Energy Storage E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 389

Veröffentlichungsjahr: 2013

Beschreibung

Energy storage examines different applications such as electric power generation, transmission and distribution systems, pulsed systems, transportation, buildings and mobile applications. For each of these applications, proper energy storage technologies are foreseen, with their advantages, disadvantages and limits. As electricity cannot be stored cheaply in large quantities, energy has to be stored in another form (chemical, thermal, electromagnetic, mechanical) and then converted back into electric power and/or energy using conversion systems. Most of the storage technologies are examined: batteries, hydrogen, super capacitors, SMES, flywheels, CAES, thermal storage and hydraulic gravitational storage.

Leseprobe

Table of Contents

Foreword

Chapter 1. Energy Storage for Electrical Systems

1.1. Introduction

1.2. Energy storage for the producer

1.3. The special case of intermittent generation

1.4. Energy storage for transmission systems

1.5. Energy storage for distribution networks

1.6. Energy storage for retailers

1.7. Energy storage for consumers

1.8. Energy storage for the balancing responsible party (BRP)

1.9. Conclusion

1.10. Bibliography

Chapter 2. Transport: Rail, Road, Plane, Ship

2.1. Introduction

2.2. Electrical energy is a secondary energy

2.3. Electrical energy: principal or unique source

2.4. Electrical energy complementing another source – hybridization

2.5. Conclusion

2.6. Bibliography

Chapter 3. Energy Storage in Photovoltaic Systems

3.1. Introduction

3.2. Stand alone photovoltaic systems

3.3. Limited lifespan for lead acid battery technology

3.4. Grid connected systems

3.5. Bibliography

Chapter 4. Mobile Applications and Micro-Power Sources

4.1. The diverse energy needs of mobile applications

4.2. Characteristics due to the miniaturized scale

4.3. Capacitative storage

4.4. Electrochemical storage

4.5. Hydrocarbon storage

4.6. Pyroelectricity

4.7. Tribo-electricity

4.8. Radioactive source

4.9. Recovering ambient energy

4.10. Associated electronics: use of electricity – onboard EP

4.11. Bibliography

Chapter 5. Hydrogen Storage

5.1. Introduction

5.2. Generalities regarding hydrogen storage

5.3. Pressurized storage

5.4. Cryogenic storage

5.5. Solid storage

5.6. Other modes of storage

5.7. Discussion: technical/energy/economic aspects

5.8. Bibliography

Chapter 6. Fuel Cells: Principles and Function

6.1. What is a cell or battery?

6.2. Chemical energy

6.3. The unfolding of a reaction

6.4. Proton-exchange membrane fuel cells (PEMFCs)

6.5. The solid oxide fuel cell (SOFC)

6.6. The alkaline fuel cell (AFC)

6.7. Comparison of the different types of fuel cell

6.8. Catalysis

6.9. Critical points

6.10. Conclusion: the storage application

Chapter 7. Fuel Cells: System Operation

7.1. Introduction: what is a fuel cell “system”?

7.2. Air supply system

7.3. Gas humidification system

7.4. The static converter at the stack terminals

7.5. Lifespan, reliability and diagnosis

7.6. Bibliography

Chapter 8. Electrochemical Storage: Cells and Batteries

8.1. Generalities of accumulators: principle of operation

8.2. Applications

8.3. Technological histories: lead, Ni-Cd, Ni-MH...then lithium ion

8.4. Application needs

8.5. Focusing on lithium-ion technologies

8.6. Processing and recycling of lithium batteries

8.7. Other batteries

8.8. Bibliography

Chapter 9. Supercapacitors: Principles, Sizing, Power Interfaces and Applications

9.1. Introduction

9.2. Supercapacitor: electric double-layer capacitor

9.3. Sizing a bank of supercapacitors

9.4. Power interfaces

9.5. Applications

9.6. Bibliography

List of Authors

Index

First published 2011 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc. Adapted and updated from Problématiques du stockage d’énergie published 2009 in France by Hermes Science/Lavoisier © LAVOISIER 2009

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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The rights of Yves Brunet to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Energy storage / edited by Yves Brunet.

p. cm.

  Includes bibliographical references and index.

  ISBN 978-1-84821-183-4

  1. Energy storage. 2. Electric power supplies to apparatus. I. Brunet, Yves.

  TK2980.E54 2010

  621.31'26--dc22

2010022199

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-84821-183-4

Foreword1

Sources of energy: density of stored energy

Energy sources are all stored, whether on a geological scale or greater (the sun), and the stores are used up according to need (the idea of “renewables” therefore is only meaningful when considering human timescales). We can distinguish the primary source of fossil fuels that exist “naturally” and for which we only pay the cost of extraction, from secondary sources, which are man-made, and for which we must pay for both storage and extraction.

