Supercapacitors: The Innovation of Energy Storage

Abstract

In addition to the accelerated development of standard and novel types of rechargeable batteries, for electricity storage purposes, more and more attention has recently been paid to supercapacitors as a qualitatively new type of capacitor. A large number of teams and laboratories around the world are working on the development of supercapacitors, while their ever-improving performances enable wider use. The major challenges are to improve the parameters of supercapacitors, primarily energy density and operating voltage, as well as the miniaturization, optimization, energy efficiency, economy, and environmental acceptance. This chapter provides an overview of new techniques and technologies of supercapacitors that are changing the present and future of electricity storage, with special emphasis on self-powering sensor and transmitter systems. The latest achievements in the production, modeling, and characterization of supercapacitor elements (electrode materials, electrolytes, and supporting elements) whose parameters are optimized for long-term self-supply of low power consumers (low voltage, high energy density, and low leakage current, etc.) are considered.

1. Introduction

For decades, science has been intensively researching electrochemical systems that exhibit extremely high capacitance values (in the order of hundreds of Fg⁻¹), which were previously unattainable. The early researches have shown the unsuspected possibilities of supercapacitors and traced a new direction for the development of electrical energy storage systems [1]. In recent times, with the development of new materials and technologies, very large developed surfaces and very small inter-electrode distances have been achieved. In many materials, enormous pseudocapacitance is also expressed, which achieves extremely large capacitances (several orders of magnitude larger than standard capacitors), so such systems are called supercapacitors (supercapacitors or, more rarely, ultracapacitors) [2].

There are two types of supercapacitors, depending on the energy storage mechanism: electric double-layer capacitors and pseudocapacitors [3]. In the first case, it is an electrostatic principle, and in the second one, the charge storage is caused by fast redox reactions [4]. Some electrode materials have both one and the other mechanism, thus so-called hybrid capacitors are formed on their basis.

High needs for powering isolated systems (sensor networks and IoT), covering peaks of electricity consumption, filtration, and new more efficient topologies in power electronics require further development and improvement of supercapacitor properties. More on this topic is given in Section 2 of this chapter.

New technologies and new materials in this area are presented in Section 3 of this chapter. Fabrication, modeling, and characterization are presented in Section 4.

Supercapacitors are widely used due to their high-power density, that is, fast charge and discharge, and a huge number of charge/discharge cycles [5]. Increasing the performance of supercapacitors opens up new fields of application and attention will be paid to this in Section 5. The growth of the industry in this area causes a drop in prices, which will be discussed in Section 6.

2. Need for supercapacitors

Since the energy harvesting from renewable energy sources is highly actual today, the studies are also focused on the diverse methods for storing this energy in the form of electricity. Supercapacitors are one of the most efficient energy storage devices. As they have many advantages, supercapacitors are continuously being used in devices and systems that are eager for a high-power supply, opposite to the batteries. Recently actual, supercapacitors’ applications are driven by their high performance and market potential, placing them in many fields of interest, such as industrial control, power, transportation, consumer electronic products, national defense, communications, medical equipment, electric and hybrid vehicles [6, 7, 8].

Nowadays, with the rapid development of intelligent electronic devices, have placed flexible energy storage devices in the focus of researchers. The industry requires energy storage that are flexible and optimized but endowed with high electrochemical properties [8, 9, 10]. The advantages of the supercapacitors, such as charge-discharge cycle life, size and weight, and environmentally oriented, suiting them for various applications. Supercapacitors are being used more and more as applications require storing and releasing high amounts of energy in short periods. Current industry applications include the automotive industry, hybrid transportation systems around the world, grid stabilization, utility vehicles, and rail-system power models [11].

The storing of energy is one of the main applications of supercapacitors. Following their outstanding power characteristics, supercapacitors are vital for the energy sector and their stationary applications. Additionally, the low maintenance requirements, as well as the extreme conditions that supercapacitors are able to withstand, make them suitable for renewable energy-related applications [12, 13]. Furthermore, the supercapacitors provide substantial benefits to railway electricity systems and the aerospace industry, since these sectors are trying to achieve a more electric power supply [13, 14]. Furthermore, many systems in the industrial sector are using supercapacitors, including small vehicles, such as forklifts, shovel trucks, agricultural machinery, excavators, mining shovels, harbor cranes, and industrial lasers. Consumer electronics are relying on supercapacitors, especially in real-time clock or memory backup, power failure backup, storage applications in which supercapacitors are used instead of batteries, and high load assistance to the primary electrical energy storage systems [13].

3. New technologies and materials for supercapacitors

Supercapacitors are increasingly used for energy storage due to their large number of charge and discharge cycles, high power density, minimal maintenance, long life span, and environmental friendliness [15]. The only disadvantage over batteries, the lower energy density, is decreasing more and more thanks to the intensive development of new technologies and new materials. Hybrid electrodes that combine double-layer (electrostatic) capacitance and pseudo (redox) capacitance are increasingly being used [16].

