New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Measures of energy and power in terms of solar input
A. P. FRENCH
This short note is based on an article by A. Rose entitled ‘A Global View of Solar Energy in
Rational Units’.’ Rose points out that most measures of energy are presented in units that have
little or no meaning even for the technically trained reader, He proposes a simple scale based on
the solar unit - a unit of power: ‘1 solar unit equals the solar power striking the earth, averaged
throughout the day and throughout the year’.
The solar unit is elastic in the following sense: if applied to the whole earth, it is the average
solar power striking the whole earth’s surface. If applied to just a part of the earth (e.g. the
United States) it equals the average solar power falling on that part of the earth. The world
average of one solar unit is about 200 watts per square metre.2
The generation and use of power by humans is added to the solar input. This will raise the
mean temperature of the earth, or of particular portions of the earth, because the mean temperature
is the result of a balance between the energy being received or generated at the earth’s surface
and the energy being radiated away into space. Actually these man-produced changes of mean
temperature are very tiny - it would be disastrous if it were otherwise - but the solar input still
provides a very convenient measuring stick with which other energy sources or consumptions can
be compared.
The present mean temperature of the earth is close to the freezing point of water. This is just
about what one would expect theoretically from the balance between radiation loss and one
solar unit of input. Now suppose that world energy generation added a second solar unit of input.
What would happen to the mean temperature? The answer is that it would rise close to the
boiling point of water, and almost all life would be eliminated. What if world energy generation
were one tenth of a solar unit? This would produce a world-wide temperature rise of about
8°C. Regarded simply as a temperature rise, this would not be disatrous for life in general; it
would simply imply a tropical climate for most regions. But it would be intolerable because the
1. A. Rose, ‘A Global View of Solar Energy in Rational Units’, Physica Status Solidii (A). Applied Research, Vol. 56, 1979,
p. 11-26.
2. The solar power input at the earth’s surface is less than that measured by spacecraft in orbit above the earth’s atmosphere
which plays a major role in the reflection, absorption and reemission of the radiation.
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