A Novel Highly Integrated Hybrid Energy Storage System for Electric Propulsion and Smart Grid Applications A Novel Highly Integrated Hybrid Energy Storage System for Electric Propulsion and Smart Grid Applications

This chapter addresses potentialities and advantages of a highly integrated hybrid energy storage system (HESS) for electric propulsion and smart grids. This configuration consists of a highly integrated battery-ultracapacitor system (HIBUC) and aims to benefit from the advantages of both passive and active HESS configurations. Particularly, the integration of the ultracapacitor module (UM) within the DC-link of the DC/AC multilevel converter enables the decoupling between DC-link voltage and energy content without the need for any additional DC/DC converter. As a result, HIBUC benefits from simplicity and energy flow management capabilities very similar to those achieved by passive and active HESS configurations, respectively. This is highlighted properly by a theoretical analysis, which also accounts for a comparison between HIBUC and both passive and active HESS configurations. Some HIBUC application examples are also reported, which highlight the flexibility and potentialities of HIBUC for both electric propulsion systems and smart grids.


Introduction
Electric energy storage systems (ESSs) are widely recognized as one of the most promising technology for enabling the transition toward a sustainable energy system [1][2][3]. Particularly, transportation electrification is pushing toward progressive improvements of ESS technologies, especially for light-and heavy-road electric vehicles: these have to rely on on-board ESSs batteries (Li-ion) are surely the best solution for modern electric vehicles due to their very highenergy density and specific energy. However, sodium-based batteries may be preferred when high capacity is required, namely for load leveling and renewable energy sources integration. Despite their advantages, electrochemical batteries generally suffer from low power capabilities; thus, even if research is focused on improving power density, they are not yet the best solution for high-power applications [13,14].
Differently from electrochemical batteries, high-power density ESSs can provide much little amount of energy but in very short times, which is the case of flywheel energy storage system (FESS), superconducting magnetic energy storage system (SMES), and ultracapacitors (UCs). Particularly, FESS is characterized by very high efficiency, long life expectancy, and low environmental impact. However, it presents a quite high self-discharge rate and also suffers from safety issues as far as high speeds are concerned [15][16][17]. Regarding SMES, a cryogenic system ensures the superconducting state of coils. Consequently, losses are due to power converters only, leading to a very high overall efficiency. Other advantages consist of fast response and wide power range, as well as long lifespan. Nevertheless, SMES is very expensive due to the high costs of both superconductors and cryogenic system; thus, although it has been recently tested for power quality and voltage stabilization in both transmission and distribution systems, it is still employed in military applications mostly [18][19][20]. The main advantages of UCs are high-power capability, quite long life cycle and no memory effect, but they cannot store great amount of energy unless big and costly UC modules are used. Thus, UCs have been used for power quality applications and for handling small regenerative braking on electric propulsion systems [21][22][23].
Based on the previous considerations, it can be stated that a single ESS technology hardly matches both energy and power application requirements. Particularly, electrochemical batteries are very suitable for providing energy services, in which high-energy storage capability is mandatory. On the other hand, FESS, SMES, and UCs are more appropriate for power services, when high-power rates are required for very short times. In addition, further constraints may regard efficiency, life cycle, and cost, which might make one ESS technology unsuitable. In this regard, a viable and promising solution is the employment of a hybrid energy storage system (HESS), which consists of combining high-energy and high-power density ESSs in order to benefit from the advantages of different ESS technologies [24][25][26][27]. As a result, HESS may bring increased performances, higher efficiency, longer lifetime, reduced costs, and more appropriate design and sizing. Among all the ESS combinations, HESSs made up of electrochemical batteries and UCs are the most popular and promising solutions because of the perfect complementarity between their features [28][29][30].
Consequently, this chapter focuses on HESS made up of a battery pack (BP) and an ultracapacitor module (UM). Particularly, a brief overview of main HESS configurations and management approaches is provided in Section 2. Then, Section 3 focuses on a highly integrated HESS configuration [31][32][33], in which BP and UM are coupled only by means of a multilevel converter. This highly integrated battery-ultracapacitor system (HIBUC) is also compared to those described in Section 2, highlighting its most important advantages. In Section 3, two HIBUC application examples are also presented and discussed based on numerical simulations. Concluding remarks are reported in Section 4.

