dnvgl-rp-0043 safety, operation and performance of grid ...dnv gl as acknowledgements this rp was...

The electronic pdf version of this document, available free of chargefrom http://www.dnvgl.com, is the officially binding version.

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RECOMMENDED PRACTICE

DNVGL-RP-0043 Edition September 2017

Safety, operation and performance of grid-connected energy storage systems

FOREWORD

DNV GL recommended practices contain sound engineering practice and guidance.

© DNV GL AS September 2017

Any comments may be sent by e-mail to [email protected]

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of thisdocument. The use of this document by others than DNV GL is at the user's sole risk. DNV GL does not accept any liability or responsibilityfor loss or damages resulting from any use of this document.

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Recommended practice — DNVGL-RP-0043. Edition September 2017 Page 3Safety, operation and performance of grid-connected energy storage systems

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CHANGES - CURRENT

This document supersedes the December 2017 edition of DNVGL-RP-0043Changes in this document are highlighted in red colour. However, if the changes involve a whole chapter,section or sub-section, normally only the title will be in red colour.

Main changes— The following topics were added or significantly expanded:

— [4.3] Tendering and procurement— [4.4.1] and [4.4.2] Conformity assessment including FAT/SAT testing— [4.4.3] Warranty— [4.9.2] Decommissioning— [6.2.2] Microgrids— [6.5.10] Communication protocols— [7.4.9] Cyber security— [8.5] Greenhouse gas emissions calculation— [10.3] Bankability— [10.4] Residual value.

— Technology-specific recommendations and background were added on (sub-)technologies:

— [7.5.1.2] Lead crystal batteries.— [7.5.1.3] Inorganic lithium ion batteries— [7.5.2.1] Supercapacitors— [7.5.2.4] Compressed air energy storage (CAES)— [7.5.2.5] Liquid air energy storage (LAES).

— [7.5.1.3]: Technology-specific safety recommendations for Li-ion batteries were updated

— [7.4.5] and [7.4.6]: Recommendations regarding first responders, fire considerations and thermalmanagement were updated

— [B.3]: Overview of normative and informative references was actualised and expanded to include therecommended practice (RP) updates described above

— Sec.3 has been expanded with three applications, previously belonging to other categories: ramp ratecontrol, generation peak shaving, capacity firming

— Sec.7 has been reorganised for clarity and a more elaborate introduction was added

— Former Sec. 10 has been turned into App.B

— Former paragraph 5.2's content on levelised cost of storage (LCOS) and life cycle costs were moved to thenew Sec.10

— Paragraph [B.2] (formerly 10.2) was updated and expanded to better reflect the American and Europeansituation.

Editorial correctionsIn addition to the above stated changes, editorial corrections may have been made.

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AcknowledgementsThis RP was developed in 2015 and updated in 2016-2017 in two respective Joint Industry Projects (JIPs).The work was performed by DNV GL and discussed in regular project meetings and workshops withindividuals from the organisations participating in these projects. They are hereby acknowledged for theirsignificant, valuable and constructive input. In case consensus has not been achievable, DNV GL has soughtto provide acceptable compromise.The JIP consortium of the 2015 edition of this document included the following organisations: JSR Micro,REDT Energy Storage, Energy Canvas, Joulz, Institute for Mechatronic Systems in Mechanical Engineering(Technische Universität Darmstadt), Institute for Power Generation and Storage Systems (RWTH AachenUniversity), Cumulus Energy Storage.The JIP consortium of the 2017 edition of this document included the following organisations: Alaska Centerfor Energy and Power, Alfen, Alevo, ATEPS, Conergy, Datawatt, Enexis, Gaelectric, Highview Power, HybridEurope, Maxwell, PNNL (in support of the U.S. DOE OE energy storage safety strategy), RWTH, Wärtsilä.Furthermore, DNV GL is grateful for the valuable feedback from individuals in the organisations who haveparticipated in the review (hearing) process in 2015 and 2017: Alaska Center for Energy and Power, Alliander,Clean Energy Council, Denchi Power, Ecovat Renewable Energy Technologies, Electricity Storage Network,Enel Ingegneria & Ricerca SpA, GE Energy Storage, GNB Industrial Power (a division of Exide Technologies),Hybrid Europe BV, muGrid Analytics, New York Battery and Energy Storage Technology Consortium (NY-BEST), Norton Rose Fulbright LLP, PNNL, Scholt Energy Control, Scottish and Southern Energy PowerDistribution (SSEPD), UK Power Networks, Younicos and 125 other organisations not explicitly mentionedhere.

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CONTENTS

Changes - current...................................................................................................3Acknowledgements.................................................................................4

Section 1 General....................................................................................................81.1 Introduction......................................................................................81.2 Objective...........................................................................................81.3 Scope................................................................................................ 81.4 Structure of this document...............................................................81.5 Relationship to other standardisation activities................................91.6 Abbreviations..................................................................................10

Section 2 Electrical energy storage systems - introduction and definitions...........122.1 Definition of electrical energy storage system................................122.2 Definitions of other terms.............................................................. 22

Section 3 Applications of stationary electrical energy storage systems................ 303.1 General........................................................................................... 303.2 Bulk energy services.......................................................................323.3 Ancillary services............................................................................333.4 Transmission infrastructure services.............................................. 363.5 Distribution infrastructure services................................................ 373.6 Customer energy management services......................................... 373.7 Renewables integration.................................................................. 41

Section 4 Electrical energy storage system life cycle phases................................ 434.1 General........................................................................................... 434.2 Design and planning....................................................................... 444.3 Tendering and procurement............................................................454.4 Manufacturing.................................................................................464.5 Transport and warehousing............................................................ 484.6 Installation and commissioning...................................................... 484.7 Operation........................................................................................ 494.8 Maintenance and repair.................................................................. 504.9 Repurposing and end of life............................................................504.10 Documentation requirements........................................................52

Section 5 Performance indicators......................................................................... 545.1 Power..............................................................................................54

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5.2 Energy.............................................................................................565.3 Dynamics........................................................................................ 565.4 Efficiency........................................................................................ 575.5 Reliability, availability.....................................................................59

Section 6 Operation, maintenance, repair.............................................................616.1 General........................................................................................... 616.2 Grid connection aspects..................................................................616.3 Automation..................................................................................... 636.4 Documentation................................................................................646.5 Monitoring and control functions....................................................656.6 Repair............................................................................................. 74

Section 7 Safety.................................................................................................... 757.1 Introduction....................................................................................757.2 Risk assessments............................................................................757.3 Hazards, failure modes and effects.................................................787.4 Safety requirements - technology agnostic.....................................787.5 Safety requirements - technology specific...................................... 877.6 Safety requirements for specific life cycle phases...........................97

Section 8 Environmental analysis........................................................................1018.1 General......................................................................................... 1018.2 Effects of electrical energy storage systems on the environment..1018.3 Effects of environment on electrical energy storage systemoperation............................................................................................ 1048.4 End of life decommissioning and recycling................................... 1068.5 Calculation of greenhouse gas emissions for electrical energystorage systems................................................................................. 107

Section 9 Sizing considerations...........................................................................1099.1 General......................................................................................... 1099.2 Power and energy requirements...................................................1109.3 Siting considerations - geographic circumstances........................ 1119.4 System parameters influencing sizing.......................................... 1129.5 Economic considerations for sizing...............................................1179.6 Configuration of cells, modules and racks.................................... 1179.7 Accommodation, housing, containment of the system and sub-levels.................................................................................................. 119

Section 10 Economics..........................................................................................120

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10.1 Life cycle costs........................................................................... 12010.2 Levelised cost of energy and levelised cost of storage................12110.3 Bankability.................................................................................. 12110.4 Residual value.............................................................................122

Appendix A Life cycle cost background information............................................124

Appendix B Regulations, standards and legal issues...........................................126B.1 General......................................................................................... 126B.2 Structure of existing legal frameworks, regulations and codes..... 126B.3 List of normative and informative references............................... 137B.4 Recommendations for storage systems regarding legalframeworks, regulations and standards............................................. 144

Changes - historic...............................................................................................146


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SECTION 1 GENERAL

1.1 IntroductionThe market for grid-connected energy storage systems is rapidly maturing. The successful deployment oftomorrow’s smart electricity grids requires clarity and widespread agreement on issues concerning energystorage systems. Stakeholders agree that joint guidelines, agreements and standards are essential to enableenergy storage to provide the benefits it promises and achieve mass deployment throughout the grid. Thisrecommended practice (RP) aims to accelerate safe and sound implementation of grid-connected energystorage by presenting a guideline for safety, operation and performance of electrical energy storage systems.The information and recommendations in this document comprehensively cover and link all aspects relevantfor grid-connected energy storage.This RP will:

— cover a broad range of energy storage technologies, instead of focusing on one (e.g. batteries)— have a system-level approach, instead of being limited to a small number of key components— have a comprehensive and structured approach.

