A pile of coal is energy waiting on demand. Sunshine and wind are not: they arrive when the weather decides. As grids rely more on these sources, the central engineering question becomes how to have power when the sun is down and the air is still.

Stocks and flows

Fossil fuels and uranium are stocks of energy that can be burned when needed. Sunlight and wind are flows. Wind and solar are called variable because their output depends on the weather and the time of day, and operators cannot simply turn them up when demand rises. Their output can be forecast fairly well a day or two ahead, which helps, but a forecast is not control.

The duck curve

In 2013 California's grid operator published a chart that became famous. It showed demand minus solar output over a typical spring day. The duck curve shows the demand left for other plants dipping at midday and then climbing steeply at sunset, when solar output fades just as people come home, cook and switch on lights. Drawn out, the line looks like the belly and neck of a duck. The deep midday belly means solar power is sometimes wasted, a practice called curtailment, and the steep neck means other plants must ramp up very quickly.

Ways to store electricity

The oldest large method is pumped hydro. Pumped hydro storage pumps water uphill into a reservoir when power is cheap and lets it flow back down through turbines when power is needed. Pumped hydro still holds most of the electricity stored in the world's grids. It needs suitable hills and water, and new projects take many years.

The fastest growing method is the lithium-ion battery, the same kind found in phones and electric cars. Battery pack prices have fallen by roughly ninety percent since 2010, following a learning curve like solar's. Grid batteries are best at shifting energy over a few hours and responding within fractions of a second, which suits the duck's neck perfectly. They are far too costly to store energy for weeks.

Other ideas fill other gaps. Some solar plants focus sunlight to heat molten salt, which can keep making steam after dark. Compressed air can be stored in underground caverns. For long gaps, many engineers look to hydrogen made from electricity. Turning electricity into hydrogen and back loses most of the energy, but hydrogen can be stored for months. Whether that trade pays is an open question.

Hours, days and seasons

The daily cycle is the easy part, since solar comes back every morning. Harder are winter spells of several days with little wind and weak sun. Germans call these spells Dunkelflaute, roughly "dark doldrums", and they can stretch across much of northern Europe at once. Hardest of all is the seasonal gap in places where solar is plentiful in summer but heating demand peaks in winter.

Tools beyond storage

Storage is only one answer. Wider grids help because weather differs across regions, so a calm spell in one place is often offset by wind elsewhere. Wind and solar also tend to complement each other, since winds are often stronger in winter and at night. Building more wind and solar than the average need, and accepting some waste, can be cheaper than storing every surplus. Demand can bend too: electric cars can charge at midday, water heaters can warm when power is cheap, and factories can shift some work.

Finally there are firm low-carbon sources that run on demand, such as reservoir hydropower, geothermal heat, nuclear plants, and gas plants that capture their carbon. Several modeling studies have found that keeping some firm low-carbon power makes a nearly carbon-free grid much cheaper. Other researchers argue that wind, solar, storage and wide grids can do the whole job at reasonable cost. The debate is active, and the answer probably differs from place to place.

Worked example: South Australia

South Australia has some of the highest shares of wind and solar of any large grid. On September 28, 2016, violent storms toppled transmission towers, and several wind farms disconnected because of protective settings that were triggered by the repeated faults. The whole state went dark. Politicians argued furiously over whether renewables were to blame. The operator's inquiry found that the settings, not the wind itself, were the key problem, and they were changed.

In 2017 a large lithium-ion battery was built at Hornsdale in under a hundred days, then the biggest in the world. It proved extraordinarily fast at steadying frequency, and it cut the cost of those services. More batteries followed, along with a new link to the neighboring state of New South Wales. Today, on sunny days, rooftop panels alone can sometimes meet all of the state's demand.

Apply the ideas. The blackout came from settings and storms, not from variability as such. Batteries handled the fast balancing that spinning plants once provided, and wider links spread out the weather. The case shows that a high-renewable grid can work, but only through deliberate engineering, not by adding panels and hoping.