Cogeneration, Topping and bottoming cycles, benefits,

1. CogenerationLounge
Nishkam Dhiman
Asst Prof : Electrical and Electronics
Engineering
Chitkara University, Punjab

2. Introduction
Cogeneration 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.

3. 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.

4. 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.

5. 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.

6. Benefits
(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.

7. Cogeneration Technologies
• The cogeneration technologies used for
topping cycles are:
Steam Turbine System
Gas Turbine System
Combined Cycle system
Diesel Engine System

8. Steam Turbine System
• 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

9. 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.

10. 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.

11. 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.