DEVELOPMENT OF WATER-POWER FOR ELECTRIC STATIONS.
Electrical transmission has reduced the cost of water-power development.
Without transmission the power must be developed at a number of
different points in order that there may be room enough for the
buildings in which it is to be utilized. This condition necessitates
relatively long canals to conduct the water to the several points where
power is to be developed, and also a relatively large area of land with
canal and river frontage.
With electrical transmission the power, however great, may well be
developed at a single spot and on a very limited area of land. The canal
in this case may be merely a short passageway from one end of a dam to a
near-by power-house, or may disappear entirely when the power-house
itself forms the dam, as is sometimes the case.
These differences between the distribution of water for power purposes
and the development by water of electrical energy for transmission may
be illustrated by many examples.
A typical case of the distribution of water to the points where power is
wanted may be seen in the hydraulic development of the Amoskeag
Manufacturing Company at Manchester, N. H. This development includes a
dam across the Merrimac River, and two parallel canals that follow one
of its banks for about 3,400 feet down stream. By means of a stone dam
and a natural fall a little beyond its toe a water head of about
forty-eight feet is obtained at the upper end of the high-level canal.
Below this point there is little drop in the bed of the river through
that part of its course that is paralleled by the two canals. All of the
power might be thus developed within a few rods of one end of the dam,
if means were provided for its distribution to the points where it must
be used.
Years ago, when this water-power was developed, the electrical
transmission or distribution of energy was unheard of, and distribution
of the water itself had therefore to be adopted. For this purpose the
two canals already mentioned were constructed along the high bank of the
river at two different levels.
[Illustration: FIG. 4.--Hydraulic Development on the Merrimac River.]
The high-level canal, so called, was designed to take water directly
from the basin or forebay a little below one end of the dam, so that
between this canal and the river there is a full water head of about
forty-eight feet. Over nearly its entire course the nearer side of this
high-level canal runs between 450 and 750 feet from the edge of the
river wall, and thus includes between it and the river a large area on
which factories to be driven by water-wheels may be located. It was
thought, however, that this strip of land between the high-level canal
and the river was too wide for a single row of mill sites, and the lower
level canal was therefore constructed parallel with that on the higher
level, but with about twenty-one feet less elevation.
Between these two canals a strip of land about 250 feet wide was left
for the location of mills. By this arrangement of canals it is possible
to supply wheels located between the high and the low levels with water
under a head of about twenty-one feet, and to supply wheels between the
lower canal and the river with water under a head of about twenty-nine
feet. The entire area of land between the high canal and the river is
thus made readily available for factory buildings.
Water for the lower canal is drawn mainly from the high canal through
the wheels in buildings that are located between the two canals. It is
desirable in a case of this sort to have as much water flow through the
wheels between the high and low canal as flows through the wheels
between the low canal and the river, but this is not always possible. A
gate is therefore provided at the forebay where the two canals start, by
which water may pass from the forebay directly into the low canal when
necessary, but the head of twenty-one feet between the forebay and the
low canal is lost as to this water. Between the high and low canal, and
between the low canal and the river twenty-three turbine wheels or pairs
of wheels have been connected, and these wheels have a combined rating
of 9,500 horse-power.
To carry out this hydraulic development it thus appears that about 1.3
miles of canal have been constructed; one-half this length of
river-front has been required, and about one-sixth square mile of
territory has been occupied. Contrast with this result what might have
been done if electrical transmission of power had been available at the
time when this water-power was developed. All but a few rods in length
of the existing 1.3 miles of canal might have been omitted, and an
electric generating station with wheels to take the entire flow of the
river might have been located not far from one end of the dam. Factories
utilizing the electric power thus developed might have been located at
any convenient points along the river-front or elsewhere, and no space
would have been made unavailable because of the necessity of head- and
tail-water connections to scattered sets of wheels.
Compare with the foregoing hydraulic development that at Cañon Ferry on
the Missouri River, in Montana, where 10,000 horse-power is developed
under a water-head of 32 feet. At Cañon Ferry the power-house is 225
feet by 50 feet at the floor level inside, contains turbine wheels
direct-connected to ten main generators of 7,500 kilowatts, or 10,000
horse-power combined capacity, and is built on the river bank close to
one end of the 500-foot dam. The canal which runs along the land side of
the power-house, and takes water at the up-stream side of the abutment,
is about twice the length of the power-house itself. The saving in the
cost of canal construction alone, to say nothing of the saving as to the
required area of land, is evidently a large item in favor of the
electrical development and transmission. In its small area and short
canal the Cañon Ferry plant is not an exception, but is rather typical
of a large class of electric water-power plants that operate under
moderate heads.
A like case may be seen in the plant at Red Bridge, on the Chicopee
River, in Massachusetts, where a canal 340 feet long, together with
penstocks 100 feet long, convey water from one end of the dam and
deliver it to wheels in the electric station with a head of 49 feet.
