Energy storage systems in modern grids—Matrix of technologies and applications

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Journal of Energy Storage

Volume 6, May 2016, Pages 248-259

Energy storage systems in modern grids—Matrix of technologies and applications

Omid Palizban. Author links open the author workspace.Opens the author workspaceOpens the author workspaceKimmo Kauhaniemi. Author links open the author workspace.Opens the author workspace

Department of Electrical Engineering and Energy Technology, University of Vaasa, Vaasa, Finland

Department of Electrical Engineering and Energy Technology, University of Vaasa, Vaasa, Finland

Received 9 December 2015, Revised 1 February 2016, Accepted 2 February 2016, Available online 13 February 2016.

https://doi.org/10.1016/j.est.2016.02.001Get rights and content

Highlights

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The available technologies and applications of energy storage system in the modern grid.

•

The possibility of integrating different types of energy storage system into the modern grid.

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Batteries are the most commonly used technique to cover many applications.

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Batteries can integrate with most other storage types to provide system support.

Abstract

Energy storage technologies are used in modern grids for a variety of applications and with different techniques. The range of applications and technologies is very broad, and finding the right storage solution for the job at hand can be difficult. In order to simplify the selection, this paper presents a matrix of the available technologies and applications. Along with proposing the matrix, the technologies and applications of Energy Storage Systems (ESSs) are described thoroughly and are compared on the basis of many different parameters, such as capacity, storage power, response time, discharge time, and life time. Moreover, the structure of energy storage, which is constituted of different steps and parts, is investigated. Since the implementing of an ESS is expensive, this paper also analyzes the possibility of integrating different types of ESSs and presents a comprehensive diagram to show the ESS technologies that can be integrated together in order to provide the needed performance in a cost-optimal way. Finally, the key results of this comprehensive study are summarized in a number of tables.

Keywords

Applications of energy storages

Energy storage system

Modern grid

Renewable energy source

Energy storage technologies

1. Introduction

Power management and stability assurance are critical tasks in modern grids because of the variables involved in generation and on the demand side. There are many different methods of approaching these problems, such as de-loading the operation of Renewable Energy Sources (RESs) when the generation of power is greater than demand and load shedding during power shortages [1–5]. However, the absorption and injection of energy by energy storage systems may be the best solution for managing this issue well [6–9]. Investigations of the challenges and barriers to power systems indicate that ESSs should aim at the following three targets [10–12]:

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Enhancing the reliability of renewable energy sources;

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Improving the resilience of the grid and resolving its issues;

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Realizing the benefits of smart grids and optimizing generation to suit demand.

Indeed, by storing energy when it is easily available and dispatching it during shortages, the combination of energy storage technology and RESs can help to stabilize power output while also enhancing the reliability of RESs. Moreover, energy storage can increase the resilience of systems during weather variations, natural disasters, and so on [13–16].

In fact, determining the best arrangement of ESS can be the first critical issue in designing a system. From this point of view, storage systems may be either distributed or aggregated. In distributed arrangements, the energy storage systems are connected via individual power electronic interfaces to each RES. In this method, each storage system has responsibility for the control and optimization of the power output of the source to which it is connected [17–19]. The aggregated model operates so that the whole system—for example, a microgrid (MG)—is supported through a central energy storage system. Depending on the arrangement, such a system may be connected to the DC bus either directly or through a power-electronic interface [20–22].

The second critical issue for storage systems may be the control of each application and of the optimum storage type. Indeed, in implementing an optimum storage project, three different steps need to be considered:

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Investigating the type and size of the storage system and selecting the one that is best for the system.

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Defining the best control strategy for the application considering the selected storage system.

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Investigating the net present value of the storage system.

In the second step, control methodologies for ESSs can be classified as either central or decentralized and can cover both of the arrangements. Indeed, the control methodologies of storage units, and also the power-electronics interfaced DG system with them, are investigated in [23–25] for centralized methods and in [26–31] for decentralized methods. Moreover, the control strategy is comprehensively discussed by the authors in [32,33].

