Keywords

1 Battery Energy Storage Systems Composition

Wind and photovoltaic generation systems are expected to become some of the main driving technologies toward the decarbonization target [1,2,3]. Globally operating power grid systems struggle to handle the large-scale interaction of such variable energy sources which could lead to all kinds of disruptions, compromising service continuity. Electricity storage systems can help reduce some of the inefficiencies and gaps in the system, helping to increase its reliability, helping to facilitate the integration of renewables, and effectively managing energy production. Furthermore, through the reasoned use of electric energy storage systems, it is possible to facilitate the regulation between supply and demand of electric energy, through a decoupling of electricity production from the load or from the user. Finally, the use of storage systems allows distributed accumulation for the increasingly widespread microgrids, which significantly increase the certainty of electricity supply [4].

There are therefore different types of storage systems, and they are defined as mechanical, electrical, thermal, and electrochemical. Among the categories of storage systems, in recent years, the one linked to electrochemical systems, also known as batteries [5], is becoming more and more interesting. Battery uses are commonly divided into two categories—in front of the meter (FTM) and behind the meter (BTM)—depending on where they are placed within the electrical supply chain. FTM batteries can be found in distribution and transmission networks, utilities, substations, and generation plants. In general, the sizes in terms of capacity of FTM storage systems are in the order of MWh. On the other hand, talking about BTM storage systems, generally they are batteries that are positioned in the final part of the supply chain, the user side. Typically, these batteries are smaller in terms of capacity size and are used in residential, industrial (as backup generators), commercial, and transportation [6,7,8]. Among the various categories of batteries, those based on lithium ions are increasingly used; however, there are different chemical compositions that are used depending on the applications. Lithium-ion batteries, among the most common today, thanks to their high specific energy value (3.86 Ah/g), are used in electric vehicles and also as storage systems to support the grid and can be of different sizes. With that type of chemistry, it is also easy to avoid the memory effect of the batteries; they also have a low self-discharge and are also safe in environmental terms. In addition to high specific energy and high load capacity, power cells have long cycle life and long service life, with little need for replacement. They are characterized by their high specific energy density, low internal resistance, and relatively short recharging time. Among the disadvantages, however, there are the high temperatures and charge levels, which accelerate the degradation in terms of accumulation and, moreover, require a protection circuit that prevents heat dispersion during overloads. This means that high value of C-rate must be avoided, in order to prevent undesired temperature raise, able to generate thermal runaway phenomena, which affect the storage system with fire risk. Another type of battery is lead-acid, cheaper than the previous ones, but less efficient in charge, less durable, and with a limited specific energy and power compared to other technologies [9, 10]. Even if the treatment for their disposal is easier with respect to Li-ion, where innovative methods are studied to recover materials, also lead-acid batteries require a special operation for the disposal; otherwise, they risk becoming harmful to the environment. Nickel batteries, on the other hand, have longer life cycles than lead-acid battery and have a higher specific energy; however, they are more expensive than lead batteries [11,12,13]. Open batteries, usually indicated as flow batteries, have the unique capability to decouple power and energy based on their architecture, making them scalable and modular with moderate cost of maintenance. They are used as energy backup, covering long duration energy storage timeframes up to 1 or 2 weeks, but also load leveling and peak shaving applications for the transmission and distribution of electricity. These batteries have a specific energy significantly lower with respect to Li-ion, generally used for shorter timeframes (up to 8 hours), but flow batteries are simple to update and easily integrated, however, they are an innovative technology and are still being studied and improved today. There are currently new flow batteries in development, but also more mature technologies such as vanadium redox flow batteries (VRFB). In this case for high capacity to power ratio, the cost per stored kWh is lower than for lithium-ion batteries [14]. The batteries are then integrated with other systems, with which they create a more complex architecture defined as battery energy storage system (BESS), which can work with a centralized or distributed architecture. Conventional centralized architectures consist of the following:

  • The battery pack: the electrochemical storage system, which transforms electrical energy into chemical energy during the charge phase, while the opposite occurs during the discharge phase. The energy released during discharging can be used by the user for the various purposes previously described.

  • The battery management system (BMS): The BMS takes care of the correct and safe functioning of the battery. Since each battery has preferential operating conditions, the BMS ensures that these conditions are met. Furthermore, the BMS takes care of monitoring the residual energy inside the battery and its state of health (SOH), so as to optimize performance.

