Advances in Energy Systems and Technology. Volume 1

389 Pages·1978·4.902 MB·English

by  Peter Auer (Auth.)

Advances in
Energy Systems and Technology
Volume 1
PETER AUER
Upson Hall
Cornell University
Ithaca, New York
ACADEMIC PRESS New York San Francisco London 1978
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1978, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
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List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions
begin.
Harry Perry (243), Resources for the Future, National Economic Research
Associates, Washington, D.C. 20036
David Pimentel (125), College of Agriculture and Life Sciences, Comstock
Hall, Cornell University, Ithaca, New York 14853
Vasel Roberts (175), Electric Power Research Institute, P.O. Box 10412,
Palo Alto, California 94304
D. G. Shepherd (1), Sibley School of Mechanical and Aerospace Engineer-
ing, Upson Hall, Cornell University, Ithaca, New York 14853
Richard H. Tourin (327), Stone & Webster Engineering Corporation, One
Penn Plaza, New York, New York 10001
Walter Vergara (125), Department of Agricultural Engineering, Riley-
Robb Hall, Cornell University, Ithaca, New York 14853
vii
Preface
The subject of energy is now highly topical. The very rapid growth in
the number of published articles and books on the subject provides clear
proof of this. Is there a need for more books on energy, one might well
ask. Our answer to this is a firm yes, for it is our expectation that the serial
publication we are embarking on will serve a unique purpose.
The chapters appearing in this volume, and in each subsequent volume,
are intended to furnish detailed critical reviews of specific topics within
the general field of energy. They address largely technological issues, or
issues in a somewhat broader systems context, which in turn are closely
related to technological issues. We intend to have each article provide a
breadth of coverage greater than that generally found in review articles
prepared for journal publication or a standard review series, yet less than
what may be expected from a textbook devoted to the same subject.
Thus, each of the articles appearing here should serve as a valuable refer-
ence work for an extended period of time.
The publication is addressed both to the serious student and research
investigator engaged in some aspect of energy study, as well as to policy
analysts and energy planners who seek a fuller understanding of the tech-
nical factors underlying energy developments. It may well serve as a
reference text in university level courses.
The scope of this serial publication will be broad, encompassing recent
developments in the technologies of energy supply and delivery, end use
and conservation, as well as in methodologies for policy analysis. The
present volume includes topics ranging from such renewable resources as
wind and biomass, to the assessment of the potential of geothermal
energy, the prospects of clean fuels from coal, and, finally, to the possibil-
ity of introducing modern district heating systems to the United States.
Future volumes of this publication may concentrate to a greater degree on
a common theme or select a group of topics that appear to be particularly
timely.
Finally, it is our intent to attract an international audience of readers to
the series, for the energy problems of concern are inherently international
in nature, though they may be punctuated by important considerations
that vary from one region of the globe to another.
PETER AUER
ix
ADVANCES IN ENERGY SYSTEMS AND TECHNOLOGY, VOL. 1
Wind Power
D. G. Shepherd
Sibley School of Mechanical
and Aerospace Engineering
Cornell University
Ithaca, New York
I. Introduction and Background 2
II. Energy, Power, and Momentum Considerations 5
A. Energy Available 5
B. Performance Boundaries 7
III. Turbine Types and Terminology 9
IV. Aerodynamics of Wind Turbines 13
A. Flow through a Rotor 13
B. Aerodynamic Analysis of Rotors—Propeller Type . .. 18
C. Aerodynamic Analysis of Rotors—Cross-Wind
Axis Type 24
V. Wind Energy Concentrators and Augmentors 31
VI. Wind Characteristics and Siting 7 4
A. Wind Data, Velocity, and Power Duration,
Rated Values 47
B. Statisical Methods 57
C. Effect of Height 0 6
D. Effect of Gusts and Turbulence 5 6
E. Arrays of WECS 70
VII. Environmental Considerations 73
VIII. Structural Considerations 76
IX. Testing and Test Procedure 81
A. Background 81
B. Free-Air Testing 84
C. Wind Tunnel Testing 86
D. Tow Testing 87
X. Applications and Systems 88
XI. Economics of Wind Power 96
XII. State of the Art 1° 6
XIII. Wind and Weather Permitting 115
References 117
1
Copyright © 1978 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-014901-X
2 D. G. Shepherd
I. INTRODUCTION AND BACKGROUND
The development of wind power has been characterized generally by
periods of progress and longer periods of neglect, but nevertheless the
employment of wind power has never been absent from the time of its
inception many centuries ago in ancient Babylon and the Middle East, and
possibly before that to a suggested genesis from the prayer mills of Tibet.
