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Original Research Article

Review of energy storage technologies for sustainable power networks

D.O. Akinyele , R.K. Rayudu

School of Engineering and Computer Science, Victoria University of Wellington, Wellington 6140, New Zealand

a r t i c l e i n f o

Article history:

Received 9 September 2013

Revised 26 June 2014Accepted 22 July 2014

Keywords:

Adiabatic-CAES

Renewable integration

Sub-surface pumped hydro

Tri-generation

Underwater-CAES

a b s t r a c t

A significant percentage of the global energy demand is expected to be met through widespread supply of

renewable electricity in the near future. However, renewable energy outputs are variable due to a sto-

chastic characteristic of their sources. Electrical power system operators around the world are faced withdifficulties of integrating these variable power sources into the existing power grids. Energy storage sys-

tems are one of the possible solutions for mitigating the effects of intermittent renewable resources on

networks, allowing increased renewable energy utilization, and providing flexibility and ancillary ser-

vices for managing future electricity supply/demand challenges. This paper presents a comprehensive

review of energy storage technologies that are currently engaged for power applications, including

pumped hydro, compressed-air, battery, flywheel, capacitor, supercapacitor, superconducting magnetic

and thermal systems. The study compares the characteristics of these systems, and presents their

technological development status and capital costs. Some directions for future work are also highlighted.

Furthermore, particular attention is paid to some new storage technologies such as: adiabatic, underwa-

ter, isothermal and small-scale compressed-air; sub-surface, seawater and variable-speed pumped hydro,

and pumped heat systems, which hold opportunity for future smart electrical grid applications, but there

is need for more research to actualize their promising potentials.

2014 Elsevier Ltd. All rights reserved.

Introduction

Recent concerns on the increase of carbon dioxide emissions

(CDE) in the environment, rising energy demands, and the liberal-

ization of the electricity sector have informed the attention of glo-

bal community to renewable energy technologies[1]. Renewable

energy, such as solar, wind, hydro, biomass, geothermal, wave or

tidal is an energy obtained from natural and persistent flows of

energy occurring in the immediate environment, with a huge

potential for alternative electricity generation[2]. Though integra-

tion of intermittent renewable energy (RE) generation is new in the

evolution of electrical power systems, it is receiving increasing

attention around the world, due to certain technical benefits itoffers, including improvement of power quality and voltage profile,

enhancement of voltage stability and reliability, and grid support,

etc. [3]. However, grid-integrated renewable systems affect the

operational characteristics of existing power networks, because

of the stochastic nature of renewable energy sources[4,5]. In cer-

tain cases, the grid may not be able to accommodate the entire

RE generation at the PCC, and this will lead to curtailment of power

generation, to a level that could be allowed by the network

characteristics [6]. This development will restrict renewable

energy production, and limit its further uptake in the future. Com-

pensation for variability in power networks at different time scales

is being achieved by some generators which are dedicated for

intermediate and peaking generation, and operating reserves[7].

However, the application of demand response (DR) and ESSs have

been identified as possible solutions for intermittency of renew-

able energy resources[7]. Renewable energy resources are depen-

dent on weather conditions, and because of this, they cannot be

dispatched; this means that if renewable electricity is not stored,

it must be utilized as soon as it is generated[8]. ESSs have a capac-

ity to absorb the variability of renewable energy sources[8], and

can allow renewable power to be dispatched. They are useful inevery section of electrical power systems; for large (GW), medium

(MW), or micro (kW) scale applications, depending on the function

and location. At generation, they are employed for energy arbi-

trage, balance and reserve; for frequency regulation and invest-

ment deferral at transmission level; for voltage control,

investment deferral, grid capacity support at distribution level;

and for peak shaving, ToU cost management, etc. at the cus-

tomer-side[9,10]. They are also a necessary component for smart

grid systems, which are expected to thrive in the future for wide-

spread deployment of distributed generation technologies.

Some recent scholarly research has been conducted on the

applications of energy storage systems for electrical power

http://dx.doi.org/10.1016/j.seta.2014.07.004

2213-1388/ 2014 Elsevier Ltd. All rights reserved.

Corresponding author. Tel.: +64 223891364.

E-mail address:daniel.akinyele@ecs.vuw.ac.nz(D.O. Akinyele).

Sustainable Energy Technologies and Assessments 8 (2014) 7491

Contents lists available at ScienceDirect

Sustainable Energy Technologies and Assessments

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s e t a

http://dx.doi.org/10.1016/j.seta.2014.07.004mailto:daniel.akinyele@ecs.vuw.ac.nzhttp://dx.doi.org/10.1016/j.seta.2014.07.004http://www.sciencedirect.com/science/journal/22131388http://www.elsevier.com/locate/setahttp://www.elsevier.com/locate/setahttp://www.sciencedirect.com/science/journal/22131388http://dx.doi.org/10.1016/j.seta.2014.07.004mailto:daniel.akinyele@ecs.vuw.ac.nzhttp://dx.doi.org/10.1016/j.seta.2014.07.004http://crossmark.crossref.org/dialog/?doi=10.1016/j.seta.2014.07.004&domain=pdf

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applications. One of such is a technical report in[11]by NREL on

the role of energy storage technologies with RE electricity genera-

tion, focusing on large-scale deployment of intermittent RE

resources. Jiang et al. proposed a robust unit commitment with

wind power, in which pumped hydro storage technology is applied

for absorbing the variability of wind energy resource[4]. An exec-utive overview of energy storage options for a sustainable energy

future has been presented in[12], which deals with the economics

and competitive edge of energy storage technologies. A hybrid

energy storage system (HESS) controlled by wavelet-based capac-

ity configuration, for smoothing out wind power intermittency

has been presented in [13]; where the author discussed a hybrid

option consisting of an ultra-capacitor and lithium-ion battery

banks, for better storage performance. Tewari et al. presented the

analysis and results of capacity of sodium sulfur (NAS) battery

for grid integration of wind power [7]. Competitive storage sys-

tems with RE applications have also been presented in [14]. In

[15], the author considered an energy management approach of

employing flywheel storage for smoothing fluctuation of wind

power output. Also, a critical review of the progress in electricalenergy storage system was discussed by Chen et al., focusing on

the descriptions, applications, and comparison of energy storage

systems [16]. The application of Vanadium Redox flow battery

has been proposed in [17] for enhancing integration of wind

power. An optimal power flow has also been discussed for distribu-

tion power networks with distributed generation (DG) and battery

energy storage[8]. While the author in Ref. [18] presented ade-

quacy and economy analysis of distribution systems integrated

with electric energy storage and renewable energy resources,

[19] proposed how distribution network systems could be

expanded with DG and energy storage units, by applying a modi-

fied Particle Swarm Optimization (PSO) algorithm. A study has

been conducted on the economics of compressed air energy stor-

age (CAES) system to interconnect wind power with the grid fora case study of Electric Reliability Council of Texas (ERCOT) [20].

An overview, applications, technologies and economical evaluation

of energy storage system has also been presented in Ref.[21].Elec-

tricity energy storage technology options with emphasis on the

applications, costs and benefits are reported in Ref. [22]. The

research in Ref. [23]investigates the contribution of electric vehi-

cles to primary frequency response of the Great Britains powersystem. The history, evolution, and the future status of energy stor-

age technologies are discussed in Ref.[24], focusing on large- and

small-scale storage options, and possible prognosis. The research

work in Ref.[25]focuses on energy storage technologies for trans-

port and grid applications. The author in Ref.[26]studies the global

energy scenario and impact of power electronics in 21st century;

the impact of power electronics in RE, storage technologies and

electric/hybrid vehicles has also been discussed. The study in Ref.

[27] presents the role of energy storage in power networks, and

how the capacity of power networks will be met in the future,

and also suggests other possible solutions apart from storage sys-

tems. The seasonal energy storage in a RE system devoid of fossil

fuels has also been presented[28]. The research in Ref.[29]studies

how energy storage is considered as the core of RE technologies.Battery storage for enabling integration of distributed solar power

generation has been proposed in Ref.[30], with focus on mitigation

of the negative effects of integrating photovoltaic systems. A schol-

arly research on electric vehicles charge forward has been dis-

cussed in Ref. [31], which reviews the global status of electric

vehicles and hybrid electric vehicles. Carrasco et al. presents a sur-

vey of power-electronic systems for grid integration of RE sources

[32].A review of configurations, control and applications of hybrid

renewable/alternative energy systems for electric power genera-

tion has also been studied[33]. The research in Ref. [34]deals with

the history, present state, and future prospects of underground

pumped hydro system for achieving massive energy storage. An

investigation of optimized thermal and electrical scheduling of a

large-scale virtual power plant in the presence of ESSs has beenconducted in Ref. [35]. The research in Ref. [36] proposes an

Nomenclature

ATES aquiferous thermal energy storageBES battery energy storageBP bridging powerCAES compressed air energy storageCES cryogenic energy storage

CEST chemical energy storage technologiesCDE carbon dioxide emissionCS capacitor storageCSP concentrated solar plantDER distributed energy resourcesDG distributed generationDR demand responseESS energy storage systemEEST electrical energy storage technologiesEstorage energy storedEM energy managementEV electric vehicleFES flywheel energy storageHEB high energy batteryHS hydrogen storage

HTES hot thermal energy storageHupper height of upper hydro reservoirHlower height of lower hydro reservoirLi-ion lithium-ionMEST mechanical energy storage technologiesNaS sodium sulfurNBC novel battery chemistries

NiCd nickelcadmiumPHS pumped hydro storagePHES pumped heat electrical storagePHEV plug-in hybrid electric vehiclePCC point of common coupling

PSBB polysulphide bromine batteryPCM phase change materialsRE renewable energyRTE round trip efficiencySCES super capacitor energy storageSMES superconducting magnetic energy storageSoS state-of-chargeSPHS seawater pumped hydro storageSSPHS sub-surface pumped hydro storageTES thermal energy storageTEST thermal energy storage technologiesT/D transmission and distributionToU time-of-useVSPHS variable speed pumped hydro storageV2G vehicle-to-grid

VRB vanadium redox batteryZEBRA zero emissions batteries research activityZnBr zinc brominex velocity of flywheelI moment of inertia of flywheel rotorq density of water

D.O. Akinyele, R.K. Rayudu/ Sustainable Energy Technologies and Assessments 8 (2014) 7491 75

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intelligent home energy management system for the purpose of

improving demand response. Also, the study in Ref. [37] focuses

on balancing wind intermittency by employing hydro reserves

and demand response.