In recent years, there has been much interest in investigations into technologies and applications of ESS. Researchers have produced comprehensive reviews in this area, such as those by Tan et al. [17], Carnegie et al. [34], Bradbury et al.[35], and Cavanagh et al. [36] The first objective of the present paper is thus to cover the first step by creating a matrix of different storage technologies and their applications. Such a matrix may be beneficial in allowing industry and researchers to quickly determine the optimum storage technique for a given application. The second objective of this paper is to analyze the possibility of integrating different ESS technologies. Indeed, such an analysis can help to obviate the high cost of storing energy in certain applications.

The present paper is organized as follows: The structure of energy storage is discussed in Section 2. The energy storage technologies and applications are investigated in Sections 3 and 4, respectively. A comparison of these two issues and the matrix appear in Section 5. The possibility of integrating ESS is discussed in Section 6. Finally, the conclusion is presented in Section 7.

2. Structure of energy storage

To store the generated power, it is necessary to convert it into other forms of energy, such as chemical or mechanical energy. As was presented by Gazarian[37], and based on the above definitions, energy storage consists of three different steps: charge: absorbing electrical energy from sources; storage: converting electrical energy to other types of energy and storing it and discharge: injecting the stored electrical energy back into the system. Moreover, storage systems can be divided into three different parts: central storage, the repository in which the energy is stored after conversion; power transformation, the interface between the central storage and the power system with bidirectional transfer; and control, which uses sensors and other measuring devices to determine the level of charge or discharge of the stored energy. Since energy storage is not an ideal energy source, but just a repository of energy, there are always losses at each step of the storage process. The energy generated by the sources given the energy delivered to the system during shortages is described by Eq. (1),

(1)$E_{\text{generate}}-\text{Δ}{E}_{\text{loss}}=E_{\text{out}}$

And the energy losses in this process are explained by Eq. (2).

(2)$\text{Δ}{E}_{\text{loss}}=\text{Δ}{E}_{\text{ch}}+\text{Δ}{E}_{\text{st}}+\text{Δ}{E}_{\text{disch}}$

Indeed, a significant parameter in electrical storage is the efficiency of each step. Taking into account Fig. 1, which shows the energy flow in a storage system, the efficiency of the charge step can be calculated as

(3)${\eta }_{\text{ch}}=\frac{E_{\text{st}}}{E_{\text{ch}}}$

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Fig. 1

The storage period can be expressed as

(4)${\eta }_{\text{st}}(t)=\frac{E_{\text{st}}^{\ast }}{E_{\text{st}}}$

Regarding Eq. (4), it should be noted that the energy losses, and also the efficiency of the storage, depend on the storage time; for this reason, the time t between charging and discharging need to be considered. Finally, the discharge steps can be obtained:

(5)${\eta }_{\text{dish}}=\frac{E_{\text{st}}^{\ast }}{E_{\text{disch}}}$

Moreover, the total energy storage efficiency $\left({\eta }_{\text{st}}^{\text{total}}\right)$ is shown in Eq. (6)[37].

(6)${\eta }_{\text{st}}^{\text{total}}=\frac{E_{\text{out}}}{E_{\text{generate}}}={\eta }_{\text{ch}}\times {\eta }_{\text{st}}(t)\times {\eta }_{\text{disch}}$

In these equations and Fig. 1, the losses of energy are shown by $\text{Δ}{E}_{\text{loss}}$and the energy losses during storage, charge, and discharge are presented as$\text{Δ}{E}_{\text{st}}$,$\text{Δ}{E}_{\text{ch}}$ and $\text{Δ}{E}_{\text{disch}}$respectively. The stored energy in the central part, represented by $E_{\text{st}}$ and$E_{\text{st}}^{\ast }$, is the existing energy from this part.$E_{\text{generate}}$,$E_{\text{out}}$,$E_{\text{ch}}$, and $E_{\text{disch}}$are the generated, output, charging, and discharging energy respectively. The efficiencies of charging, discharging, and storage are represented by${\eta }_{\text{ch}}$,${\eta }_{\text{disch}}$, and.${\eta }_{\text{st}}$.

