Energy

15.1 INTRODUCTION

Human societies have always required energy—for cooking and heating, for transportation, and for industrial activities. For most of human existence, wood has provided the main fuel for cooking, heating, and such high-temperature techniques as pottery and metallurgy. Human and animal power have provided the mechanical energy, supplemented from some ancient time by wind and water power as windmills and water wheels were developed. But it was only with the invention of the steam engine and later the internal combustion engine and electrical generators, all with a need for an energy supply, that power for technological development became freely available. Transportation similarly was by muscle or wind power until these other power sources were developed. Use of wood as a major energy source resulted in deforestation and large environmental changes when population density became large; some third-world nations have serious problems today from demands for wood as cooking fuel that outstrip regeneration. For other uses, wood is a relatively inefficient and variable fuel. Even primitive technologies attempted to improve on it by the use of charcoal: partially burned wood from which the volatile components have been driven off and the solid converted to nearly pure carbon.

Industrialization required the use of a superior fuel, coal, which became the main fuel for industrial and mechanized transportation applications. Coal is abundant, but much less convenient than liquid fuels such as petroleum. Widespread availability of the latter permitted it to replace coal in most transportation and some stationary power generation and heating applications by the mid-twentieth century. More recently, natural gas has been playing a larger role. After 1945, nuclear energy from fission began to be considered as a major component of power generation, and in some nations (e.g., France) fills that role today, although in other nations public concern about safety and general fear of radiation make its future questionable. Still more recently, attention has shifted to liquid fuels that are readily prepared from biological sources (e.g., alcohols), mainly for environmental reasons.

All these sources of energy are finite, and in the case of petroleum in particular, reserves are projected to have limited lifetimes at present rates of use, although predictions of when we will exhaust them have repeatedly been shown to be wrong as more reserves have been discovered. The need to conserve these limited resources is one reason to use them with more efficiency and to find alternative, preferably renewable sources. Another reason, and one that has driven a great deal of research and development in recent years, has been the realization that availability of much of these resources is controlled by a small number of nations, which consequently wield considerable power to influence world events. This was brought out in the "oil crisis" of the early 1970s, when the large Middle East producers cut production to drive up prices. Not only did prices for petroleum products and materials dependent on petroleum for their production go up, but severe shortages developed that resulted in restrictions on driving and, in some places, gasoline being available only on alternate days of the week. Still another reason to look for alternatives to the traditional fossil fuels is concern about environmental degradation that accompanies their use, as we have discussed many times. Human need for energy has had and will have major impacts on the environment.

Much of our energy is based on combustion of carbon-based fuels. The energy released upon combustion is essentially the difference between the energy of the bonds that are formed in the product molecules (C=0 and O—H) and those in the reactants that are broken (0=0 in dioxygen, and C—C, C—H, C—O, etc., depending on the fuel). Some illustrative values of heats of combustion (heating values) are given in Table 15-1 for equal masses of material. Wood and some coals can be highly variable because of moisture content. Note that heating values for oxygenated fuels are lower than for others because some of the bonds that appear in the products (O—H) are already present in the reactants.

TABLE 15-1

Heating Value for Various Fuels

TABLE 15-1

Heating Value for Various Fuels

Fuel

Heating value (kj/g)

Methane

55.6

Propane

50.3

«-Butane

49.5

Gasoline, kerosene

41-48

Fuel oil

44.4-49.4

Coal

anthracite

32.5

Bituminous

25.5-34.8

Lignite

11.6-17.4

Wood (12% moisture)

17.5-18.3

Methanol

22.7

Ethanol

29.7

In this chapter, we shall discuss some thermodynamic considerations underlying efficiency, and then consider the various sources of energy that are in use or that are reasonable proposals for future use. Not all these are chemical, but we shall include them for completeness. We shall also comment on some predictions for future energy needs.

15.2 THERMODYNAMIC CONSIDERATIONS

15.2.1 The First and Second Laws of Thermodynamics

Energy flow and energy use are governed by the law of conservation of energy, which states that energy can neither be created nor destroyed, although it can be changed from one form to another. This law appears sometimes as the first law of thermodynamics:

where AU is the change in internal energy of the system being studied when heat q flows in or out, and work w is done on or by the system. The system being studied can be any part of the universe—a person, a city, a forest, a gasoline engine, a nuclear power plant—within certain limits that will not be discussed further here.1 All the rest of the universe is called the surroundings. When AU is positive, the internal energy of the system is increasing; when it

1For rigorous application of the laws of thermodynamics, the system in question must have either isothermal or adiabatic boundaries at any time.

is negative, the internal energy of the system is decreasing. When q is positive, heat is being absorbed by the system from the surroundings; when it is negative, heat is being evolved by the system to the surroundings. When w is positive, work is being done on the system by the surroundings; when it is negative, work is being done by the system on the surroundings.

There are many forms of work. For example, mechanical work can be done on the system by compressing it, or the system can do mechanical work on the surroundings by expanding against a resisting pressure. Electrical work can be done on a charged particle by moving it through a potential difference. Most of the time we will be concerned here only with the mechanical work of expansion or compression of a system from volume Vj to Vf against a resisting pressure P, in which case

If the resisting pressure P is constant, w = —PAV, where AV ( = Vf — Vj) is the change in volume of the system when it expands (AV > 0, w < 0) or is compressed (AV < 0, w > 0).

We shall be discussing the heat released during various chemical reactions—for example, the combustion of coal or oil. The enthalpy H of the system is defined as H = U + PV, so that the change in enthalpy for a process is AH = AU + A(PV). If we consider only the mechanical work of expansion or compression, it is easily shown from equation (15-2) that at constant pressure the heat absorbed for a process is equal to the change in enthalpy:

A heat engine is a device that generates mechanical work from heat. An example is the automobile internal combustion engine, in which chemical energy is released as heat at about 1000 K when gasoline is burned in the combustion chamber above the pistons. Some of this heat appears as work as the pistons move and drive the transmission, generator, and so on through a complex series of linkages, and much heat is "rejected," some at the temperature of the exhaust, and some at the temperature of the cooling system. When a car is cruising down a highway at about 50mph (80km/h), about 75% of the chemical energy is lost as "rejected" heat, and only about 25% of the chemical energy is eventually converted to useful mechanical work to move the automobile. The automobile internal combustion engine operates through a six-process cycle called the Otto cycle.2 In the following discussion, however, we

2M. W. Zemansky and R. H. Dittman, Heat and Thermodynamjcs: An Intermedjate Textbook, 7th ed., McGraw-Hill, New York, 1997.

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