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GENERATION OF ELECTRIC POWER
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Calculation Procedure
Consider the Energy Source and Generating System
As indicated in Table 8.2, a single energy source or fuel (e.g., oil) is often capable of
being used in a number of different types of generating systems. These include steam
cycles, combined steam- and gas-turbine cycles (systems where the hot exhaust gases are
delivered to a heat-recovery steam generator to produce steam that is used to drive a
steam turbine), and a number of advanced technology processes such as fuel cells (i.e.,
systems having cathode and anode electrodes separated by a conducting electrolyte that
convert liquid or gaseous fuels to electric energy without the efficiency limits of the
Carnot cycle).
Similarly, at least in the planning stage, a single generic type of electric-power generating
system (e.g., a steam cycle) can be designed to operate on any one of a number of
fuels. Conversion from one fuel to another after plant construction does, however, generally
entail significant capital costs and operational difficulties.
As Table 8.3 indicates, each combination of energy source and power-generating-system
type has technical, economic, and environmental advantages and disadvantages that are
unique. Often, however, in a particular situation there are other unique considerations that
make the rankings of the various systems quite different from the typical values listed in
Table 8.3. In order to make a determination of the best system, it is necessary to quantify
and evaluate all factors in Table 8.3 (see page 8.4).
Select the Plant, Unit Rating, and Site
The choice of plant, unit rating, and site is a similarly complex, interrelated process.
As indicated in Table 8.3 (and 8.9), the range of unit ratings that are commercially
available is quite different for each of the various systems. If, for example, a plant is
needed with a capacity rating much above 100 MW, combustion turbine, diesel, and
geothermal units could not be used unless multiple units were considered for the
installation.
Similarly, the available plant sites can have an important impact upon the choice of
fuel, power-generating system, and rating of the plant. Fossil-fuel or nuclear-energy
steam-cycle units require tremendous quantities of cooling water [50.5 to 63.1 m3/s
(800,000 to 1,000,000 gal/min)] for a typical 1000-MW unit, whereas gas-turbine units
require essentially no cooling water. Coal-fired units rated at 1000 MW would typically
require over 2.7 million tonnes (3 million tons) of coal annually, whereas nuclear units
rated at 1000 MW would typically require only 32.9 tonnes (36.2 tons) of enriched uranium
dioxide (UO2) fuel annually.
Coal-fired units require disposal of large quantities of ash and scrubber sludge,
whereas natural-gas-fired units require no solid-waste disposal whatsoever. From each of
these comparisons it is easy to see how the choice of energy source and power-generating
system can have an impact on the appropriate criteria to be used in choosing a plant site.
The location and physical characteristics of the available plant sites (such as proximity to
and availability of water, proximity to fuel or fuel transportation, and soil characteristics)
can have an impact on the choice of fuel and power-generating system.
Consider the Electrical Load
The electrical load, on an electric-power system of any size generally fluctuates considerably
on a daily basis, as shown by the shapes of typical daily load curves for the months
April, August, and December in Fig. 8.1. In addition, on an annual basis, the system electrical
load varies between a minimum load level, below which the electrical demand never
falls, and a maximum or peak, load level which occurs for only a few hours per year. The
annual load duration curve of Fig. 8.1a graphically shows the number of hours per year that
the load on a particular power system exceeds a certain level.
For example, if the peak-power system load in the year (100 percent load) is 8100 MW,
the load duration curve shows that one could expect the load to be above 70 percent of the
peak (i.e., above 0.7 8100 MW 5760 MW) about 40 percent of the year. The minimum
load (i.e., load exceeded 100 percent of the time) is about 33 percent of the peak value.
Typically, for U.S. utility systems the minimum annual load is 27 to 33 percent of the
peak annual load. Generally, the load level exceeds 90 percent of the peak value 1 to 5
percent of the time, exceeds 80 percent of the peak value 5 to 30 percent of the time, and
exceeds 33 to 45 percent of the peak 95 percent of the time. Annual load factors [(average
load/peak annual load) 100 percent] typically range from 55 to 65 percent.
CONSTRUCTION OF SCREENING CURVE
A screening curve provides a plot of cost per kilowattyear as a function of capacity factor
or operating load. An example is the screening curve of Fig. 8.2 for a coal-fired steamcycle
system, based on the data in Table 8.7 (see pages 8.16 and 8.17). Assume the total
installed capital cost for a 600-MW system is $450 million and the fixed-charge rate is 16
percent. In addition, assume the total fixed operation and maintenance cost is $3,750,000
per year for the unit. Verify the figures given in Fig. 8.2.