The Potential Roles of Energy Storage in Power Systems

Since this is CIGRE’s first dedicated effort on the use and application of energy storage in power system development and economics, it lays the foundation for future Working Groups to develop upon the recommendations of this TB.

Successfully decarbonising the electric power system requires increased flexibility.  Most grid operators agree that storage technologies will play an important role within their networks in the coming years, as they have the potential to support and facilitate the integration of non-dispatchable renewable energy sources into the grid while also resolving other grid constraints and bottlenecks.  Some storage roles are already clear, while others lie in the future. 

This Technical Brochure reports on energy storage technologies that do not emit CO2 when used.  The WG organised the TB into six chapters, as outlined below. 

Energy storage

Chapter One introduces the broad concept of energy storage and provides a brief overview of its role in history.  Storage, both as a concept and a practice, is ancient.  Humanity has used it for millennia, whether by storing grain during good seasons to stabilise the food supply during inevitable poor harvests or by saving water in wet seasons for crops and drinking water during dry periods.  Electric energy storage, however, presents a challenge, mainly because electricity is a phenomenon that is produced and consumed instantaneously, rather than being a tangible commodity.  So far, humankind has not found an effective way to store large amounts of electricity directly; instead, it must be stored indirectly. 

Ideally, energy storage in electric power systems should be affordable, compact (with high energy density), have charge/discharge cycle efficiency close to 100%, be easily scalable, move seamlessly between locations, charge and discharge rapidly (within milliseconds), last for tens of thousands of charge/discharge cycles over its lifetime (durability), and have a practical lifetime of at least 40 years (comparable to today’s generating plants).

Currently, no storage solution can fulfil these requirements.

Storage technologies

Chapter Two outlines the types of energy storage technologies that do not emit CO2.  The types are grouped into five main categories.

1. Electric: Supercapacitors and inductors are the most obvious methods for storing electrical energy.  They are ideal for applications that require rapid current changes.  However, they are unsuitable for delivering energy beyond very short bursts.
2. Electrochemical: The most common electrochemical technology is the battery.  While all batteries operate on the same fundamental principles, they vary significantly in size, energy density, and other characteristics determined by the materials used.  Flow batteries could shape the future of utility applications, especially for long-term storage. 
3. Mechanical:  Ranges from the familiar, such as pumped hydro, to the unexpected, such as compressed air energy storage and gravity energy storage.  
4. Thermal energy:  Stores heat or cold for future use, either directly in dedicated devices or integrated with the thermal inertia of certain manufacturing processes.  
5. Chemical:  Primarily hydrogen (H2) and its derivatives can be used in the same ways as natural gas and oil are used today.  The advantage of H2 is that the main by-product of burning it is water. 

The TB discusses various universal storage comparison factors, including initial costs, net operating costs, decommissioning, cycle efficiency, physical size, electrical capacity (MW), usable energy capacity (MWh), capability degradation over time, durability, scalability, mobility, life expectancy, siting and approval issues, and other technical specifications.

Storage applications

Chapter Three discusses the applications of energy storage in three main categories:

1. Overall, system energy storage applications that support or provide the most basic functions of the power system.  These applications supply the entire customer load and firm generation by providing backup, long-duration storage, and more.  As electric systems transition to higher levels of intermittent renewable resources like wind and solar, energy storage will play an increasingly important role in overall system support.  Storage can balance the overall system power supply and provide non-location-dependent support, such as buffering supply and demand, generation firming, supporting system stability, supplying short-circuit current, and facilitating black start. 

    

2. Balancing generation with load and frequency control applications involves providing basic frequency control, spinning and operating reserves, ramp-rate control, and managing the rate of change of frequency.

• Many energy storage technologies are ideal providers of ancillary services in power grids, such as frequency regulation. 
• Storage types capable of responding swiftly are well-suited to provide spinning reserves—a backup power supply that remains online but is not loaded, ready to respond quickly to a supply shortfall. 
• Because most inverter-based energy storage systems can respond within milliseconds, they may reduce the required reserve capacity to maintain the same level of grid reliability, leading to a more efficient overall system.
• Inertial response functions to counteract any immediate imbalance between power supply and demand in electric power systems.  Storage with rotating synchronous generators—mainly mechanical storage—can naturally provide an inertial response.  Electromechanical, thermal, and chemical sources are capable of delivering synthetic inertial response and generally respond more quickly than conventional resources.  
• Energy storage providing synthetic inertia can also help mitigate high rates of change of frequency (RoCoF), allowing the governors of conventional generators and other slower-acting frequency control mechanisms time to respond to arrest, stabilise, and restore system frequency. 
• Generator ramping is becoming a growing concern for system operators as they integrate more renewable energy.  All the storage types discussed in this TB can provide ramp-rate services to some extent.

3. Location-based applications support the system’s operation by improving the economic efficiency or effectiveness through the specific location of energy storage.

