Our daily use of electricity is largely sourced from thermal power plants. The basic operational cycle used by all power plants today is the Rankine cycle. All of the work-producing processes in this cycle are reversible, and the operating fluid goes through a continuous cycle of evaporation and condensation.

What is Rankine Cycle?

To fully define the cycle, let’s examine the past.Scottish engineer William John Macquorn Rankine continued his research and created a comprehensive theory of the heat engines in addition to the steam engine in 1859. In recognition of his contributions to this topic, the Rankine cycle bears his name. A constant pressure heat engine that transforms heat into mechanical work is the key component of the Rankine cycle, an ideal thermodynamic cycle. This cycle uses water or any other organic fluids (Pentane or Toluene) as the working fluid and receives the heat from an external source in a closed loop.

Power plants operate on a theoretical cycle called the Rankine cycle. The cycle that underlies Steam turbines is also referred to as a modified Carnot cycle. The most effective thermodynamic cycle is the Carnot cycle. This cycle eliminates the shortcomings of the Carnot engine, such as its inability to operate practically or utilize superheated steam.

Components of the Rankine Cycle

We must comprehend the elements of this cycle before learning how it operates. See the illustration below.

Fig 1: Schematic of Rankine Cycle

(1) Pump: Centrifugal pumps may be used in industrial settings. When water enters the pump as a saturated liquid, it is compressed.

(2) Boiler: In thermal power plants, boilers are typically heat exchangers. The boiler receives the compressed liquid to produce superheated steam from it.

(3) Turbines: Machines called turbines or steam turbines use pressurized steam to generate mechanical work. As it enters the turbine, the superheated steam expands and rotates the shaft, creating work that turns into electricity.

(4) Condenser: A set of tubes and a cooling medium surround the condenser. Depending on where the power plant is located, the cooling medium may be either air or water. At constant pressure, saturated liquid-vapour steam condenses, rejecting heat to a cooling medium.

In places where there is a scarcity of water, the power plants are cooled by air. In car engines, this type of cooling is known as “dry cooling.”

Working Principle of Rankine Cycle

We can better understand how the cycle functions in a closed loop with the help of the study of its individual parts. To comprehend how the Rankine cycle works, let’s look at the P-v and T-s diagrams along with the h-s diagram.

Fig 2: Temperature-Entropy (T-s) Diagram

And Fig 3: Pressure-Volume (P-v) Diagram

Fig 4: Specific Enthalpy-Specific Entropy (h-s) Diagram

Four thermodynamic processes that make up a typical Rankine cycle are described below with reference to all the diagrams. Assume for the moment that the cycle is running at a temperature range of 0 °C to 400 °C.

• Process 1-2: The working fluid (saturated liquid) is pumped from low to high pressure as it enters the pump. Also known as isentropic compression, this phenomenon. At this point, the input energy is necessary.
• Process 2-3: An external heat source with constant pressure heats the liquid entering the boiler at a high pressure. Through the addition of constant pressure heat in the boiler, the liquid is transformed into dry, saturated steam.
• Process 3-4: As it enters the turbine, the dry, saturated steam from the boiler expands. It also goes by the name of isentropic expansion. As a result, the steam’s temperature and pressure drop.
• Process 4-1: At this point, the wet vapour entering the condenser is condensed under a constant pressure. The result is a saturated liquid. The condenser’s constant pressure heat rejection is another name for this procedure. The cycle is continued by circulating this saturated liquid once more back to the pump. Q out is used to represent the heat that was rejected or the exhaust heat after the decisive stage.

Efficiency of Rankine Cycle

Let’s take a look at these concepts that are crucial for calculating the cycle’s efficiency.

Q = Rate of heat flow in the system (towards or away)

W_T = Mechanical work done by the turbine

W_P = Mechanical work done by the pump

h₁, h₂, h₃, and h₄ = Specific enthalpies of water at states 1, 2, 3, and 4 respectively referring to the T-s diagram in figure 2.

h_f = Specific enthalpy of water = h (In this cycle)

Applying steady flow energy equation (SFEE) to pump, boiler, turbine, and condenser,

(1) For Pump: h₂+W_P=h₁⟹W_P=h₂−h₁

(2) For Boiler: h₂+Q_in=h₃⟹Q_in=h₃−h₂

(3) For Turbine: h₃=W_T+h₄⟹W_T=h₃−h₄

(4) For Condenser: h₄=Q_out+h₁⟹Q_out=h₄−h₁

Now, the Rankine cycle efficiency with pump is given as,

= {(h₃−h₄)−(h₂−h₁)}/(h₃−h₂)

Similarly, the Rankine cycle efficiency without pump work is given as,

η Rankine={1−(Q_out/Q_in)}=1−{(h₄−h₁)/(h₃−h₂)}

The calculation of this efficiency is less accurate in practical applications than the Rankine cycle’s ideal efficiency. Let’s examine the differences between the ideal and actual cycles.