Future updates of this RP are likely to expand the range of technology-specific recommendations.

1.2 ObjectiveThe objective of this RP is to provide a comprehensive set of recommendations for grid-connected energystorage systems. It aims to be valid in all major markets and geographic regions, for all applications, on alllevels from component to system, covering the entire life cycle. End users, operators and other stakeholderswill be able to take this RP as their single all-encompassing document for such systems, providing them withdirect guidance or referencing through other guidelines and standards.

1.3 ScopeThis RP focuses on three main aspects of grid-connected energy storage: safety, operation and performance.These aspects are assessed for electricity storage systems in general, i.e. a technology agnostic approach).Furthermore, recommendations applying only to specific energy storage technologies are provided wherevernecessary.The RP concerns electrical energy storage systems and technologies. The energy going into and out ofthese systems is electrical energy (electricity), whereas the energy stored in the storage medium may havedifferent forms (see Sec.2). Explicitly out of scope is chemical and thermal energy storage, such as hydrogenstorage or heat storage. Electric vehicles are not currently considered to be grid-connected storage and arenot considered within scope. With respect to the application types of energy storage systems, only homestorage systems are not considered within scope given their distinctly different nature. All electricity storageapplications at the high, medium and low voltage grid level as well as industrial behind-the-meter storageand storage in microgrids and island grids are considered within scope.The proposed guidelines are limited to common requirements, based on worldwide accepted regulation andbest practices like IEC, ISO and IEEE standards. Applying this RP will not guarantee a fully secure storagesystem: new technology will always invalidate previous designs and safe components will not automaticallyresult in a safe system.

1.4 Structure of this documentThe structure of this RP is as follows:

— Sec.2 contains the definitions and abbreviations, including short descriptions of the storage technologieswithin scope.


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— Sec.3 lists the applications of stationary storage systems, grouped in: bulk energy services, ancillaryservices, transmission and distribution infrastructure services, customer energy management andrenewables integration.

— The life cycle phases of a storage system are mentioned in Sec.4: design and planning, transport,installation and commissioning, operation, maintenance and repair, end of life management. Sec.4 alsolists the documentation requirements in each of these phases.

— Sec.5 defines the main performance indicators for qualifying or comparing storage systems for a certainapplication, including performance indicators for economic analyses.

— In Sec.6, operation, maintenance and repair, subjects such as automation considerations, documentationrequirements and requirements for monitoring and control functions are covered.

— Sec.7 on safety design covers generic issues and technology-specific issues, risk analysis methods (e.g.FMEA), and safety considerations with respect to second life (reuse) applications.

— Environmental analysis is addressed in Sec.8, i.e. the effects of storage systems on the environment andeffects of the environment on storage system operation. Special attention is given to the decommissioningand recycling phase.

— In Sec.9 the recommended method of sizing the storage system for a certain application is presented,covering topics like power and energy requirements, siting considerations and economic considerations.

— Recommendations regarding economic considerations of energy storage can be found in Sec.10.— App.A contains background information on life cycle cost calculations.— App.B elaborately addresses existing legal frameworks, regulations and standards, including an overview

list..

1.5 Relationship to other standardisation activitiesThe topics addressed in this RP are (partially) covered by a number of existing standards. This RP aimsto collect the most relevant requirements of all these standards to present a framework guide for grid-connected energy storage with a system-level approach, but including technology-specific aspects whereneeded.This RP is aligned with ongoing international standardisation activities. A special cooperation with IEC TC 120– the technical committee that is also dealing with electrical energy storage systems, but with a technology-agnostic approach – has been established, with exchange of information and shared personnel. IEC TC 120expects to issue its standards and technical specifications in parts in late 2017 through early 2018. DNVGL-RP-0043 (this document) contains more detailed information about technology-specific aspects, as theTC 120 documents are primarily technology-agnostic. The RP serves as input to the TC 120 workinggroups, ensuring its further integration into the international standardisation community. A similar relationto IEC TC 21 / SC 21A (committees addressing electrochemical batteries) exists, based on informationexchange. Furthermore, through information exchange and shared personnel, the development of this RP hasalso had close interaction with US developments such as the Department of Energy’s Energy Storage SafetyWorking Group (DOE ESSWG), the Inventory of Safety Related Codes and Standards for Energy StorageSystems (September 2014, PNNL-23618) and other national and state-based standardization activities.DNVGL-RP-0043 (this document) will most likely be further adapted and extended in the near future, forexample by updating references to newly issued or updated external standards and guidelines, includingupdated properties and developments, and by adding technology-specific aspects of other storagetechnologies not yet covered. Such short- to mid-term flexibility offers added value over standards whichare generally slower to create and adjust, and at the same time updates might serve as further input tostandardisation committees (like IEC TC 120).


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1.6 AbbreviationsTable 1-1 Abbreviations used in this document

Abbreviation Description

AC alternating current

ACE area control error

AGM absorbed glass mat

AHJ authority having jurisdiction

BMS battery management system

CAES compressed air energy storage

CAPEX capital expenditures

DC direct current

CMS capacitor management system

DOD depth of discharge

DSO distribution system operator

E2P energy to power ratio

EDLC electric double layer capacitor

EES electrical energy storage

EMC electromagnetic compatibility

EMS energy management system

EOL end of life

EPC engineering, procurement and construction

FAT factory acceptance test

FMEA, FMECA failure mode effects (and criticality) analysis

HAZMAT hazardous materials

HSE health safety and environment

HV high-voltage

HVAC heating, ventilation and air conditioning

IGBT insulated-gate bipolar transistor

ISO independent system operator

IT information technology

KESS kinetic energy storage system

LAES liquid air energy storage

LCC life cycle costs


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Abbreviation Description

LCOE levelised cost of energy

LCOS levelised cost of storage

LFL lower flammability limit

LIC lithium-ion capacitor

LV low-voltage

MSDS material safety data sheet

MTBF mean time between failures

MV medium-voltage

PCC point of common coupling

PHS pumped hydro(-electricity) storage

PM pro memoria

POC point of connection

PPE personal protective equipment

PSDS product safety data sheet

PV photovoltaic

RES renewable energy source

RP recommended practice

RTO regional transmission organization

SAT site acceptance test

SCADA supervisory control and data acquisition

SCBA self-contained breathing apparatus

SDA specification, design and assessment

SDS safety data sheet

SG specific gravity

SMES superconducting magnetic energy storage

SOC state of charge

SOE state of energy

SOH state of health

TEL threshold exposure limit

TSO transmission system operator

UPS uninterruptible power supply

VRLA valve-regulated lead-acid


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SECTION 2 ELECTRICAL ENERGY STORAGE SYSTEMS -INTRODUCTION AND DEFINITIONS

