16.4 Energy storage systems for stability support

Energy storage systems are game-changers for power grid stability. They act like shock absorbers, smoothing out the bumps caused by renewable energy's ups and downs. From batteries to flywheels, these technologies help keep the lights on when the wind stops or clouds roll in.

Choosing the right storage system is crucial. It's all about matching the tech to the task. Need quick responses? Batteries or flywheels are your go-to. Looking for long-term storage? Pumped hydro might be the answer. It's a balancing act of power, speed, and duration.

Energy storage systems for stability

Classification and characteristics

• Energy storage systems can be classified into various categories based on their underlying technology and operating principles:
  • Mechanical storage systems (pumped hydro, compressed air, flywheels)
  • Electrochemical storage systems (batteries, flow batteries)
  • Electrical storage systems (supercapacitors, superconducting magnetic energy storage)
  • Thermal storage systems
• Key characteristics of energy storage systems that determine their suitability for stability support applications include:
  • Power rating: The maximum amount of power that the storage system can deliver or absorb
  • Energy capacity: The total amount of energy that can be stored in the system
  • Response time: The speed at which the storage system can respond to changes in power demand or supply
  • Cycle life: The number of charge-discharge cycles the storage system can undergo before its performance degrades
  • Round-trip efficiency: The ratio of energy output to energy input during a complete charge-discharge cycle
• Pumped hydro and compressed air energy storage systems offer large energy capacities (hundreds to thousands of MWh) but have slower response times (minutes to hours), making them suitable for long-duration energy shifting and peak shaving applications
• Flywheels and supercapacitors provide high power ratings (MW-scale) and fast response times (milliseconds to seconds) but have limited energy capacities (kWh to MWh), making them ideal for providing short-duration power quality and frequency regulation services
• Battery energy storage systems, such as lithium-ion and flow batteries, offer a balance between power rating (MW-scale) and energy capacity (MWh to GWh), with response times in the range of milliseconds to seconds, making them suitable for a wide range of stability support applications

Selection criteria for stability support

• The choice of energy storage technology for stability support depends on several factors related to the specific requirements of the power system:
  • Duration of support needed: The length of time for which the storage system needs to provide stability support (seconds, minutes, hours)
  • Speed of response: The required response time of the storage system to effectively mitigate stability issues (milliseconds, seconds)
  • Frequency of cycling: The expected number of charge-discharge cycles the storage system will undergo during its lifetime
  • Environmental and safety considerations: The impact of the storage technology on the environment and the safety risks associated with its operation
• For applications requiring long-duration support (hours), such as energy shifting or peak shaving, storage technologies with high energy capacities like pumped hydro or compressed air are preferred
• For applications demanding fast response times (milliseconds to seconds), such as frequency regulation or voltage support, storage technologies with high power ratings and fast response times like flywheels, supercapacitors, or batteries are more suitable
• The frequency of cycling is an important consideration for storage technologies with limited cycle life, such as some battery chemistries, to ensure their economic viability over the project lifetime
• Environmental and safety aspects, such as the use of toxic materials or the risk of fire or explosion, must be carefully evaluated when selecting a storage technology for a specific application

Energy storage for renewables

Mitigating variability and intermittency

• Renewable energy sources, particularly wind and solar, are characterized by their inherent variability and intermittency:
  • Wind power output varies with wind speed and direction, leading to fluctuations in power generation on time scales ranging from seconds to hours
  • Solar power output depends on the available solar irradiance, which can be affected by factors such as cloud cover, atmospheric conditions, and diurnal and seasonal variations
• The variability and intermittency of renewables pose challenges for maintaining power system stability:
  • Fluctuations in renewable generation can cause imbalances between power supply and demand, leading to frequency deviations
  • Rapid changes in renewable output can lead to voltage fluctuations and impact power quality
  • The unpredictability of renewable generation makes it difficult to schedule and dispatch conventional generation units effectively
• Energy storage systems can help mitigate the variability and intermittency of renewables by acting as a buffer between renewable generation and the grid:
  • During periods of high renewable generation, excess energy can be stored in the storage system, preventing curtailment and improving the utilization of clean energy resources
  • During periods of low renewable generation or high demand, the stored energy can be released to supplement the power supply and maintain the balance between generation and load
  • By providing a more consistent and controllable power output, energy storage helps to smooth out the fluctuations in renewable generation and reduce their impact on grid stability

Benefits of energy storage integration with renewables

• Integrating energy storage systems with renewable generation offers several benefits for power system operation and stability:
  • Improved power quality: Energy storage can help to regulate the voltage and frequency of the power generated by renewables, ensuring a more stable and consistent power supply to the grid
  • Reduced cycling of conventional generators: By absorbing excess renewable energy during periods of high generation, storage systems can reduce the need for conventional generators to operate at part-load or to ramp up and down frequently, improving their efficiency and reducing wear and tear
  • Increased renewable energy penetration: Energy storage enables a higher penetration of renewable energy sources in the power system by mitigating their variability and providing a more dispatchable and reliable power output
  • Reduced greenhouse gas emissions: By facilitating the integration of renewables and displacing fossil fuel-based generation, energy storage contributes to the reduction of greenhouse gas emissions and supports the transition to a cleaner energy mix
• Energy storage can also provide additional benefits beyond stability support, such as:
  • Energy arbitrage: Storing energy during periods of low electricity prices and discharging during periods of high prices to generate revenue
  • Transmission and distribution deferral: Reducing the need for investments in new transmission and distribution infrastructure by providing localized power support
  • Ancillary services: Participating in markets for frequency regulation, voltage support, and other ancillary services to generate additional revenue streams

