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3 Grid Terms to Know: Reliability, Resiliency and Flexibility

3 Grid Terms to Know: Reliability, Resiliency and Flexibility

| Aung Thant, Power Systems Engineering Architect

Grid operators seek to achieve a grid that is reliable, resilient and flexible. Industry news outlets often refer to these terms when discussing management strategy, policy, energy storage technologies and grid network failures. Often we see the words reliability and resiliency used interchangeably. Flexibility is a new trending term. What are their nuanced differences in meaning?

In this post, I will explore the terms and provide examples for how ultracapacitor (supercapacitor) storage systems support each one.

Grid Reliability

A reliable grid provides consistent, quality power flow

Everyone wants the lights to stay on. This is the essence of a reliable grid—power continues to flow, and brownouts/blackouts are avoided.

Ultracapacitor storage contributes to reliability by:

  • Providing peak power when the grid needs additional support to prevent a cascading grid outage

Example scenario:
Peaker generators at different locations provide additional power to the grid when needed. When this doesn’t happen quickly enough or peaker capacity is not sufficient, the risk of system collapse increases.

Ultracapacitor (supercapacitor) storage contributes reliability to this scenario by providing the short-term power until the rest of the system can readjust. Essentially, the ultracapacitor system buys highly valuable time for the greater system to be able to reconfigure itself in the event of sudden change in the system. 

Grid Resiliency

A resilient grid is able to easily "bounce back" from a disruptive event

Resiliency is a term that refers to the grid’s ability to recover from disruptive events, such as a lightning strike or unexpected increase in load.

In the US, the grid frequency must be maintained at 60 hertz. In Europe, at 50 hertz. Any small deviation from these baselines means trouble for the grid. The more resilient the grid, the less it deviates from the baseline when a storm, a fallen tree, or a software bug hits the grid.

Ultracapacitor storage contributes to resiliency by:

  • Assisting black start generators with load pick-up
  • Helping the grid to recover faster from unpredictable events

Example scenario:
A hydro power plant provides reliable energy generation, but it is slow to start up and slow to respond to increases or decreases in grid power demands. Hydro power cannot provide a black start (a restoration of power after a total or partial power loss).

If the hydro plant is paired with an ultracapacitor storage system, the ultracapacitor system provides instant power to cover the load change while the hydro plant ramps up or down. Ultracapacitors help the grid recover faster.

The grid does have ways to black start, but it doesn’t include hydro. Current methods include gas turbines and diesel generators placed at certain locations, which could reenergize the grid from a black start. Grid resiliency improves with an increased number of black start units, and ultracapacitor storage is a viable option to achieve faster black start recovery.

A resilient grid bounces back from disruptive events such as lightning strikes or sudden increases in load.

Grid Flexibility

A flexible grid adapts to variable generation and load to maintain balance

A flexible grid is one that can handle renewable energy integration and maintain stability between generation and load demands despite variability such as renewables. Uncertainty is usually associated with renewable energy because solar or wind energy can become unavailable.

Ultracapacitor storage contributes to flexibility by:

  • Providing system designers with flexibility to increase power of the greater system without having to oversize batteries
  • "Firming" solar and wind energy
  • Complementing  pumped hydroelectric storage

Example scenarios:
I’ll use an analogy for this first scenario: Let’s say if when you buy insurance, the deal includes every kind of insurance—life, health, car and homeowner’s insurance. That’s a really inflexible package. What if you don’t own a car or a house? What you want is to buy the types of insurance you need.

Oftentimes, system designers have to "buy" more energy (batteries) just to gain more power in an energy storage system. This results in an inflexible, less efficient system because the additional batteries take up more real estate and add needless energy. Ultracapacitors increase system flexibility by providing short-term, high power without having to oversize the battery system.

As for the uncertainty of solar and wind, ultracapacitor storage helps make renewable energy and power output smoother and more predictable. Ultracapacitors provide near real-time solar and wind firming and rapid response to the changing power flow, helping to increase the general stability of the grid. By mitigating the inherent variations of renewable energy, ultracapacitors add predictability to delivered power quality.

  Renewable energy generation is unpredictable. A flexible grid maintains stability despite the variability. 


The way the ultracapacitor storage mechanism works is the same for reliability, resiliency and flexibility (it provides short-term, high power), but how it impacts the system is different. Ultracapacitor storage helps prevent the grid from having a blackout (reliability). If a blackout occurs, ultracapacitors can help restart the grid faster (resiliency). Ultracapacitors can be considered a surgical tool for the system designer’s toolbox so that they can add more power to the system without having to take on nonessential energy and space (flexibility).

Maxwell’s ultracapacitor storage technology contributes to greater reliability, resiliency and flexibility for the grid and microgrid. Explore our system solutions or email to connect with our expert grid storage team.

Also by this author: How Australia Came Close to Wide-Scale Blackout – And Insights Into Energy Storage for Grid Resiliency

AungAung Thant
Power Systems Engineering Architect
About this author

Aung Thant is a seasoned power systems engineer currently performing advanced architecture and designs, modeling and simulations of energy storage utilizing Maxwell’s ultracapacitor grid modules and commercially available advanced batteries. He works with utility, developers and C&I customers to identify the best strategic approach for their energy storage requirements, which may include a stand-alone ultracapacitor ESS or a hybrid ultracapacitor-battery ESS system. Thant received his education from New York University – Polytechnic/Tandon School of Engineering with BSEE and MSEE degrees in electrical engineering.

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