It is convenient, if an oversimplification, to date the modern era from the
time of World War I, when powered flight led to the burgeoning of the
science of aerodynamics and its adaptation to the development of fluid
machines in general. Application of propeller theory to wind turbines by
Betz, dauert, and others still furnishes the basis of modern analysis.
The 25 years from the end of World War II have seen increased, al-
though sporadic, interest in the western countries, of which Denmark,
Germany, France, Great Britain, and the United States may be perceived
to have made major contributions.
In Denmark, following the earlier pioneering work of La Cour at As-
kov, particularly in the use of wind power for electricity generation, the
concept of wind power for making a significant impact was energetically
revived. The complete absence of fossil fuels and the vicissitudes of two
world wars led to a not-too-common united effort of government, indus-
try, and public utility to carry out experimental work and to finance,
design, operate, and test a number of wind energy conversion systems
(WECS), culminating in the Gedser Mill of 200 kW placed in continuous
operation in 1958 (Juul, 1956, 1964). Danish activity died down in 1967
after the Gedser Mill suffered a mechanical breakdown after 10 years use,
and an adverse economic survey led to the conclusion that wind power
was not competitive and the mill was idle for 10 years. However, the
current energy shortage has revived interest and recommendations have
been made to the government for a five-year, $9,000,000 program in ex-
periment and pilot operation (Hinrickson and Cawood, 1976; Danish
Academy of Technical Sciences, 1975). The Gedser Mill itself was re-
commissioned in 1977 for further experimental work, particularly more
detailed measurements of the rotor performance (Merriam, 1977-1978). A
2-MW wind turbine has been designed, built, and placed in operation at
Tvind by a local community headed by a school group, with consultant
help (see Section XII).
In Germany the tradition of Betz (see Section II) has been carried on
with Hütter as the major contributor. His work covers many years and
stands out, along with that of Juul in Denmark, as having a wide scope in
all areas: analysis, design, construction, testing, operation, and eco-
nomics (Hütter, 1964a, 1977). His name is associated with the Allgaier
Wind Power 3
units produced commercially for many years and with the design of the
ERDA-NASA 100-kW MOD-0 test unit at Plum Brook, United States.
He has also pioneered in the use of composite materials for rotor blades.
French work for the period in question includes that of Lacroix (1969)
and Vadot (1957), plus the manufacture and testing of two large units.
One, the plant at Nogent-Le-Roi, was rated at 800 kW at a wind speed of
16.7 m/sec (—37 mph), and this appears to the third largest output wind
turbine yet operated. However, the blade diameter was only 30.2 m (~ 100
ft) owing to the high wind speed for the rated output. The second unit, at
Saint-Remy-des-Lourdes, was rated at 132 kW at 13.2 m/sec (—30 mph),
with a blade diameter of 21.2 m (—70 ft). Tests were also made in Algeria
on the Andreau-type plant designed and manufactured in Britain (see
later) and small output units for lighthouse duty, etc. An account of
French work is given by Bonneville (1974).
