Series-connected Cells Research Articles

Upscaling Inverted Perovskite Solar Cells: Optimization of Laser Scribing for Highly Efficient Mini-Modules.

The upscaling of perovskite solar cells is one of the challenges that must be addressed to pave the way toward the commercial development of this technology. As for other thin-film photovoltaic technologies, upscaling requires the fabrication of modules composed of series-connected cells. In this work we demonstrate for the first time the interconnection of inverted modules with NiOx using a UV ns laser, obtaining a 10.2 cm2 minimodule with a 15.9% efficiency on the active area, the highest for a NiOx based perovskite module. We use optical microscopy, energy-dispersive X-ray spectroscopy, and transfer length measurement to optimize the interconnection. The results are implemented in a complete electrical simulation of the cell-to-module losses to evaluate the experimental results and to provide an outlook on further development of single junction and multijunction perovskite modules.

Dual-Junction GaAs Photovoltaics for Low Irradiance Wireless Power Transfer in Submillimeter-Scale Sensor Nodes.

Dual-junction GaAs photovoltaic (PV) cells and modules at sub millimeter scale are demonstrated for efficient wireless power transfer for Internet of Things (IoT) and bio-implantable applications under low-flux illumination. The dual-junction approach meets demanding requirements for these applications by increasing the output voltage per cell with reduced area losses from isolation and interconnects. A single PV cell (150 μm × 150 μm) based on the dual-junction design demonstrates power conversion efficiency above 22% with greater than 1.2 V output voltage under low-flux 850 nm near-infrared LED illumination at 6.62 μW/mm2, which is sufficient for batteryless operation of miniaturized CMOS IC chips. The output voltage of dual-junction PV modules with 4 series-connected cells demonstrates greater than 5 V for direct battery charging while maintaining a module power conversion efficiency of more than 23%.

Automotive Battery Equalizers Based on Joint Switched-Capacitor and Buck-Boost Converters

A series of integrated equalizers based on joint buck-boost (BB) and switched-capacitor (SC) converters are proposed for balancing the voltages of series-connected battery packs. All these equalizers realize the any-cells-to-any-cells (AC2AC) equalization mode without increasing the count of MOSFETs and drivers. Corresponding operational principles are analyzed and the expressions of balancing currents are derived by analytical methods and verified by experimental waveforms. According to the comparative balancing experiments for four and six series-connected Li-ion cells, one proposed CBB-PCSC equalizer, which achieves the dual AC2AC balancing modes through the integration of both coupled buck-boost (CBB) and parallel-connected switched-capacitor (PCSC) converters, leads to the highest balancing speed and efficiency. Moreover, compared with several conventional equalizers, this CBB-PCSC topology also has the compact size and low cost, making it become a well-performing integrated topology for automotive battery voltages equalization.

Cascaded Droop and Inverse Droop Regulation for Two-Layer Coordinated Power Flow Control in Series-Connected Power Cells

To interconnect string power cells into high voltage grid in a distributed manner, a cascaded power flow regulator is proposed for simultaneous control of the injected power to the grid and flexible power sharing among string power cells. First, the traditional frequency and voltage magnitude droop control at the central controller behaves as the outer power control loop for an accurate regulation of the system power flow to main grid. Then, by setting the frequency reference and voltage magnitude reference from outer power control loop as base values for each power cells local controller, inner voltage magnitude droop for apparent power S control and the frequency inverse droop for P/E control are implemented at each power cell local controller to achieve well-decoupled real and reactive power sharing among series-connected power cells. With this control architecture, the power cells and the entire grid-tied system can obtain the flexible power regulation in a fully voltage control manner. Accordingly, seamless grid-tied to islanding operation mode transfer and power sharing of the system with other converters in autonomous islanding operation can be easily obtained.