Table 1. Time required to replenish sources (source: W.A. Hermann, Quantifying Global Energy Resources, Science direct, Elsevier 2005)

Sources

Unit of time

Biomass

Years

Oceanic thermal gradients

Hundreds of years

Fossil fuels

Millions of years

Tides/waves

Hours

Geothermal

Days - years

Thermal mass

Hours

Batteries

Minutes

SMES

Seconds

Capacities

Seconds

Hyraulic pumping

Hours

In this work we will primarily be interested in these secondary forms of storage.

The energy that can be exploited is not only stored in nature under various forms, but is also stored with very different densities (Figure 1).

Figure 1. The density of energy stored in materials or storage components varies greatly. The figure above shows the great advantage of fossil fuel sources over secondary storage sources. Nuclear sources are even more concentrated as we can obtain 108 Wh/kg from fission of natural uranium

The range in the amounts of energy usage is such that it is good to consider a few simple applications: what can be done with 1 kWh?

We can:

– drive 1 km with a car that consumes 8 liters per 100 km;

– run a refrigerator for a day;

– light a house for an evening;

– make 200 g of steel or 100 g of plastic.

On average, the total amount of energy consumed in France, for each inhabitant, comes to 40 MWh/year, which is 4.5 kWh/hour per person.

Conversion of stored energy

Stored energy is released, according to target applications, either in the form of power (W), or in the form of energy 1 (J or Wh), which is sustained power over a certain amount of time. Storage sources, which combine a quantity of stored energy with power that is instantaneously available, are often useful.

The storage strategy may lead to a range of different solutions (Figure 2).

Figure 2. The requirements of an electric vehicle. A hybrid vehicle essentially needs stored power, whereas an electric vehicle will need both power and energy. For a hybrid vehicle, the amounts required are of the order of 12 Wh/kg and 500 W/kg, with available energy of 300 Wh and available power of 10 kW over 2 seconds and a lifetime of 15 years

Energy is brought to the user by an energy carrier, after transformation and conversion to the most suitable form possible for the target application. Electricity is one of these forms, without doubt the most flexible form known to this day (Figure 3).

Figure 3. Principal energy carriers

The problem of energy storage is both technical and economic, and the solutions depend very much on the target applications (see Chapters 1-5). Regarding energy storage for technologies linked to the electricity carrier, this is not of immediate interest, particularly in the case of networks, and at least two opposing situations can be distinguished:

–  onboard systems (mobile or portable applications, etc.), which carry their energy with them in order to ensure autonomous functioning, or pulsing systems for which storage acts as a “buffer” that releases the necessary high power;

– coupled systems (networks), which put into play high energy and high power.

The special case of pulsing systems2

A pulsing system stores energy and releases it in a very short time. In general, the energy is stored in electromagnetic form (an electric or magnetic field) and delivered in a very brief time (several milliseconds) as a result of a rapid switch. Therefore, for an amount of stored energy W, the power, P=W/t, can be very large.

In the case where energy is stored by a series of capacitors (Marx generators), several parameters are involved in the release of energy:

– the electrical characteristics of the storage circuit (R, L, C);

– the electrical characteristics of the charge impedance (R, L, C);

– the initial conditions;

– the characteristics of the switching system (R, L, t).

The voltage can reach several megavolts, for currents of several mega-amperes. Pulsing systems can be single shot or can go up to several kilohertz.

A capacitive system includes capacitances and a closing switch (V). An inductive system includes inductances and both closing (I) and opening (V) switches.

Switching devices can be of the following types:

– gaseous: pressurized spark gaps, ignitrons, thyratrons, etc.;

– semi-conductor: thyristor, GTO, IGBT, MOSFET, SRD diodes, etc.;

– solid: fuse.

Figure 4. Inductive storage for a voltage generator: the principle is to generate current in an inductance, and then to force the current to cross an impedance in a given instant. This technology requires a sharp opening switch

In the case of capacitive storage, Marx generators enable a high voltage to be generated by charging capacitors in parallel and discharging them in series.

Pulsed energy is used in several domains, industrial as well as research based. Among the applications using pulsed power-based systems, we can cite radar, particle accelerators, creation of very high magnetic fields, lasers, electric cannons (railgun) etc.

The special case of electrical networks

This case will be detailed in Chapter 1. Here, we outline the principal characteristics of storage in electrical networks. The storage problem takes on a greater level of seriousness when looking at electrical networks. As electricity is not readily stored in an efficient manner and in useful quantities, it is necessary to constantly adapt the power supplied to the power demanded, whilst recognizing that this fluctuates according to the time of day and season (Figure 6). Storage technologies break this link by allowing production and storage of electricity for later use.

Figure 6. Electricity consumption in France (source: RTE)

It is therefore necessary to store energy in an intermediary physical form (mechanical, thermal, chemical, etc.) and to convert this stored energy into electricity (battery, generator, etc.) by incorporating energy converters, based on power electronics, whose efficiency (of the order of 80% to 90%) nevertheless has energy and financial costs.

