Keywords

1 Introduction

One of the most important challenges to modern society is the availability of energy at a reasonable cost without adverse environmental consequences. The worldwide energy demand is projected to double by the middle of the century and triple by the end of the century as because billions of third world consumers gaining access to modern services and products. Major sources of energy for human activity are fossil fuels and nuclear sources. Currently, the burning of fossil fuels for thermal power plants, transportation and domestic heating represents more than 80 % of the world energy production. We nearly reach 87 % by the addition of nuclear sources and it is expected to peak around the year 2050 [1, 2]. Although the stored carbon fossil has some advantages including high energy density and stability but our planet will not support for much longer such massive energy consumption from this resource. The massive utilization has induced two major disadvantages: The first one is that the availability of fossil fuel is limited and eventually it will be exhausted. The second one is even more serious and concerns anthropogenic heavy greenhouse gas emissions, carbon dioxide in particular. Every kWh of electricity production by burning fossil carbon (coal) generates an average 1000 g CO2, a greenhouse gas that is generally considered as the primary contributor to global warming [3].

To reduce greenhouse gas (GHG) emissions, many countries already implemented emission regulations. The issues of carbon footprint and carbon tax, in the context of pollution caused by fossil fuels are vigorously debated and truly controversial. If we do not think to control those CHG emissions we will have to face the penalties of an abnormally high concentration of CHGs in the Earth atmosphere, which will lead to deep alteration of the climate and particularly global warming [4]. To date most reports forecast an increase in temperature from 2 to 3 °C by the end of twenty-first century, suggesting dramatic changes on the environment, such as the rise in sea level [5]. From August 1992 to April 1999, the Topex–Poseidon satellite has been recorded an annual average sea level rise of 3.1 mm [6]. Clearly, climate change could entirely reshuffle our natural features within a few decades. However, we cannot expect immediate change of this complex situation. If it takes 4–8 years for CO2 to spread into the atmosphere, it will take 50–100 years to get clear of half of it.

The constant increase in the world population and in order to reach the living standard of modern societies, mankind will face the fundamental challenge in the twenty-first century, which is definitely energy supply, its storage and conversion. The finite supply of conventional energy sources and constantly increasing energy demand in the context of environmental pollution, have stimulated the development of alternative, sustainable and clean energy technologies. Renewable energy sources, such as solar, wind, geothermal and tidal energy have been developed. Among various renewable energy sources, the solar energy is most abundant and potentially readily available. The solar radiation energy the Earth receives in 1 h is enough to meet worldwide energy consumption for a year. Currently, the output from PV module installations is growing 40 % per year globally [7]. In the coming decades, the solar energy is expected to play a fundamental role in the massive generation of decarbonized electricity. However, the solar energy is generated only in day time and often relies on the weather or climate to work effectively. It exhibits large fluctuations in power output in monthly or even annual cycles. Thus, energy from this source must be stored when excess is produced and then released, when production levels are less than the required demand. Therefore, energy storage technologies are an integral and indispensable part for a reliable and effective solar PV system.

2 Electricity and Its Storage

As shown in Fig. 1, various technologies have been developed within the energy framework, which includes mechanical, electrical, thermal, chemical, radiant and nuclear energy. Among the different forms of energy, electricity is the most convenient form of energy, which is easy to distribute by simple conversion processes without any strong harmful scattering. The physical access to electricity through power grid or mobile sources particularly increases the standard of living [8]. The blackout episode on August 2003 and the recent September 2011 power failure which extended from southern California to Mexico and Arizona are two of the more widely publicized examples in which 50 million people were in a critical situation demonstrating how life get somewhat cumbersome without electricity.

