MIL-HDBK-1003/11
Section 4:
COGENERATION CONSIDERATIONS
4.1 Introduction. Cogeneration is the simultaneous on-site generation of
electric energy and process steam or heat from the same plant. Use of heat
recovery can increase overall efficiency of diesel-electric generation from
around 33 percent, which is available for most diesel engine-generators, to
a theoretical 75 percent. Heat which would otherwise be wasted is recovered
for use in building heating, ventilating and air conditioning systems and,
in special cases, to generate additional power. Process thermal loads can
also be served where practicable. Guidelines for assessing the potential
for cogeneration, the circumstances when it should be considered, and
discussions on the types of equipment to utilize are addressed in the
following paragraphs.
4.2 Design Considerations. Cogeneration applications should be considered
for all new designs of prime duty diesel-electric generating plants.
Cogeneration may be considered for existing plants if proven economically
viable. Standby/emergency plants will rarely justify use of cogeneration,
although in some cases heat recovery systems may be economical. Packaged
cogeneration units may be considered for stand-alone installations; however,
the system and components must comply with the applicable criteria.
4.2.1 Fuel Availability.
Fuel availability should be assured for the life
of the project.
4.2.2 Load Sizing Criteria. The following criteria shall be used in the
design of cogeneration installations:
4.2.2.1 Electric and Thermal Loads. Electric and thermal loads should be
continuous to satisfy economic criteria. Only limited fluctuations in
thermal loads are permitted unless adequate thermal storage systems or
standby boilers are provided.
4.2.2.2 Load Balance. The electric load should be in reasonable balance
with both the heating peak and average load. The ratio of peak to average
load for cogeneration installations should be in the range from 2:1 to 3:1.
4.2.2.3 Load Coincidence. Time and quantity demands for electric power and
thermal energy should have a coincidence of not less than 70 percent.
Coincidence is defined as the ratio of the maximum coincident total demand
of a group of loads to the sum of the maximum demands of individual loads
comprising the group, both taken at the same point of supply at the same
time.
4.2.3 Prime Mover Sizing. Size the cogeneration prime mover for heat
recovery equivalent to 50 to 75 percent of the maximum thermal load.
4.2.4 Thermal Product Properties. Design cogeneration installations
producing steam and/or hot water as thermal products and to provide these
products at the same pressures and temperatures as existing distribution.
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