Overview on hybrid energy storage systems
Considering a hybrid energy storage system (HESS) made up of a battery pack (BP) and an ultracapacitor module (UM), a number of configurations have been proposed in the literature, which differ from each other mainly due to the number of power electronic converters involved. Particularly, no or few power electronic converters entail simple configurations, but poor flexibility and energy management. As the number of power electronic converters increases, energy management capability, and, thus, HESS exploitation increase, but at the cost of increased complexity, costs, volumes, and weights. The following sections investigate the most important HESS configurations, as well as their management and control strategies, by highlighting their most important advantages and drawbacks.

HESS configurations
HESS configurations can be roughly classified into passive and active configurations; in passive HESS, BP and UM are directly coupled to the DC-link of the DC/AC converter, as depicted in Figure 3 [34,35]. Therefore, BP and UM share the same voltage, which generally varies with BP state-of-charge. As a result, UM voltage cannot vary independently, resulting in its poor energy exploitation. Still referring to Figure 3, the following relationships can be introduced: Particularly, a simple but effective BP model has been considered, which consists of a voltage source V b series-connected with an internal resistance r b , while i b is the battery current. BP is coupled to the DC-link through an inductive filter, whose inductance L should prevent i b from unsuitable sudden variations, V DC being the DC-link voltage. Referring to Eq. (2), C u denotes the capacitance of the UM, whereas i P denotes the power current, which is proportional to the power drawn or delivered by the DC/AC converter. The DC-link energy content E DC and its time-variation can be expressed, respectively, as: The comparison between Eq. (2) and Eq. (4) reveals a significant coupling between E DC and V DC , which cannot be varied within a wide range. This results in a poor UM exploitation, as well as in an unsuitable energy management, which are the main drawbacks of this HESS configuration. Consequently, in spite of its simplicity and cheapness, passive HESS configuration is rarely used due to weak performances that make it not suitable for most applications.
Much better performances can be achieved by semi-active and active HESS configurations, which exploit one or more DC/DC converters for decoupling BP and UM [36][37][38][39][40][41][42][43]. Particularly, Figure 4 shows an active parallel configuration, in which BP and UM are parallel-connected to the DC-link both through DC/DC converters. Referring to this configuration, the following relationships can be introduced: in which, C is the capacitance of the DC-link, which is much smaller than C u . Whereas Particularly, Eqs. (6) and (7) have been achieved by assuming ideal DC/DC converters, which have been modeled by simple gains (k b and k u for BP and UM, respectively). Therefore, the combination of Eq. (5) with Eqs. (6) and (7) leads to the following expressions: Regarding the DC-link energy content, it should account not only for C, but also for C u in order to make the comparison with all HESS configurations consistent. Consequently, E DC and its time derivative can be expressed as Therefore, considering both Eqs. (9) and (11), it can be seen that V DC and E DC can be controlled independently by setting the duty cycles of the DC/DC converters properly. However, Eqs. (9) and (11) are characterized by increased complexity compared to passive HESS configurations. Furthermore, these equations highlight a significant coupling among all system variables, which makes control system design a not trivial issue. Consequently, advanced management and control systems are required in order to exploit active HESS configurations properly.

HESS management and control
As far as semi-active or active HESS configurations are concerned, the management and control system cover a fundamental role for exploiting the HESS at the maximum extent. Particularly, an appropriate selection of HESS management strategy is of paramount importance, even from the design stage, especially for sizing BP and UM properly in accordance with target performances, and technical and economic constraints. It is fundamental also for assuring HESS efficiency, reliability, and durability.
Referring to Figure 5, HESS management consists mainly of a sharing criterion for splitting the overall HESS energy flow between BP and UM. Literature review reveals that several approaches have been proposed in order to exploit BP and UM inherent features to the maximum extent, preventing them from unsuitable operation as well. In this regard, UM generally handles fast power fluctuations, whereas BP copes with the average power demand. This principle is the basis of the simplest HESS management strategy known as frequency-based management (FBM); this consists of splitting the overall power demand into high-and low-frequency components, which have to be tracked by UM and BP, respectively [44,45]. An alternative approach is the so-called rule-based management (RBM), which exploits the single ESSs in accordance with an appropriate order of priority by means of a pre-set of rules [46,47]. In this regard, it is worth noting that FBM and RBM may be combined to each other or with fuzzy logic algorithms in order to account for ESS constraints and to improve overall HESS performances [48][49][50][51].
Although FBM and RBM are intuitive, simple, and easy to implement, they generally do not lead to optimal solutions. For this reason, another popular approach is determining BP and UM reference power profiles by minimizing suitable cost functions over a given time horizon.
Hence, different optimal solving techniques can be used, such as model predictive control, mixed-integer/linear programming, nonlinear programming, and dynamic programming [52][53][54][55]. However, such solving techniques are generally complex to implement and quite time-demanding. Consequently, heuristic approaches have been also proposed, like genetic algorithms and particle swarm optimization, which achieve sub-optimal solutions but faster and with less computational efforts [56][57][58]. As a result, very complex and sophisticated cost functions can be considered, which can account for many system constraints and goals. The main advantage of these approaches consists of enabling HESS to provide multiple services in an optimal manner, by both economic and technical points of view; this aspect makes HESS very competitive, especially for smart grid applications.