2.1 Definition of electrical energy storage system

2.1.1 Block diagram of systemAn electrical energy storage (EES) system reversibly converts energy into electrical energy, vice versa, andstores energy internally (see Table 2-5). An EES system consists of numerous components, all of which arevital to the operation of the system. Although minor differences exist between storage technologies, a blockdiagram similar to Figure 2-1 can be mapped to every EES system.The heart of the EES system is the energy storage device itself where the physical process of storingenergy takes place. In most practical applications, this process relies on an electrical (e.g. capacitors),electrochemical (e.g. batteries) or mechanical (e.g. flywheels) working principle. In many cases of grid-connected energy storage, a power converter between the electric power of the grid and the physical energystorage is required; this may be a single converter or a distributed conversion system. In other instances, amotor-generator is connected to the grid through a variable frequency drive or directly; in the latter case,the power converter is used to generate an excitation voltage or it is absent. Furthermore, a transformer isgenerally present between the grid and the EES system.The state of the physical energy storage is monitored and controlled by the system’s low-level controls,the storage management system, which in case of batteries (cell-based and redox flow) and capacitors isreferred to as battery management system (BMS) and capacitor management system (CMS), respectively. Itreads all relevant data from the physical storage; for example, in case of batteries or LICs voltages, currentsand temperatures, in case of flywheels rotating speed and temperatures, etcetera. Furthermore, it ensuresthat the system is working within its operating range and checks whether the electric power requested iswithin the operating range of the current system status.The high-level controls (energy management system, EMS) of the EES system determine its functionality.They determine when and at what rate the storage system shall be charged, idle or discharged. Dependingon the functionality of the system this can happen locally with minimal response times (milliseconds andbelow) based on locally measured data (e.g. current, voltage, power, frequency), or within an externalenergy management system, connected via a digital protocol (DNP3, modbus, etc.), which leads to slowerresponse times (seconds). When the system is set up for multifunctional performance a combination of localand external high-level controls is possible.


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Figure 2-1 Top-level block diagram of an EES system showing EES device, converter, auxiliariesand management systems1)

1)Based on: Schaede, Hendrik; Dezentrale elektrische Energiespeicherung mittels kinetischer Energiespeicherin Außenläufer-Bauform; Shaker, Aachen, Dissertation 2014Figure 2-1 is a functional drawing; certain components such as protection and safety components have notbeen drawn. Furthermore, a transformer may not be present, especially for smaller systems.Furthermore, several peripheral components (auxiliary equipment) are needed to run the system. Dependingon the physical working principle of the energy storage system these range from a cooling system (manystorage technologies) to liquid pumps (redox flow batteries) or vacuum pumps (flywheels). The low-levelcontrols monitor these peripherals.Consequently, the system boundaries of the energy storage system include all components needed to makethe system perform as required. This is especially important concerning the losses of the EES system (see[5.4] and [9.4.6]).A block diagram including sub-system drawings should be provided for each EES system.


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2.1.2 Electrical energy storage technologiesThere are five broad storage classes according to the form of energy inside the storage medium: electrical1,electrochemical, mechanical, chemical and thermal. There are technologies from each class already deployedin the grid and there are others at various stages of maturity. The technologies mainly applicable for grid-connected storage are shown in Figure 2-2. Short descriptions of the main technology classes for EESsystems are given below.Chemical storage and thermal storage are not considered in this RP. An example of chemical storage ishydrogen (e.g. in the electrolyser/hydrogen storage/fuel cell combination). Examples of thermal storage are(hot) water, ice, molten salt and ceramics.Within the electrochemical class, technologies are generally distinguished by the key chemical compound(s)in their active components. It should be noted that within each technology, many subtechnologies may exist,with widely differing properties. For example, lead acid batteries can be of the AGM, flooded, gel or leadcrystal types, with significant differences in performance, lifetime, safety, etc.

Figure 2-2 Main electrical energy storage technologies for the purpose of grid-connected storage

1 The storage class "Electrical", referring to the form of energy inside the storage medium, should not beconfused with the higher-level categorisation "Electrical energy storage", referring to the form of energygoing into and out of the system.


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2.1.2.1 Electrochemical technologies1. Room-temperature batteryExamples: Lead-acid (LA or Pb-A), copper-zinc (CuZn), nickel-cadmium (Ni-Cad or NiCd), nickel metalhydride (NiMH) and lithium-ion (Li-ion) batteries. A general schematic is shown in Figure 2-3 and corecomponents are listed and described in Table 2-2.

Table 2-1 Room-temperature battery core components

Term Definition

battery system parallel connection of several racks (or strings) forming the battery.The battery system is part of the EES system and includes the disconnect devices and protectivecircuitry. The battery system-BMS collects and aggregates all information from the connected racksand sends them to a superior control, i.e. the energy management system of the EES system.

block usually a parallel connection of a few cells; sometimes referred to as ‘battery’.

cell smallest subpart of an electrochemical EES system. Stores the chemical energy.

module aggregation of several cells or blocks.The number of serial and/or parallel cells or blocks should be provided. One positive and one negativeterminal. May contain a basic BMS (module-BMS) that checks voltages, currents and temperature,and balances the SOC of the aggregated cells. The module-BMS delivers information on internalstates to a superior BMS (and may receive control signals).

Note: The module is normally the smallest interchangeable part of the battery system.

Note: A module may also be referred to as ‘pack’ or ‘tray’.

rack aggregation of several modules.Information on rack layout should be provided. One positive and one negative terminal. Maycontain a superior BMS that controls the interactions of the modules inside. The rack-BMS deliversinformation on the state of the rack to a superior control system.

Note: A rack may also be referred to as ‘string’.


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Figure 2-3 General schematic and components of a cell-based battery EES system

The subdivision mentioned in Table 2-2 and Figure 2-3 (block, module, …) may differ according to technology.Some technologies may also have other differences from this figure, e.g. the modularity of the BMS or thepresence of the transformer. Furthermore, the converter in Figure 2-3 may be a single converter connectedto the battery system or a distributed conversion system with converters connected directly to racks orstrings.2. High-temperature batteryExamples: NaS and NaNiCl batteries. The general schematic (Figure 2-3) and core components (Table 2-2)for room-temperature battery also apply to high-temperature batteries.3. Redox flow batteryExamples: all-vanadium (VRFB) and zinc-bromine (Zn-Br) redox flow batteries. A general schematic is shownin Figure 2-4 and core components are listed and described in Table 2-3.

Table 2-2 Redox flow battery core components

Term Definition

cell smallest subpart of an electrochemical EES system, where chemical energy is converted into electrical energy.

stack series connection of redox flow cells. In general, the electrical connections in a stack are in series and theliquid flows are in parallel.


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Figure 2-4 General schematic and components of a redox flow battery EES system.

Several configurations of a redox flow battery EES system are possible. In a generic system, each stack hasits own converter, but alternatively, multiple stacks may be connected in parallel to a single converter. One ormore converters may be connected to a transformer, thus forming the redox flow battery EES system.4. General system layout for all battery types

— Li-ion battery: cell measurement, module BMS, rack BMS. Multiple racks are connected to one conversionsystem which may have multiple converters. The converter has converter controls. Overall it is the systemdemand controller that communicates with the grid operator.

— Other room-temperature batteries: similar, but may need less stringent safety functions.— High-temperature battery: similar, with additionally a heating system.— Redox flow battery: cell stacks could be connected in series to form a string and connect to converter.

Converters/strings are connected in parallel to form the system.— All battery types have a tailored auxiliary system for heating, ventilation and air conditioning (HVAC).

Furthermore, there are enclosures, fire suppression systems and site controllers.

2.1.2.2 Electrical1. Capacitor


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Capacitors store electrical energy in the electric field between a positively and a negatively charged plate.The two plates are parallel and separated by an insulator, the dielectric. When accumulating energy, theelectric charges on the plates build up. Power generation corresponds to discharging the plates.The basic electrostatic capacitor has limited properties concerning power and energy density, nominal powerand nominal voltage. For application in grid-connected energy storage supercapacitors are widespread(the terms ultracapacitor and supercapacitor are treated as synonyms in this document). These can besubdivided into two types: an electric double-layer capacitor (EDLC) stores the charge electrostatically,a pseudocapacitor stores the charge electrochemically, i.e. similar to a battery. A hybrid capacitor is acombination of the two. The Li-ion capacitor (LIC) is a hybrid capacitor: one electrode is similar to theelectrode of an electrostatic capacitor, the other electrode is similar to the electrode (i.e. the anode) of a Li-ion battery.