In Britain, the years since World War II have been dominated by the
work of E. W. Golding and his colleague and successor A. Stodhart at the
Electrical Research Association. Golding is the author of the classic text
on wind power, which although published in 1955 is still valid in most
respects and after being out of print for many years has now been reissued
with some revisions by Harris (Golding, 1976). The work is particularly
valuable for its emphasis on aspects of the motive power itself, i.e., the
characteristics, distribution, surveys, measurement, and general data re-
lating to the wind and its behavior. The aerodynamic design of blades is
treated lightly, but on the whole is an excellent introduction to all aspects
of wind power. During the 1950s and 1960s, three 100-kW units were built
and operated in the United Kingdom; two being conventional propeller
types, one being in the Orkney Islands north of Scotland and the other in
the Isle of Man, with the third being an Andreau type wherein hollow
blades ejected air from the tips, with the resulting suction used to drive
the turbine at ground level via a connecting duct. The latter unit was
originally erected at St. Albans near London, and later removed to
Algeria, where it was operated for a period. These machines were re-
garded as experimental and as models for larger units of possibly 1-MW
output (Golding, 1955; BEAMA J., 1955). Again the time was not right for
economic development and so technical development likewise came to a
halt.
In the United States, the period is highlighted first of all by a negative
development, the almost complete disappearance of the rural windmill
used for pumping water and for low-voltage dc lighting. These units, of
0.5-3 kW, were ubiquitous in the Middle West, where distances were too
great to afford the distribution cost of public utility power to scattered
farms. Their disappearance was due to the Rural Electrication Acts of the
4 D. G. Shepherd
1930s, but it is said that up to about 1930 there were still some 50,000 units
in operation. Ironically, these well-engineered, reliable windmills are now
a collector's item and those extant are being vigorously sought for over-
haul and resale. Their basic design still appears to be the equal of anything
yet proved in quantity production for small-output plants.
The second development is that of the Smith-Putnam 1250-kW wind
turbine erected in 1941 on "Grandpa's Knob" in New Hampshire, which
operated (intermittently and with one 2-year outage due to war shortages)
until March 1945 (Putnam, 1948). This ambitious project, with an output
far greater than hitherto attempted, still remains the second largest unit
operated to this day and this was 35 years ago. The abandonment of the
wind power project as a whole, although precipitated by mechanical fail-
ure, was due to economic reasons pertinent at the time, and not to basic
unsoundness in either mechanical or aerodynamic design.
The third major event was the reports by Thomas (1945, 1946, 1949) for
the Federal Power Commission on the possibilities and design features of
wind power plants. These were comprehensive studies and still relevant
in many respects, although no action resulted at the time.
Thus the quarter-century 1945-1970 was a period of activity but an
uneven one, with interest stimulated by individuals or small groups and in
nearly every case with promising developments arrested by virtue of ad-
verse or nonproven economic merit. By the last half of the 1960s, activity
was at a low point, judging by the lack of reported work during this
period. The following quotation is pertinent:
The recent resurgence of interest in wind power generation is due to a
number of causes. Among these are the costs of fuels and their high rate of
exhaustion in some countries, the need for alternative sources of energy in
countries where the end of the exploitation of economic water power sources is
in sight, the desire for independence of imported fuels and the urge to make
fuller use of some of the under-developed areas of the world where a main
supply of electricity would be out of the question in the early stages of
development.
Thus Golding (1955)—in 1954. Prophetic words unheeded then, repeated
by scores 20 years later. An additional reason for the resurgence in these
times is that of concern for the environment and the search for "clean"
sources of energy. Wind power plants have access to an inexhaustible
source of energy and yield no chemical or thermal pollution. It would
seem that their environmental impact is limited to visual aspects, to noise,
and to interference with high-frequency broadcast transmission. An indi-
vidual wind turbine can be esthetically satisfying but it is unlikely that
rows of turbines on the passes and peaks of the White Mountains of New
Hampshire or the Pennines of Northern England will lack fierce opposi-
Wind Power 5
tion from environmentally concerned groups or indeed from the public at
large.
