Post on 05-Apr-2018
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1 .INTRODUCTION
1.1Definition
By definition, Cogeneration is on-site generation and utilisation of energy in different formssimultaneously by utilizing fuel energy at optimum efficiency in a cost-effective and
environmentally responsible way. Cogeneration systems are of several types and almostall types
primarily generate electricity along with making the best practical use of the heat,which is an
inevitable by-product. The most prevalent example of cogeneration is the generation of electric
power and heat.
The heat may be used for generating steam, hot water, or for cooling through absorption chillers.
In a broad sense, the system, that produces useful energy in several forms by utilising the energy
in the fuel such that overall efficiency of the system is very high, can be classified as
Cogeneration System . The concept is verysimple to understand as can be seen from followingpoints
(1) Conventional utility power plants utilise the high potential energy available in thefuels at the
end of combustion process to generate electric power. However,substantial portion of the low-
end residual energy goes to waste by rejection tocooling tower and in the form of high
temperature flue gases.
(2)On the other hand, a cogeneration process utilizes first the high-end potentialenergy to
generate electric power and then capitalizes on the low-end residualenergy to work for heating
process, equipment or such similar use.
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1.2 Background
Cogeneration first appearedin late 1880s in Europe and in the U.S.A. during the early
parts of the 20th century, when most industrial plants generated their own electricity using
coal-fired boilers and steam-turbine generators
When central electric power plants and reliable utility grids were constructed andthe costs of
electricity decreased, many industrial plants began purchasing electricity andstopped producing
their own. Other factors that contributed to the decline of industrialcogeneration were the
increasing regulation of electric generation, low energy costswhich represent a small percentage
of industrial costs, advances in technology suchas packaged boilers, availability of liquid or
gaseous fuels at low prices, and tighteningenvironmental restrictions
The afore mentioned trend in cogeneration started being inverted after the first dramaticrise of
fuel costs in 1973. Systems that are efficient and can utilise alternative fuels havebecome moreimportant in the face of price rises and uncertainty of fuel supplies.
In addition to decreased fuel consumption, cogeneration results in a decrease of
pollutantemissions. For these reasons, governments in Europe, U.S.A. South East Asia ,INDIA
and Japan are taking an active role in the increased use of cogeneration.
In India, the policy changes resulting from modernized electricity regulatory rules have
induced710MW of new local power generation projects in Sugar Industry. Other core sector
industriesare also already moving towards complete self generation of heat and electricity
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2. PRINCIPLE OF COGENERATION
Cogeneration or Combined Heat and Power (CHP) is defined as the sequential generation of two
different forms of useful energy from a single primary energy source, typically mechanical
energy and thermal energy. Mechanical energy may be used either to drive an alternator for
producing electricity, or rotating equipment such as motor, compressor, pump or fan for
delivering various services. Thermal energy can be used either for direct process applications or
for indirectly producing steam, hot water, hot air for dryer or chilled water for process cooling.
Cogeneration provides a wide range of technologies for application in various domains of
economic activities. The overall efficiency of energy use in cogeneration mode can be up to 85
per cent and above in some cases.
For example in the scheme shown in Figure1 an industry requires 24 units of electrical energy
and 34 units of heat energy. Through separate heat and power route the primary energy input inpower plant will be 60 units (24/0.40). If a separate boiler is used for steam generation then the
fuel input to boiler will be 40 units (34/0.85). If the plant had cogeneration then the fuel input
will be only 68 units (24+34)/0.85 to meet both electrical and thermal energy requirements. It
can be observed that the losses, which were 42 units in the case of, separate heat and power has
reduced to 10 units in cogeneration mode.
Fig 1: advantage of cogeneration
Along with the saving of fossil fuels, cogeneration also allows to reduce the emission of
greenhouse gases (particularly CO2 emission). The production of electricity being on-site, the
burden on the utility network is reduced and the transmission line losses eliminated.
