SOURCES OF ENERGY
INTRODUCTION
Staff Contribution
Global energy requirements are supplied primarily by fossil fuels, nuclear
fuels, and hydroelectric sources; about 1 to 2 percent of global
requirements are supplied from other miscellaneous sources. In the
United States in 1994, total domestic power requirements were supplied
approximately as follows: 70 percent fossil fuel (of which coal accounted
for 58 percent), 20 percent nuclear fuel, 10 percent from hydroelectric
sources, and less than 1 percent from all other sources. In spite
of the large increase in nuclear generated power, both in the United
States and globally, coal continues to be the major fuel consumed.
In the United States, new power plants constructed at this time are
designed to consume fossil fuels—primarily coal, many with gas, and a
few with petroleum. The situation with regard to nuclear power is complicated
by a number of circumstances; see Sec. 9.8. Energy statistics
and accompanying data stay current for a short time. Bear in mind that
when quantities of known reserves of fuel of all types are stated, there is
implied the significant matter of whether they are, indeed, producible in
a given economic climate. Estimates for additional reserves remaining
to be discovered are available, by and large, only for the United States.
At any given time, the situation with regard to estimates of recoverable
fuel sources is subject to wide swings whose source is manifold: national
and international politics, environmental concerns, significant
progress in energy conservation, unsettled political and social conditions
in locations within which reside much of the world reserves of
fossil fuels, economic impact of financing, effects of inflation, and so
on. The references cited, in their most current form, will provide the
reader with realistic and authoritative compilations of data.
Fossil Fuels
Petroleum Proved reserves of crude oil and natural gas liquids in
the United States, based upon estimated discovered quantities which
geological and engineering data demonstrate with reasonable certainty
to be recoverable in future years from presently known reservoirs under
existing economic and operating conditions, are published annually by
the American Petroleum Institute. Estimates of additional remaining
producible reserves which will be discovered, proved, and produced in
the future from the total original oil in place, are derived by U.S. Geol.
Surv. Circ. 725 from present and projected conditions in the industry.
Estimates of proved crude oil reserves in all countries of the world are
published by Oil and Gas Journal. New discoveries are continually
adding to and changing proved reserves in many parts of the world, and
these estimates are indicative of producible quantities.
Natural Gas Proved reserves of natural gas in the United States,
based upon the same definition as for crude oil and natural gas liquids,
are estimated annually by the American Gas Association. The estimated
total additional potential supply remaining to be discovered is prepared
by the Potential Gas Committee, sponsored by the Potential Gas
Agency, Colorado School of Mines Foundation, Inc.
Estimates of proved reserves of natural gas in all countries of the
world are published by Oil and Gas Journal. As with crude oil, large
additional natural gas reserves are currently being discovered and developed
in Alaska, the arctic regions, offshore areas, northern Africa,
and other locations remote from consuming markets. Valid estimates of
additional probable remaining reserves in the world are not available.
Coal (See also Secs. 7.1 and 7.2.) Authoritative information about
reserves of coal is presented in Geol. Surv. Bull. 1412, Coal Resources
of the United States. Remaining U.S. proved reserves (1974) of bituminous,
subbituminous, lignite, and anthracite have been estimated by
mapping and exploration of areas with 0 to 3,000-ft overburden. The
U.S. Geological Survey (USGS) estimates probable additional resources
in unmapped and unexplored areas with 0 to 3,000-ft overburden
and in areas with 3,000- to 6,000-ft overburden. Slightly more than
one-half of the proved reserves are considered producible (at this time)
because of favorable depth of overburden and thickness of coal strata.
Approximately 30 percent of all ranks of coal are commerically available
in beds less than 1,000 ft deep. The USGS estimates that about 65
percent contains less than 1 percent sulfur; most of the low-sulfur coals
are located west of the Mississippi. USGS Bull. 1412 also estimates
global coal resources, but in view of the questionable validity of much
of the global data, it can but offer gross approximations. (See Sec. 7.1.)
Shale Oil The portion of total U.S. reserves of oil from oil shale,
measured or proved, considered minable and amenable to processing is
estimated to be over 150 billion bbl (30 billion m3), based upon grades
averaging 30 gal/ton in beds at least 100 ft thick (USGS Bull. 1412).
Most oil shale occurs in Colorado. No commercial production is expected
for many years. World reserves occur largely in the United States
and Brazil, with small quantities elsewhere.
Tar Sands Large deposits are in the Athabasca area of northern
Alberta, Canada, estimated capable of producing 100 to 300 billion bbl
(15.9 to 47.7 billion m3) of oil. About 6.3 billion bbl (1.0 billion m3) has
been proved economically recoverable within the radius of the present
large mining and recovery plant in Athabasca. Commercial quantities of
oil have been produced there since the 1960s. Sizable deposits are located elsewhere; they have not been exploited to date, meaningful data
for them are not available, and there is no report of those other deposits
having been worked. (See Sec. 7.1.)
