Energy and Human Ambitions on a Finite Planet, 2021a

11 Hydroelectric <strong>Energy</strong> 178
On average, terrain is about 800 m above sea level, so each gram that
falls on l<strong>and</strong> has an average of 8 J left as available energy. But only 29%
of the earth’s surface is l<strong>and</strong>, so that the gram of water we’re tracking
preserves about2Jofenergy, on average. 21
We’re down to only 0.1% of the input solar energy—2 J out of 2,300 J
input—so that the theoretical hydroelectric potential might be about
44 TW: reduced from the 44,000 TW input. But only a small fraction
of rain flows into rivers suitable for damming. And once dammed, a
typical dam height is in the neighborhood of 50 m, knocking us down
even further. Much of the journey from terrain to reservoir involves
losing elevation in streams too small to practically dam, or just seeping
through the ground. In the end, perhaps it is not surprising that we end
up in the sub-TW regime globally.
Detailed assessments [67] of hydroelectric potential globally estimate
a technically feasible potential 22 around 2 TW, but only half of this is
deemed to be economically viable. Recall that 477 GW, or about 0.5 TW,
is delivered globally, which is therefore about half of what we believe to
be the practical limit of ∼1 TW. Thus we might not expect more than a
factor-of-two expansion of current hydroelectricity as possible/practical.
The low-hanging fruit has been plucked already, capturing about half of
the total practical resource.
21: . . . reduced from 8 J since most rain falls
back onto ocean
The 90% efficiency of a hydroelectric dam
is now contextualized a bit better. That last
step is pretty efficient, but the overall process
is extremely inefficient. Still, it takes relatively
little effort to exploit, <strong>and</strong> provides
real power. Efficiency is not everything.
[67]: (1997), Study on the Importance of Harnessing
the Hydropower Resources of the World
22: ...ifcostisnobarrier
Compared to the 18 TW global scale of energy use, hydroelectricity
is not poised to assume a large share at this level, unless the overall
scale of energy use is reduced substantially. Let’s say this more visibly:
hydroelectric power cannot possibly come close to satisfying present
global power dem<strong>and</strong>.
11.3 Hydropower in the U.S.
Hydroelectric power is not available to the same degree everywhere.
Geography <strong>and</strong> rainfall are key factors. This brief section serves to
present a snapshot of the distribution <strong>and</strong> qualities of hydroelectric
power generation in the United States. We start with Figure 11.5, showing
the average hydroelectric power generated in each state, the top four
states being listed in Table 11.1. These four states account for 56% of
hydroelectricity in the U.S., <strong>and</strong> the next states on the ranked list drop
to 1 GW or lower. Most of the California generation is in the northern
part of the state, effectively as part of the Pacific Northwest region.
To get a sense for how concentrated different sources are, we will make
a habit of examining power density for renewable resource implementations.
Figure 11.6 indicates the state-by-state density of hydroelectric
power generation, 23 just dividing generation by state area. No state
exceeds 0.05 W/m 2 , which can be contrasted to insolation values (see
Ex. 10.3.1; p. 167) of∼200 W/m 2 . Globally, total l<strong>and</strong> area is about
Table 11.1: Top hydroelectric states.
State
Production (GW)
Washington 8.9
Oregon 3.8
California 3.0
New York 2.9
U.S. Total 33
23: . . . based on actual generation, not installed
capacity
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.