1. INTRODUCTION:
1.1 MEMS:
Micro-Electro-Mechanical Systems
(MEMS) is an integration of mechanical elements, sensors, actuators, and
electronics on a common silicon substrate through the utilization of
microfabrication technology. MEMS is truly an enabling technology allowing the
development of smart products by augmenting the computational ability of
microelectronics with the perception and control capabilities of microsensors
and microactuators. MEMS technology makes possible the integration of
microelectronics with active perception and control functions, thereby, greatly
expanding the design and application space.
Although
MEMS devices are extremely small (e.g. MEMS has enabled electrically-driven
motors smaller than the diameter of a human hair to be realized), MEMS
technology is not about size. Furthermore, MEMS is not about making things out
of silicon, even though silicon possesses excellent materials properties making
it a attractive choice for many high-performance mechanical applications.
Instead, MEMS is a manufacturing technology; a new way of making complex
electromechanical systems (like power generation) using batch fabrication
techniques. Already, MEMS is used for everything ranging from in-dwelling blood
pressure monitoring to active suspension systems for automobiles.
Recent
examples of the advantages of MEMS technology consider the MEMS accelerometers,
which are quickly replacing conventional accelerometers for crash air-bag
deployment systems in automobiles.
Microturbine is one of
the best examples of the recently used MEMS. The technology is to generate
power for at a small level for a few houses or as a stand-by power source. It
is given hype now days and further research work is also in progress. Now let us know what exactly the microturbine
is.
1.2 Gas Turbine:
Gas
turbines are Brayton cycle engines, which extract energy from hydrocarbon fuels
through compression, combustion, and hot gas expansion. Air is drawn in to a
compressor, which increases the air pressure. The compressed air is mixed with
fuel and ignited in a combustor. Then, the hot gas is expanded through a
turbine, which drives the compressor and gives useful work through rotation of
the compressor- turbine shaft. The shaft power can be used to drive a
electrical generator, thereby providing electricity.
1.3 Microturbine:
Microturbines are small
gas turbines used to generate electricity. Occupying a space no larger than a
telephone box, they typically have power outputs in the range of 25 to 300kW.
In comparison, large powerstations are entire buildings and have much higher
power outputs of around 600MW to 1000MW. The small size of microturbines is a
major advantage that allows them to be situated right at the source of
electricity demand. This eliminates energy losses that usually occur when
transmitting electricity from power stations. Such transmission losses are
quite significant and can easily amount to 7% of the power generated.
Microturbines
are a new class of small gas turbines used for distributed generation of electricity.
Microturbines are small version of gas turbines emerged from four different
technologies viz. small gas turbines, auxiliary power units, automotive
development gas turbine and turbochargers. Microturbines are new class of gas
turbines used for distributed generation of electricity. Microturbine
development is based on turbines used for aircraft auxiliary power units, which
have been used in commercial airlines for decades.
One way in which microturbines can
be distinguished from larger turbines is that microturbines use a single shaft
to drive the compressor, turbine and generator. Where as in large power plants,
the turbines and generator are on separate shafts and are connected by gears
that slow down the high-speed rotation of the gas turbines, simultaneously
increasing the torque sufficient to turn much large electric generators. Some
microturbines even include the ability to generate electricity from heat of
exhaust gases.
2.0 HISTORY:
In 1900 when a 2 MW steam
turbine was installed at Hartford, its size was 4 times bigger than any of the
existing steam turbines. From then on economy of scale meant bigger and bigger.
By the end of the 1970s and largely driven by nuclear power plants, steam
turbines exceeded 1000 MW. The electric efficiency of steam turbine power
plants eventually reached 34%.
That trend was broken in
the 1980s. More efficient gas turbines combined with steam turbines could
produce electric power with efficiencies up to 55%. This new technology,
combined cycle power plants, was the technology of choice for independent power
producers. It was now possible to build competitive power plants down to the
range of 100-200 MW.
Microturbines
have been experimented with since 1945, when Rover tried to develop one for a
vehicle application. Since that time, automobile, aerospace, aircraft and
military contractors have tried to develop an economical and functional
microturbine for different industrial and commercial applications.
