2.1 Steam Turbine Cogeneration System
Steam turbines are one of the most versatile and oldest prime mover technologies still in
general production. Power generation using steam turbines has been in use for about 100
years, when they replaced reciprocating steam engines due to their higher efficiencies and
lower costs. The capacity of steam turbines can range from 50 kW to several hundred MWs
for large utility power plants. Steam turbines are widely used for combined heat and power
(CHP) applications. The thermodynamic cycle for the steam turbine is the Rankine cycle. The
cycle is the basis for conventional power generating stations and consists of a heat source
(boiler) that converts water to high-pressure steam. In the steam cycle, water is first pumped
to medium to high pressure. It is then heated to the boiling temperature corresponding to the
pressure, boiled (heated from liquid to vapor), and then most frequently superheated (heated
to a temperature above that of boiling). A multistage turbine expands the pressurized steam to
lower pressure and the steam is then exhausted either to a condenser at vacuum conditions or
into an intermediate temperature steam distribution system that delivers the steam to the industrial or commercial application. The condensate from the condenser or from the steam
utilization system returns to the feedwater pump for continuation of the cycle.
The two types of steam turbines most widely used are the backpressure and the extractioncondensing types. The choice between backpressure turbine and extraction-condensing
turbine depends mainly on the quantities of power and heat, quality of heat, and economic
factors. The extraction points of steam from the turbine could be more than one, depending
on the temperature levels of heat required by the processes