  • The power conversion system (PCS): The PCS is the interface with the grid and allows the DC terminal of the battery to communicate with the AC terminal of the grid. Since the AC current has a certain mains frequency, an electronic circuit called phase-lock-loop (PLL) is used to synchronize the current leaving the battery with that of the mains.

  • The energy management system (EMS): The EMS control unit is the equivalent of the BMS applied not to the battery but to the entire BESS. EMS links all elements of the BESS together and optimizes the performance of the entire system.

  • The safety system: It is generally structured on several levels, each responsible for a specific task.

Subsequently, it should be remembered that each type of BESS has certain technical specifications that characterize the efficiency of the system [15]. It is clear that the first characteristic parameter is the storage capacity, i.e., the amount of electric charge that the battery can accumulate and that the BESS can make available. Another parameter of primary importance is the nominal power, a characteristic that specifies the amount of power that the BESS can transmit. The round-trip efficiency represents the ratio between the energy emitted during the discharge phase and the energy supplied during the battery charge phase. The depth of discharge (DoD) represents the percentage of energy discharged with respect to the maximum capacity. Battery lifetime is also a relevant parameter for choosing the storage system and is calculated through the number of battery charge and discharge periods; otherwise, it can be expressed as the total amount of energy that a battery can supply during its life. Finally, the safety parameter is important in determining the suitability of the battery for a particular use.

2 BESS Application as Grid Integration

Therefore, considering the decarbonization trend in the field of electricity production, it is clear that the development of these storage systems can facilitate the energy transition. In fact, following the decarbonization trend of the various sectors, the national electricity requirement is only increasing, rising the electrical demand. The typical electrical demand curve has peaks at certain times of the day, one in the morning and one in the evening. This demand is compensated for by various energy sources, in a manner compatible with the national energy production mix. Energy sources, both renewable and nonrenewable, have precise start-up times; in fact, depending on the time of day, a specific energy source is used. For example, coal-fired plants require very long start-up times; therefore, the fund of energy demand is met through the use of these plants. Conversely, systems whose start-up is much faster, such as gas cycles, for example, can compensate for the momentary peaks encountered during the day. Qualitatively, the shape of the demand curve is similar in the various days of the week; therefore, the presence of these peaks is almost constant. Finally, the production curve will have to be capable of punctually following the demand curve. However, starting up these gas-fired plants is both expensive and polluting. Therefore, the integration of storage systems within the electricity grid could contribute to the damping of these peaks, making it possible to avoid the start-up of gas-fired plants. This attempt to dampen peak loads is also called peak shaving. The application of the peak shaving technique, through the use of accumulation systems, also helps to avoid grid oversizing, which would be necessary for peak hours. Furthermore, it must also be considered that sudden variations in demand generate grid instability, in terms of voltage and frequency [16]. Such fluctuations would risk damaging the quality of the supply service. The use of the peak shaving technique would therefore make it possible to absorb electricity when demand is lower, and then release it when it is higher. However, it should be considered that the integration of the various renewable resources within the electricity grid, however, only increases the variability of the production curve. Following the principle described above, there is another technique that is establishing itself to facilitate the integration of renewables. If peak shaving technique aims to remove the generation peaks, load leveling is a technique that is used to level the load curve [17, 18]. The operating principle is similar to that of peak shaving, absorbing power from the grid when demand is lower and then returning it when demand reaches its maximum daily values. Therefore, the damping of peak power demand can be facilitated, if accompanied by a prudent tariff policy which, by combining the price of power and energy supplied, makes it convenient for a user to purchase such storage systems. Therefore, the two techniques end up combining perfectly, since the user’s peak shaving operation leads to load leveling for the supplier. Therefore, the user, through an automatic BMS, applies a daily peak shaving, optimizing the management of electricity and, consequently, saving the user. In fact, in doing so it is possible to stipulate a contract with the distributor at a lower peak power, since the peak beyond the maximum established level required by the loads would be filled by the accumulation system. For the supplier, however, this procedure means a leveling of demand (load leveling), improving efficiency in the power generation and transmission phases, avoiding construction costs linked to infrastructural upgrades.