A Novel System for Measuring Alternating Current Impedance Spectra of Series-Connected Lithium-Ion Batteries With a High-Power Dual Active Bridge Converter and Distributed Sampling Units

Alternating current (ac) impedance spectra facilitate lithium-ion battery management. Realizing a low-cost and low-complexity onboard impedance measuring system is a vital issue for the management based on the ac impedance. In the article, a novel impedance measuring system combined with a high-power dual active bridge (DAB) converter and distributed sampling units is proposed and verified. The DAB converter is designed to generate the ac disturbance to ensure a quasi-steady measurement of the battery impedance. The distributed signal sampling units simultaneously measure the voltage and current of all the series-connected battery cells in a module to measure their impedance. The measured impedance in a frequency range of 0.1-500 Hz shows the feasibility of the system. The root-mean-square errors of the measured impedance phase from 0.2 to 200 Hz and the magnitude from 0.1 to 500 Hz are less than 6.9% and 4.0%, respectively. The errors in some frequency ranges are slightly larger, which are analyzed. The novelty is reflected in that the system is easily integrated into a bidirectional onboard charger and compatible with the battery management system, thus reducing costs and complexity. It provides a basis for the onboard application of the battery impedance.

A Relative State of Health Estimation Method Based on Wavelet Analysis for Lithium-Ion Battery Cells

The state of health (SOH) is essential for the safety and reliability of lithium-ion batteries in different applications. Moreover, the inconsistency of battery cells in battery packs is inevitable. To determine the SOH difference is even more challenging without the calculation of the SOH of each battery cell. To solve the problem, this article proposes a novel relative SOH (rSOH) estimation method based on wavelet analysis, and the SOH differences for series-connected lithium-ion battery cells in the battery pack can be obtained. It should be noted that no battery model is needed in this method. The more accessible battery voltage of each battery cell is sufficient to obtain the SOH differences, without SOH estimation for each battery cell. With the wavelet analysis, more detailed information of the battery cells is obtained. The rSOH is then introduced as the index of the SOH difference, and the estimation procedure is further analyzed. Finally, experimental validations are conducted by different aged battery cells. The different state of charge conditions and different driving cycles are applied to show the robustness of the proposed method. The experimental results show that the rSOH estimation error is limited within 5%, equivalently with less than 1% SOH estimation error, thus proving the effectiveness of the proposed rSOH estimation method.

A Modularized Bidirectional Charge Equalizer for Series-Connected Cell Strings

In this article, a modularized charge equalizer is proposed. In the proposed topology, N series cells are divided into K modules and each module consists of J series cells and its multiplexer switches. A multioutput transformer is used as temporary energy storage. Each of the transformer outputs is connected to one of the K modules and targets the cells in its module. In this way, at the same time K cells are targeted simultaneously and the speed of balancing has increased. In the proposed circuit, energy can be transferred by string to cell, cell to string, and cell to cell methods. In addition, it can be transferred between modules and equalize them. Therefore, the proposed topology is very flexible and can circumvent the limitations of each method and use the appropriate method in the right place. Also, a step-by-step method for command the multiplex switches is proposed. To illustrate the correctness of the circuit operation, a prototype is implemented to balance the charge of four lithium-ion battery cells. The experimental results confirm the correctness of the proposed circuit operation.

Phase Enlargement of Semi-Markov Systems without Determining Stationary Distribution of Embedded Markov Chain

A phase enlargement of semi-Markov systems that does not require determining stationary distribution of the embedded Markov chain is considered. Phase enlargement is an equivalent replacement of a semi-Markov system with a common phase state space by a system with a discrete state space. Finding the stationary distribution of an embedded Markov chain for a system with a continuous phase state space is one of the most time-consuming and not always solvable stage, since in some cases it leads to a solution of integral equations with kernels containing sum and difference of variables.
 For such equations there is only a particular solution and there are no general solutions to date. For this purpose a lemma on a type of a distribution function of the difference of two random variables, provided that the first variable is greater than the subtracted variable, is used.
 It is shown that the type of the distribution function of difference of two random variables under the indicated condition depends on one constant, which is determined by a numerical method of solving the equation presented in the lemma.
 Based on the lemma, a theorem on the difference of a random variable and a complicated recovery flow is built up. The use of this method is demonstrated by the example of modeling a technical system consisting of two series-connected process cells, provided that both cells cannot fail simultaneously. The distribution functions of the system residence times in enlarged states, as well as in a subset of working and non-working states, are determined. The simulation results are compared by the considered and classical method proposed by V. Korolyuk, showed the complete coincidence of the sought quantities.

