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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

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