Furthermore, as previously mentioned, the network may be subjected to instability. Among these instabilities are voltage dips. Voltage dip is defined as the temporary reduction of voltage below 90% of the declared voltage for a period greater than or equal to 10 milliseconds and not greater than 1 minute, where the conditions for interruption do not exist (definition taken from standard CEI EN 50160); the unipolar voltage dip is a voltage dip that affects only one phase [19]. These instabilities generally originate from faults in the public network or in network systems, sometimes linked to overloads of starting transients of large motors or to the insertion of significant loads. These holes are unpredictable, and their annual frequency is variable and is not attributable to the network operator or local distributor. In addition, data processing and control equipment can experience data loss and require time-consuming maintenance in the event of a significant voltage sag. In addition to this, voltage sags lead to economic losses. If previously diesel generators such as UPS (uninterrupted power supply) were used to compensate for the existence of voltage dips, today the use of electrochemical storage systems would facilitate the management of voltage dips, thanks to BMS systems [4]. Similarly, it has been studied that when there are fluctuations in the network frequency, due, for example, to the variation of the rotation speed of synchronous generators, the use of batteries considerably facilitates frequency regulation [20,21,22]. Therefore, to condense the various uses applied to the network, it is possible to state the FTM applications to which the BESSs should be applied:

  • Bulk energy services: mass energy service process, which increases the capacity that can be supplied by the electricity system, thanks to the accumulation of massive quantities of energy to meet the peaks. At the same time, it is possible to accumulate electricity in the time slots in which it has a low cost, to sell it at a time in which the demand is higher and the cost of energy is higher.

  • Ancillary services: e.g., all those systems that support the transmission of energy from the production site to the user and that help to maintain the usability of the system. Ancillary services can be generators or connected service providers capable of rapidly increasing the output of potential reserves, regulation, and flexibility (e.g., frequency regulation, voltage regulation, black start).

  • Transmission and distribution network support: Expansion of the transmission network to avoid unwanted congestion can be expensive, and installing a BESS can be more cost-effective. Furthermore, with the growing contribution of renewables to the local distribution system, the power fluctuations in their output can create voltage fluctuations and damage the equipment connected to the system. A BESS can take part in voltage regulation, stabilizing the system.

  • Integration of renewables: The integration of batteries into variable renewable energy production systems helps to give greater stability to the electricity grid. Capacity firming is widely used, for example, with the production of wind and solar energy and has advantages such as the optimization of the generation profile, supply, and exchange of reactive power in support of renewables and balancing of load currents [23].

3 BESS Integration in Transport Sector

If on the side of the electricity system, electrochemical storage systems represent a great opportunity, within the transport sector they represent a necessity. Indeed, the use of batteries within the mobility sector has become the engine of the decarbonization of this sector. Although electric means of transport have already existed for some time, electrification through battery-mounted vehicles has undergone significant development in recent years. Furthermore, the different needs related to the different types of vehicles have also favored the study of new accumulation chemistries related to the different types of vehicle that use them. NMC (nickel-manganese-cobalt), LFP (lithium-iron-phosphate), and NCA (nickel-cobalt-aluminum) batteries are among the most used onboard road vehicles. Meanwhile, other studies of electric vehicles that require more effort in power use other types of chemical compositions such as LTO (lithium-titanate) [24]. In any case, batteries represent a fundamental prerogative for the electrification of electric vehicles, as much as they represent for the creation of a recharging infrastructure capable of supporting the electrification of the mobility system. With the increase in the number of electric vehicles on the market and in use, local distribution networks risk running into overloads. This is due to the fact that some users, such as those who are driving along a motorway, suddenly need high power to quickly recharge their exhausted vehicle through fast charging. The use of a distributed storage system helps to reduce the maximum load that must be supported by the transmission and distribution infrastructure by implementing BESSs in the vicinity of the electrical loads. The integration of BESS systems within the electricity grid brings various advantages, such as the provision of ancillary services for the distribution system operator (DSO) and transmission system operator (TSO). Based on these considerations and with the increase in the use of electric vehicles, the attention on the integration of BESSs, based on Li-Ion batteries, in charging stations has increased. Interest has been brought both to the domestic use of these batteries, to integrate the solar home systems (SHS), for vehicles, and with public recharging infrastructures.