In the energy chain, storage can be used in every one of these steps (Figure 7).

Figure 7. Points where storage can be inserted within a network

Storage technologies must demonstrate technical viability and economic interest. The cost of energy, linked to its variability according to time of day and of year (due to the supply and demand law in a market, which is increasingly open) and the difference between this cost at peak and off-peak times, are parameters that determine the degree of interest in adding storage. Storage is a means of adding flexibility competing with other factors:

– the value of the storage depends very much on the technology used and on its sizing compared to the predicted usage;

– the same type of storage can have a different value on different markets and for different agents;

–  several factors have a strong influence on the value that agents can give to storage, such as the energy mix, the level of congestion on the network, etc.

A storage system can play different roles and can be, for example:

– a peak-time electric power station;

– a source of charge smoothing (harnessing transits over targeted work);

– a way to maintain the quality of the current, voltage, and frequency;

– a support to the network during downgraded function;

– a promotion permitting investment;

– a stabilizing function in a context where renewables have properly penetrated the market.

Figure 8. Storage brings answers to problems in electrical networks (source Energy Storage, The Missing Link in the Electricity Value Chain: An ESC White Paper, Published by the Energy Storage Council, May, 2002)

There are also intermediary situations (micro-networks, isolated systems, etc.), which often use intermittent energies (wind power, solar energy, etc.) for which the storage solutions must be studied according to technical and economic criteria. Storage, therefore, enables us to resolve the problem of intermittence of renewables by allowing us to:

– maximize the use of photovoltaic electricity;

– consume energy at the place of production and increase energy efficiency;

– increase the flexibility and efficiency of energy management;

– ensure safety of the user in the case of network outage.

Following the target applications, several technical and economic parameters (investment costs, energy or power densities, cyclability, impact on the environment, etc.) influence the choice of storage technologies (Figure 9). These different technologies will be detailed in Chapter 6 and later chapters.

Figure 9. Constraints and criteria for choosing storage technology based on applications

If we look again at Figure 6, we see that using storage to account for increases beyond the average daily consumption of electricity leads to a requirement to store several tens of gigawatt hours. At the user level, the problems are different as the quantities of power are much lower, and it might be interesting to consider storage solutions closer to where they are needed 3 (Figure 10). Storage is a way to guarantee the quality of the energy at user level (UPS, Uninterruptible Power System).

However, the financial cost resulting from instantaneous interruptions or from prolonged interruptions (blackouts) of the electrical network is very important as the network today touches all sectors of the economy (it is calculated to be several tens of billions of dollars per year in the USA)5 and this cost must be compared to that of the storage systems that could reduce the risks of interruption.

Power and energy must be globally managed using network management systems that use ICT (Information and Communication Technology) at the network-operator level, even more in the presence of distributed production. In addition to their traditional function of control-command, these systems are also capable of managing the entire production, storage, and charge using virtual power stations.

Storage technologies

The two tables below summarize the different storage technologies alongside their domains of application.

Table 2. Storage technologies of high capacity (source CEA)

Table 3. Storage technologies of average and low capacity (source CEA)

1. Foreword written by Yves BRUNET. We may also refer to the chapter “Energy storage: applications related to the electricity vector” by the same author, in Low Emission Power Generation Technologies and Energy Management, ISTE / John Wiley, 2009.

2. I would like to thank Jean-Claude BRION (Europulse) for help with the editing of this section.

3. EPRI 2.4 kW, 15 kWh Salt River Residential Photovoltaic-Battery Energy Storage System Project 1997.

4. Doc GIE IDEA (Tuan Tran Quoc).

5. Communication J ETO EESAT 2004.

Chapter 1

Energy Storage for Electrical Systems1

1.1. Introduction

This chapter addresses the potential applications for energy storage in electrical networks or, more specifically, in “electrical systems”. The term “electrical network” tends to refer mostly to transmission and distribution networks, whereas the more general term of “electrical systems” encapsulates the entire electric power supply chain, comprising:

– electricity generation, not only by centralized power stations (whether they be nuclear, fossil fueled, hydraulic, etc.), but also by smaller decentralized generation units (cogeneration, diesel, etc.), or from renewable energy (RE) sources (wind, photovoltaic power, etc.);

– the transmission and distribution networks, with different levels of voltage (from 400 kV for very high voltage transmission networks up to 400 V on low voltage feeders);

– electricity consumpti on by different types of customers connected to these networks: industrial, commercial and tertiary sectors, residential customers, etc.

