In the US, fully half of the water withdrawn from sources such as lakes and aquifers ends up being used for generating electricity. Most of that water is converted to steam, cooled, and returned to its original source. Even in those cases, however, losses during the cooling process reduce the total amount of water available.

The end result is that electricity generation competes with other potential uses for the water. In cases of severe drought, power generation may end up losing, reducing the amount of electricity we can generate. This situation is a problem given that climate change is expected to exacerbate droughts in a number of regions of the US.

To find out how much of a problem this can be, three MIT researchers have looked into the balance between water use and carbon emissions, using the Texas power grid as their test tube. Their model shows that taking carbon emissions into account is bad news for coal, but limiting water use essentially forces coal off the grid. While nuclear looks great for limiting carbon emissions, its heavy water requirements cut down on its role when both factors are considered.

Currently, most US facilities use what are called open-loop or once-through cooling. In these systems, water is removed from a source, used for energy generation (e.g., heated and used as steam), and then cooled and returned to the original source at a higher temperature. Although a majority of the water is immediately put back into circulation, these systems have a fairly high rate of loss.

There are a number of alternative approaches that require far less water to be extracted. The simplest is closed-loop, which passively cools the water in the facility and uses it again. Dry cooling, which uses air to actively cool the water, requires more in the way of both up-front construction costs and ongoing operating costs. Hybrid systems use both of these approaches, but they have even larger up-front costs. All of them require periodic withdrawals of water due to losses, but they end up requiring far less to be withdrawn for a given period of activity than an open-loop system would.

Different types of energy production can use the different technologies to various degrees. For example, natural gas can be used to heat water in a traditional boiler plant, or the hot exhaust gasses can be used to drive turbines directly, significantly limiting the amount of water involved. Current nuclear plants have cooling towers designed to lower the water's temperature before returning it to the environment; these could instead be merged with air cooling to create hybrid, closed-loop cooling systems.

The authors considered the Texas grid of 2050 since all existing generating capacity would be expected to be replaced by then anyway, allowing them to work with a blank canvas (albeit one with a larger electricity demand than the present). They considered three different scenarios: business as usual, a 75 percent reduction of carbon emissions, and the same 75 percent emissions reduction coupled to a 50 percent reduction in water used while generating electricity.

With no restrictions, the grid of 2050 looks a lot like it does today: mostly coal and natural gas, with a healthy chunk of nuclear. In fact, because the authors seem to be looking only at costs largely as they exist in the near future, wind is largely left out of this scenario, even though it already accounts for a substantial fraction of Texas' generating capacity. In fact, even with heavy carbon restrictions, the percentage of wind barely budges—instead, most of the coal is displaced by nuclear. Photovoltaics are effectively missing.

(If this seems unreasonable to you, it does to us as well—more on that in a moment.)

Adding water limits turns the whole thing upside down. Nuclear power use drops dramatically, and most of the remaining generation capacity is linked to a hybrid closed-loop system despite their high costs. The vast majority of electricity is generated by some form of natural gas, most of which doesn't use water at all. Here, wind finally makes an appearance, although only at five percent of the total capacity. Even though, like wind, photovoltaics use no water to generate their electricity, they have almost no presence in this scenario.

Is any of this realistic? Not entirely. Nuclear plants require huge capital outlays for years before they generate power; in an era of cheap natural gas, we're not likely to see many being constructed. And the authors found essentially no presence of wind systems, while Texas is already seeing times where wind is approaching 30 percent of the electricity feeding into the grid—and the Department of Energy is expecting the cost of installing wind systems to drop further in the upcoming years.

All of which suggests that there is something seriously amiss with the economic assumptions in the authors' model.

Nevertheless, the study highlights a real challenge. Texas recently experienced its worst drought on record, and climate change is expected to put a strain on the water supply of several regions in the US. The prospect of a competition between electricity and other water uses—or an actual shortage of water for generation purposes—isn't out of the question in the coming decades. Any plans for making the US energy economy sustainable had better consider it, which will hopefully motivate a more detailed look at the challenge.

Nature Climate Change, 2013. DOI: 10.1038/NCLIMATE2032 (About DOIs).