Note: the terms ultracapacitor and supercapacitor are synonyms.Note: An EDLC stores the charge electrostatically; a pseudocapacitor stores the charge electrochemically; ahybrid capacitor is a combination of the two above.

Figure 2-5 Classification of supercapacitors

System layout: cell measurement, module CMS, module (or pack) CMS; multiple modules (or packs) in arack (or string) are connected to one conversion system (may have multiple converters); converter hasconverter controls; overall is the system demand controller that communicates with the grid operator. Table2-2 and Figure 2-3 (see [2.1.2.1]) also apply to supercapacitors. Generally, what is called BMS in a batterysystem is called a CMS (cell or capacitor management system) in a supercapacitor system.2. Superconducting magnetic energy storageSuperconducting magnetic energy storages (SMES) stores electrical energy in a magnetic field arounda superconducting coil. Superconducting properties of the coil wire, resulting in a permanent flow ofdirect current without losses, are used to maintain the magnetic field. Cryogenic cooling of the coil below100 Kelvin is required to maintain the superconducting state. Also, a protection system is needed to protectthe magnet from local overheating, thereby losing its superconducting state (quench protection). The coolingsystem translates into self-discharge of the storage system. SMES is still in the R&D phase, mainly becauseof the superconducting wire technology. Several demonstrations have been deployed of up to 10 MW. The


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technology is expected to be ideal for maintaining power quality due to its fast response time and highpower-to-energy ratio.SMES is not scope for this RP.

2.1.2.3 Mechanical technologies1. FlywheelA flywheel is the most prominent example of a kinetic energy storage system (KESS). It stores energy as therotational kinetic energy of a spinning mass, i.e. the rotor. The rotor is accelerated by an electric machineacting as a motor during charging, and decelerates when energy is extracted (discharging mode) by thesame machine acting as a generator. To reduce friction losses during rotation, in general the rotor spins in avacuum and magnetic bearings are used to keep the rotor in position.The amount of energy than can be stored is proportional to the mass, the square of the rotational speed andthe square of the radius of the rotor. Power rating is determined by the electric motor/generator. Flywheelsrequire external power to maintain its rotational velocity. These idling losses incur a relatively high self-discharge rate. Self-discharge rate is mainly influenced by the bearing technology and the quality of thevacuum.Figure 2-6 shows the general structure and components of a KESS. To stabilize the rotating mass bearingsare needed. Modern flywheels often operate fully contact-free levitated by magnetic bearings or acombination of magnetic bearings and high speed roller bearings. Often the bearing system requiresperipheral systems like an electronic controller for the active magnetic bearing system.

Figure 2-6 General schematic and components of a flywheel-based EES system or KESS1)


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1)Based on: Schaede, Hendrik; Dezentrale elektrische Energiespeicherung mittels kinetischer Energiespeicherin Außenläufer-Bauform; Shaker, Aachen, Dissertation 2014Figure 2-6 is a functional drawing, showing EES device, converter, auxiliaries and control systems. Certaincomponents such as protection and safety components have not been drawn. Further, a transformer may notbe present.An electric machine converts the electrical energy to the mechanic energy. A converter and transformercouple the motor/generator to the grid. The high power density of most motor/generators requires a coolingsystem.The flywheel-mass rotates under low pressure (often vacuum or even high vacuum) in a containment toreduce friction losses. On the one hand the containment acts as the low-pressure vessel, on the other hand itacts as a safety measure in case of a disintegration of the flywheel (see Sec.7).System layout: each flywheel has its own converter; multiple converters in a flywheel-based ESS system maybe connected to one transformer.2. Pumped hydro storagePumped hydro storage (PHS) involves the pumping of water from a lower basin into a higher one. Whenpower is delivered (discharging mode), the water is run through water turbines in identical fashion to anormal hydropower plant, generating power. In charging mode, the turbines are used as pumps. Maximumpower generation is determined by the maximum flow of water from the upper to the lower storage reservoirand the height difference between the two reservoirs (i.e. the head), if the generators and turbines aresized accordingly. The amount of electricity that can be stored is determined by the size of the (smallest)reservoirs. The cycle efficiency of a PHS is around 70-85%, the losses are incurred through the losses inpumping, generation and water evaporation.System layout: multiple turbines on one connecting rail; one upper and one lower reservoir; low-level controlper turbine; 1 high-level controlPHS is out of scope of this RP.3. Compressed air energy storageCompressed air energy storage (CAES) stores electricity by compressing air into a reservoir and generateselectricity by expanding the compressed air in a gas turbine. The compression is performed by a compressorunit. Depending on the type of CAES, the heat produced during the compression is stored or released intothe atmosphere. The compressed air is stored in a suitable geological formation such as salt domes, aquifersor depleted gas fields. The air is released for power generation: it is heated by combustion of natural gas andthen expanded in the gas turbine.The generation capacity of the CAES is determined by the size of the gas turbines. The compressor and thegas turbines can be dimensioned independently. The size of the geological formation determines the amountof energy that can be stored.System layout: one turbine/compressor with compressed air storage system4. Liquid air energy storageLiquid air energy storage (LAES) stores electricity by liquefying air into tanks and generates electricity byexpanding the liquefied air in a turbine (Figure 2-7). Air turns to liquid when refrigerated to -196°C, andcan be stored in standard insulated vessels, either unpressurised or pressurized. Exposure to ambienttemperatures above the boiling point of liquid air causes re-gasification and an expansion in volume, whichis used to drive a turbine and create electricity. The charging, energy store and power recovery parts of thesystem can be scaled independently. An optimised LAES also includes a heat and/or cold storage system. ALAES system can be integrated with external sources of (waste) heat and/or cold. The system does not havegeographical constraints.The main components of the system layout are (Figure 2-6): one or more air liquefiers; one or more liquidair (cryogenic) storage tanks and heat storage and cold storage; one or more expansion turbines/generators(i.e. power recovery units).


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Figure 2-7 General schematic and components of a liquid air energy storage system

2.1.3 Hybrid energy storage systemsHybrid energy storage systems combine different classes of EES to one system (e.g. a combination offlywheels and Li-ion batteries, or supercapacitors with a redox flow battery). The aim of hybrid systems isto combine the strengths of the different storage technologies into an improved storage system, such ascombining a power-intensive short-duration technology with an energy-intensive long-duration technology.The critical point with such systems is the functional integration of the storage technologies to one workingsystem within the energy management system (high-level controls).

Guidance note:Ambiguity exists in the usage of the term hybrid system. A hybrid energy system may also refer to a system with a combination ofa diesel genset and a PV-system (or grid connection) as electricity sources, for example.

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The technology-specific requirements from this RP for all the technologies used in a hybrid energy storagesystem shall be addressed.


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2.1.4 Monitoring and control systemEach EES system needs a monitoring and control system. There are two basic types of functions for thissystem:

— safety management functions (e.g. monitor the values of temperature, voltage and current and takecorrective actions (e.g. decrease power) or emergency actions (e.g. switch off) if one or more of theseparameters get out of the safety limits)

— operational management functions (e.g. monitor the values of power, SOC, current and voltage andcontrol the system in such a way that the desired values over time are realised).

For optimal system safety, the safety management functions should be embedded at different levels of theEES system (e.g. monitor string current and system current; monitor cell voltage and module voltage). Formore information see Sec.7.For optimal operational behaviour of the system, the operational management functions should beincorporated at different levels of the EES system. For example: the EMS of the EES system will communicatewith the grid operator’s system about the desired output power profile of the EES system at the POC; theEMS will communicate this internally with the energy storage management system, and the latter mighttranslate this and communicate it at a lower control level to parts of the EES.