Indeed, it is evident that, despite all the benefits that follow the electrification of means of transport, there is an increase in the demand for electricity to power them, with a considerable impact on the electricity grid. In fact, the presence of multiple charging infrastructure systems that require very high powers easily creates voltage dips and voltage instability, which represent one of the main causes of blackouts, since, as mentioned, the supply system works very close to the limit of stability, since the power demand is very high. The electrical loads associated with the rapid recharging of vehicle batteries, in addition to requiring significant amounts of power from the grid, are also highly nonlinear. The characteristics of nonlinear loads have a non-negligible impact on the network; therefore, it is advisable to recognize them. Such nonlinearities, for example, affect voltage stability, making in-depth studies in this regard fundamental. It is recognized that careful planning of the use of charging infrastructure can smooth out voltage fluctuations [25]. At the same time, careful planning of the charging phases of electric vehicles prevents the emergence of unwanted peaks in the demand curve [26]. Among the other critical issues that can be encountered are those that are identified as power quality (PQ) problems, deriving from harmonics and voltage imbalances that occur in the event of crowding in the recharge. In addition to voltage dips, unwanted peaks in demand, and PQ problems, an increase in the EV penetration rate can increase grid power losses, proportional to the number of feeders in the destruction system, augmented by the additional losses inherent in the recharge [27, 28]. The last of the critical issues to be submitted is the overload and overheating of distribution transformers, which are particularly stressed in the event of a request for high powers. Operation of transformers at temperatures higher than the nominal ones causes premature aging, reducing the useful life of the transformer [29]. However, despite the critical issues reported, electric mobility, if properly integrated with the smart grid, can assist in the dynamics of peak shaving. In fact, it is known that for most of the time during a day, the cars of private users remain parked and consequently unused or are used for limited trips. Based on this consideration and on the recent developments of the intelligent grid, which has evolved in a bidirectional perspective, thanks to the advent of distributed generation, we thought about how to optimize the times in the stall phase of the cars. The vehicle-to-grid (V2G) technology was born from this idea.

4 Electric Vehicle and Infrastructure Interaction

With the increase in technological availability and sensors in the field of mobility, it is correct to say that the entire sector, by imitating the distributed generation of dispatching, has evolved from a bidirectional perspective. If previously, the vehicle had a one-way interaction with the infrastructure, this is no longer the case. In fact, previously four charging methods have always been considered, which differ in power and protections, which transferred electric power from the charging infrastructure to the vehicle, stationed in a dedicated space. The traditional one-way charging procedure was then renamed as V1G, which means intelligent charging. Through intelligent recharging, the vehicle is able to change the recharging timing dynamically, since in addition to power, the vehicle and the infrastructure exchange information, which allows recharging to be regulated, helping to minimize its costs. Among the advantages of V1G, in addition to charging monitoring and timing optimization, there are also the infrastructure localization tool and the possibility of charging the vehicle at certain times of the day, so as to have greener and at the same time cheaper energy [30]. If sensoring and artificial intelligence (AI) have been fundamental in the development of intelligent recharging, the integration of a bidirectional power inverter connected to the car battery and to the grid represents the heart of V2G technology. The improvement that V2G brings to V1G is the possibility of having a vehicle that is not only able to draw power from the grid to recharge the battery but is also able to return power to the grid at times of the day when it is most stressed. These energy flows are obviously managed by a control unit, which ascertains the needs of both the network manager and the user. Following the description of the increase in electricity demand, the need to dampen peaks, the need to level the load, and the value that electrochemical storage systems will have to meet all the needs of the network, the V2G technology, which exploits the power that could be supplied by vehicles that remain parked and unused for most of the day, is perfectly combined with the demands of the network. In doing so, from an electrical point of view, the vehicles are seen by the network as many distributed accumulation systems from which it is possible to draw power to level the load curve [31, 32]. If this technology is seen from the point of view of a broad distribution network, in the various bibliographic studies, its applicability at the local level has not been overlooked. When the V2G technology is applied locally, and therefore on a building or at home, it takes the name of vehicle-to-building (V2B) or vehicle-to-home (V2H), respectively. The operating principle of V2B and V2H is the same and very similar to that of V2G but limited to a single building: with the integration of renewables and their variability, the generated power becomes fluctuating, sometimes creating power excesses or imbalances; therefore, vehicles are used as real batteries to receive or deliver energy according to the demand curve. In that case, the power transmitted by the vehicle can be applied to prevent service interruptions and blackouts locally [33]. Finally, to complete the description of the interactions between the vehicle and the surrounding world, vehicle-to-everything (V2X) should be mentioned. The V2X is based on the interaction of the vehicle with any object that surrounds it—vehicles, infrastructures, people, and traffic—in order to improve the driving experience. This technology is also strictly dependent on the high level of sensing. What prompted private companies to invest in V2X technology was the better management of roads and traffic, which are supported by AI through various algorithms and predictive models, but at the same time it encourages safer driving, thanks to continuous communication of data with the surrounding vehicles and infrastructure and energy savings. Today with V2X, we mean the possibility that the vehicle has to interact, in a bidirectional way, with any object that surrounds it [34]. It is clear that, with the increase in the diffusion of renewables and the electrification of the transport sector, the possibility of having energy storage systems available in a distributed manner represents an important push toward decarbonization, since they would help to combat the variability of the production and demand. At the moment, however, a remuneration policy has not yet been clearly defined for those who decide to join the sharing of energy toward the grid. Remuneration to those who subscribe to the V2G service is important, not only because users are actively supplying energy to the grid rather than recharging but also because users are subjecting their vehicle to continuous charge and discharge cycles in the process, potentially contributing to battery aging [35].