3. Energy storage technologies

As mentioned earlier, energy storage can be achieved by converting electrical energy into another form. A complete classification of ESS types is presented in Fig. 2.

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Fig. 2

3.1. Electrochemical storage

In this technology, the chemical energy contained in the active material is converted directly into electrical energy [38,39]. Batteries are an advanced technique for storing electrical energy in electrochemical form. The possibility of using batteries in a wide range of different sizes is the main advantage of this technique [40,41].

Indeed, the operational voltage and current levels are generated through series or parallel connections of cells [43]. A simplest equivalent circuit of a battery and an explanation of its operation is presented by Patel [42], and shown in Fig. 3. The operating point is the intersection of the source line, which has the terminal voltage drop $(V_{\text{b}})$, and the load line $\left(V_{\text{L}}\right)$. The quantity of electrical charge in the cell from the fully charged state to the discharged state is called the capacity of the battery. Moreover, the state of charge (SoC) is the ratio between remaining capacity and the full charge, equal to 100% for full charge and 0% for full discharge. The variation in SoC $(dS\text{o}\text{C})$ is based on time and its relation to capacity $(C_{\text{i}})$ is outlined in Eq. (7).

$d\text{S}\text{o}\text{C}=\frac{idt}{C_i}$

(7)$\mathrm{S}\mathrm{o}\mathrm{C}=\mathrm{S}\mathrm{o}\mathrm{C}-\int \frac{idt}{C(i)}$

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Fig. 3

Lead–acid batteries [44,45] are available in large quantities and in a variety of sizes and designs. They have high performance and possess the highest cell voltages of all aqueous electrolyte battery technologies. For MGs (especially when large), they are the most economic option [38,46]. Despite their suitability for a wide range of applications, they cannot equal the storage capacity of pumped hydro [47]. Nickel–Cadmium (NiCd) [48,49] and Nickel–Metal Hydride (NiMh) [50–52] batteries are much more expensive to implement than lead-acid batteries, but they provide good charge retention and energy density. They also have a long life cycle. Lithium-ion batteries [53] have rapid charge capability and high energy density. They need no maintenance during operation, and their energy loss is very low (5% per month). On the other hand, their performance decreases at high temperatures and protective circuitry is needed [54–56]. Sodium sulfur (NaS) is a type of electrochemical energy storage [57,58] that needs to operate at high temperature (350 °C/623 K) in order to ensure that the sodium is liquid. This condition leads to some difficulty and increases the cost of implementation [48,59,60]. However, the energy efficiency is high and these systems have very flexible operation [60]. Finally, flow batteries are another type of storage method; this is a class of electrochemical energy storage that uses ions dissolved into liquid electrolytes [61–63]. There exist both redox and hybrid flow batteries. Hybrid flow batteries include zinc–bromine models, while vanadium batteries are a good example of the redox type. This method is characterized by its long life cycle (around 40 years) and its adaptability: increasing the tank sizes and adding more electrolytes allows the capacity to be increased [47,64,65]. However, further development is still needed; these batteries are expensive to use and an external power is also required to operate [36].

3.2. Mechanical storage

Electrical energy can also be stored in the form of mechanical energy. Some major methods of this type are described in the following:

3.2.1. Flywheel energy storage (FES)

This technique employs the mechanical energy of a spinning rotor to store energy. There are two types of FES: low speed (under 10,000 rpm) and high speed (above 10,000 rpm). Low-speed systems are much more popular in industry [66–68]. FES systems have low maintenance, long life cycles of up to 20 years, no carbon emissions, no toxic components, and very fast response. However, they suffer from high rates of self-discharge (3–20% percent per hour), low storage capacity, and high cost. The available range of energy storage for a FES system is 0.2–25 kWh; however, this is expected to increase to 200 Wh/kg and 30 kW/kg respectively over the next few decades [42]. The energy stored by a flywheel (E) can be calculated using (8). This equation shows that the total mass of the flywheel (m) and the angular velocity ($\omega$) squared have a direct impact on the energy stored by this device. In this equation, the radius of flywheel is shown by (r).