Energy storage applications

Frequency regulation

• Frequency regulation is a critical ancillary service that maintains the balance between power generation and demand in real-time to keep the grid frequency within acceptable limits (typically ±0.5 Hz)
• Energy storage systems can provide fast-responding frequency regulation services by rapidly absorbing or injecting power in response to frequency deviations:
  • When the grid frequency drops below the nominal value due to a sudden increase in load or loss of generation, the storage system can quickly discharge power to compensate for the power deficit and restore the frequency
  • When the grid frequency rises above the nominal value due to a sudden decrease in load or excess generation, the storage system can rapidly absorb power to reduce the power surplus and bring the frequency back to the desired level
• Battery energy storage systems, flywheels, and supercapacitors are particularly well-suited for frequency regulation applications due to their fast response times (milliseconds to seconds) and ability to provide both upward and downward regulation
• Compared to conventional generators, energy storage systems can provide frequency regulation services with higher accuracy, faster response times, and lower environmental impact
• Energy storage-based frequency regulation can also help to reduce the wear and tear on conventional generators by reducing the need for them to constantly adjust their output to follow load variations

Voltage support

• Voltage support refers to the maintenance of voltage levels within prescribed limits across the power system to ensure stable and reliable operation
• Voltage instability can occur due to various factors, such as changes in load, variations in renewable generation, or contingencies like line or generator outages
• Energy storage systems can provide voltage support by injecting or absorbing reactive power as needed:
  • When the voltage level drops below the desired range, the storage system can inject reactive power to boost the voltage and maintain it within acceptable limits
  • When the voltage level rises above the desired range, the storage system can absorb reactive power to lower the voltage and keep it within the prescribed boundaries
• Battery energy storage systems equipped with power electronic converters can provide dynamic voltage support by rapidly adjusting their reactive power output in response to voltage fluctuations
• The fast response times and controllability of energy storage systems make them effective in providing voltage support during disturbances or fluctuations in renewable generation
• Placing energy storage systems at strategic locations, such as near renewable generation sites or at weak points in the network (areas with low short-circuit capacity or high voltage sensitivity), can enhance their effectiveness in providing localized voltage support

Sizing and placement of energy storage

Optimal sizing considerations

• The sizing of energy storage systems involves determining the appropriate power rating (in MW) and energy capacity (in MWh) to meet the specific stability requirements of the power system
• Factors to consider when sizing energy storage systems for stability support include:
  • Expected magnitude and duration of power imbalances: The storage system should have sufficient power rating and energy capacity to cover the anticipated range and duration of power imbalances in the system
  • Desired level of stability improvement: The sizing should be based on the targeted improvement in stability metrics, such as frequency nadir, rate of change of frequency, or voltage deviation
  • Technical constraints of the storage technology: The sizing must account for the limitations of the chosen storage technology, such as maximum power rating, depth of discharge, or cycle life
  • Economic considerations: The sizing should strike a balance between the cost of the storage system and the benefits it provides in terms of stability improvement and other revenue streams
• Optimization techniques can be used to determine the optimal sizing of energy storage systems considering multiple objectives and constraints:
  • Mathematical programming methods, such as linear programming or mixed-integer programming, can be employed to formulate and solve the sizing problem as an optimization model
  • Heuristic methods, such as genetic algorithms or particle swarm optimization, can be used to search for near-optimal sizing solutions in large and complex solution spaces
• The sizing optimization problem can be formulated to minimize the total cost of the storage system while satisfying the stability requirements and other technical and operational constraints
• Sensitivity analysis can be performed to assess the impact of different sizing parameters on the performance and cost-effectiveness of the storage system

Optimal placement strategies

• The placement of energy storage systems in the power network is crucial for maximizing their impact on stability enhancement
• Optimal placement strategies aim to locate energy storage systems at nodes or buses where they can most effectively mitigate stability issues and improve overall system performance
• Factors to consider when determining the optimal placement of energy storage systems include:
  • Network topology: The electrical connectivity and configuration of the power system, including the location of generators, loads, and transmission lines
  • Power flow patterns: The distribution of power flows across the network under different operating conditions and contingencies
  • Stability indices: Metrics that quantify the stability margin or vulnerability of different nodes or regions in the network, such as voltage stability indices or frequency response indices
  • Renewable generation penetration: The location and capacity of renewable energy sources in the network, as they can introduce localized stability challenges
• Optimization algorithms can be employed to solve the complex problem of optimal energy storage placement considering multiple objectives and constraints:
  • Genetic algorithms: These evolutionary algorithms can search for optimal placement solutions by evolving a population of candidate solutions over multiple generations based on fitness evaluation and selection
  • Particle swarm optimization: This swarm intelligence technique can explore the solution space by adjusting the position and velocity of a swarm of particles representing potential placement solutions
  • Multi-objective optimization: The placement problem can be formulated as a multi-objective optimization problem to simultaneously optimize multiple conflicting objectives, such as stability improvement, cost minimization, and environmental impact
• The optimization process typically involves simulating the power system under various scenarios and evaluating the impact of different storage placement options on the selected stability indices and performance metrics
• The optimal placement solution should provide the maximum stability enhancement benefits while considering practical constraints such as land availability, grid interconnection capacity, and environmental regulations