3.0 NEED OF MICROTURBINE:
In today's energy
economy, most electricity is produced using fossil fuel-burning generators.
These machines consist of a motor and a dense coil of copper wires that
surround a shaft containing powerful magnets. To get that power to a home or
factory typically requires a local utility to run a heavy copper cable to the
residence or business site.
But
what if the site requiring energy is in a remote mountain location, or it's an
offshore oil rig where electricity is scarce and hookups don't exist? Here the
microturbines come into the picture. It is one of the best options to set up a
local power-generation plant, perhaps using a microturbine -- a small,
sometimes portable, fossil fuel-burning system that can provide enough
electricity to power anywhere from 10 to 5,000 homes.
Also it has an important
application as a turbocharger in vehicles when more energy is required from the
engine in less amount of fuel.
4. CONSTRUCTION OF MICROTURBINE:
Microturbines are
typically single shaft machines with the compressor and turbine mounted on the same
shaft as the electrical generator. It therefore consists of only one rotating
part, eliminating the need for a gearbox and associated numerous moving parts.
Microturbines
are miniature versions of the huge machines used to generate power from natural
gas, and evolved from aircraft engines and automotive turbochargers.
A cutaway view of a
microturbine is shown in Figure1. The single stage Turbine and Compressor
wheels are inertia welded to the shaft, which supports the generator alternator
rotor and provides for a cold end drive. A block diagram showing a complete
cycle of the microturbine is shown in Figure2. The inner bearing is a
hydrodynamic bearing and the outer bearing utilizes a ceramic ball race. A
device called recuperator plays an important role in completing the cycle of
microturbine.
Fig 1: Sectional view
of a typical microturbine [1].
5. PRINCIPLE AND WORKING OF MICROTURBINES:
The high velocity exhaust gases
coming from the combustor rotate the turbine used in the microturbine. The
basic principle of working of the microturbine is that the compressor as well
as the electric generator is mounted on the same power shaft as that of the
turbine. Because of this the compressor and the generator also rotate with the
turbine.
The generator rotates
with the same speed as that of the turbine and generates the electricity. The
electricity is first given to the power conditioning devices and then it is
supplied to the required areas. The combustor is supplied with the fuel in the
gaseous form by the gas compressor. Also fresh and compressed air is supplied
to the combustor by the compressor through the recuprator.
Fig 2: Working cycle
of a microturbine [2].
Here
the recuprator plays an important role of heat exchanger. It absorbs the heat
from the hot gases coming from the turbine. Then it gives this heat to the
compressed air coming from the compressor. Thus the air supplied to the
combustor is hot and compressed. This helps to increase the overall efficiency
of the cycle.
6. PERFORMANCE:
The
performance of the microturbines is given in the tabular form as below,
|
Table
No.1 MICROTURBINE EFFICIENCY for 25 to
500KW [1]
|
|
|
CONFIGURATION
|
EFFICIENCY
|
|
Unrecuperated
|
15%
|
|
Recuperated
|
20-30%
|
|
With
Heat Recovery
|
Up
to 85%
|
Commercial
microturbines used for power generation range in size from about 25KW to 500KW.
They produce both heat and electricity on a relatively small scale. The energy
to electricity conversion efficiencies are in the range of 20 to 30%. These
efficiencies are attained when using a recuperator. Cogeneration is an option
in many cases as a microturbine is located at the point of power utilization.
The combined thermal electrical efficiency is 85%. Unrecuperated microturbines
have lower efficiencies at around 15%.
7.0 FEATURES:
Microturbines offer many potential
advantages for distributed power generation. Selected strengths and weaknesses
of microturbine of the microturbine technology are listed below:
Advantages :-
Ø
Small number of moving parts.
Ø
Compact size.
Ø
Light weight.
Ø
Good efficiency in cogeneration.
Ø
Low emission.
Ø
Can utilize waste fuel.
Ø
Long maintenance intervals
Limitations:-
Ø
Low fuel to electricity efficiency.
