POWER PLANT COGENERATION
Definition. Cogeneration is the simultaneous generation of electricity (or
the same fuel (or energy) source.
Cycles. Cogeneration cycles consist of energy conversion equipment such as
boilers, turbines, and electric generators arranged to produce both electricity and
steam or other thermal energy.
Basic Cycle. The basic conventional cycle consists of a steam boiler and
turbine, which drives either an electrical generator or other mechanical equipment, and
from which steam is extracted or exhausted to environmental heating or processes.
Combined Cycle. See Figures 13, 19, and 48 for typical cogeneration cycles.
The combined cycle consists of a gas turbine which exhausts to a heat recovery steam
generator (HRSG). The HRSG in turn produces steam to drive a steam turbine. Both
turbines can drive a single or separate electrical generator. Low pressure turbine
exhaust steam can be used directly for process or heating purposes or the steam can be
otherwise condensed and returned to the HRSG.
Efficiency. The overall efficiency of a cogeneration cycle is a ratio of all
usable energy (electricity, steam, hot water, etc.) obtained from the cycle to the
energy (fuel, solar, etc.) input to the cycle.
Parallel Operation. Under parallel operation, the cogeneration plant is
electrically interconnected and synchronized with an electric utility distribution or
transmission system, with both the cogenerator and electric utility generating
electricity simultaneously. Under parallel operation, some electricity will be flowing
either to or from the cogeneration system.
Reasons for Paralleling
a) Selective use of electrical energy from the electric utility by the
cogenerator. The cogenerator purchases electrical energy during periods and in amounts
as needed to supplement its cogeneration capabilities.
b) Sale of excess electricity by the cogenerator to the electric
utility. The cogenerator has a large heat demand for process use or environmental
heating and can cogenerate electricity in excess of facility needs. The excess
electricity generated is sold to the electric utility generally at a rate which is less
than the generating costs; this is usually not economical.