch, the trigger current in the form of a damped sine wave with an amplitude of over 600 A and a total duration of up to 90 μs.

An Autonomous Piezoelectric Energy Harvesting IC Based on a Synchronous Multi-Shot Technique

This paper presents a fully autonomous integrated circuit (IC) dedicated to piezoelectric harvesters. The novel Multi-Shot Synchronous Electric Charge Extraction (MS-SECE) performed by the IC optimizes the energy transfer from a highly charged piezoelectric harvester to a low voltage storage element. The system deals with piezoelectric powers in the 10 µW to 1 mW range and handles very high piezoelectric voltage values ( $>$ 100 V) thanks to the off-chip components around the IC. The IC has been fabricated in AMS 0.35 µm 3.3 V technology and its low power consumption (1 µW @ 5 Hz) is particularly suitable for low frequency harvesters. MS-SECE increases the extracted power compared to the Maximum Power Point Tracking (MPPT) technique but it also improves the efficiency by up to 25% compared to standard SECE. Furthermore, MS-SECE allows the use of small off-chip components: an efficiency of 61% has been reached with a 125 mm 3 coupled-inductor. Finally, the proposed power management circuit self-starts, works without any battery and its overall volume is expected to be less than 1 cm 3 .

Multilevel modular DC/DC converter for regenerative braking using supercapacitors

Regenerative braking is presented in many electric traction applications such as electric and hybrid vehicles, lifts and railway. The regenerated energy can be stored for future use, increasing the efficiency of the system. This paper outlines the benefits of the MMC (modular multilevel converter) in front of the cascaded or series connection of converters to achieve high voltage from low voltage storage elements such as supercapacitors. The paper compares three different solutions and shows that the MMC can benefit from weight and volume reduction of the output inductance when shifted switching modulation strategy is used. Using this modulation strategy, not only the output frequency is increased, but also the magnitude of the inductor applied voltage is reduced, reducing inductor size and volume.

Current-fed isolated PFC pre-regulator for multiple LED lamps with extended lifetime

To emit high-quality LED light, one or a bin of LED lamp(s) is normally driven by a PWM-controlled constant current source to minimize device variation and achieve accurate brightness control. Powered from offline AC mains, the front-end power converters should provide a relatively low DC voltage bus for the inputs of post-end LED current regulators. To match the long lifetime of LEDs, the whole LED driver (ballast) should work as durably as the LEDs. However, the lifetime of the driver is usually limited by the high-voltage electrolytic charge storage capacitors used in conventional PFC pre-regulators. In this paper, our previously proposed resonant current-fed isolated PFC pre-regulator is extended to operate in discontinuous conduction mode (DCM). It allows the use of the low-voltage storage capacitors on the transformer secondary, and therefore extends the overall lifetime of the LED lighting system. A detailed procedure for finding the expected lifetime of the low-voltage electrolytic capacitor is given. As before, the high-voltage stress on the main switches, which is typical in current-fed isolated converters, is reduced substantially by taking advantage of the transformer leakage inductance necessary for resonance. Additionally, high efficiency is ensured by the use of dual non-cascading structures. Steady-state state-space averaging analysis is performed for designing the converter in DCM operation. A prototype converter is built to verify performance of the proposed PFC LED pre-regulator. Copyright © 2011 John Wiley & Sons, Ltd.