Tom Murphy - Having now sorted solar, wind, and tidal power into three “boxes,” let’s keep going and investigate another source of non-fossil energy and put it in a box. Today we’ll look at hydroelectricity. As one of the earliest renewable energy resources to be exploited, hydroelectricity is the low-hanging fruit of the renewable world. It’s steady, self-storing, highly efficient, cost-effective, low-carbon, low-tech, and offers a serious boon to water skiers. I’m sold! Let’s have more of that! How much might we expect to get from hydro, and how important will its role be compared to other renewable resources?
Last week, as soon as I put tidal power into a box labeled “waste of time,” I received some deserved howls of protest. I saw it coming, and had built in words to soften the “waste of time” label. But it was a poor choice from the start. A better set of labels is “abundant,” “potent,” and “niche.” The last could also be thought of as “boutique,” in that it is cute, perhaps decorative, may serve some function, but will never be a heavy lifter. The “potent” label—formerly “useful”— is meant to indicate 
a source that could supply a healthy fraction (say over a quarter) of our global demand if fully exploited. We will neverfully exploit any resource, so the numbers at least need to support ¼-scale before we can believe that it may play a major role.
...Global Hydro Potential
Now the fun part. How much hydro power is theoretically achievable? Hydroelectricity is cashing in on residual potential energy provided by the rain cycle. A look at the Earth’s energy budget shows that a whopping 23 percent of the solar budget goes into evaporating water! The water cycle is a big deal.
...At present, the U.S. has 78 GW of installed hydro power (out of which we get 31 GW, averaged annually). The world has about 1 TW installed, likely realizing 400 GW on an annual average. The realized capacity therefore undershoots our crude estimate of global potential by a factor of 10 or more. Does this mean we could go nuts and expand hydro to amazing new levels? Should I ask for water skis for Christmas?
Realistic Assessments
I don’t want to discount the top-down approach we did here. After all, if anyone tried to tell me that hydro could deliver much more than 25 TW of power, I would know that the basic physics of the planet does not allow it. But at the same time, the upper limit we established does not account for a whole host of practical considerations, like actual rivers with known flow rates and geographic potential for damming. So I turn to a study that has put some more time into the question than I can afford personally, outside of my day job. Specifically, a report by the Eurelectric group assessed global hydro potential in four cascading steps:
- Gross potential if all runoff is developed to sea level with no loss;
- Technical potential, ignoring economic limitations;
- Economically viable potential, cost competitive with other sources;
- Exploitable potential, considering environmental and other restrictions.
For the first step, they come away with 5.8 TW — not far at all from my estimate (I’m not cheating I swear: I did not look at any estimates prior to writing the above sections). Other assessments get 4.4 TW, 4.6 TW, and 5.1 TW.
For technical feasibility, these same sources estimate 1.6–2.3 TW globally. Economic feasibility (in today’s economic climate) drops this to 1.0–1.4 TW. Environmental restrictions (in today’s climate) reduce this number further. Thus, having developed 0.4 TW worldwide (using average annual output for proper comparison to studies), the world may be able to expand by a factor of 2–5. This is a large range: a factor of two isn’t that much, while a factor of 5 is a pretty big jump. Where is it, really?
For the U.S., the Idaho National Laboratory estimates a gross potential of 0.3 TW, and a technical potential of 0.17 TW. The latter was determined after a study of 500,000 potential sites, out of which 130,000 made the cut. It is also estimated that existing dams with no hydroelectric capacity could add 0.013 TW (13 GW).
So here in the U.S., we could expand by a factor of 5 according to this report—ignoring economic and environmental barriers. Such a boost would bring hydro up to 5 percent of our gross energy, or 12 percent if we correct for the heat-engine effect (40 percent of our electricity). I have seen other reports less optimistic about our expansion potential, coming in closer to a doubling of current capacity—likely factoring in economic and environmental considerations, and consistent with the lower end of the range estimated for global potential...