In electrical systems, the need to maintain the balance between production and consumption of electricity at each instant has made energy storage an issue for a long time. In fact storage systems have been present for a very long time, such as, for example, pumped hydro energy storage known in French as STEP (Stations de transfert d’énergie par pompage/hydraulic pumping stations). However, the economic conditions for the majority of energy storage systems, comprising high costs, economic constraints related to the access to the grids, insufficient financial returns, etc., have prevented the level of development that would have been expected in this area.

However, the current situation and future evolution scenarios for the electric sector bring new perspectives on energy storage, and reasons for modification of the economic conditions include [JAC 08]:

– the need to reduce carbon dioxide (CO2) emissions;

– the development and integration of intermittent RE;

– acknowledgment of the fact that traditional energy sources are dwindling;

– the rise in prices of fossil fuels that should result;

– volatility of the markets;

– networks being operated closer and closer to their limits and the difficulties encountered in developing further network infrastructures;

– technological evolution;

– regulatory evolution.

As a result there is a revival of interest and a large number of research projects are underway on different aspects of energy storage relating to electrical systems. In this context, every electricity system participant (or type of participant) has his own needs, and these lead to different applications for storage.

In this chapter, therefore, we review the main functions of electrical systems and the possible applications for storage. More precisely, we will consider energy storage:

– for the producer (section 1.2) and for the special case of integration of RE (section 1.3);

– for transmission (section 1.4) and distribution (section 1.5) networks;

– for the energy supplier or the retailer (see section 1.6);

– for consumers of electricity (section 1.7);

– for the balance responsible parties (section 1.8).

We will end with a brief summary (section 1.9).

1.2.  Energy storage for the producer

The activity of “production” (or generation) consists of exploiting power stations and selling, at every instant, the produced energy on wholesale markets (for example, on spot markets where it will be bought by suppliers) or directly on retail markets (i.e. to the final customers).

In France, this activity takes place in the deregulated sector and, therefore, by participants that are in competition. “Pure” producers sell their entire production on wholesale markets. Integrated producers, which are both producers and retailers (see section 1.6), use all or part of their production in order to satisfy the energy needs of the portfolio “customers” on whom they rely commercially.

Whatever the nature of the markets considered by a producer, the volume and the revenue generated by its production are subject to different hazards:

– the volume sold at every instant by a producer depends on the availability of its power stations, on the size of the demand to be met, and on the competitiveness of its production costs;

– the produced energy, sold on a spot market or exported via interconnections, is paid for at a price depending on all the events and fluctuations occurring in electrical systems.

Faced with these uncertainties, the key issue for the producer is to optimize and secure its production and the associated revenues.

1.2.1.  High-power energy storage” to maximize revenues associated with production

The remuneration associated with the sale of produced energy fluctuates, especially according to hourly, weekly or seasonal variations in demand. In order to maximize production revenues, it is important for a producer to be able to sell the maximum amount of energy at times when the remunerations are most profitable.

Storage management allows energy to be stored when electricity prices are low, so that that energy can be sold when the electricity prices are higher. Therefore, storage is a lever that can allow a producer to increase the revenues associated with its production.

To fully profit from opportunities to carry energy forward, the storage capacities should be sized so that cycles of several dozen hours of use are possible and they should be of high power (order of magnitude: several hundreds of megawatts). Such capacities will enable energy to be carried forward from night to day, over the weekend, and on working days. Typically, the mature technologies that are targeted for this purpose are bulk storage systems, such as hydraulic pumping stations (STEP) and compressed-air energy storage (CAES).

Seasonal energy transfers from summer to winter can also present opportunities for increased revenue for a producer with hydraulic dams sized for storing major quantities of energy (cycles of several hundred hours of use). However, note that the sites (valleys), which are most suitable for developing such storage systems, are for the most part already used up in France.

In summary, by profiting from opportunities to carry forward production according to patterns in consumption (summer-winter, weekend-week, day-night), and by smoothing the residual load curve, energy storage allows a producer to maximize production revenues and to profit from new margins for exploitation. These new areas for exploitation correspond essentially:

– to a reduction in variable fuel charges: substitution of the most expensive fuels (fossil fuels), which are consumed at peak times by the least expensive fuels, which are consumed at off-peak times.

– to a better optimization of sale on the markets: selling more energy when the market conditions are more favorable;

– to the relaxation of the dynamic constraints affecting the operation and management of the generation park: smoothing the load curve enables better optimization of the generation unit commitment by limiting the weight of these constraints (for example, limiting the number of costly stop-starts of certain power generators);

– to a reduction in the level of CO2 emissions which can be valued at the price of emission licenses: in particular in the case of inserting storage as a supplement to a generation mix with low CO2 emissions in base-load (for example, hydraulic and nuclear) but with high CO2 emissions at peak times (due to fossil fuelled stations).

