UTILITY OF WATER-POWER IN ELECTRICAL SUPPLY.
In comparatively few systems is the available water-power sufficient to
carry the entire load at all hours of the day, and during all months of
the year, so that the question of how much fuel can be saved is an
uncertain one for many plants. Again, the development of water-power
often involves a large investment, and may bring a burden of fixed
charges greater than the value of the fuel saved.
In spite of these conflicting opinions and factors, the application of
water-power in electrical systems is now going on faster than ever
before. If a saving of fuel, measured by the available flow of water
during those hours when it can be devoted directly to electrical supply,
were its only advantage, the number of cases in which this power could
be utilized at a profit would be relatively small. If, on the other
hand, all of the water that passes down a stream could be made to do
electrical work, and if the utilization of this water had other
advantages nearly or quite as great as the reduction of expense for
coal, then many water-powers would await only development to bring
profit to their owners.
No part of the problem is more uncertain than the first cost and
subsequent fixed charges connected with the development of water-power.
To bring out the real conditions, the detailed facts as to one or more
plants may be of greater value than mere general statements covering a
wide range of cases.
On a certain small river the entire water privilege at a point where a
fall of fourteen feet could be made available was obtained several years
ago. At this point a substantial stone and concrete dam was built, and
also a stone and brick power-house with concrete floor and steel truss
roof. In this power-house were installed electric generators of 800
kilowatts total capacity, direct-connected to horizontal turbine wheels.
The entire cost of the real estate necessary to secure the water-power
privilege plus the cost of all the improvements was about $130,000. More
than enough water-power to drive the 800-kilowatt generators at full
load was estimated to be available, except at times of exceptionally low
water. At this plant the investment for the water-power site,
development, and complete equipment was thus $162 per kilowatt capacity
of generators installed.
Allowing 65 days of low water, these generators of 800 kilowatts
capacity may be operated 300 days per year. If the running time averages
ten hours daily at full load, the energy delivered per year is 2,400,000
kilowatt hours. Ten per cent of the total investment should be ample to
cover interest and depreciation charges, and this amounts to $13,000
yearly. It follows that the items of interest and depreciation on the
original investment represent a charge of 0.54 cent per kilowatt hour on
the assumed energy output at this plant. This energy is transmitted a
few miles and used in the electrical supply system of a large city.
On another river the entire water privilege was secured about four years
ago at a point where a fall of more than 20 feet between ledges of rock
could be obtained and more than 2,000 horse-power could be developed. At
this point a masonry dam and brick power-house were built, and
horizontal turbine wheels were installed, direct-connected to electric
generators of 1,500 kilowatts total capacity. The entire cost of real
estate, water rights, dam, building, and equipment in this case was
about $250,000.
Assuming, as before, that generators may be operated at full capacity
for 10 hours per day during 300 days per year, the energy delivered by
this plant amounts to 4,500,000 kilowatt hours yearly. The allowance of
10 per cent on the entire investment for interest and depreciation is
represented by $25,000 yearly in this case, or 0.56 cent per kilowatt
hour of probable output. Energy from this plant is transmitted and used
in a large system of electrical supply.
If, through lack of water or inability to store water or energy at times
when it is not wanted, generators cannot be operated at full capacity
during the average number of hours assumed above, the item of interest
and depreciation per unit of delivered energy must be higher than that
computed. With the possible figure for this item at less than six-tenths
of a cent per kilowatt hour, there is opportunity for some increase
before it becomes prohibitive. At the plant last named the entire
investment amounted to $166 per kilowatt capacity of connected
generators, compared with $162 in the former case, and these figures may
be taken as fairly representative for the development of water-power in
a first-class manner on small rivers, under favorable conditions. In
both of these instances the power-houses are quite close to the dams. If
long canals or pipe lines must be built to convey the water, the expense
of development may be greatly increased.
