Cogeneration Cycle By K.BABA SAHEB (0209-1602) ME (CAD/CAM) 2nd SEM

Department of Mechanical EngineeringChaitanya Bharathi Institute of Technology

(Affiliated to Osmania University, Hyderabad)Gandipet, HYDERABAD- 500075

MAR - 2009

AbstractAbstract

CogenerationCogeneration (also (also combined combined heat and powerheat and power, , CHPCHP) is the use of ) is the use of a heat engine or a power station to a heat engine or a power station to simultaneously generate both simultaneously generate both electricity and useful heat.electricity and useful heat.

Thermal power plants and heat engines in Thermal power plants and heat engines in general, do not convert all of their general, do not convert all of their thermal energy into electricity. In most thermal energy into electricity. In most heat engines, a bit more than half is lost heat engines, a bit more than half is lost as excess heat.as excess heat.

By capturing the excess heat, CHP uses By capturing the excess heat, CHP uses heat that would be wasted in a heat that would be wasted in a conventional power plant, potentially conventional power plant, potentially reaching an efficiency of up to 89%, reaching an efficiency of up to 89%, compared with 55% for the best compared with 55% for the best conventional plants. conventional plants.

This means that less fuel needs to be This means that less fuel needs to be consumed to produce the same amount consumed to produce the same amount of useful energy.of useful energy.

A car engine becomes a CHP plant in A car engine becomes a CHP plant in winter, when the reject heat is useful winter, when the reject heat is useful for warming the interior of the for warming the interior of the vehicle. This example illustrates the vehicle. This example illustrates the point that deployment of CHP point that deployment of CHP depends on heat uses in the vicinity depends on heat uses in the vicinity of the heat engine.of the heat engine.

By-product heat at moderate By-product heat at moderate temperatures (212-356°F/100-180°C) temperatures (212-356°F/100-180°C) can also be used in absorption chillers can also be used in absorption chillers for cooling. A plant producing for cooling. A plant producing electricity, heat and cold is sometimes electricity, heat and cold is sometimes called called trigenerationtrigeneration or more or more generally: generally: polygenerationpolygeneration plant. plant.

ContentsContents

IntroductionIntroduction Types of PlantsTypes of Plants Gas TurbinesGas Turbines Steam Turbine Power PlantSteam Turbine Power Plant Gas Turbine With RegenerationGas Turbine With Regeneration Combined CycleCombined Cycle Design Principle for combined cycleDesign Principle for combined cycle Combined Cycle Power PlantCombined Cycle Power Plant ResultsResults Conclusion Conclusion BibliographyBibliography

IntroductionIntroduction Conventional power plants emit the heat Conventional power plants emit the heat

created as a by-product of electricity created as a by-product of electricity generation into the environment through generation into the environment through cooling towers, flue gas, or by other cooling towers, flue gas, or by other means. CHP or a bottoming cycle means. CHP or a bottoming cycle captures the by-product heat for captures the by-product heat for domestic or industrial heating purposes, domestic or industrial heating purposes, either very close to the plant, or—either very close to the plant, or—especially in Scandinavia and eastern especially in Scandinavia and eastern Europe—as hot water for district heating Europe—as hot water for district heating with temperatures ranging from with temperatures ranging from approximately 80 to 130 °C. approximately 80 to 130 °C.

This is also called decentralized energy. This is also called decentralized energy.

Some tri-cycle plants have utilized a Some tri-cycle plants have utilized a combined cycle in which several combined cycle in which several thermodynamic cycles produced thermodynamic cycles produced electricity, and then a heating system electricity, and then a heating system was used as a condenser of the power was used as a condenser of the power plant's bottoming cycle. Tri-cycle plant's bottoming cycle. Tri-cycle plants can have thermal efficiencies plants can have thermal efficiencies above 80%.above 80%.

CHP is most efficient when the heat CHP is most efficient when the heat can be used on site or very close to can be used on site or very close to it. Overall efficiency is reduced when it. Overall efficiency is reduced when the heat must be transported over the heat must be transported over longer distances. longer distances.

This requires heavily insulated pipes, This requires heavily insulated pipes, which are expensive and inefficient; which are expensive and inefficient; whereas electricity can be whereas electricity can be transmitted along a comparatively transmitted along a comparatively simple wire, and over much longer simple wire, and over much longer distances for the same energy loss.distances for the same energy loss.