Fig. 1
figure 1

Illustration of the six forms of energy and related examples of their inter-conversions [9]

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Such power outage events highlight the complex set of issues associated with the generation and use of electricity: the use of fossil fuels and related carbon emissions, the reliability of grid, the development of electric vehicles to decrease dependence of foreign oil and the increased deployment of renewable energy resources. Besides, most of the issues are international in scope as previously mentioned that worldwide demand for electricity is projected to double by 2050. Electrical energy storage (EES) cannot possibly address all of these problems. However, energy storage can offer a well-established approach for improving grid reliability and overall use of the entire power system (generation, transmission and distribution). As illustrated in Fig. 2, EES can be employed for providing many grid services, including a set of ancillary services such as (1) frequency regulation and load following, (2) cold start services, (3) contingency reserves and (4) energy services that shift generation from peak to off-peak periods. Moreover, it can provide services to solve more localized power quality issues and reactive power support.

Fig. 2
figure 2

Schematic of applications of electricity storage for generation, transmission, distribution, and end customers and future smart grid that integrates with intermittent renewables and plug-in hybrid vehicles through two-way digital communications between loads and generation or distribution grids [10]

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3 Renewable Energy from PV Generation and Storage

The development and use of renewable energy from PV generation has experienced rapid growth over the past few years. Electrical production from PV source yields a more assured supply for consumers with less environmental hazards. As previously mentioned the major problem of this resource is its fluctuation from demand. Although it is abundant and conversion systems are becoming more and more reasonable, significant contribution to sustainable energy use will, however, require considerable further development of energy storage systems. Energy storage represents the ultimate solution to the problem of intermittent generation. Energy storage and its utilization in the electrical grid add value to renewable energy sources such as solar energy, allowing for more intense use of these technologies. Its use includes applications in load levelling, integration of renewable sources, peak-shaving and energy trading, making the system more stable and reliable. Figure 3 shows a schematic diagram of PV systems connected to the grid with and without energy storage systems, which show the undeniable increase in flexibility with the insertion of the energy storage system [11]. Generally, energy storage increases the usefulness of PV in the way that it absorbs excess PV and allows PV energy to be used when it is not produced in the evenings, on cloudy days etc. This will open up a new field of application, especially due to the growth of electrical production from solar PV, along with decentralized production.

Fig. 3
figure 3

Photovoltaic systems interconnected to the grid: a without energy storage, b utilizing energy storage with the different options 1 local load management, 2 load management for the utility, and 3 considering critical emergency loads [11]

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4 Energy Storage Technologies

The history of the stationary EES dates back to the turn of the twentieth century, when power stations were often shut down overnight, with lead–acid accumulators supplying the residual loads on the direct current networks [12]. Electrical energy storage systems are devices that store electricity after its conversion in some other forms of energy that can be converted back to electricity when needed. This process enables electricity to be produced at times of either low generation cost, low demand or from intermittent energy sources and to be used at times of high generation cost, high demand or when no other generation is available [13]. There are many possible techniques for energy storage, found in practically all forms of energy: mechanical, electrical and chemical. The storage technologies that answer to specific technical and economic criteria, which vary considerably as a function of the applications and requirements, will obviously be of different types. A number of utility-scale energy storage systems are currently deployed such as, pumped hydro storage (PHS) and compressed air energy storage (CAES). Both technologies are generally large scale (tens to hundreds of MW) and have unique geological and geographic requirements [14]. Batteries are more scalable in size and do not depend on availability of water or air storage. The efficiency of these technologies ranges from 70 to 95 % [15]. Table 1 presents the main features of selected EES systems.

Table 1 Energy storage technologies
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In terms of their applications, the energy storage technologies are divided in two categories:

  • Low to medium power applications where the energy could be stored as kinetic energy (flywheel), compressed air, chemical energy, hydrogen (fuel cells) or in supercapacitors.

  • Large-scale power applications where the energy could be stored as potential energy, thermal energy, chemical energy (batteries) or compressed air.