Real Rankine Cycle

The isentropic compression and expansion caused by the pump and turbine do not occur in a real Rankine cycle or a non-ideal cycle used in actual power plants. In contrast to the ideal cycle, these processes are irreversible, and the entropy increases, as shown in the T-s diagram below.

Fig 3: Real Rankine Cycle T-s Diagram

As can be seen from the aforementioned figure, the actual cycle differs from this one in that it experiences pressure drops in the condenser and boiler and irreversible processes in the pump and turbine. These circumstances lead to higher required power levels and lower generated power levels.

Types of Rankine Cycle

By increasing the heat input to the cycle, the Rankine cycle’s thermodynamic efficiency can be raised. To achieve this, raise the temperature until superheated steam is the predominant steam phase. So This is just one of many variations that aim to boost the cycle’s thermodynamic effectiveness. The cycles listed below were created to improve the cycle’s thermal efficiency.

Rankine Cycle with Reheat

The moisture carried by the steam during the cycle’s final stages of expansion is eliminated by rewarming. The steam turbines are kept in series to complete the work with this cycle design. An overview of this cycle’s operation is given below.

1. The first turbine is filled with high-pressure steam from the boiler.
2. After being reheated, the steam is transferred to the boiler.
3. This heated steam is directed toward the second turbine, which operates at low pressure.

The Rankine cycle with reheat is used to raise the average steam temperature throughout the process. Only half as much as the previous stage’s efficiency is gained by rewarming the steam through an additional stage.

Advantages of Reheat Rankine Cycle

• Steam condenses during expansion, increasing the cycle’s thermal efficiency and decreasing damage to the turbine blades.
• increases the turbine’s overall work output while taking into account the total work input.

Disadvantages of Reheat Rankine Cycle

• Long piping setup is necessary for these cycles. As a result, high installation costs at first are followed by high maintenance costs.
• The condenser’s size could grow as reheating occurs.

Regenerative Rankine Cycle

In this cycle, a regenerator is used to heat the condenser’s subcooled liquid by trapping steam in the hot part of the cycle. To change a fluid into a saturated liquid, this process is combined with it. The cycle is therefore known as the “regenerative Rankine cycle,” and the process is known as “Regeneration.”

By removing the heat addition from the boiler that is typically present in a typical Rankine cycle, the regeneration process raises the heat input temperature of the cycle. The efficiency of the cycle rises as a result of the high temperature at which heat enters the cycle.

Advantages of Regenerative Cycle

• The temperature stresses in this cycle decrease along with the range of steam’s working temperatures,
• The erosion of the turbine is reduced due to the nature of the working fluid.

Disadvantages of the Regenerative Cycle

• Because there is a regenerator, there is less work done in the boiler, which increases the steam rate.
• Costly power plants are a result of larger components adhering to the cycle’s natural process.

Organic Rankine Cycle

The organic Rankine cycle, or ORC, is a thermodynamic cycle that substitutes toluene or pentane for water and steam in place of those two fluids. These fluids have a higher vaporization temperature than water and a high molecular mass. It enables the extraction of heat from low-temperature sources like geothermal heat, biomass combustion, and solar ponds.

Advantages of Organic Rankine Cycle

•  The inlet temperature and pressure of the turbine will be low.
• There is no need for superheating, hence no water-treatment system of fluid

Disadvantages of Organic Rankine cycle

• ORCs generate less power than a regular cycle at the same operating conditions.
• This cycle uses organic fluids, which can catch fire. Environmental dangers can result from even a small leak.

Supercritical Rankine Cycle

The heat is transferred to a high-pressure supercritical fluid in a supercritical Rankine cycle, which transforms it into a supercritical phase. In this phase, fluid is sent to a turbine where it expands and aids in power production. So The used steam is returned to complete the cycle by being condensed into a liquid. A fluid that is above its critical point in both temperature and pressure is referred to as a supercritical fluid, or SCF. So At this temperature, gas and liquid cannot coexist. Black smokers, a particular kind of hydrothermal vent, are where these liquids are discovered on Earth.

Advantages of Supercritical Rankine Cycle

• The outflow of the turbine is also of high quality.
• The efficiency of the cycle is greater than all other cycles.

Disadvantages of Supercritical Rankine Cycle

• High temperatures and pressures might be too much for standard boilers to handle. Consequently, a unique boiler needs to be created.
• Due to this, there is an overall cost bump.

Applications

Knowing the benefits and drawbacks piques our interest in the applications of the various cycles. Let’s examine how these cycles are used in real-world situations.

1. Thermal power stations, such as nuclear power plants, currently use the regenerative Rankine cycle with a minor variation.
2. Power plants that produce energy using fluids under supercritical pressure use the Rankine cycle with reheating.
3. In medium power applications like supercritical power plants (such as the Philo Power Plant in Ohio and the majority of the coal-fired power stations in China), the supercritical Rankine cycles are used.
4. So The waste heat recovery plant, biomass power plants, geothermal power plants, solar thermal power plants, and wind-thermal energy stations are just a few of the industries that use ORC.