The latest nanotechnologies have given rise to nanomaterials, such as 2D graphene, 1D CNT, and 0D fullerene [17]. There is a high trend in the research of carbon-based electrode materials (CNT, graphene, fullerene and their composites with metal oxides, etc.), copper sulfide and other metals, and metal oxides, all in combination with appropriate electrolytes.

For years, the authors of this chapter have researched the possibility of using natural copper minerals, primarily chalcosine [18] and coveline [19].

The latest research in the field of perovskite oxide applications for supercapacitor electrode materials deserves special attention. Perovskite oxides based on lanthanum, strontium, and cerium, etc. are being researched [20].

The researches with polymer materials are of great interest as well, which, in addition to high power density, also has an acceptable energy density (Figure 1). Conductive polymers are materials that contain a conjugated double bond, which places them in the group of materials that exhibit good electrical conductivity. Apart from showing electrical conductivity, these materials are characterized by redox reactions that take place when the electrode is polarized in a certain potential range. As a consequence of the redox reaction, constant currents are recorded in a wide range of potentials, which indicates the continuous development of the redox reaction in the investigated potential range. The resulting response is similar to the electrochemical response of charging/discharging the double layer and therefore these reactions are called pseudocapacitive reactions. This property enables the application of these materials in supercapacitors. In order to further improve the properties of conducting polymers, attempts are being made to increase their electrical conductivity and porosity [21].

Figure 1.

Ragone plot shows the energy vs. power density comparison of supercapacitors with the other energy storage devices [21].

In Figure 2 a comparative review of current supercapacitor electrode materials has been provided. Carbon materials have a specific capacitance of up to 300 F/g, while polymer and metal oxide materials can have over 1000 F/g. Composites of two or more of the above materials can have a very high specific capacitance of over 2000 F/g. Recently, the predominant approach is the development of binary or ternary nanocomposites of different capacitive materials to determine and optimize the structures and physical and mechanical properties of the electrode materials, in order to achieve improved supercapacitor performance. However, the properties of composite electrodes, in addition to the individual active components, also depend on the morphology and characteristics of the interphase [21].

Figure 2.

Comparison of various materials according to their specific parameter for supercapacitor applications [21].

Special, often complex technologies are developed for the production of superior electrode material. In Figure 3 one of them is shown. A schematic diagram of the entire process of MnNi₂O₄@MnNi₂S₄ electrode materials is presented. Ni²⁺ and Mn²⁺ form Mn-Ni precursors in the reactor and are then calcined at high temperature to produce oxides. Then, under the influence of sodium sulphide, ion exchange is carried out at the appropriate temperature, i.e. oxygen is replaced by sulphur, which is less electronegative. A core-shell structure is formed without changing the morphology of the oxide. Metal ions react with KOH electrolyte in the Faraday redox reaction [22].

Figure 3.

The schematic diagram of the construction process for MnNi₂O₄@MnNi₂S₄ electrode [22].

4. Production, modeling, and characterization of supercapacitors

Supercapacitors fill a wide area between storage batteries and conventional capacitors. Both from the aspect of energy density and from the aspect of power density this area covers an area of several orders of magnitude. Materials, construction, production technology, and test methods are chosen depending on the part of the area covered by a certain type of supercapacitor.

4.1 Construction and production of SC

The construction of the laboratory prototype and zero series begins with the choice of material and type of supercapacitor (asymmetrical - polarized, or symmetrical - nonpolarized). The variant with a solid active material, which the authors of this chapter have often used, is shown in Figure 4. The connection of the active material with the power supply is achieved by using conductive silver glue. The role of a separator is provided by a Nafion foil soaked in a selected electrolyte.

Figure 4.

Construction of supercapacitor prototype [23].

Today, the active material is applied from a suspension, as well as the printing techniques that are also applied for micro SC (Figures 5 and 6).

Figure 5.

Configuration of supercapacitors in various casings: (a) in laminated cells and glass cells; and (b) in coin cells [24].

Figure 6.

(a–c) Schematic diagram of the fabrication process for micro-supercapacitors by laser scribing method. (d, e) Flexible micro-supercapacitors with high areal density [25].

After testing and optimization, a zero series is made, where the technological processes of production are elaborated (and often simplified) and only then the mass production could start. The appearance and structure of a commercial supercapacitor of low power and capacitance are shown in Figure 7, and for higher powers, voltages, and capacitances, it is shown in Figure 8.

Figure 7.

Structure of Murata’s supercapacitor (cross-section) [26].

Figure 8.

Supercapacitor engine start module [27].

