T6. Available And Unavailable Energy

The consistency of Carnot’s form of the second law (version 5 in the foregoing Essay T5) with the general principle of entropy increase can best be appreciated by thinking in terms of order and disorder. The complete conversion of heat to work would represent a transformation away from disordered energy, a replacement of random molecular motion by orderly bulk motion. This violates the second law of thermodynamics. But a partial conversion of heat to work is possible because a small heat flow into a cool region may increase the entropy there by more than the decrease of entropy produced by a larger heat flow out of a hot region. At absolute zero, the hypothetically motionless molecules¹ have maximum order. Greater temperature produces greater disorder. Therefore heat flow into a region increases its entropy, heat flow out of a region decreases its entropy. Fortunately for the feasibility of heat engines, it takes less heat at low temperature than at high temperature to produce a given entropy change. To make an analogy, a pebble is enough to bring disorder to the smooth surface of a calm lake. To produce an equivalent increase in the disorder of an already rough sea requires a boulder.

The reverse transformation, total conversion of work to heat, is not only possible but is commonplace. Every time a moving object is brought to rest by friction, all of its ordered energy of bulk motion is converted to disordered energy of molecular motion. This is an entropy-increasing process allowed by the second law of thermodynamics. In general, the second law favors energy dissipation, the transformation of energy from available to unavailable form. Whenever we make a gain against the second law by increasing the order or the available energy in one part of a total system, we can be sure we have lost even more in another part of the system. Thanks to the constant input of energy from the Sun, the Earth remains a lively place and we have nothing to fear from the homogenizing effect of the second law.

In many applications of the second law of thermodynamics, the concept of available energy is the easiest key to understanding. In general, the trend of nature toward greater disorder is a trend toward less available energy. A jet plane before takeoff has a certain store of available energy in its fuel. While it is accelerating down the runway, a part of the energy expended is going into bulk kinetic energy (ordered energy), a part is going into heat that is eventually dissipated into unavailable energy. At constant cruising speed, all of the energy of the burning fuel goes to heat the air. Thermodynamically speaking, the net result of a flight is the total loss of the available energy originally present in the fuel. A rocket in free space operates with greater efficiency. Being free of air friction, it continues to accelerate as long as the fuel is burning. When its engine stops, a certain fraction (normally a small fraction) of the original available energy in the fuel remains available in the kinetic energy of the vehicle. This energy may be “stored” indefinitely in the orbital motion of the space vehicle. If it reenters the atmosphere, however, this energy too is transformed into the disordered and unavailable form of internal energy of the air. To get ready for the next launching, more rocket fuel must be manufactured. The energy expended in the chemical factory that does this job is inevitably more than the energy stored in the fuel that is produced.

In general the effect of civilization is to encourage the action of the second law of thermodynamics. Technology greatly accelerates the rate of increase of entropy in our immediate environment. Fortunately the available energy arriving each day from the Sun exceeds by a very large factor the energy degraded by human activity in a day. Fortunately too, nature, with no help from humankind, stores in usable form some of the Sun’s energy—for periods of months or years in the cycle of evaporation, precipitation, and drainage; for decades or centuries in lumber; for millennia in coal and oil. It is also humankind’s good fortune that as we deplete the long-term stored supply of available energy and better understand the deleterious effects of using it, we are learning how to rely more heavily on the short-term stores such as wind and on direct sunlight itself.

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