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3.NEED FOR COGENERATION
Thermal power plants are a major source of electricity supply in India. The conventional
method of power generation and supply to the customer is wasteful in the sense that only
about a third of the primary energy fed into the power plant is actually made available to the
user in the form of electricity (Fig2). In conventional power plant, efficiency is only 35% and
remaining 65% of energy is lost. The major source of loss in the conversion process is the heat
rejected to the surrounding water or air due to the different thermodynamic cycles employed
in power generation. Also further losses of around 10-15% are associated with the transmission
and distribution of electricity in the electrical grid.
Fig2 :Balance in typical coal fired power station(for an input energy of 100GJ )
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4.CLASSIFICATION OF COGENERATION SYSTEM
4.1 TOPPING CYCLE
In a topping cycle , fuel is burnt in the boiler to produce high temperature steam .This steam is
expanded in a turbine coupled to a generator to give electric power. The rejected from the
turbine is used for manufacturing process .
Fig 3 : Topping Cycle
4.2 BOTTOMING CYCLE
In bottoming cycle , fuel is burnt in the boiler to produce steam . This steam is used for
manufacturing process. The reject heat from the process is used to generate electricity.
Thus in a topping cycle electrical energy is produced first whereas in bottoming cycle heatgenerated is used first. Generally the steam required for industrial process is at low
temperature whereas high temperature steam is needed for electric power generation.
Therefore only the topping cycle is used .the bottoming cycle has very limited utility
Fig 4: bottoming cycle
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5. FACTORS INFLUENCING COGENERATION CHOICE
The selection and operating scheme of a cogeneration system is very much site-specific and
depends on several factors, as described below:
5.1- Base electrical load matching
In this configuration, the cogeneration plant is sized to meet the minimum electricity demand
of the site based on the historical demand curve. The rest of the needed power is purchased
from the utility grid. The thermal energy requirement of the site could be met by the
cogeneration system alone or by additional boilers. If the thermal energy generated with the
base electrical load exceeds the plants demand and if the situation permits, excess thermal
energy can be exported to neighbouring customers.
5.2- Base thermal load matching
Here, the cogeneration system is sized to supply the minimum thermal energy requirement of
the site. Stand-by boilers or burners are operated during periods when the demand for heat is
higher. The prime mover installed operates at full load at all times. If the electricity demand of
the site exceeds that which can be provided by the prime mover, then the remaining amount
can be purchased from the grid. Likewise, if local laws permit, the excess electricity can be sold
to the power utility.
5.3-Electrical load matching
In this operating scheme, the facility is totally independent of the power utility grid. All the
power requirements of the site, including the reserves needed during scheduled and
unscheduled maintenance, are to be taken into account while sizing the system. This is also
referred to as a stand-alone system. If the thermal energy demand of the site is higher than
that generated by thecogeneration system, auxiliary boilers are used. On the other hand, when
the thermal energy demand is low, some thermal energy is wasted. If there is a possibility,
excess thermal energy can be exported to neighbouring facilities.
5.4-Thermal load matching
The cogeneration system is designed to meet the thermal energy requirement of the site at any
time. The prime movers are operated following the thermal demand. During the period when
the electricity demand exceeds the generation capacity, the deficit can be compensated by
power purchased from the grid. Similarly, if the local legislation permits, electricity produced in
excess at any time may be sold to the utility.
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6. IMPORTANT PARAMETERS OF COGENERATION
While selecting cogeneration systems, one should consider some important technical parameters
that assist in defining the type and operating scheme of different alternative cogeneration
systems to be selected.
6.1- Heat-to-power ratio
Heat-to-power ratio is one of the most important technical parameters influencing the selection
of the type of cogeneration system. The heat-to-power ratio of a facility should match with the
characteristics of the cogeneration system to be installed.
It is defined as the ratio of thermal energy to electricity required by the energy consuming
facility. It is presented on the basis of the energy unit (kW).
