Civil Engineering Reference
In-Depth Information
9
Cogeneration Plants
9.1
Introduction
In conventional utility power plants, electric energy is generally
produced with an overall efficiency, that is, the ratio between useful
output and input power as fuel, in the range 35-60 %, because of the
large quantity of heat discharged into the atmosphere without recovery
through cooling towers, lakes, or rivers.
Differently, in cogeneration plants (heat and power plants), either
useful heat and mechanical or electric power are generated from fuel,
or power is produced by recovering heat from processes. An overall
efficiency ranging from 60 to 85 % can be achieved, depending on the
type of cogeneration plant. If useful heat is transformed into cooling
media by an absorption system, the plant is called trigeneration plant.
Cogeneration is an effective method of primary energy conservation; from this
point of view, cogeneration can be applied whenever it is economically justified.
The correct use of this system implies a balance between electric-power
requirements and process-heat requirements in quantity and in quality. If this
balance does not exist, electric power must be exchanged with utility and boiler
plants must produce additional heat. The discharge of unnecessary heat should be
avoided; otherwise, the system works with lower efficiency like a conventional
utility plant.
Before the widespread diffusion of utilities producing and distributing electric
energy to end users, cogeneration plants were very common in industry.
