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

Energy degradation
Finally, we note that the description of the life cycles (see above) driven by the dissipation of
sunlight could be further simplified by assigning a value to the different energy forms instead of
considering the capabilities of some processes to reverse other processes. For example, photosynthesis
could be described, by analogy with the heat engine, as upgrading heat radiation to
chemical energy to be stored in plants. The emission of heat to the surroundings by the plants
(through evaporation etc.) may be considered as the corresponding energy degradation, necessary
to compensate that upgrading. (The far-reaching analogy between the green leaf and a heat engine
has been worked out by Schlichting et a1 [ 41 .)
QUANTITATIVE ASPECTS OF ENERGY DEGRADATION (ENTROPY)
On the connection be tween energy degradation and entropy
The explanations above have shown that the qualitative concept of degradation is able to describe
a multitude of phenomena from a single viewpoint and to contribute to a deeper understanding
of energy problems.
If, in addition, one wishes to estimate certain degradations quantitatively and to make
measurements, it is necessary to quantify the concept.
As a description of an appropriate measuring procedure does not contribute significantly new
aspects, we shall confine ourselves to a rough sketch showing how the qualitative aspects of
degradation may be related in an intuitive way to the quantitative concept of entropy (see
Backhaus et al, [7, 81 ). The starting point for the quantification is the fact, described above, that
the overall degradation is the smaller the-more strongly one spontaneous process is harnessed
to drive another backwards.
To be more precise, we must first demonstrate the fact implicit in the notion of degradation
that the energy degradation increases with increasing energy consumption. If two otherwise
equal processes differ in the quantity of dissipated energy (e.g. the cooling of two different
masses of water from, say, 100°C to the temperature of the surroundings), the degradation of
energy increases as the quantity of dissipated energy increases.
If we assume that energy degradations are additive we may state:
The energy degradation of a process is proportional to
the energy degraded. (6)
On the other hand, the different temperatures of the systems involved in the process have to
be taken into account. Considering, for example, the ‘cooling of hot water in cold surroundings’,
it is obvious that the degradation depends intimately on the temperature 8 of the water and the
temperature O2 (< 8 ) of the surroundings; and we may write
This may be shown as follows:
(a) The higher the temperature 8 of the hot water and
(b) the lower the temperature O2 of the surroundings
the greater the down-grading of the energy. (7)
(a) Let a be the process ‘cooling down of a body at temperature 8 to temperature O2 by the
transfer of energy to the surroundings’. The process 0 differs from a only in that the body has a
temperature 8 < 8 1. a may be used to drive 0 in reverse, As a result of cooling one body from
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