While some of these studies have highlighted the roles of major

energy storage systems being currently used, a major part of the

literature concentrates on the grid applications of storage devices.

However, none of these research works have considered newsystems such as: adiabatic-CAES, underwater/ocean-CAES, isother-

mal-CAES, small-scale CAES, sub-surface PHS, seawater PHS, vari-

able-speed PHS and pumped heat electrical storage systems,

which hold opportunity for future power applications.

Energy storage devices may be applied in other systems, such as

portable devices and electric vehicles[16], however, the intent of

this study is to review the state-of-the-art development of ESSs,

which are currently engaged for power applications including

pumped hydro storage (PHS), compressed-air energy storage

(CAES), battery energy storage (BES), capacitor storage (CS), sup-

ercapacitors energy storage (SCES), flywheel energy storage (FES),

superconducting magnetic energy storage (SMES) and thermal

energy storage (TES) technologies. The utilization of ESSs for elec-

trical applications is referred to as stationary application. The

development status, comparison and capital cost metrics for these

technologies are also discussed. While some of these storage tech-

nologies are matured and have gained commercial acceptance,

some are still in the early stages of development, demonstration,

pilot, and laboratory or idea stage [22]. There is need for more

research to improve the performance of some of these technologies

at optimum costs, and also develop innovative options for those

ones having negative environmental influence, including CAES,

PHS, NiCd. Furthermore, in this paper, we paid particular atten-

tion to recent technological breakthroughs leading to the develop-

ment of some new storage technologies including adiabatic-,

underwater/ocean-, isothermal-, and small-scale CAES; sub-sur-

face, seawater and variable-speed pumped hydro storage, and

pumped heat electrical storage systems, which require further

research to realize their promising potentials for addressing futureelectricity supply/demand challenges.

Energy storage systems

An electrical ESS includes a means by which electricity

imported from a power grid, is converted into a form that could

be stored at off-peak demand, when energy cost is usually low or

during surplus generation, and converted back to electricity at

peak demand or when needed. Its benefits are well summarized

in Refs. [16,21,22,25,38], some of which are discussed later in this

section. A storage system is made up of four main components

including the following[38]:

Storage medium: this is a means or system through whichenergy is stored such as BES, CAES, CS/SCES, FES, PHS, SMES

and TES.

Charging: this unit permits the flow of energy from the

electrical network to the storage medium.

Discharging: it ensures the delivery of the stored energy when

demanded.

Control: it governs the entire storage system.

An ESS is illustrated in Fig. 1. The interface links the energy

flows from the storage system with the electrical network during

charging and discharging.

The conventional electricity system links the following sections[16]: fuel/energy source, generation, transmission, distribution and

customer-side. ESSs are expected to play a major role across these

sections in the future, offering flexibility to power networks. There-

fore, integrating storage devices into power systems provides some

benefits such as: hedge risk, energy arbitrage, high-energy utiliza-

tion, stability and improved power quality; however, electrical

networks without storage systems face the technical challenges

of: unpredictability, low-utilization of energy generation, conges-

tion, security and poor power quality, respectively [16,39]. Fig. 2

shows the role of a large-scale energy storage system with base-

load, intermediate and peak generation, at different periods of

the day[39]. The large-scale storage system could provide electric-

ity during peak periods, at optimum costs; it has potential to

reduce the requirements for expensive peaking generation; and

could also facilitate reductions in transmission losses, and

improvement of system power quality and reliability[24,39].

Energy storage technologies

Though it is difficult to achieve a cost-effective means of storing

electricity directly, it is possible to store it in other forms and

convert back to electrical energy when demanded [16]. Storage

technologies for power applications are categorized into the

following[16]:

EEST include CS, SCES and SMES systems.

MEST include FES, CAES and PHS systems.

CEST include BES, FC systems.

TEST include ATES, CES, HTES, PHES systems.

However, energy storage technologies are also classified based

on their discharge capacity. Classification of ESSs in terms of their

discharge duration is shown in Fig. 3. The storage systems with

discharge time of seconds to minutes are used for achieving power

quality (PQ); those with discharge capacity of minutes to an

hour are employed for bridging power (BP), and systems with dis-

charge time of hours are used for energy management (EM) appli-

cations [11,16]. The following are some applications of ESSs

[25,38,127129]:

(1) Energy arbitrage: It involves storing electricity at off-peaks

when the cost is low, and selling it at peak demand periods

when the cost is high.(2) Load leveling:It is the utilization of the stored energy at peak

periods, thereby reducing the requirements of peaking

generators.

(3) Renewable integration: Storage systems could minimize the

effect of intermittency of renewable energy resources and

increase their penetration in power grids, thus, allowing

renewable generation to be dispatched.

(4) Spinning reserve:Storage systems could reduce the require-

ment for idlinggenerators in power systems. Such generators

are dedicated to take over of any sudden failure of a major

generator, but ESSs could defer the option of operating them.

(5) Customer-side peak shaving:This involves reliability back-up

support by ESSs through the use of uninterruptible power

supply (UPS) to address short- and long-period interrup-tions, voltage peaks and flickers.Fig. 1. Components of an energy storage system [38].

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(6) Primary frequency regulation: This involves the provision of

frequency stability support for power networks through

the charge/discharge characteristics of ESSs, thereby

regulating the voltage and frequency.

(7) Investment deferral: Involves the use of storage to

defer transmission and distribution (T/D) infrastructure

investment.

Energy storage for power quality support

Power quality is of great importance to electrical power systems

operation, due to requirements of generating undistorted (sinusoi-

dal) power and supplying the end-users at acceptable voltage

levels[40]. The PQ support by ESSs requires a rapid response whichincludes frequency regulation and transient stability, for a dis-

charge time up to 10 min, and response of milliseconds

[11,16,27,30]. The typical power rating for CS in this application

is less than 1 MW, SCES up to 1 MW; FES between 10 kW and

1 MW, and SMES from 1 MW to 10 MW; these technologies have

a high power density but a relatively small energy density.

Capacitors

Capacitors offer a direct storage of electricity. They store elec-

tricity at a very fast rate, possessing several thousand charge/dis-

charge cycles without material degradation compared to

batteries, and are suitable for transient voltage stability applica-

tions [11,16,21,24]. However, conventional capacitors are limited

by their low energy density; this shortcoming has triggered inno-vative research towards the development of electrochemical

capacitors, referred to as supercapacitors. SCES systems possess

capacitance and energy density values which are thousands of

times higher than those of conventional capacitors, due to their lar-

ger surface area[21]. Structurally, conventional electrolytic capac-

itors store energy through two parallel plates separated by a

dielectric material, but in SCES, energy is stored through an elec-

trolyte solution (propylene carbonate) between two plates, insteadof dielectric between electrodes that is employed in conventional

capacitors [16,25]. The challenges with capacitors are: short dis-

charge time and high self-discharge losses[16], which have limited

them to applications with shorter timescales. Eq. (1) shows the

relationship between the energy stored in a capacitor, its capaci-

tance (C) and voltage (V). CS and SCES have an expected lifetime

of about 15 and 20 years, respectively [16], their structural

schematics are shown inFig. 4.Though SCES systems possess high

power and energy densities compared to conventional CS systems,

their energy density is still lower than that of leadacid battery

[21,24,25]. It has been suggested that power capacity of capacitors

will be most suitable for stationary applications rather than their

relatively low energy capacity[24].

Estorage 0:

5CV2 1

The energy storage capacity of a capacitor is directly proportional to

the square of its voltage[12]. Increase in capacitance and voltage

values of capacitors will also lead to an increase in energy.

