ABSTRACT
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
TECHNOLOGY DISCRIPTION OF MICROTURBINES
2.1 Basic Processes
Microturbines are small gas turbines,
most of which feature an internal heat exchanger called a recuperator. In a microturbine, a radial
flow (centrifugal) compressor compresses the inlet air that is then preheated
in the recuperator using heat from the turbine exhaust. Next, the heated air
from the recuperator mixes with fuel in the combustor and hot combustion gas
expands through the expansion and power turbines. The expansion turbine turns
the compressor and, in single shaft models, turns the generator as well.
Two-shaft models use the compressor drive turbine’s exhaust to power a second
turbine that drives the generator. Finally, the recuperator uses the exhaust of
the power turbine to preheat the air from the compressor. Single-shaft models generally
operate at speeds over 60,000 revolutions per minute (rpm) and generate
electrical power of high frequency, and of variable frequency (alternating
current --AC). This power is rectified
to direct current (DC) and then inverted to 60 hertz (Hz) for U.S. commercial
use. In the two-shaft version, the power turbine connects via a gearbox to a
generator that produces power at 60 Hz. Some manufacturers offer units
producing 50 Hz for use in countries where 50 Hz is standard, such as in Europe and parts of Asia.
2.2 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.
2.3 Basic Components
2.3.1 Turbo-Compressor Package
The
basic components of a microturbine are the compressor, turbine generator, and
recuperator Figure 2.3.1. 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.
2.3.2 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.
2.3.3 Recuperators
Recuperators are heat
exchangers that use the hot turbine exhaust gas (typically around 1,200ºF) to
preheat the compressed air (typically around 300ºF) going into the combustor,
there by reducing the fuel needed to heat the compressed air to turbine inlet
temperature. Depending on microturbine operating parameters, recuperators can
more than double machine efficiency. However, since there is increased pressure
drop in both the compressed air and turbine exhaust sides of the recuperator,
power output typically declines 10 to 15% from that attainable without the
recuperator. Recuperators also lower the temperature of the microturbine
exhaust, reducing the micro turbine’s effectiveness in CHP applications.
2.3.5 Air bearings
They allow the turbine to spin
on a thin layer of air, so friction is low and rpm is high. No oil or oil pump
is needed. Air bearings offer simplicity of operation without the cost,
reliability concerns, maintenance requirements, or power drain of an oil supply
and filtering system. Concern does exist for the reliability of air bearings
under numerous and repeated starts due to metal on metal friction during
startup, shutdown, and load changes. Reliability depends largely on individual
manufacturers' quality control methodology more than on design engineering, and
will only be proven after significant experience with substantial numbers of
units with long numbers of operating
hours and on/off cycles.
2.3.6 Power Electronics
.
The high frequency AC is rectified to DC, inverted back to 60 or 50 Hz
AC, and then filtered to reduce harmonic distortion.. To allow for transients
and voltage spikes, power electronics designs are generally able to handle
seven times the nominal voltage. Most icroturbine power electronics are
generating three phase electricity. Electronic components also direct all of
the operating and startup functions.
FIG 2.3.1 MICROTURBINE BASED CHP SYSTEM ( SINGLE
SHAFT )
FIG 2.3.2
BASIC PARTS OF MICROTURBINE
FIG 2.3.3 MICROTURBINE CONSTRUCTION
Chapter 3
Design Characteristics 0f microturbines
Thermal
output: Microturbines produce thermal output at temperatures in the
400 to 600°F range, suitable for supplying a variety of building thermal needs.
Fuel
flexibility:
Microturbines can operate using a number of different fuels:
Sour gases
(high sulfur, low Btu content), and liquid fuels such
as
gasoline, kerosene, natural gas and diesel fuel/heating oil.
Life Design life is estimated
to be in the 40,000 to 80,000 hour range.
