Energy and Human Ambitions on a Finite Planet, 2021a

11 Hydroelectric <strong>Energy</strong> 181
◮ Natural source, solar-driven, without waste products or pollution;
29
◮ Technologically simple, therefore straightforward to implement
<strong>and</strong> maintain;
◮ High efficiency, turning 90% of available energy into useful electricity;
◮ Good baseline of steady power over daily timescales; 30
◮ Life-cycle CO 2 emissions only 4% that of traditional fossil fuel
electricity [68];
◮ Facilities 31 can last a century or longer;
◮ While not directly related to energy, dams can provide flood
control <strong>and</strong> reliable water supplies.
29: . . . aside from construction <strong>and</strong> decommissioning
aspects
30: ...noimposed short-term fluctuations
in available power, as happens for solar or
wind
31: ...thedamitself, at least; turbines <strong>and</strong>
generators will need periodic replacement
And some of the downsides that may discourage further development:
◮ Silt can build up behind dams displacing the reservoir, eventually
rendering them useless <strong>and</strong> dangerous;
◮ Requires the semi-permanent flooding of an ecological habitat, of
varying quality <strong>and</strong> value;
◮ Seasonal variability of available power, often by a factor of ten;
◮ Defunct or poorly maintained facilities represent a dam-burst
danger to downstream residents;
◮ Blocks salmon runs <strong>and</strong> impacts the health of both oceanic <strong>and</strong>
forest ecosystems;
◮ As the distribution maps show, hydroelectric is not a viable option
everywhere: the combination of terrain 32 <strong>and</strong> rainfall is needed.
On balance, our society has embraced hydroelectricity as a clean <strong>and</strong>
reliable resource. One can view it as nature’s low-hanging fruit, partly
evidenced by how early it was adopted on a large scale. It is likely to
remain an attractive form of energy as we face increasing pressures to
migrate away from carbon-based fuels. 33 It will not, however, be able
to provide an avenue for wholesale replacement of fossil fuels given its
limited scale <strong>and</strong> electric-only nature. Only if our overall energy dem<strong>and</strong>
is reduced substantially will it form a large fraction of our portfolio.
32: . . . mountains or canyons to hold the
reservoir
33: . . . whether due to resource limits or
climate change action
11.6 Problems
1. If a 70 kg person climbs 10 flights of stairs, each flight 3 m high,
how much potential energy have they gained?
2. If an 80 kg person is capable of delivering external mechanical
energy at a rate of 200 W sustained over several minutes, 34 how
high would they be able to climb in two minutes?
3. A 10 kg box is lifted 2 m off the floor <strong>and</strong> placed on a frictionless
horizontal conveyor to take it 30 m across a warehouse. At the end
of the conveyor, it is lowered 1 m where it ends up on a shelf. 35
34: It is hard to keep up 200 W for too long.
35: The shelf is therefore 1 m off the same
(original) floor.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.