Ø
Loss of power output and efficiency with higher
ambient temperature and elevation
8. FUTURE SCOPE:
Extensive field test data collected
from units currently in use at commercial and industrial facilities will
provide the manufacturers with the ability to improve the microturbine design,
lowering the cost and increasing performance, in order to produce a competitive
distributed generation product. Utilities, government agencies, and other
Organizations
are involved in collaborative research and field-testing.
Development is
ongoing in a variety of areas:
1. Heat recovery/coregeneration
2. Fuel
flexibility
3. Vehicles
4. Hybrid
systems (e.g. fuel cell/microturbine, flywheel/microturbine)
9. APPLICATIONS:
While
the simplest application for a microturbine prime mover is of power generation
other application exists. Microturbine prime movers can be used for cooling,
refrigeration; air compression and pump drive application whereby the inherent
high speed of the power shaft can be used to drive high efficiency and low cost
centrifugal compressors.
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 produce between 25kw to 500kw of power and are well
suited for small commercial building establishments such as restaurants,
hotels/motels, small offices, retail stores and many others.
The development of the
microturbine technology for the transportation application is also in progress.
One of the major applications used is the turbocharger in the small vehicles.
Automotive companies are interested in microturbines to provide a light weight and
efficient fossil fuel- based energy source for hybrid electric vehicles,
especially buses.
9.1 CASE STUDY ON DISTRIBUTED
GENERATION:
9.1.1 Introduction:
Distribution
generation is a concept of installing and operating small electric generators,
typically less than 20MW, at or near electrical load. The premise of
distributed generation is to provide electricity to a customer at a reduced
cost and more efficiently with reduced losses than the traditional utility
central generating plant with transmission and distribution wires.
9.1.2 Microturbine in distributed
generation:
Microturbine
is small scale combustion turbines ranging inside from 28 kW to 500kW, which
include a compressor, combuster, turbine, alternator, recuperator and
generator. Microturbines are smaller, lighter and operate with no vibration and
less noise. All of these features help to make on - site installation possible
without compromising the environmental aspects. They have potential to be
located on site having space limitations to produce power.
9.1.3 Working principle:
The
technology used for distributed generation is that of microturbine. The three
basic equipments, viz. the turbine, the generator and compressor are mounted on
a single shaft. The core of the microturbine is a high-speed compressor
-turbine section, which rotates very fast - 96000rpm in Capston model 330. On
the same shaft is a high-speed generator using permanent magnets. A key element
for designs of microturbine is air bearings (or more correctly gas bearings).
Air bearings enable the high speed only air cooling a long life almost
maintenance free.
The
high speed generator delivers a high frequency power. To "gear it
down" to useful 50/60 Hz power, electronics is brought into
application.
The
following table shows the speed of microturbine for different power generation
capacities.
Table
No.3 Speed of Turbine For Different
Power Ratings [3].
|
POWER
|
SPEED
|
|
45
KW
80
KW
200
KW
|
90,000 TO 1,16,000 (RPM)
70,000 (RPM)
50,000 (RPM)
|
Fig
3: Cutaway of the Capston 330 turbine
[3].
9.1.4 Advantages:
The
general advantages of microturbine are that there are small number of moving
parts are compact in size, light weight and have opportunities for greater
efficiencies, lower emissions, lower electricity costs and use renewable fuels
such as land fill or sewage treatment gases. Microturbine in general offer to
be advantages
1 lower emission and
2 low
maintenance.
As illustrated below (Table no.3),
the Capston microturbine has one of the best emission performances of any
fossil fuel combustion.
Table
no.3 COMPARISION OF EMISSIONS [3]
|
Item
|
NO (ppm)
|
CO (ppm)
|
THC (ppm)
|
|
Reciprocating
Engines (500kW)
|
100
|
340
|
150
|
|
Gas Turbines
(4.5MW)
|
25
|
50
|
10
|
|
Coal Fired
Steam (500MW)
|
200
|
N/A
|
N/A
|
|
Microturbine
|
9
|
25
|
9
|
Source: Cambridge Energy Research Associates.