Guidance note:In case of a Li-ion battery and a supercapacitor storage system, each individual cell voltage should be monitored for safety andoperational reasons. In case of paralleled cells, each group of paralleled cells may be monitored as if it were a single cell. Themodule-BMS, respectively the CMS for supercapacitor systems, will perform the safety checks on cell and module level and itwill transfer the aggregated data (e.g. module voltage and current) to the BMS/CMS that resides one level up. In a complicatedbattery storage system, there may be a module-BMS/CMS, rack-BMS/CMS and system-BMS/CMS, complemented by a convertermanagement system and the EMS at the highest control level.

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2.2 Definitions of other terms

2.2.1 Roles concerning electrical energy storage systemsThe roles and relationships contained in Table 2-4 are for general guidance. A party can have more than onerole simultaneously, e.g. an aggregator who may both own and operate an EES system. The actual division ofresponsibilities should be outlined in detail within the contract terms.

Table 2-3 Roles concerning EES systems

Term Definition

customer the end user of electrical energy and the user of a grid connection, who is a customer of thegrid operator

DSO** the distribution system operator is responsible for the distribution of electrical power at lowand medium voltage level and for (local) power distribution system operation within a certainarea

EES system integrator the system integrator provides/sells the EES system to the owner and is responsible for fullEES system functionality

EES system operator the operator is responsible for the operation of the grid-connected EES system; the operatormanages the EES system side of the POC

EES system owner the legal owner of the grid-connected EES system


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Term Definition

EES system user the user realizes the benefits of the EES system; the user typically receives services from theoperator

EPC contractor engineering, procurement and construction (EPC) firm responsible for the site layout,preparation and usually the installation of the EES system equipment

grid operator the operator of the grid can be either the TSO, ISO or DSO and manages the grid-side of thePOC

ISO* the independent system operator is responsible for the transmission of electrical power athigh voltage level and for power system operation within a certain area

manufacturer the manufacturer of a subset of the different EES system components, responsible for safetyand functional aspects of his products

supplier the vendor of the EES system; either the EES system integrator or the EPC contractor

TSO* the transmission system operator is responsible for the transmission of electrical power athigh voltage level and for power system operation within a certain area

* In certain regions, the term TSO (and DSO) is used, whereas in others, ISO is used (e.g. Europe and the USA,respectively).** The distribution network operator (DNO) is responsible for the distribution of electrical power at low and mediumvoltage level and for (local) network operation within a certain area. The DNO formally has fewer system operationresponsibilities than a DSO. Within the scope of this RP, no distinction between DNO and DSO has been made. Whereapplicable, the term DSO can be replaced by DNO.

2.2.2 Terms and definitions for electrical energy storage systemclassificationTable 2-4 Terms and definitions for EES system classification

Term Definition Reference

EES system installation or multiple installations, comprising at least oneEES, whose purpose is to receive (charge) electrical energyas a grid-connected system, store this energy internally insome manner, and discharge electrical energy as a grid-connected system, and which includes civil engineeringworks, energy conversion equipment and all the necessaryauxiliary equipment. The EES system is coordinated toprovide services to the grid.

IEC 62933-1 CDV1) (Mar 2016)

electrical energy storage(EES)

installation that reversibly converts energy into electricalenergy, vice versa, and stores energy internallyNote 1 to entry: EES may also be used to indicate theactivity of an apparatus described in the definition, i.e. whileperforming its functionality.

IEC 62933-1 CDV

grid-connected to be connected to a public grid with one or more POCs IEC 62933-1 CDV


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Term Definition Reference

microgrid a group of interconnected loads and distributed energyresources within clearly defined electrical boundaries thatacts as a single controllable entity with respect to the publicgrid. A microgrid can connect and disconnect from the publicgrid to enable it to operate in both grid-connected or island-mode.

DOE Microgrid WorkshopReport (DOE, 2011)

point of connection(POC)

reference point on the grid where the EES system isconnectedNote 1: an alternative term often used is ‘point of commoncoupling’ (PCC), defined by IEC as “point in an electric powersystem, electrically nearest to a particular load, at whichother loads are, or may be, connected”.

IEC 62933-1 CDV, IEC 60050

public grid part of an electrical network used for electricity transferfrom/to users and/or other grids, within an area ofcompetence. The area of competence is defined by nationallegislation/regulation. The public grid is managed by a gridoperator.

Note 1: in this RP, we speak of the POC as the connectionpoint to the grid. The grid operator is responsible from thegrid side of the POC.

Note 2: IEC 62933-1 uses the term ‘utility grid’ for the publicgrid

1) CDV = Committee Draft for Voting; the CDV status means that the document is subject to change before becoming thefinal version of the standard (International Standard = IS).

2.2.3 Terms and definitions for electrical energy storage systemspecificationTable 2-5 Terms and definitions for electrical energy storage system specification

Term Definition Unit2) Reference

actual capacity* actual value (i.e. present value) of the capacity of the EES

Actual capacity is also called ‘remaining capacity’.

The actual (i.e. remaining) capacity could be lower than the nominalcapacity of the EES due to ageing.

Ah

actual energycapacity*

actual value (i.e. present value) of the energy capacity of the EES

Actual energy capacity is also called ‘remaining energy capacity’.

The actual (i.e. remaining) energy capacity could be lower than thenominal energy capacity of the EES due to ageing.

Wh


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C-rate rate at which a battery EES is charged/discharged. A C-rate of ‘xC’means that the battery (dis)charging current Ib (with unit A) equals:

Ib = x It,

with It = (nominal capacity) / (1 h),

with nominal capacity given in Ah.

A C-rate of ‘C/y’ is equal to a C-rate of 1/y C.

In IEC the charging rate is defined as a multiple of It (IEV 482-05-45).

IEC TC120 does not use the term C-rate, but rather the EES systempower, which may be compared relative to the energy capacity of theEES to calculate charging/discharging times.

C (**) (**) This isnot an SIunit.

calendar lifetime* theoretically expected lifetime if the EES is not cycled at all, caused byEES degradation over time

years [5.5.1]

capacity* amount of electric charge a fully charged battery EES can deliver at aspecified discharge current (or discharge current profile), at specifiedenvironmental conditions, between its full state and its empty state

The full and empty state of the battery EES may be specified byphysical or electrical boundary conditions, such as the minimum andmaximum operating voltage of the battery system.

Capacity is a typical metric for electrochemical batteries. EEStechnologies other than batteries normally use energy capacityinstead of capacity as a measure for energy storage capability.

Ah

cycle charge/discharge cycle consisting of four controlled phases startingfrom an initial SOC, in particular, either: a charge phase, thena pause, then a discharge phase and finally a new pause; or: adischarge phase, then a pause, then a charge phase and finally a newpause

The patterns of the charge and discharge phases are generally linear(constant active power); however different patterns can be defined.

A pause means zero active power into and out of the EES.

1 IEC 62933-1CDV

cycle lifetime* theoretically achievable number of cycles when the EES is cycled withequal full charge-discharge cycles 1[5.5.2]

depth ofdischarge

energy discharged from the EES during a cycle (discharge phase)expressed as a percentage of the nominal energy capacity

%

EES systemefficiency*

useful energy output at the POC divided by the energy inputs to theEES system including all parasitic energies needed to run the system,such as heating or cooling, etc. and expressed as percentage, atspecified service conditions

% IEC 62933-1CDV


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efficiency* energy delivered by the EES divided by energy received by the EES,presented as a percentage

Efficiency may be defined at different levels of the system:

— related to the EES behind the converter— related to the EES system at the POC including auxiliary power

demand— related to the EES system at the POC without taking the auxiliary

power demand into account.

The efficiency may depend on the charging and/or dischargingpower, the initial and final SOC, the pauses between charging anddischarging, and the operating conditions (temperature, humidity, airpressure).

Efficiency should either be defined for a number (> 1) of equal pre-defined cycles or for a certain pre-defined application duty-cycleprofile.

Efficiency related to power parameters is out of scope of this RP.

