POWER June 2010 - 62

Plant efficiency
Table 1. ISO baseload performances of three combined-cycle power
plant variants (1 x 1 with an E-Class GT). Plant A is for electric power generation
only and has a net electric power output of 130 MW. Plant B is a combined-cycle
combined heat and power (CC CHP) plant that provides saturated steam at 125 psia to an
industrial customer. Plant C is also a CC CHP plant that provides superheated steam at
1,200 psia and 800F to an industrial customer. Note that, in terms of total useful thermal
energy supply to the customer, Plants B and C are equivalent at 50 MWth. The total power
production of Plants A and B reflects comparable ST technology after extracting the specified
amount of steam from the bottoming cycle. Source: GE Energy
Performance measure
Gas turbine output (MWe)
Steam turbine (ST) output (MWe)
Duct firing?
Fuel input (LHV), MWth
Combined cycle (MWgross)
Combined cycle (MWnet)
Process steam flow (lb/hr)
Process steam pressure (psia)
Process steam temperature (F)
Thermal energy (MWth)
Thermal exergy (MW)
β
MET, F (Equation [2])
Rational ST efficiency (%)-Figure 3
Plant A
85
47
Unfired
254.5
132
130
NA
NA
NA
Plant B
85
31
Unfired
254.5
116
114
143,250
125
Saturated
50
15.5
Plant C
85
31
Unfired
254.5
116
114
123,750
1,200
800
50
21.1
436
Plant C
85
38
Fired
311.4
123
121
247,500
1,200
800
100
42.1
31.10% 42.10% 42.10%
293
436
67.90%
74.40%
74.40%
Table 2. CHP efficiencies for the CC cogeneration power plants described
in Table 1 using different definitions. The thermal or first-law efficiency
is calculated by adding the electricity production to the thermal energy production
and dividing by the fuel supplied to the plant on a HHV basis. The PURPA efficiency is
calculated in the same manner, but the thermal energy is weighted, by legislative definition,
by a factor of one-half. The heat chargeable to power or HCP, in units of Btu/kWh, is
the energy charged to net electric power after subtracting the amount of fuel that would
be required to generate an equivalent amount of steam (thermal energy) in a conventional
fired boiler with an assumed LHV efficiency of 90%. The HCP efficiency is found by dividing
HCP into 3,412.12 Btu/kWh, a conversion factor. The origin or justification for factors
such as one-half or 90% is unclear.
Efficiency measure
Net power only (%)
Duct firing?
First law or thermal efficiency (%)
PURPA efficiency (%)
HCP (Btu/kWh)
HCP efficiency (%)
Proposed second law or CHP (%)-from
equation [1]
When the information contained in Figure
3 is recast in terms of MET using [2],
in which β = E/Q, the result is a simple
curve, which is depicted in Figure 4. The
rational efficiency found from Figure 4 is
universally valid when comparing the efficiency
of any combined heat or standard
steam power plant.
62
Plant A
51
Unfired
45.9
51
6,695
51
51
Plant B
44.7
Unfired
58
54.5
5,969
57.2
48.8
Plant C
44.7
Unfired
58
54.5
5,968
57.2
50.8
Plant C
38.7
Fired
63.9
54.8
5,666
60.2
48.8
An Example Clarifies the Concepts
The advantage of the proposed CHP plant
rational efficiency is best demonstrated by
using a simple numerical example. Consider
a 1 x 1 CC power plant based on an
E-Class GT and condensing ST. Three
variants of this CC plant, one for electric
power generation and two for cogenerawww.powermag.com
tion
applications, are summarized in Table
1. Note that the numbers in Table 1 are for
illustrative purposes only and do not reflect
an existing or commercially offered
product.
Plant A is clearly the most valuable
variant from a thermal efficiency perspective
from the options listed in Table 1.
Furthermore, although both Plants B and
C deliver the same amount of thermal energy,
50 MWth, Plant C is the more valuable
variant because its steam product is
of a much higher grade of energy. This
is reflected by the calculated exergy and
MET values of the process steam provided
by Plants B and C.
As seen from the data in Table 2, all definitions
are equivalent as far as Plant A is
concerned, with the exception of thermal
efficiency, which uses the gross heating
value (HHV) of the fuel as a basis. This
is not surprising, because the only product
of Plant A is electric power supplied
to the grid. However, neither the thermal
efficiency nor the PURPA (Public Utility
Regulatory Policies Act) efficiency or heat
chargeable to power (HCP) are able to provide
a correct measure of the much higher
grade of the steam product from Plant C
vis-à-vis Plant B. All three conventional
CHP efficiencies mistakenly assign the
same value to the thermal energy products
of Plants B and C. They do not make a distinction
between different grades of steam.
In other words, 100 psia (saturated) steam
is implicitly considered as equivalent
to 1,200 psia superheated steam at 800F
(Plants B and C).
Furthermore, these CHP definitions
evaluate Plants B and C, which are based
on an E-Class GT, at an efficiency that is
equivalent to an F-Class GT. This is an example
of mismeasurement, which would
be even more glaring in the presence of
supplementary firing in the heat-recovery
boiler. As can be seen in Tables 1 and
2, when Plant C is duct-fired to generate
twice the amount of original process
steam, two definitions return a CHP plant
efficiency that is equivalent to or even better
than an H or J-Class GT.
What should be emphasized is the fact
that the conventional measures not only
mix " electricity apples with thermal energy
oranges, " but they also totally ignore
the variation in quality among the " thermal
energy oranges " themselves. One can
certainly make the reasonable argument
that " beauty is in the eye of the beholder. "
In other words, an industrial customer that
needs 125 psia (saturated) steam for his
manufacturing plant is really not interested
in the fact that 1,200 psia and 800F steam
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POWER June 2010

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