Common Medium. Steam power cycles are particularly applicable to cogeneration
situations because the actual cycle medium, steam, is also a convenient medium for area
distribution of heat.
a) The choice of the steam distribution pressure should be a balance
between the costs of distribution, which are slightly lower at high pressure, and the
gain in electrical power output by selection of a lower turbine exhaust or extraction
b) Often, the early selection of a relatively low steam distribution
pressure is easily accommodated in the design of distribution and utilization systems,
whereas the hasty selection of a relatively high steam distribution pressure may not be
recognized as a distinct economic penalty on the steam power plant cycle.
c) Hot water heat distribution may also be applicable as a district
heating medium with the hot water being cooled in the utilization equipment and returned
to the power plant for reheating in a heat exchange with exhaust (or extraction) steam.
Relative Economy. When the exhaust (or extraction) steam from a cogeneration
plant can be utilized for heating, refrigeration, or process purposes in reasonable
phase with the required electric power load, there is a marked economy of fuel energy
because the major condensing loss of the conventional steam power plant (Rankine) cycle
is avoided. If a good balance can be attained, up to 75 per cent of the total fuel
energy can be utilized, as compared with about 40 percent for the best and largest
Rankine cycle plants and about 25 to 30 percent for small Rankine cycle systems.
Cycle Types. The two major steam power cogeneration cycles,
which may be combined in the same plant or establishment, are the back pressure and
Back Pressure Cycle. In a back pressure turbine, the entire flow to the
turbine is exhausted (or extracted) for heating steam use. This cycle is more effective
for heat economy and for relatively lower cost of turbine equipment, because the prime
mover is smaller and simpler and requires no condenser and circulating water system.
Back pressure turbine generators are limited in electrical output by the amount of
exhaust steam required by the heat load and are often governed by the exhaust steam
load. They, therefore, usually operate in electrical parallel with other generators.
Extraction-Condensing Cycle. Where the electrical demand does not correspond
to the heat demand, or where the electrical load must be carried at times of very low
(or zero) heat demand, then condensing-controlled extraction steam turbine prime movers,
as shown in Figure 12, may be applicable. Such a turbine is arranged to carry a
specified electrical capacity either by a simple condensing cycle or a combination of
extraction and condensing. While very flexible, the extraction machine is relatively
complicated, requires complete condensing and heat rejection equipment, and must always
pass a critical minimum flow of steam to its condenser to cool the low pressure buckets.