One advantage of water- over steam-power is the smaller cost of the
building with the former for a given capacity of plant. The building for
direct-connected electric generators, driven by water-wheels, is
relatively small and simple. Space for fuel, boilers, economizers,
feed-water heaters, condensers, steam piping, and pumps is not required
where water-power is used. No chimney or apparatus for mechanical
draught is needed.
The model electric station operated by water-power usually consists of a
single room with no basement under it. One such station has floor
dimensions 27 by 52 feet, giving an area of 1,404 square feet, and
contains generators of 800 kilowatts capacity. This gives 1.75 square
feet of floor space per kilowatt of generators. In this station there is
ample room for all purposes, including erection or removal of machinery.
Next to the saving of fuel, the greatest advantage of water-power is due
to the relatively small requirements for labor at generating stations
where it is used. This is well illustrated by an example from actual
practice. In a modern water-power station that contributes to electrical
supply in a large city the generator capacity is 1,200 kilowatts. All of
the labor connected with the operation of this station during nearly
twenty-four hours per day is done by two attendants working alternate
shifts.
These attendants live close to the station in a house owned by the
electric company, and receive $60 each per month in addition to house
rent. Considering the location, $12 per month is probably ample
allowance for the rent. This brings the total expense of operation at
this station for labor up to $132 per month, or $1,584 per year, a sum
corresponding to $1.32 yearly per kilowatt of generator capacity.
At steam-power stations of about the above capacity, operating
twenty-four hours daily, $6 is an approximate yearly cost of labor per
kilowatt of generators in use. It thus appears that water-power plants
may be operated at less than one-fourth of the labor expense necessary
at steam stations per unit of capacity. On an average, the combined cost
of fuel and labor at electric stations driven by steam-power is a little
more than 76 per cent of their total cost of operation. Of this total,
labor represents about 28, and fuel about 48 per cent. Water-power, by
dispensing with fuel and with three-fourths of the labor charge, reduces
the expense of operation at electric stations by fully 69 per cent.
But this great saving in the operating expenses of electric stations can
be made only where water entirely displaces coal. If part water-power
and part coal are used, the result depends on the proportion of each,
and is obviously much affected by the variations of water-power
capacity. In such a mixed system the saving effected by water-power must
also depend on the extent to which its energy can be absorbed at all
hours the day. By far the greater number of electric stations using
water-power are obliged also to employ steam during either some months
in the year or some hours in the day, or both.
[Illustration: ENERGY CURVES FROM WATER POWER ELECTRIC STATIONS.
FIG. 3.]
It is highly important, therefore, to determine, as nearly as may be,
the answers to three questions:
First, what variations are to be expected in the capacity of a
water-power during the several months of a year?
Second, if the daily flow of water is equal in capacity to the daily
output of electrical energy, how far can the water-power be devoted to
the development of that energy?
Third, with a water-power sufficient to carry all electrical loads at
times of moderately high water, what percentage of the yearly output of
energy in a general supply system can be derived from the water?
To the first of these questions experience alone can furnish an answer.
Variations in the discharge of rivers during the different months of a
year are very great. In a plant laid out with good engineering skill
some provision will be made for the storage of water, and the capacity
of generating equipment will correspond to some point between the
highest and lowest rates of discharge.
Curve No. 1 in the diagram on the opposite page represents the energy
output at an electric station driven entirely by water-power from a
small stream during the twelve months of 1901, the entire flow of the
stream being utilized. During December, 1901, the output of this station
was 527,700 kilowatts, and was greater than that in any other month of
the year. Taking this output at 100 per cent, the curve is platted to
show the percentage attained by the delivered energy in each of the
other months. At the lowest point on the curve, corresponding to the
month of February, the output of energy was only slightly over 33 per
cent of that in December. During nine other months of the year the
proportion of energy output to that in December was over 60 and in three
months over 80 per cent. For the twelve months the average delivery of
energy per month was 73.7 per cent of that during December.
