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THE WATER MAINS of Paris burst in August. This sounds like the title of a Hemingway pastiche, but actually it’s a truism of infrastructure. What’s more, it has implications in one of the more pesky problems concerning renewable energy: What to do with an overabundance of it? How to store a resource, especially when it’s a renewable one that’s only intermittently available?
About those Parisian water mains: In August, Parisians leave for a month’s vacation. And, in fact, the sudden drop in water demand causes a spike in water supply, thus increasing pressure in the mains and causing the weaker ones to fail.
In the larger sense, an overabundance of any resource raises the matter of storage, and some resources are easier to store than others. Electricity, for one, is particularly challenging. Utilities prefer to produce it as needed, not necessarily when wind or solar conditions encourage it.
Storing the electrical energy in batteries isn’t a particularly good option. Battery chemistry is not adept at quick charge and discharge; nor is it especially durable in the long-term utility sense. Instead, electric companies prefer to power down generators, to the detriment of system efficiency.
Water is one means of storing electricity. During times of overproduction, use the excess electricity to pump water into a reservoir at higher level. When demand warrants, route the water back down to operate turbines generating the necessary electricity. Gravity does the work, abetted by appropriate topography.
Another innovative approach is described in the September 11, 2015, issue of Science, published by the American Association for the Advancement of Science. “Tailpipe to Tank,” by Robert F. Service, describes research striving to suck CO2 out of the air and turn it into a liquid fuel. A neat trick, eh?
In overview, this also offers benefits for renewable sources because liquids have a high energy density and they’re particularly easy to store and distribute. There’s nothing akin to electricity’s 8-10-percent loss in long-line transmission, and storing a liquid hasn’t any battery tradeoffs. The wind or solar energy generates electricity; the electricity produces a liquid fuel; and the fuel returns the energy when and where needed.
Researchers at George Washington University, Washington, D.C., start with CO2 in the air and H2O, er… water. Then they apply solar-generated electricity to split these two into H2 and CO, only one step away (albeit a non-trivial one) from producing a hydrocarbon fuel, for example CH3HO, methanol.
In a sense, it’s running the combustion process in reverse. Plants do this, though they get by with an efficiency of only 1 percent. By contrast, the researchers have used solar panels to produce high-voltage electricity at an efficiency of 38 percent. Then this electricity is shunted to split water molecules in one electrochemical cell and CO2 in another. Last, as an example, the researchers note they could make a kilogram of hydrogen, the energy equivalent of a gallon of gasoline, at a cost of $2.41. (It’s far from setting up lucrative neighborhood fuel-cell refueling, but certainly a start.)
A daunting challenge of the process is its extreme heat, around 1000 degrees Celsius (more than 1800 degrees Fahrenheit). Another is the chemical stability of the CO2 molecule. New catalysts and techniques are in the works.
Syngas is one name for these artificially concocted hydrocarbons, most derived today from natural gas, not CO2 and water. The George Washington researchers like to call theirs “sungas.”
Iceland makes use of another natural resource, its abundant geothermal energy, to produced syngas. The path is geothermal heat generating electricity, in turn producing electrolysis of water and CO2, the resulting syngas turned into methanol.
A last example, one with a potential economic twist, concerns Denmark’s overabundance of wind power. The country already produces 30 percent of its electricity from wind farms, with a goal of reaching 50 percent by 2020. During a particularly blustery day in July, the farms put out 140 percent of the country’s needed electricity, with the excess sent to neighboring Norway, Sweden and Germany. An enviable situation?
Science notes, “But the oversupply added to utilities’ fears that in times of peak renewable power production, the value of electricity could fall to zero or even below, as producers would have to pay others to take it so as not to damage their grid.”
As Oliver Hardy used to say to Stan Laurel, “Well, here’s another nice mess you’ve gotten me into.” ds
© Dennis Simanaitis, SimanaitisSays.com, 2015