Could the Great Blackout Reoccur? The Keys to a Renewable Energy-Based System with a Critical Role for Nuclear Energy

The historic total blackout suffered by the peninsula served, in a way, to remind us how robust the national system is: outages like yesterday's are very rare. The electricity mix, largely supported by renewable sources, is capable of meeting demand. However, something went wrong.

What is the composition of the Spanish system?

In terms of generation, the system is composed of a multitude of sources. In 2024, the most important were wind power (producing 22.9% of the total), nuclear power (19.6%), and solar photovoltaic (16.7%), which last year surpassed gas-fired combined-cycle plants (13%) for the first time. Hydropower has also historically had a significant impact (13% in 2024), although it is affected by droughts.

In terms of installed capacity, solar photovoltaic is already the leading source, with 33.6 GW, ahead of wind (32.3 GW) and combined cycle (26.2 GW). However, although greater capacity means greater production capacity, the availability of a technology itself does not imply that it produces more, only that it has the capacity to do so. Nuclear, for example, only has 7.1 GW (approximately 1 GW per reactor), but the fact that the plants operate practically year-round boosts their contribution to the mix.

What is the role of Red Eléctrica?

As a system operator, you must balance electricity demand with supply. The two must be in harmony: excess production can also bring down the system. Thus, you have the capacity to disconnect producers if you detect an unexpected drop in demand, or to activate other sources if there is a peak in demand at any given time.

Are all sources suitable for this?

Yes, but not all of them work the same. Intermittent power—wind and solar photovoltaic—is, of course, only available when the sun is shining or the wind is blowing. Meanwhile, nuclear power works more like a staple: it's always there. The plants shut down during refueling—always scheduled well in advance—or occasionally, but they are almost always producing. They do this to provide stability to the system, but also because it takes a long time to restart from a complete shutdown. Although they have practically been decommissioned in Spain, the role of coal-fired thermal power plants was very similar (in theory, nuclear power plants will end operations between 2027 and 2035).

Thus, the two sources with the greatest capacity to respond to specific increases in demand or, in general, system stresses are hydroelectric and combined-cycle power, which burns fuel (usually gas). In the case of dams, they depend on the availability of water, so in drought years they can be more affected. In the case of combined-cycle power, the key is to have it already active, even if at idle: otherwise, it will take some time to get up and running, although not as long as nuclear power.

Can batteries be used to store energy?

In a way. In fact, there are already pumped-storage turbine plants that function like giant "batteries": when there's excess production, electricity is used to raise water to a higher dam; when generation is needed, it's allowed to fall.

Solar thermal power also allows the sun's heat to be stored in molten salts, which can be released hours later to generate electricity.

In the very long term, the only way to store energy is potentially, either through these dams or in the form of natural gas... which in the future should be able to be replaced by green hydrogen. This is created by separating oxygen and hydrogen molecules by applying electrical energy to water. If this electricity comes from renewable sources, it takes on that epithet, green, which can also be pink if it's produced by a nuclear power plant.

Would this help the system?

Yes, and not only because it extends renewable production beyond the operating hours of wind and photovoltaic energy. As we said, the system must maintain the balance between supply and demand. Currently, it's very common for what is known in the sector as "dumps," which is unused production; a wind turbine or panel that can't enter the system, despite being capable of producing at that moment, because there's no demand. Using that electricity to power batteries of any kind would facilitate system management.

Are national systems connected?

Yes, countries generally send and receive electricity to their neighbors. Spain tends to export to Morocco and Portugal, while with France the situation varies more frequently. In fact, the neighbors—except for Portugal, which also failed—were key to getting the peninsular system back up and running.