MODELS GENERALS DE PERTORBACIÓ REGENERACIÓ OBSERVATS ALS DIAGRAMES POL ÜNICS

1.3.1.1 Lead-acid batteries

Lead acid (LA) battery represents the oldest and more widespread BESS technology, being adapted during years for use in various applications. It is mainly used as automotive starting, lightning, ignition (SLI) and main storage system in light-duty electric vehicles (e.g. golf kart and lift trucks) as well as for stationary storage applications, such as uninterruptible power supply (UPS) and emergency ESS in power stations [27, 28]. The worldwide diffusion of this battery is primarily related to its easy manufacture in a wide range of capacity sizes and to its relatively low cost. However, the high specific weight, moderate cycle-life and reduced discharging capabilities have limited their range of applications, LA batteries being unsuitable for renewable integration and smart grid projects.

1.3.1.2 Lithium-ion batteries

The Lithium-Ion (Li-Ion) battery technology has recently emerged as the most promising BESS on the market, specifically because of several characteristics that make this battery type more suitable than other technologies in a variety of

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tions. In fact, Li-Ion batteries show the highest energy density, conversion efficiency and cycle-life as well as the lowest weight among all rechargeable BESS [27]. There-fore, besides being already widely employed for consumer and portable electronics, Li-Ion batteries are now becoming the leading storage technology for plug-in hybrid electric vehicles (PHEVs) and all-electric vehicles (EVs), as well. However, there are still some challenges for developing large-scale Li-ion batteries. In particular, cost and safety issues are the two main factors that obstacle the deployment of lithium-ion for widespread use in power systems. Cell degradatlithium-ion and thermal runaway due to overcharge and underdischarge phenomena require special packaging and inter-nal protection circuits, increasing complexity and costs. In order to overcome such criticalities and meet the cost and safety requirements for ESS applications, Li-ion batteries have been designed in a range of different chemistries, each with unique cost and performance characteristics. Considering cathode materials, lithium iron phosphate (LiFePO₄) based batteries are emerging as promising candidate for both EES stationary and vehicular applications, due to its low cost and reduced environ-mental impact as well as high stability [11, 29, 30]. The latter is in fact essential in order to avoid safety issues due to thermal runaway at high temperatures. Thanks to the improvements in Li-Ion technologies, stationary systems have been recently deployed, demonstrating the feasibility of this battery type for frequency regulation and spinning reserve services, RES integration and distribution grid support [31].

1.3.1.3 Molten salt batteries

Sodium-sulphur (NaS) and Sodium-nickel chloride (NaNiCl2) batteries are based on a liquid sodium negative electrode and operate both at high temperatures, above 250^◦C. The development of these electrochemical cells have been driven by the need of identifying combinations of materials characterised by both low cost and high specific energy. Moreover, because of their high internal temperature, operations of molten salt batteries are independent from the ambient thermal conditions making them suitable for applications in extreme climates, where other BESS typologies can be not effectively employed.

NaS batteries are a relatively commercial mature and attractive candidate for large-scale EES systems, finding applications in wind power integration and high-value grid services, such as load-leveling or peak shaving [31]. In fact, they are gen-erally used for energy-density applications, being able to sustain discharge processes for long time-periods . This system is composed of a liquid sodium anode separated

20 1. Smart Grids, Distributed Generation and Energy Storage Systems

from the sulphur cathode by a ceramic electrolyte (β⁰⁰-alumina), which allows the migration of sodium-ions while acting as a good electric insulator. The cell needs to be operated at high temperatures, between 300^◦C and 350^◦C, which are required to maintain the electrodes in molten states and ensure efficient ion transportation through the ceramic solid electrolyte. Although NaS features very interesting prop-erties for stationary ESS, such as very high specific energy and cycling flexibility, it is still facing some challenges especially related to safety problems and incident risk [32]. In fact, the direct contact of electrodes can results in fires and explosion, mining the feasible and secure deployment of this technology in large-scale storage systems.

In Sodium-nickel chloride batteries, the positive electrode is in solid state and based on nickel element. This configuration requires the presence of a second elec-trolyte in liquid state (sodium chloroaluminate, NaAlCl4) in order to improve the ion conductivity of the electrode. In case of damage or solid electrolyte structure failure, the sodium chloroaluminate reduces the effects of the exothermic reaction, leading to continuous and safe operations of the ESS plant, without risk of fire or explosion.

The intrinsic safety and the wide variety of applications, ranging from residential to grid support services, are encouraging the installation of such a technology in sev-eral projects, including RES integration and frequency/voltage regulation [31, 33], However, further improvements are needed in order to increase energy density and power capability, and to lower specific costs.

1.3.1.4 Flow batteries

Redox flow battery (RFB) represents one of the most recent and advanced BESS technology, being based on a novel electrochemical structure, which make them more similar to fuel cell storage systems than traditional batteries. In fact, RFBs is con-stituted of two external electrolyte tanks storing the solvable redox couples, and of reaction stacks where the liquid electrolytes are pumped in, converting chemical energy in electricity. This peculiar electrochemical structure results in one of the more interesting features of such a technology: the capability of coming with inde-pendent size of power and energy. Particularly, power is a function of the number of cells that are stacked, whilst energy depends on the electrolyte volume, which is circulated by pumps. Moreover, RFBs are characterised by high flexibility, high depth-of-discharge, long durability and fast responsiveness [34]. Such features allow for a variety of power and energy applications, ranging from power quality to energy

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management services. Among various RFB technologies, the leading chemistries at the present is the Vanadium Redox battery (VRB), which is in fact the sole that has reached effective commercial maturity. Although VRB systems are already been successfully demonstrated in test and commercial plants, capital and cycle life cost reductions as well as improvements in power and energy densities and further devel-opment in electrochemical materials are still required in order to obtain a commercial product widely deployed for smart-grid-oriented applications [27, 34].