Which Numbers Are The True Measure Of Ecological Virtue?

I recently came across a site that compared commonly used metals in terms of the energy required to produce them. This raises interesting questions. Shall we criticize auto and motorcycle manufacturers for using more aluminum in their vehicles and less steel? The energy cost to win a kilogram of aluminum from its aluminum oxide (bauxite) ore is roughly 10 times greater than what is consumed in transforming iron into steel. Or shall we praise them because the lighter weight of the resulting vehicles requires less fuel to accelerate them in stop-and-go driving?

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Imagine that I win the lottery and open the fabled Ducati accessory catalog in a whole new frame of mind. This time I'm here to buy, not just to stare. And I circle every gorgeously tantalizing titanium item I can find. But my delight turns to wormwood as I realize that the energy cost to produce 1 kilo of titanium is 25 times that of a kilo of steel. Hmm, do you suppose there's such a thing as a Ural accessory catalog?

During WWII a large part of the output of the then-new Grand Coulee hydroelectric dam was used to produce aluminum, mainly for the production of military aircraft. But since hydroelectric dams supply only 6 percent of US electricity, the real story on aluminum production is not just the volts and amps sent to the truly impressive Hall process reactors. Much of the power to produce aluminum comes from electric plants fired by coal or from nuclear plants. Both operate at an average efficiency of about 35 percent, so the actual energy consumed at the electricity-generating plant is almost three times greater than just the kilowatt-hours.

But then part of the power comes from the natural-gas-fired turbine plants whose average efficiency (in the US) was 43 percent in 2016. Much better.

Thinking about this, I know that natural gas is mostly methane, consisting of a single carbon atom bonded to four hydrogen atoms. That means that burning natural gas produces only half as much carbon dioxide as does burning coal or hydrocarbon liquids for a given energy output. But before I start to feel good about that (and about the fact that the combined-cycle gas turbine plants now steadily replacing US coal plants can closely approach 60 percent efficiency), I remember that US natural gas now in large part comes from the new combination of horizontal drilling and hydro-fracturing (“fracking”)—which are roundly disliked by many environment-minded persons.

Hydro-fracturing injects mostly water carrying sand plus viscosity additives into well bores at pressures up to 15,000 psi. The pressure causes fractures in the surrounding rock (shale that is oil- or gas-bearing), and the sand or aluminum oxide grains (which are called “proppants” because their presence props open the fractures) is carried into them. The result is a network of fine cracks through which oil or gas can move to the borehole for recovery to the surface. Many oil-bearing formations lack the porosity to flow to conventional vertical boreholes (they are called “tight plays”) but have now been made to produce economic amounts of energy via the new techniques.

The techniques and equipment that enable drill strings to be steered through 90 degrees to be horizontally positioned in oil-bearing formations are wonderfully clever, whatever we may think of their environmental effects. Just as GPS and IMUs enable racing and sports motorcycles to reach higher performance, so they are also able to report the exact position of down-hole “mud motors” far below the surface (those mud motors are powered by drilling mud pumped down through the drill pipe and employ the same principle as the familiar two-element “gerotor” oil pumps found in many motorcycle engines). The mud returns to the surface carrying with it the rock chips produced by the drilling bit. The chips are removed and the filtered mud is returned to the well bore.

Clever as all this is, we are warned that such "fracking" is environmentally hazardous—it can contaminate ground water or even possibly trigger earthquakes. Can we officially consider this process "good" because natural gas cuts carbon dioxide emissions by half or more when burned in efficient combined-cycle gas turbine electric plants? Or must we consider it "bad" because a) it is not a renewable energy source, b) some methane, a much more potent greenhouse gas than carbon dioxide, is inevitably released to atmosphere during recovery, processing, and delivery, and c) there are other disagreeable environmental effects?

Okay, these are hard questions, like trying to work out whether it’s hotter in the city or in the summer. What can we meaningfully compare in order to know what to do?

This past July, Elon Musk (Tesla, Hyperloop, SpaceX) mentioned in passing a favorite of the solar power industry—the idea of powering the whole US by covering an area of, say, Arizona, 100 miles by 100 miles with solar panels. I decided to estimate how much structural material (almost certainly aluminum because steel rusts if not painted) might be needed for this.

To have another familiar large-scale application of aluminum for comparison, I looked up the total weight of military aircraft produced in the US 1940–1945 and used the standard fraction of 60 percent of that as aluminum. The result was 2.8 billion pounds. That included the 17,000 B-24s built, the 14,000 B-17s, 9,500 B-25s, 10,000 P-47s, 3,960 B-29s, and so on.

Then I estimated that if the support structure necessary to hold up our solar panels were melted down and rolled into sheet, it might form a layer of the same area as the panel but one-tenth of an inch thick. For the 10,000 square miles (100 x 100) of solar panels mentioned by Musk, the total weight of aluminum required would be 240 times that of the aluminum in the US WWII military aircraft fleet of 303,000 machines.

But not to worry—if we can find enough recycled aluminum for this, the energy cost to produce the metal will be only one-twentieth as much as it would be if we used virgin aluminum won from its ore.

Renewables are coming. In 2014 4.1 percent of US electricity came from wind. In 2015 it increased to 4.7 percent, and last year to 5.6 percent. Solar in 2016 supplied 0.9 percent. It’s complicated, but little by little, environmental perfection is coming.