(8)$E=\frac{1}{2}\text{m}{\text{r}}^2{\omega }^2$

As presented by Östergård [69], a model of a general FES system consists of two voltage source converters (VSC), an electrical machine, a step-up transformer, and a main network. Indeed, the frequency decreases as the flywheel slows down. For this reason, the generation of AC power by the FES system should be converted to DC (constant frequency)—hence the use of the VSCs, for two back-to-back converters. The controller operation of these converters varies depending on the requirements of the FES application. For example, control of both active and reactive power may be needed for a FES system connected to an AC grid.

3.2.2. Pumped hydro storage

Electrical energy may be stored through pumped-storage hydroelectricity, in which large amounts of water are pumped to an upper level, to be reconverted to electrical energy using a generator and turbine when there is a shortage of electricity. The infinite technical lifetime of this technique is its main advantage [70], and its dependence on topographical conditions and large land use are the main drawbacks [43,71]. Pump storage projects throughout the world are significantly contributing to balancing the massive increase in future volatile regenerative energy production (wind and solar). The technology is well-established and commercially available on a large scale (sized up to 4000 MW), and the efficiency of the storage type is usually around 70–85%. [37]. The energy stored by this technique can thus be calculated through (9); the general equation for the output power (P) is shown in Eq. (10)[47].

In these equations, $Q$ is the volume flow rate passing the turbine $[{\mathrm{m}}^3/\mathrm{S}]$, $\rho$ is density of the water $[\mathrm{k}\mathrm{g}/{\mathrm{m}}^3]$, and $\eta$ is the hydraulic efficiency of the turbine $[\%]$. Gravitational acceleration and height are shown by g$[\mathrm{m}/{\mathrm{s}}^2]$ and $h$ [m] respectively.

(9)$W=\text{mgh}$

(10)$P=Qh\eta g\rho$

3.2.3. Compressed air energy storage

Compressing air to a pressure of around 70 bar is used to store electrical energy in the technology called compressed air energy storage (CAES). The method is, however, very expensive. In practice, large volumes of cheap natural storage, such as aquifers, salt caverns, and hard rock caverns, are used. An expansion turbine and generator are used to reconvert the compressed air to electrical energy [67]. CAES systems have typical capacities of about 50–300 MW and can store energy for longer than other methods—typically for more than a year—due to the very low losses involved. Like the pumped hydro method, CAES systems are capable of storing large amounts of energy. The efficiency of CAES is also similar to that technique (at around 70%). Moreover, the response time of the method is very high, but the technology is still not fully developed [47].

3.3. Electrical storage

Electric double-layer capacitors and superconducting magnetic energy storage (SMES) are electrical storage types discussed in the following:

3.3.1. Double layer capacitor (DLC)

In double-layer capacitor storage—which is also called ultra-capacitor or super-capacitor storage—the dielectric gap between two conductors is employed. This technique has a high energy storage capability due to its high power ability [72]. As presented in Ref. [37], based on a simple series RC circuit and the total energy which can be stored in a capacitor by this technique is calculated with (11). In fact, the energy stored in a capacitor is divided into two different parts: one part is retained in the capacitor, while the other is converted to heat and is wasted. The electrical energies stored in capacitors must be used very quickly, because the self-discharge rate of this method of energy storage is around 5% per day [54]. In this equation, $Q$is the charge stored in the capacitor [C], $V_{\text{q}}$ is voltage across the capacitor [V], and $C$ is its capacitance [F].