A novel highly integrated HESS
In order to overcome the issues arising from both passive and active HESS configurations, a highly integrated solution has been proposed by the authors in [31][32][33], whose schematic representation is depicted in Figure 6. It consists of coupling a BP with an UM through a multilevel converter, namely a three-level neutral-point-clamped converter (NPC). The key feature of the proposed highly integrated battery-ultracapacitor system (HIBUC) is the full integration of UM within the DC-link of the NPC, which decouples the overall DC-link voltage (V DC ) from its energy content (E DC ). As a result, HIBUC energy flow management is quite similar to that achieved with active HESS configurations without resorting to any DC/DC converter, as detailed in the following sections.

HIBUC modeling
Still referring to Figure 6, the DC-link of HIBUC is split into high-side and low-side due to the three-level configuration: high-side consists of the UM, whose overall voltage and capacitance are denoted by V u and C u , respectively, while the low-side is made up of conventional capacitors, V and C being the corresponding voltage and capacitance. Hence, HIBUC main equations can be expressed as Figure 6. The highly integrated HESS configuration proposed in [31][32][33].
Whereas, the overall DC-link energy content can be determined easily as Based on both Eqs. (12) and (13), the dynamic equation of the DC-link voltage can be achieved as where C DC and i DC are defined as the equivalent DC-link capacitance and current, respectively. Furthermore, by time-differentiating Eq. (14) and combining the result with (13), the following relationship is achieved: Hence, Eq. (16) reveals that E DC depends on both i b and i P . However, since the latter is generally imposed by the application requirements, E DC can be regulated successfully through i b only, while V DC can be driven independently by means of i DC , as pointed out by Eq. (15). This occurs as far as i DC and i P differ from each other. Otherwise, both V DC and E DC time variations would be proportional to the difference between i b and i P , thus E DC and V DC control decoupling cannot be achieved. Therefore, considering both Eqs. (15) and (16), the following relationship can be introduced: Consequently, since A must be non-singular, the following constraint is achieved: Hence, if Eq. (18) is satisfied, i DC and i P can differ from each other; this does not occur in conventional NPC configurations, which are characterized by equal voltages and capacitances. Whereas, HIBUC suitably exploits DC-link capacitance and voltage unbalances, leading to a decoupled control of V DC and E DC .

Comparison with passive and active HESS configurations
In order to highlight the advantages of HIBUC compared to passive and active HESS configurations, reference can be made to the per unit DC-link energy content (e DC ), which is defined as follows:  (14), the following results are achieved: where the superscripts (P), (A), and (H) denote passive, active and HIBUC configuration, respectively. In addition, α is the capacitance factor, which is much greater than 1 because C u is much greater than C. Furthermore, ξ is the UM voltage share, which is equal to k u in the case of active HESS configuration. It is worth noting that ξ ranges from 0 to 1 for the HIBUC configuration and that the same reasonably occurs also for active HESS configurations.
Hence, considering the e DC evolutions with ξ depicted in Figure 7, different considerations can be made for each HESS configuration and for a given α value. Particularly, referring to passive HESS configuration at first, e DC does not depend on ξ, as already pointed out by Eq. (20). This is because the voltage across UM always equals V DC , thus ξ = 1 over any operating conditions. As a consequence, once α has been set, E DC can vary only with V DC . However, since V DC should be kept almost constant in order to supply the converter properly, a poor UM exploitation is achieved, as expected.
Considering now active HESS configuration, minimum and maximum e DC values are always achieved for ξ = 0 and ξ = 1, respectively, as pointed out in the following: Therefore, the maximum exploitation of the DC-link energy content can be achieved by varying ξ within [0,1] as Hence, differently from passive HESS configuration, active configuration enables the DC-link energy content to vary with ξ without the need of changing V DC . However, it is worth noting that the operating range of ξ is much narrower than [0,1] because ξ equals k u , which is constrained by DC/DC converter maximum and minimum duty cycle capabilities.
Focusing now on the HIBUC configuration, the minimum e DC value is achieved in correspondence of the following ξ value: Whereas, given that α is greater than 1, the maximum e DC value is reached for ξ = 1, as pointed out in the following: Consequently, maximum exploitation of the DC-link energy content is achieved if ξ varies as follows: The evolutions of both Δξ and Δe DC with α are depicted in Figure 8. Particularly, for low α values, active HESS configuration shows superior performances compared to HIBUC, which is characterized by limited UM voltage range, and thus, poor DC-link energy exploitation. However, HIBUC performances rapidly increase with α, they becoming very close to those achieved by active HESS configurations for relatively high α values. Since α should be quite high due to the huge capacitance of UM, e.g., hundreds or even thousands, HIBUC is a very competitive solution even versus active HESS configuration because it can assure very similar performances in terms of energy flow management. In addition, HIBUC benefits also from a multilevel converter, whose increased complexity and costs are counterbalanced by improved output voltage and current waveforms due to the availability of multiple voltage levels.