This station contains electric generators with a combined capacity of
4,800 kilowatts or 6,400 horse-power, and its floor area is 141 by 57
feet.
[Illustration: FIG. 5.--Canal at Red Bridge on Chicopee River.]
So, again, at Great Falls, on the Presumpscot River, in North Gorham,
Me. (see cut), the electric station sets about 40 feet in front of the
forebay wall, which adjoins one abutment of the dam, and there is no
canal whatever, as short penstocks bring water to the wheels with a head
of 35 feet. In ground area this station is 67.5 by 55 feet, and its
capacity in main generators is 2,000 kilowatts or 2,700 horse-power.
A striking illustration of the extent to which electrical transmission
reduces the cost of water-power development may be seen at Gregg’s Falls
on the Piscataquog River, in New Hampshire, where an electric station of
1,200 kilowatts capacity has been built close to one end of the dam, and
receives water for its wheels under a head of 51 feet through a short
penstock, 10 feet in diameter, that pierces one of the abutments.
[Illustration: FIG. 6.]
Perhaps the greatest electric water-power station anywhere that rests
close to the dam that provides the head for its wheels is that at
Spier Falls (see cut), on the upper Hudson. One end of this station is
formed by the high wall section of the dam, and from this wall the
length of the station down-stream is 392 feet, while its width is 70
feet 10 inches, both dimensions being taken inside. The canal or forebay
in this case, like that at Cañon Ferry, lies on the bank side of the
power-station, and is about equal to it in length. From this canal ten
short penstocks, each 12 feet in diameter, will convey water under a
head of 80 feet to as many sets of turbine wheels in the station. These
wheels will drive ten generators with an aggregate capacity of 24,000
kilowatts or 32,000 horse-power.
Sometimes the slope in the bed of a river is so gradual or so divided up
between the number of small falls, or the volume of water is so small,
that no very large power can be developed at any one point without the
construction of a long canal. In a case of this sort electrical
transmission is again available to reduce the expense of construction
that will make it possible to concentrate all the power from a long
stretch of the river at a single point. This is done by locating
electric generating stations at as many points as may be thought
desirable along the river whose energy is to be utilized, and then
transmitting power from all of these stations to the single point where
it is wanted.
A case in point is that of Garvins Falls and Hooksett Falls on the
Merrimac River and four miles apart. At the former of these two falls
the head of water is twenty-eight feet, and at the latter it is sixteen
feet. To unite the power of both these falls in a single water-driven
station would obviously require a canal four miles long whose expense
might well be prohibitive. Energy from both these falls is made
available at a single sub-station in Manchester, N. H., by a generating
plant at both points and transmission lines thence to that city.
At Hooksett the present capacity of the electric station is 1,000
horse-power, and at Garvins Falls the capacity is 1,700 horse-power. The
river is capable of developing larger powers at both of these falls,
however, and construction is now under way at Garvins that will raise
its station capacity to 5,000 horse-power.
A similar result in the combination of water-powers without the aid of a
long canal is reached in the case of Gregg’s Falls and Kelley’s Falls,
which are three miles apart on the Piscataquog River. At the former of
these two falls the electric generating capacity is 1,600 horse-power,
as previously noted, and at the latter fall the capacity is 1,000
horse-power. In each case the station is close to its dam, and no canal
is required. Electrical transmission unites these two powers in the same
sub-station at Manchester that receives the energy from the two
stations above named on the Merrimac River.
Instead of transmitting power from two or more waterfalls to some point
distant from each of them, the power developed at one or more falls may
be transmitted to the site of another and there used. This is, in fact,
done at the extensive Ludlow twine mills on the Chicopee River, in
Massachusetts. These mills are located at a point on the river where its
fall makes about 2,500 horse-power available, and this fall has been
developed to its full capacity. After a capacity of 2,400 horse-power in
steam-engines had been added, more water-power was sought, and a new dam
was located on the same river at a point about 4.5 miles up-stream from
the mills. The entire flow of the river was available at this new dam,
and a canal 4.5 miles long might have been employed to carry the water
down to wheels at the mills in Ludlow.
Such a canal would have meant a large investment, not only for land and
construction, but also, possibly, for damages to estates bordering on
the river, if all of its water was diverted. Instead of such a canal, an
electric generating station was located close to the new dam with a
capacity of 6,400 horse-power, and this power is transmitted to motors
in the mills at Ludlow.
