rankine cycle

Rankine cycle

                                                           

The Rankine cycle closely describes the process by which steam-operated heat engines commonly found in thermal power generation plants generate power. The heat sources used in these power plants are usually nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil.
The efficiency of the Rankine cycle is limited by the high heat of  vaporization of the working fluid. Also, unless the pressure and  temperature reach super critical levels in the steam boiler, the temperature range the cycle can operate  over is quite small: steam turbine entry temperatures are typically  around 565°C and steam condenser temperatures are around 30°C. This  gives a theoretical maximum Carnot efficiency for the steam turbine alone of about 63% compared with an actual  overall thermal efficiency of up to 42% for a modern coal-fired power  station. This low steam turbine entry temperature (compared to a gas turbine) is why the Rankine (steam) cycle is often used as a bottoming cycle to recover otherwise rejected heat in combined-cycle gas turbine power stations.
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with condensed droplets often seen billowing from power stations is  created by the cooling systems (not directly from the closed-loop  Rankine power cycle) and represents the means for (low temperature)  waste heat to exit the system, allowing for the addition of (higher  temperature) heat that can then be converted to useful work (power).  This 'exhaust' heat is represented by the "Qout" flowing out of the lower side of the cycle shown in the T/s diagram below. Cooling towers operate as large heat exchangers by absorbing the latent heat of vaporization of the working fluid and simultaneously evaporating cooling water to  the atmosphere. While many substances could be used as the working fluid  in the Rankine cycle, water is usually the fluid of choice due to its  favorable properties, such as its non-toxic and unreactive chemistry,  abundance, and low cost, as well as its thermodynamic properties.

There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the above T-s diagram.

• Process 1-2: The working fluid is pumped from low to high  pressure. As the fluid is a liquid at this stage, the pump requires  little input energy.
• Process 2-3: The high pressure liquid enters a boiler where  it is heated at constant pressure by an external heat source to become a  dry saturated vapour. The input energy required can be easily  calculated graphically, using an enthalpy-entropy chart , or numerically, using steam tables.
• Process 3-4: The dry saturated vapour expands through a turbine,  generating power. This decreases the temperature and pressure of the  vapour, and some condensation may occur. The output in this process can  be easily calculated using the chart or tables noted above.
• Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become a saturated liquid.

In an ideal Rankine cycle the pump and turbine would be isentropic,  i.e., the pump and turbine would generate no entropy and hence maximize  the net work output. Processes 1-2 and 3-4 would be represented by  vertical lines on the T-s diagram and more closely resemble that of the Carnot cycle. The Rankine cycle  shown here prevents the vapor ending up in the superheat region after  the expansion in the turbine, which reduces the energy removed by the condensers.