635
33
Energy from the Sun
A list of criteria for the ideal energy source would likely include these: It should be
environmentally friendly, causing little or no harm to the environment; it should
be inexhaustible, or least renewable, eliminating worry about issues of reserves,
resources, and lifetimes; and it should be domestic, eliminating worry about geo-
politics and possible import restrictions.
Hydroelectricity has many advantages (such as no “fuel” costs and the fact that it is
essentially pollution free). While the actual electricity generation itself—water ow-
ing through a turbine connected to a generator—is very clean, the impact of dams on
ecosystems is now of great concern. Wind energy shares many of the advantages of
hydro and is becoming an increasingly cost-effective and reliable source of energy.
Yet another energy source meets the criteria set out above: solar energy. Solar energy
shares many of the advantages of hydropower: there is no cost for the “fuel,” it is cer-
tainly inexhaustible, and there is virtually no pollution. In addition, solar energy can
be used in any area of abundant sunshine, whereas hydropower is restricted to sites
where there are water sources. It can be used by any country or any region, eliminat-
ing many of the geopolitical issues concerned with unevenly distributed resources
such as petroleum.
The suns energy derives from the conversion of mass to energy according to
Einsteins equation, E = mc
2
(Chapter 23). We will defer a discussion of why this
equation applies until the next chapter. The mass loss associated with the nuclear
processes in the sun amounts to about four million tonnes per second. Since c
2
is
itself a huge number, the energy production per second in the sun is so large that we
have no way of translating it into our normal experience. The sun radiates this energy
into all directions in space (Figure 33.1). Since Earth is a tiny “dot” 150 million
kilometers from the sun, virtually all of the energy produced by the sun misses the
Earth. In fact, the fraction of the suns energy hitting the Earth is only one two-
billionth (1/2,000,000,000th!) of the suns total output.
This extremely tiny fraction would not seem worth bothering with. And yet, almost
all the  that we use derives in some fashion from the sun. Growing plants
require sunlight to provide the  for photosynthesis. Our use of food, and of
rewood or other biomass products for , depends directly on the sunlight-
driven photosynthesis process. Fossil fuels derive from the remains of once-living
organisms preserved in the Earths crust; those organisms once required sunlight
and photosynthesis. Water-driving waterwheels or hydroelectric turbines are sup-
plied by Earths hydrological cycle, driven by the  in sunlight. Wind results
from differences in the amount of heating of Earths atmosphere by the sun. Only the
 provided by nuclear ssion reactors, and the comparatively tiny amount of
 derived from chemical processes in batteries do not derive, directly or indi-
rectly, from the  of the sun. Furthermore, if we could collect completely this

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