Flywheel energy storage

A flywheel is essentially an electromechanical system which

stores energy in the form of kinetic energy; it is made up of a rotat-

ing cylinder on magnetic bearings coupled with an electrical

machine, which behaves as a motor during charging, and as a gen-

erator during discharging[11,15,21]. The levitated magnetic bear-

ings are employed to minimize frictional effects and also increase

the lifespan of the system; FES systems are operated in vacuums

to ensure minimization of drag and maintain good performance[11,15,21]. To store energy, the machine operates as an electrical

0 6am

Baseload

generation

6pm midnight

Time of day

Generation

profile without

storage

Storage used to

maintain frequency and

voltage by balancing

supply and demand

Storage charged

from baseload

generating plant

Storage charged

from baseload

generating plant

Generation profile

with storage

Generation profile

without storage

Peak demand for power

supplied by peaking plant,

running only a few hours

each day

Storage discharginginto network

midday

Midmerit

generation

Peaking

generation

SystemDemand/MW

Fig. 2. Load profile of a large-capacity energy storage system[39].

Fig. 3. Classification of storage technologies based on time scales.

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motor with a very high speed (10,000 r.p.m for low-speed rotors,

>10,000100,000 r.p.m for high-speed rotors), to spin the flywheel;

to discharge the energy, the motor regenerates through the drive

(reverses), and supplies power to the network through a suitable

interface power electronic converter [21,42]. Eq. (2)representsthe energy stored in a FES system[2,42]. The moment of inertia is

a function of mass and radius of the rotor[2], and the higher the

velocity of FES, the larger the energy stored. Flywheels have an

expected lifetime of15 years[16]with several thousand charge/

discharge cycles compared to batteries, but possess short discharge

duration like capacitors; they have high self-discharge and fric-

tional losses. FES systems have a high efficiency, which is usually

in the range of 90% and 95% [2,10,16,2426].Fig. 5shows the bea-

cons generation 4 flywheel storage technology, which has up to

100,000 charge/discharge cycles and a lifespan of >20 years [43].

Estorage 0:5Ix2 2

Superconducting magnetic energy storageWhen a direct current (d.c) passes through a normal coil, the

resistance of the coil will make the current to die out quickly.

However, when a d.c flows through a superconducting coil, the

electrical energy will certainly not die out, in this way, the energy

is stored in a magnetic form till it is needed [120]. The direct

current induces a magnetic field in the superconductors, thus,

allowing energy to be stored in SMES [16,21,25]. SMES is made

up of the following components as shown inFig. 6[16,42,120]:

Superconductor: is a superconducting coil made of an alloy,called niobiumtitanium which operates at about the boiling

point of liquid helium (269C).

Cryogenic refrigerator: it utilizes helium as a refrigerant and

maintains the operating temperature of SMES by keeping the

superconducting feature of the coils.

Power conversion system (PCS): it ensures effective charging

and discharging by providing a positive voltage across the

superconductor in order to store electrical energy, and a

negative voltage to release the stored energy when needed.

Control: it ensures the coordination between the electrical net-

work and the energy flow to and from the superconducting

coils.

Eq.(3)represents the energy stored in the magnetic field of thesuperconducting wire, where I and L are current and inductance of

the wire, respectively[16,42]. The energy stored is a function of the

amount of direct current allowed in the inductive coil; SMES has an

efficiency of 98% with a switching time of 17 ms between charge

and discharge (fast response); it also has exceptional power den-

sity and expected life span of >20 years; however, short discharge

duration, high system cost and the environmental concerns of

strong magnetic field, are major challenges of SMES [12,16,25].

Estorage 0:5LI2 3

Energy storage for bridging power

The bridging power applications include load following,spinning reserve and forecast uncertainty [11], requiring a rapid

response in seconds to minutes and discharge time of up to 1 h,

and ensuring the reliability of power supply to customers. It has

a typical power rating of 1 kW10 MW, as reported in Ref. [46].

Batteries are the most commonly used storage technologies; they

Fig. 4. Schematic diagrams of three types of capacitors [41].

Fig. 5. Beacons Gen 4 Flywheel system [43]. Fig. 6. Components of SMES system [120].

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are suitable for BP applications due to less cycling requirements

compared to PQ applications; BES for BP include leadacid, Ni

Cd, NiMH and Li-ion, and these systems can respond to load

changes within 20 ms; they have a round trip efficiency (RTE)

between 60% and 90% depending on the technology [8,11,12,16].

Leadacid batteries

Leadacid is the oldest and most commonly used rechargeableBES [16]. It consists of series-connected cells, an electrolyte, and

the positive and negative electrodes [16,24]. In the charged state,

the battery consists of lead (Pb) and lead oxide (PbO2) in 37% tet-

raoxosulphate (VI) acid (H2SO4); however, in the discharged state,

lead sulphate (PbSO4) is produced both at the anode and the cath-

ode, while the electrolyte changes to water[16]. A porous separa-

tor is placed between the electrodes to prevent them from having

a contact [24]. The chemical reactions at anode and cathode are

represented by Eqs.(4) and (5),respectively[16]. Because the tet-

raoxosulphate (VI) acid is consumed in the discharged state, SoC of

the battery could be determined by measuring the concentration of

the electrolyte through the specific gravity (SG) method[16,42].

For batteries designed for temperate regions, SG for the fully-

charged and the fully-discharged states is 1.21.3 and 1.01.2

[42], respectively. Furthermore, there are two types of leadacid

batteries namely: flooded or vented and sealed or valve-regu-

lated[42]. By Ohms law, the same current flows through the ser-

ies-connected cells, and the total voltage is the sum of the voltages

across the cells. The nominal voltage of a single leadacid battery

cell is 2 V. Therefore, stringing 6 units of 2 V cells will produce a

12 V battery. Eqs.(6) and (7)show the current and voltages of ser-

ies-connected cells. Battery units are arranged in series or parallel

to achieve a desired electrical configuration voltage and ampere-

hour (Ah) capacity. For instance, a parallel connection of 4 units of

12 V 100 Ah batteries will produce a 12 V 400 Ah configuration. On

the contrary, a series arrangement of 4 units of 12 V 100 Ah batter-

ies will yield a 48 V 100 Ah capacity. Because batteries store direct

current, power electronic converters are required to interface them

with power networks. Leadacid batteries possess a low cycle life(20002500), and a RTE of 7090% with an expected lifetime of

515 years; they also have a negative influence on the environ-

ment by generating toxic remnants [16]. They are suitable for

cost-sensitive applications such as automotive starting, lighting

and ignition and uninterruptible power supplies[25]. A Leadacid

battery is illustrated inFig. 7(a) and (b).

Pb SO24 $ PbSO4 2e

4

PbSO2 SO24 4H

2e $ PbSO4 2H2O 5

I I1 I2 . . . . . . . . . . . . . . . IN 6

V V1 V2 . . . . . . . . . . . .VN 7

where I, I1, I2and INis the current through the battery, cell 1, cell 2,

and cell N, respectively; V, V1, V2and VNis the output voltage of the

battery, voltage across cell 1, cell 2 and cell N, respectively.

Lithium-ion batteries

This technology is

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Nickel cadmium batteries

There are five BES technologies which use the nickel-electrode

[42], these include nickeliron (NiFe), nickel cadmium (NiCd),

nickel hydrogen (NiH2), nickel metal hydride (NiMH), and

nickelzinc (NiZn); NiCd and NiMH are popular than the oth-

ers, but NiCd is the most utilized nickel-electrode technology in

modern utility industries. The drawbacks of NiCd batteries are a

low cycling capacity of 20002500, a memory effect and the toxiccadmium material which poses a threat to the environment

[16,24,25]. The cathode of this battery is nickel hydroxideNi(OH)2in a spongy form, which is converted to nickel oxyhydroxide NiO-

OH during the charging process; its anode is a metallic cadmium,

which is converted to cadmium hydroxide Cd(OH)2 by oxidation

when it is charged in the presence of aqueous potassium hydroxide

KOH(H2O) electrolyte; during discharge, NiOOH reacts with H2O

to produce Ni(OH)2 and hydroxide ion at the positive electrode

[42]. Eq. (10) represents the reversible reaction in the battery

system[16]. NiCd battery is illustrated inFig. 8(b).

2NiOOH Cd 2H2O $ 2NiOH2 CdOH2 10

Metalair batteriesThese batteries are a type of fuel cell which employs metal as

the fuel and air as the oxidizing agent[16]; they are environmen-

tally friendly and have a potential to offer a cost-effective storage

option in the future; however, the major challenges with metal

air batteries are that they have a low RTE (100 MW1GW; NaS batteries are utilized for

applications >10 MW; large-scale batteries, flow batteries and

high-temperature TES, which are also termed medium-scale ESSs

are used for systems with a storage capacity of 10 to 100 MW

[16,22,46].

Compressed-air energy storage

CAES systems store energy in the form of a compressed air in

underground caverns[93], and the energy is later supplied to elec-

trical networks through a conversion process. The air is com-

pressed into an underground reservoir, which facilitates the

storage of energy in a pressure gradient; the energy is discharged

through a combustion process to operate an expansion turbine

which spins an electrical generator[20]. The heat energy generated

during air compression (charging) is released to the atmosphere,

and during decompression (discharging), the air needs to be

reheated, usually with a fuel [97]. The components of a CAES are

shown inFig. 9. CAES systems have a RTE of85%, and an expected

life time of 2040 years; they are a suitable option for large-scale

storage applications [11,12,16,24,48]. The challenges with CAES

Fig. 8. (a) Schematic diagram of Li-ion. (b) Diagram of NiCd batteries [53,58].

Fig. 9. Schematic diagram of CAES[20].

