On April 17, Easter Sunday, electricity demand plunged in Spain and photovoltaic solar generation reached a peak power of almost 13.5 GW, which means that it covered up to 62.5% of national demand. The wholesale price of electricity went from €168.50 ($182)/MWh to just €3.70/MWh, according to data provided by Spanish energy forecasting service AleaSoft.

Power generation exceeded what had been programmed and the first PV curtailment in Spain took place – that is, a cut or stoppage of production. Renewable energy generation has been affected by these adjustment mechanisms since 2015, but mostly wind power generation.

“Curtailment occurred last Sunday. A considerable amount of renewable energy was discarded that could have been generated with the existing capacity and the solar resource available that day,” a spokesperson from Aleasoft told pv magazine.

The production curve of solar for Sunday shows that between 2:50 p.m. (CEST) and 4:00 p.m. there is a decrease in production with this technology, falling from 10,710 MWh registered at 2:40 p.m. to 9,689 MWh at 3:50 p.m. Additionally, some producers posted on social media that the grid operator, Red Eléctrica de España (REE), told them to stop production.

The Aleasoft spokesperson explained how this mechanism is activated.

“REE monitors the operation of the electrical system in real-time to guarantee the generation-demand balance. When it detects a situation like the one on Sunday, in which the demand was low and the renewable energy production was higher than scheduled, it first activates all the existing adjustment mechanisms, such as the regulation bands, and, if it is not enough, it communicates with generators to disconnect or reduce production,” he stated.

He said there has been a failure in the REE forecasting mechanisms. If there had been a better forecast of demand and generation, the adjustment mechanisms could have been used to a greater extent and at the right time.

According to a recent study from the US Department of Energy's National Renewable Energy Laboratory (NREL), regulations that completely prevent PV curtailment could end up restricting the ability of grid operators to use solar power for ancillary services, according to new research by NREL scientists.

In “Too much of a good thing? Global trends in the curtailment of solar PV,” which was recently published in Solar Energy, the NREL team analyzed how PV curtailment is handled in four mature solar energy markets: Germany, Chile, the United States, and China. They found that in 2018, the curtailment of solar power in these countries hit 6.5 million MWh, to account for around 1% of total PV power generation.

“Some policies require generators to be compensated for reduced production, and some interconnection policies prohibit systems from interconnecting if those systems are going to lead to curtailment,” the researchers said.

According to Aleasoft, the opportunities for participation and the role that storage technologies are going to play in the electrical system in the coming years are becoming more obvious and clearer. Hourly price differences of more than €100/MWh within the same day in the Iberian electricity market have become relatively common. These price differences are an opportunity for energy storage technologies. In addition, these will also be very important in the adjustment services to provide a rapid response to the needs of balancing the electrical system.