Given the motivation for the use of wind power quoted above, but on
the other hand being aware of the disappointments of the recent past, are
there any new reasons for reexamination at this time? It would seem that
the interest of much of the general public is engendered by the environ-
mental aspects, which fit the new awareness of the need for restraint in the
spread of hard technology, while the not-inconsiderable support of many
governmental institutions throughout the world is based primarily on the
rapidly diminishing availability of the convenient fossil fuels and their
equally rapidly increasing cost. With respect to technological reasons, the
continuous development of new materials and manufacturing methods
would be expected to allow greater freedom in design and lower produc-
tion cost, while the advances in the aerodynamics of wings and blading,
particularly as related to experience with helicopter rotors, together with
greatly improved techniques in stress analysis applicable to vibration and
flutter of rotor blades, should provide substantial help in areas posing
particularly severe problems for wind turbines. All technical and eco-
nomic concerns now have today's vastly more sophisticated computer
techniques to help in obtaining optimal solutions for performance, reliabil-
ity, and life, and, equally important, in load control and scheduling, It is
the purpose of the following material to first provide a framework by
which recent and current work may be related and assessed and then to
summarize such work as a starting point for evaluation of the present
status of wind energy systems, and for appraising some of the major
problems for continuing development.
II. ENERGY, POWER, AND MOMENTUM CONSIDERATIONS
A. Energy Available
The power available in the wind is taken as the flux of kinetic energy
through the active cross-sectional area intercepting the apparatus which is
utilizing this energy for mechanical or electrical output. This flux ismV2J2
and with m = pAV, it is expressed as pAV3J2. The density cannot be
x
controlled except within the limits of altitude siting, and usually other
considerations prevail. However, it might be noted that the density of the
standard atmosphere is nearly 16% lower at an altitude of 1500 m (—5000
ft) than it is at sea level. Using the sea-level value of density, 1.225 kg/m3
(0.0765 lb/ft3), the available power W in kW is given by 6.125 χ 10~4AV:i,
6 D. G. Shepherd
where Λ is in m2 and V in m/sec (or W = 5.05 x 10~6AV^, with A in ft2
x
and Voo in mph). This relationship is the key to the whole problem of
utilizing wind power, in the first place because of the very low energy
density, and secondly, because of the powerful effect of the cube relation-
ship. Evaluating the above expression for power with V» = 5 m/sec
(«11.2 mph), then an area of 13.15 m2 («142 ft2) is required for 1 kW
available power, or a circle of 4.09 m diameter («13.4 ft). Average wind
speeds of only 4-5 m/sec are typical for many regions of the world where
wind power might be desirable and although diligent effort can sometimes
find individual sites on a generally poor area which can be more favorable,
economic application in such areas is likely to be minimal. The depen-
dence of power on V'i is thus of paramount importance, because it implies
that only a small variation of estimated wind behavior can be the differ-
ence between success and failure in economic terms. Thus siting is all-
important and this requires long-term detailed data for wind behavior to
ensure reliable system behavior. Such data are very meager and unlikely
ever to be available in kind and degree to the desired extent and hence
much effort is going toward developing generalized statistical relation-
ships which may be used to estimate reliable probable values from a
minimum of measurements.
This power relationship provides the motif for the design of wind tur-
bines and the problems associated with its economic development. In the
first place, the energy density is very low, comparable to that of solar
radiation, thus requiring a large machine for appreciable output. It must
also be noted that the power levels quoted are for available power and not
that actually delivered by a wind power unit. A combination of an inher-
ent physical limitation and the aeromechanical efficiency means that at
best, only about 40% of the available power is likely to be utilized. The
large size carries with it the concomitant need for low rotational speed,
owing to stress restrictions, and this in turn connotes a speed increasing
device for electrical generation. The cube law for wind velocity stresses
the importance of siting and of setting a design condition optimal between
failure to utilize fully the energy available and high first cost due to over-
rating of the unit. There is also the fact that there is a minimum windspeed
for which usable power is delivered (cut-in speed) and a maximum value
beyond which the unit must be shut down for reasons of structural safety
(cut-out speed), also sometimes called "furling speed." The advantages
are a free and inexhaustible energy supply and low maintenance, thus
obviating problems of cost inflation.
It will be these characteristics that guide the following discussion.
There is no doubt of the technical feasibility of the use of wind power—
the problems lie in providing cost-effective systems.