Size range: Microturbines available and under
development are sized
From 25 to 350 KW
Emissions: Low inlet temperatures and high
fuel-to-air ratios result in NOx
Emissions of less than 10 parts per million (ppm) when
Running on natural gas
Modularity: Units may be connected in parallel
to serve larger loads and
Provide power reliability
Dimensions: About 12 cubic feet.
Chapter 4
Microturbines and
distributed generation
Distributed generation, a concept first
promoted by Thomas Edison in the 19th century, is rewiring the way
facility. Operators and environmental mangers think about how electric power
can be produced and distributed. For decades, energy users have waited for the
promise of fuel cells, solar panels, and wind turbines to translate into
reliable and economically viable sources of power. The table shown below
compares the microturbines with other D.G.
resources. Microturbines are quietly delivering on those promises and
proving to be a supplement to traditional forms of power generation. Moving away from 100% dependence on the
utility power grid to having an onsite microturbine power supplement is,
admittedly, a Para diagram shift. But for
progressive environment mangers worldwide, microturbines are quickly becoming
an energy management solution that saves money, resources, and the environment
in one compact and scalable package- is it stationary or mobile, remote or
interconnected with the utility
Chapter 5
Economics of
Microturbines[
Micro
turbine capital costs range from $700-$1,100/kW. These costs include all
hardware, associated manuals, software, and initial training. Adding heat
recovery increases the cost by $75-$350/kW. Installation costs vary
significantly by location but generally add 30-50% to the total installed cost.
Micro turbine manufacturers are targeting a future cost below $650/kW. This
appears to be feasible if the market expands and sales volumes increase.
With fewer
moving parts, micro turbine vendors hope the units can provide higher
reliability than conventional reciprocating generating technologies.
Manufacturers expect that initial units will require more unexpected visits,
but as the products mature, a once-a-year maintenance schedule should suffice.
Most manufacturers are targeting maintenance intervals of 5,000-8,000 hours.
Maintenance
costs for micro turbine units are still based on forecasts with minimal
real-life situations. Estimates range from $0.005-$0.016 per kWh, which would be
comparable to that for small reciprocating engine systems.
Micro turbine Cost
|
|
Capital
Cost
|
$700-$1,100/kW
|
O&M
Cost
|
$0.005-0.016/kW
|
Maintenance
Interval
|
5,000-8,000
hrs
|
Chapter 6
Fuel Flexibility of microturbines[2][3]
Microturbines are small
power plants operate on natural gas, diesel, gasoline or other similar
high-energy, fossil fuel. However, research is progressing on using lower
grade; lower energy fuels such as gas produced from biomass to power the
microturbine. This gas, called biogas, is a combustible gas derived from
decomposing biological wastes that have undergone conversion by biological
decomposition called anaerobic digestion or by thermal decomposition in a
gasifier which is called pyrolysis.
In a forest, a gasifier could be used to
convert wood chips and pine needles to a biogas on site. By making
modifications, the turbine will be able to utilize low pressure fuels with
lower energy content than traditional fuels. Natural gas-fired turbines have
fuel with a heating value of 1,000 British thermal units per cubic foot.
Biogases typically have between 10 and 20 percent of the heating value of
fossil fuels. The thrust of current research is concentrated on fuel
flexibility. The goal is to modify microturbines so they can utilize low
energy, low pressure biogases. In order to do this, a key change is to add a
catalytic combustor. An added benefit of the catalytic combustor is that it
will eliminate the formation of nitrogen oxides, a technology breakthrough.
These modified microturbines have been nicknamed "Flex-microturbines".
Chapter
7
MIROTURBINES
TEST PROCEDURE & INSTALLATION
7.1 Test Procedures [3]
To fully evaluate the MTGs, a
series of tests were developed. Testing of MTGs has been categorized into the following phases:
- Installation and Startup.
- Operation and Maintenance.
- Performance.
7.2 Installation and Startup Procedures
Each MTG
delivered to the test site was inspected and confirmed to include:
·
Operating
instructions.
·
Repair
parts or a recommended spare parts list.