Depending on the field of application and the set of parameters, the final structure, production technology, and housing are selected. If the supercapacitor is used as a replacement for a battery to power small consumers, in addition to the capacitance, it is very important that it has a low leakage current. In contrast, when powering larger consumers, it is much more important that the supercapacitor has a low series resistance due to losses at high currents. Thus, the type of supercapacitor is defined. As an example, in Figure 9. the types of supercapacitors are given for the different needs of power supply backup.

Figure 9.

Performance for supercapacitor selection [28].

4.2 Modeling and characterization of SC

In order to predict the behavior in different conditions, electrochemical processes are modeled and simulated on the computer [2]. Most often, an equivalent electric circuit with two or more RC branches is taken as a model (Figure 10) [29].

Figure 10.

EDLC models: A - ideal capacitor, B - series RC model, C - model B with added leakage resistor, D - model C with added high-frequency inductance component, and E - model D expanded with n-branch RC circuits and voltage-dependent main capacitance [29].

For many years, the authors of this chapter have done research in this area and have developed their models. The simplified model will be presented here for the case of sulfide minerals in an aqueous solution of sulfuric acid. The equivalent circuit, shown in Figure 11, was adopted. Capacitors represent double-layer and diffusion capacitance, and resistors correspond to electrolyte resistance, diffusion resistance, and leakage current.

Figure 11.

The equivalent electric circuit.

For the assumed equivalent electric circuit of the observed electrochemical system and a short voltage pulse (of the order of 0.1 s for such systems), the response of the system (current in this case) will be as shown in Figure 12. The following parameters are:

1. $I=\frac{E}{R_0+R_{123}}$ - quasi-stationary charging current.

2. $R_{123}=\frac{R_1{R}_2{R}_3}{R_1{R}_2+R_1{R}_3+R_2{R}_3}$ - eq. resistance of parallel connection R1, R2, and R3.

3. $I_{10}=\frac{-U_{C_{10}}}{R_1+R_{023}}$ - initial discharge current.

4. $R_{023}=\frac{R_0{R}_2{R}_3}{R_0{R}_2+R_0{R}_3+R_2{R}_3}$ - eq. resistance of parallel connection R0 , R2, and R3.

5. $U_{C_{10}}=\frac{I_2\bullet ∆t}{C_1}\frac{R_2}{R_1+R_2}$ - initial discharge voltage of capacitor C1.

6. $I_{20}=\frac{-U_{C_{20}}}{R_2+R_{03}}$ - quasi-stationary discharge current.

7. $R_{03}=\frac{R_0{R}_3}{R_0+R_3}$ - eq. resistance of parallel connection R0 and R3.

8. $U_{C_{20}}=\frac{I_2∆t}{C_2}\frac{R_1}{R_1+R_2}$ - initial discharge voltage of capacitor C2.

9. ${\tau }_1=\left(R_1+R_{023}\right)C_1$ - time constant of the first discharge phase.

10. ${\tau }_2=\left(R_{21}+R_{03}\right)C_2$ - time constant of the second discharge phase.

Figure 12.

The circuit excitation and the response.

Based on the given analysis, the parameters of the electric circuit can be determined, and therefore, in the same manner, the physical parameters of the electrochemical system [30].

In order to check the provided model, an electrochemical system was formed. Several experiments were performed using this method and it has been verified and determined that it can be applied for rapid characterization of electrochemical systems. The experiments were carried out using a system for electrochemical tests based on a PC and the LabVIEW software package (Figure 13) [31]. The system is designed to cover most electrochemical tests in wide ranges, both at the level of the electrochemical cell, and at the level of the completed supercapacitor (Figure 14) [32].

Figure 13.

System for electrochemical testing.

Figure 14.

Block diagram of the system for supercapacitor prototype characterization.

Modeling can also be done on a physical level. For example, a finite element model for charge transport in conjugated polymers has been developed, but it is still being refined [33].

5. Use of supercapacitors

From the user’s point of view, the most environmentally friendly form of energy is electricity. However, if we take into account the way in which this energy was obtained, then it fully retains its ecological advantage only if it originates from solar energy, wind, and wave energy, and to a considerable extent, from hydropower. In the latter case, namely, significant harmful ecological effects may occur due to disturbance of water regimes, such as surface waters. However, even when the electricity comes from the burning of fossil fuels or from nuclear reactions, problems related to the negative effects of by-products can be solved much more efficiently in one place than, for example, in all vehicles that move using the appropriate energy. One of the conditions for the use of electric energy in vehicles is the existence of such a device that would have high specific energy but at the same time a high specific power, which standard electric devices could not provide. The advent of supercapacitors has made this application much more realistic.

In the case of supplying consumers with energy that comes from solar or wind energy, it is necessary to have an appropriate method of energy storage for the period when there is no sun or wind. In this case, supercapacitors have an advantage over standard batteries because they can withstand a much greater number of charging and discharging cycles.

From a very large number of possible applications of supercapacitors, current examples will be listed where their characteristics are irreplaceable.

