Environmental Engineering Reference
In-Depth Information
3
CHAPTER
Thermodynamic Principles
of Energy Conversion
3.1
INTRODUCTION
The development of the steam engine, an invention that powered the first two centuries of the
industrial revolution, preceded the discovery of the scientific principle involved—namely, the
production of mechanical work in a device that utilizes the combustion of fuel in air. The scientific
understanding that explains the production of work in different kinds of combustion engines is
derived from the laws of thermodynamics, developed in the nineteenth century. In the twentieth
century these principles aided the development of engines other than the steam engine, such as the
reciprocating gasoline and diesel engines, the gas turbine, and the fuel cell. With the aid of this
scientific hindsight, in this chapter we will review how the laws of thermodynamics determine the
functioning of these sources of mechanical energy, and especially how they limit the amount of
mechanical work that can be generated from the burning of a given amount of fuel.
The source of mechanical power developed from the combustion of fossil fuels or the fission
of nuclear fuel in an engine is the energy released by the change in molecular or nuclear compo-
sition. This released energy is never lost but is transformed into other forms, appearing as some
combination of mechanical or electric energy, internal energy of the molecular or nuclear products
of the reactions, or energy changes external to the engine caused by heat transfer from it. This
conservation of energy is explicitly expressed by the first law of thermodynamics.
The principle of energy conservation, or first law, places an upper limit on the conversion of
chemical or nuclear energy to mechanical work; that is, the work of an engine cannot exceed the
energy available. Experience shows that the work is very significantly less than the energy released
by fuel reactions in engines, a matter having important practical consequences. The scientific
principle that explains why, and by how much, there is a work shortfall is called the second law
of thermodynamics. In combination with the first law, it enables us to understand the limits to
producing work from fuel and to improve the engines that have been invented to accomplish
this task.
The laws of thermodynamics cannot substitute for invention, but they do inform us of the
performance limits of a perfected invention. In this chapter we review the principles of operation of
the major inventions that transform chemical to mechanical energy: the steam engine, the gasoline
and diesel engines, the gas turbine, and the fuel cell. In each case we show that the laws of
thermodynamics provide limits on how much of the fuel's energy can be converted to work and
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