Nuclear Fuels
Uranium Reserves of uranium in the United States are reported by
the Department of Energy (DOE). The proved reserves, usually
presented in terms of quantity of U3O8 , refer to ore deposits (concentrations
of 0.01 percent, or 0.0016 oz/lb ore, are viable) of grade, quantity,
and geological configuration that can be mined and processed profitably
with existing technology. Estimated additional resources refer to uranium
surmised to occur in unexplored extensions of known deposits or
in undiscovered deposits in known uranium districts, and which are
expected to be discovered and economically exploitable in the given
price range. The total of these uranium reserves would yield about 3,000
tons of U3O8 . United States uranium resources are located mainly in
New Mexico, Wyoming, and Colorado.
Thorium Total known resources of thorium, the availability of
which is considered reasonably assured, are estimated in the millions of
tons of thorium oxide. Additional actual reserves will increase in response
to the demand and concomitant market price. Most of the larger
known resources are in India and Brazil. There seems to be little prospect
of significant requirements for thorium as a nuclear fuel in the near
future.
Hydroelectric Power
Hydroelectric and Pumped Storage for Electric Generation Although
most available sites for economical production of hydroelectric
energy have been developed, some additional hydroelectric capacity
will be provided at new sites or by additions at existing plants. Increased
pumped storage capacity will be limited by the availability of suitable
sites and a dependable supply of economical pumping energy. The flexibility
of operation of a pumped storage plant in meeting sudden load
changes and its ability to provide high inertia spinning reserve at low
operating cost are additional benefits that can weigh heavily in favor of
this type of installation, particularly in the future if (when) the proportion
of nuclear capacity in service increases. At this time, hydro and
pumped storage account for about 10 percent of electricity generated by
all sources of energy in the United States.
World installed hydropower capacity presently is located about 40
percent in North America and 40 percent in Europe.
ALTERNATIVE ENERGY, RENEWABLE ENERGY,
AND ENERGY CONVERSION:
AN INTRODUCTION
Staff Contribution
REFERENCES: AAAS, Science. Hottell and Howard, ‘‘New Energy Technology
—Some Facts and Assessments,’’ MIT Press. Fisher, ‘‘Energy Crises in Perspective,’’
Wiley-Interscience. Hammond, Metz, and Maugh, ‘‘Energy and the Future,’’
AAAS.
Many sources of raw energy have been proposed or used for the generation
of power. Only a few sources—fossil fuels, nuclear fission, and
elevated water—are dominant in practical applications today.
A more complete list of sources would include fossil fuels (coal,
petroleum, natural gas); nuclear (fission and fusion); wood and vegetation;
elevated water supply; solar; winds; tides; waves; geothermal;
muscles (human, animal); industrial, agricultural, and domestic wastes;
atmospheric electricity; oceanic thermal gradients; oceanic currents.
There are others.
Historically, wood, muscles, elevated water, and wind were prominent.
These sources were superseded in the industrial era by fossil fuels,
with nuclear energy the most recent addition. This dominance rests in
the suitability of the thermal sources for practical stationary and transportation
power plants. Features of acceptability include reliability,
flexibility, portability, maneuverability, size, bulk, weight, efficiency,
economy, maintenance, and costs. The plant for transportation service
must be self-contained. For stationary service there is wider latitude for
choice.
The dominant end product, especially for stationary applications, is
electricity, because of its favorable distribution and control features.
However, there is no practical way of storing electric energy. Electricity
must be generated at the instant of its use. Reliability and continuity of
service consequently dictate the need for reserve, alternate, and interconnection
supports. Pumped storage, coal piles, and tanks of liquid and
gaseous fuels, e.g., offer the necessary continuity, flexibility, and reliability.
Raw energy sources, other than fuels (fossil and nuclear) and elevated
water, are particularly deficient in this storage aspect. For example,
wind power is best for jobs that can wait for the wind, e.g., pumping
water or grinding grain. Solar power, to avoid foul weather and
the darkness of night, could call for desert locations or extraterrestrial
satellites.
Despite such limitations an energy-intensive society can expect to see
increasing efforts to harness many of the raw energy sources cited.
Several of these topics are treated in the following pages to show the
factual and technical progress that has been made to adapt sources to
practicality.
MUSCLE-GENERATED POWER
by Ezra S. Krendel, Amended by Staff
REFERENCES: Whitt and Wilson, ‘‘Bicycling Science,’’ 2d ed., MIT Press.
Harrison, Maximizing Human Power Output by Suitable Selection of Motion
Cycle and Load, Human Factors, 12, 1970. Krendel, Design Requirements for
Man-Generated Power, Ergonomics, 3, 1960. Wilkie, Man as a Source of Mechanical
Power, Ergonomics, 3, 1960. Brody, ‘‘Bioenergetics and Growth,’’
Reinhold.