Figure 1.1. Example of energy being carried forward in the daily cycle, as a result of a storage system based on a combustion turbine and the resulting advantages

1.2.2. High-power energy storage” to alleviate physical and financial risks of production

Availability of stored energy can help to limit the effect of unfavorable physical hazards, such as lack of available capacity in conventional production, cold fronts, lack of wind, etc. Energy storage constitutes an effective solution for alleviating these physical risks and enables producers to avoid having to take recourse in other forms of cover, such as buying energy on forward markets or investing in supplementary capacity.

In particular, storage enables to smooth the extreme peaks in the residual load curve, which reduces the need to invest in extra production capacity just to accommodate those load peaks that occur only a few hours each year. Therefore, storage permits the delay of, indeed even the reduction in need for, investing in new production capacity, which constitutes a saving for the producer.

In addition, availability of stored energy can cover financial risks, by limiting the level of exposure to the volatility of market prices. By providing the required energy to cover these risks, storage limits recourse to other ways of covering this risk, and this is yet another source of value of storage.

1.2.3. Storage for ancillary services

Depending on their size and their technical characteristics, storage systems, which are (or would be) at the disposal of a producer, can enable the producer’s obligations to be fulfilled; for example, relating to regulation of frequency or network restoration. A succinct description of these services is given below.

1.2.3.1. Frequency regulation

The characteristics of frequency regulation depend very much on the country. As a result, there is much ambiguity in the terminology. For convenience, only the French case is considered in this chapter. We distinguish three types of frequency regulation: primary, secondary, and tertiary.

1) Primary frequency regulation: the goal of primary frequency regulation is to maintain real-time generation-consumption balance by acting directly and automatically on the governing systems of the generating units participating in the regulation. In particular, it is about stabilizing the frequency on a time scale of several seconds in the case of variations in frequency following an incident on the interconnected network.

Primary regulation of frequency is ensured due to the existence of an active power reserve (primary reserve) on a part of the generating units connected to the network, which therefore operate at reduced power. Therefore, the steady state power/frequency characteristic of the generating units contributing to the regulation is illustrated in the following figure.

Figure 1.2. Steady state power/frequency characteristic of a generating unit contributing to primary frequency regulation

The primary reserve, established at the scale of the UCTE (Union for the Coordination of Transmission of Electricity) zone, must permit compensation for the reference incident, namely the sudden loss of 3,000 MW of production. In addition, in order to reduce the frequency gap in steady state, it is necessary to fulfill a minimum value for the ratio,,where ∆Pis the change in power of the ∆f generating units participating in the regulation and ∆fis the change in frequency in steady state. This ratio is the regulating energy of the system, and it must be at least 18,000 MW/Hz for the first synchronous zone of the UCTE (Western Europe) and at least 3,000 MW/Hz for the second synchronous zone (Balkan countries). As a result, it is convenient to allocate the primary reserve to a sufficient number of generating units in order to guarantee the level of regulating energy required for the system.

In order to stabilize the frequency, the entire required reserve should be able to be supplied in 15 seconds for a gap in the production of less than or equal to 1,500 MW, and in 30 seconds for a gap of 3,000 MW (reference incident).

2) Secondary frequency regulation: this is an automatic centralized regulation that adjust the production of the generation units participating in the regulation in order to bring the frequency and the exchanges of power with neighboring electric networks to their target values. In contrast to the primary frequency regulation, secondary regulation is not a local regulation and requires sending of a signal to the generating units. This signal is calculated at the control center of the transmission system operator (TSO). Secondary frequency regulation is ensured thanks to the existence of an active power reserve (secondary reserve) on the generating units participating in this regulation. Only generating units of a sufficient size (greater than 120 MW in France) can participate in this regulation.

3) Tertiary frequency regulation: this is a manual regulation that serves to:

– feed the primary and secondary power reserves and to bring the frequency back to its required value when secondary regulation has not been able to do so (following a secondary reserve deficit);

– rebalance the system when there is slow growth in the gap between supply and demand.

Depending on the country, tertiary regulation can also be used to solve congestion on the transmission network.

Tertiary frequency regulation calls for tertiary active power reserves that can be mobilized at different time scales [RTE]. In France, these reserves are mobilized by a telephone call to the production facilities from the control center of the TSO.

The tertiary reserves are often associated with the balancing mechanism organized by the TSO. This is like a permanent call for tenders where balancing responsible parties (see section 1.8) submit “adjustment” offers, i.e. production offers upward or downward with some precise characteristics. The TSO then chooses the offers that fit the needs of the system in order to face the imbalances and ensure generation-consumption balance as well as system security.

1.2.3.2. Restoration of the network

After a total or partial collapse of the network, the objective of network restoration is to restore the supply as soon as possible, first for the priority customers, and then progressively for all customers and to return the electrical network to normal operation [RTE 04].

Restoration of the network comprises a number of steps and relies on generating units. Depending on their size and their technical characteristics, energy storage systems could contribute to restoration in the same way as generating units. In France, only generating units of more than 40 MW can participate in this service.

