MIL-HDBK-1003/7
Economics. Cogeneration plants are capital intensive and high maintenance
21.6
facilities. However, cogeneration can be economically justified if the savings in
electrical energy costs, resulting from the use of cogeneration as compared to purchase
of electricity from a utility, offset the costs of the cogeneration facility capital
investment, added fuel usage, added operation, and added maintenance.
Fuel Savings. Using cogeneration, there is no fuel saving when compared to
21.6.1
the use of boilers for the production of steam for process or environmental
heating/cooling systems. There is a fuel saving for production of electricity, by use
of cogeneration as compared to non-cogeneration systems, if the exhaust heat or steam
from the prime mover is used by other process or environmental heating/ cooling systems.
Competitive Systems. The basic cogeneration systems that provide the means to
21.6.2
utilize exhaust heat from the prime mover are the back pressure steam turbine, gas
turbine, combined cycle, or reciprocating combustion engine. A cogenerator using only
condensing turbine generator equipment cannot compete economically with an electric
utility using the same type of equipment because of the economics of large scale
operations. The utility's large condensing steam turbine generators operating at high
initial pressure and temperature are much more efficient than would be a cogenerator's
small condensing steam turbine generators operating at lower initial pressures and
temperatures. However, condensing steam turbine generators can be economically combined
with noncondensing or extraction turbine generators if the noncondensing equipment is
utilized to the extent that fuel savings more than offset the increased cost of
operation of the condensing equipment. The noncondensing and condensing equipment may
be combined into single machines such as automatic extraction condensing turbine
generators.
Power to Heat Ratio. A gauge for match of the cogeneration system with the
21.6.3
facility heat or steam and electric demands is often expressed as the power to heat
ratio. This ratio is defined as the ratio of the power generated to heat available for
process or environmental heating/cooling systems. The higher the power to heat ratio,
the higher will be the efficiency of cogeneration or economic return on investment
capital. Table 34 shows typical power to heat ratios of cogeneration systems.
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