Page 80 - Sustainable On-Site CHP Systems Design, Construction, and Operations

58 CHP B a s i c s

needed for the thermal loads served. The steam pressure is then reduced through a
backpressure turbine to the pressure needed to serve the thermal loads.
A condensing steam turbine is a steam turbine that exhausts into a condenser where
the exhausted steam is condensed. The condenser will be in a vacuum allowing much
more enthalpy to be obtained from each pound of steam, making the steam turbine
thermodynamic process much more efficient. Most condensers are water cooled but
some condensers are air cooled. Condensing turbines are the usual choice in commer-
cial electrical power plants since the only need is for electrical power. A condensing
turbine provides more rotational power for the steam available but most of the energy
is lost in condensing. With a backpressure turbine, the condensing occurs in serving the
thermal loads and is therefore beneficial. The overall efficiency in serving both power
and thermal need is therefore much greater for a backpressure turbine. Of course, a
power plant that does not serve a thermal load is not a CHP plant.
Extraction condensing turbines allow steam to be removed from the turbine at any
reduced pressure, including multiple reduced pressures. So, for example, steam could
enter the STG at 200 psig and a portion of the steam could be extracted at 100 psig to
feed the medium pressure steam system, a second extraction port could also bleed off
steam at 15 psig for use in the low-pressure steam system, and the remaining steam
would drive the turbine to produce useful work as it expands through the rest of the
turbine. Since the steam bleed off serves beneficial thermal needs, such a plant is a
CHP plant.
The existence of multiple steam turbine types offers mechanical engineers several
options to consider when analyzing and designing the most efficient CHP plants. Steam
turbine exhaust, when reduced in both pressure and temperature, can be used to supply
heat exchangers, absorption chillers, pumps, or other equipments that are designed to
operate with steam and that are installed in place of electrically driven equipments.

Size Range
Steam turbines are commonly available in practically any size with some units installed
in power generation plants exceeding 100 MW. Steam turbines, since there is no com-
bustion process, have no environmental impacts as do combustion turbines. Steam
turbines are likely to be available to make use of CHP produced steam of any quantity,
and combustion turbine exhaust duct firing can very efficiently increase the steam
production in a combined CTG CHP system.

Electrical Efficiency Range
Steam turbine thermodynamic efficiencies are directly related to the efficiency of the
Carnot cycle; therefore, the temperature of the heat source and the temperature of the
heat sink set the maximum possible theoretical efficiency. The higher the steam tem-
perature and the colder the condenser water, the higher the theoretical thermodynamic
efficiency. Due to irreversibility (entropy), real systems will be less efficient than that
predicted by the theoretical Carnot cycle. The efficiency of the turbine design at con-
verting the energy of the steam into shaft energy is also an important factor. Steam tur-
bines of high-quality construction can have isentropic efficiencies as high as 90 percent.
Note, the isentropic efficiency is the efficiency of the steam turbine to convert steam
energy into shaft power, and is not the same as overall thermodynamic cycle efficiency,
which is much lower. To achieve high thermodynamic cycle efficiency, commercial power
plants have boilers able to produce very high-pressure steam (often 1000 psig or more)