Storage technologies are characterized by factors such as storage capacity, available power, depth of discharge, discharge time, cycling performance, self-discharge, efficiency, mass and volume densities of energy, reliability, operational constraints and environmental aspect. The size of an energy storage system has two components: energy (how much energy may be stored) and power (what is the rate of charge and discharge). The relative size of the energy and power components may be independent of one another, depending on the storage technology. The relationship between energy and power in an energy storage system may be expressed by the energy/power ratio. Energy storage can increase performance ratio of the PV system. Energy storage helps to reduce power injection to the grid during the peak times.

5 Role of Energy Storage Technology

For decades, people have argued that electricity differs from all other products and markets because it cannot be stored. This is basically correct, but future developments have the prospective to remove this limitation and to combine storage with other energy technologies, e.g. solar PV to create a new energy pattern. Energy storage and demand side management are two options that can be used to increase the penetration level of grid-connected distributed generation. The fact is that energy generation from solar PV is seldom constant over time and also electricity demand is never constant. Thus, using an energy storage technology into solar PV generating system is important. Energy storage technologies provide opportunity for the generation side to meeting the level of power quality as well as consistency needed by the demand side. Energy storage can also offer emergency power and peak saving opportunity. Energy storage is especially important for decentralized power supply system by giving the more load-following capability, which is an important factor from generation side management.

6 Different Energy Storage Technologies

As previously mentioned, electricity is not easy to be stored cheaply but it can be easily stored in other forms and converted back to electricity when required. Different storage technologies are discussed in the following sections.

6.1 Mechanical Energy Storage

6.1.1 Pumped Hydroelectric Storage

PHS is the most widely implemented large-scale EES. The main advantage of this technology is that it is readily available. This technology is currently the most used for high power applications. It uses the power of water, a highly concentrated renewable energy sources. As shown schematically in Fig. 4, this system generally consists of (1) two reservoirs located at different elevations, (2) a unit to pump water to the high elevation and (3) a turbine to generate electricity with the water returning to the low elevation. During periods when demand is low, these stations use electricity to pump the water from low reservoir to the upper reservoir. When demand is very high, the water flows out of the upper reservoir and activates the turbines to generate high-value electricity for peak hours. Clearly, the amount of stored energy is proportional to the height difference of between the two reservoirs and the volume of water stored. Pumped hydroelectric storage is a mature technology with large volume, long storage period, high efficiency and relatively low capital cost per unit of energy. The conversion efficiency of PHS is about 70–85 %, which depends on equipment characteristics. Owing to the small evaporation and penetration, the storage period of PHS can be varied from typically hours to days and even years. The typical rating of PHS is about 1000 MW (100–3000 MW) and facilities continue to be installed worldwide at a rate of 5 GW per year. The major shortcoming of this technology is the need for available sites for two large reservoirs. A high cost (typically hundreds to thousands of million US dollars) and long lead time (~10 years) for construction and environmental issues (e.g. removing trees and vegetation from the large amounts of land prior to the reservoir being flooded) [16].

Fig. 4
figure 4

Illustration of pumped hydro storage with the pumping energy supplied by PV array [17]

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6.1.2 Compressed Air Energy Storage

CAES is a technology that stores energy as compressed air for later use. Besides the PHS, the CAES is only other commercially available technology, which is capable of providing very large energy storage deliverability (above 100 MW with single unit). Figure 5 presents a schematic diagram of a CAES system. It consists of five major components: (1) Air compressor of two or more stages with inter-coolers and after-coolers to achieve economy of compression and reduce the moisture content of the compressed air. (2) A cavity or container for storing compressed air. (3) A turbine train, containing both high and low pressure turbines. (4) A motor/generator that employs clutches to provide alternate engagement to the compressor or turbine trains. (5) Equipment controls and auxiliaries, such as fuel storage and heat exchanger units.