(11)$W={\int }_0^Q{V}_{q}dq=\frac{C{V}^2}{2}$

3.3.2. Superconducting magnetic energy storage

The SMES technique involves a cryogenic refrigerator, a superconducting coil, a helium vessel, and a power conditioning system which is presented by Salameh in Ref. [73]. In this method the voltage is stored in the superconducting coil after being switched to DC by an AC–DC convertor. The temperature of the coil is kept low in order to avoid resistive loss. With this method, the current is stored in the coil until it is injected into the system. The responsibility of the power conditioning system is to control the stored energy and to inject power into the system.

A simple diagram of an SMES system is usually equal to the series RL circuit. Hence, the principle of the mathematical model is similar to the double layer capacitor (DLC), and the amount of energy stored by SMES per coil volume [J/m3] can be calculated by Eq. (12)[37]. In this equation, $B$ is the magnetic flux density [T] and $\mu$ is the permeability [H/m].

(12)$W=\frac{1}{2}\frac{B^2}{\mu }$

3.4. Thermal storage

The thermal storage method is based on converting the energy to ice or hot water. There are many different approaches to use such thermal energy storage, a comprehensive review of which is presented in Refs. [74–76]. The most common version of a thermal energy storage system stores ice during the night and use the water to cool an air conditioning system during the day, thus reducing the use of power from the main grid or microgrid. On the other hand, the heat storage method can also use water to store heat energy and inject this energy into the system whenever it is needed [34]. There are many other thermal methods, such as geothermal energy systems, solar thermal energy conversion systems, and phase-changing materials, which are discussed in more detail in Refs. [77–79].

4. Energy storage applications in the power system

The principle of system control that classifies loads by priority and employs load shedding is not suitable for achieving high reliability in modern systems. Hence, one benefit of ESSs may be that they result in improved reliability for these systems. Akhil et al. [79] and Eyer et al. [80] have presented different applications that can be provided with ESS. The applications of ESS to MG are classified into four different groups, which are shown in Fig. 4 and discussed in the following.

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Fig. 4

4.1. Bulk energy applications

Bulk energy is a key application for integrating a large amount of variation in modern grids. The two major types are represented in the following:

4.1.1. Energy arbitrage

Energy generation is very expensive, and storing the energy can both increase the efficiency of a system and optimize it economically. Storage of energy when the price is low and selling energy at peak times when electricity is expensive is the main goal of the application. In a MG with RESs, the application also stores energy when the amount generated exceeds demand and inject power during shortages [81].

4.1.2. Peak shaving

The principle of peak shaving is very similar to energy arbitrage. The difference is that peak shaving is installed to cover the peak load, and does not have an economic target, as energy arbitrage does [82]. The application helps to improve the system design, based on a normal capacity and supporting the peak demand through the ESS. The peak shaving application is usually installed at the consumer, whereas energy arbitrage is used on the supply side [80,83].

4.2. Ancillary service applications

In modern grids, providing support to the system during the transmission of power from its generation to the consumer can be referred to as an ancillary service and involves adjustments and flexible reserves. The different approaches to this application are discussed below:

4.2.1. Load following

As compared to generation types, ESSs have a rapid response to changes in load [84]. Since the load can undergo frequent variations, energy storage is more suitable for load-following applications. In fact, in this application, the responsibility of energy storage is to create a balance between the generation part and the load [85]. Another reason for supporting load changing using energy storage is so that the system can cover both sides of the variations, following the load both up and down [80].

4.2.2. Spinning reserve

As mentioned by Gonzalez et al. in Ref. [58], the spinning reserve is a part of the capacity of the source that is not used in normal operation. However, the source can cover a power shortage in the system by injecting power for specific period. Indeed, the power shortage thus is covered by sources operating in this extra operations mode. Since power generation must continue until the backup system reaches its nominal value, the storage system in this application must be able to discharge over a long time (at least one hour) [83,86].