Even where the power is to be utilized at some point distant from each
of several waterfalls, it may be convenient to combine the power of all
at one of them before transmitting it to the place of use. This is
actually done in the case of two electric stations located respectively
at Indian Orchard and Birchem Bend on the Chicopee River, whose energy
is delivered to the sub-station of the electrical supply system in
Springfield, Mass. At the Indian Orchard station the head of water is 36
feet, and at Birchem Bend it is 14 feet, while the two stations are
about 2 miles apart. A canal of this length might have been built to
give a head of 50 feet at the site of the Birchem Bend dam, but instead
of this an electric station was located near each fall, and a
transmission line was built between the two stations. Each generating
station was also connected with the sub-station in Springfield by an
independent line, and power is now transmitted from one generating plant
to the other, as desired, and the power of both may go to the
sub-station over either line. In the Indian Orchard station the dynamo
capacity is about 2,000 kilowatts, and at Birchem Bend it is 800
kilowatts.
Another case showing the union of two water-powers by electrical
transmission, where the construction of an expensive canal was avoided,
is that of the electrical supply system of Hartford, Conn. This system
draws a large part of its energy from two electric plants on the
Farmington River, at points that are about 3 miles apart in the towns of
Windsor and East Granby, respectively. At one of these plants the head
of water is 32 feet, and at the other it is 23 feet, so that head of 55
feet might have been obtained by building a canal 3 miles long. Each of
these stations is located near its dam, and the generator capacity at
one station is 1,200 and at the other 1,500 kilowatts. Transmission
lines deliver power from both of these plants to the same sub-station in
Hartford.
Sometimes two or more water-powers on the same river that are to be
united are so far apart that any attempt to construct a canal between
them would be impracticable. This is illustrated by the Spier and
Mechanicsville Falls on the Hudson River, which are 25 miles apart in a
direct line and at a greater distance by the course of the stream. At
Spier Falls the head is 80 feet, and at Mechanicsville it is 18 feet.
Union of the power of these two falls is thus out of the question for
physical reasons alone. Electrical transmission, however, brings energy
from both of these water-powers to the same sub-stations in Schenectady,
Albany, and Troy.
In another class of cases electrical transmission does what could not be
done by any system of canals, however expensive; that is, unites the
water-powers of distinct and distant rivers at any desired point. Thus
power from both the Merrimac and the Piscataquog rivers is distributed
over the same wires in Manchester; the Yuba and the Mokelumne contribute
to electrical supply along the streets of San Francisco; and the Monte
Alto and Tlalnepantla yield energy in the City of Mexico.
It does not follow from the foregoing that it is always more economical
to develop two or more smaller water-powers at different points along a
river for transmission to some common centre than it is to concentrate
the water at a single larger station by more elaborate hydraulic
construction, and then deliver all of the energy over a single
transmission line. The single larger hydraulic and electric plant will
usually have a first cost larger than that of the several smaller ones,
because of the required canals or pipe lines. A partial offset to this
larger hydraulic investment is the difference in cost between one and
several transmission lines, or at least the cost of the lines that would
be necessary between the several smaller stations in order to combine
their energy output before its transmission over a single line to the
point of use.
Against the total excess of cost for the single larger hydraulic and
electrical plant there should be set the greater expense of operation
at several smaller and separate plants. Even a small water-driven
electric station that can be operated by a single attendant at any one
time must have two attendants if it is to deliver energy during the
greater part or all of every twenty-four hours. But a single attendant
can care for a water-power plant of 2,000 horse-power or more capacity,
so that two plants of 750 horse-power each will require double the
operating force of one plant of 1,500 horse-power. If two such plants
are constructed instead of one that has their combined capacity, the
monthly wages of the two additional operators will amount to at least
one hundred dollars. If money is worth six per cent yearly, it follows
that an additional investment of $1,200 ÷ 0.06 = $20,000 might be made
in hydraulic work to concentrate the power at one point before the
annual interest charge would equal the increase of wages made necessary
by two plants.
Reliability of operation is one of the most important requirements in an
electric water-power plant, and its construction should be carried out
with this in view. Anchor ice is a serious menace to the regular
operation of water-wheels in cold climates, because it clogs up the
openings in the racks and in the wheel passages. Anchor ice is formed in
small particles in the water of shallow and fast-flowing streams, and
tends to form into masses on solid substances with which the water comes
in contact.
At the entrance to penstocks or wheel chambers, steel racks with long,
narrow openings, say one and one-quarter inches wide, are regularly
placed to keep all floating objects away from the wheels. When water
bearing fine anchor or frazil ice comes in contact with these racks, it
rapidly clogs up the narrow openings between the bars, unless men are
kept at work raking off the ice as it forms. At the Niagara Falls
electric station, in some instances, when the racks become clogged, they
have been lifted, and the anchor ice allowed to pass down through the
wheels. This is said to have proved an effective remedy, but it would
obviously be of no avail in a case where the ice clogged the passages of
the wheels themselves.
