2.2.2. Hybrid Electric Vehicles (HEVs)

SCs also have application in hybrid electric vehicles. This type of vehicle combines an ICE group and an electric drive. The aim of the electrical system is to operate at the point of maximum efficiency of the motor at each situation. The main elements are a fossil combustion engine, an electric traction machine (generally a permanent magnet machine), a power converter and an ESS. According to the arrangement of these elements, four different configurations are distinguished: (1) series based on ICE, (2) parallel based on ICE, (3) series-parallel based on ICE, and (4) plug-in based on ICE.

In the series configuration the heat engine and the electric drive share the traction shaft (see Figure 11a). In the parallel configuration, the thermal motor and the electric drive are connected through a transmission element (see Figure 11b). In ICE based on series-parallel, the hybrid double conversion, the heat engine and the electric drive are interconnected through an electrical link as shown in Figure 11c. The ICE-based plug-in has the same characteristics as the parallel-based ICE, but it allows one to recharge the battery with an external charger (see Figure 11d).

**Figure 11.** Simplified electrical diagrams of different traction drive configurations in HEV adapted from [44]: (**a**) ICE-based series; (**b**) ICE-based parallel; (**c**) ICE-based series-parallel; (**d**) ICE-based plug-in.

In the literature, there are several studies which propose to include SCs in HEVs to improve their efficiency and reduce the cost of their powertrain. Passalacqua, M. [45] describes the future possible advantages of a serial versus parallel architecture due to the development of the technology of the power elements (power electronics and ESS) that make up the powertrain of a medium size HEV. An SC-based ESS is proposed as a storage system. Results obtained in simulations are presented where fuel consumption is shown for different speed profiles (road missions) and serial vehicle configurations. The vehicle studied is based is equipped with a diesel engine, SiC power converters and SCs in series configuration. The paper concludes that the serial configuration and the proposed energy management system (EMS) achieve an energy saving of 35–48% compared to conventional configurations.

Passalacqua, M. [46] also described and analysed how the sizing of SCs affects engine number of starts (comfort) and the amount of energy that can be stored during braking. A review of the characteristics of a series configuration in HEVs is made and SCs are studied as the only storage system to make series configuration more efficient compared to the parallel configuration in a HEV. An EMS and seven experimentally measured road missions are proposed to calculate the required energy from the ESS to recover all the braking energy. From the point of view of the need for storage energy, mountain mission is the most demanding. The energy needs for the road missions studied are 150–200 Wh with

an approximate weight of 40–50 kg. It is suggested that, if LiCs are used instead of EDLC technology, the storage weight would be reduced to 25–35 kg.

#### 2.2.3. Electric Vehicles (EV)

EVs are vehicles with a pure electric powertrain, i.e., without another power source of a nature other than electric [47]. Figure 12 shows an overall diagram of the traction system in this type of vehicle. In general, the parts that make up the electric drive of an EV are: an ESS (electrochemical battery, fuel cell), a power converter, and an electric traction drive. Depending on the ESS, a braking resistor is necessary (e.g., fuel cells as power source) or not (battery) to dissipate the braking energy.

**Figure 12.** Simplified electrical diagram of the traction drive of EVs.

As in the previous sections, there are studies which analyse the inclusion of SCs in the electric drive of an EV to improve its dynamic response and reduce the cost of the ESS because of the higher battery lifetime. One of these studies is [48], in which a HESS made up of batteries and SCs is analyzed to be used in EVs. Two options of EMSs are proposed, one based on a low pass filter with a fuzzy logic controller and another based on an adaptive proportional integrator. This control distributes the power supplied between the battery and the SCs during acceleration and stores the energy during braking in the SCs considering their SoC. A simulation study has been done for different drive cycles (New York City cycle, Artemis urban cycle, and New York composite cycle). The conclusions of the paper are as follows. The proposed control strategy allows to reduce the variation of the voltage, higher SoC of the battery and efficiency, lower losses in the battery, a reduction of the current ripple through the battery, and a slight increase in temperature. In this case, no economic analysis is performed to compare this configuration to one that uses only batteries.

Sadeq, T. [49] also proposed an EMS for a HESS (battery/SC) to improve the dynamic response of EVs. The proposed topology is a battery/SCs system in semiactive configuration [50]. In order to distribute the power delivered by each ESS, two adaptive algorithms (optimal adaptive and fuzzy adaptive controller) are compared taking into account the profile traveled by the vehicle. Three profiles are simulated in MATLAB-Simulink at three different speeds, demonstrating that the useful life of the batteries is increased due to the reduction of the stress on the battery during high load operations. On the other hand, it is concluded that the response of the vehicle with an optimal adaptive controller is better than with a fuzzy adaptive controller in most cycles.