Table 1 Heat-to-power ratios and other parameters of cogeneration systems
CogenerationSystem Heat-to-power
ratio(kWth/ kWe)
Power output(asper cent of fuelinput)
Overall efficiency(per cent)
Back-pressuresteam turbine
4.0-14.3 14-28 84-92
Extraction-condensing steamturbine
2.0-10.0 22-40 60-80
Gas turbine 1.3-2.0 24-35 70-85
Combined cycle 1.0-1.7 34-40 69-83
Reciprocating
engine
1.1-2.5 33-53 75-85
Cogeneration uses a single process to generate both electricity and usable heat or cooling. The
proportions of heat and power needed (heat: power ratio) vary from site to site, so the type of
plant must be selected carefully and appropriate operating schemes must be established to match
demands as closely as possible. The plant may therefore be set up to supply part or all of the site
heat and electricity loads, or an excess of either may be exported if a suitable customer is
available.
The ratio of heat to power required by a site may vary during different times of the day and
seasons of the year. Importing power from the grid can make up a shortfall in electrical output
from the cogeneration unit and firing standby boilers can satisfy additional heat demand. Many
large cogeneration units utilize supplementary or boost firing of the exhaust gases in order to
modify the heat: power ratio of the system to match site loads.
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6.2 -Quality of thermal energy needed
The quality of thermal energy required (temperature and pressure) also determines the type of
cogeneration system. For a sugar mill needing thermal energy at about 120C, a topping cycle
cogeneration system can meet the heat demand. On the other hand, for a cement plant requiring
thermal energy at about 1450C, a bottoming cycle cogeneration system can meet both high
quality thermal energy and electricity demands of the plant.
6.3 -Load patterns
The heat and power demand patterns of the user affect the selection (type and size) of the
cogeneration system. For instance, the load patterns of two energy consuming facilities shown in
figure 5 would lead to two different sizes, possibly types also, of cogeneration systems.
Fig 5: Different heat to power demand patteren in two factories
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6.4-Fuels available
Depending on the availability of fuels, some potential cogeneration systems may have to be
rejected. The availability of cheap fuels or waste products that can be used as fuels at a site is oneof the major factors in the technical consideration because it determines the competitiveness of
the cogeneration system.
A rice mill needs mechanical power for milling and heat for paddy drying. If a cogeneration
system were considered, the steam turbine system would be the first priority because it can use
the rice husk as the fuel, which is available as waste product from the mill.
6.5- System reliability
Some energy consuming facilities require very reliable power and/or heat; for instance, a pulp
and paper industry cannot operate with a prolonged unavailability of process steam. In such
instances, the cogeneration system to be installed must be modular, i.e. it should consist of more
than one unit so that shut down of a specific unit cannot seriously affect the energy supply.
6.6-Local environmental regulation
The local environmental regulations can limit the choice of fuels to be used for the proposedcogeneration systems. If the local environmental regulations are stringent, some available fuels
cannot be considered because of the high treatment cost of the polluted exhaust gas and in some
cases, the fuel itself.
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7. TECHNICAL OPTIONS FOR COGENERATION
7.1 -Steam turbine based cogeneration system
Steam turbines systems can use a variety of fuels, including natural gas, solid waste, coal, wood,
wood waste, and agricultural by-products. Steam turbines are highly reliable and can meet
multiple heat grade requirements. Steam turbines typically have capacities between 50 kW and
250 MW and work by combusting fuel in a boiler to heat water and create high-pressure steam,
which turns a turbine to generate electricity.The low-pressure steam that subsequently exits the
steam turbine can then be used to provide useful thermal energy. Ideal applications of steam
turbine-based cogeneration systems include medium- and large-scale industrial or institutional
facilities with high thermal loads and where solid or waste fuels are readily available for boileruse.
