Energy and Human Ambitions on a Finite Planet, 2021a

12 Wind <strong>Energy</strong> 194
◮ Harnessing wind is relatively low-tech <strong>and</strong> straightforward;
◮ Wind has decent efficiency—typically 40–50%—in extracting energy
from the oncoming wind;
◮ Life-cycle CO 2 emissions for wind is only 2% that of traditional
fossil fuel electricity [68];
◮ Growth in the wind sector points to economic viability;
◮ Wind is able to scale up to cover a meaningful fraction of energy
dem<strong>and</strong>.
[68]: (2020), Life Cycle GHG Emissions
And the downsides:
◮ Wind is intermittent: power when nature allows, not when people
dem<strong>and</strong>;
◮ Wind is regionally variable: many places do not produce enough
wind to support development;
◮ Wind can cause environmental disruption to habitats—especially
dangerous to birds <strong>and</strong> bats;
◮ Esthetic objections to noise <strong>and</strong> degradation of scenery hamper
expansion.
12.5 Problems
1. A modest slap 32 might consist of about 1 kg of mass moving at 32: ...howpainful can a few Joules be?
2 m/s. How much kinetic energy is this?
2. A hard slap might consist of about 1 kg of mass moving at 10 m/s.
How much kinetic energy is this, <strong>and</strong> how much warmer would
10 g of skin 33 33: . . . corresponding to a volume of 10 mL
get if the skin has the heat capacity properties of appropriate to a slap area of 10 cm by 10 cm
water, as in the definition of a calorie (Sec. 5.5; p. 73 <strong>and</strong> Sec. 6.2; <strong>and</strong> to a depth of 1 mm
p. 85 are relevant)?
3. A 10 kg bowling ball falls from a height of 5 m. Using the convenient
g ≈ 10 m/s 2 , how much gravitational potential energy does it have?
Just before it hits the ground, all of this potential energy has gone
into kinetic energy. 34 What is the speed of the bowling ball when
it reaches the ground, based on kinetic energy?
4. Did the final answer for the speed of the bowling ball at the end of
its drop depend on the mass? 35 Write out the math symbolically 36
<strong>and</strong> solve for velocity, v. Does the result depend on mass?
5. Thermal energy is just r<strong>and</strong>omized kinetic energy on a microscopic
scale. To gain some insight into this, consider one liter (1 kg) of
water, <strong>and</strong> figure out how much energy it would take to heat it from
absolute zero temperature 37 to 300 K assuming that the definition
of the calorie (Sec. 5.5; p. 73) applies across this entire range. If
this same amount of energy went into kinetic energy—hurling the
water across the room—what would the corresponding velocity
be?
34: . . . neglecting any energy flow to air
resistance
35: Try it using a different mass.
36: . . . using variables/symbols
37: ...0K,when the kinetic energy is effectively
frozen out, or stopped
i As large as the number is, it is representative
of the speeds of individual molecules
within the water, <strong>and</strong> is, not coincidentally,
similar to the speed of sound in water.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.