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systems include the requirement for an underground cavern, dissi-

pation of heat into the atmosphere, consumption of fossil fuels, and

generation of pollutant emissions from their combustion processes

[16,97,98]. The diabatic-CAES system has been described above;

however, recent technological efforts have led to development of

the following systems:

Adiabatic-CAES: this system has been designed to eliminate theneed for fuel in CAES technologies, by storing the heat energy

generated during compression, and then use it to reheat the

air during expansion[97,98]. In this case, the combustion cham-

ber in the conventional CAES has been replaced by a thermal

energy storage (TES) system. Adiabatic-CAES is a promising

technology because of its scalability, environmental friendli-

ness, hybrid storage property (CAES/TES) and high energy stor-

age efficiency[98,99]; the elimination of combustors and fossil

fuels also gives it an economic advantage over the conventional

CAES. The major components of an adiabatic-CAES plant include

compressor, TES, underground cavern and air turbine [98].

These components could be configured into the desired storage

scale. Salt and concrete are suitable thermal energy storage

materials in adiabatic-CAES system[99]. The worlds first adia-

batic-CAES plant, referred to advanced adiabatic-CAES (AA-

CAES) demonstration plant, is expected to go on stream in Ger-

many by 2016, by RWE Power and its partners [99101]. The

focus of the project is to achieve system efficiencies of 70%.

The AA-CAES by RWE is shown inFig. 10.

Isothermal CAES: this emerging technology attempts to address

some of the challenges with conventional diabatic- or adiabatic-

CAES[98,102], by eliminating the requirement for fuel and high

temperature heat energy storage, thus, offering an improved

RTE of (7080%) and a relatively low cost. The air is com-

pressed without a change in temperature, allowing minimal

work for compression while maximizing the work needed for

expansion, through effective heat transfer with surroundings

of the air vessel[98,99].

Small-medium scale CAES: large-scale CAES plants usuallyrequire suitable geological formations for storing air [16,98

101]. However, small-medium scale CAES, with a capacity of

110 MW, having artificial pressure vessels is a more flexible

CAES option without caverns and TES[103,104]. This technol-

ogy can be used for a tri-generation purpose, including

distributed electricity generation and storage, air-cycle heat-

ing and cooling in a single process [98], which can signifi-

cantly reduce energy costs and greenhouse gas emissions in

the future.

Underwater/ocean-CAES: this is a promising storage option in

the absence of underground cavern, which could be integrated

with offshore renewable energy resources such as wind, tides

and waves[99]. The compressed air is stored in an underwaterair storage chamber installed on the seabed; the pressure of the

compressed air is kept constant requiring no pressure throt-

tling, thus, allowing efficient extraction of energy from the com-

pressed air [99]. The authors in Ref. [98,99] have proposed

constant-pressure CAES combined with pumped hydro storage,

and conceptual design of ocean compressed air energy storage

(OCAES), respectively. Also, an energy storage solution has been

patented by Hydrostor, based in Canada. This technology uti-

lizes semi-adiabatic underwater-CAES (UW-CAES) with a

potential to store large-scale electricity for durations of

448 h, at applications between 1 and 50 MW[105]. UW-CAES

is promising for future applications of microgrids and DERs. It is

also expected to be delivered at optimum costs for the intended

markets.

Pumped-hydro storage

The PHS is also a means of storing off-peak electricity from

power grid and delivering it at peak demand periods. It is a low

cost and widely employed option for large-scale storage applica-

tions; it consists of two interconnected reservoirs at different ele-

vations (lower and upper), a means to pump water to the upper

reservoir at off-peak hours and a turbine to produce electricity

when water is released to the lower reservoir at peak demand peri-

ods[12,16,24,26]. PHS has a RTE of 7080%, but usually at 75% [12]

depending on its scale, design and the technical arrangement; it

has an expected lifetime of 4060 years[16]. Eq.(11)shows that

the energy stored in a PHS is proportional to the product of volume

of water, and the difference in height between upper and lower

reservoirs [21]. The constant of proportionality is the product ofthe density of water and the acceleration due to gravity. If a big

volume of water or a large difference between the two heights

could be achieved, a large energy will be stored by PHS.

Estorage aVHupper Hlower 11

Fig. 10. Diagram of adiabatic-CAES[100].

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Eq.(11)is re-written as: E = qgV(Hupper Hlower), where q, g and V

are density of water, acceleration due to gravity and volume of

water, respectively. PHS is illustrated inFig. 11. The major problems

with PHS technologies include the need for a suitable site (usually

large land mass), high capital cost, long construction time, and envi-

ronmental concerns[16]. However, innovative research has led to

the development of some systems which have potentials to reduce

the environmental impacts of PHS technologies. These systems

include sub-sea PHS, seawater PHS, and variable-speed PHS.

Sub-surface pumped hydro storage (SSPHS): PHS systems nor-

mally involve upper and lower reservoirs, and while some

pumped hydro systems use rivers as the lower reservoir, others

have employed ocean or massive lakes [106]. The SSPHS con-

cept attempts to position either the lower or upper reservoir, or

both reservoirs at sub-surface (below ground) [106]. Due to

high cost of underground excavation and construction, poten-

tial developers make use of existing subsurface structures for

building SSPHS plants[106]. The schematic diagram of a sub-

surface PHS is shown inFig. 12. Elmhurst Quarry pumped stor-

age project developed by DuPage County, Illinois, US, has been

proposed to use an abandoned mine and quarry for both reser-

voirs, with a rating of 50250 MW and energy storage capacity

of 708.5 GWh [106]; other SSPHS projects are from Riverbank

Wisacasset Energy Centre (RWEC) and Gravity, LLC. While

RWEC proposed a large underground PHS reservoirs

(1000 MW), positioned 2200 feet beneath the earth in Wisacas-

set, Maine; Gravity Power, LLC is working towards developing a

Gravity Power Grid-Scale electricity storage system [106].

Though SSPHS systems have not yet been completed at the

moment, they are promising systems for future applications,

because of the availability of location for upper and/or lower

reservoirs, and potential to minimize environmental influences.

Seawater pumped hydro storage (SPHS): this system uses the

ocean as its lower reservoir[107]. The Okinawa Yanbaru SPHS,

having a capacity of 30 MW, located in Japan is currently the

only PHS plant which utilizes seawater in the world[107], withPacific ocean as its lower reservoir and a man-made upper res-

ervoir. However, other SPHS projects have recently been pro-

posed including 480 MW SPHS plant in Glinsk, Ireland;

300 MW SPHS system in Lanai Hawaii[108]. The proposed SPHS

in Ireland is expected to store about one-third of the surplus

power from 5000 MW wind generators that will be in operation

by 2020; also, the SPHS plant in Hawaii is expected to increase

renewable energy penetration by storing excess electricity in

the near future, from a proposed 400 MW wind turbines

[108]. Though SPHS systems suffer from additional operations

and maintenance (O and M) costs due to a highly corrosive envi-

ronment for pumping turbines and marine growth on hydraulic

structures [107], they utilize less land (only one reservoir is

needed on land), and can also be sited close to renewable power

generating plants such as wind and solar, which can lead to

integration of a greater proportion of renewable energyresources into power grids, in the future[108].

Variable Speed Pumped Hydro Storage (VSPHS): majority of PHS

systems around the world employ fixed-speed pump turbines

[109]; the reversible single-stage Francis pump turbine

developed in the US, is an example of a fixed-speed turbine used

for PHS. This proven system operates as a pump in one direc-

tion and as a turbine in the other; however, it cannot provide

frequency regulation support to the grid because of its fixed-

speed; also, when operating as a turbine, the unit will be unable

to run at maximum efficiency at part load[109]. However, var-

iable-speed turbines will address these draw backs by ensuring

that the power consumed in the pumping mode is varied over a

range of outputs, thus, allowing the machine to operate at peak

efficiency over a wide range of speeds [109]. The widespread

application of variable-speed machines (i.e., doubly-fed induc-

tion motor/generator), and other variable-speed technology

such as cyclo-converters in PHS will enable greater renewable

power penetration in the future, by providing ancillary service

to electrical grids in the presence of a large intermittent renew-

able power input[109].

Thermal energy storage

TES utilizes materialsthat canbe maintained at high or lowtem-

peratures in effectively insulated containments[16]; the heat or

cold that is extracted in the process is utilized for producing elec-

tricity through a heat engine. The thermal storage may be classified

into low- and high-temperature systems, but the working principle

of each one of these depends on the operating temperature of the

corresponding storage medium compared to room temperature[16]; temperature below 18C is used for industrial cooling,

012 C employed for cooling in buildings, between 25C and

50Cforheatinginbuildings,and>175 C forindustrial heat storage.

Low-temperature thermal energy storage: this technology is

classified into ATES and CES systems[16]. ATES involves cooling

or icing of water by a refrigerator, at low energy demand peri-

ods, which can be utilized to meet the cooling requirements

Fig. 11. Schematic diagram of pumped-hydro system [56,57].

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at periods of peak energy demand; CES involves boiling liquid

nitrogen (cryogen) by surrounding heat, and the heated

cryogen is utilized for generating electrical energy through a

cryogenic heat engine system [16]. CES has an expected life

span of 2040 years [16], and an efficiency of 4050%.

Fig. 13(a) illustrates the CES.

