MIL-HDBK-1003/7
c) Peak Shaving.
Cogeneration is used to minimize demand charges from the
electrical utility.
Isolated Operation. Under isolated operation, the cogeneration plant
21.4.2
generates, independently of the electric utility, all electricity and steam needed and
used by the facilities that it serves.
21.4.2.1
Reasons for Isolation
a) The facilities and cogeneration plant are in a remote location and
electricity is not or cannot feasibly be made available from an electric utility to the
facilities.
b)
Critical mission requires self-contained system, including onsite
standby.
c) Facility heat and electrical needs coincide to permit self-contained
(total energy) cogeneration system.
d)
Agreement for parallel operation cannot be reached with electric
utility.
21.4.2.2 Electric Utility Crosstie. A cogenerator may operate in isolation and may
also be interconnected to an electric utility for service during maintenance down times
and/or for standby service for use during an emergency outage of the cogeneration plant.
If a cogenerator is not crosstied to an electric utility, backstart capability must be
provided.
Base Load Thermal. In this mode of operation, the cogeneration equipment is
21.4.3
sized and installed to generate electricity at a constant (base) load equal to that
afforded by minimum steam demand, as determined from periodic swings in steam load.
Supplemental electrical demands above the base load would be supplied by the electric
utility through a parallel arrangement. Supplemental steam demands would be supplied by
the use of supplementary firing of heat recovery steam generators or separate boilers.
Base Load Electric. In this mode of operation, the cogeneration equipment is
21.4.4
sized and installed to generate electricity at a constant (base) load equal to the
minimum annual (or some other chosen period) electrical demand. Some of the electricity
included in the base load may not be cogenerated but may serve to reduce demand supplied
by an electric utility. Because of the duality of steam production and utilization,
automatic extraction condensing turbine generators lend themselves to base load electric
operation.
Electric Peak Shaving. Peak shaving is accomplished by the use of onsite
21.4.5
generation or cogeneration equipment to limit the demand, during peak electrical use
periods, of electricity purchased from an electric utility through a parallel
arrangement. Peak shaving is usually done on a daily cyclic basis. The objective of
peak shaving is to economically reduce electric utility demand charges and overall
electrical energy costs. If there is a high demand for steam during peak electric
demand periods, cogeneration can be used to supplement purchased power to achieve peak
shaving, if it proves to be economical.
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