What is the Rankine cycle?
The Rankine cycle is the ideal thermodynamic cycle for vapor power plants. Water is the working fluid and is present as liquid and vapor in different stages. The graph below shows the relationship between temperature and entropy*. This cycle produces power from steam turning turbines. Some methods of producing the heat needed include nuclear fission, burning fossil fuels, and even through geothermal energy. The steps for the (ideal) Rankine cycle are:
1-2 (Isentropic) Compression: Saturated water enters the pump and is compressed to the correct temperature for the boiler. Saturated means that the water is at the temperature at which liquid water and steam can exist together. It is compressed so that it is completely liquid. Since entropy always increases, this is actually an adiabatic process (no heat added or removed).
- 2-3 (Isobaric) Heat Addition: Heat is added to the boiler to heat the water into steam. The steam is superheated, meaning that it is above the saturation temperature. I.e., there is no liquid water left. In reality, there is some liquid in the vapor. The goal is to minimize this as much as possible.
- 3-4 (Isentropic) Expansion: The steam turns a turbine which turns a shaft in a generator. Here, the temperature and pressure of the steam reduces and it becomes a saturated mixture of liquid and vapor. This process is actually adiabatic.
- 4-1 (Isobaric) Heat Rejection: The saturated steam goes into a condenser. Vapor is condensed into liquid, and the final saturated liquid enters the pump. The condenser is usually located in a heat sink like a body of water or the atmosphere. Oil or air could be used instead of water for cooling. Dry cooling, or cooling with air, is less efficient and requires more air since it transfers heat poorly. This increases the size of the components since more air is needed, it increases cost, and reduces efficiency. In areas where water conservation is critically important, there may be no other choice.
As always, there are deficiencies which cause the actual cycle to deviate from the ideal. There are three ways to increase the Rankine cycle’s efficiency, and an improved Rankine cycle called the Ideal Reheat Rankine Cycle.
*The entropy shown is actually the specific entropy, or entropy per unit mass.
Improving the cycle
- Decreasing the condenser pressure: Steam enters the condenser as a saturated vapor. Decreasing the pressure will further decrease the temperature and increase the amount of energy output of the entire cycle. More heat needs to go into the working fluid. However, the increase amount of heat needed is so small, that the overall efficiency still increases. Air can also leak into the condenser, and the moisture content of the steam can be higher than it should be near the end of the turbine cycle. Moisture decreases the turbine efficiency and erodes its blades. Superheating the steam can fix this.
- Superheating the steam: It is possible to heat the steam without increasing the boiler pressure. Doing this increases the overall energy produced and decreases the moisture content in the turbine’s outlet steam.
- Increasing the boiler pressure: Increasing the operating pressure of the boiler will increase the temperature of the working fluid, thus increasing the efficiency.
Ideal Reheat Rankine Cycle
It is advantageous to heat the steam again and use it to turn another turbine. The steam is held under the same pressure as it passes through the boiler, thus heating it back to the same temperature as it was when entering the first turbine. Using more than two reheat stages is not practical as it would decrease the efficiency. Adding a second reheat stage would increase the efficiency by half of that of the first reheat stage. A third would only increase it by half of that (one quarter of the first reheat stage), so the increase in efficiency would not justify the added costs and design complexity. Even double reheating is not always efficient. It would superheat the exhaust steam if the inlet pressure isn’t high enough. This is because the steam would have to cool much more than possible to put out the same energy, reducing the efficiency. Double reheat is is used only on supercritical-pressure power plants (pressures over 218 times atmospheric).