5.1 Energy harvesting

Monitoring of environmental parameters requires the installation of systems in inaccessible and dangerous terrains. After installation, the system is expected to have a lifetime as long as possible with minimal maintenance. In addition, energy consumption is directly related to the lifetime of a wireless sensor network (WSN). It is similar to other scattered and/or remote systems. Primary (non-rechargeable) batteries, despite the application of modern energy management algorithms, have the greatest impact on the limited lifetime of a wireless sensor node. Also, regular technical interventions in the field, primarily battery replacement, drastically increase the cost of maintenance. With the aim of increasing the life span and reducing maintenance costs, current research studies involve the use of secondary (rechargeable) batteries and the so-called collecting energy from the environment, that is, “energy harvesting” (EH), which contributes to WSM getting the self-powered prefix. Due to the characteristics of secondary batteries that degrade over time, the increase in lifetime is insufficient for multi-year monitoring of certain environmental parameters. This is the reason why, instead of rechargeable batteries, capacitors with very large capacities— supercapacitors—can be used to power the node. They represent reversible electrochemical systems, and they are increasingly used to power sensor nodes. For several reasons, supercapacitors are favorable for power supply, one of them being the exceptional scalability that allows increasing capacity and performance with increasing dimensions and weight. The characteristics of supercapacitors, such as high-power density, fast charging, large number of charging cycles, temperature stability, small equivalent series resistance, and low leakage current, favor the operation mode of most wireless sensor nodes. However, the lower energy density compared to batteries contributes to the fact that they are discharged relatively quickly and require frequent recharging. That is why it is necessary to provide a constant or at least intermittent source of energy in the natural environment. It can be a solar panel, piezo vibration transducer, thermoelectric generator, antenna, etc. [34]. Possible sources of environmental energy are shown in Figure 15.

Figure 15.

Environmental energy sources [35].

5.1.1 Piezoelectric effect

Piezoelectric materials have the property of converting energy through the direct piezoelectric effect, the energy of mechanical deformations of the piezoelectric structure into the electric field, that is, voltage [36]. Two modes, that is, mode 33 and mode 31, are used in most developed EH applications (Figure 16). In both of these modes, the electric field, and thus, the generated voltage on the electrodes, are oriented along the direction of polarization 3, while external forces cause stresses in a single direction. In mode 33, this is the same direction as the stress (3), while in mode 31 the stresses are along the normal (1) [37].

Figure 16.

Operational modes of piezoelectric material for EH applications [37].

5.1.2 Electromagnetic conversion

Electromagnetic induction, described by Faraday’s law, is the creation of electromotive force (EMF), that is, voltage on an electric conductor in a changing magnetic field, a phenomenon that forms the basis of electric generators. The induced EMF is proportional to the strength of the magnetic field, the speed of the relative movement, and the number of turns of the conductor. If a conductor is connected to an electrical load, the current will flow, thus generating electricity. This system is often used as an effective tool for the realization of kinetic EH systems, where the relative displacement of the permanent magnet in relation to the coil is caused by the vibrations of the energy generation base [38].

5.1.3 Comparison of piezoelectric and EH devices with electromagnetic vibrations

The results of a thorough comparison of electromagnetic and piezoelectric vibration EH systems, with identical volumes, seismic masses, natural frequencies, quality factors, and excitation conditions, are given in [39]. Appropriate mathematical models of both types of vibration EH devices were used for the calculation of output voltages and powers (Figure 17) and depending on the intensity and frequency of harmonic dynamic excitation, the recommended configuration of the most efficient vibration EH systems [39].

Figure 17.

Model of electromagnetic (a) and piezoelectric (b) vibration EH systems for output power analysis [39].

5.1.4 Magneto-strictive effect

Physically in a way similar to the piezoelectric effect. A characteristic property of magneto-strictiveness is that the magneto-elastic coupling induces mechanical elongations if they are subjected to a magnetic field, while conversely, their magnetization will change due to changes in the applied mechanical stresses. This effect can be used in EH devices, however, an additional coil is required to obtain electrical energy [35].

5.1.5 Photovoltaic panels

Solar panels are easy to purchase and install. The price is getting lower, so they are a good choice for collecting energy from the environment. Since the panels provide a DC output, they can be plugged directly into the electronics’ power system. The output power can be from mW to MW depending on the size of the panels as well as the intensity of the light they receive. However, the main limitation is that they need to be exposed to sunlight, so they can only work during the day. Batteries and/or supercapacitors are necessary for power supply at night. Energy storage is also necessary for cloudy or snowy days [40].

5.1.6 Thermoelectric systems for energy harvesting

In addition to mechanical energy, a temperature difference is also a very rich source of energy; therefore, often considered a viable option for the development of EH systems. To convert heat into electricity, thermoelectric generators (TEG) use the available temperature differences between two surfaces [35].

