The gas turbine is a power plant, which produces a great amount of energy for
its size and weight. The gas turbine has found increasing service in the past
40 years in the power industry both among utilities and merchant plants as well
as the petrochemical industry, and utilities throughout the world. Its compactness,
low weight, and multiple fuel application make it a natural power plant
for offshore platforms. Today there are gas turbines, which run on natural gas,
diesel fuel, naphtha, methane, crude, low-Btu gases, vaporized fuel oils, and
biomass gases.
The last 20 years has seen a large growth in Gas Turbine Technology. The
growth is spearheaded by the growth of materials technology, new coatings,
and new cooling schemes. This, with the conjunction of increase in compressor
pressure ratio, has increased the gas turbine thermal efficiency from about 15%
to over 45%.
Table 1-1 gives an economic comparison of various generation technologies
from the initial cost of such systems to the operating costs of these systems.
Because distributed generation is very site specific the cost will vary and the
justification of installation of these types of systems will also vary. Sites for
distributed generation vary from large metropolitan areas to the slopes of the
Himalayan mountain range. The economics of power generation depend on the
fuel cost, running efficiencies, maintenance cost, and first cost, in that order. Site
selection depends on environmental concerns such as emissions, and noise, fuel
availability, and size and weight.
Gas Turbine Cycle in the Combined Cycle or Cogeneration Mode
The utilization of gas turbine exhaust gases, for steam generation or the heating
of other heat transfer mediums, or in the use of cooling or heating buildings or
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parts of cities, is not a new concept and is currently being exploited to its full
potential.
The Fossil Power Plants of the 1990s and into the early part of the new
millennium will be the Combined Cycle Power Plants, with the gas turbine
being the centerpiece of the plant. It is estimated that between 1997–2006 there
will be an addition of 147.7 GW of power. These plants have replaced the large
Steam Turbine Plants, which were the main fossil power plants through the 1980s.
The Combined Cycle Power Plant is not new in concept, since some have been
in operation since the mid-1950s. These plants came into their own with the new
high capacity and efficiency gas turbines.
The new marketplace of energy conversion will have many new and novel
concepts in combined cycle power plants. Figure 1-1 shows the heat rates of
these plants, present and future, and Figure 1-2 shows the efficiencies of the
same plants. The plants referenced are the Simple Cycle Gas Turbine (SCGT)
with firing temperatures of 2400 ◦F (1315 ◦C), Recuperative Gas Turbine (RGT),
the Steam Turbine Plant (ST), the Combined Cycle Power Plant (CCPP), the
Advanced Combined Cycle Power Plants (ACCP) such as combined cycle
power plants using Advanced Gas Turbine Cycles, and finally the Hybrid Power
Plants (HPP).
Table 1-2 is an analysis of the competitive standing of the various types of
power plants, their capital cost, heat rate, operation and maintenance costs, availability
and reliability, and time for planning. Examining the capital cost and
installation time of these new power plants it is obvious that the gas turbine is
the best choice for peaking power. Steam turbine plants are about 50% higher
in initial costs—$800–$1000/kW—than combined cycle plants, which are about
$400–$900/kW. Nuclear power plants are the most expensive. The high initial
costs and the long time in construction make such a plant unrealistic for a
deregulated utility.
In the area of performance, the steam turbine power plants have an efficiency
of about 35%, as compared to combined cycle power plants, which have an
efficiency of about 55%. Newer Gas Turbine technology will make combined
cycle efficiencies range between 60–65%. As a rule of thumb a 1% increase in
efficiency could mean that 3.3% more capital can be invested. However one must
be careful that the increase in efficiency does not lead to a decrease in availability.
From 1996–2000 we have seen a growth in efficiency of about 10% and a loss in
availability of about 10%. This trend must be turned around since many analyses
show that a 1% drop in the availability needs about a 2–3% increase in efficiency
to offset that loss.
The time taken to install a steam plant from conception to production is about
42–60 months as compared to 22–36 months for combined cycle power plants.
The actual construction time is about 18 months, while environmental permits
in many cases takes 12 months and engineering 6–12 months. The time taken
for bringing the plant online affects the economics of the plant, the longer capital
is employed without return, the plant accumulates interest, insurance, and
taxes.
It is obvious from this that as long as natural gas or diesel fuel is available the
choice of combined cycle power plants is obvious.
Gas Turbine Performance
The aerospace engines have been the leaders in most of the technology in the
gas turbine. The design criteria for these engines was high reliability, high performance,
with many starts and flexible operation throughout the flight envelope.
The engine life of about 3500 hours between major overhauls was considered
good. The aerospace engine performance has always been rated primarily on
its thrust/weight ratio. Increase in engine thrust/weight ratio is achieved by the
development of high-aspect ratio blades in the compressor as well as optimizing
the pressure ratio and firing temperature of the turbine for maximum work output
per unit flow.
The Industrial Gas Turbine has always emphasized long life and this conservative
approach has resulted in the Industrial Gas Turbine in many aspects
giving up high performance for rugged operation. The Industrial Gas Turbine
has been conservative in the pressure ratio and the firing temperatures. This has
all changed in the last 10 years; spurred on by the introduction of the “Aero-
Derivative Gas Turbine” the industrial gas turbine has dramatically improved its
performance in all operational aspects. This has resulted in dramatically reducing
the performance gap between these two types of gas turbines. The gas turbine
to date in the combined cycle mode is fast replacing the steam turbine as the
base load provider of electrical power throughout the world. This is even true
in Europe and the United States where the large steam turbines were the only
type of base load power in the fossil energy sector. The gas turbine from the
1960s to the late 1980s was used only as peaking power in those countries. It
was used as base load mainly in the “developing countries” where the need for
power was increasing rapidly so that the wait of three to six years for a steam
plant was unacceptable.
Figures 1-3 and 1-4 show the growth of the Pressure Ratio and Firing Temperature.
The growth of both the Pressure Ratio and Firing Temperature parallel
each other, as both growths are necessary to achieving the optimum thermal
efficiency.
The increase in pressure ratio increases the gas turbine thermal efficiency when
accompanied with the increase in turbine firing temperature. Figure 1-5 shows