Microturbine generators(MTG) are small, high speed power plants
that are usually include the turbine, compressor and power electronics to
deliver the power to the grid. These small power plants typically operate on
natural gas. Future units may have the potential to use lower energy fuels such
as gas produced from landfill or digester gas. Microturbine generators are
classified into two types:
UNRECUPERATED
MTG
In a simple cycle or unrecuperated systems the compressed air is
mixed with fuel and burned under constant pressure conditions. The resulting
hot gas is allowed to expand through a turbine to perform work. Simple cycle
MTGs have lower efficiency at around 15%, but also lower capital costs, higher
reliability and more heat available for co-generation applications than
recuperated units.
RECUPERATED
MTG
Recuperated units use a thin
sheet-metal heat exchanger that recovers some of the heat from an exhaust stream
(1,200ºF) and transfers it to the incoming air stream, boosting the temperature
of the air stream (around 300ºF) supplied to the combustor. Further exhaust
heat recovery can be used in a co-generation configuration. The fuel-energy to
electrical conversion efficiencies are in the range of 20 to 30%. In addition,
recuperated units can produce 30 to 40% fuel savings from preheating. Depending
on the microturbine operating parameters, recuperators can more than double
machine efficiency.
The entire microturbine generator
system can be divided into three primary sub-systems:
Mechanical
Internal view showing the part of an MTG Block
Generator/Gearbox
The standard Power Works (NREC’s microturbine)
package incorporates a single-stage helical gear set to transfer power from the
turbine to the 3600 RPM generator. The low-torque, highsliding- velocity
results in exceptional design-life margins. At the conditions specified for the
PSOFC, the gear and bearing life exceed one million hours.
A commercial 2-pole 3600 RPM induction
generator is standard with the Power Works package, and for a production
version of the proposed system would be the probable choice. The manufacturer
predicts a B10 life of 160,000 hours for normal service. The generator has been
conservatively selected and operates in a cool, clean, low-vibration
environment. For cold weather and extended peaking-power operation, a higher
power rated generator can be provided. An optional synchronous generator can
also be substituted for grid-isolated operation, as proposed in connection with
the current experimental program.
Combustor
The combustor proposed
for the integrated PSOFC package would be a modification of the standard
patented Power Works (NREC’s microturbine) design, originally developed in 1990
in collaboration with SoCal Gas. It has consistently demonstrated NOx levels
below 9ppmv, with exceptionally good turndown stability and proven durability.
Departure from the
standard Power Works (NREC’s microturbine) design is needed to limit combustor
pressure loss during unfired operation. Combustor inlet temperature under these
conditions will be in the vicinity of 1600F, whereas the current running
condition is around 1200F. The design change needed to accommodate this
difference is straightforward, and is roughly a matter of increasing the
effective flow area of the combustor.
Recuperator
Recuperator is a heat exchanger which transfers
heat from the exhaust gas to the discharge air before it enters the combustor
to reduce the amount of fuel required to raise the discharge air temperature to
that required by the turbine.
Turbine
There are two kinds of
turbines, high speed single shaft turbine and split shaft turbines. All are
small gas turbines .Turbine
Isometric
view of an MTG
Electrical
The electrical
system includes main control software, inverter and power firmware.
Engine controller
Engine
controller is a digital system which controls the entire process of the
microturbine generator. They provide the provision of automated starting and
all. And we can set the delay using this system. They will also locate the
fault occurred and perform the safety functions & speed can be controlled.
Engine controller will reduce the power output produced if the engine is
running near its maximum permitted temperature. They also have the ability to
interact with the other parts of the generator control systems.
Power Conditioning System
We
know the power output of a microturbine generator will be between the frequency
ranges 1.5-4 kHz. For our usage it must have to be converted to the useable
standard mode.
The power
conditioning system converts the unregulated, variable-frequency output of the
generator into a high quality, regulated waveform. The waveform quality
surpasses the general utility standards and is suitable for supplying sensitive
equipment. Output voltage and frequency are adjustable between desirable
ranges, allowing the system to be easily configured for the operation anywhere.
In the
figure, we can see that an MTG feeding 3-phase power to a rectifier and the dc
is then fed to a high frequency, a single-phase inverter so that a compact,
high frequency transformer can be used. The secondary of the transformer feeds
an ac/ac converter that takes the single phase, high frequency voltage to
produce a 3-phase voltage at a frequency and phase needed to make a direct
connection to the grid.
Power
Controller
They are
mostly on-board, pc-based, a processor linked to pc, etc., depending on constraints and factors such as MTG
packaging, desired versatility, type of available features, and the
sophistication/maturity of the system design. A power controller control and
co-ordinates the operation of the power conditioning circuit by ensuring that
the functions such as voltage following, current following, phase matching,
harmonic suppression, etc are performed reliably and at high efficiency.
Fuel
system
Microturbine
generator have fuel flexibility and are capable of using alternative fuels
including natural gas, diesel, ethanol, landfill gas and other bio-mass derived
liquids & gases. The microturbine generators are fitted with fuel boosters
which reduce the fuel consumption. For 2 kW power, the machine consumes only 25
icfm.
WORKING
Mechanically
the microturbine generator is a single shaft gas turbine with the entire
compressor, power turbine and the permanent magnet generator being mounted on
the same shaft. The power turbine drives the generator which produces the
electrical power and speed of rotation of this power turbine is from
50000-120,000 rpm.
During
engine operation, air is drawn into the compressor unit through an air filter.
The air filter will filter out unwanted components in the air. The compressor
unit will then compress in taken air and raises its pressure to a heavy value.
The high pressure air then is introduced into a recuperator arrangement where
the heat exchanging process takes place. Inside the recuperator, the exhaust
air from the turbine after burning the fuel, possessing a temperature around
650 degrees Celsius will then transfer the heat to the compressed air and thereby
increase the temperature by 200 degree Celsius.
Figure illustrating the working of an MTG
Now, the
hot air is passed into the combustion chamber. Simultaneously the fuel which is
also get compressed in a gas compressor is introduced and mixed with high temperature
air and due to this burning of fuel will occur, producing high temperature gas
or steam. This gas is then taken into the power turbine by means of a nozzle.
As a result the thermal energy holding by the gas is used effectively to rotate
the turbine to high speed.
Thus the
generator which is coupled to the turbine wheel is get rotated and eventually
the electrical power is produced at higher frequencies which is later get
regulated.
Advantages
·
MTG has small number of moving parts, therefore maintenance is comparably
less.
·
It has compact size.
·
Most of the parts are light weight.
·
Good efficiency.
·
Low emission & less noise and vibration than reciprocating systems.
·
Can utilize waste fuels.
·
Strengthens energy security.
·
Cheap and easy installation.
·
Wide range of benefits in terms of operational and fuel flexibility,
service performance and maintainability.
Disadvantages
·
Low power output & efficiency with higher ambient temperatures
·
Time-variable electrical and thermal demand distorts MTG’s energy balance
sometimes leading to larger fuel requirement.
Applications
Microturbines
can be used for stand-by power, power quality and reliability, peak shaving,
and cogeneration applications. In addition, because microturbines are being
developed to utilize a variety of fuels, they are being used for resource
recovery and landfill gas applications. Microturbines are well suited for small
commercial building establishments such as: restaurants, hotels/motels, small offices, retail stores, and
many others.