Thermoelectric devices for energy production or cooling consist of two types of thermoelectric materialsthermoelements. These are p-type and n-type semiconductors. Thermocouples are electrically connected in series and thermally connected in parallel. The working principle of TEG is shown in Figure 18.

Figure 18.

Thermoelectric device principle [40].

5.1.7 Wind energy harvesting

Wind-based energy harvesting is increasingly pursued due to the ubiquitous nature of the source, as well as complementarity with other sources (e.g., solar). In particular, vortex-induced vibrations (VIV) are investigated [41].

Vortex-induced vibration energy harvesting mechanisms can be divided into five categories, namely, flutter, VIV, galloping, wake-galloping, and hybrid-type flow-induced vibrations. VIV energy harvesters are designed for regions with low wind speed [42].

5.1.8 Storaging of harvested energy by supercapacitors

Regardless of the source of clean renewable energy, it is necessary to have a circuit to store the energy generated from the energy harvesting source. When a DC voltage is applied to a discharged supercapacitor, it is charged, and thus stores electrical energy. Since these are small consumers (sensors, transmitters, and IoT), today’s supercapacitor can often replace batteries and be more durable and environmentally friendly.

Most consumers require a higher operating voltage than a single supercapacitor can provide. In systems requiring higher voltages, supercapacitors are usually connected in series. In series-connected supercapacitors, however, a balancing circuit such as this, is required to distribute the voltages across the individual elements equally. There are two types of balancing circuits: passive balancing and active balancing [43].

A team of scientists from the American UCLA and the University of Connecticut [44] designed a system that is powered by electrical impulses from the human body. It is a “biological supercapacitor” that uses charged particles and ions, from the fluids in the human body. The device is not dangerous for the body and it can be used in pacemakers and other implants that require a power supply (Figure 19).

Figure 19.

Energy harvesting power management [44].

The block diagram of a system for collecting the energy of light radiation (natural or artificial) is shown in Figure 20. A supercapacitor with a capacity of 400 mF was used.

Figure 20.

Energy harvesting power management [45].

Another example of ultra-low power management, with a supercapacitor for energy storage (1.5 F) is shown in Figure 21. MOSFETs are used to rectify the output voltage of a wind energy harvester exposed to low wind. The proposed algorithm enables the monitoring of the maximum output power at time-varying wind speeds. A microcontroller was used to provide a source and sink impedance matching [46].

Figure 21.

Interface circuit for the harvester [46].

5.2 Smart cities

Following the smart city concept, supercapacitors have the potential to be involved in the creation of greener, sustainable, and efficient powering systems. One of the most prominent examples is public transport. By using the distributed energy sources in the urban smart environments, the power sources become DC based including the photovoltaics cell, and fuel cell, etc. As the urban environments are designed with many distributed power sources connected to the distribution lines, energy storage takes a significant place in the system. Battery energy storage systems and supercapacitor energy storage systems, as well as hybrid ones, may be installed both on large and small scales, which makes them the ideal fit for the smart city concept [47].

The smart city concept cannot be imaginable without sensor networks and Internet of Things devices and applications. As the energy requirement in sensor devices is increasing, the energy has to be stored for the blackout periods. Considering that the batteries are not a permanent solution, the supercapacitors serve as a solution for high-energy storage applications that require high-voltage and high-current drive [48]. Recent studies show that the supercapacitors are well suited for a wide range of applications, such as IoT, consumer products, white goods, office automation, long-term battery backup, and energy harvesting [48]. In order to overcome the powering issues that may occur at the remote nodes, as well as in the extreme weather conditions, fully functional IoT devices have been designed based on energy harvesting with supercapacitors and batteries as storage elements [47].

5.3 Smart grid

In recent years, the economic trends have been dictating the renewable energy sources generation of electric power. Therefore, the concept of the microgrid has been introduced as an off-grid or grid-connected energy system that can work independently or collaboratively with other microgrids [49]. In general, such a system can provide electric power either from a single source or multiple sources, such as wind and solar energy, adding energy storage to the system [50, 51].

The supercapacitors are being used to regulate the microgrid voltage and to improve the system stability. In recent studies, the supercapacitor provides the error component of the battery current in the proposed control scheme. This is an addition to the microgrid, as it improves the microgrid voltage regulation capability, as well as extends the battery lifetime [52, 53].

5.4 Energy systems

Supercapacitors are increasingly used in both AC power systems (EES) and DC power sources. With the development of the voltage balancing technique of serially connected SCs, a great improvement in the HVDC transmission system is expected. The large capacity of SC provides enough energy storage for small consumers in a short time, and their main advantage in energy systems is high power density, so they can cover large consumption peaks. In combination with power electronics circuits, SCs can inject energy into the EES at the right moment, thus opening a whole new field of development of circuits and control algorithms. A large field of application of SC in DC power supplies is low-pass filters with previously unimaginable parameters.

