Chart of the Day: Batteries step in as solar eclipse sweeps across US

The spectacular total eclipse that swept across Mexico and the US – and the partial eclipse in many other parts of the country – have given researches and grid engineers a fascinating insight into the management of grid heavily dependent on solar in the daytime.

According to researchers at NREL, the eclipse was expected to cause a 93 per cent reduction in solar output in Texas, where solar has recently eclipsed coal for a whole month for the first time, a 71 per cent reduction in the last and a 45 per cent reduction in the west.

NREL showed a live stream of the passing of the eclipse and its impact on the grid. As expected, the lights didn’t go out – in fact they got switched on as the eclipse swept across the nation – and NREL has yet to release its analysis of the event.

Grid operators had said they expected no issues, although they saw it as a test of the ability to “ramp” – increase or decrease the level of supply quickly.

However, others have jumped in quickly. A graph of the California grid (top) showed that the downturn in solar power was almost entirely made up by a jump in battery storage – the state does have the biggest battery fleet in the country.

In Texas, some battery storage was used (see graph above) but the operators of the Ercot grid chose instead to turn to the state’s large fleet of gas peaking plants to respond to the bulk of the solar downturn, although the state is planning to double its battery storage capacity in 2024.

“What happens to the grid with just Wind-Water-Solar, when there is an eclipse? Won’t the grid fail?” wrote Stanford University professor of civil and environmental engineering Mark Jacobsen on LinkedIn.

“No. Batteries kick in,” he said of the California grid. “This is the 25th day out of the past 32 that California WWS (wind, water and solar) supply exceeded demand for 0.25-6 hours per day.”