Balancing-Aware Charging Strategy for Series-Connected Lithium-Ion Cells: A Nonlinear Model Predictive Control Approach

Charge unbalancing in series-connected cells can lead to lower storage capacity and shorter battery life. Model-based optimization strategies have proven to be very effective in addressing this problem. In this article, we propose a general nonlinear model predictive control (NMPC) scheme for obtaining a balancing-aware optimal charging. The presented method relies on an electrochemical model, tailored for control purposes. In view of the possibility of practical implementation, the concepts are subsequently specialized for an easily implementable power supply scheme. Finally, the NMPC approach is validated on commercial cells using a detailed battery simulator, with sound evidence of its effectiveness both under the assumption of full state availability and in the presence of an observer scheme.

Light-Driven Electrochemical Carbon Dioxide Reduction to Carbon Monoxide and Methane Using Perovskite Photovoltaics

Summary Storing solar energy in chemical bonds is an effective strategy to overcome the intermittency of sunlight as an energy source. Here, we demonstrate unassisted light-driven electrochemical aqueous carbon dioxide reduction to carbon monoxide and methane using p-i-n double-cation lead halide perovskite solar cells in combination with catalytic electrodes for carbon dioxide reduction and water oxidation at near-neutral pH. Three series-connected photovoltaic cells and gold and ruthenium(IV) oxide electrodes provide carbon monoxide with >8% solar-to-carbon monoxide conversion efficiency for 4.5 h. Including concomitant hydrogen formation, the total solar-to-fuel conversion efficiency remains >8.3% for 10 h. Four series-connected cells with copper and ruthenium(IV) oxide electrodes provide methane. The longevity of the copper electrode improves by setting the cell to open circuit for 1 min every 15 min. The solar-to-methane conversion efficiency is close to 2%, and including 3% solar-to-hydrogen conversion efficiency, the solar-to-fuel conversion efficiency is 5% for 8 h.

A Low-Cost and High-Efficiency Integrated Device toward Solar-Driven Water Splitting.

Achieving the spontaneous evolution of fuel from integrated devices by solar-driven water splitting is an attractive method for renewable energy conversion. However, their widespread implementation is hindered by their immature architectures and inferior performances. Here, we propose a real integrated device consisting of two series-connected perovskite solar cells (PSCs) and two CoP catalyst electrodes, which can be immersed into the aqueous solution directly for solar-driven water splitting. Benefiting from the low-cost and facile encapsulation technique, this integrated device possesses a compact structure and well-connected circuits for the process of charge carriers generation, transfer, and storage. Moreover, although all expensive components in this integrated device are eliminated, the two series-connected carbon-based PSCs still exhibit a high solar-to-electric efficiency of 10.6% as well as the integrated devices display a solar-to-hydrogen efficiency of as high as 6.7%. This integrated device serves as a model architecture toward future development and optimization of the integrated device that can be immersed into the aqueous solution directly for water splitting.

Unassisted Water Splitting Using Standard Silicon Solar Cells Stabilized with Copper and Bifunctional NiFe Electrocatalysts.