5.4.1 Quality of electricity in AC systems

Experts realized a long time ago that the quality of electricity affects the quality of work and life. Many norms from that area were applied, but the real progress started at the end of the last century. The problem began to be approached globally. CENELEC (European Committee for European Standardization) was established [54]. Most European countries have accepted the CENELEC standard EN 50160 [55] for voltage monitoring at the point of delivery to the consumers under normal conditions.

The LEM corporation (NORMA, ELMES, ELSIS, and HEME) developed a series of measuring devices MEMOBOX for monitoring the quality of electricity, and thus, began a new practice supported by the EN 50160 standard, but also by newly created standards (e.g., IEC 1000-3-6/71).

As connecting the national and European power systems is a necessity, it is also necessary to adapt local legislation and standardization in this area. The specificity of electricity is that its quality is influenced to a greater extent by consumers (non-linear loads) than by producers. That is why the consumer is, to a considerable extent, a partner of the supplier in ensuring the quality of electricity. At the same time as electrical energy becomes dirty, the consumer is also sensitive to this kind of contamination. From the point of view of the application of supercapacitors for the elimination of short-term disturbances, the following terms should be highlighted [55]:

5.4.2 Frequency change

Under normal operating conditions in the distribution network connected to the power system, the ten-second mean value of the frequency during 99.5% of each week must be within 50 Hz ± 1% and 50 Hz + 4% / - 6% in the remaining 0.5% of the week.

In disconnected (island) networks, the limits are 50 Hz ± 2% during 95% of the week and 50 Hz ± 15% during the remaining 5% of the week.

5.4.3 Flicker

The need to define and measure that parameter is created from the fact that the change in light intensity in the working or living environment negatively affects people’s health, that is, their work and other efficiencies. Headaches, nervousness, depression, and vision impairment, etc. occur. Flicker is defined as follows: If there are 100 people in a room under equal conditions, and if the light intensity changes so much that 50 of the 100 people notice it, the flicker is said to have an intensity of 1. Flicker is a consequence of amplitude modulation supply voltage with frequencies in the range from 1 to 33 Hz, where the amplitude is a direct function of that frequency. For example, at a frequency of 8 Hz, the nominal voltage fluctuation amplitude is about 0.256 % of the nominal value (e.g., 0.59 V from 230 V∼).

5.4.4 Voltage failure

Voltage failures occur most often due to faults in the consumer’s facilities or in the public distribution network. They are defined as follows: failure (partial loss of voltage) is sudden (unpredicted), short-lived (from 10 ms, up to 1 minute) reducing the supply voltage to one of the values in the range of 90%, and up to 1% of the nominal voltage, after which the nominal voltage is restored. The permissible guideline number of voltage drops during one year is ranged from 10 to 1000. Most of them must have a duration of less than 1s and an amplitude of less than 60% of the nominal voltage.

5.4.5 Power failure

It is a state in which the voltage at the point of transmission is less than 1% of the nominal voltage. The following power interruptions are distinguished:

• Planned interruptions of supply, about which consumers are informed in advance, in order to enable the planned works to be carried out in the network, and

• Accidental interruptions caused by permanent or transient disturbances they usually appear in conjunction with other disorders.

The following random interruptions are distinguished:

• Long-term interruptions (longer than 3 minutes), caused by permanent failure

• Short-term supply interruptions (up to and including 3 minutes), caused by a transient fault.

Figure 22 shows an example of a short-term power failure.

Figure 22.

Example of a short-term power failure [54].

Figure 23 shows the configuration of the system supercapacitor in the control area of the power system. The converter (VSC) consists of a rectifier/inverter with 6-pulse control and pulse width modulation (PWM) with an IGBT bridge. The PWM converter and the DC-DC converter (chopper) are connected by a DC link capacitor. A bidirectional DC-DC converter operates in step-up mode if electrical power is supplied to the supercapacitor bank from the power system. Smoothing inductance is used for current transfer and filtering [56].

Figure 23.

Configuration of supercapacitor bank in the control area [56].

5.4.6 Supercapacitors in DC systems

Supercapacitors are most often polarized, and due to their enormous capacitance, they are most often used in DC systems. The most application is in the accumulation of electrical energy to cover shorter production stops and large consumption peaks, but they are widely used in DC voltage filtration and other purposes in power electronics circuits.

An example of an independent photovoltaic system with supercapacitors for energy storage is shown in Figure 24.

Figure 24.

PV panel and supercapacitor controller system scheme [57].

The operating voltage of the supercapacitor can be kept within range by properly sizing the supercapacitor and monitoring the upper and lower limits of the charge and discharge controller (SOC). The rate of change in the supercapacitor is proportional to the charge current isc. To extract the maximum available power from a PV panel, it is necessary to operate the PV at its maximum power point (MPP). The MPP monitoring device is a high-frequency DC-DC converter, a chopper-voltage booster inserted between the PV panel and the DC bus [57].