4.2.3. Voltage support

Stability is an important issue in the power system, and can be achieved through maintaining the voltage within the permissible limits. As discussed in Ref. [79], the management of reactive power is a requirement for achieving this, and can regulate accurately with an ESS as a voltage support resource. As reactive power cannot reasonably be transferred over long distances, a voltage support application is used locally to manage the problem [79,80].

4.2.4. Black start

Unplanned events can lead to interruptions in power throughout the whole system or in a single part [87]. The result of this may be a black out [88], compromising the stability of the system [89–91]. The system is restored through a process called a black start, the responsibilities of which are power management, voltage control, and balancing. In this application, the energy storage system generates active power that can be used for energizing distribution lines or as start up power for large power plants [79].

4.2.5. Frequency regulation

Frequency control is crucial in power systems for dealing with the many small variations that occur. The energy storage system in a frequency regulator serves power systems by correcting the frequency deviations to within the permissible limits—for example to ±0.1 Hz in Nordel (North of Europe) or ±0.2 Hz in UCTE (Continental Europe) [92–94]. As mentioned in Refs. [95,96], there are three types of frequency regulation: primary, secondary, and tertiary. These are shown in Fig. 5. The responsibility of the primary reserve control is to create a balance between generation and demand and to restore the frequency within 5–30 s for the generator control [39,95–97]. The secondary reserve has two objectives: it serves as a backup for primary regulation and ensures that the frequency is set to 50 Hz, while also avoiding any imbalance in the interconnection. This control level reacts to the primary control reserves for 5–15 min, and should then be ready for frequency correction to within the permitted limits [96–98]. In the last level, Tertiary reserve has the same objective as the secondary reserve and also aims to balance load, generation, and sales, thus helping to keep the system synchronized. This reserve level is operated manually, and should reach its target in 15–60 min, depending on the country [96].

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Fig. 5

4.3. Customer energy management applications

Energy management applications are based on the quality and reliability of power delivery to the consumer which are discussed as follow:

4.3.1. Power quality

It is clear that there are some variations in generation and in energy sources, especially when it comes to RESs, which are dependent on environmental conditions [99–101]. Indeed, the fluctuations in power generation systems lead to concerns about power quality, especially in terms of voltage harmonics and voltage variation [102]. To manage this problem, energy storage is the first alternative to cover the variation. The application helps to protect downstream loads against short-duration events and to improve the quality of delivered power [83].

4.3.2. Power reliability

The principle of power reliability is similar to power quality [103,104], but power reliability follows power quality in sequence. This means that the time for restoring power with this application is longer than the time taken by the power quality application. The energy storage system in this application should have high reliability power with the best quality. Moreover, the power reliability application is under customer control and is installed in customer locations [105].

4.4. Renewable energy integration applications

There are many fluctuations in the power generated by RESs which can be covered by ESS. These applications are divided in two different categories: time shifting and capacity firming. The time-shift application manages the problem through different energy storage techniques [11]. It stores energy when demand is lower than generation, and injects this power into the system during shortages. In this application, energy storage can be installed anywhere in the system, whether near to the source or to the load [106–108]. The responsibility of the capacity firming application is to smooth the power and voltage output from renewable energy over a short period using an ESS [40]. The output power from RESs is added to the energy storage and supports the load. This mixing also can help to improve power quality.

4.5. Location of each application

In optimizing the amount of stored energy, the utilization of the energy storage system is important, as is its application in related parts. There are several applications which can be used in different parts of a power system. Fig. 6 demonstrates the locations of each energy storage application in power system, from the point of generation to the customer.

