Chapter 5 - Publications, US Army Corps of Engineers

EM 1110-2-1701
31 Dec 1985
elevation by means of a hydraulic turbine connected to a generator.
Electrical energy is usually measured in kilowatt-hours, but it can
also be defined in terms of average kilowatts. Three classes of
energy are of interest in hydropwer studies: average annual, firm,
and secondary.
b= ~ A hydro projectts average annual
energy is an estimate of the average amount of energy that could be
generated by that project in a year, based on examination of a long
period of historical streamflows. In sequential streamflow analysis,
average annual energy is calculated by taking the mean of the annual
generation values over the period of record. In non-sequential
analysis, it is computed by measuring the area under.the annual powerduration
curve. In many power studies, energy benefits are based
directly on average annual energy. In other cases, it is necessary to
evaluate firm and secondary energy separately (see Section 9-100).
(1) AS defined from the marketing standpoint, firm energy is
electrical energy that is available on an assured basis to meet a
specified increment of load. For hydroelectric energy to be marketable
as firm energy, the streamflow used to generate it must also be
available on an assured basis. Thus, hydroelectric firm energy (also
sometimes called primary energy) is usually based on a projectrs
energy output over the most adverse sequence of flows in the existing
streamflow record. This adverse sequence of flows is called the
critical period (see Section 5-10d).
(2) Where a hydro plant or hydro system carries a large portion
of a power system~s load, the hydro plantts firm energy output must
closely follow the seasonal demand pattern. Reservoir storage is
often required to shape the energy output to fit the seasonal demand
pattern. Where hydro comprises only a small part of a power systemts
resource base, a hydro plantts output does not necessarily have to
match the seasonal demand pattern. Its firm output can frequently be
utilized in combination with other generating plants and in this way
will serve to increase the total system firm energy capability.
However, in some systems, marketing constraints may preclude taking
advantage of this flexibility.
(3) In the Pacific Northwest and parts of Alaska, where hydropower
is the predominant source of generation, generation planning is
based primarily on system energy requirements rather than peak load
requirements (see Sections 2-2b and S-sb). Thus, to determine a
proposed hydro projectts value to the system, it is necessary to
compute that projectfs firm energy capability. Capacity consid-
5-2