Types of PlantsTypes of Plants Topping cycle plantsTopping cycle plants

Bottoming cycle plants Bottoming cycle plants

Common CHP plant types are:Common CHP plant types are:

Gas turbine CHP plantsGas turbine CHP plants

Gas engine CHP plantsGas engine CHP plants

Steam turbine CHP plantsSteam turbine CHP plants

Combustion (Gas) Combustion (Gas) TurbinesTurbines

Combustion turbine plants operate on the Combustion turbine plants operate on the Brayton cycle. They use a compressor to Brayton cycle. They use a compressor to compress the inlet air upstream of a compress the inlet air upstream of a combustion chamber. Then the fuel is combustion chamber. Then the fuel is introduced and ignited to produce a high introduced and ignited to produce a high temperature, high-pressure gas that temperature, high-pressure gas that enters and expands through the turbine enters and expands through the turbine section.section.

The turbine section powers both the The turbine section powers both the generator and compressor. Combustion generator and compressor. Combustion turbines are also able to burn a wide turbines are also able to burn a wide range of liquid and gaseous fuels from range of liquid and gaseous fuels from crude oil to natural gas. The combustion crude oil to natural gas. The combustion turbine’s energy conversion typically turbine’s energy conversion typically ranges between 25% to 35% efficiency as ranges between 25% to 35% efficiency as a simple cyclea simple cycle..

Simple Cycle Power Plants Simple Cycle Power Plants (Open Cycle)(Open Cycle)

Steam Turbine Power Steam Turbine Power PlantsPlants

Steam turbine power plants operate on a Steam turbine power plants operate on a Rankine cycle. The steam is created by a Rankine cycle. The steam is created by a boiler, where pure water passes through boiler, where pure water passes through a series of tubes to capture heat from the a series of tubes to capture heat from the firebox and then boils under high firebox and then boils under high pressure to become superheated steam.pressure to become superheated steam.

The superheated steam leaving the The superheated steam leaving the

boiler then enters the steam turbine boiler then enters the steam turbine throttle, where it powers the turbine and throttle, where it powers the turbine and connected generator to make electricity. connected generator to make electricity.

Steam Turbine Power Steam Turbine Power PlantPlant

Gas Turbine with Gas Turbine with RegenerationRegeneration

The simple cycle efficiency can be The simple cycle efficiency can be increased by installing a recuperator increased by installing a recuperator or waste heat boiler onto the or waste heat boiler onto the turbine’s exhaust. turbine’s exhaust.

A recuperator captures waste heat in A recuperator captures waste heat in the turbine exhaust stream to the turbine exhaust stream to preheat the compressor discharge air preheat the compressor discharge air before it enters the combustion before it enters the combustion chamber chamber

Combined CycleCombined Cycle A A combined cyclecombined cycle is characteristic of a power is characteristic of a power

producing engine or plant that employs more producing engine or plant that employs more than one thermodynamic cycle. Heat engines than one thermodynamic cycle. Heat engines are only able to use a portion of the energy are only able to use a portion of the energy their fuel generates (usually less than 50%).their fuel generates (usually less than 50%).

The remaining heat from combustion is The remaining heat from combustion is generally wasted. Combining two or more generally wasted. Combining two or more "cycles" such as the Brayton cycle and "cycles" such as the Brayton cycle and Rankine cycle results in improved overall Rankine cycle results in improved overall efficiency.efficiency.

In a combined cycle power plant (CCPP), In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates plant, a gas turbine generator generates electricity and the waste heat is used to electricity and the waste heat is used to make steam to generate additional make steam to generate additional electricity via a steam turbine; this last electricity via a steam turbine; this last step enhances the efficiency of electricity step enhances the efficiency of electricity generation. Most new gas power plants in generation. Most new gas power plants in North America and Europe are of this type. North America and Europe are of this type.

In a thermal power plant, high-In a thermal power plant, high-temperature heat as input to the power temperature heat as input to the power plant, usually from burning of fuel, is plant, usually from burning of fuel, is converted to electricity as one of the converted to electricity as one of the outputs and low-temperature heat as outputs and low-temperature heat as another output. another output.

As a rule, in order to achieve high As a rule, in order to achieve high efficiency, the temperature efficiency, the temperature difference between the input and difference between the input and output heat levels should be as output heat levels should be as high as possible.high as possible.

This is achieved by combining the This is achieved by combining the

RankineRankine (steam) and (steam) and BraytonBrayton (gas) (gas) thermodynamic cycles. Such an thermodynamic cycles. Such an arrangement used for marine propulsion arrangement used for marine propulsion is called is called Combined Gas (turbine) And Steam (turCombined Gas (turbine) And Steam (turbine)bine) (COGAS) (COGAS)..

Design PrincipleDesign Principle

In a thermal power station water is the In a thermal power station water is the working medium. High pressure steam working medium. High pressure steam requires strong, bulky components. requires strong, bulky components. High temperatures require expensive High temperatures require expensive alloys made from alloys made from nickelnickel or or cobaltcobalt, , rather than inexpensive rather than inexpensive steelsteel. .