Fig. 5
figure 5

Schematic diagram of compressed air energy storage [18]

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It works on the basis of conventional gas turbine generation. It decouples the compression and expansion cycles of a conventional gas turbine into two separated processes and stores the energy in the form of elastic potential energy of compressed air. When demand is low, energy is stored by compressing air into an airtight space, typically 4.0–8.0 MPa. To extract the stored energy, compressed air is drawn from the storage vessel, heated and the expanded through high pressure turbine, which captures some of the energy in the compressed air. The air is then mixed with fuel and combusted with the exhaust expanded through a low pressure turbine. Both the low and high pressure turbines are connected to a generator to produce electricity. The waste heat of the exhaust is potentially captured via recuperator before being released.

CAES systems have been considered for a number of applications, most notably for electric grid support for load-levelling applications. In such systems, energy is stored during periods of low demand and then converted back to electricity when the electricity demand is high. CAES systems are designed to cycle on a daily basis and to operate efficiently during partial load conditions. Compared to the conventional intermediate generating units, CAES systems have improved environmental characteristics. CAES has low capital cost, high storage efficiency (70–89 %) and relatively long storage period. The typical rating for a CAES system is in the range of 50–300 MW, which is much higher than other storage technologies except for the PHS. The major barrier to the implementation of CAES is the reliance on favourable geography, hence it is only economically feasible for the power plants that nearby rock mines, salt caverns, aquifer or depleted gas fields. In addition, it is not an independent system and has to be associated with a gas turbine plant.

6.2 Electrical Energy Storage

6.2.1 Superconducting Magnetic Energy Storage

Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current through a superconducting coil of nearly zero resistance. This is the only technology which store electrical energy directly into electric current [19]. A typical SMES system includes three parts: superconducting coil, cryogenically cooled refrigerator and power conversion system as shown in Fig. 6. To maintain the inductor (coil) in its superconducting state, it is immersed in liquid helium (at 4.2 K or super fluid helium at 1.8 K) contained in a vacuum-insulated cryostat. The inductor is generally made of niobium-titanium (NbTi), that operate cryogenically cooled (−270 °C) temperature. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil.

Fig. 6
figure 6

Supermagnetic energy storage system [20]

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Compared to the other methods of energy storage, SMES loses the least amount of electricity in the energy storage process. SMES provides one of the highest densities of any power storage method. Its main advantage is high energy storage efficiency (˃95 %). The energy output of an SMES system is much less dependent on the discharge rate compared with batteries. It also has a high cycle life and suitable for applications that require constant, full cycling and a continuous mode of operation. These features make SMES suitable for use in solving voltage stability and power quality problems for large industrial consumers. The energy stored in the SMES coil can be calculated by E = 0.5LI 2, where L is the inductance of the coil and I is the current passing through it [21]. However, the major problem confronting the implementation of SMES units are the high cost and the environmental issues associated with strong magnetic fields.

6.2.2 Electrochemical Capacitors (Supercapacitors)

Electrochemical capacitors (also called supercapacitors or ultracapacitors) are passive and static electrochemical devices for storing and releasing energies rapidly and reversibly [22]. This is another form of energy storage devices with high power density and long life cycle. Supercapacitors have higher energy densites (about 5 Wh kg−1) than conventional capacitors. Although supercapacitors have the lower energy densities than popular lithium-ion batteries, their ultrafast charged and discharged capability leads to high power densities (10 kW kg−1). The principle of energy storage in a supercapacitor is through the ion adsorption on an electrode/electrolyte interface which make electric double layer [electrical double layer capacitors (EDLC)] or due to electron transfer between the electrolyte and electrode through fast Faradic redox reaction (pseudocapacitors) [23].

