Steam Power Plant
Posted under Energy, Power Source
Conventional thermal power plants can be categorized by the thermodynamic cycles they utilize when converting heat into work. Utility-scale thermal power plants are based on either the Rankine cycle, in which a working fluid is alternately vaporized and condensed, or the Brayton cycle, in which the working fluid remains a gas throughout the cycle. Most baseload thermal power plants, which operate more or less continuously, are Rankine cycle plants in which steam
is the working fluid.
Most peaking plants, which are brought on line as needed to cover the daily rise and fall of demand, are gas turbines based on the Brayton cycle. The newest generation of thermal power plants use both cycles and are called combined-cycle plants.
The basic steam cycle can be used with any source of heat, including combustion of fossil fuels, nuclear fission reactions, or concentrated sunlight onto a boiler. In the steam generator, fuel is burned in a firing chamber surrounded by a boiler
that transfers heat through metal tubing to the working fluid. Water circulating through the boiler is converted to high-pressure, high-temperature steam.
During this conversion of chemical to thermal energy, losses on the order of 10% occur due to incomplete combustion and loss of heat up the stack.
High-pressure steam is allowed to expand through a set of turbine wheels that spin the turbine and generator shaft. For simplicity, the turbine is shown as a single unit, but for increased efficiency it may actually consist of two or sometimes three turbines in which the exhaust steam from a higher pressure turbine is reheated and sent to a lower-pressure turbine, and so forth.
The generator and turbine share the same shaft allowing the generator to convert the rotational energy of the shaft into electrical power that goes out onto transmission lines for distribution. A well-designed turbine may have an efficiency approaching 90%, while the generator may have a conversion efficiency even higher than that.
The spent steam is drawn out of the last turbine stage by the partial vacuum created in the condenser as the cooled steam undergoes a phase change back to the liquid state. The condensed steam is then pumped back to the boiler to be reheated, completing the cycle.
The heat released when the steam condenses is transferred to cooling water, which circulates through the condenser. Usually, cooling water is drawn from a river, lake, or sea, heated in the condenser, and returned to that body of water,
in which case the process is called once-through cooling.
A more expensive approach, which has the dual advantages of requiring less water and avoiding the thermal pollution associated with warming up the receiving body of water, involves use of cooling towers that transfer the heat directly into the atmosphere. In either case, if we think of the power plant as a heat engine, it is the environment that acts as the heat sink so its temperature helps determine the overall efficiency of the power cycle.