High-temperature thermal energy storage: a system which

stores heat energy from a solar thermal plant, in a double tank

of molten salt [25] is illustrated inFig. 13(b). Other potential

media for HTES are concrete, phase change materials, saturated

water/steam and high purity-graphite [16,124]. HTES has an

expected lifetime of 515 years, and a potential for low cost

storage in the future; $3060/kWh based on[16]. Though the

RTE of a CSP may be close to 100%, it is restricted to solar ther-

mal applications, and cannot store electrical energy from any

other source[11]. The worlds largest solar thermal plant, called

Solana Generating Station, has a capacity of 280 MW with 6 h

storage duration in molten salt[110]. A PHES requires two large

heat reservoirs; it stores electrical energy by means of a heat

pump, which pumps thermal energy from cold to hot store

[111]. In order to recover the thermal energy, the operation of

the heat pump is reversed, and it becomes a heat engine in

the power-producing engine cycle to spin a generator for elec-

tricity generation [111,112]. PHES uses gravel as the storage

material, thereby offering a relatively low cost option; it is in

development stage, and its efficiency is also expected to be

between 75% and 80% [111,112]. This technology is scalable

and cost-effective compared to PHS; it can also be deployed

close to habitation with less environmental concerns [112].

These benefits are likely to attract developers and electricity

operators towards PHES in the future.

High-energy batteries

HEB systems that are used for EM applications include NaS,

VRB, sodiumnickel chloride (ZEBRA) and ZnBr. These technologies

are classified into high-temperature and liquid-electrolyte flow

batteries [11], and they may also be employed for all the time-

scales. They could store electricity in a smaller package compared

Fig. 12. Schematic diagram of a sub-surface pumped hydro storage[106].

Fig. 13. Schematic diagrams of: (a) CES; (b) hot-TES[16,54,59].

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to leadacid batteries[11]. The commercially available high-tem-

perature batteries include NaS and ZEBRA; while VRB and ZnBr

are the flow batteries. Flow batteries are scalable, suitable for

large-scale applications, and environmentally benign (non-toxic)

[25,42]. The flow battery is an ESS in which the electrolyte con-

tains one or more dissolved electro-active species flowing through

a power cell/reactor in which the chemical energy is converted to

electricity [16]. Its shortcomings are possession of a low energydensity (though higher than leadacid), and the need for pumping

mechanisms, which might limit their use for stationary applica-

tions in the future [24,25].

VRB: this technology stores energy by electron transfer between

different ionic vanadium materials. During the charge process at

the anode, V3+ is converted to V2+ by accepting an electron, but

during discharge, V2+ ions are converted back to V3+ leading to

thereleaseofanelectron [42]. Thesame process ofelectrontrans-

fer occurs at the cathode between ionic V5+ and V4+ [24,42]. VRB

hasa lifetimeof 510 years and an efficiencyof85%, butposesa

negative effect to the environment by generation of toxic rem-

nants[16].Fig. 14(a) is a schematic diagram for VRB.

ZnBr: this technology is a form of hybrid system in which a

high proportion of the energy is stored through plating solid

zinc onto the negative plates, in the electrochemical stack,

when it is being charged [114]. During discharge, Zn and Br

react together to form ZnBr [16]. The chemical reactions at

the cathode and anode are represented by Eqs. (12) and (13),

respectively[42].

NaS: this technology has a high power and energy density more

than four times that of leadacid BES[25]; it consist of molten

sulphur at the cathode, and sodium at the anode, which are sep-

arated by beta alumina membrane ceramic electrolyte (NaO and

Al2O3); the electrolyte allows the sodium to pass through it and

then combine with sulfur to produce sodium polysulphides

[16,24,25,46]. The battery is maintained at a temperature of

300350 C[16].The shortcoming of NaS is that there is need

for an external heat source for its efficient operation.Fig. 14(b) is a schematic diagram for NaS.

ZEBRA: this BES is called sodium nickel chloride; it has a high

temperature property (300C), and uses nickel chloride and

liquid sodium as cathode and anode, respectively[16,97]; it

can operate between 40C and 70C without the need for

cooling, and also possesses a high cell voltage of 2.58 V. Every

component of ZEBRA batteries may be recycled and processed

into new BESs; however, they possess a lower power and energy

density compared to NaS technologies[97]. Eq.(14)represents

the chemical reactions which occur in sodium nickel chloride

technologies[16].

Br2aq 2e $ 2Braq 12

Zn $ Zn2aq 2e 13

2NaCl Ni $ NiCl2 2Na 14

Comparison of characteristics of storage technologies

The following comparisons for the storage technologies are

based onTables 1 and 2 [16,9396]andFig. 15[46]:

PHS, CAES and CES technologies are appropriate for energy

management (EM) application with power ratings >100 MW

[16,46]. Large-scale (leadacid, Li-ion, NiCd, NiMH), VRB,

ZnBr BESs and HTES are suitable for EM applications with power

rating of 10100 MW[16]. Also, BES technologies for EM appli-cations could also be employed for all the storage timescales

and purposes[11].

BESs including Leadacid, Li-ion, NiCd, NiMH and metalair

with typical power rating of 0.00110 MW [16,46] and dis-

charge duration of seconds to hours are used for bridging power

applications because of less cycling requirements.

CS, SCES, FES, and SMES technologies have discharge duration of

milliseconds to minutes, and are employed for power quality

supports. While the power rating of capacitors and FES is

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It is evident that Li-ion, FES, SMES, and SCES technologies

possess a very high RTE (>90%); lead acid, NiCd, NiMH, NaS,ZnBr, ZEBRA,VRB BESs and conventional CS technologies have

a RTE in the range of 6090%; CES, HES and metalair technol-

ogies have a low RTE (>60%)[16].

ESSs could also be categorized into high-energy and high-power

devices[25]; technologies with slow response are referred to as

high-energy devices, while the ones with fast response are

high-power devices.

Status of storage systems

PHS, CAES, NaS, Li-ion, advanced Leadacid, NiCd, FES and VRB

systems have the following global installed capacities [47];

127 GW, 440 MW, 304 MW, >100 MW, 70 MW, 27 MW,

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Table 2

Additional comparison and cost metrics for the storage technologies.

Technology Energy and power density Round trip efficiency (%) Cost

Wh/kg W/kg Wh/L W/L $/kWh

PHS 0.51.5[16] 0.51.5[16] 7185[16], 7580[93], 5100[16]

6580[95]

CAES 3060[16] 36[16] 0.52[16] 7089[16], 7085[95] 250[16]

CES 150250[16] 1030[16] 120200[16] 4050[16] 330[16]

HTES 80200[16] 120500[16]

NaS 150240[16,94] 150230[16,94] 150250[16,94] 7590[16], 7090[94] 300500[16],

250500[94]

VRB 1030[16] 1633[16] up to 85[16], 6085[94] 1501000[16,94]

ZEBRA 100 120[16,94] 150200[16,94] 150180[16,94] 220300[16] 8590[94] 100200[16,94]

ZnBr 3050[16], 60[93] 3060[16]

75[16], 6075[94],7585[93] 1501000[16,94],500[93]

NiCd 5075[16] 150300[16] 60150[16] 72[93] 8001500[16]

Leadacid 3050[16],

3550[93]

75300[16], 180[93] 5080[16] 10400[16] 7090[16], 7080[93] 200400[16]

Li-ion 75200[16,94],

120200[96]

150315[16,94] 200500[16,94] 100[16], 8589[94] 6002500[16],

5002500[94]

Fuel cells 80010,000[16] 500+[16] 5003000[16] 500+[16]

Metalair 15 0 30 00[16] 50010,000[16] 97[16], >95[95] 100010,000[16]

FES 1030[16] 4001500[16] 2080[16] 10002000[16] 9095[16], >80[95], 90[93] 10005000[16]

SCES 2.515[16], 5 [95],

25[96]

5005000[16],

500010,000[95],

20005000[96]

1030[16] 100,000+[16] 95[95], 7580[93] 3002000[16], 2000[93

CS 0.055[16] 100, 000[16] 210[16] 100,000+[16] 6090[16] 200400[16]

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Fig. 15. Characteristics of storage[46].

Table 3

Storage technologies development status.

System Development

status

Description

PHS, leadacid battery[22] Mature The system is having a significant commercial experience

CAES[22], leadacid[22], NiCd[22], NaS[22], ZEBRA[16],Li-ion[16] Commercial The system has a nascent commercial experience

FES[22], adiabatic-CAES[22,103]ZnBr[22], VRB[22], Li-ion [22], advanced lead

acid[22],HTES[16], PHES[104]

Demonstration The system concept is being verified by integrated demonstration

unit

Fe/Cr[22] Pilot The system concept is being verified by small pilot facility

Zn/air[22], advanced Li-ion[22],NBC Laboratory The system concept is being verified by laboratory studies and

initial hardware development

Nano-capacitors[22], other novel battery chemistries[22] Idea No system, component or device test available

Fig. 16. Market opportunity and price points for application-specific storage options[46,49].