% [5.4]

efficiency map* graphical overview of the EES system efficiency for all relevant systemstates, defined by charging and discharging power and the initial andfinal state of charge

% [5.4.1]

end of life moment in time after commissioning of the EES system when itsperformance, whether technical, financial or otherwise, has degradedto the point of being no longer usable in its current application

End of life may be reached before the expected lifetime because ofadditional criteria beyond technical calendar life and cycle life.

(notapplicable)

[4.9] and[6.5.3]

energy capacity* amount of electrical energy a fully charged EES can deliver at aspecified discharge power (or discharge power profile), at specifiedenvironmental conditions, between its full state and empty state

The full and empty state of the EES may be specified by physical orelectrical boundary conditions, such as the minimum and maximumoperating speed of a flywheel or the minimum and maximumoperating voltage of a battery or capacitor system.

Wh

expectedlifetime*

technical design duration for which the EES system performancecharacteristics are valid at nominal operation

Expected lifetime is strongly related to calendar lifetime and cyclelifetime

years [5.5.1] and[5.5.2]

full cycle nominal cycle, with a charging phase at maximum continuouscharging power (and typical charging profile) from 0% SOC to 100%SOC and a discharging phase at maximum continuous dischargingpower (and typical discharging profile) from 100% SOC to 0% SOC

1


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initial delay time an EES system requires from a trigger to provide power (such asa command or a grid event) until it starts to ramp up power

This time is called ‘dead time’ in IEC 62933-1 CDV.

ms [5.3.3]

maximumcontinuouspower*

maximum value of the active power the ESS system is designed for incontinuous operation, i.e. in constant charging mode until a specifichigh SOC is reached or in constant discharging mode until a specificlow SOC is reached

Maximum continuous charge power and maximum continuousdischarge power may have different values.

kW [5.1.1]

maximum peakpower*

maximum value of the active power at which the ESS system is ableto operate during a short period of time and during a specific SOCwindow; this period of time and the specific SOC window shall bespecified together with the maximum peak power

Maximum peak charge power and maximum peak discharge powermay have different values.

kW [5.1.2]

nominal capacity initial value of the capacity of the EES as stated by the manufacturerand by which the EES is designated and identified

Ah IEC 62933-1CDV

nominal cycle pre-defined charge/discharge cycle used in the characterization,specification and testing of the EES system

A typical nominal cycle will show the same SOC before and after thecycle, have relatively short pauses and have constant charging anddischarging power equal to the nominal charging and dischargingpower, respectively. Also, its DOD will be (close to) the maximum DODallowed.

A variation on the nominal cycle may be a cycle with power levelsequal to the nominal cycle, but a different initial SOC and/or adifferent SOC at the first pause. Another variation on the nominalcycle may be a cycle with SOC values at the start and at the pausesequal to the nominal cycle, but different charge and discharge powerlevels.

IEC 62933-1 CDV uses the term ‘standard charging/discharging cycle’

1

nominal energycapacity*

initial value of the energy capacity of the EES as stated by themanufacturer and by which the EES is designated and identified

kWh IEC 62933-1CDV

nominalfrequency

value of the frequency by which the POC is designed and identified

The nominal frequency can also be a range of values at the POCwhere the EES system is able to remain connected to the grid andperform all its duties without risk of damage.

Hz IEC 62933-1CDV

nominal operation EES operation at nominal voltage and frequency, rated charging anddischarging power and normal environmental conditions

(notapplicable)


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nominal round-trip efficiency

round-trip efficiency at nominal operation %

nominal voltage value of the AC voltage by which the POC is designed and identified

The nominal voltage can also be a range of voltage values at the POCwhere the EES system is able to remain connected to the grid andperform all its duties without risk of damage.

V IEC 62933-1CDV

environmental conditions specified by the EES system integratorunder which the EES system will operate as intended. For eachperformance indicator, these normal environmental conditions need tobe specified:

— ambient air temperature range— ambient relative humidity range.

°C%

normalenvironmentalconditions

ambient air pressure range:

— expressed as pressure range, or— expressed as altitude range.

When specifying performance parameters or test conditions, referenceshall be made to the normal environmental conditions or to differentlyspecified environmental conditions.

If not specified otherwise, the normal environmental conditions shouldbe:

— ambient air temperature: 20-25°C— ambient relative humidity: 20-80%— ambient air pressure: 101.325 kPa (1 atm).

Pa

m

partial cycle cycle in which the SOC at the end may be different from the SOC atthe start

1

ramp rate rate of change of EES system power at POC

From a system point of view, the maximum ramp rate of the EESsystem at the POC is the most important ramp rate parameter.

The ramp rate is defined for the change of power between 10% and90% of the difference between the final and the initial power valueand the corresponding ramping time interval.

Maximum ramp rate may or may not have different values for:

— increase of charging power— decrease of charging power— increase of discharging power— decrease of discharging power.

kW/s [5.3.2]


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ramp-up time time period starting from the moment the EES system starts to rampup power (after the initial delay) and ending when the power at POCcomes to (and stays) within a certain accuracy band of the targetvalue (for example within 2%)

ms [5.3.2]

response time the total time the EES system needs to reach the state where thepower at POC comes to (and stays) within a certain accuracy band ofthe target value (for example within 2%), after an internal or externaltrigger (such as a setpoint command or a grid event)

The response time (also called ‘step response time’) is the initial delayplus the ramp-up time.

In IEC 62933-1 CDV, the response time is called ‘settling time’.

ms [5.1.3.2]

round-tripefficiency*

EES system efficiency over one cycle with equal final and initial SOCand with a specified DOD (either positive or negative)

% [5.3.2]

self-dischargerate*

reduction of the energy content (relative to actual energy capacity) ofan EES while the system idles

% / month

state of charge(SOC)

degree to which an electrochemical EES has been charged relativeto a reference point (defined as SOC = 100%) indicating the totalelectrical charge that can be stored by the EES.

The SOC reference point should be the actual charge capacity of theEES.

The SOC reference point may alternatively be defined using the EESvoltage.

The point of SOC = 0% is generally defined by a certain EES voltagerelated to a certain lower boundary value of the cell voltage.

% [6.5.1]

state of energy(SOE)

available energy inside an EES relative to the actual energy capacity,given as a percentage.

% [6.5.1]

state of health(SOH)

actual capacity relative to the initial rated capacity of the EES, givenas a percentage

Actual and rated capacity may be charge capacity or energy capacity

This is the narrowest definition of SOH; additional indicators may beincluded in the SOH definition, i.e. maximum power relative to theinitial rated power.

% [6.5.2]

2) For some units, a commonly used SI metric prefix is listed here; a different prefix may also be used, e.g. W or MWinstead of kW. Time units may also be interchanged, e.g. days or hours instead of months.

* Terms marked with an asterisk (*) should be carefully specified wherever they occur in any document, i.e. in eachoccasion it should be stated under which conditions the stated value(s) are valid.


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SECTION 3 APPLICATIONS OF STATIONARY ELECTRICAL ENERGYSTORAGE SYSTEMS

3.1 GeneralThe aim of this section is to provide self-contained and concise descriptions of EES applications as appliedto the energy sector. The descriptions of storage applications provided in this section shall be used as areference for other sections of this RP.The current markets and technologies of EES are constantly evolving. It should thus be noted that the EESapplications listed in this section may not be exhaustive. New applications may arise, some applications maybecome unviable and/or specific applications may not be covered. In future updates of this document, suchdevelopments will be covered.

3.1.1 Application overview and reading instructionsTwenty different EES applications are sub-divided in six different umbrella groups, as listed in Table 3-1below and further explained in corresponding (sub-)sections in this section. Note that EES systems mayserve several different applications simultaneously or sequentially, thereby increasing the profitability ofthe EES system, which may be referred to as revenue stacking, application stacking or benefit stacking.Per EES application, an example power range and discharge duration has been provided. It should be notedthat these example values reflect EES applications in combination with the USA interconnected grid. Thespecifications of EES systems incorporated in an electricity grid strongly depend on the grid properties andexisting grid regulations. The EES manufacturer should always be consulted for precise system specifications.Per application, example technologies are stated. From a technical perspective, for most applications hybridenergy storage systems (see [2.2.3]) may be equally or better suited than single technologies, in particularwhen the load profile consists of a combination of high power peaks and long duration loads.The estimated maximum discharge duration range provides a range in which an EES system should beable to completely discharge. The estimated maximum discharge duration does not provide an EES systemminimum time of operation. In many applications, the EES system will only be partly discharged before beingrecharged again. Please note that all values in Table 3-1 are indicative examples, both location-dependentand case-specific; higher or lower values may be possible, just less common.

