Renewables: Variable, Not Intermittent

António Ferreira dos Santos

Senior Advisor - Power, Oil & Gas

The blackout of 28 April 2025 placed security of electricity supply at the centre of public debate. Following the publication of the final technical investigation reports, it is important to analyse its causes rigorously and draw the correct conclusions.

The electricity system requires a constant, real-time balance between generation and consumption. Any deviation affects the nominal frequency of the grid — in Europe, 50 Hz — and may trigger automatic actions by protection systems and, in extreme situations, cause a cascading collapse.

That is what happened on 28 April. The ENTSO-E panel of experts concluded that the blackout resulted from a combination of factors: oscillations in the grid, shortcomings in voltage and reactive power control, and a rapid sequence of generator disconnections in Spain. In addition, the power electronics converters of the Spanish generators did not react adequately to voltage variations — a failure to meet technical requirements that the Spanish transmission system operator should have anticipated. It is important to underline this point: the report does not attribute the cause of the blackout to renewables as a technology. Rather, it identifies failures in operation and in the behaviour of equipment under specific grid conditions.

On the other hand, the blackout gave ammunition to those who try to associate renewables with electrical instability. It is worth clarifying: renewables are not intermittent. They are variable over time. The difference is fundamental. “Intermittent” describes something that operates in an “all-or-nothing” mode, switching on and off abruptly — like a flashing light. “Variable” describes something whose output fluctuates gradually and predictably, following wind and solar cycles. Solar and wind generation fall into the second category: their production curve can be forecast with increasing accuracy and incorporated into daily grid planning.

Who is genuinely intermittent? Nuclear generation. A nuclear power plant operates in baseload mode: it is either online or offline. It does not shape its output according to demand. An unplanned shutdown instantly removes blocks of more than 1,000 MW from the system — a disturbance of enormous magnitude for a grid the size of Portugal’s. Indeed, during the blackout of 28 April itself, the Spanish nuclear power plants were the first to be disconnected for safety reasons and did not serve for black start. That role fell to a gas-fired combined-cycle power plant, Tapada do Outeiro, and to a hydroelectric power plant, Castelo de Bode, on the Portuguese side.

However, the integration of variable renewable sources requires more sophisticated management — not their replacement with twentieth-century technologies. The generation portfolio combines complementary sources: hydropower stored in reservoirs, which is highly dispatchable; wind and solar, which are variable but predictable and have zero marginal cost; battery storage, with an almost instantaneous response; and natural gas in combined-cycle plants, which is flexible and essential during the transition.

Grid management operates at several levels: long-term planning, to ensure that the portfolio covers scenarios of renewable variation; daily management, adjusting production and consumption and activating reserves; ancillary services markets, where flexibility has growing economic value; and predictive maintenance, supported by digital decision-support tools. To these are added demand-side flexibility — consumers adapting their consumption to the availability of generation — distributed generation and microgrids. The Technical Advisory Group created in Portugal after the blackout also underlined the urgency of implementing converters with synthetic inertia, namely grid-forming converters, which emulate the stabilising behaviour of the rotating masses of conventional generators. These technologies exist and can be implemented.

In the debate on nuclear energy in Portugal, it is important to be direct: nuclear generation does not solve the challenges of integrating renewables — it aggravates them. Construction costs have soared. Flamanville and Hinkley Point C are examples of cost overruns that have made nuclear the most expensive source in the European market. Portugal does not have the technical know-how or the necessary supply chain. High-level radioactive waste still has no definitive solution. And its real intermittency — abrupt shutdowns of large-capacity units — would pose a serious threat to the stability of the National Electricity System.

The blackout was a warning about the need to adapt operation and regulation to increasing renewable penetration. It was not an argument against renewables. The International Energy Agency is clear: electricity security depends not only on generation, but also on the quality of grid operation and the behaviour of all connected assets.

To decarbonise the Portuguese economy — by electrifying transport, heating and industrial processes — there is no alternative to expanding renewables. They are the cheapest, the fastest to deploy and the most abundant across the national territory. The challenge lies in building, in parallel, the infrastructure, markets and operational tools that make it possible to manage that variability with robustness and security.

That is the path. Not the flashing light.

(translated from the original published in Jornal Económico)