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Integration of renewable energy resources

Renewable energy technologies are environmentally benign

and abundant. These benefits make them an attractive electricity

generation alternative for mitigating climate change and address-

ing energy crises. The European Wind Energy Association (EWAE)

estimated that the worlds electricity needs could be met by har-

nessing 10% of the global wind energy resources [26]. However,renewable energy systems are not load-following because of their

intermittent and unpredictable characteristic. Also, the integration

of variable renewable power sources affects the operation of elec-

trical networks[25,29,30]. It is required that when the input to

power networks from renewable generation (i.e., wind) is up to

10%, balancing supports from other generators of 24% of the

renewable power capacity is required [24,25,37,38]. Thus, the

integration of energy storage devices into the electrical grids is

expected to mitigate the effects of intermittency of renewable

resources, and also facilitate a higher penetration of renewable

electricity. Many scholarly works have unraveled the potentials

of energy storage systems; however, the economic viability of

these systems is also one of the major factors which determine

their deployments. The estimation and recovery of value of energy

storage in the grid still remains a challenge which needs further

research[11].

Vehicle-to-grid application

The concept of V2G focuses on the utilization of the potentials

of a large number of EVs for providing ancillary services to power

networks [11,115]. PHEVs are mobile agents that have the prop-

erty of operating either with an electrochemical battery which

runs on electric power obtained from the grid, or an internal com-

bustion engine that uses petrol as its main source of fuel[115]; its

plug-in charging system allows electricity from the power network

to be stored in the battery bank during off-peak periods. The stored

electricity is later delivered to the network to reduce demand pres-sures at peak periods. The type of battery utilized is an important

factor in the operation of PHEVs. A high-energy storage capacity of

these vehicles requires the use of larger battery sizes, thereby cre-

ating the possibility of reducing the utilization of fuel [115]. In

BESs, there are trade-offs, mostly between energy and power den-

sity[115,116]. For instance, leadacid batteries have a low energy

density for PHEV applications, usually around 30 Wh/kg, while the

NiMH battery has an energy density around 80 Wh/kg [115].

Though NiMH batteries have a better energy density when com-

pared to leadacid batteries, they possess a lower RTE. The power

density of NiMH is 800 W/kg [115,117,118]; however, lithium-

cobalt (Li-Co) batteries possess both a high RTE and high energy

density, of 90% and 160 Wh/kg, respectively. Lithium-based BES

technologies are currently leading for BEV and PHEV applications

[119]. However, there is still room to improve their energy storage

capacity. The widespread application of EVs in electrical networks

could provide ancillary services, and also lead to increased utiliza-

tion of renewable energy. However, the capability of EVs to provide

these services will be limited by storage capacity, capital costs and

shortened lifetime of batteries through frequent charge/discharge

cycles [11,26,29].

Future research

There is need for further improvements in energy storage

systems that will lead to the development of low-cost and high

performance solutions. The emerging technologies will also require

intensive and extensive research for them to gain industrial andcommercial acceptance in the future. The environmental issues

associated with PHS, CAES and NiCd may affect their uptake in

the future, say 1520 years upwards [11,16,24,38], except there

are technological innovations to reduce their environmental

impacts. Therefore, particular attention should be paid to the fol-

lowing storage options by research community: adiabatic-CAES,

isothermal-CAES, and small-medium scale CAES, underwater/

ocean-CAES, sub-surface PHS, seawater PHS and variable-speed

PHS [97109]. The use of adiabatic-CAES will eliminate fuel con-sumption, lead to significant reduction in pollutant emissions,

and increased adoption of DERs in the future. Isothermal-CAES

has a potential to eliminate fuel consumption; it also does not

require TES. Small-medium scale CAES with artificial pressure ves-

sels will eliminate the need for geological formations and TES, and

allow a widespread application of DGs[121,122]in the future. Also,

the tri-generation capability of small-medium scale CAES systems

will not only reduce energy costs, but will also facilitate significant

reductions in CDEs. Fundamental research is required in developing

ocean/underwater-CAES. This ESS option does not require an

underground cavern, and can also be integrated with offshore

renewable resources to harvest a greater amount of wind energy.

More work is needed for developing SSPHS systems. It has a poten-

tial to reduce environmental impacts of PHS, by positioning either

the lower or upper reservoir, or both reservoirs at sub-surface.

The use of existing sub-surface structures is also a good economic

consideration for building SSPHS plants. Also, more research is

required in bringing about developments in seawater-PHS system,

which offers an advantage of utilizing less land (only one reservoir

is needed on land), and VSPHS which has a capacity to offer ancil-

lary services to electrical grids. There is still much opportunity for

development in recycling of NiCd, and other BES systems[125].

It is expected that FES and SCES technologies will find greater

power quality and frequency regulation applications. However,

both ESSs need further development. The safety issue with fly-

wheels must also be addressed. There is much opportunity for

research to adopt SCES systems for high-power applications,

because of their exceptional power density. In 2025 years, elec-

tricity grids are likely to: have a high degree of smartness, integratea greater proportion of RE technologies, EVs and other DERs, which

will require viable, efficient and cost-effective energy storage sys-

tems. This will be a road map to actualizing sustainable power net-

works, by reducing full reliance on capital intensive peaking power

plants that are still been currently used. Future electricity grids are

expected to utilize the potentials of a large number of EVs for ancil-

lary services; however, this aspect still requires intensive R and D.

Further system investigations are also necessary in studying the

temporal characteristics of availability of EVs[11].

Though fuel cells are non-rechargeable and cannot be used for

cyclic storage of energy, they are a potential energy source for

future applications[16,24]. More work is required for developing

fuel cells, including direct methanol, molten carbonate and solid

oxide[16]. Hydrogen is also a non-polluting alternative fuel [60],which could offer a scalable storage option for renewable power

generation in the future. However, Hydrogen storage currently

suffers from a low RTE (

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ESSs with different charge/discharge characteristics and timescales

are combined for some particular applications. More studies

should target the consideration of synergy of two or more storage

options for power applications (i.e., high-power devices + high-

energy devices, etc.). Investigations into the capability of hybrid

storage technologies to provide two or all the three services: PQ,

BP and EM, will also be necessary. Such research is required to uti-

lize the combined potentials of the high-power and the high-energy systems for better performance [13,24,25,29]. As suggested

in [10], one single solution will probably not be the most cost-

optimal solution, a mix of all solutions is needed for achieving

efficient energy storage. Attention must also be paid to power

conversion systems for storage applications. This is necessary to

improve their reliability and control, and also ensure their

efficient integration of storage systems into power networks.

A next-generation smart grid without energy storage is like a

computer without a hard drive: severely limited[123].

Conclusion

This work has presented a comprehensive review of energy

storage technologies which are currently engaged for electricalpower applications. The technological progress, performance and

capital costs assessment of the systems have been discussed, and

directions for further research have also been emphasized. Also,

the discussion of some new storage systems such as: adiabatic-,

underwater/ocean-, isothermal- and small-scale CAES systems;

sub-surface, seawater and variable-speed PHS systems, and

pumped heat electrical storage has been of interest in this paper.

Based on this study, the following conclusions are made:

Energy storage technologies have potentials to offer flexibility

and ancillary supports to electrical networks, and they have

been classified as follows: PHS, CAES, VRB, ZnBr, NaS, ZEBRA,

TES and large-scale (Leadacid, Li-ion, NiCd, NiMH) technol-

ogies, are applied for energy management purposes, becauseof their long discharge timescales (up to 24 h); CS, SCES, FES

and SMES systems have a short discharge timescale (millisec-

onds to minutes), and are suitable for power quality and fre-

quency regulation supports, while battery technologies (Lead

acid, Li-ion, NiCd, NiMH and Metalair) are employed for

bridging power applications, because of less cycling require-

ment, and they possess a discharge duration of (seconds to

hours).

PHS remains a mature storage option currently engaged for

large-scale applications. However, the environmental concerns

(i.e., destruction of plants and requirement for a large land

mass) may likely affect its uptake in the future. New technolo-

gies such as sub-surface and seawater storage systems are

expected to receive much attention, due to their potentials forreducing the environmental impacts of PHS, and increasing

the utilization of renewable electricity. Also, the use of vari-

able-speed electrical machines in PHS will offer ancillary sup-

ports to power networks in the presence of a high percentage

of variable renewable energy input. Pumped heat energy stor-

age is also scalable, and has a relatively low capital cost with

less environmental impacts compared to PHS, which will prob-

ably be found a competitive storage option in the future.

Conventional CAES technologies have gained commercial

acceptance for large-scale applications. However, their require-

ments for geological formations and fossil fuels are major con-

cerns. The utilization of heat produced by air compression

and/or heat generated from renewable energy resources for

air expansion will offer an emission-free storage solution, by

eliminating fossil fuels. This will yield a better system

performance, and will likely give adiabatic-CAES option an edge

in not-too distant future. Apart from this, adiabatic-CAES sys-

tems are scalable. This makes them a suitable option for

microgrids. However, further research is needed for their wide-

spread applications in the future. Efficiency of 70% is being

targeted by RWE Power for its AA-CAES plant. Small-medium

scale CAES is also a promising version of CAES technology; it

offers a flexible solution and eliminates the need for geologicalformations. This storage system is likely to thrive in the near

future because of its tri-generation capacity, which could facil-

itate significant savings on carbon emissions and energy costs,

and increased distributed electricity generation.

