What is
co-generation of power?
Cogeneration is
the simultaneous generation of electricity and steam (or heat) in a
power plant. Cogeneration is recommended for industries and
municipalities because it can produce electricity more cheaply and/or
more conveniently than a utility.
Also, it provides
the total energy needs (heat and electricity) for the industry or
municipality. Cogeneration is beneficial if it saves energy when
compared with separate generation of electricity and steam (or heat).
Cogeneration is beneficial if the efficiency of the cogeneration
plant is greater than that of separate generation
Types of Cogeneration
The two main
categories of cogeneration are (1) the topping cycle and (2) the
bottoming cycle.
The Topping
Cycle. In this cycle, the primary heat source is used to generate
high enthalpy
steam and
electricity. Depending on process requirements, process steam at low
enthalpy is taken from any of the following:
● Extracted from
the turbine at an intermediate stage (like feedwater heating).
● Taken from the
turbine exhaust. The turbine in this case is called a back-pressure
turbine.
Process steam
requirements vary widely, between 0.5 and 40 bar. The Bottoming
Cycle. In this cycle, the primary heat (high enthalpy) is used
directly for process requirements [e.g., for a high-temperature
cement kiln (furnace)].
The low enthalpy
waste heat is then used to generate electricity at low efficiency.
This cycle has lower combined efficiency than the topping cycle.
Thus, it is not very common. Only the topping cycle can provide true
savings in primary energy.
Arrangements of
Cogeneration Plants
The various
arrangements for cogeneration in a topping cycle are as follows:
● Steam-electric
power plant with a back-pressure turbine.
● Steam-electric
power plant with steam extraction from a condensing turbine.
● Gas turbine
power plant with a heat recovery boiler (using the gas turbine
exhaust to generate steam).
● Combined
steam-gas-turbine cycle power plant. The steam turbine is either of
the backpressure type or of the extraction-condensing type.
Economics of Cogeneration
Cogeneration is
recommended if the cost of electricity is less than the utility. If a
utility is not available, cogeneration becomes necessary, regardless
of economics. The two types of power plant costs are (1) capital
costs and (2) production costs.
Capital costs are
given in total dollars or as unit capital costs in dollars per
kilowatt net. Capital costs determine if a plant is good enough to
obtain financing. Thus, it is able to pay the fixed charges against
capital costs.
Production costs
are calculated annually, and they are given in mills per kilowatt
hour (a mill is U.S $0.001). Production costs are the real measure of
the cost of power generated.
They are composed
of the following:
● Fixed charges
against the capital costs
● Fuel costs
● Operation and
maintenance costs
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