CogenerationLounge

Nishkam Dhiman

Asst Prof : Electrical and Electronics Engineering

Chitkara University, Punjab

IntroductionCogeneration means sequential conversion of energy contained in fuel into two or more usable forms. 1. Energy from coal is converted to heat in boiler to produce steam, this steam produces electricity, in addition provides heat for manufacturing process.2. Gas is used in gas turbine to generate electrical energy, the remaining heat is used to produce steam in a heat recovery boiler which can be used for manufacturing process.Thus a conventional system produces either Electrical or thermal energy.But a cogeneration system produces both the energies.

A cogeneration system can either be an inplant power generation system or a reject heat utilization system.

Inplant power generation system

• The industry needs both process steam and electricity.

• In conventional system, steam is produced by a boiler, electricity is either purchased or produced by diesel engine.

• In cogeneration: the boiler is made to produce steam at higher temperature and pressure than needed for manufacturing process, this steam is used in a turbine generator set to produce electricity.

• The exhaust steam is used for manufacturing process.

Reject heat utilization system.• The reject heat utilization system is used in power plants.

Some steam is extracted from the turbine(at suitable temp. and pressure) and supplied to an adjacent industry for manufacturing purposes.

Topping and Bottoming Cycles

• 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 reject heat from the turbine is used for manufacturing process.

Electrical energy is produced first.

Since steam required for manufacturing process need low temp therefore topping cycle is mostly used.

Efficiency: 75%

Efficiency of separate systems: 55%

• Bottoming Cycle:

Fuel burnt in the boiler to produce steam. The steam is used for manufacturing process. The reject heat from process is used to generate power.

Heat energy is produced first.

B en efits

(a) Fuel Economy: Less consumption of primary fuel.

Fuel economy results from higher thermodynamic efficiency of a cogeneration system as compared to separate power producing and heat producing systems.

(b) Lower capital cost: Industry needing steam has to invest on boilers, the extra investment needed to upgrade boiler so that electricity can also be generated is pretty small as compared to the cost of boiler. Thus cogeneration results in enormous savings in capital costs.

(C) Low gestation period: A utility needs 6yrs to add a thermal generation system. Whereas, installation of a cogeneration system by and industry needs only 3yrs. The lower gestation period results in saving in interest, early utilisation of facility, early return on investment.

(D) Saving to industry from power cuts and power supply interruptions.

Cogeneration Technologies

• The cogeneration technologies used for topping cycles are:

Steam Turbine System

Gas Turbine System

Combined Cycle system

Diesel Engine System

S team T u rb in e S y stem

• Uses system similar to thermal plant. • Fuel may be oil, coal or wood.• The higher pressure steam produced by the boiler drives turbine and then generate

electricity.• The lower pressure exhaust form steam turbine is used for industrial process application.• Back pressure steam turbines are used as they exhaust low pressure steam and are more

efficient.• 13% energy saving . If steam-turbine cogeneration is used.• Fuel flexibility

Gas Turbine System

• The exhaust from the combustion chamber is used as process heat. If necessary the hot gases can be used to raise steam in waste heat recovery boiler where the heat of gasses is transferred to water.

• Due to less availability and high cost of petroleum products, the cost of electricity produced is high.

• Gas turbines need more maintenance than steam turbines.

• Energy Savings of about 25% is obtained using cogeneration.

Combined Cycle System

• It is a combination of gas turbine and steam turbine system.

• The gas turbine exhaust is used in waste heat boiler to raise steam.

• The steam from the boiler is used in a back pressure turbine which drives another alternator to produce electricity.

• The low pressure exhaust from steam turbine is used as process steam.

• This system has greater flexibility for topping process steam.

• Energy saving of about 35% is done by combined cycle cogeneration system.

Diesel Engine System

• This system uses a diesel engine which drives the electric generator. The engine exhaust and jacket cooling are used as heat sources for the waste heat boiler which converts water into process steam.

• This system has higher electricity to thermal ratio than that required by most industries, used when electricity can be sold.

Energy saving is about 26% with diesel engine cogeneration.