1.3. The special case of intermittent generation

1.3.1. Contribution to frequency regulation in the absence of storage

One of the problems introduced by intermittent generation (such as photovoltaics, wind power, etc.) is linked to its limited capacity to participate in ancillary services, and especially in frequency regulation. Due to its variable character and because of the rules for buying back this type of generation, it is customary to exploit renewable generation at the maximum available power, without participating in frequency regulation.

The lack of participation of wind power in frequency regulation has not been a problem until now, while the level of market penetration was weak. Nevertheless, the increase in installed power makes it necessary hereafter to provide ancillary services, as wind power tends to substitute the production from classical generating units which were used to ensure frequency regulation.

1.3.1.1. Regulatory evolution

Wind power is not controllable, but the creation of reserves for wind farms is more economically penalizing than for traditional means of production, for which the reduction in production can lead to a drop in the consumption of fuel.

Nevertheless, regulatory evolution is on the way, as the growing importance of wind power production could compromise the security of the electricity system. This is why, in countries where the penetration level of wind energy is particularly high, from now on “grid codes” foresee the participation of wind energy in frequency regulation, even if this participation is complicated by the intermittence of the resource.

This is the case, for example, in Denmark, where the grid code includes advanced functionalities where wind farms participate in maintaining the frequency, and in Ireland [ESB 04], where the grid is weakly connected and the level of penetration is especially significant, fluctuations in wind energy production have a great impact on the frequency of the grid.

1.3.1.2. Regulation modes

It is possible to implement functionalities of primary frequency regulation at the level of the wind farm controller. According to the characteristic given in [SOR 05], at nominal frequency the wind farm operates at reduced power compared to the power that is available. Therefore, the offshore wind farm at Horns Rev is equipped with a control system that allows its turbines to operate at reduced power and to contribute to primary frequency regulation. There is a boundary on either side of the nominal frequency within which requests for regulation are prohibited during normal operation, and the active power follows a linear characteristic when there is a significant gap in frequency.

The order for a reduction in power during normal operation is generated by the control system of the wind farm, using the maximum power available at the level of each turbine as the basis. This order can be generated in different ways (constant order for reduced power, maintaining a constant gap between supplied and producible power, limiting the power gradients of the wind farm, etc.).

1.3.1.3. Limitations

Insofar as the primary energy source is not controllable, any voluntary reduction in power leads to suboptimal functioning from the point of view of the producer.

In addition, the intermittent character of wind energy production does not allow a guarantee of the possibility for increasing power, as the maximum power of each turbine is directly linked to the availability of wind energy (note, however, that the differences in wind power production in the different parts of the electricity network allows smoothing the variations in wind power production at a time scale corresponding to the one of the primary frequency regulation).

This fact leads to investigation of the possibility of using storage systems associated with wind power production for primary frequency regulation, in order to:

– economically optimize the operation of the wind farm; the turbines would be led to operate at a power that is closer to their maximum power while the power at the grid connection point would be modulated by the storage system as a function of the frequency of the network (at the level of a fraction of the nominal power of the turbine, and over short periods);

– better guarantee the availability of the reserve for power increase, thanks to stored energy.

1.3.2. Contribution of storage to power/frequency regulation

1.3.2.1. Impact of aggregation

A study for the US Department of Energy [KIR 04] evaluates the supplementary power required to contribute to frequency regulation for a wind farm. The farm considered in the study is large (138 turbines producing 103 MW in total), and so it is possible to see the impact of aggregation on the participation in primary frequency regulation.

To do this, the farm was split up into four groups of turbines and the power required for the regulating service was evaluated for each group. The results are given in Table 1.1.We see that the 4.8 MW of stand-alone regulation capacity required by the wind plant is about 65% of the 7.5 MW of regulation that would be required if the four sections were to address regulation independently. The author of the study deduced that aggregation has a positive effect on regulation.

Table 1.1. Impact of aggregation at the scale of a wind farm, from [KIR 04]

On the one hand, the impact of wind energy on frequency regulation on the scale of one large control zone is hardly noticeable. Therefore, the primary reserve of other means of production will not be affected by fluctuations in wind energy production. On the other hand, the positive effect of aggregation tends to reduce the need for storage on the scale of a large wind park (i.e. a group of wind farms). Therefore, the contribution of wind power plant to primary frequency regulation should be estimated at the scale of the electrical network, and not at the scale of a single wind farm. However, this poses the question as to what type of electricity system participants is going to install the storage systems (localized storage at the level of a farm, or centralized storage managed by a participant that has to be specified, e.g. such as a production aggregator).

In addition, in the conclusions of the study, the author indicated that the desired characteristics for the storage correspond well in this case with the performance of inertial flywheel systems (good cycling capacity, short response time, and reduced discharge time). In this way, it appears that the required storage is of the type “storage in power”, when the available power is the most important parameter.

