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Intermittency: Achilles' heel of renewable energy

This article first appeared in the St. Louis Beacon, Nov. 26, 2012 - William Pickard believes in long-range planning — at least 70 years into the future.  He foresees the economic end to earth’s fossil fuel supply before the close of this century and hopes to promote a smooth transition to an energy future fueled by renewables. But he is wary of what has been called the Achilles’ heel of renewable energy: intermittency.

This retired Washington University professor of electrical and systems engineering fears that in their retirements his grandchildren will have a drastically reduced standard of living if the world does not develop and implement technology to assure a constant supply of electricity.

Pickard recently co-edited a special volume of the Proceedings of the IEEE (Institute of Electrical and Electronics Engineers): “The Intermittency Challenge: Massive Energy Storage in a Sustainable Future.” In this volume the authors discuss various strategies for coping with the fact that the sun only shines during the day, and the “Wind bloweth where it listeth” (John 3:8, King James Bible).

Sun and wind can provide plenty of energy to support a modern industrial world. Gregory Wilson, director of the National Center for Photovoltaics, calculates that even with the less-than-constant sun of St. Louis, and using today’s solar panels, a field 7 percent of the size of the United States landmass could power the entire planet. Of course many areas on earth have nearly constant daily sunshine.  And many areas are prone to windy conditions.

Unfortunately, sun and wind electricity cannot be stored on a large scale at present. Without storage, and without backup generators burning fossil fuels, it could be cold and dark at night — with no television to entertain and the possibility that a visit to the emergency room might not even allow an X-ray. 

Pickard cites the most recent estimates for peak abundance of fossil fuels: natural gas peaks in 2035, oil in 2015, and coal in 2052.  Theoretically, after the peak, the remaining stores become harder and harder to extract and correspondingly more expensive.

25 million kilowatt-hours a day

Today, average Americans consume 10 kilowatt-hours (kWh) a day of electricity. Usage is high in the daytime, but significant even during the middle of the night. Streetlights are on, furnaces or air conditioners are operating, computers are backing up, trains are running. It takes about 25 million kWh a day to support the lifestyle of the 2.5 million residents of the St. Louis area. While many parts of the world use much less electricity at present, development will bring electrical and other energy needs on the same scale as ours.

Pickard reminds us that our energy use goes way beyond residential.  As he puts it, “To maintain our lifestyle, we have to consider the entire energy expenditure made within the Americas' boundaries. Only 22 percent of our energy budget is classified as residential. We are all interconnected, no matter what we see on our energy bill."

Large grids — even transnational grids — could spread locally produced energy to where it is needed. For example, some European companies and the Desertec Foundation envision solar farms in north Africa exporting power to Europe across the Mediterranean. The transmission lines need to be ultrahigh voltage (800 kilovolt) DC lines. Interconnected AC lines will not work over long distances, says Pickard, because electricity “sloshes around” in AC networks and makes them unstable electrically.

Need for energy storage

Even with transnational grids, energy must be stored on a massive scale. Rainy days and dark nights demand that the grid’s surplus energy be converted to a stored form that can be drawn upon immediately when needed.

St. Louis would need two gigawatt-days of stored power for back-up energy, says Pickard. That amount would supply minimal lighting, water and run emergency rooms and hospital-like facilities for several days.

Investigators are following a number of paths for massive storage and release of electrical energy. For example, carbon dioxide can be taken out of the air by adsorption onto a membrane and then converted into methane (natural gas). Solar-powered electricity could electrolyze water and produce hydrogen for fuel cells. Some ideas have worked on a pilot scale but may be prohibitively expensive or inefficient to scale up. One possibility, storing compressed air to turn turbines, not only would require very special geological storage conditions but would need to deal with the heat generated when the air is compressed.


Batteries will almost certainly be part of the ultimate solution to the intermittency and storage problem. Lead-acid batteries like those used in cars are already being used in parts of China, India and the area of Japan hit by last year’s tsunami. But the planet does not have enough lead to make back-up batteries for the whole world.

The lithium ion battery used in hybrid and electric cars may be sufficient to power a single house, says Wilson. The Nissan leaf battery pack has 24 kWh of storage, so it would be useful for a house using photovoltaic panels for its daytime power.

