Rodica Negroiu

Accurate modeling of supercapacitors for DC operation regime

Supercapacitors or Electrochemical Double Layer Capacitors (EDLC) have been intensively studied in the last years for promising applications in vehicle traction used in parallel with batteries or fuel cells, but also for energy storage purposes. The investigations presented in this paper offer an electric model that can be used to model the charge/discharge behavior in DC regime or low frequency pulses. In this sense, the methods for characterization are according to standard IEC 62391. It should be mentioned here that the models include a nonlinear capacitance and all elements are temperature dependent. The values of the circuit elements are valid for the specific capacitor, but the procedure for testing can be used in general situations.

Comparison between Zubieta model of supercapacitors and their real behavior

The supercapacitor is a relatively young electronic component which bases its operation on the principle of the Helmholtz double layer of charge formed on the surface of an activated carbon (with an extremely high equivalent surface) and an electrolyte which allows both a short distance between the electric layers as well as a very large useful area. For optimal use of an EDLC (Electrochemical Double Layer Capacitor) it is necessary to know the time behavior, specific electrical characteristics (discharge current, charge current, voltage variation at the terminals, equivalent series resistance, and especially leakage current (self-discharge current), as well as behavior over a long time - days or even months - under electric charge at different voltages, etc.) and the influence of environmental operating conditions (temperature, humidity, vibration, etc.). For modeling in more detail the behavior of an EDLC in time and voltage a comparison between the Zubieta model and actual behavior was performed. The research is focused on the accuracy of the Zubieta model related to the real behavior of supercapacitors.

Comparative studies on dimming capabilities of retrofit LED lamps

These days many variants for lighting systems are available on the market, and new solutions are about to emerge. Most of the new lamps are offered in form to be retrofitted to existing sockets and luminaires. In this paper, are presented some systematically investigations on different lamps as LEDs, Compact Fluorescent Lamps (CFLs), tungsten, and new available Cold Cathode Fluorescent Lamps (CCFLs), regarding the light level, dimming performances and also the resulting flicker and power distortion performances. The light level was expressed by the illuminance level, measured for all lamps in the same conditions, at the same distance and on the same surface represented by the photometer probe.

Research and development of a system for measuring electrical parameters of EDLC

The batteries used to store electricity in industrial applications or electric vehicles sold so far have the disadvantage of a limited number of charge / discharge cycles (10000 for lead-acid batteries) or high price (NiMH and Li-Ion). Also have a long time for full charge. This paper presents an automatic system and a study of the electrical energy storage in a battery EDLC (Electrochemical Double Layer Capacitor-) that allows 300,000 charge / discharge cycles. It can also study its variation over time and temperature behavior. The leakage current has a large variation in time and, especially, is dependent on ambient temperature. The electronic system developed enables automatic determination of the evolution in time of parameters (ten days) of EDLC without human supervision. The paper presents specific features of EDLC families which are very important for developers of applications in automotive and green energy storage.

Supercapacitors — An alternative electrical energy storage device

In the last time, the power supplies issues of electronic circuits had become an important issues, especially for mobile devices. The use of classical or Li-ion batteries in the industry, mainly utilized in storage systems, is not a viable solution because these devices, after 2–3 years, lose their ability to store electricity and the materials employed are polluting, even banned in some countries. The European Union regulation like RoHS 1 and RoHS represent sometimes a real bottleneck. Fortunately in the last time on the market more and more are present electronic passive components like supercapacitors. The Supercapacitors (Electrochemical Double Layer Capacitors — EDLCs) are the promising solution for the future. They do not contain lead or other polluting materials on a side and on an other side they are able to storage large quantity of electrical charge. EDLCs have a very long lifespan (> 106 unloading cycles) and very small internal resistance, which allows charging and discharging at very high currents (hundreds of A). Also, another advantage of using EDLCs battery systems in power structures is a decrease of the weight of the entire system. We have simulated and analysed the behaviour of a 100F/12V EDLC system by connecting 6 supercapacitors (600F/2.5V) in series. In this context, using an EDLC system, we have obtained interesting results after the primary power source has been disconnected. The proposed model is in the R&D phase, but the test results are promising. EDLC is a very viable solution for use in a hybrid electric system such as a UPS or electric car.

Electrochemical impedance spectroscopy for different types of supercapacitors

Electric Double Layer Capacitors (EDLCs) are relatively new electronic components developed in the late 1980s; their operation is based on a Helmholtz double layer of charge formed on an activated carbon surface (of high equivalent surface) and an electrolyte. The advantages of this constructive solution are a very small capacitive distance and large effective capacitive area, leading to remarkably large capacitances. EDLCs tend to be low voltage, low impedance devices, sustaining high currents, and with a capacitance dependent on the charge-up voltage. In this context it is important to accurately know their parameters in order to design the associated circuitry — in particular of interest are values for equivalent series resistance (ESR) and parallel resistance (self-discharge) of the EDLC. The results presented in this paper are focused on a comparison between using the electrochemical impedance spectroscopy (EIS) method and the direct method (DM) in determining super capacitor parameters. This comparison was performed in the 10−… 105 Hz range, albeit (due to parasitic components) only the 10−…103 Hz range is effective.

Investigations on balancing circuits for supercapacitor modules

Supercapacitors or Electrochemical Double Layer Capacitors, briefly named EDLC, are very promising energy storage components for applications in automotive industry where are intended to be parallel connected to lead-acid or Li-ion batteries or fuel cells. At the beginning of development are the applications in green energy domain, for storage purposes. The need for higher rated voltages of capacitors makes necessary the construction of modules using series connection. The balancing circuits that are investigated in this paper look to prevent the overcharging of capacitors that can occur in the charging process due to inherent uneven parameters of individual supercapacitors. The active balancing circuit that is presented is a cost-effective solution that brings benefits over the simple passive methods for balancing, especially for high power applications.

A research of the characteristics of materials used in the construction of EDLCs

Supercapacitors, also known as EDLC (Electric Double Layer Capacitor), are passive components with exponential accumulation of electrical charge. This paper presents the characteristics of EDLC depending on the materials used in their construction. Thus, using different materials for the electrodes and/or for the electrolyte can influence EDLC performance. The most common electrode is carbon based and is used with an organic or aqueous electrolyte. EDLC has three main forms: active carbon, carbon-based aerogels and carbon nanotubes with organic or aqueous electrolyte.