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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