An example of a PV system connected to the distribution network, where supercapacitors are also used, is shown in Figure 25. The power generated by the PV panel is connected to the grid using buck converters and inverters. A buck chopper is used to regulate the variable voltage from the PV panel. Its duty cycle is continuously adjusted to instantly locate the maximum power output from the PV panel at varying irradiance and temperature. To minimize the fluctuation in the generated power, a bidirectional buck chopper is used to connect the energy storage to the DC link. The energy storage (supercapacitor bank) is continuously charged and discharged by a buck chopper to absorb or release the required power between generated and transmitted to the grid. The step-up chopper controls the supercapacitor voltage and the DC link voltage. An independent VSC active and reactive power regulator was implemented to inject available power into the AC network [58].

Figure 25.

PV panel and supercapacitor connected on grid [58].

A hybrid system for electrical storage based on supercapacitors and batteries is shown in Figure 26. In a hybrid system, the peak load power is supplied from the supercapacitor and the batteries provide lower constant power for a longer time.

Figure 26.

Supercapacitor-battery hybrid energy storage in PV system [59].

The authors of this chapter have designed a sample PV system with supercapacitors and batteries for energy storage (Figure 27). A system for monitoring energy parameters was developed, and several algorithms of energy management and MPPT were also implemented. In Figure 28 data acquisition block diagram is given. Analog channel AI0 monitors the temperature of the PV module in which the LM35 chip is installed. Channels AI1 to AI4 monitor voltages V1 to V4 (from Figure 27). Figure 29 shows the basic LabVIEW monitoring application. Based on the measured voltages, the currents and other relevant parameters are calculated. Additionally, all the parameters and data are displayed, saved, and taken to further processing (implementation of the given algorithms).

Figure 27.

Schematic of PV/batt/SC system.

Figure 28.

Acquisition block diagram.

Figure 29.

LabVIEW block diagram of the monitoring system.

The authors of this chapter have also set up a system with supercapacitors for injecting energy into the DC link of the self-excited asynchronous generator - rectifier - DC link with the supercapacitor - inverter - asynchronous motor system. Engine start-up and optimal energy management were tested. Due to the relatively high operating voltage (up to 300V DC), a bank of series-connected supercapacitors with passive voltage balancing was used, the efficiency of which was tested by thermal imaging (Figure 30). It is obvious that at point Sp1 the heating is significantly increased (58.40C), which means that passive voltage balancing should be improved (reduce the resistance of parallel resistors).

Figure 30.

Photo and thermal image of serially connected supercapacitors.

5.5 Electric vehicles

Vehicles with electric drive represent one of the most significant ecological advances, bearing in mind the prevalence of this type of contamination of nature. In the world, there is also interest in hybrid vehicles that have lower fuel consumption and significantly lower emission of harmful products compared to classic vehicles. In the most general form, hybrid vehicles can be described as vehicles that use a combination of technologies for energy production and storage. It combines the good features of conventional vehicles (long range and acceleration and excellent fuel supply network) and electric vehicles (zero emissions, quiet operation, and use of braking energy).

Time has shown that it is not necessary to immediately build a complete network of charging stations in order to increase the sale of electric vehicles since users are ready to charge the batteries of their vehicles at home. The possibility of supermarkets, parking garages, and restaurants offering charging stations to customers is mentioned as the next step.

5.5.1 Application of supercapacitors in EV

Certain characteristics of supercapacitors make these devices suitable for purposes in which a combination of high specific energy and high specific power is required, or a long service life expressed by the number of charging and discharging cycles. Namely, supercapacitors retain the positive property of standard capacitors that they can achieve an almost unlimited number of charging and discharging cycles.

From the point of view of the application, there are several groups of supercapacitors. Depending on the place of application, different characteristics of the supercapacitor come to the advantage more or less. Some of them are of crucial importance for the selection of capacitors, and some may be unimportant. The strictest requirements are set for capacitors used in electric traction, that is, in electric vehicles. Batteries with a capacity of several hundreds of farads and with an operating voltage of several hundred volts are already being made. In addition to high capacitance and relatively high operating voltage, these capacitors must have high specific energy and power (due to limited space in the vehicle). In terms of specific power, they have a great advantage over storage batteries, but they are, therefore, incomparably weaker in terms of specific energy. That is why the ideal combination is a combination of accumulator and capacitor batteries (Figure 31). In steady mode (normal traction), the vehicle’s engine is powered from the battery, and during sudden acceleration from the supercapacitor. Especially important is the fact that during sudden braking, all mechanical energy can be returned to the system by converting it into electricity only with the presence of a supercapacitor with high specific power. For the above reasons, the internal resistance of such supercapacitors must be extremely low. The leakage current is not relevant [60, 61].