1.3.2.2. Strategy for using storage

Traditional generating units contributing to primary frequency regulation should have a power/frequency characteristic that conforms to the characteristic reported in Figure 1.3. The power is modulated around a reference value, and if the frequency remains constant and equal to 50 Hz, the production should not change.

It is evident that this cannot be verified on the scale of a single wind farm with a very fluctuating production. In order to ensure an equivalent contribution with a wind farm, the associated storage system should fulfill two functions:

– modulation of power for frequency regulation;

– maintaining the reference value (for frequency this is 50 Hz) independently from variations in production.

These require a storage size that is probably unrealistic. In contrast, it is currently known that instantaneous wind power production can be considered to be constant on the scale of the electric network for durations in the order of a few minutes1.

Figure 1.3. Power modulation at the scale of a farm and a park

In this way, the contribution to primary regulation of a wind park (i.e. a collection of wind farms) could be obtained by leaving the power of each turbine unchanged (at the maximum available power), but modulating the power at the grid connection point of the farm via the storage. The power for the entire wind park will then follow the P/f characteristic desired, thanks to the natural scattering between the different farms.

1.3.2.3. Storage charge management

Contrary to traditional generating units, primary frequency regulation is not limited by a variation range of power between some technical minimum and the specific power capacity of the limiter. The limit of the contribution of wind power to primary regulation will be linked to the charge state of the storage.

Every type of storage presents a minimal charge beyond which it is impossible to descend (risk of degrading the equipment). In addition, in order to use the same volume of reserve during upward and downward trends, it is convenient to maintain, as far as possible, a level of intermediary charge in order to avoid the critical situations reported in Figure 1.4.

Figure 1.4. Level of storage charge. At A, the charge is maximal; it is no longer possible to curtail production, whatever the power reserve available. Conversely, at B, it is no longer possible to increase production, no matter how much power reserve is available

In fact, the frequency in normal operation tends to fluctuate around its normal value with a mean value of 50 Hz. Frequency fluctuations being of weak amplitude, and the mean value of the frequency gap being zero a priori, the state of charge of the storage should remain satisfactory and close to the reference value. Nevertheless, when there is significant perturbation of the frequency, the reconstitution of the storage charge may be compromised. It will then be necessary to determine to what extent the wind power’s reserve will be able to be reconstituted both in power and in energy.

The primary regulation cannot be effective unless the available energy allows the maximal power to be supplied, which may be required for regulation over a duration of at least 15 minutes.

1.3.3. Other possible ancillary services for power storage

1.3.3.1. Secondary and tertiary frequency regulation

Up to this point, only primary regulation has been considered. Nevertheless, the contribution to secondary and tertiary regulation seems currently to be beyond the scope of wind power production.

On the one hand, the wind power itself has an impact on the reserve volumes required, and the intermittence of production can no longer be neglected. Therefore, the power of the storage devices used must cover not only the power required by the regulation, but also some of the variation of the wind power.

On the other hand, this reserve should be free to be used over longer durations and therefore, it needs higher energy. Therefore, secondary and tertiary regulations bring up applications, such as load deferral, that are inaccessible to power storage devices which would be dedicated to primary regulation.

1.3.3.2. Voltage regulation

Storage could favor voltage regulation via modulation of reactive power at the connection point. Admittedly, wind turbines more and more often present voltage regulation functionalities, but this practice would enable contribution to the voltage regulation even in absence of production. In addition, it would be necessary to determine how much such a system would permit the range of regulation of reactive power at the connection point to be extended. The possibilities for voltage regulation are detailed later.

1.3.3.3. Other applications

The use of a storage system of reduced capacity equally permits the anticipation of applications to do with power quality. Rapid fluctuations of production can induce transitory effects on the distribution voltage. Also, it would be pertinent to evaluate the capability of storage systems for smoothing these transitory effects, and to see to what extent such a function would interfere with primary regulation of frequency.

Another application could be the contribution to the restoration of the network after a blackout. One of the questions posed by the recovery from an incident is the capacity of the power generators to maintain voltage and frequency in the restored zone. Wind power generation occupies a growing place in the generating facilities, and so the regulation capacities could be a decisive point to regulate the voltage from the distribution side.

1.4. Energy storage for transmission systems

Depending on the mode of operation, storage systems can behave either as generating units or as loads (consumption). Therefore, in principle, they can ensure the same services on transmission networks. The majority of applications for transmission networks have already been considered in the two previous sections. These will not be detailed again here, and whenever needed we will refer to those sections.

1.4.1. Control of investments and congestion management

Depending on their size and their technical characteristics, energy storage systems could be used when charging or discharging, in order to control the flow of power on transmission lines. They could contribute in this way to maintaining the flows at values below the maximum acceptable.