PERCENTAGES OF ENERGY DELIVERED IN DIFFERENT MONTHS, 1901.
January 68.0
February 33.1
March 80.5
April 81.7
May 77.9
June 58.6
July 67.7
August 75.8
September 79.3
October 65.9
November 95.8
December 100.0
At a somewhat small water-power station on another river with a
watershed less precipitous than that of the stream just considered, the
following results were obtained during the twelve months ending June
30th, 1900. For this plant the largest monthly output of energy was in
November, and this output is taken at 100 per cent. The smallest
delivery of energy was in October, when the percentage was 53.1 of the
amount for November. In each of seven other months of the year the
output of energy was above 80 per cent of that in November. During
March, April, May, and June the water-power yielded all of the energy
required in the electrical supply system with which it was connected,
and could, no doubt, have done more work if necessary. For the twelve
months the average delivery of energy per month was 80.6 per cent of
that in November, the month of greatest output.
PERCENTAGES OF ENERGY DELIVERED IN DIFFERENT MONTHS, 1899 AND 1900.
July 68.6
August 69.1
September 73.3
October 53.1
November 100.0
December 87.0
January 84.9
February 91.3
March 98.5
April 85.7
May 80.8
June 74.9
The gentler slopes and better storage facilities of this second river
show their effect in an average monthly delivery of energy 6.9 per cent
higher as to the output in a month when it was greatest than the like
percentage for the water-power first considered. These two water-power
illustrate what can be done with only very moderate storage capacities
on the rivers involved. At both stations much water escapes over the
dams during several months of each year. With enough storage space to
retain all waters of these rivers until wanted the energy outputs could
be largely increased.
As may be seen by inspection of curve No. 2, the second water-power has
smaller fluctuations of capacity, as well as a higher average percentage
of the maximum output than the water-power illustrated by curve No. 1.
If the discharge of a stream during each twenty-four hours is just
sufficient to develop the electrical energy required in a supply system
during that time, the water may be made to do all of the electrical work
in one of two ways. If the water-power has enough storage capacity
behind it to hold the excess of water during some hours of the day, then
it is only necessary to install enough water-wheels and electric
generators to carry the maximum load. Should the storage capacity for
water be lacking, or the equipment of generating apparatus be
insufficient to work at the maximum rate demanded by the electrical
system, then an electric storage battery must be employed if all of the
water is to be utilized and made to do the electrical work.
The greatest fluctuations between maximum and minimum daily loads at
electric lighting stations usually occur in December and January. The
extent of these fluctuations is illustrated by curve No. 3, which
represents the total load on a large electrical supply system during a
typical week-day of January, 1901. On this day the maximum load was
2,720 and the minimum load 612 kilowatts, or 22.5 per cent of the
highest rate of output. During the day in question the total delivery
of energy for the twenty-four hours was 30,249 kilowatt hours, so that
the average load per hour was 1,260 kilowatts. This average is 46 per
cent of the maximum load.
Computation of the area included by curve No. 3 above the average load
line of 1,260 kilowatts shows that about 17.8 per cent of the total
output of energy for the day was delivered above the average load, that
is, in addition to an output at average load. It further appears by
inspection of this load curve that this delivery of energy above the
average load line took place during 12.3 hours of the day, so that its
average rate of delivery per hour was 438 kilowatts.
If a water-power competent to carry a load of 1,260 kilowatts
twenty-four hours per day be applied to the system illustrated by curve
No. 3, then about 17.8 per cent of the energy of the water for the
entire day must be stored during 11.7 hours and liberated in the
remaining 12.3 hours. This percentage of the total daily energy of the
water amounts to 36 per cent of its energy during the hours that storage
takes place.
