I just ran a quick search on Google Trends to check my hunch that the phrase "sustainable energy" has become a lot more common, lately. That seems to be the case, at least based on the volume of news references tracked by Google. While I would regard a greater focus on sustainability as a positive development, I'm much less comfortable with its indiscriminate use as a synonym for "renewable". It's dangerously simplistic to think that the only parameters of sustainability that matter for a given energy technology are the extent of the energy supply it taps and the greenhouse gas emissions associated with its use. However understandable that might be in light of concerns about climate change and energy security, the complexities it obscures could ultimately prove just as limiting, in their own way, as the depletion of finite reserves of fossil fuels or the response of the global climate to increasing concentrations of carbon dioxide in the atmosphere.
Sustainable energy means different things to different people, as even a Wikipedia definition that points mainly to standard renewables admits. All too often, though, these definitions focus on the consumption of fossil energy sources and their accompanying emissions, while ignoring the use of other scarce resources, particularly water. In a recent posting I highlighted the high water consumption associated with the production of corn ethanol, a fuel widely regarded as more or less infinitely renewable, and thus much more sustainable than the oil it is intended to displace. Ironically, petroleum production and refining on average consume far less water per gallon or BTU of marketed fuel than most biofuels. The production of biofuels from non-food sources requiring little or no irrigation would alter that comparison, but still might not close this gap.
Biofuels aren't the only components of our energy mix that use lots of water. Electricity generation also consumes huge quantities, though much of it is returned downstream without degradation. Most thermal power plants use water for cooling and steam generation. That includes both fossil and nuclear power plants. Some renewable energy sources are also subject to this constraint, including solar thermal power, as noted in a recent Washington Post article. Enhanced geothermal power, which has great potential as a low-emission energy source, requires the injection of large volumes of water underground to create artificial hydrothermal reservoirs, and to transfer heat to the surface for power generation. In fact, at least 80% of the electricity generated in the US last year involved the use of water to some degree, a dependency that attracted critical attention during the Southeast drought in 2007.
Water is hardly the only input that should be considered in a broader view of sustainable energy. The consumption of rare earths and scarce metals in the production of thin-film solar panels, advanced batteries, wind turbine generators, and other aspects of the developing green-energy economy is starting to worry some experts. While I haven't delved into it in much detail, I'd be surprised if these factors proved limiting in the near term. After all, the technologies in question have only been around for a few years, so there hasn't been much time for the sources of these exotic ingredients to ramp up to support their growing demand. This scaling issue cuts both ways, however. For example, if solar energy is to expand from its present contribution of less than 1% of renewable power generated here last year to, say, 10% of our total power supply, the use of an ingredient in proportions as small as a hundred grams per kilowatt of capacity would translate into a cumulative requirement for tens of thousands of tons. If the substance in question was the Tellurium used in Cadmium-telluride solar cells, its global output would have to expand by at least 10X within a decade or two. That might not be possible, or at least economically feasible.
The point here is not to suggest that we're stuck in some depressing dynamic in which we encounter bottlenecks and unintended consequences in every direction we turn, as we seek alternatives to conventional energy. Instead, we need to remember that oil, gas and coal aren't the only finite substances in the earth's crust. We must consider all our energy options in terms of trade-offs, and not just with regard to the aesthetics of wind farms and solar panels in our back yards vs. oil derricks and central power plants in someone else's. The choices we are making demand a thorough look at their lifecycle impacts, including all the inputs and outputs along the way. That won't be easy, and it clearly will not be convenient for those sectors that have benefited from an overly narrow view of this issue, such as the interests that are supporting legislation to block the EPA from factoring in the effect of global land-use changes in the agency's lifecycle assessment of corn ethanol.
This broader view of sustainable energy is another reason to moderate our faddish focus on all things renewable, as I noted recently when I argued that we need a low-emission electricity standard, instead of a renewable electricity standard. Tackling climate change effectively will require clear goals that address outcomes, rather than preconceived notions about pathways. And when it comes to energy security, we need a framework that recognizes that oil in tankers is not the only energy-sector import that bears watching.