Definition
Distributed generation is one of the
important field of research now a days. Market prospect for microturbine for
distributed power generation and their associated high grade heat extremely
encouraging. Therefore microturbines are becoming a point of study. Presented
paper tries to discus the microturbine concept, technology description gives
technical and practical background through basic process and thermodynamic
cycle. Various components of microturbine and their performance is briefly
analyzed.
To improve this characteristics and
efficiency the points like development of chemically recuperated gas turbine is
added. Microturbine economics is a big question .Fuel used for microturbine
gives list of all possible fuel to be used. Manufacturers and availability of microturbine
is one important discussed point. Application gives idea about customer range,
customer targeted stand by power, hybrid electric vehicles, CHP
operation, etc. At the end conclusion is drawn for the feasibility study of
microturbine .
Thermodynamic
Cycle
Microturbines operate on the same
thermodynamic cycle, known as the Brayton cycle, as larger gas turbines. In
this cycle, atmospheric air is compressed, heated, and then expanded, with the
excess power produced by the expander (also called the turbine) over that
consumed by the compressor used for power generation. The power produced by an
expansion turbine and consumed by a compressor is proportional to the absolute
temperature of the gas passing through those devices. Consequently, it is
advantageous to operate the expansion turbine at the highest practical
temperature consistent with economic materials and to operate
the compressor with inlet airflow at as low a
temperature as possible. Higher temperature and pressure ratios result in
higher efficiency and specific power. Thus, the general trend in gas turbine
advancement has been towards a combination of higher temperatures and
pressures. However, microturbine inlet temperatures are generally limited to
1,800ºF or below the use of relatively inexpensive materials for the turbine
wheel, and to maintain pressure ratios at a comparatively low 3.5 to 4.0.
Turbo-Compressor
Package
The heart of the microturbine is the
compressor-turbine package, which is commonly mounted on a single shaft along
with the electric generator. Two bearings support the single shaft. The single
moving part of the one-shaft design has the potential for reducing maintenance
needs and enhancing overall reliability. There are also two-shaft versions, in
which the turbine on the first shaft directly drives the compressor while a
power turbine on the second shaft drives a gearbox and conventional electrical
generator producing 60 Hz power. The two shaft design features more moving
parts but does not require complicated power electronics to convert high
frequency AC power output to 60 Hz.
Moderate to large-size gas turbines
use multi-stage axial flow turbines and compressors, in which the gas flows
along the axis of the shaft and is compressed and expanded in multiple stages.
However, microturbine turbo machinery is based on single-stage radial flow
compressor and turbines. Rotary vane and scroll compression are the most
commonly used technology in the microturbine industry. Second generation
gas compressor technologies are in development or being introduced. That may
reduce costs and target on-board application Rotary vane compression technology
offers a wide range of gaseous fuel flexibility Parasitic loads vary based
on type of gas and inlet pressures available, general rule 4 to 6% for
natural gas and 10 to 15% for bio gas.
Generator:
The microturbine produces electrical
power either via a high-speed generator turning on the single turbo-compressor
shaft or with a separate power turbine driving a gearbox and conventional 3,600
rpm generator. The high-speed generator of the single-shaft design employs a
permanent magnet (typically Samarium-Cobalt) alternator, and requires that the
high frequency AC output (about 1,600 Hz for a 30 kW machine) be converted to
60 Hz for general use. This power conditioning involves rectifying the high
frequency AC to DC, and then inverting the DC to 60 Hz AC. Power conversion
comes with an efficiency penalty (approximately five percent).To start-up a
single shaft design, the generator acts as a motor turning the turbo-compressor
shaft until sufficient rpm is reached to start the combustor. Full start-up
requires several minutes. If the system is operating independent of the grid
(black starting), a power storage unit (typically a battery UPS) is used to
power the generator for start-up.
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