HIBUC management and control
The overall HIBUC management and control scheme is depicted in Figure 9. The comparison with Figure 5 reveals some differences, especially due to the high degree of integration among all HIBUC components. Particularly, the NPC has to account for HIBUC needs in terms of energy flow management, thus it cannot be driven only in accordance with application requirements. Regarding the HIBUC management block, it has to synthesize the most suitable BP current profile (i b * ) based on the chosen HIBUC management approach; this is then tracked Figure 7. The e DC evolutions with ξ for α = 10: passive, active, and HIBUC configurations.
by means of the HIBUC control system, which can be designed in accordance with both Eqs. (12) and (15). As a result, a suitable i DC * is achieved, whose implementation is guaranteed by means of advanced PWM patterns that account for both HIBUC and application requirements, as well detailed in [33,59,60].
In conclusion, the structure of both HIBUC management and control blocks generally depends on the specific application, as well as on the kind of power and energy services HIBUC has to provide. For this reason, two application examples are reported in the following, for each of which a much in-depth analysis of these blocks is presented, as well as some simulation results.

Electric propulsion system
The HIBUC management and control blocks for the highly integrated electric propulsion system proposed in [31][32][33] are depicted in Figure 10. Focusing on the HIBUC energy management at first, the idea is to exploit the UM over acceleration and regenerative braking mainly. Consequently, the reference E DC profile is set in accordance with the actual motor/ vehicle speed (ω m ), namely E DC should decrease properly as ω m increases in order to enable UM to release its energy content gradually during acceleration, as well as storing it back  during regenerative braking. In this regard, different maps can be chosen depending on UM sizing and on the kind of assistance it has to provide to BP in supplying the traction motor over these operating conditions, as pointed out in [32].
Once E DC * has been set, its tracking can be accomplished through a PI regulator (R E ), which can be designed in accordance with Eq. (16), but expressed as Then, in order to overcome the dependence of P b from V DC , it is possible to multiply (12) by i b and substituting the result in Eq. (29), leading to the following expression: in which, the magnetic energy variation related to the inductive filter can be neglected safely due to the relative low inductance value. As a result, the reference i b value can be computed as follows: Regarding the HIBUC control block, it consists of two nested control loops: the external loop regulates i b through V DC by means of a PI regulator, which can be designed easily based on Eq. (12). As a result, a reference DC-link voltage profile is achieved, whose tracking is demanded to the internal loop; this determines the most suitable reference i DC profile by means of another PI regulator, which is designed in accordance with Eq. (15).
In order to highlight the effectiveness of the proposed configuration, a simulation study has been performed in MATLAB-Simulink, whose main parameters are reported in Table 1. While simulations results are depicted in Figures 11-15. Particularly, each variable is shown in per unit with reference to the corresponding base value shown in Table 1. Focusing on Figure 11 at first, different speed profiles have been considered for simulating a start and stop of the vehicle, which are characterized by decreasing ramp times (5 s for case 1, 4 s for case 2, and 3 s for case 3). While the corresponding motor torque evolutions are depicted in Figure 12. The latter reveals higher torque demands as soon as faster speed variations are required, namely from case 1 to case 3, as expected. Considering the current evolutions shown in Figure 13, it can be seen that high power demands occur during both vehicle acceleration and braking, especially in comparison with steady state vehicle power requirements. However, BP is prevented from coping with such high and fast power variations, as proved by the low battery current profiles achieved in all cases and still shown in Figure 13. This is due to UM, which is discharged and charged appropriately in accordance with the (ω m , E DC * ) map, as highlighted in Figure 14.
In addition, Figure 15 shows that V u is reduced during vehicle acceleration, but V is increased simultaneously. Consequently, V DC can be kept sufficiently high in order to prevent unsuitable and fast BP current variations. Similar considerations can be made during regenerative braking, in correspondence of which UM is recharged, while BP current is slowly driven to zero. As a result, HIBUC enables the UM to supply the motor on its own mostly during both vehicle acceleration and regenerative braking, thus preventing BP from an unsuitable exploitation.

