*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**.

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

**Figure 24.** *PV panel and supercapacitor controller system scheme [57].*

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

**Figure 25.** *PV panel and supercapacitor connected on grid [58].*

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].

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].*

**Figure 27.** *Schematic of PV/batt/SC system.*

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).

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 28.** *Acquisition block diagram.*

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

**Figure 29.** *LabVIEW block diagram of the monitoring system.*

**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].

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

#### **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.*
