Engg Thermodynamics - Lecture - 12 - Unit - 3 - Steam Power Cycle

 Rankine

Large electric power plants typically utilize a vapor power cycle. Regardless of the heat source, be it nuclear or combustion of coal, oil, natural gas, wood chips, etc., the remaining details of these plants are similar. Typically a pure working fluid, usually water, is circulated through a cycle, and that fluid trades heat and work with its surroundings. We sketch a typical power plant cycle for electricity generation in Fig.... The ideal Rankine cycle was first described in 1859 by William John Macquorn Rankine, long after the steam engine wasin wide usage. 

The cycle has the following steps:

• 1 → 2: isentropic compression in a pump,

• 2 → 3: isobaric heating in a boiler,

• 3 → 4: isentropic expansion in a turbine, and

• 4 → 1: isobaric cooling in a condenser.

Two variants of the T − s diagram are given in Fig.. The first is more efficient as it has

the appearance of a Carnot cycle. However, it is impractical, as it induces liquid water in

the turbine, which can damage its blades. So the second is more common.

The thermal efficiency is

high power output: One can enhance this by raising the fluid to a high temperature during the combustion process or by pumping the fluid to a high pressure.

Both strategies soon run into material limits; turbine blades melt and pipes burst. Another strategy is to lower the condenser pressure, which means that one must maintain a

vacuum, which can be difficult.

• high thermal efficiency: The key design strategy here lies in 1) increasing component efficiencies, and 2) rendering the overall cycle as much like a Carnot cycle as is feasible.

Modern power plants have had revolutionary increases in overall thermal efficiency because of enhancements which make the process more Carnot-like.

There are some important loss mechanisms in the Rankine cycle which inhibit efficiency. They include

• Turbine losses: These are the major losses. To avoid these losses requires detailed consideration of fluid mechanics, material science, and heat transfer and is beyond the scope of classical thermodynamics. Thermodynamics develops broad measures of turbine efficiency such as ηturbine = (h3 − h4)/(h3 − h4s).

• Pump losses: Again, fluid mechanics, machine design, and material science are required to analyze how to actually avoid these losses. Thermodynamics characterizes them by pump efficiency, ηpump = (h2s − h1)/(h2 − h1).

• Heat transfer losses from components.

• Pressure drop in pipes.

• Incomplete fuel combustion.

• Pollution removal devices.

• Loss of heat to surroundings in the condenser.

One simple design strategy to make the system more Carnot-like is to use

• Reheat: a design strategy in which steam is extracted from the turbine before it is fully expanded, then sent to the boiler again, and re-expanded through the remainder of the turbine.