6.2.2.1 Electric Double Layer Capacitors

EDLCs store electrical energy using reversible adsorption of ions from an electrolyte on the electrodes to form an electric double layer at an electrode/electrolyte interface. Supercapacitors that only involve physical adsorption, in which there is no electrochemical reactions on the electrode material (e.g. carbon) while charging and discharging process (Fig. 7a) are called EDLC. In this type of supercapacitors, no charge transfers across the electrode/electrolyte interface, and no net ion exchanges occur between the electrode and the electrolyte [23]. This implies that the electrolyte concentration remains constant during the charging and discharging processes. Since there is no physical change in the electrodes during charge/discharge process, EDLCs can sustain millions of cycles. The model of energy storage could be defined by the following equation:

Fig. 7
figure 7

The working principle of supercapacitors. a Electric double layer, b redox reaction on the surface and c redox reaction in bulk [24]

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$$ E = 1/2\,CV^{ 2} $$
(1)

where E is the entire energy delivered, C denotes the specific capacitance and V is the potential window.

However, due to the electrostatic surface charging mechanism, EDLCs suffer from a limited energy density and scientific research mainly emphasises to improve energy performance. The materials for EDLCs should be highly conductive large specific surface area. Different carbon materials, such as activated carbon, carbon nanofibres, carbon nanotubes are considered to be best electrode materials for EDLCs.

6.2.2.2 Pseudocapacitors

The pseudocapacitors are different from EDLC in the way in which charge is stored. Pseudocapacitive charge storage fundamentally relies on redox reactions between electrode materials and electrolyte ions. The electric energy is generated by fast Faradic redox reaction, which produces pseudocapacitance. Materials undergoing such redox reactions include conducting polymers and several metal oxides (RuO2, MnO2, Co3O4 etc.) [25]. Depending on the location of these redox reactions, the pseudocapacitive charge storage can be categorized into surface charge storage and bulk charge storage (Fig. 7 b and c). These pseudocapacitors can be superimposed on any electric double layer capacitors. In systems, where multiple oxidation states can be accessed, the pseudocapacitors often provide a higher energy density than EDLCs. Since the electrochemical processes occur both on the surface and in the bulk near the surface of the solid electrode, pseudocapacitors exhibit higher specific capacitance and energy density than EDLCs. However, pseudocapacitors suffer from relatively lower power density than EDLC because Faradic processes are normally slower than non-faradic processes. Moreover, due to the physical changes of the electrodes during redox reactions, pseudocapacitors have poor cycling stability compared with EDLCs [25].

Supercapacitors show potential applications in electronics, transportation, communication and aviation. It can also be used in a wide range of energy capture and storage applications either by themselves as a primary power source or in combination with batteries and fuel cells. Supercapacitors can be used as uninterrupted power supplies (back-up supplies used to protect against power disruption) and load-levellers (back-up power for memories, microcomputers, clocks, system boards etc.). In combination with batteries and fuel cells, supercapacitors are likely to be used for powering HEVs and EVs [26].

6.3 Chemical Energy Storage (Batteries)

Batteries are rechargeable electrochemical systems used to store energy. These are the oldest and most established form of electricity storage, which store electricity in the form of chemical energy [27]. Among the different types of storage methods, systems based on chemical energy storage are very attractive. A battery comprised of single or multiple electrochemical cells, connected in series or in parallel or both depending on the desired output voltage. As presented in Fig. 8, each cell consists of

Fig. 8
figure 8

Rechargeable cell/battery diagram [28]

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  • The anode is the negative electrode of a cell associated with oxidative chemical reactions that release electrons into the external circuit.

  • The cathode is the positive electrode of a cell associated with reductive chemical reactions that gain electrons from the external circuit.

  • An electrolyte is a material that provides the medium for transfer of electrons between the positive and negative electrodes of a cell.

  • A separator is a physical barrier between the positive and negative electrodes incorporated into most cell designs to prevent electrical shorting. The separator can be a gelled electrolyte or a microporous plastic film or other porous inert material filled with electrolyte. Separators must be permeable to the ions and inert in the battery environment.