A real advance could come from flow batteries. As with other batteries, electrons flow through an electrolyte solution from the negative cathode to the positive anode. Flow batteries consist of two large storage tanks of electrolytes in which metal ions in different charge states are dissolved. As shown in the illustration, the solutions are pumped into their respective reaction chambers that are separated by a semi-permeable membrane. Developers are exploring at least five different chemical combinations that could regenerate when the stored electricity is discharged.

When they become commercially feasible, flow battery tanks would be of a size to fit into a utility sub-station and would be able to store electricity for a town the size of Webster Groves.

Underground pumped hydro

Pickard calls his favorite solution to the intermittency problem underground pumped hydro (UPH). Conventional pumped hydro is an old, well-understood technology. When supplementary power is required, the water contained in a large reservoir flows steeply downhill into a second reservoir, generating hydroelectric power.  When electricity is abundant, the water is pumped back uphill.  Efficiency is about 75 percent, which is quite high. The Taum Sauk power station in southern Missouri is a pumped hydro facility. 

Pickard estimates that to store two gigawatt-days of energy, reservoirs need to be the volume of 10 Great Pyramids — about 25 million cubic meters. Such a reservoir would be about one square kilometer and about 25 meters deep. He proposes that for UPH, a second 25 million cubic meter reservoir be excavated about 800 meters directly below the ground level reservoir. The excavated rock can be used to dike the upper reservoir. Both reservoirs need to be dug out of rock that can hold water.

A nationwide UPH system would require several hundred such paired reservoirs. Pickard thinks finding suitable sites would not be a problem. The sites should be close to, but not inside, cities and built upon land neither suitable for farming nor of great natural beauty. 

Pickard concedes that the up-front capital outlays would be enormous — even as much as $7.7 billion for each facility. 

What about nuclear?

Missouri’s Callaway plant produces a steady amount of electrical power — so- called “base power.” To deal with peak demands, Ameren uses coal and some natural gas to create the heat that powers its turbines. Wilson believes that nuclear might well play a bigger role in the future, with new generation plants considerably more stable and somewhat more flexible. Pickard feels that until the nuclear waste disposal problem is solved, building more nuclear plants should not be an option.

Thoughts and solutions for the medium term

Michael Kintner-Meyer of the DOE’s Pacific Northwest National Laboratory doubts that underground pumped hydro will be the ultimate solution chosen to solve the intermittency problem. He cites a “lock-in” mechanism because of the initial cost. “Lock-in” means that if a society decides to invest heavily in a particular approach, it is difficult to deviate. 

Kintner-Meyer feels that the U.S. will take an incremental approach to dealing with its energy problems. He points out that even appliances can help solve the intermittency problem by using electricity in a smarter manner. An electric hot-water heater could be regulated to heat its tank only when the energy supply is high.

As our use of renewable sources increases, capital outlays will go down. Wilson points out that putting a photovoltaic system on a roof is five to six times more expensive in the U.S. than in Germany.  Permits cost a lot, there is a shortage of installation equipment and of companies with solar expertise. Since Germany started committing itself to solar and other renewables in 2003, many citizens make a living installing and maintaining photovoltaic systems.

The use of biofuels is another approach to stretching the supply of fossil fuels.

Jim Umen, associate member of the Enterprise Rent-a-Car Institute for Renewable Fuels at the Donald Danforth Plant Science Center, points out that biofuels in many instances can be substituted for fossil fuels with little or no change in the vehicles using them. “You can’t fly a plane on solar electricity, but the Navy has already tested biofuels in their jets.” In a future where the main source of electricity is solar or wind energy, it is certainly feasible that biofuels could be burned in back-up generators. Burning biofuels is carbon-neutral, and if the carbon dioxide is captured it could be used to grow algae for oil.

An expensive proposition

Lengthening the available lifetime of fossil fuels gives scientists and engineers more time to solve the intermittency problem, but that problem must be solved. 

Finding the best solutions will be expensive. Kintner-Meyer suggests it will cost trillions of dollars worldwide. It may take 10,000 tries for two to three successes. Discovery and technology development never go in a straight line. Most experiments fail; many laboratory successes cannot be scaled up.

Yet, as Pickard puts it, what is at stake is no less than the future of our society.

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