Energy and Human Ambitions on a Finite Planet, 2021a

12 Wind <strong>Energy</strong> 185
Often, we use energy sources to deliver kinetic energy, as in moving
planes, trains <strong>and</strong> automobiles.
Example 12.1.2 A 1,500 kg car moving at freeway speeds (30 m/s)
has a kinetic energy around 675 kJ.
Getting up to this speed from rest in 5 seconds would require a power
of 135 kW, 1 equating to 180 horsepower. 2
But we can also go the other direction <strong>and</strong> convert kinetic energy into
different forms of energy 3 for versatile use. Most commonly, we turn
kinetic energy into electrical potential energy (a voltage) that can drive a
circuit. At this point, the energy can be used to toast a bagel, charge a
phone, or wash clothes.
The method for converting kinetic energy into electricity is usually
accomplished by transferring kinetic energy in a moving fluid 4 into
rotation of a shaft by way of a turbine—essentially fan blades. The
spinning shaft then turns an electric generator, which consists of the
relative motion between magnets <strong>and</strong> coils of wire, <strong>and</strong> is essentially
the same construction/concept as an electric motor run in reverse.
1: Do the calculation yourself to follow
along.
2: Recall that 1 hp is 746 W. Indeed, it takes
a powerful engine to provide this level of
acceleration.
3: See Table 5.2 (p. 70), for examples.
4: In this sense, “fluid” is a general term
that can mean a liquid or even air.
Hydroelectric installations do the same thing—turning a shaft via blades
of a turbine—even though we framed the energy source as one of
gravitational potential energy. Within the dam’s turbine, the water
acquires kinetic energy as it flows from the reservoir to the outlet.
Wind energy acts in much the same way, converting kinetic energy in
the moving air into rotational motion of a fan/turbine whose shaft is
connected to a generator located behind the blades.
12.2 Wind <strong>Energy</strong>
It is tempting to think of air as “empty” space, but at sea level air has
a density of 1.25 kg per cubic meter (ρ air ≈ 1.25 kg/m 3 ). Let this sink
in visually: imagine a cubic meter sitting next to you (as in Figure 12.1).
The air within has a mass of 1.25 kg (about 2.75 lb). Now draw a square
meter on the ground—either literally or in your imagination. The many
kilometers of air extending vertically over the top of that square meter
has a mass of ∼10,000 kg! For context, figure out how many cars that
would be (typ. 1,500 kg ea.), or what kind of animal would be this
massive.
What this means is that air in motion can carry a significant amount of
kinetic energy, since neither its mass nor velocity are zero. If the entire
earth’s atmosphere moved at 5 m/s—a noticeable breeze—at a total
mass 5 of 5 × 10 18 kg,we’dhave6 × 10 19 J of kinetic energy in air currents.
If we somehow pulled all this energy out of the air—stopping its motion
entirely—we might expect the atmosphere to revive its normal wind
1 m 3
1.25 kg
atmos.
~10,000 kg
1 m 2
Figure 12.1: The mass of a cubic meter of
air is 1.25 kg, <strong>and</strong> the mass of atmosphere
over one square meter is an astounding 10
metric tons.
5: 10 4 kg per square meter times the surface
area of Earth (4πR 2 ⊕ )
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.