Fig 6: Steam turbine based cogeneration system
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7.2 -Gas turbine based cogeneration systems
Gas turbines typically have capacities between 500 kilowatts (kW) and 250 megawatts (MW),
can be used for high-grade heat applications, and are highly reliable.Gas turbines operate
similarly to jet enginesnatural gas is combusted and used to turn the turbine blades and spin
an electrical generator. The cogeneration system then uses a heat recovery system to capture
the heat from the gas turbines exhaust stream. This exhaust heat can be used for heating (e.g.,
for generating steam for industrial processes) or cooling (generating chilled water through an
absorption chiller). About half of the CHP capacity in the United States consists of large
combined cycle systems that include two electricity generation steps (the combustion turbine
and a steam turbine powered by heat recovered from the gas turbine exhaust) that supply
steam to large industrial or commercial users and maximize power production for sale to the
grid. Fig 7 shows how a simple-cycle gas turbine cogeneration system recovers heat from the
gas turbines hot exhaust gases to produce useful thermal energy for the site.
Fig 7: Gas turbine based cogeneration system
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8. ADVANCED COGENERATION USING MICROTURBINE
8.1 Introduction
A new class of small gas turbines called microturbines is emerging for the distributed resource
market. Several manufacturers are developing competing engines in the 25-250 kW range,
however, multiple units can be integrated to produce higher electrical output while providing
additional reliability. Most manufacturers are pursuing a singleshaft design wherein the
compressor, turbine and permanent-magnet generator aremounted on a single shaft supported on
lubrication-free air bearings. These turbinesoperate at speeds of up to 120,000 rpm and are
powered by natural gas, gasoline, diesel,and alcohol. The dual shaft design incorporates a power
turbine and gear for mechanical drive applications and operate up to speeds of 40,000 rpm.
Microturbines are a relatively new entry in the CHP industry and therefore many of the
performance characteristics are estimates based on demonstration projects and laboratory testing.
8.2 Technology Description
The operating theory of the microturbine is similar to the gas turbine, except that most designs
incorporate a recuperator to recover part of the exhaust heat for preheating thecombustion air. As
shown in (fig 8) air is drawn through a compressor section, mixedwith fuel and ignited to power
the turbine section and the generator. The high frequencypower that is generated is converted to
grid compatible 50/60HZ through power conditioning electronics. For single shaft machines, astandard induction or synchronous generator canbe used without any power conditioning
electronics.
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8.3 Design Characteristics
Fig 8: schematic diagram of micro turbine
8.3.(a) Compact: Their compact and lightweight design makes microturbinesan attractive
option for many light commercial/ industrialapplications.
8.3.(b) Right-sized: Microturbine capacity is right sized for many customerswith relatively
high electric costs.
8.3.(c) Lower noise: Microturbines promise lower noise levels and can belocated adjacent to
occupied areas.
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8.4 Performance Characteristics
8.4.(a) Efficiency
Most designs offer a recuperator to maintain high efficiency while operating at combustiontemperatures below NOx formation levels. With recuperation, efficiency iscurrently in the 20%-30% range.
8.4.(b) Capital Cost
Installed prices of $500-1000/kW for CHP applications is estimated when microturbines aremass produced.
8.4.(c) Availability
Although field experience is limited, manufacturers claim that availability will be similarto othercompeting distributed resource technologies, i.e. in the 90->95% range.
8.4.(d) Maintenance
Microturbines have substantially fewer moving parts than engines. The single shaftdesign with
air bearings will not require lubricating oil or water, so maintenance costsshould be below
conventional gas turbines. Microturbines that use lubricating oil shouldnot require frequent oilchanges since the oil is isolated from combustion products. Onlyan annual scheduled
maintenance interval is planned for micoturbines. Maintenancecosts are being estimated at
0.006-0.01$/kW.
8.5 Heat Recovery
Hot exhaust gas from the turbine section is available for CHP applications. As
discussedpreviously, most designs incorporate a recuperator that limits the amount of heat
availablefor CHP. Recovered heat can be used for hot water heating or low pressure steam
applications.
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8.6 Emissions
NOx emissions are targeted below 9 ppm using lean pre-mix technology without any postcombustion treatment.