The use of human muscles to generate work will be examined from two
points of view. The first is that of measuring the energy expended in
gross, long duration physical activities such as marching, forestry work,
freight handling, and factory work. The second is that of determining
the useful mechanical work which can be performed by specified muscle
groups for brief or extended periods of time in well defined work
situations, such as pedaling or cranking.
Labor
Over an 8-h day for a 48-h week, a useful norm for a 35-year-old laborer
for total power expenditure, including basal metabolism energy, is
0.49 hp (366 W). Of this total expenditure, approximately 0.1 hp
(75 W) is available for useful work. A 20-year-old man can generate
about 15 percent more power than this norm, and a 60-year-old man
about 20 percent less. The total energy or power expenditure is needed
for determining nutritional requirements for classes of labor. A rule of
thumb for power developed by European males can be expressed as a
function of age and duration of effort in minutes for work lasting from 4
to about 480 min, assuming that 20 percent of the total output is useful
power.
For pedaling efforts of from 1 to about 100 min, the useful power
generated may be expressed as hp 5 0.53 2 0.13 log t (t is in minutes).
Work scheduling, either as rhythmic work activity or with rest stops
for recuperation, the temperature and humidity of the environment, and
the detailed nature of the laborer’s diet are factors which influence
ability to generate and maintain the above nominal power values. These
considerations should be factored in for specific work situations.
Steady State and Transient
When a human and a passive mechanism are working together to generate
power, the following conditions obtain: Energy is available both
from stores residing in the muscles [a total usefully available energy of
about 0.6 hp?min (27 kJ), usually applied in transient bursts of activity]
and from the oxidation of foods (for producing steady state power). For
an aerobic transient activity, energy production depends on the mass of
muscle which can be brought into effective contact with the power
transmission mechanism. For example, bicycle pedaling is an effective
use of a large muscle mass. For steady-state activity, assuming adequate
food for fuel energy, power generated depends on the oxygen supply
and the efficiency with which oxygenated blood can be transported to
the muscles as well as on the muscle mass.
The physiological limit, determined by oxygen-respiration capacity,
for steady-state useful mechanical power generation is between 0.4 and
0.54 hp (300 and 400 W), depending on the man’s physical condition.
Useful power production may be achieved by such methods as rowing,
cranking, or pedaling. The highest values of human-generated
horsepower using robust subjects have been achieved using a rowing
assembly which restrained nonuseful motions of the torso and major
limbs. Under these conditions up to 2 hp (1,500 W) was generated over
intervals of 0.6 s, and averages of about 1 hp (750 W) were generated
over 2 min.
In order to approach an optimal conversion efficiency (mechanical
work/food energy) of 25 percent, a mechanism would be required to
store and to transmit energy from the body muscle masses when they
were operating at optimal efficiency. This condition occurs when the
force exerted by the muscle is about one-half of its maximum and the
speed of muscle movement one-quarter of its maximum. Data on both
force and speed for a given set of muscles are best measured in situ.
Optimal conversion efficiency and maximum output power do not occur
together.
Examples of High Output
Data for human-generated power come from measurements of subjects
with different kinds of training, skill, body builds, diets, and motivation
using a variety of mechanical devices such as bicycles, ergonometers,
and variations on rowing machines. For strong, healthy young men,
aggregations of such data for power produced in an interval t of 10 to
120 s can be approximated as follows:
hp 5 2.5t20.40
For world-class athletes this becomes hp 5 0.25 1 2.5t20.40. These
values can be exceeded for bursts of power of less than 10 s. For longterm
efforts of from 2 to about 200 min, the aggregated data for useful
power generated by strong, healthy young men can be approximated as
follows (t in minutes):
hp 5 0.50 2 0.13 log t
For world-class athletes this becomes hp 5 0.65 2 0.13 log t. The pilot
of the Gossamer Albatross, who flew 22 mi from England to France in
2 h 55 min on August 12, 1979 entirely by pedal-generated power,
sustained an output of about 1⁄3 hp (250 W) during the flight.
Maximum power output occurs at a load impedance of 5 to 10 times
the size of the human being’s source impedance.
Brody has developed detailed nomograms for determining the energetic
cost of muscular work by farm animals; these nomograms are
useful for precise cost-effectiveness comparisons between animal and
mechanical power generation methods. A 1,500- to 1,900-lb horse can
work continuously for up to 10 h/day at a rate of 1 hp, or equivalently
pull 10 percent of its body weight for a total of 20 mi/day, and retain its
vigor to an advanced age. Brody’s work allows the following approximations
for estimating the useful power output of work animals of
varying sizes: The ratio of the power exerted in maximal energy production
for a few seconds to the maximum steady-state power maintained
for 5 to 30 min to the power produced in sustained heavy work
over a 6- to 10-h day is approximately 25 : 4 : 1. For any one of these
conditions, it has been found that, for healthy, mature specimens,
hpanimal 5 hpman(mass of animal/mass of man)0.73
Thus, from the previously given horsepower magnitudes for men, one
can compute the power generated by ponies, horses, bullocks, or elephants
under the specified working conditions.

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