Smart grid
The HIBUC management and control blocks for a smart grid are depicted in Figure 16.
Particularly, it differs from Figure 10 only in terms of HIBUC management because the tracking of both i b * and V DC * does not depend on the specific application. Therefore, focusing on the HIBUC management only, it can be seen that a frequency-based management approach has been followed [61], namely the smart grid reference active power profile (P e *) is processed by an appropriate low-pass filter in order to extract low-frequency components only. These are further processed by the energy management block, which has to synthesize the reference BP   power profile in accordance with BP energy and power constraints. As a result, the UM has to cope with high-frequency power components only, which are generally characterized by poor energy content. However, since UM has to compensate for system losses, forecasting errors, and relatively low BP dynamic performances, the energy management block accounts also for the DC-link and, thus, the UM energy content in defining the reference BP power profile. Hence, if UM energy level is too low, additional power is delivered by BP with the aim of restoring an intermediate UM energy content. The opposite occurs when UM is almost fully charged, namely BP should draw energy from UM in order to preserve its continuous operation. As a result, the HESS configuration is exploited properly, not only by differentiating the kind of services BP and UM have to provide, but also by enabling a mutual support between the single ESSs.
A simulation study has been carried out in MATLAB-Simulink with reference to the main parameters shown in Table 2. Particularly, HIBUC has been sized differently from the previous application (electric propulsion system), especially in terms of voltage and capacitance ratings, in order to better match the new application requirements. Simulation results are depicted in Figures 17-21; all the results are expressed in per unit with reference to the corresponding base values shown in Table 2. Simulations refer to several active and/or reactive smart grid power variations, as highlighted in Figure 17. In addition, after 3 s, a noise signal has been added to the reference active power in order to test HIBUC performances in preventing BP from coping with such high-frequency power fluctuations. Still focusing on Figure 17, it can be seen that a very fast and suitable reference power tracking is achieved. This is mainly due to the fast UM dynamic response, while a much slower BP power response is achieved due to the employment of a low-pass filter, as highlighted in Figure 18. Particularly, the low-pass filter prevents BP from quickly reacting to reference active power variations, as well as from coping with the noise signal added to the reference active power; this is handled by UM on its own mostly, as still highlighted in Figure 18. Figure 16. The HIBUC management and control scheme for a smart grid. Focusing now on the BP reference power profile, it is made up of two contributions, as pointed out in Figure 19: one comes from the low-pass filter (P̅ b * ), whereas the other contribution (δP b * )

MG parameters and rated values
is provided by a DC-link energy loop similar to that shown in Figure 10. Particularly, δP b * enables BP to slowly drive the UM energy to a suitable intermediate reference value, as shown in Figure 20. This occurs by varying V u and V suitably, as highlighted in Figure 21. It is worth noting that UM may be fully charged or discharged if this control loop is not employed and, thus, unable to cope with fast active power decrease or increase, respectively. In addition, δP b * accounts also for system losses that, if ignored, would force UM to be fully discharged. In conclusion, it is also worthy of note that reactive power variations do not affect both BP and UM power profiles significantly, as highlighted by the comparison between Figure 17 and Figure 18.

Conclusion
This chapter has addressed the potentialities and advantages of a highly integrated batteryultracapacitor system (HIBUC). Particularly, HIBUC benefits from the advantages of both passive and active hybrid energy storage system (HESS) configurations, namely a relative simple structure and an effective energy flow management of both the battery pack (BP) and the ultracapacitor module (UM). This has been proved by the analytical comparison among all the above-mentioned configurations, which reveals HIBUC as a very suitable and competitive solution, especially when high UM capacitance is concerned. In addition, HIBUC is also very flexible, as proved by the two application examples, namely an electric propulsion system and a smart grid. For each of these, HIBUC management and control approaches have been presented and discussed, providing some simulation results as well. These highlight the high level of integration achieved in each application, especially in the design of the HIBUC management stage, which should be done in accordance with the specific application requirements.

Acknowledgements
This work has been developed within the project "Design and Implementation of a Novel Hybrid Energy Storage System for Microgrids," which is funded by the Sardinian Regional Government