Figure 31.

Block diagram of an electric vehicle with a hybrid power supply.

B - accumulator, SC - supercapacitor; DC/DC - converters of direct voltage; R - regulator; M-G - motor-generator (depending on the operation mode); and W - wheels.

The wheels are driven by a regulated electric drive. Regulated electric motor drives are developing very quickly and are placing ever stricter demands on speed (and position) and torque regulation before designers. From the energy point of view, it is desirable that their participation be as large as possible, because, by setting the speed of the drive to the optimal or necessary one, it is possible to save on the energy consumed.

A typical variable speed electric motor drive contains [62]: A big challenge with electric vehicles is the necessary increased operating voltage to power the electric motor (due to the reduction of losses), so supercapacitors are connected in large series strings. This reduces the equivalent capacitance, and also leads to voltage balancing problems on individual cells. The first problem is solved by connecting several series strings in parallel, and the second by passive (cheap and bad) or active (expensive and excellent) voltage balancing.

• Between the source and the motor, a converter that adapts the characteristic sizes of the source: frequency, voltage, current, and number of phases to the needs of the motor. By dosing that energy, engine control is also achieved;

• Between the engine and the load, a mechanical transmission that adjusts the speed and torque of the engine to the speed and torque of the working mechanism (load);

• Information from all the mentioned elements (source, converter, motor, transmission, and from the load) is collected by the regulator (controller), which is based on the set (desired) parameters, performs automatic control of the drive

In an asynchronous motor, at a constant frequency and amplitude of the supply voltage, the rotor speed depends on the load moment, which requires complicated control algorithms in cases where precise speed and/or position control is required. This phenomenon is a consequence of the principle of operation of an asynchronous motor, which is electromagnetic induction and requires a difference in speed between the rotor and the rotating magnetic field generated by the stator in order for the electromagnetic torque to exist. The electronics that realize the aforementioned algorithms were expensive, therefore, it made it difficult to use asynchronous motors for such purposes. However, today, with the low cost of electronic components and the use of computers in the realization of regulation algorithms, they are increasingly used.

The introduction of supercapacitors and power electronics assemblies based on DC voltage interfaces lead to a significant improvement in the performance of electric vehicles, such as acceleration, use of braking energy, and reduction of dimensions. Blocks of high-power supercapacitors are now also installed in large vehicles (buses and rail vehicles). In Figure 32 a complete green energy system is presented, where supercapacitors play an important role.

Figure 32.

Green transport [63].

5.6 Power electronics

Supercapacitors are, and in the future will be, increasingly used in power electronics assemblies of medium power, where they serve as reservoirs of electrical energy in the transition mode. There is a real possibility that they will soon replace bulky inductances, which are also huge sources of electromagnetic interference. In such an application, the supercapacitor must have both high capacitance and a relatively high operating voltage (which implies regular binding of the cells and all related problems). The internal resistance must be quite small, and the leakage current is not of major importance.

In Figure 33 a schematic of a buck-boost converter with a supercapacitor for accepting braking energy is given.

Figure 33.

A typical modern supercapacitor system with a bi-directional converter with SC [64].

In Figure 34 the scheme of the SMPS converter with supercapacitor is given and Figure 35 shows the supercapacitor in the inverter circuit.

Figure 34.

Supercapacitor system with SMPS converter [65].

Figure 35.

Supercapacitor system with switch mode rectifier in the inverter [66].

6. Economy efficiency

Nowadays, the industry is focusing on improving product performance and reducing production costs. The supercapacitor components improve themselves, not only in the manufacturing process and technology, but in the direction to find stable and effective electrode and electrolyte materials, and to improve the performances, as well as to reduce the cost [8].

Following their properties, various industries and applications use supercapacitors, since they bring features, such as a safe, eco-friendly, and economical source of energy to the industry. It has been noted that the global supercapacitor market is supposed to attain US$ 8.3 billion by 2025. This market is predicted to enhance at a compound annual growth rate (CAGR) of 30% until 2025 [65, 66]. As provided in the recent reports, the cost of the material is a major constraint of market growth. Even though these disadvantages inhibit the supercapacitors’ market growth, there is a high number of research groups that are focusing on minimizing the overall cost, and improving the supercapacitors adoption rate in the current market [66, 67]. CO₂ emission regulations that have been regulated in many countries worldwide, also have stimulated the usage of supercapacitors in the industry. This green agenda secures the future development of the supercapacitors, improvement of their properties, and their further research [13].

Acknowledgments

This research has resulted from the Projects funded by the Ministry of Education, Science and Technological Development, Government of the Republic of Serbia, grant numbers 451-03-9/2021-14/ 200135, 451-03-9/2022-14/ 200131, and 451-03-9/2022-14/ 200223.