Silicon photovoltaic cells functionalized with water-splitting electrocatalysts are promising candidates for unassisted water splitting. In these devices, the total surface of silicon solar cells is covered with electrocatalysts, causing issues with (i) stabilizing silicon solar cells in water and (ii) device efficiency due to parasitic optical absorption in electrocatalysts. We describe and validate a water-splitting device concept using a crystalline silicon solar cell where the front side is covered with an insulating Si3N5 antireflection coating. The Ag contacts, fired through the antireflection coating, are removed and subsequently substituted with NiFe layered double hydroxide (LDH) or Cu/NiFe-LDH electrocatalysts. In this device, only the site of Ag contacts, nearly 2% of the total device area, is covered by the electrocatalyst. We found that this small area of the catalyst does not limit device performance and the addition of a Cu interlayer between Si and NiFe-LDH improves device performance and stability. The unassisted water-splitting efficiency of 11.31%, measured without separating the evolved gases, is achieved using a device composed of three series-connected silicon solar cells and an NiFe-LDH/Cu/Ni-foam counter electrode in a highly alkaline electrolyte.

Cu Nanoparticle Array-Mediated III–V/Si Integration: Application in Series-Connected Tandem Solar Cells

The integration of III-V materials with crystalline Si (c-Si) is a promising pathway to design high-performance optoelectronic devices, including solar cells. We have previously reported high-effic...

Predicted annual energy yield of III-V/c-Si tandem solar cells: modelling the effect of changing spectrum on current-matching.

High efficiencies of >30% are predicted for series-connected tandem solar cells when current-matching is achieved between the wide-bandgap top cell and silicon bottom cell. Sub-cells are typically optimized for current-matching based on the standard AM1.5G spectrum, but in practice, the incident radiation on a solar cell can be very different from this standard due to the effects of the sun's location in the sky, atmospheric conditions, total diffuse element etc. The resulting deviations in spectral content from optimum conditions lead to current mismatch between tandem cell layers that adversely affects the device's performance. To investigate the impact of this issue the energy yield (%) of tandem solar cells comprising a III-V wide-bandgap solar cell connected electrically and optically in series with a silicon bottom cell was simulated over a full year using measured spectral data from Denver, CO. Top cells with bandgaps from 1.5-1.9 eV were modelled using an external radiative efficiency method. The predicted annual energy yields were as high as 31% with an optimum 1.8 eV top cell, only 2.8% lower (absolute) than the AM1.5G predicted efficiency. The annual energy yield of tandem cells with no current-matching constraint, i.e. parallel-connected devices, was also simulated. Here the difference between series and parallel connections were only significant for non-optimum bandgap combinations. Our results indicate that AM1.5G based optimization of sub-cells can be effectively employed to achieve high energy yields of >25% for III-V/Si tandem solar cells in mid-latitude US locations, despite the continuous variation in spectra throughout a calendar year.

A Broad Frequency Range Harmonic Reduction for Cascaded-Power-Cell-Based Islanded Microgrid With Lumped PCC Filter

Series connection of low-voltage power cells has been considered as an alternative microgrid configuration to supply rated voltage power to point of common coupling (PCC) loads. However, in the previous studies, the major focus was on the power regulation of system. Similar to the conventional shunt-distributed generation (DG)-unit-based islanded microgrid, the PCC voltage harmonic problem can be serious without any active control. In order to provide enhanced power quality in this system, a flexible PCC harmonic voltage compensation approach is developed via the hierarchical control of series-connected power cells. First, the selected low-order PCC harmonic voltage components are mitigated by using the closed-loop equivalent impedance shaping method, via cooperation among the PCC central and power cell local controllers. Second, the medium-range lumped output LC filter resonance is suppressed with the help of internal virtual impedance, which is implemented only at the power cell local controllers. Finally, the high-frequency switching ripples of PCC voltage are mitigated by phase-shifting pulsewidth modulation in distributed power cells, through exploring the output current signal of power cells as an inherent synchronizer for carrier shifting control. It has been experimentally validated that with the proposed approaches, the PCC voltage quality can be significantly improved in broad frequency range without any interferences.

Vertically integrated high voltage Zn-Air batteries enabled by stacked multilayer electrodeposition

We report a scheme for the integrated fabrication of MEMS-scale, series-connected battery cells. The approach exploits multilayer electrodeposition of active and sacrificial layers to generate structures that can be formed into high voltage power sources. Aqueous, non-vacuum-based fabrication, via electrodeposition, enables high throughput and deterministic control of layer thicknesses and battery performance. Batteries manufactured using this technique offer the potential to match supply voltages to system needs, ranging from conventional electronics to direct drive of high voltage electrostatic or piezoelectric MEMS.