·
Consumable
supplies.
·
Troubleshooting
and maintenance procedures/guides.
·
Drawings
and diagrams sufficient to support maintenance.
7.3 Performance Procedure
For the test program, MTGs were operated
for as long as practicable at the full load the units were capable of producing
under ambient conditions. Daily operating parameters: fuel flow, ambient air
pressure, temperature and humidity, energy output, operating temperatures, and
pressures were recorded. The recorded MTG parameters were used to determine
heat rate and efficiency, gross and net
peak kilowatts, operating hours, capacity factor and availability. Capacity
factor for an operating period is the ratio of the actual kilowatt hours
generated to the maximum potential kilowatt hours for the rating of the unit.
Availability for an operating period is the ratio of the hours the unit was not
restricted from operation due to maintenance or repairs unit to the maximum
possible hours of operation. Peak gross power is defined as the peak power
output by the MTG inverter.
Chapter 8
Development of Chemically Recuperated
Micro gas turbine [1]
outline of the
chemically Recuperated Gas Turbine (CRGT) System
Figure 8.1 shows a block diagram of a
CRGT system. In an MGT system, turbine exhaust temperature is about 600°C, and
the power generation efficiency is increased by heat recovery of air
recuperator, but in CRGT system, the reformer recovers the turbine heat first.
As shown in Fig. 8.1 .The equipment
after the reformer is the air recuperator, and next is the evaporator, but the
system works if sequence of lie equipment after the reformer is arranged in
accordance with layout of the engine system of micro gas turbines The amount
of heat recovered by the reformer is
determined by enthalpy change of chemical reaction which converts fuel and
steam into hydrogen rich gas. The principal reactions in a reformer are
expressed as follows:
The reaction rate of water gas
shift is larger than that of steam reforming, and it is regarded as
equilibrium. Thus, the molar stoicometric ratio between methane and steam
(i.e., steam carbon ratio, SIC) in the above reactions is 2.0, but SI\C is
operated from 3 to 4 in order to avoid deposition of carbon by Boudouard’s
reaction expressed as follows:
The steam reforming
reaction occurs partially below 600°C, methane of 100% is not converted into
hydrogen-rich gas. The maximum conversion of methane is limited
thermodynamically, which is influenced
by physical conditions: temperature T, total pressure P. and SI\C For example, equilibrium conversion is about
35% under the conditions of ‘I’ 500°C, P=0.4 MPa, and S/C = 4.0. The reaction
pressure P in the reformer is higher than that in the combustor and it ranges
between 0.3 and 0.5 Mpa. The more temperature or S/C increases, the more
equilibrium conversion increases. Therefore, it is preferable that temperature
and SIC in the reformer are set high to enlarge heat recovery.
However, configurations of the
gas turbine and the evaporator restrict the amount of steam, and S/C has an
upper limit. In the gas turbine, surge limit of the compressor determines the
maximum flow rate including steam through the turbine. In the evaporator, the
upper limit of steam generation is
determined because the pinch point temperature difference becomes critical.
Hence, the maximum S/C is set to approximately 7 Besides, the temperature in
the reformer is influenced by the
configuration of the reformer because the reaction occurs in the reformer
exchanging heat with turbine exhaust. Hence, the design of the reformer is very
important
Figure
8.1 shows the principal stream conditions of a chemically recuperated MGT
based on a commercial 75 kW MTG system using natural gas as fuel. Figure8.2 shows a schematic
illustration of heat balance in the system .Figure 8.2 also shows each energy percentage of natural gas LHV
(lower heating value) .. In the system shown in Fig. 8.1, as much steam possible is generated to enlarge heat
recovery by large conversion and S/C is set to 6.3. The heat duty of the
reformer is estimated considering reactions as equilibrium, and the reformer
uses exhaust heat from 656°C to 561°C. The mole fraction of hydrogen is the
reformer output is 24%, and the conversion rises to 51%. The heat recovery at
the reformer is 82.4 kW, which recovers 25.6% of fuel LHV. The air recuperator is placed after reformer and it
heats compressed air until 520°C using exhaust heat from 561°C to 274°C. This
recovers 40.5 kW. The evaporator uses the bottom of exhaust heat from 274°C to
121°C. According to this system analysis, the output and the efficiency were
expected to improve up to 98 kW and 30.4%, respectively, comparing with 75 kW
and 28% of the original MGT.