Additionally, the research described in [51] proposes a traditional topology of a HESS (battery/SC) to improve the drawbacks of using a ESS made up only of batteries (high cost, low power density and short cycle life). For the sizing of the ESS and the power distribution, a bilevel multiobjective design and control framework with the nondominated sorting genetic algorithm (NSGA-II) and fuzzy logic control (FLC) is described. The authors conclude that a good EMS allows reducing the dimensioning of the SCESS (size and volume), achieving the same dynamic response as with a larger mass of the HESS.

Finally, regarding the most common topologies, Shi, R. [52] proposed a new topology of a HESS comprising batteries and SCs is proposed to distribute the power needed by an open winding motor. This topology connects both ESS to the electric motor through a power converter (dual in-verter drive), which makes it easier for both the batteries and the SCs to deliver the active/reactive power requirements (see Figure 13). This allows the use of machines with a higher voltage level than those that can be used with a traditional configuration. During the periods in which the required power is low, SCs remain in stand-by to improve the efficiency of the set. Experimental tests are carried out with a 10-kW liquid-cooled EV motor.

**Figure 13.** Single stage HESS topology adapted from [52] for its integration in EVs.

#### 2.2.4. Wireless Charging of EVs

This section describes the existing papers in the bibliography that propose the use of SCs in wireless power transfer (WTP) systems for EV recharging. The selected papers have been those considered most relevant in the last three years. In general, the use of an ESS, in this case SCs, is justified to reduce the high fluctuations in power consumption and allow the power flow in the charger to be bidirectional, increasing its versatility. The papers that have been considered most relevant are described below.

Lu, W. and Geng, Y. [53,54] propose the inclusion of a HESS (SCs/lithium-ion batteries) in WTP systems. Lu, W. [53] proposed a methodology for sizing the HESS for recharging a 100kW electric bus during the time the vehicle is at one of the stops on the route. The proposed methodology is applied to a laboratory prototype in which the output power is analyzed based on the power transfer distance, reaching an efficiency of 89.6%. Additionally, Geng, Y. [54] focused on the control strategy from the point of view of the efficiency and cost of the system. The proposed strategy is based on the dynamic distribution of power between both ESS to get the WPT system to work at the optimum power point throughout the entire operating range. The strategy is validated in a 27.8 kW laboratory prototype. The efficiency achieved of the WPT System is 93% with a reduction of the required power of 27.6% compared with non optimal control charging.

Azad, A. [55] proposes the use of SCs as the only ESS to dampen power grid fluctuations that occur in dynamic and wireless power transfer (DWPT) in EVs charging. It seeks to ensure the stability of the network to which the charging point is connected. A complete analysis of the system is carried out through simulation including the design of the regulator and the modeling of the converter, achieving a reduction of grid transients by 75%. Ruddell, S. [56] presents a new topology of a WTP system, also with SCs, for the dynamic recharging of EVs. Experimental tests are carried out on a 3.8 kW prototype highlighting the advantages of the proposed topology over the conventional ones due to its reduced complexity and lower number of required switches. Finally, Wu, Y. [57] also proposes SCs as an energy buffer for dynamic loading in WPT systems. The ESS stores energy when the coupling is strong and discharges when the coupling is weak. To

optimize the dimensioning of the ESS (smallest capacitance) and the pole spacing between two adjacent transmission coils and capacitance of SCs (maximum distance) a genetic algorithm is used. The proposed method is verified by simulations to demonstrate that the WPT system allows for increasing the power density and reducing the construction cost compared with WPT systems without SCs.

### **3. Renewable Generation Applications**

There are two main problems in the operation of generation plants based on renewable energy sources. The first is related to the fluctuating nature of the renewable resource and, as a consequence, the fluctuation of the electrical power generated. Power fluctuation injected into the grid can cause the variation of the point of common coupling (PPC) voltage and affect the system's stability [58]. The second problem is related to the operation and behavior of the renewable generation system when there are disturbances at the grid point where it is connected. In general, this section describes the existing papers in the bibliography that include SCs as a possible solution to the problems discussed. This section is divided into three main points depending on the renewable resource: wind, solar energy, and wave energy.
