Criteria For Cogeneration. For minimum economic feasibility, cogeneration
cycles will meet the following criteria:
Load Balance. There should be a reasonably balanced relationship between the
peak and normal requirements for electric power and heat. The peak/normal ratio should
not exceed 2:1.
Load Coincidence. There should be a fairly high coincidence, not less than 70
percent, of time and quantity demands for electrical power and heat.
Size. While there is no absolute minimum size of steam power plant which can
be built for cogeneration, a conventional steam (cogeneration) plant will be practical
and economical only above some minimum size or capacity, below which other types of
cogeneration, diesel, or gas turbine become more economical and convenient.
Distribution Medium. Any cogeneration plant will be more effective and
economical if the heat distribution medium is chosen at the lowest possible steam
pressure or lowest possible hot water temperature. The power energy delivered by the
turbine is highest when the exhaust steam pressure is lowest. Substantial cycle
improvement can be made by selecting an exhaust steam pressure of 40 psig (276 kPa gage)
rather than 125 psig (862 kPa gage), for example. Hot water heat distribution should
also be considered where practical or convenient, because hot water temperatures of 200
to 240 degrees F (93 to 116 degrees C) can be delivered with exhaust steam pressure as
low as 20 to 50 psig (138 to 345 kPa gage). The balance between distribution system
and heat exchanger costs, and power cycle effectiveness should be optimized.
High Pressure Extraction Type. Turbines with throttle pressures generally
above 400 psig (2758 kPa gage) are considered high pressure machines; however, the exact
demarcation between high, intermediate, and low pressure turbines is not definite.
Turbines built with provisions for extraction of steam from the turbine at intermediate
pressure points below the throttle pressure are called extraction turbines. The
extracted steam may be used for process systems, feed water heating, and environmental
heating. A typical cycle using a high pressure extraction type turbine is shown in
High Pressure Non-Extraction Type. The high pressure non-extraction
type of turbine is basically the same as the extraction type described in 22.214.171.124 above,
except no steam is extracted from the turbine. High pressure steam enters the turbine
throttle and expands through the turbine to the condenser. The condenser pressure is
comparable to that with high pressure extraction machines.