5 BESS Lifetime

Although today we have a thorough knowledge of electrochemical storage systems, still today there are several limitations related to BESSs, and the most relevant is precisely the useful life of rechargeable batteries, which degrade with aging. This event represents a problem for applications such as electric vehicles, since battery degradation implies a reduction in capacity and consequently a limitation of the vehicle’s travel range. As a result, EV manufacturers have a tendency to oversize the battery, to make the driving range appear constant, over the life of the battery. However, this technique only increases the cost and mass of the battery. Complicating matters is the fact that the degradation process of lithium-ion batteries is nonlinear, requiring knowledge of the materials that make up the battery, internal reactions and knowledge of aging processes. Normally, the useful life of the battery is characterized by a predefined number of charge and discharge cycles to which it can be subjected, which can vary depending on some factors during the life cycle, decreasing its efficiency. For batteries, there is a specific parameter that indicates the condition of the battery, called state of health (SOH). SOH indicates the level of performance of the storage system, based on voltage, self-discharge, and internal resistance. This parameter varies in the range 0–1, and an SOH equal to 1 indicates a battery at the beginning of its useful life, in which the capacity, in kWh, is maximum. Among the factors that influence the useful life of the battery, the first is the aging and degradation of materials. After that, the working environment is one of the factors that influence the useful life of the batteries. Specifically, if a battery worked in an environment with a temperature that was too high or too low for its operating range, the activity of the electrode would end up decaying. Therefore, maintaining an operating temperature range in line with the nominal one has a positive impact on battery life. Maintenance and cleaning factors, which facilitate the functioning of the components, should not be overlooked. Another factor that impacts battery life is the charge and discharge cycle. The succession of charge and discharge cycles implies a decrease in capacity, also due to the internal degradation of the materials. The factor that most significantly impacts the useful life of the batteries is the depth of discharge (DoD). The higher the DoD, the shorter the useful life of the battery; therefore, a charge and discharge cycle with a controlled and optimal DoD helps to significantly extend the useful life of the battery. The different chemistries of the storage systems will then have operating ranges and different DoD [36,37,38]. For example, for lithium-ion batteries, which have a wide range of uses since they are excellent for both power and energy applications, they have an optimal state of charge (SoC) operating range between 20% and 80%. Within this range, the duration of the useful life of the lithium-ion battery is maximized. Furthermore, by respecting this range, the amount of energy stored in the batteries is optimized with respect to the recharge time [39]. Current also has a major impact on the life span of the cells and consequently on the battery and the number of cycles it can withstand. Batteries that are subjected to higher discharge currents have a shorter life.