• The location of energy storage can reduce transmission loading and increase transmission capacity, but it may also require additional transmission capacity for charging or delivering energy.  Energy storage can also enhance transmission capacity by alleviating the effects of local contingencies.  
• Storage located close to the load can supply power and ancillary services that are more difficult to deliver from a distant location.  Industrial and commercial energy users can use energy storage systems to reduce peak electricity demand and improve power quality.

The chapter also discusses the role and importance of inverters.

Implementation issues and problems

hapter Four describes implementation issues and problems, presents issues and challenges introduced by energy storage, and discusses energy storage effects:

1. Energy storage analysis presents challenges where energy storage solves problems but complicates analysis by introducing new variables and uncertainties.  The process requires tools to assess the many possible technical configurations.  Analyses must also consider how market designs influence the operation of energy storage assets.

    

2. Inverter dynamic capabilities introduce greater complexity and maintenance requirements compared to conventional resources.  CIGRE study committees are addressing these issues from multiple perspectives.

    

3. Planning for storage with limited capacity and energy that includes all forms of energy storage.  Planning and operating such energy storage requires attention to charging and discharging times and cycle efficiency, including insipient losses that occur during storage and the energy consumed by auxiliaries to maintain storage conditions, such as temperature.  System operators must include these factors in planning and operating the system.  Complicating matters further, energy storage may be used in multiple ways, simultaneously offering several types of services. 

    

4. Providing sufficient transmission capability for large storage installations. This can be especially challenging when system operators lack visibility of the state of charge, as well as the forecast of charge or discharge flows. 

    

5. Specifying and verifying inverter specifications—CIGRE study committees are addressing these issues on multiple fronts.

    

6. Management and control of customer energy storage, including residential and commercial storage, electric vehicles (EVs), and behind-the-meter applications.  EVs commonly seen today can function as a form of demand response (DR).  Some newer EVs enable bidirectional energy transfer, known as vehicle-to-grid (V2G).  Planning for and managing both EVs and behind-the-meter devices will pose challenges for system planners and system and distribution operators.

    

7. Investment decisions regarding storage devices are complicated by the trade-offs between generation, transmission, and storage.

• Generation vs transmission vs storage
  • Energy storage can provide various grid-related services that can be offered through storage as a grid, generation, or dual-use asset. 
  • In some settings, a single entity owns, plans, and manages generation, transmission, and storage units within a vertically integrated utility that can optimise the operation of generation, transmission, and storage together to reduce system-wide costs. 
  • In other settings, energy storage can be planned, operated, and/or owned by independent private companies whose interests are to optimise revenues from the energy storage operation.  
• Lifespan and decommissioning (retirement) 
  • Expected equipment life is an obvious factor in investment decisions. 
  • These factors impact all types of energy storage, including sediment buildup in hydroelectric plants, the disposal of typical electrical equipment with various contaminants, and restoring sites to their original conditions. 
  • Decommissioning certain battery types can be particularly challenging. 

8. Integrating energy storage into power markets 

• In market environments, energy storage will only be developed and participate if the right market services exist, allowing developers to profit from building and operating the storage.  The most profitable approach for a developer may not align with the optimal needs of the system operator. 
• Comparing generation, transmission, and storage in a market environment presents a new challenge for planners, as they must identify a mix of solutions that ensure adequate service across various time scales and risk levels.  Evaluating the different combinations of these functions and assumptions may be daunting.
• In many cases, market rules and connection codes were developed based on the generation technologies and load characteristics of the time.  However, applying some of these can disadvantage energy storage, and are not always aligned with global or local system constraints.  Grid codes must be flexible and adaptable to the evolving market environment.

9. Storage operating challenges for the grid

• Grid operation is complex, as it must respect overall system needs, balance generation and consumption, and maintain grid security.  While energy storage introduces technical flexibility, it also introduces new levels of complexity in a market environment.
• The dynamic capabilities of inverter-based storage are a significant advantage, enabling rapid responses to changing frequency.  However, this can also pose a serious drawback when the speed of power changes is incompatible with the system's command, protection, and control systems.
• Capacity (MW) and energy (MWh) limits need to be managed in all types of storage. 
• Under certain conditions, inverters can suddenly switch between their maximum and minimum output, which may cause voltage, frequency, and stability problems.

Utility experience

Chapter Five describes utility experiences with and applications of energy storage.  The examples are grouped into five major sections: 1. Battery energy storage, including bulk storage, transient support, black-start support, grid congestion management, and V2G; 2. Flywheel energy storage, 3. Gravity energy storage, 4. Pumped-hydro energy storage, 5. Compressed air energy storage, and 6. Thermal energy storage.

Conclusions and recommendations

Chapter Six presents the conclusions and recommendations regarding energy storage, including overall observations regarding energy storage in electric...