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 end 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) entitled “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% 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 per day
Today, average Americans consume 10 kilowatt-hours(kWh) /day of electricity in their homes. 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, etc. It takes about 25 million kWh per 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 with it 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 America’s boundaries. In 2011, this was 97.30 quads (quadrillion btu’s) of which only 21.62 were residential. So only 21.62/97.30 = 22% of our energy budget is classified as residential. In a renewable society, every resident has to be aware that, for instance, the energy the US Navy expending patrolling our coast has to be prorated. We are all interconnected, no matter what we see on our energy bill.
What are some of the solutions to the intermittency problem?
Upgrading the Electrical Grid
First of all, 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 (800kilovolt) 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.
The Need for Energy Storage
But even with trans-national grids to distribute it, 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.
Photo by Dennis Schroeder, National Renewable Energy Laboratories
Saint Louis would need 2 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.
What technologies would allow the storage and release of electrical energy on a massive scale?
Investigators are following a number of paths, but for many the journey has just begun. 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
Batteries will almost certainly be part of the ultimate solution to the intermittency/storage problem. Your car’s familiar lead-acid battery is already being used in parts of China, India, and the area of Japan hit by last year’s tsunami. Unfortunately, there is not enough lead on the planet to make back-up batteries for the whole world.
Photo courtesy of Pacific Northwest National Laboratory
The lithium ion battery used in hybrid and electric cars may be sufficient to power a single house, says Gregory Wilson. The Nissan leaf battery pack has 24 kWh of storage, so would be useful for a house that uses 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 semipermeable membrane. Developers are exploring at least five different chemical combinations that will be able to 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 favored 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%, which is quite high. The Taum Sauk power station in southern Missouri is a pumped hydro facility.
Pickard estimates that to store his 2 gigawatt days of energy, reservoirs need to be the volume of ten Great Pyramids—about 25 million cubic meters. Such a reservoir would be about 1 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. Obviously 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 of cities, and built upon land neither suitable for farming nor of great natural beauty.
He concedes that the up-front capital outlays would be enormous—even as much as $7.7 billion for each facility. But he points out that $7.5 billion is a small amount relative to the $165 billion spent annually on the war in Iraq.
What about Nuclear?
Saint Louis’s Calloway 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. (For a good primer on electricity, see Electricity 101.) Without fossil fuel to use, Saint Louis would need several more Calloway’s to meet its electrical energy demands. Gregory 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 becomes 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. France, for example, has committed to nuclear power as its energy source. It is simply too difficult to shut all the power plants down.
Kintner-Meyer feels that the United States 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 5-6 times more expensive in the United States than in Germany. Permits cost a lot, there is a shortage of installation equipment, and there is a shortage 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.
Biofuels
Obviously, increased reliance on solar and wind power will lessen the pressure on fossil fuels. 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 Place Science Center, points out that biofuels in many instances can be substituted for fossil fuels with little or no change in the vehicles that use 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 their 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 nevertheless be solved eventually.
All the scientists agree that finding the best solutions will be expensive. Kintner-Meyer suggests that we will invest trillions of dollars worldwide. The reason is that often you need to try 10,000 approaches for 2-3 successes (a la Thomas Friedman). 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. We do not want to consign our grandchildren or great grandchildren back to the “dark” ages.
This article was originally published in the St. Louis Beacon.