6 BESS Dismission and Second Life

Downstream of the knowledge of how the useful life of the batteries works and is interpreted, it is useful to describe how these storage systems are decommissioned. When it comes to decarbonization and reducing the environmental impact through electrification, it is often mentioned that the disposal of batteries is a complicated and not always green process. In addition, despite the lowering of battery costs relative to capacity, batteries still have a significant cost, for example, for an electric vehicle the battery can be worth 50% of the total cost. Therefore, to respond to these two needs, various stakeholders have undertaken various studies on the second life of batteries. Second life has the purpose of ensuring a recovery of the functionality of the batteries at the end of the life cycle, converting them into stationary accumulation systems. For example, the useful life of a lithium-ion battery applied to electric vehicles has a duration in charge and discharge cycles equivalent to 8–10 years. After this time, the battery is removed from the vehicle even if it still has some remaining capacity, as this is not sufficient to meet the standards for electric vehicles. However, the battery can still be useful for other energy storage purposes, such as, for example, the inclusion of storage systems in the charging infrastructure for electric vehicles, which help to sustain the grid. The three main benefits that can be generated to the smart grid by reusing batteries after their first life are as follows:

  • Defer and limit expenses related to the production and sale of new batteries.

  • Provide energy reserves that allow continuity of service, especially in industrial processes powered by other energy sources.

  • Use the available energy previously accumulated in times of absence or high cost of raw materials.

Typically, end of life (EOL) is considered to occur when actual capacity reaches 80% of rated capacity. Similarly, the end of the second life is considered to occur when the actual capacity reaches 30% of the nominal capacity. For this reason, half the nominal effective capacity is considered for stationary applications where batteries are used during their second life, since it is considered the middle ground between 80% maximum nominal capacity and 30% minimum rated capacity. The theoretical value 50% of the nominal capacity is considered for practical purposes, since it is approximately possible to have a desired capacity value with a number of second life batteries equal to twice the number of first-life batteries that would be needed to have the same capacity [40].

Among the BTM areas of application with the greatest interest in the second life of batteries are the fast-charging systems (DC fast-charging stations) with which it is possible to reduce charging times. Using batteries during their second life to assist recharging stations, it is possible to guarantee high peak currents, accelerating recharging times, avoiding oversizing of the network. This solution also represents an opportunity for savings or income for car manufacturers, which have resalable batteries in proportion to the range of vehicles they put on the market. Instead, the FTM applications that best lend themselves to a second life are those related to transmission services and energy shifting, a practice that aims to distribute energy production throughout the day. These applications present marginal installation space problems; in fact, it should be remembered that second life batteries occupy twice as much space as first-life ones.

On the basis of these considerations, therefore, it is possible to deduce that some BESSs are particularly suitable for use as storage systems during their second life, such as those based on lithium ions. Among the applications that these storage systems can perform during their second life are as follows:

  • Peak shaving: process of damping power demand peaks through the activation of local energy sources or using an accumulation system.

  • Power upgrade deferral: Through the BESS, it is possible to provide additional capacity, and with low growth rate loads, it is possible to postpone infrastructural interventions.

  • Time-of-use energy cost management: BESSs can help end users reduce the cost of the service by smoothing demand into preset daily peaks.

  • Self-production optimization: A BESS can assist standby generators by extending uninterrupted service time, reducing inefficient start-ups, and reducing fuel requirements for diesel powered UPS systems.

  • Integration of renewables: Renewable energy sources are fluctuating by nature and depend on several variable parameters. These variations cause frequency and voltage fluctuations, causing grid instability. A BESS system even in its second life can help compensate for grid imbalances.

  • Energy independence applications: Batteries during their second life can be integrated into microgrids, useful for powering users, which can range from buildings to neighborhoods. Microgrids are often powered by renewables; therefore, BESSs prevent grid imbalances.

  • Support for fast-charge recharges: The possibility of integrating BESSs during their second life into high-power infrastructures helps to reduce energy costs and avoid grid peaks. The station could also move toward net-zero energy consumption with the assistance of a BESS.

Despite the existence of different applications for batteries during their second life, there are applications where high-power density and instantaneous service with a high C-rate are required, which do not make second life batteries suitable for the task. Among the high-performance applications, there are those of backup or support to the network to increase the power quality. Therefore, even reactive power compensation operations require too high performance and significant stress levels for batteries during their second life [41,42,43].