The best safeguard against anchor ice is a large, deep forebay next to
the racks, where the water, being comparatively quiet, will soon freeze
over after cold weather commences. The anchor ice coming down to this
forebay and losing most of its forward motion, soon rises to the surface
or to the under side of the top coating of solid ice, and the warmer
water sinks to the bottom. Good construction puts the entrance ends of
penstocks well below the surface of water in the forebay, so that they
may receive the warmer water that contains little or no anchor ice.
[Illustration: FIG. 7.--Cross Section of Dike on Chicopee River at Red
Bridge.]
Illustrations of practice along these lines, as to size, depth of
forebay, and location of penstocks may be seen in many well-designed
plants. One instance is that at Garvins Falls, on the Merrimac River,
where the new hydraulic development for 5,000 horse-power is now under
way. Water from the river in this case comes down to the power-station
through a canal 500 feet long, and of 68 feet average width midway
between the bottom and the normal flow line. In depth up to his flow
line the canal is 12 feet at its upper and 13 feet at its lower end,
just before it widens into the forebay. In this forebay the depth
increases to 17 feet, and the width at the rack is double that of the
canal. The steel penstocks, each 12 feet in diameter, terminate in the
forebay wall at an average distance of 7 feet behind the rack, and each
penstock has its centre 10.6 feet below the water level in the forebay.
As there is a large pond created by the dam in this case, and as the
flow of water in the canal is deep rather than swift, enough opportunity
is probably afforded for any anchor ice to rise to the surface before it
reaches the forebay in this case.
Penstocks for the electric station at Great Falls, on the Presumpscot
River, whence energy is drawn for lighting and power in Portland, Me.,
are each 8 feet in diameter, and pierce the forebay wall behind the rack
with their centres 15 feet below the normal water level in the forebay.
In front of the forebay wall the water stands 27 feet deep, and the pond
formed by the dam, of which the forebay wall forms one section, is 1,000
feet wide and very quiet. Though the Maine climate is very cold in
winter and the Presumpscot is a turbulent stream above the dam and pond,
there has never been any trouble with anchor ice at the Great Falls
plant. An excellent illustration is thus presented of the fact that
deep, still water in the forebay is a remedy for troubles with ice of
this sort.
Maximum loads on electrical supply systems are usually from twice to
four times as great as the average loads during each twenty-four hours.
A pure lighting service tends toward the larger ratio between the
average and maximum load, while a larger motor capacity along with the
lamps, tends to reduce the ratio. Furthermore, by far the greater part
of the energy output of an electrical supply system during each
twenty-four hours must be delivered between noon and midnight. For these
reasons there must be enough water stored, that can flow to the station
as wanted, to carry a large share of the load during each day, unless
storage batteries are employed to absorb energy at times of light load,
if the entire normal flow of the river is to be utilized.
It is usually much cheaper to store water than electrical energy for the
daily fluctuations of load, and the only practicable place for this
storage is most commonly behind the dam that maintains the head for the
power-station. This storage space should be so large that the drain upon
it during the hours of heavy load will lower the head of water on the
wheels but little, else it may be hard to maintain the standard speed of
revolution for the wheels and generators, and consequently the
transmission voltage.
[Illustration: FIG. 8.--Valley Flooded above Spier Falls on the Hudson
River.]
At the Great Falls plant, water storage to provide for the fluctuations
of load in different parts of the day takes place back of the dam, and
for about one mile up-stream. This dam is 450 feet long in its main
part, and a retaining wall increases the total length to about 1,000
feet. For half a mile up-stream from this dike and dam the average width
of the pond is 1,000 feet, and its greatest depth is not less than 27
feet. As the station capacity is 2,700 horse-power in main generators,
with a head of 35 feet at the wheels the storage capacity is more than
ample for all changes of load at different times of day.
The dam at Spier Falls, on the Hudson River, is 1,820 feet long between
banks, 155 feet high above bedrock in its deepest section, and raises
the river 50 feet above its former level. Behind the dam a lake is
formed one-third of a mile wide and 5 miles long. Water from this
storage reservoir passes down through the turbines with a head of 80
feet, and is to develop 32,000 horse-power. As a little calculation will
show, this lake is ample to maintain the head under any fluctuation in
the daily load. At Cañon Ferry, where electrical energy for Butte and
Helena, Mont., is developed, the dam, which is 480 feet long, crosses
the river in a narrow canyon that extends up-stream for about half a
mile. Above this canyon the river valley widens out, and the dam, which
maintains a head of 30 feet at the power-station, sets back the water in
this valley, and thus forms a lake between two and three miles wide and
about seven miles long. At the station the generator equipment has a
total rating of 10,000 horse-power. From these figures it may be seen
that the storage lake would be able to maintain nearly the normal head
of water for some hours, when the station was operating under full load,
however small the flow of the river above.
[Illustration: FIG. 9.--Canal at Bulls Bridge on Housatonic River.]