The growing need for flexibility in electrical systems

European electrical systems—and, more broadly, electrical systems around the world—will need a greater level of flexibility in the coming years, both on the supply and demand sides. This need is on the horizon. Flexibility already provides a significant source of additional income for consumers and producers, and its economic and operational importance continues to grow.

To understand this topic, it is necessary to review a few fundamental principles relating to the physical operation of electrical grids.

Stability of electrical grids: frequency and voltage

Flexibility is directly related to the stability of electrical grids. This stability depends on the quality of their regulation, i.e., all the mechanisms that enable the grid to quickly return to a state of equilibrium or near equilibrium when it is disrupted.

These regulatory mechanisms aim to keep two essential variables close to their setpoint values: frequency and voltage.

• Frequency is a global variable: it is identical at every point in an interconnected grid.
• Voltage is a local variable, which may differ depending on the area of the grid.

Frequency is regulated by adjusting the active power on a global scale, while voltage is regulated by modulating the reactive power locally. The rest of this text focuses on frequency stability.

On the European grid, the setpoint frequency is 50 Hz. When it deviates too far from this, automated protection systems isolate equipment to prevent damage. This can create a domino effect leading to massive outages or even a blackout.

A simple principle sums up the dynamics:

• If production is instantly lower than consumption, the frequency drops.
• If production exceeds consumption, the frequency increases.

These imbalances must be corrected very quickly to prevent the grid from collapsing.

Primary, secondary, and tertiary reserves

To maintain frequency, grid operators use three levels of control, each associated with a corresponding reserve:

1. Primary reserve

The first line of defense, it activates automatically in less than 30 seconds to stop frequency drift. It adjusts production or consumption to temporarily stabilize the system.

2. Secondary reserve

This takes over from the primary reserve. It is also activated automatically, within 30 seconds to 15 minutes, to bring the frequency back to its setpoint.

3. Tertiary reserve

Activated as a last resort, it supplements the secondary reserve or replaces it if necessary. It takes 15 to 30 minutes to activate. In France, the tertiary reserve is managed via the adjustment mechanism.

The value of flexibility for the electricity system

The “value” of flexibility can be approximated by the cost of failure, also known as the cost of undistributed energy. In France, it is set by regulation at €33,000/MWh. This amount illustrates the huge gap between:

• the marginal cost of a peak power plant (€100 to €150/MWh),
• the economic loss to the community in the event of a power cut.

Thus:

• One hour without electricity in France represents €1.6 billion in losses.
• 24 hours represent €40 billion.
• 70 days would be enough to wipe out the equivalent of the country’s annual GDP.

These figures show how essential flexibility is.

Three historic blackouts illustrating the importance of flexibility

1. France – December 1978

A trip on a 400 kV line in the east of the country caused a domino effect. A significant part of the grid collapsed, leading to a widespread blackout except in border areas. The outage lasted about two to three hours, with an estimated loss of 100 GWh, or 13% of daily consumption.

2. Europe – November 4, 2006

A German line was de-energized to allow a ship to pass, overloading neighboring lines. The European grid split into three zones. Approximately 15 million people were without electricity for more than two hours. The incident had a lasting impact on the way networks are supervised today.

3. Spain and Portugal – April 28, 2025

A major outage plunged almost the entire territory of both countries into darkness. The interconnection with France automatically disconnected, preserving the French network. The investigation points to a lack of flexibility in the Spanish system, particularly conventional power plants that did not provide the expected reserve.

These accidents show that current systems are based on robust mechanisms but are facing growing demands.

A growing need for flexibility

This need is increasing for two main reasons:

1. Consumption is increasing, albeit slightly, compared to levels observed 30 or 40 years ago.
2. The massive development of intermittent renewable energies (wind, solar) reduces the resilience of the system due to their variability.

Renewable energies contribute to decarbonization, but they require greater flexibility to ensure the balance between production and consumption.

Different time horizons for flexibility

1. Intraday flexibility

This responds to variations in consumption between day and night, as well as to photovoltaic production. The resources that can be mobilized include:

• peak power plants,
• short-term storage (batteries),
• load modulation programs (residential, tertiary, industry).

2. Intra-weekly flexibility

Consumption differs between weekdays and weekends. Typical resources are:

• gravity-fed hydroelectric power,
• industrial modulation.

3. Inter-seasonal flexibility

Consumption is traditionally higher in winter. The technologies used are:

• gravity-fed hydroelectric power,
• seasonal pumping,
• semi-base production resources (combined cycle gas).

Nuclear power could, in the long term, provide greater modulation, but this is not yet an optimized option.

Flexibility on the production side

The main sources are:

• Gas-fired power plants, capable of rapid load tracking.
• Gravity-fed hydroelectricity, highly responsive but limited by the volume of water available.
• Renewable energies, which can reduce their production or be coupled with batteries to smooth out their intermittency.

Flexibility on the demand side

Demand offers significant potential that is still under-exploited:

Industry

• thermal inertia,
• capacity for load shedding or deferral,
• multi-energy arbitrage.

Tertiary

• cooling units,
• heat pumps,
• modulation systems already in operation in large retail outlets.

Residential

• connected thermostats,
• heat pumps,
• electric vehicles,
• old “peak day” load shedding systems.

The role of storage

Storage is at the heart of flexibility. There are several technological horizons:

Short term

• batteries,
• flywheels.

Medium term

• redox flow batteries,
• thermal storage.

Long term

• gravity hydropower,
• seasonal pumping.

France has significant assets (hydropower), but is lagging behind in battery development.