Offshore power systems hold promise for significant electricity

generation in the future. This is because of the availability of

huge wind energy resources in offshore sites. An underwater-

CAES system will be an attractive option for offshore power

applications, because of its capability to facilitate a higher pen-

etration of wind energy without the need for underground

caverns.

Li-ion, FES, SMES, and SCES technologies possess a very high

round trip efficiency, above 90%; Lead acid, NiCd, NiMH,

NaS, ZnBr, ZEBRA, VRB and conventional electrolytic capacitor

technologies have a high round trip efficiency, between 60%

and 90%. CES, HES and Metalair technologies have a low round

trip efficiency, below 60%.

The possession of a very low self-discharge loss by PHS, CAES,

VRB, ZnBr and Metalair systems makes them a suitable option

for stationary applications, requiring long storage duration

(hours to months). Leadacid, NiCd, Li-ion, HTES and CES sys-

tems have a low energy dissipation of (

7/23/2019 1-Main ESS 2014.pdf

17/18

dling future electricity supply/demand challenges. The develop-

ment of low-cost and high performance energy storage solutions

will therefore require intensive and extensive R and D efforts.

Acknowledgment

The authors wish to acknowledge Victoria University of Wel-

lington for its support for this work through its Research Trust.

References

[1] Teng JH, Luan SW, Lee DJ, Huang YQ. Optimal charging/dischargingscheduling of battery storage systems for distribution systemsinterconnected with sizeable PV generation systems. IEEE Trans Power Syst2013;28(2):142533.

[2] Twidell J, Weir T. Renewable energy resources. 2nd ed. Taylor Francis E-Library; 2006.

[3] Hernandez JC, Ruiz-Rodriguez FJ, Jurado F. Technical impact of photovoltaicdistributed generation on radial distribution systems: stochastic simulationsfor a feeder in Spain. Electr Power Energy Syst 2013;50:2532.

[4] Jiang R, Wang J, Guan Y. Robust unit commitment with wind power andpumped storage hydro. IEEE Trans Power Syst 2012;27(2):80010.

[5] Singh M, Khadkikar V, Chandra A, Varma RK. Grid interconnection ofrenewable energy sources at the distribution level with power-quality

improvement features. IEEE Trans Power Del 2011;26(1):30715.[6] Barton JP, Infield DG. Energy storage and its use with intermittent renewable

energy. IEEE Trans Energy Convers 2004;19(2):4418.[7] Tewari S, Mohan N. Value of NAS energy storage toward integrating wind:

results from the wind to battery project. IEEE Trans Power Syst2013;28(2):53241.

[8] Gabash A, Li P. Active-reactive optimal power flow in distribution networkswith embedded generation and battery storage. IEEE Trans Power Syst2012;27(4):201635.

[9] Gayme D, Topcu U. Optimal power flow with large-scale storage integration.IEEE Trans Power Syst 2013;28(2):70917.

[10] DG ENER working paper the future role and challenges of energy storage.European Commission Directorate-General for Energy, 2013. p. 136.

[11] Denholm P, Ela E, Kirby B, Milligan M. The role of energy storage withrenewable electricity generation. NREL technical report, NREL/TP-6A2-47187,2010.

[12] Schainker R. Executive overview: energy storage options for a sustainableenergy futures. IEEE PES Gen Meet 2004:16.

[13] Jiang Q, Hong H. Wavelet-based capacity configuration and coordinatedcontrol of hybrid energy storage system for smoothing out wind powerfluctuations. IEEE Trans Power Syst 2013;28(2):136372.

[14]Taylor JA, Callaway DS, Poolla K. Competitive energy storage in the presenceof renewable. IEEE Trans Power Syst 2013;28(2):98596.

[15] Diaz-Gonzalez F, Sumper A, Gomis-Bellmunt O, Bianchi FD. Energymanagement of flywheel-based energy storage device for wind powersmoothing. J Appl Energy 2013;110:2079.

[16]Chen H, Cong TN, Yang W, Tan C, Li Y, Ding Y. Progress in electrical energystorage system: a critical review. Prog Nat Sci 2009;19:291312.

[17] Banham-Hall DD, Taylor GA, Smith CA, Irving MR. Flow batteries forenhancing wind power integration. IEEE Trans Power Syst2012;27(3):16907.

[18] Xu Y, Singh C. Adequacy and economy analysis of distribution systemsintegrated with electric energy storage and renewable energy resources. IEEETrans Power Syst 2012;27(4):233241.

[19]Sedghi M, Aliakbar-Golkar M, Haghifam MR. Distribution network expansionconsidering distributed generation and storage units using modified PSOalgorithm. Electr Power Energy Syst 2013;52:22130.

[20]Fertig E, Apt J. Economics of compressed air energy storage to integrate windpower: a case study in ERCOT. Energy Pol 2011;39(5):233042.[21] Teleke S. Energy storage overview: applications, technologies and economical

evaluation. White Paper, Quanta Technology, 2011. p. 111.[22] Electricity energy storage technology options, a white paper primer on

applications, costs and benefits. Electric Power Research Institute, Report1020676, 2010.

[23]Mu Y, Wu J, Ekanayake J, Jenkins N, Jia H. Primary frequency response fromelectric vehicles in the Great Britain power system. IEEE Trans Smart Grid2013;4(2):114250.

[24]Whittingham MS. History evolution and future status of energy storage. ProcIEEE 2012;100:151834.

[25]Vazquez S, Lukic SM, Galvan E, Franquelo LG, Carrasco JM. Energy storagesystems for transport and grid applications. IEEE Trans Ind Electron2010;57(12):388195.

[26] Bose BK. Global energy scenario and impact of power electronics in 21stcentury. IEEE Trans Ind Electron 2013;60(7):263851.

[27]Manz D, Piwko D, Mille N. Look before you leap the role of energy storage inthe grid. IEEE Power Energy Mag 2012:7584.

[28]Converse AO. Seasonal energy storage in renewable energy system. Proc IEEE2012;100(2):4019.

[29] Li P. Energy storage is the core of renewable energy technologies. IEEENanotechnol Mag 2008:138.

[30]Hill CA, Such MC, Chen D, Gonzalez J, Grady WM. Battery energy storage forenabling integration of distributed solar power generation. IEEE Trans SmartGrid 2012;3(2):8507.

[31]Chan CC, Wong YS. Electric vehicles charge forward. IEEE Power Energy Mag2004:2433.

[32]Carrasco JM, Franquelo LG, Bialasiewicz JT, Galvan E, Guisado RCP, Prats MaAM, Leon JI, Moreno-Alfonso N. Power-electronic systems for the gridintegration of renewable energy sources: a survey. IEEE Trans Ind Electron

2006;53(4):100226.[33] Nehrir MH, Wang C, Strunz K, Aki H, Ramakumar R, Bing J, Miao Z, Salameh Z.

A review of hybrid renewable/alternative energy systems for electric powergeneration: configurations, control and applications. IEEE Trans SustainableEnergy 2011;2(4):392403.

[34]Pickard WF. The history, present state and future prospects of undergroundpumped hydro for massive energy storage. Proc IEEE 2012;100(2):47383.

[35]Giuntoli M, Poli D. Optimized thermal and electrical scheduling of a largescale virtual power plant in the presence of energy storages. IEEE Trans SmartGrid 2013;4(2):94254.

[36] Ozturk Y, Senthilkumar D, Kumar S, Lee G. An intelligent home energymanagement system to improve demand response. IEEE Trans Smart Grid2013;4(2):694700.

[37]Chakrabarti, Rayudu RK. Balancing wind intermittency using hydro reservesand demand response. IEEE POWERCON 2012:16.

[38]Electric energy storage systems. Quanta Technology; 2013. p. 112.[39]Makansi J, Abboud J. Energy Storage: the missing link in the electricity value

chain-An ESC White Paper. Energy Storage Counc 2002:123.[40]Bollen MHJ. What is power quality. Electr Power Syst Res 2003;66:514.[41]Supercapacitor diagram. Maxwell Technologies; 2006.[42] EPRI-DOE handbook of energy storage for transmission and distribution

applications. Report 1001834, 2003.[43] Generation 4 Flywheel Energy Storage System. Beacon Power, LLC, 2013.[44]Lithium-ion battery materials. SigmaAldrich Material Science; 2013.[45]Yang Z, Liu J, Baskaran S, Imhoff CH, Holladay JD. Enabling renewable energy

and the future grid with advanced electricity storage. JOM 2010;62(9):1423.[46] Minns D. The prospects for grid energy storage technology, invited paper

presentation to the Association of Professional Engineers and Geoscientists ofBC, 2013.

[47] Rastler D. Electricity energy storage technology options: system costbenchmarking. Electr Power Res Inst 2012:133.

[48]Forley A, Lobera ID. Impacts of compressed air energy storage plant on anelectricity market with large renewable energy portfolio. Energy2013;57:8594.

[49] Energy storage for the electricity grid: benefits and market potentialassessment guide. Sandia National Laboratories Report-SAND2010-0815,2010. p. 1160.

[50] Myst ery of car b at tery s current s olved- Lead acid b att erydiagram. Physicsforme; 2011.

[51] Madaeni SH, Siosshansi R, Delhom P. Estimating the capacity value ofconcentrating solar power plants with thermal energy storage: a case studyof t he southwes tern United States . I EEE Trans Power Sys t2013;28(2):120515.