Table 3-1 EES applications, in six umbrella groups

Example power range Estimated maximumdischarge duration range Reference

Bulk energy services [3.2]

1. Electrical energy time-shift 1 MW – 500 MW 2 hours – 6 hours [3.2.1]

2. Power supply capacity 1 MW – 500 MW 2 hours – 6 hours [3.2.2]

Ancillary services [3.3]

3. Load following 1 MW – 100 MW 15 min – 4 hours [3.3.1]

4. Regulation 10 MW – 40 MW Seconds to hours, dependson market

[3.3.2]

5. Frequency response 2 MW – 50 MW Seconds to minutes [3.3.3]

6. Spinning, non-spinning and supplementalreserve

10 MW – 100 MW Minutes to hours [3.3.4]

7. Voltage support 1 MVAr – 10 MVAr Minutes to an hour [3.3.5]


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Example power range Estimated maximumdischarge duration range Reference

8. Black start 5 MW – 50 MW Seconds to hours [3.3.6]

Transmission infrastructure services [3.4]

9. Transmission congestion relief 1 MW – 100 MW 1 hour – 4 hours (cannotbe generalised easily)

[3.4.1]

10. Transmission upgrade deferral 10 MW – 100 MW 1 hour – 8 hours [3.4.2]

Distribution infrastructure services [3.5]

11. Distribution upgrade deferral 500 kW – 10 MW 1 hour – 4 hours [3.5.1]

Customer energy management and microgridservices

[3.6]

12. Power quality 100 kW – 10 MW Milliseconds – 15 minutes [3.6.1]

13. Power reliability (grid-connected) 50 kW – 10 MW 1 hour – 8 hours [3.6.2]

14. Power reliability (microgrid operation) 50 kW – 10 MW 1 hour – 8 hours [3.6.3]

15. Retail electrical energy time-shift 1 kW – 1 MW 1 hour – 6 hours [3.6.4]

16. Demand charge management 50 kW – 10 MW 15 minutes– 4 hours [3.6.5]

17. Renewable power consumptionmaximisation

50 kW – 10 MW 1 hour – 4 hours [3.6.6]

Renewables integration [3.7]

18. Ramp rate control 1 MW – 500 MW 1 minute cyclic-repetitive –4 hours

[3.7.1]

19. Generation peak shaving 1 MW – 500 MW 1 hour – 4 hours [3.7.2]

20. Capacity firming 1 MW – 500 MW 1 hour – 4 hours [3.7.3]

3.1.2 ReferencesThe following documents were used in order to construct the content of this subsection:

— Sandia National Laboratories, DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA,USA, Jul 2013

— Sandia National Laboratories, Energy Storage for the Electricity Grid: Benefits and Market PotentialAssessment Guide

— Ecofys, Energy Storage Opportunities & Challenges – A West Coast Perspective White Paper— Energy Storage Operating Forum, A Good Practice Guide on Electrical Energy Storage, EA Technology,

United Kingdom, Dec 2014— Luo X., et al., Overview of current development in electrical energy storage technologies and the

application potential in power system operation, Applied Energy 137, 2015, p511-536— European Association for the Storage of Energy (EASE) and European Energy Research Alliance (EERA);

Joint EASE/EERA recommendations for a European Energy Storage Technology Development Roadmaptowards 2030


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3.2 Bulk energy servicesBulk electrical energy storage is used to store relatively large amounts of energy in order to be madeavailable (often locally) at another, usually more convenient, time.

3.2.1 Electrical energy time-shiftEES systems operating within an electrical energy time-shift application are charged with inexpensiveelectrical energy and discharged when prices for electricity are high. On a shorter timescale EES systems canprovide a similar time-shift duty by storing excess energy production from, for example, renewable energysources with a variable energy production, as this might otherwise be curtailed. If difference in energy pricesis the main driver, this application is often referred to as arbitrage.Storing energy (i.e. in charge mode) at moments of peak power to prevent curtailment or overload is calledpeak shaving. Peak shaving can be applied for peak generation and also – in discharge mode – for peakdemand (e.g. in cases of imminent overload). Peak shaving implicates that the energy charged or dischargedis discharged or recharged, respectively, at a later stage. Therefore, peak shaving is a form of the energytime-shift application.An EES system used for energy time-shift could be located at or near the energy generation site or in otherparts of the grid, including at or near loads. When the EES system used for time-shift is located at or nearloads, the low-value charging power is transmitted during off-peak times.Important for an EES system operating in this application are the variable operating costs (non-energy-related), the storage round-trip efficiency and the storage performance decline as it is being used (i.e. ageingeffects).

Example EES system power range: 1 MW – 500 MW

Response time: Milliseconds to minutes

Estimated maximum discharge duration range: 2 hours – 6 hours

Minimum cycles per year: >250

Experienced EES technology options: PHS, CAES, LAES and batteries

3.2.2 Power supply capacityAn EES system could be used to defer or reduce the need to buy new central station generation capacityand/or purchase capacity in the wholesale electricity market. In this application, the EES system suppliespart of the peak capacity when the demand is high, thus relieving the generator by limiting the requiredcapacity peak. Following a (partly) discharge, the EES system is recharged when the demand is lower. Thepower supply capacity application is a form of electrical energy time-shift. An EES system participating in theelectrical capacity market may be subject to restrictions/requirements of this market, for example requiredavailability during some periods.


Response time: Minutes

Estimated maximum discharge duration range: 2 hours – 6 hours

Minimum cycles per year: 50 – 365

Experienced EES technology options: PHS, CAES, LAES and batteries


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3.3 Ancillary servicesEES systems used as an ancillary service are used to facilitate and support the electricity grid providinga continuous flow of electricity and matching supply and demand. Providing start-up power after a totalblackout is also considered an ancillary service.

3.3.1 Load followingLoad following is one of the ancillary services required to operate a stable electricity grid. EES systems usedin load following applications are used to supply (discharge) or absorb (charge) power to compensate for loadvariations. Therefore, this is a power balancing application. In general, the load variations should stay withincertain limits for the rate of change, or ramp rate. Therefore, this application is a form of ramp rate control.The same holds for generation variations, which is very applicable to renewable energy sources (RES).Conventional power generation can also operate with a load following (or RES compensating) application.Within these applications, the benefits of EES over conventional power generation are that:

— most EES systems can operate at partial load with relatively modest performance penalties— most EES systems can respond quickly with respect to a varying load— EES systems are suitable for both load following down (as the load decreases) and load following up (as

the load increases) by either charging or discharging.

Note that an EES system operating with a load-following or ramp rate control application within a marketarea needs to purchase (when charging) or sell (when discharging) energy at the going wholesale price. Assuch the EES efficiency is important when determining the value of the load following application.