This service can be used by the control center of the transmission system to solve congestion on the transmission network (as is already done using generating units) and to postpone certain investments. Moreover, the energy storage system can appear as a solution in cases where difficulties in the development of network infrastructures arise (for example, strong local opposition).

It takes cycles of a few hours duration to smooth over peaks of power in the transmission lines. If the duration of use of the storage system is much longer, reinforcement of the network will be inevitable. Regarding congestion management, depending on the country, the control center of the transmission system can call upon the tertiary reserve or upon the balancing mechanism (section 1.2.3.1).

1.4.2. Frequency regulation and the balancing mechanism

Just as for generating units, storage systems can participate in frequency regulation. This application has already been the subject of section 1.2.3.1 where participation in primary, secondary, and tertiary frequency regulation was presented.

We will not copy the description here. We recall only that participation in frequency regulation requires having access to reserve volumes upward as well as downward, i.e. the reserves should be used to inject power into the network as much as to store power from the network. Therefore, particular technical characteristics and specific operation modes might be required.

Moreover, storage systems can also participate in balancing mechanisms. These were also briefly described in section 1.2.3.1 and will be discussed again in section 1.8, which discusses energy storage for balance responsible parties who submit offers on the balancing mechanism (or on the balancing market – see section 1.8). Generally, a minimum volume is required in order to have access to the balancing mechanism. In France, the minimum volume of an offer is 10 MW [RTE 09].

1.4.3. Voltage regulation and power quality

Voltage regulation is not the primary function of a storage system. Other more efficient and less costly specialized systems exist for these kinds of applications. Nevertheless, when a storage system is present on the transmission network (to fulfil another function), voltage regulation can be promoted using the alternator or the power electronics interface with which it may be equipped. However, resizing may be necessary.

Following the example of frequency, three types of voltage regulation exist: primary, secondary, and tertiary:

– Primary voltage regulation is an automatic local regulation that maintains the voltage at a given point of the network at a regulated value. In order to carry out this task, the generating units are equipped with automatic voltage “regulators”. Other types of equipment on the transmission network can also carry out this function, for example static reactive power compensators, STATCOMs, etc. In France, every generating unit linked to the transmission network should be equipped with a primary voltage regulation system.

– Secondary voltage regulation is an automatic centralized regulation that coordinates the actions of the voltage regulators of the generating units that contribute to the secondary regulation in a such a way to control the voltage schemes of predefined zones. In France, only generating units linked to voltage levels from 225 kV to 400 kV are required to contribute to secondary voltage regulation.

– Tertiary voltage regulation is a manual regulation undertaken by network operators in order to coordinate the voltage scheme between different secondary regulation zones.

In the same way, even if it is not its primary function, a storage system can contribute to improving the power quality on a network, insofar as it has the technical capacity (for example, using its power electronics interface, if it is equipped with such a device).

1.4.4. System security and network restoration

Beyond their participation in regulating frequency and voltage, energy storage systems can also contribute to the security of the electricity system when they are in charging mode, and in particular:

– to load shedding: in case of “frequency collapse”, when ordinary regulation actions do not enable control of the downward trend, the TSO cuts off the loads when the frequency reaches certain thresholds. In France, four power cut thresholds are fixed: 49 Hz, 48.5 Hz, 48 Hz, and 47.5 Hz. A curtailment level (the volume of load to be cut off) is associated with each threshold. Storage systems that are charging can be cut off, and this is exactly what happens with STEP operating on the network;

– to maintain voltage stability: in the case of voltage collapse, some action on the load is again a possibility that can be used by the TSO. The storage could contribute to this action.

Finally, as was already mentioned in section 1.2.3.2, after a total or partial blackout of the network, depending on their size and their technical characteristics, energy storage systems could contribute to the restoration of the network in the same way as generating units.

1.4.5. Other possible applications

Islanded networks: in some circumstances, parts of the transmission system can operate as islanded networks. For example, in the case of a blackout or a long-duration power cut following problems on the network, functioning as a separate (or islanded) network may be authorized while waiting for complete re-establishment of the network.

Islanded networks are generally less stable and are susceptible to frequency and voltage fluctuations that are more significant than for the normally operating interconnected network. However, the TSO should ensure the supply-demand balance in real-time; in this framework, because of the flexibility offered by the existing storage systems, they can contribute to maintaining this balance and the stability of the separate network.

1.5. Energy storage for distribution networks

Traditional use of storage in distribution networks consists of providing emergency power to certain infrastructures in the network. A good example is batteries in a substation for control/command systems and electrical protection. In the following sections we describe other more innovative services that may be facilitated by storage units installed through the distribution network.

1.5.1. Storage advantages in planning phase

1.5.1.1. Storage for load smoothing

In order to manage investments by the network operator, load smoothing is one of the services considered for storage and even, on a wider scale, for distributed energy resources (including distributed generation and load management).