With very low emissions and
maintenance, microturbines hold promise to enable small-scale cogeneration. To
exhaust heat can be use water heating, absorption cooling, dehumidification,
etc.
It
is possible to reach efficiencies of 70-80%. Because of the three exhausts with
no risk of any oil fuel (due to the air bearing) it should be possible to use
the exhaust gas directly in some industrial processes.
9.1.5 Benefits of Distributed
generation:
Thus the
benefits that distributed generation could potentially provide, depending on
the technology, include reduced emissions, utilization of waste heat, improved
power quality and reliability and deferral of transmission or distribution
upgrades.
9.2 HYDROPOWER GENERATION:
A typical
use of microturbine is the hydropower generation. Microturbine technology
equipments harness the best possible energy source that is the discharge flow
of even minor streams, in that sense; they constitute cleaner, superior
environmental alternatives to the less acceptable fossil fuel powered
generators.
The technology is very simple as
shown in figure .A turbine with a generator on its shaft is fitted in the way of
water flowing in a river or a stream.
In most cases, microturbine views
only a small portion of a stream's flow that is channeled through a penstock.
As it is clear from the figure, we can say that reliable and renewable
hydro-energy generation (with the help of microturbines of course) does not
require a reservoir or the flooding of low-lying areas. Microturbine technology
turbines may be installed low discharge flow streams and rivers. They are
efficient even in cases of low drop river fall, as microturbine technology
turbines generate electricity from as little as one meter of hydraulic
head.
9.3 TURBOCHARGER:
Turbocharger is one of the applications of microturbine. It uses the
principle of mounting the compressor on same shaft as that of turbine. Here
also the exhaust gases drive the turbine. Today with precise control offered by
the computers, turbochargers are making small engines more efficient and
capable of producing more power.
Microturbines
are evolved from automotive and truck turbochargers, auxiliary power units for
airplanes, and small jet engines and are comprised of a compressor, combustor,
turbine, alternator, recuperator, and generator.
9.3.1 Turbocharging principle:
A
turbocharger is a device that uses exhaust gases, rather than the engine power
to run an air pump or compressor. The air pump then forces an increased amount
of air into the cylinders. Both the diesel and gasoline engines in the market
use turbochargers. Figure 3 shows a typical schematic of air and exhausting a
turbocharged engine. High velocity exhaust gases pass out of the exhaust ports.
From there they pass through a turbine driven pump. Here the exhaust gases
cause the exhaust turbine to turn very rapidly.
The
exhaust turbine causes the intake compressor to run very rapidly. As the
compressor turbine runs it draws in a large amount of fresh air. The intake air
is pressurized and forced into the intake port. The increase in the pressure in
the intake manifold is called as boost. Boost may produce pressure in the
intake manifold of about 6 to 10 psi or more depending on the manufacturer.
Figure
shows a chart that compares a turbocharged and a normally aspirated engine.
Note that both the torque and the horse power are increased at all rpm. For
example at 5000 rpm the normally aspirated engine produces about 80hp. at this
rpm, the turbocharged engine can produce about 140hp.
10. CONCLUSION:
As
a breakthrough technology, allowing unparalleled synergy between hitherto
unrelated fields of endeavor such as biology and microelectronics, many new
MEMS applications will emerge, expanding beyond that which is currently
identified or known. In the industrial sector, MEMS devices are emerging as
product performance differentiators in numerous markets with a projected market
growth of over 50% per year.
Microturbines (MEMS) also promises a
lot of further development. The introduction of competition into the electric
marketplace has driven the development of new electrical generation
technologies. Most technologies being developed for distributed generation
application are currently too costly, and can only be utilized in some applications.
For this the microturbine is one of the best applications.
Microturbines are capable of
generating power even with the availability of low grade fuel or low head of
water. It is rightly said that the microturbine will start eating the market
share that diesel engine has so far enjoyed.
REFRENCES
1.
www.microturbine.com
2.
www.distributed generation.com
3.
www.memsnet.org
4. E.Schwaller,
Turbocharger, Automotive Technology, Delmar publications.
5. Electrical
Review, Microturbine, Vol.235, 22nd January 2002.
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