During discharge electrochemical reactions occur at the two electrodes generating a flow of electrons through an external circuit. The reactions are reversible, allowing the battery to be recharged by applying an external voltage across the electrodes. The batteries are rated in terms of their energy and power capacities. For most of the battery types, the energy and power capacities are not independent and are fixed during the battery design. Some of the other essential features of a battery are cycle life, efficiency, depth of discharge, operating temperature, self-discharge and energy density. At present, significant development is going on in battery technology. Different types of batteries are being developed of which some are available commercially while some are still in the experimental stage [29]. They not only provide environmental advantages and fuel flexibility, but also offer a number of important advantages to the electricity utility. They can respond very quickly to load changes and accept co-generated and/or third-party power, thus enhancing the system stability. Various battery technologies for power system applications are discussed in the following sections.

6.3.1 Lead–Acid Battery

Over the past hundred years, lead–acid battery technology has been the most widely used of any electrochemical storage medium. Lead–acid batteries that store and release electricity via a reversible electrochemical conversion. It is made of stacked cells, immersed in a dilute solution of sulfuric acid (H2SO4) as an electrolyte. The positive electrode (Cathode) of each cell is composed of lead dioxide (PbO2), while the negative electrode (anode) is sponge lead (Pb). On discharging, both electrodes are converted into lead sulphate (PbSO4). During charge, both electrodes return to their initial state. The electrochemical reactions are

  1. (I)

    Anode:

    $$ {\text{Pb}} + {\text{SO}}_{4}^{2 - } \leftrightarrow {\text{PbSO}}_{ 4} + 2 {\text{e}} $$
    (2)
  2. (II)

    Cathode:

    $$ {\text{PbO}}_{ 2} + {\text{SO}}_{4}^{2 - } + 4 {\text{H}}^{ + } + 2 {\text{e}}^{ - } \leftrightarrow {\text{PbSO}}_{ 4} + 2 {\text{H}}_{ 2} {\text{O}} $$
    (3)

These batteries have been applied to almost every area of industry, and their sales constitute 40–45 % of all battery sales in the world [30]. Since their availability, lead–acid batteries have been tested for utility applications, especially for power quality, reliability control and power regulation. There are two major types of lead–acid batteries: flooded batteries, which is the most common topology, and valve regulated batteries, which are subject of extensive research and development. Lead–acid battery has low cost and a high reliability with a round trip efficiency of 70–90 %. It is a popular storage choice for uninterrupted power supply (UPS), power quality and some spinning reserve applications. However, lead–acid batteries have a short cycle life of 500–1000 cycles and low energy density (30–50 Wh/kg), which limits its application for energy management. Lead–acid batteries also have a poor low temperature performance. In reality, high operation temperature (up to 45 °C) can improve the battery performance in terms of higher capacity, but reduce the life time of the system [28].

6.3.2 Nickel–Cadmium Battery

Development of nickel–cadmium (Ni–Cd) rechargeable batteries has been carried out since 1950. The main components of Ni–Cd batteries are nickel hydroxide (positive electrode), cadmium hydroxide (negative electrode), a separator and an alkaline electrolyte (KOH) [31]. During discharge, Ni(OH)2 is the active material of the positive electrode, and Cd(OH)2 is the active material of the negative electrode. During charge cycle, nickel oxyhydroxide (NiO(OH)) is the active material of positive electrode and metallic Cd is the active material of the negative electrode. The overall electrochemical reaction is

$$ 2 {\text{NiO}}\left( {\text{OH}} \right) + {\text{Cd}} + 2 {\text{H}}_{ 2} {\text{O}} \leftrightarrow 2 {\text{Ni}}\left( {\text{OH}} \right)_{ 2} + {\text{Cd}}\left( {\text{OH}} \right)_{ 2} $$
(4)

This type of battery can be found in two different forms, depending on the application: (i) sealed form in portable equipment and (ii) flooded form in general industrial applications. Ni–Cd batteries have a higher energy density (50–75 Wh/kg) and robust reliability. They have longer cycle life (more than 3500 cycles) than lead–acid batteries combined with very low maintenance requirements. These advantages over lead–acid batteries make them favourable for emergency lighting, UPS, power tools, generator starting and telecoms. The major drawbacks of Ni–Cd batteries are their toxicity and the fact that they suffer from the “memory effect”. Cadmium is a toxic heavy metal which can cause health risk of humans.