Fig 8.1 The principle stream conditions for
CRGT system
Fig 8.2 heat balance in CRGT
system expressed as % of natural gas LHV
Chapter 9
Applications
of Microturbines [4]
Combined
heat and power (co-generation) (chp)
Waste heat from the microturbine can be transferred via a heat exchanger to produce steam or provide hot water for local area. The hot water can be used in a greenhouse to grow plants; water can be ducted to provide central heating in buildings in winter. Thermal hosts can be found easier because the heat produced by each microturbine unit is so much smaller than that by a large power station.
Waste heat from the microturbine can be transferred via a heat exchanger to produce steam or provide hot water for local area. The hot water can be used in a greenhouse to grow plants; water can be ducted to provide central heating in buildings in winter. Thermal hosts can be found easier because the heat produced by each microturbine unit is so much smaller than that by a large power station.
Distributed power generation
Hospitals, hotels, factories and holiday resorts can install distributed power systems on site to supplement power supplied by grid. Also, electricity can be generated at remote sites without grid access. Distributed generation provides a wide range of services to consumers and utilities, including standby generation, peak shaving capability, baseload generation and co-generation.
Hospitals, hotels, factories and holiday resorts can install distributed power systems on site to supplement power supplied by grid. Also, electricity can be generated at remote sites without grid access. Distributed generation provides a wide range of services to consumers and utilities, including standby generation, peak shaving capability, baseload generation and co-generation.
Hybrid (microturbine connected to high
speed alternator):[4]
In hybrid vehicle applications, the power produced by a microturbine is converted into electricity by a high-speed alternator. The power is used to drive electric motors connected to the wheels. Any excess energy is directed to an energy storage system such as batteries or flywheels.
In hybrid vehicle applications, the power produced by a microturbine is converted into electricity by a high-speed alternator. The power is used to drive electric motors connected to the wheels. Any excess energy is directed to an energy storage system such as batteries or flywheels.
Hybrid
vehicle (microturbine and fuel):
Hybrid systems take advantage of an increase
in fuel cell efficiency with an increase in operating pressure. The microturbine
compressor stage is used to provide this pressure. The fuel cell produces heat
along with power, and this heat energy is used to drive the microturbine’s
turbine stage. If the fuel cell produces enough heat, the microturbine can
generate additional wer. For the hybrid combination, efficiency is expected to
be as much as 60% and emissions less than 1.0 ppm NOx, with negligible Sox and
other pollutants.
Chapter 10
Conclusion
The drawbacks of centralized power generation and shortage of power
leading to concept of Distributed generation (DG).DG tends to several
advantages and concept of DG is more feasible. Microturbine is the application
of DG .The history of IC engine . Shows several year research works for today’s better result. Therefore microturbine is
tomorrow’s world. Microturbine can use low grade of
fuel very effectively like waste gases, sour gases etc.
Thus
microturbine gives chance of low fuel cost and less emission. The dimensions of
Microturbine comparatively small by which it can be installed at field where
power is consumed. It has few efficiency problems. Due to chemical recuperation
the thermal efficiency increases sharply. Microturbine is also effective in CHP
operation .It is having problem of Starting time and that’s why it fails as standby power generator
compared to IC engines. In India
the microturbine is quite useful. The power shortage effect can be solved using
microturbine, using fuels like biogas, etc .But in India the technology is still
underdevelopment so the present seminar is an honest attempt to introduce
microturbine technology in India
for solving the problem of power generation in future.