These alloys limit practical steam These alloys limit practical steam temperatures to 655 °C while the temperatures to 655 °C while the lower temperature of a steam plant is lower temperature of a steam plant is fixed by the boiling point of water. fixed by the boiling point of water. With these limits, a steam plant has a With these limits, a steam plant has a fixed upper efficiency of 35 to 42%fixed upper efficiency of 35 to 42%

An open circuit gas turbine cycle has a An open circuit gas turbine cycle has a compressorcompressor, a , a combustorcombustor and a and a turbineturbine. . For gas turbines the amount of metal For gas turbines the amount of metal that must withstand the high that must withstand the high temperatures and pressures is small, temperatures and pressures is small, and lower quantities of expensive and lower quantities of expensive materials can be used. In this type of materials can be used. In this type of cycle, the input temperature to the cycle, the input temperature to the turbine (the turbine (the firing temperaturefiring temperature), is ), is relatively high (900 to 1,400 °C).relatively high (900 to 1,400 °C).

The output temperature of the flue gas The output temperature of the flue gas is also high (450 to 650 °C). This is is also high (450 to 650 °C). This is therefore high enough to provide heat therefore high enough to provide heat for a second cycle which uses steam as for a second cycle which uses steam as the working fluid; (a Rankine cycle).the working fluid; (a Rankine cycle).

Combined cycle power Combined cycle power plantplant

In a combined cycle power plant, the In a combined cycle power plant, the heat of the gas turbine's exhaust is heat of the gas turbine's exhaust is used to generate steam by passing it used to generate steam by passing it through a heat recovery steam through a heat recovery steam generator (HRSG) with a live steam generator (HRSG) with a live steam temperature between 420 and 580 °C. temperature between 420 and 580 °C.

The condenser of the Rankine cycle is The condenser of the Rankine cycle is usually cooled by water from a lake, usually cooled by water from a lake, river, sea or cooling towers. This river, sea or cooling towers. This temperature can be as low as 15 °Ctemperature can be as low as 15 °C

ResultsResults By combining both gas and steam cycles, By combining both gas and steam cycles,

high input temperatures and low output high input temperatures and low output temperatures can be achieved. The temperatures can be achieved. The efficiency of the cycles add, because they efficiency of the cycles add, because they are powered by the same fuel source. are powered by the same fuel source.

So, a combined cycle plant has a So, a combined cycle plant has a thermodynamic cycle that operates thermodynamic cycle that operates between the gas-turbine's high firing between the gas-turbine's high firing temperature and the waste heat temperature and the waste heat temperature from the condensers of the temperature from the condensers of the steam cycle.steam cycle.

This large range means that the Carnot This large range means that the Carnot efficiency of the cycle is high. The actual efficiency of the cycle is high. The actual efficiency, while lower than this is still efficiency, while lower than this is still higher than that of either plant on its own. higher than that of either plant on its own. CHP is one of the most cost efficient CHP is one of the most cost efficient methods of reducing carbon emissions of methods of reducing carbon emissions of heating in cold climates.heating in cold climates.

The thermal efficiency of a combined The thermal efficiency of a combined cycle power plant is the net power cycle power plant is the net power output of the plant divided by the output of the plant divided by the heating value of the fuel. If the plant heating value of the fuel. If the plant produces only electricity, efficiencies of produces only electricity, efficiencies of up to 60% can be achieved. In the case up to 60% can be achieved. In the case of combined heat and power of combined heat and power generation, the Energy Utilisation Factor generation, the Energy Utilisation Factor (overall efficiency) can increase to 85%. (overall efficiency) can increase to 85%.

ConclusionsConclusions By this, we can conclude that more By this, we can conclude that more

thermal efficiency can be achieved. thermal efficiency can be achieved. This approach provides with superior This approach provides with superior

performance, decreased operating performance, decreased operating expenses and increased return on expenses and increased return on investment.investment.

Better utilisation of fuel will be Better utilisation of fuel will be possible using this combined cycle. It possible using this combined cycle. It means that less fuel needs to be means that less fuel needs to be consumed to produce the same consumed to produce the same amount of useful energyamount of useful energy

BibliographyBibliography

www.hitachi.comwww.hitachi.com www.www.cogenerationcogeneration.net.net catalog.asme.org/books/PrintBook/catalog.asme.org/books/PrintBook/Handbook_Handbook_CogenerationCogeneration

www.team-bhp.com/forum/technical-www.team-bhp.com/forum/technical-stuff/35859-honda-researching-stuff/35859-honda-researching-advanced-hybrid-drive-rankine-advanced-hybrid-drive-rankine-cyclecycle--co-generationco-generation

www.www.gasgas--turbinesturbines.com.com

www.howstuffworks.comwww.howstuffworks.com www.gepower.comwww.gepower.com igti.asme.orgigti.asme.org www.kawasakiwww.kawasakigasturbinesgasturbines.com.com www.Siemens.comwww.Siemens.com

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