If all of the storage is done with water, the electric generators must
be able to work at the rate of 2,720 kilowatts, the maximum load. If all
of the storage is done in electric batteries, the use of water may be
uniform throughout the day, and the generator capacity must be enough
above 1,260 kilowatts to make up for losses in the batteries. Where
batteries are employed the amount of water will be somewhat greater than
that necessary to operate the load directly with generators, because of
the battery losses.
In spite of the large fluctuations of electrical loads throughout each
twenty-four hours, it is thus comparatively easy to operate them with
water-powers that are little, if any, above the requirements of the
average loads.
Perhaps the most important question relating to the use of water-power
in electrical supply is what percentage of the yearly output of energy
can be derived from water where this power is sufficient to carry the
entire load during a part of the year. With storage area for all surplus
water in any season, the amount of work that could be done by a stream
might be calculated directly from the records of its annual discharge of
water. As such storage areas for surplus water have seldom, or never,
been made available in connection with electrical systems, the best
assurance as to the percentage of yearly output that may be derived from
water-power is found in the experience of existing plants.
The question now to be considered differs materially from that
involving merely the variations of water-power in the several months,
or even the possible yearly output from water-power. The ratio of output
from water-power to the total yearly output of an electrical system
includes the result of load fluctuations in every twenty-four hours and
the variable demands for electrical energy in different months, as well
as changes in the amount of water-power available through the seasons.
In order to show the combined result of these three important factors
curve No. 4 has been constructed. This indicates the percentages of
total semi-yearly outputs of electrical energy derived from water-power
in two supply systems. Each half-year extends either from January to
June, inclusive, or from July to December, inclusive, and thus covers a
wet and dry season. Each half-year also includes a period of maximum and
one of minimum demand for electrical energy in lighting. The period of
largest water supply usually nearly coincides with that of heaviest
lighting load, but this is not always true.
Electrical systems have purposely been selected in which the water-power
in at least one month of each half-year was nearly or quite sufficient
to carry the entire electrical load. The percentage of energy from
water-power to the total energy delivered by the system is presented for
each of five half-years. Three of the half-years each run from July to
December, and two extend from January to June, respectively. The half
years that show percentages of 66.8, 80.2, and 95.6, respectively, for
the relation of energy from water-power to the total electrical output
relate to one system, and the half years that show percentages of 81.97
and 94.3 for the energy from water-power relate to another system.
For the half-year when 66.8 per cent. of the output of the electrical
system was derived from water-power, the total output of the system was
3,966,026 kilowatt hours. During the month of December in this half-year
more than 98 per cent of the electrical energy delivered by the system
was from water-power, though the average for the six months was only
66.8 per cent from water.
In the following six months, from January to June, the electrical supply
system delivered 4,161,754 kilowatt hours, and of this amount the
water-power furnished 80.2 per cent. For the six months just named, one
month, May, saw 99 per cent of all the delivered energy derived from
water-power.
The same system during the next half-year, from July to December,
without any addition to its water-power development or equipment, got
95.6 per cent of its entire energy output from water-power, and this
output amounted to 4,415,945 kilowatt hours. In one month of the
half-year just named only 0.2 per cent of the output was generated with
steam-power.
These three successive half years illustrate the fluctuations of the
ratio between water-power outputs and the demands for energy on a single
system of electrical supply. The percentage of 81.9 for energy derived
from water-power during the half-year from July to December represents
the ratio of output from water to the total for an electrical supply
system where water generated 94 per cent of all the energy delivered in
one month.
In the same system during the following six months, with exactly the
same water-power equipment, the percentage of output from water-power
was 94.3 of the total kilowatt-hours delivered by the system. This
result was reached in spite of the fact that the total outputs of the
system in the two half-years were equal to within less than one per
cent.
The lesson from the record of these five half-years is that
comparatively large variations are to be expected in the percentage of
energy developed by water-power to the total output of electrical supply
systems in different half-years. But, in spite of these variations, the
portion of electrical loads that may be carried by water-power is
sufficient to warrant its rapidly extending application to lighting and
power in cities and towns.