ETL 1110-2-550
30 May 97
D-11. Dependable Capacity Gained by New
(2) The variability in usable capacity caused by
Runners
variations in the availability of streamflow, which in
turn causes variations in the amount of energy avail-
able to support the capacity.
The amount of energy available in each week will be
increased due to the higher runner efficiency. In
b. A variety of different techniques are used to
some weeks, sufficient energy is already available to
estimate dependable capacity. The Corps presently
support the existing capacity. But in some of the
uses the average availability method for projects
lower flow weeks, this additional energy will permit
which operate in thermal-based power systems and
more capacity to be supported. The average gain in
the critical month method for projects in
capacity over all of the peak demand weeks in the
hydroelectric-based power systems.
period of record defines the gain in dependable
capacity attributable to the new runners. Typically,
c. For this example, the average availability
this gain is relatively small for runner replacement,
method was used. Space does not permit a detailed
and for this example, the new runners increase the
discussion of the procedure, but, in brief, it involves
dependable capacity from 185 MW to 190 MW
computing the amount of capacity that can be sup-
(compared with an installed capacity of 200 MW).
ported with the available energy for each week in
the peak demand months for each year in the
D-12. Dependable Capacity Gained by
hydroelectric period of record. The average
Generator Rewind
capacity that can be supported over that period
defines the project's dependable capacity.
The generator rewind increases the maximum
d. Supportable capacity is defined as the
capacity of the plant. This in turn permits more
capacity to be supported in those weeks where more
amount of capacity that can be supported for a
energy is available than is needed to support the
specified number of hours per week. The number of
existing capacity. In the example case, if the gener-
hours required varies from project to project and
ator capacity is increased by 40 MW, the depend-
from system to system, depending on the system
able capacity increases from 190 MW to 226 MW
resource mix and hourly load shape. A typical
(compared with the new installed capacity of
example might be 4 hours per day, 5 days per week
240 MW).
(or 20 hours per week).
e. Some examples will illustrate this concept.
D-13. Computation of Capacity Benefits
Taking the 200-MW example project and using the
20-hr/week criterion, assume that in a particular
a. The average annual gain in capacity bene-
month, sufficient stream flow is available to
fits that accrues to a rehabilitation plan is computed
produce 5,000 MWh/week. Applying the 20-hr
by applying a unit capacity value to the gain in
criteria, (5,000 MWh)/(20 hr/week) = 250 MW
dependable capacity creditable to that plan. Assum-
could theoretically be supported. However, the
ing a capacity value of /kW-year, the gain in
installed capacity of the plant is only 200 MW, so
capacity benefits for the runner replacement and
the supportable capacity for that month is limited to
rewind measures would be:
200 MW. However, if the generators were rewound
to 240 MW, the supportable capacity would
Runner replacement benefits
increase to 240 MW. Assume that in another
(5,000 kW x /kW-year)
=
$ 475,000
month, 3,000 MWh/week can be generated. In this
Generator rewind benefits
month, only (3,000 Mwh)/(20 hr/week) = 150 MW
(36,000 kW x /kW-year)
=
,420,000
can be supported, either with or without the rewind.
Total capacity benefits
=
,895,000
D-7
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