Active cell balancing control strategy for parallel connected LiFePO4 batteries

While several recent studies have focused on eliminating the imbalance of energy stored in series-connected battery cells, very little attention has been given to balancing the energy stored in parallel-connected battery cells. As such, this paper aims at presenting a new balancing approach for parallel LiFePO 4 battery cells. In this regard, a Backpropagation Neural Network (BPNN) based technique is employed to develop a Battery Management System (BMS) that can assess the charging status of all cells and control its operations through a DC/DC Buck-Boost converter. Simulation results demonstrate the effectiveness of the proposed approach in balancing the energy stored in parallel-connected battery cells in which the state of charge (SoC) estimation error is found to be only 1.15%.

The Influence of Solar Spectrum and Concentration Factor on the Material Choice and the Efficiency of Multijunction Solar Cells

In this work, we revisit the theoretical study on the conversion efficiency of series-connected multijunction solar cells. The theoretical method, based on the detailed balance model, is then applied to devices with 2 to 6 junctions under different illumination conditions. As results, (i) we show that the peaks in the efficiency distribution occur for recurrent values of bottom junction bandgap energy corresponding to atmospheric absorption in the solar spectrum, and (ii) we demonstrate that variations in the number of junctions, in the incident solar spectrum, and in the concentration factor lead to changes in the optimum bandgap energy set but that the bottom junction bandgap energy only changes among the recurrent values presented before. Additionally, we highlight that high conversion efficiencies take place for a broad distribution of bandgap energy combination, which make the choice of materials for the device more flexible. Therefore, based on the overall results, we propose more than a hundred III-V, II-VI and IV semiconductor material candidates to compose the bottom junction of highly efficient devices.

Cell Voltage Equalizer Using a Selective Voltage Multiplier with a Reduced Selection Switch Count for Series-Connected Energy Storage Cells

Cell voltage equalization is mandatory to eliminate voltage imbalance of series-connected energy storage cells, such as lithium-ion batteries (LIBs) and electric double-layer capacitors (EDLCs), to ensure years of safe operations. Although a variety of cell equalizers using selection switches have been proposed, conventional techniques require numerous switches in proportion to the cell count and are prone to complexity. This paper proposes a novel cell voltage equalizer using a selective voltage multiplier. By embedding selection switches into the voltage multiplier-based cell voltage equalizer, the number of selection switches can be reduced in comparison with that in conventional topologies, realizing the simplified circuit. A prototype for twelve cells was built, and an equalization test using LIBs was performed. The voltage imbalance decreased down to approximately 20 mV by the proposed equalizer, and the standard deviation of cell voltages at the end of the equalization test was as low as 10 mV, demonstrating its equalization performance.

Wireless sensor node remote supply using a compact stacked rectenna array with voltage multipliers at 2.45 GHz

AbstractThis paper presents compact rectenna arrays for ambient RF energy harvesting on the 2.45 GHz ISM band. The arrays are based on four and nine series-connected rectenna cells. Each cell is composed of a stacked fractal antenna and an RF-to-dc conversion circuit. The antenna is a compact third Koch fractal shape, fed by a coaxial probe for more compactness. The conversion circuit is a full-wave rectifier with a differential output, each DC polarity is provided by a two-stage Dickson voltage multiplier. Measurement results show a significant increase of the output DC voltage for the one, four, and nine cells rectenna arrays. They provide, for power density of 1.7 μW/cm2, an output DC voltage of 0.9, 2.2, and 4.1 Volts, respectively. The 9 cells rectenna array is used in a remote supply experiment of a temperature and acceleration wireless sensor, where the data are transmitted via a Bluetooth low energy link to a distant smartphone every 1 min.