Turbine Heat Rates

a)  Gross Turbine Heat Rate.  The gross heat rate is determined by dividing
the heat added in the boiler between feedwater inlet and steam outlet by the
kilowatt output of the generator at the generator terminals.  The gross heat rate is
expressed in Btu per kWh.  For reheat cycles, the heat rate is expressed in Btu per
kWh.  For reheat cycles, the heat added in the boiler includes the heat added to the
steam through the reheater.  For typical values of gross heat rate.

b)  Net Turbine Heat Rate.  The net heat rate is determined the same as for gross
heat rate, except that the boiler feed pump power input is subtracted from the
generator power output before dividing into the heat added in the boiler.

c)  Turbine Heat Rate Application.  The turbine heat rate for a regenerative turbine
is defined as the heat consumption of the turbine in terms of "heat energy in steam"
supplied by the steam generator, minus the "heat in the feed water" as warmed by
turbine extraction, divided by the electrical output at the generator terminals.  This
definition includes mechanical and electrical losses of the generator and turbine
auxiliary systems, but excludes boiler inefficiencies and pumping losses and loads.
The turbine heat rate is useful for performing engineering and economic
comparisons of various turbine designs.
Plant Heat Rates.  Plant heat rates include inefficiencies and losses external to the
turbine generator, principally the inefficiencies of the steam generator and piping
systems; cycle auxiliary losses inherent in power required for pumps and fans; and
related energy uses such as for soot blowing, air compression, and similar services.

a)  Gross Plant Heat Rate.  This heat rate (Btu/kWh) is determined by dividing the
total heat energy (Btu/hour) in fuel added to the boiler by the kilowatt output of the
generator

b)  Net Plant Heat Rate.  This heat rate is determined by dividing the total fuel
energy (Btu/hour) added to the boiler by the difference between power
(kilowatts/hour) generated and plant auxiliary electrical power consumed.  Both
turbine and plant heat rates, as above, are usually based on calculations of cycle
performance at specified steady state loads and well defined, optimum operating
conditions.  Such heat rates are seldom achieved in practice except under
controlled or test conditions.
Plant operating heat rates are actual long term average heat rates and include other
such losses and energy uses as non-cycle auxiliaries, plant lighting, air
conditioning and heating, general water supply, startup and shutdown losses, fuel
deterioration losses, and related items.  The gradual and inevitable deterioration of
equipment, and failure to operate at optimum conditions, are reflected in plant
operating heat rate data.
Plant Economy Calculations.  Calculations, estimates, and predictions of steam
plant performance shall allow for all normal and expected losses and loads and
should, therefore, reflect predictions of monthly or annual net operating heat rates
and costs. Electric and district heating distribution losses are not usually charged to
the power plant but should be recognized and allowed for in capacity and cost
analyses.  The designer is required to develop and optimize a cycle heat balance
during the conceptual or preliminary design phase of the project.  The heat balance
depicts, on a simplified flow diagram of the cycle, all significant fluid mass flow
rates, fluid pressures and temperatures, fluid enthalpies, electric power output, and
calculated cycle heat rates based on these factors.  A heat balance is usually
developed for various increments of plant load such as 25, 50, 75, 100 percent and
VWO (valves, wide open).  Computer programs have been developed which can
quickly optimize a particular cycle heat rate using iterative heat balance
calculations.  Use of such a program should be considered.
5.1.8
Steam Rates
Theoretical Steam Rate.  When the turbine throttle pressure and temperature
5.1.8.1
and the turbine exhaust pressure (or condensing pressure) are known, the
theoretical
steam rate can be calculated based on a constant entropy expansion or can be
determined
from published tables.  See Theoretical Steam Rate Tables, The American Society
of
Mechanical Engineers, 1969.  See Table 8 for typical theoretical steam rates.
The turbine heat rate of a steam turbogenerato is the ratio of thermal input:
power generated. It is often expressed in kJ/kWh. The efficiency of the
turbogenerator is simply calculated from this.

The plant heat rate is the ratio of fuel energy into the plant: power generated. It is
greater than the turbine heat rate, because not all of the fuel's thermal energy
can be captured by the boiler, and also power station services such as fuel
handling, flue gas cleaning etc consume power. Consequently, more fuel is
needed for each unit of useful net power produced. Plant heat rate is often
expressed in kJ/kWh or Btu/kWh.

The fuel energy input used in the plant heat rate calculation may be on a higher
heating value (HHV) or a lower heating value (LHV) basis, and the plant power
output, although usually on a net (net of plant own consumption) is sometimes on
the basis of that at the generator terminals. Whatever is used should be made
clear, but it often is not.