[52]Gevorkian P. Solar power in building design. 6th ed. New York: McGraw Hill;2008.

[53]Nickelcadmium battery clinic. Midland R/C; 2013.[54]Chaitali D. Cryogenic energy storage. Stanford University; 2012.[55]Xie X. Vanadium Redox-flow battery. Tennessee Valley Authority; 2012.[56] Levine JG. Pumped hydroelectric energy storage and spatial diversity of wind

resources as methods of improving utilization of renewable energy resources[Masters Degree Thesis]. University of Colorado, 2007. p. 78.

[57]Pumped-storage facility diagram. Tennessee Valley Authority; 2013.[58] Dunn et al. Electrical energy storage for the grid: a battery choices. Sci Mag

2011;334(6058):92835.[59]Barile C. Solar thermal energy storage systems. Stanford University; 2010.[60] Alternative Fuels Data Centre- Hydrogen basics. US Department of Energy;

2014.[61] Cui J, Li K, Sun Y, Zou Z, Ma Y. Distributed energy storage system in wind

power generation. In: Proceedings of the 4th Intl Conf DRPT, 2010. p. 153540.

[62] Pietsch A, Lynch G, Sutherland SB, Goodwin TW. Enabling high-densityenergy storage: design characteristics of thermal mix energy storage and ahighly conductive gas mixture. In: Proceedings of the IEEE conf smart grideng, 2012. p. 18.

[63] Shim JW, Cho Y, Kim S, Min SW, Hur K. Synergistic control of SMES andbattery energy storage for enabling dispatchability of renewable energysources. IEEE Trans Appl Supercond 2013;23(3):15.

[64] Lee J, Jeong S, Han YH, Park BJ. Concept of cold energy storage forsuperconducting flywheel energy storage system. IEEE Trans on ApplSupercond 2011;21(3):22214.

[65]Carter R, Cruden A, Hall PJ, Zaher AS. An improved leadacid battery packmodel for use in power simulations of electric vehicles. IEEE Trans EnergyConvers 2012;27(1):218.

[66]Zhu G, Loo KH, Lai YM, Tse TK. Quasi-maximum efficiency point tracking for

direct methanol fuel cell in DMFC/supercapacitor hybrid energy system. IEEETrans Energy Convers 2012;27(3):56171.

90 D.O. Akinyele, R.K. Rayudu / Sustainable Energy Technologies and Assessments 8 (2014) 7491

http://refhub.elsevier.com/S2213-1388(14)00070-8/h0005http://refhub.elsevier.com/S2213-1388(14)00070-8/h0005http://refhub.elsevier.com/S2213-1388(14)00070-8/h0005http://refhub.elsevier.com/S2213-1388(14)00070-8/h0005http://refhub.elsevier.com/S2213-1388(14)00070-8/h0010http://refhub.elsevier.com/S2213-1388(14)00070-8/h0010http://refhub.elsevier.com/S2213-1388(14)00070-8/h0015http://refhub.elsevier.com/S2213-1388(14)00070-8/h0015http://refhub.elsevier.com/S2213-1388(14)00070-8/h0015http://refhub.elsevier.com/S2213-1388(14)00070-8/h0015http://refhub.elsevier.com/S2213-1388(14)00070-8/h0020http://refhub.elsevier.com/S2213-1388(14)00070-8/h0020http://refhub.elsevier.com/S2213-1388(14)00070-8/h0025http://refhub.elsevier.com/S2213-1388(14)00070-8/h0025http://refhub.elsevier.com/S2213-1388(14)00070-8/h0025http://refhub.elsevier.com/S2213-1388(14)00070-8/h0030http://refhub.elsevier.com/S2213-1388(14)00070-8/h0030http://refhub.elsevier.com/S2213-1388(14)00070-8/h0035http://refhub.elsevier.com/S2213-1388(14)00070-8/h0035http://refhub.elsevier.com/S2213-1388(14)00070-8/h0035http://refhub.elsevier.com/S2213-1388(14)00070-8/h0040http://refhub.elsevier.com/S2213-1388(14)00070-8/h0040http://refhub.elsevier.com/S2213-1388(14)00070-8/h0040http://refhub.elsevier.com/S2213-1388(14)00070-8/h0045http://refhub.elsevier.com/S2213-1388(14)00070-8/h0045http://refhub.elsevier.com/S2213-1388(14)00070-8/h0060http://refhub.elsevier.com/S2213-1388(14)00070-8/h0060http://refhub.elsevier.com/S2213-1388(14)00070-8/h0060http://refhub.elsevier.com/S2213-1388(14)00070-8/h0065http://refhub.elsevier.com/S2213-1388(14)00070-8/h0065http://refhub.elsevier.com/S2213-1388(14)00070-8/h0065http://refhub.elsevier.com/S2213-1388(14)00070-8/h0070http://refhub.elsevier.com/S2213-1388(14)00070-8/h0070http://refhub.elsevier.com/S2213-1388(14)00070-8/h0075http://refhub.elsevier.com/S2213-1388(14)00070-8/h0075http://refhub.elsevier.com/S2213-1388(14)00070-8/h0075http://refhub.elsevier.com/S2213-1388(14)00070-8/h0075http://refhub.elsevier.com/S2213-1388(14)00070-8/h0080http://refhub.elsevier.com/S2213-1388(14)00070-8/h0080http://refhub.elsevier.com/S2213-1388(14)00070-8/h0085http://refhub.elsevier.com/S2213-1388(14)00070-8/h0085http://refhub.elsevier.com/S2213-1388(14)00070-8/h0085http://refhub.elsevier.com/S2213-1388(14)00070-8/h0090http://refhub.elsevier.com/S2213-1388(14)00070-8/h0090http://refhub.elsevier.com/S2213-1388(14)00070-8/h0090http://refhub.elsevier.com/S2213-1388(14)00070-8/h0095http://refhub.elsevier.com/S2213-1388(14)00070-8/h0095http://refhub.elsevier.com/S2213-1388(14)00070-8/h0095http://refhub.elsevier.com/S2213-1388(14)00070-8/h0100http://refhub.elsevier.com/S2213-1388(14)00070-8/h0100http://refhub.elsevier.com/S2213-1388(14)00070-8/h0115http://refhub.elsevier.com/S2213-1388(14)00070-8/h0115http://refhub.elsevier.com/S2213-1388(14)00070-8/h0115http://refhub.elsevier.com/S2213-1388(14)00070-8/h0120http://refhub.elsevier.com/S2213-1388(14)00070-8/h0120http://refhub.elsevier.com/S2213-1388(14)00070-8/h0125http://refhub.elsevier.com/S2213-1388(14)00070-8/h0125http://refhub.elsevier.com/S2213-1388(14)00070-8/h0125http://refhub.elsevier.com/S2213-1388(14)00070-8/h0125http://refhub.elsevier.com/S2213-1388(14)00070-8/h0130http://refhub.elsevier.com/S2213-1388(14)00070-8/h0130http://refhub.elsevier.com/S2213-1388(14)00070-8/h0130http://refhub.elsevier.com/S2213-1388(14)00070-8/h0135http://refhub.elsevier.com/S2213-1388(14)00070-8/h0135http://refhub.elsevier.com/S2213-1388(14)00070-8/h0140http://refhub.elsevier.com/S2213-1388(14)00070-8/h0140http://refhub.elsevier.com/S2213-1388(14)00070-8/h0145http://refhub.elsevier.com/S2213-1388(14)00070-8/h0145http://refhub.elsevier.com/S2213-1388(14)00070-8/h0150http://refhub.elsevier.com/S2213-1388(14)00070-8/h0150http://refhub.elsevier.com/S2213-1388(14)00070-8/h0150http://refhub.elsevier.com/S2213-1388(14)00070-8/h0150http://refhub.elsevier.com/S2213-1388(14)00070-8/h0155http://refhub.elsevier.com/S2213-1388(14)00070-8/h0155http://refhub.elsevier.com/S2213-1388(14)00070-8/h0160http://refhub.elsevier.com/S2213-1388(14)00070-8/h0160http://refhub.elsevier.com/S2213-1388(14)00070-8/h0160http://refhub.elsevier.com/S2213-1388(14)00070-8/h0160http://refhub.elsevier.com/S2213-1388(14)00070-8/h0160http://refhub.elsevier.com/S2213-1388(14)00070-8/h0165http://refhub.elsevier.com/S2213-1388(14)00070-8/h0165http://refhub.elsevier.com/S2213-1388(14)00070-8/h0165http://refhub.elsevier.com/S2213-1388(14)00070-8/h0165http://refhub.elsevier.com/S2213-1388(14)00070-8/h0170http://refhub.elsevier.com/S2213-1388(14)00070-8/h0170http://refhub.elsevier.com/S2213-1388(14)00070-8/h0175http://refhub.elsevier.com/S2213-1388(14)00070-8/h0175http://refhub.elsevier.com/S2213-1388(14)00070-8/h0175http://refhub.elsevier.com/S2213-1388(14)00070-8/h0175http://refhub.elsevier.com/S2213-1388(14)00070-8/h0180http://refhub.elsevier.com/S2213-1388(14)00070-8/h0180http://refhub.elsevier.com/S2213-1388(14)00070-8/h0180http://refhub.elsevier.com/S2213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