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Fig. 6

5. Technologies and applications

As aforementioned, there are many different options for using energy storage in conventional or modern grids (DG, MG, Smart grid). As is well known, the choice of energy storage technique directly depends on the applications [106]. To correctly choose storage techniques, it is first necessary to distinguish two important parameters: energy (kWh) and power (kW) [34]. Thus, to design the ideal ESS, the power and energy of the system should be determined in the first instance. In Table 1, the discharge and response times, as well as the power and desired life cycles, are presented for each application separately. Indeed, the important parameter for energy storage applications is the length of discharge, which can be divided into three different categories: second–minute, minute–hour, hours. It is clear that the two first categories are related to customer energy management and to the ancillary services of energy storage application. The hours category can be used for long-term storage and discharge, such as for bulk energy, or in renewable energy integration applications [106]. In Table 2 presents comprehensive information regarding these energy storage techniques, such as their capacity, power, response and discharge time, life time, and efficiency. Taking into account the objective of this paper, and the contents of Tables 1 and 2, Table 3has been developed on the basis of [10,34] to provide a matrix of the relationships between the available energy storage technologies and their application in ESSs. As shown in the matrix, battery technologies come in different shapes and sizes and can be used in many different applications.

Table 1

Applications			Storage power (MW)	Response time	Discharge time	Cycle		Desired life time (years)	Recommendation grid
Bulk energy	Energy arbitrage		≤500	minutes	≤10 h		300–400/yr	≤20	MV
	Peak shaving		≤500		≤6 h		50–250/yr	≤20	MV
Ancillary service	Load following		≤100		≤4 h		N/A	≤20	MV, LV
	Spinning reserve		≤100	≤4 h	≤5 h		N/A	≤20	HV
	Voltage support		≤10	≤100 ms	≤1 h		5000/yr	≤20	HV
	Black start		≤50	≤2 h	≤16 h		10–20/yr	≤25	HV, MV
	Frequency regulation	Primary	≤40	Instantaneous	30 min ≥ t ≥ 15 min		8000/yr	≤15	MV
		Secondary	≤40	minute	1 h ≥ t ≥ 30 min				MV
		Tertiary	≤100		≥1 h				MV
Customer energy management	Power quality		≤10	≤200 ms	≤2 h		50/yr	≤10	HV, MV, LV
	Power reliability		≤10	minutes	≤4 h		≤400/yr	≤15	MV, LV
Renewable energy integration	Time shift		≤500	≤30 min	≤5 h		≤4000/yr	≤15	MV
	Capacity firming		≤500	≤30 min	≤4 h		300–500/yr	≤20	MV

Table 2

Technologies				Capacity (MWh)	Power (MW)	Response time	Discharge time	Maturity	Life time (Years)	Efficiency (%)	Advantage	Disadvantage
Electrochemical	Lead–acid			0.25 ∼ 50	≤100	millisecond	≤4 h	Demo ∼ Commercial	≤20	≤85	Inexpensive High recyclable Reality available	Very heavy Limited usable energy Poor energy density
	Lithium-ion			0.25 ∼ 25	≤100		≤1 h	Demo	≤15	≤90	High capacity Great stability in calendar and cycle life
	NaS			≤300	≤50		≤6 h	Commercial	≤15	≤80	High storage capacity Inexpensive	Working only when the sodium and sulfur are liquids 290 ∼ 390 °C
	Vanadium Redox			≤ 250	≤50	≤10 min	≤8 h	Demo	≤10	≤80	Possible to use for many different renewable energy sources
Mechanical	FES			≤10	≤20	≤10 ms	≤1 h	Demo ∼ Mature	≤20	≤85	High power density Nonpolluting High efficiency	Not enough safe Noisy High speed operation let to vibration
	PHS	small		≤5000	≤500	sec ∼ min	6 ∼ 24 h	Mature	≤70	≤85	Remote operation is possible Low man power factor Relatively low maintenance	Silt build-up Impedance to the movement of environmental issues
		large		≤14000	≤1400	sec ∼ min
	CAES	underground	small	≤1100	≤135	≤ 15 min	≤8 h	Demo ∼ Commercial	≤40	≤85	High power capacity Low losses(can be storage energy for more than a year) Fast startup	It is not possible to install everywhere and the location is depend on a geological structure
			large	≤2700	≤135	≤ 15 min	≤20 h
		above ground		≤250	≤50	≤15 min	≤5 h	Demo
Electrical	DLC			0.1 ∼ 0.5	≤1	≤10 ms	≤1 min	Commercial	≤40	≤95	High power density Low resistance high efficiency	Low energy density Low voltage per cell Incomplete capacity utilization
	SMES			1 ∼ 3	≤10	≤10 ms	≤1 min	Commercial	≤40	≤95	High power High efficiency Environmentally safe	For sizing of high energy storage need to long loop Cooling system in needed expensive
Thermal				≤350	≤50	≤10 min	N/A	Mature	≤30	≤90	Nonpolluting Unlimited energy source	Expensive Depend on a geological structure

