22-03-2014, 04:18 PM
Technology Characterization – Steam Turbines
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Introduction and Summary
Steam turbines are one of the most versatile and oldest prime mover technologies still in
general production used to drive a generator or mechanical machinery. Power generation using
steam turbines has been in use for about 100 years, when they replaced reciprocating steam
engines due to their higher efficiencies and lower costs. Most of the electricity produced in the
United States today is generated by conventional steam turbine power plants. The capacity of
steam turbines can range from 50 kW to several hundred MWs for large utility power plants.
Steam turbines are widely used for CHP applications in the U.S. and Europe.
Unlike gas turbine and reciprocating engine CHP systems where heat is a byproduct of power
generation, steam turbines normally generate electricity as a byproduct of heat (steam)
generation. A steam turbine is captive to a separate heat source and does not directly convert
fuel to electric energy. The energy is transferred from the boiler to the turbine through high
pressure steam that in turn powers the turbine and generator. This separation of functions
enables steam turbines to operate with an enormous variety of fuels, varying clean natural gas
to solid waste, including all types of coal, wood, wood waste, and agricultural byproducts (sugar
cane bagasse, fruit pits and rice hulls). In CHP applications, steam at lower pressure is
extracted from the steam turbine and used directly in a process or for district heating, or it can
be converted to other forms of thermal energy including hot or chilled water.
Applications
While steam turbines themselves are competitively priced compared to other prime movers, the
costs of complete boiler/steam turbine CHP systems are relatively high on a per kW of capacity
basis because of their low power to heat ratio; the costs of the boiler, fuel handling and overall
steam systems; and the custom nature of most installations. Thus, steam turbines are well
suited to medium- and large-scale industrial and institutional applications where inexpensive
fuels, such as coal, biomass, various solid wastes and byproducts (e.g., wood chips, etc.),
refinery residual oil, and refinery off gases are available. Because of the relatively high cost of
the system, including boiler, fuel handling system, condenser, cooling tower, and stack gas
cleanup, high annual capacity factors are required to enable a reasonable recovery of invested
capital.
Technology Description
Basic Process and Components
The thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle is the basis for
conventional power generating stations and consists of a heat source (boiler) that converts
water to high pressure steam. In the steam cycle, water is first pumped to elevated pressure,
which is medium to high pressure depending on the size of the unit and the temperature to
which the steam is eventually heated. It is then heated to the boiling temperature corresponding
to the pressure, boiled (heated from liquid to vapor), and then most frequently superheated
(heated to a temperature above that of boiling). The pressurized steam is expanded to lower
pressure in a multistage turbine, then exhausted either to a condenser at vacuum conditions or
into an intermediate temperature steam distribution system that delivers the steam to the
industrial or commercial application. The condensate from the condenser or from the industrial
steam utilization system is returned to the feedwater pump for continuation of the cycle.
Performance Characteristics
Electrical Efficiency
The electrical generating efficiency of steam turbine power plants varies from a high of 36
percent HHV 4 for large, electric utility plants designed for the highest practical annual capacity
factor, to under 10 percent HHV for small, simple plants which make electricity as a byproduct of
delivering steam to industrial processes or district heating systems for colleges, industrial parks
and building complexes.
Steam turbine thermodynamic efficiency (isentropic efficiency) refers to the ratio of power
actually generated from the turbine to what would be generated by a perfect turbine with no
internal losses using steam at the same inlet conditions and discharging to the same
downstream pressure. Turbine thermodynamic efficiency is not to be confused with electrical
generating efficiency, which is the ratio of net power generated to total fuel input to the cycle.
Steam turbine thermodynamic efficiency is a measure of how efficiently the turbine extracts
power from the steam itself and is useful in identifying the conditions of the steam as it exhausts
from the turbine and in comparing the performance of various steam turbines. Multistage
(moderate to high pressure ratio) steam turbines have thermodynamic efficiencies that vary
from 65 percent for very small (under 1,000 kW) units to over 90 percent for large industrial and
utility sized units. Small, single stage steam turbines can have efficiencies as low as 50 percent.
Maintenance
Steam turbines are very rugged units, with operational life often exceeding 50 years.
Maintenance is simple, comprised mainly of making sure that all fluids (steam flowing through
the turbine and the oil for the bearing) are always clean and at the proper temperature. The oil
lubrication system must be clean and at the correct operating temperature and level to maintain
proper performance. Other items include inspecting auxiliaries such as lubricating-oil pumps,
coolers and oil strainers and checking safety devices such as the operation of overspeed trips.
In order to obtain reliable service, steam turbines require long warmup periods so that there are
minimal thermal expansion stress and wear concerns. Steam turbine maintenance costs are
quite low, typically around $0.005 per kWh. Boilers and any associated solid fuel processing
and handling equipment that is part of the boiler/steam turbine plant require their own types of
maintenance.