6.3.3 Sodium–Sulphur Battery

Sodium–sulphur batteries (NaS) are one of the promising options for high power energy storage applications. As shown in Fig. 9, the NaS batteries consist of liquid (molten) sulphur as the positive electrode and liquid (molten) sodium as the negative electrode [32]. A solid ceramic Beta alumina (Al2O3) acts as both the separator and the electrolyte simultaneously. During the discharge cycle, the metallic sodium is oxidized, releasing Na+ ions. The electrolyte enables the transfer of sodium ions to the cathode where they combine with sulphur to form sodium polysulfides: 2Na + S ↔ Na2S4. During the charge cycle, the sodium polysulphide is decomposed into sodium and sulphur. The battery operating temperature is in the range of 300–350 °C. Therefore, NaS batteries need to be heated externally for optimal operation.

Fig. 9
figure 9

NaS battery [28]

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NaS batteries exhibit high energy density (150–240 Wh/kg) and power density (150–230 W/kg), good temperature stability, high Coulombic efficiency (75–95 %), long cycle life (~2500 cycles), good safety and inexpensive. These batteries can be used for load levelling, UPS applications, emergency power supply, being suitable to a number of markets, such as industrial applications and wind power generating systems. The batteries are made of abundant and low-cost materials, making them suitable for large-scale industrial production. However, the major disadvantage is that an external heat source is required to operate NaS batteries, which partially reduces the battery performance. NaS batteries are currently used in electricity grid-related applications such as peak-shaving and improving power quality.

6.3.4 Sodium Nickel Chloride Battery

Sodium nickel chloride battery is a high temperature (~300 °C) system and also known as ZEBRA battery [33]. In this system, nickel chloride is used as the positive electrode. The chemical reaction occurring in a ZEBRA battery is

$$ 2 {\text{NaCl}} + {\text{Ni}} \leftrightarrow {\text{NiCl}}_{ 2} + 2 {\text{Na}} $$
(5)

Compared to NaS batteries, ZEBRA batteries can withstand limited overcharge and discharge and have potentially better safety characteristics and high cell voltage (2.58 V). The drawbacks with respect to NaS batteries are their low energy density (~120 Wh/kg) and power density (~150 W/kg). Another disadvantage is that only one company (Beta R&D, UK) is trying to develop this technology. At present, Beta R&D is developing a high power version of the ZEBRA battery for hybrid electric vehicles, a high energy version for storing renewable energy and a load-levelling battery for industrial applications [33].

6.3.5 Lithium-Ion Battery

Lithium-ion batteries store electrical energy in electrodes made of lithium intercalation (or insertion) compounds. Lithium intercalation compounds are used as positive and negative electrode materials in lithium-ion batteries [34]. In a typical lithium-ion cell, graphite and lithium metal oxide (LiCoO2) are used as a negative electrode (anode) and a positive electrode (cathode), respectively. A non-aqueous Li+ conducting medium is used as electrolyte. The positive and negative electrodes are separated by an electrolyte-filled porous polymer membrane that allows lithium-ion transfer but prevents electrolyte from direct contact. The lithium ions are shuttled between two host electrodes (anode and cathode) during the charge–discharge process. When the battery is discharging, Li+ ions de-intercalate from the anode, pass through the electrolyte and intercalate into the cathode. Meanwhile, the electrons pass through the external circuit from the negative electrode to the positive electrode. On charging, the process is reversed when an external voltage is applied to the battery. During charge–discharge process, Li ions move between the negative and positive electrode, empowering the conversion of chemical energy into electrical energy and storage of electrochemical energy within the battery.