Table 3

There are three major parameters that are important in defining the battery types suitable for an application: the high or low rate service, the response and discharge times, and the environmental matching. As shown in Table 3, the battery’s energy storage can support the system in ancillary service and customer energy management applications. The technique is also possible for renewable energy integration, such as time shifting and capacity firming, while the technique cannot support the system in bulk energy applications. Another technique that has been described is the flywheel; this is used for low-energy applications, emergency devices, and load levelers. It cannot be used on the large scale, but it may be useful when it gives an economic advantage [73]. The other mechanical energy storage techniques (CAES, PHS) are also suitable for most of the applications expected of customer management and voltage support in ancillary service categories. Electrical energy storage techniques can be used just for emergency devices and applications that need very rapid responses.

6. The integration of energy storage technologies

The cost of the energy storing process is high. However, because of the variation in generation and the need to balance power and regulate voltage and frequency, the use of energy storage systems is unavoidable in the modern grid. One solution to the problem of the high cost of energy storage may be the integration of different technologies for implementing specific application. To cover this methodology, the characteristics of each technology should be analyzed. Table 2 shows specific information on energy storage technologies—namely, the minimum and maximum capacities and powers, the high and low response rates, and the discharge times. The requirement characteristics for implementation of each application are shown in Table 1. Finally, the agreement of these data with each other (shown in Table 3 as a matrix) can be used to create a categorization of energy storage systems that can be integrated together. Based on the results of this work in Tables 1–3, Fig. 7 is a comprehensive diagram of technologies and applications that can be integrated together. Based on the provided figure, batteries and FES can integrate together to cover the system for voltage support and power reliability applications. Moreover, power quality application can be supported by integrating electrical energy storage with batteries and FES. Mixing CAES with PHS and thermal techniques can cover capacity firming and spinning reserve, respectively and using these three methods (PHS, CAES, and Thermal) can maintain energy arbitrage, peak shaving, and time shifting. Finally, the black start application can be managed by combining PHS, CAES, and batteries.

1. Download full-size image

Fig. 7

7. Conclusion

To design an optimum energy storage system, selecting the ESS type most closely related to the application is the most significant issue, but control methodologies should not be neglected either. There are many different characteristics of energy storage systems that can help to match the different techniques with applications. This paper provides a matrix of the relations between these, along with a comprehensive diagram of ESS solutions that can be integrated together. To provide the matrix, storage technologies and application have been compared on the basis of many different parameters, such as capacity, storage power, response time, discharge time, life time, efficiency, cycle life, and maturity. Electrical energy storage techniques have only a limited number of potential applications, focusing on power system transient issues, such as improving power quality. On the other hand, electrochemical storage is the most commonly used technique and covers many applications, such as voltage support, black start, and frequency regulation. Mechanical storage techniques can also be useful for bulk energy applications and for supporting renewable integration on a large scale. Finally, based on the provided integration of ESS, the integration between thermal, CAES, and PHS is most supportive as cover energy arbitrage, peak shaving, and time shifting. Moreover, the integration of batteries and FES is another method that can cover voltage support and power reliability applications.

Since the integration of ESS with the aim of reducing the cost has been investigated in this paper only from the technical point of view, analyzing it from the economics side, as well as the issue of determining the precise energy and cost savings, remains a good research question for future work.

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