Response time:


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3.3.3 Frequency responseFrequency response is a general term for a component in the power system that responds to a change inthe grid frequency, with the purpose to counteract this change in frequency. A passive form of frequencyresponse is inertia that physically limits the rate of change of frequency. An active form of frequencyresponse is power/frequency control (or transmission system frequency control), performed by all largesynchronous generators, meaning that they dynamically increase (or decrease) their power when thefrequency decreases (or increases, respectively). See also[3.3.4].Inertia is needed in the power system to counteract grid frequency fluctuations, thereby stabilising thesystem. Causes of frequency fluctuations could be the loss of a generation unit or a transmission line(causing a sudden power imbalance). Various generator response actions are needed to counteract a suddenfrequency deviation, often within seconds. Physical inertia is provided by conventional power generators, i.e.synchronous generators.Synthetic inertia behaviour is the increase or decrease in power output proportional to the change of gridfrequency, thereby mimicking physical inertia, i.e. counteracting the grid frequency excursion. If the totalamount of physical inertia decreases in a power system, the amount of synthetic inertia should be increasedto maintain a certain minimum amount of total inertia for power system stability. Many grid-connectedrenewable energy sources do not provide additional synthetic inertia. Therefore, larger grid frequencydeviations may occur as the total inertia in the power system decreases. Keeping track of the total systeminertia could be a future task of TSOs or ISOs. Some TSOs in the USA and Europe (e.g. EirGrid and SONI inIreland) are procuring a new service called Synchronous Inertial Response (SIR), that is specifically meant toensure physical inertia in the power system.Electromechanical EES systems with a synchronous generator, like PHS, CAES and LAES, can provide physicalinertia, including SIR. Converter-connected EES systems can add synthetic inertia to the system.EES within a frequency response application could support the grid operator and thereby assure a smoothertransition from an upset period to normal operation. For a frequency response type of application the EESis required to provide support within milliseconds. Aside from this quick response, the frequency responseapplication is similar to load following (see [3.3.1]) and regulation (see [3.3.2]).


Response time: Milliseconds – 0.1 second

Estimated maximum discharge duration range: seconds to minutes

Minimum cycles per year: PM (application results in micro-cycles)

Experienced EES technology options: Batteries, flywheels, supercapacitors

3.3.4 Operating reservesA certain reserve capacity is usually available when operating an electric power system. This reserve capacitycan be called upon in case of a significant frequency disturbance, for example upon unexpected unavailabilityof generation capacity, thus ensuring system operation and availability. Subdivisions can be made based onhow quickly a reserve capacity is available. In the US for example, the following division is made:

— Spinning reserve is reserve generation capacity connected and synchronised with the grid and canrespond to compensate for generation or transmission outages. In remote grids spinning reserve ismainly present to cover for volatile consumption. In case a reserve is used to maintain system frequency,the reserve should be able to respond quickly. Spinning reserves are the first type of backup that isused when a power shortage occurs. Spinning reserve may be provided by all synchronous generators,including PHS, CAES and LAES systems with a synchronous generator.

— Non-spinning reserve is connected but not synchronised with the grid and usually available within 10minutes. Examples are offline generation capacity or a block of interruptible loads.


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— Supplemental reserve is available within one hour and is usually a backup for spinning and non-spinningreserves. Supplemental reserves are used after all spinning reserves are online.

In Europe, the following categorisation applies:2

— Frequency Containment Reserve (FCR) stabilizes the frequency after the disturbance at a steady-statevalue

— Frequency Restoration Reserve (FRR) controls the frequency towards its setpoint value by activation ofFRR and replaces the activated FCR. FRR can be automatic or manual (aFRR resp. mFRR).

— Reserve Replacement (RR) replaces the activated FRR (and in some countries FCR as well) and/orsupports the FRR activation by activation of RR.

The need for reserve generating capacity is signaled in the power system by the frequency deviating from itsnominal value. Therefore, reserves are linked to frequency response.Stored energy reserves are usually charged EES backup systems that have to be available for discharge whenrequired to ensure grid stability. PHS, CAES and LAES can act as operating reserves. Another example of anoperating reserve is an uninterruptible power supply (UPS) system, generally containing one or more EESsystems, which can provide near to instantaneous power in the event of a power interruption or a protectionfrom a sudden power surge. Large UPS systems can sometimes maintain a whole local grid in case of apower outage; this application is called island operation and is discussed in more detail in [3.6.2].Note that from a grid balancing perspective it is also possible to reduce the loads connected to the grid(instead of increasing generation power). Thus, interruptible loads can also be considered operating reserve.An EES can thus provide reserve capacity in different ways: it can simultaneously stop charging (reduce theload) and start discharging. Furthermore, EES reserve capacity could also be negative, e.g. in case of loss ofa large load or an exporting interconnection.


Response time: Milliseconds to seconds

Estimated maximum discharge duration range: Minutes to hours


Experienced EES technology options: Batteries, flywheels, LAES, CAES, redox flow batteries

3.3.5 Voltage supportGrid operators are required to maintain the grid voltage within specified limits. This usually requiresmanagement of reactive power (but also active power, e.g. in the LV grid), therefore also referred to asVolt/VAr support. Voltage support is especially valuable during peak load hours when distribution lines andtransformers are the most stressed. An application of an EES system could be to serve as a source or sinkof the reactive power. These EES systems could be placed strategically at central or distributed locations.Voltage support typically is a local issue at low voltage (LV), medium voltage (MV) or high voltage (HV) level.The distributed placement of EES systems allows for voltage support near large loads within the grid.Voltage support can be provided by operation of generators, loads and other devices, such as dedicatedSTATCOMs or synchronous condensers. A possible advantage of EES systems over generators and loads isthat EES systems are available to the grid even when not generating or demanding power (like STATCOMsand condensers).Note that no (or low) real power is required from an EES system operating within a voltage/VAr supportapplication, so cycles per year are not applicable for this application and storage system size is indicated inMVAr rather than MW.In case of a synchronous generator-connected EES system, the generator can provide reactive power, likeany other synchronous generator. The desired reactive power capability is a design criterion for the generator.

2 Supporting Document for the Network Code on Load-Frequency Control and Reserves, EuropeanNetwork of Transmission System Operators for Electricity (ENTSOE), 28th June 2013)


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For example, the power recovery unit of a LAES system can be operated as a synchronous condenser andthis service could be provided even when the system is in charging mode, because the air liquefier (charger)and PRU (discharger) are independent from each other.In case of a converter-connected EES system, the converter needs to be capable of operating at a non-unitypower factor in order to source or sink reactive power. The nominal duration needed for voltage support isestimated to be 30 minutes, which allows the grid time to stabilize and/or begin orderly load shedding orgeneration curtailment.

Example EES system power range: 1 MVAr – 10 MVAr

Response time: Seconds

Estimated maximum discharge duration range: A few minutes to an hour

Minimum cycles per year: Not applicable

Experienced EES technology options: Batteries and redox flow batteries, LAES, CAES

3.3.6 Black startFollowing a catastrophic failure of a grid, the EES system provides an active reserve of power and energywhich can be used to energize transmission and distribution lines, provide start-up power for one or morediesel generators and/or (larger) power plants and provide a reference frequency.A consequence of the black start application is that the EES system waits, fully charged, for the (sporadic)moment that a black start is required. However, a better solution would be to have an EES system with aminimum state of charge available for black start, and in the meantime use it for other applications.


Response time: Minutes

Estimated maximum discharge duration range: Seconds to hours


Experienced EES technology options: Small-scale CAES, LAES, batteries, redox flow batteries

3.4 Transmission infrastructure servicesStrategically placed electrical energy storage used within a transmission infrastructure service may act as anenergy buffer, thereby reducing power congestion and/or defer transmission grid upgrades.

3.4.1 Transmission congestion reliefDuring moments of peak demand, it may occur that the available transmission lines do not provide enoughcapacity to deliver the least-cost energy to some or all of the connected loads. This transmission congestionmay increase the energy cost.EES systems at strategic positions within the electricity grid help avoiding congestion-related costs andcharges. The EES system can be charged when there is no congestion and discharged when congestionoccurs. As such, an EES can help avoiding congestion-related additional costs. This application is a form of(transmission) electrical energy time-shift, in addition to reactive power management; an EES can typicallyprovide both.


Response time: Milliseconds


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Estimated maximum discharge duration range: 1 hour – 4 hours (cannot be generalised easily)


Experienced EES technology options: Batteries, LAES and SMES

3.4.2 Transmission upgrade deferralEES can delay – and sometimes avoid – the need to upgrade a transmission system, e.g. in case ofcongestion problems. Installing an EES system downstream of a nearly overloaded transm

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