Figure 10 illustrates the basic lithium-ion battery system that leads the current battery market. The cell reactions are reversible lithium-ion intercalation and de-intercalation cycles between two layered structures. The redox reactions involved in LiCoO2/graphite cell are presented as follows [35]:

Fig. 10
figure 10

Schematic of a LIB [36]

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Cathode:

$$ {\text{LiCoO}}_{ 2} \leftrightarrow {\text{Li}}_{{ 1- {\text{x}}}} {\text{CoO}}_{ 2} + {\text{xLi}}^{ + } + {\text{xe}}^{ - } \quad \left( { 1< {\text{x}} < 0} \right) $$
(6)

Anode:

$$ 6 {\text{C}} + {\text{xLi}}^{ + } + {\text{xe}}^{ - } \leftrightarrow {\text{Li}}_{\text{x}} {\text{C}}_{ 6} \quad \left( { 1< {\text{x}} < 0} \right) $$
(7)

Among rechargeable batteries, lithium-ion batteries have become popular with many advantages such as high energy density, low maintenance, long cycle life, no memory effect and low self-discharge. In the past two decades since the first commercial products manufactured by SONY, lithium-ion batteries (LIBs) have been widely applied for portable electronic devices. These include notebook computers, cell phones, digital cameras, etc. Its energy density is 2–3 times and power density 5–6 times higher than traditional nickel–metal hybrid, nickel–cadmium and lead–acid batteries. The advantages in high energy capacity and power over other technologies have made lithium-ion batteries the most promising option for transportation application. Extensive research and development were carried out in the past decades. Significant progress has been made in materials and chemistries to improve the battery technologies for the applications. The worldwide market of lithium-ion batteries is valued at 10 billion US dollars per annum. Lithium-ion batteries are now being intensively pursued upcoming large-scale transportation applications including hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV) [37]. They are also seriously considered for the efficient storage and utilization of intermittent renewable energies such as, solar and wind.

Referring the disadvantages of lithium-ion batteries, it can be mentioned that the lithiated graphite electrode operates at a potential close to that of metallic lithium, leading to Li-dendrite growth and potential electrical shorting. In the presence of flammable organic electrolyte solvents currently in use, there is a risk of heat generation, thermal runway and fire. An additional challenge is the high cost that may not be critical to small portable electronic applications but is very important for large-scale applications. There are some other battery technologies, including lithium sulphur batteries, sodium ion batteries, lithium oxygen batteries are in experimental stage. Many research groups and industries around the globe are continuing intensive research to develop new environmentally benign battery technologies for large-scale energy storage applications.

7 Conclusions and Future Perspective

Today climate changes and energy storages are confronting our society with considerable technological, economic, safety, health, and national security challenges. The current trend towards reducing greenhouse gas emission and increasing penetration of renewable energy, along with growing demands of high-quality power, calls for urgent development and implementation of EES systems. EES systems are the key enabling technologies for utility and transport applications. In particular, the proliferation of energy storage will enable the integration and dispatch of renewable PV generation. Without suitable EES, the current electrical grid could allow for only limited level of penetration of renewable energy generation from PV. Storage is a key element for the growth of renewable energies, particularly for PV generation. Installing EES into the grid would not only facilitate increasing penetration of renewable energy, but insure quality power for a society. Implementing EES would help to reduce greenhouse gas emissions by replacing fossil-burning turbines currently employed to stabilize the grid.

The success of these applications of energy storage will depend on how well storage technologies can meet key expectations. The most important of these are low installed cost, long life, high efficiency and high reliability. A number of potential technologies for EES exist and some of these have demonstrated for utility applications. However, these technologies are facing either challenges in meeting the performance and economic matrix for the stationary applications, or limits in environment, site selection etc. This also calls for both basic and applied research to further develop current technologies and discover new technologies that can address the requirements for renewable and utility applications.