8.7 Applications
Markets for the microturbine include commercial and light industrial facilities. Sincethese
customers often pay more for electricity than larger end-users, microturbines mayoffer these
customers a cost effective alternative to the grid. Their relatively modest heatmoutput may be
ideally matched to customers with low pressure steam or hot waterrequirements. Manufacturers
will target several electric generation applications,including standby power, peak shaving and
base loaded operation with and without heatrecovery.
One manufacturer is offering a two shaft turbine that can drive refrigeration chillers (100-350
tons), air compressors and other prime movers. The system also includes an optionalheat
recovery package for hot water and steam applications.
8.8 Technology Advancements
Microturbines are being developed in the near term to achieve thermal efficiencies of30% and
NOx emissions less than 10 ppm. It is expected that performance andmaintenance requirements
will vary among the initial offerings. Longer term goals are toachieve thermal efficiencies
between 35-50% and NOx emissions between 2-3 ppmthrough the use of ceramic components,
improved aerodynamic and recuperator designsand catalytic combustion.
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9. BENEFITS OF COGENRATION
(a) FUEL ECONOMY :
Cogeneration results in substantial economy in consumption of primary fuel .i.e., coal, oil,
gas.
The fuel economy results from higher thermodynamic efficiency of cogeneration system
ascompare to separate power producing and heat producing systems . Moreover the extra
fuelneeded to generate electricity for same quantity of steam produced for process
requirement is only about 10%.
(b) LOWER CAPITAL COST
It is seen that an industry needing steam for processing has to invest in boilers . The extra
investment needed to upgrade boilers so that electricity can also be generated is pritty small
as compared to the cost of boiler. It has been estimated that incremental statement in
cogeneration system is only about 50% of the investement needed by an electric utility tosupply the same power to industry . Thus cogeneration results in enormous saving in capital
cost.
(c) SAVING INDUSTRY FROM POWER CUTS
In all developing countries including india the generation capacity is much less than the
demand. The electricity supply authorities impose severe power cuts on industry especially
when electricity demand for agriculture is high. The power cuts and supply interruption
result in huge losses to industry. Many industries install diesel generating sets to keep their
process running .The generation cost per KWh of these sets is very high
(d)EFFICIENCY BENIFITS
By using waste heat recovery technology to capture a significant proportion of this wasted
heat, CHP systems typically achieve total system efficiencies of 60 to 80 percent for
producing electricity and thermal energy.Because CHP is more efficient, less fuel is
required to produce a given energy output than with separate heat and power. Higher
efficiency translates into:
Lower operating costs Reduced emissions of all pollutants Increased reliability and power quality Reduced grid congestion and avoided distribution losses
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10.APPLICATION OF COGENERATION SYSTEM
Cogeneration technology exists in a wide variety of energy-intensive facility types and sizes
nationwide, including:
Industrial manufacturers - chemical, refining, pulp and paper, food processing, glass
manufacturing
Institutions - colleges and universities, hospitals, prisons, military bases
Commercial buildings - hotels and casinos, airports, high-tech campuses, large office
buildings, nursing homes
Municipal - district energy systems, wastewater treatment facilities
Residential - multi-family housing, planned communities
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CONCLUSION
Cogeneration or combined heat power generation is higher in energy efficiency thanconventional thermal generation because it reuses heat. Energy that would otherwise be wasted isput to some useful work.
Because of its efficient use of energy, cogeneration is more economic and environmentally
attractive than conventional fossil fuel power plants. Cogeneration can be located close to
electric consumers, thereby reducing transmission line losses. Cogeneration or combined heat
power generation is well-suited to facilities with higher thermal loads, consistent electric and
thermal energy requirements, and round-the-clock operations.
Campus institutions, such as universities and hospitals, often benefit from aggregating energyneeds in a district energy combined heat power generation system
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REFERENCES
http://www.cogen.org/Downloadables/Projects/EDUCOGEN_Cogen_Guide.pdf
Generation of electrical energy by BR GUPTA/chapter 18