Understanding the Rankine Cycle: A Comprehensive Guide

Introduction

The Rankine cycle is a fundamental thermodynamic cycle that underpins the operation of steam power plants, a cornerstone of electricity generation worldwide. In this blog, we will delve into the intricacies of the Rankine cycle, exploring its principles, components, efficiency factors, and applications in power generation.

Figure 1

Understanding the Rankine Cycle

The Rankine cycle is thermodynamic cycle, which promotes the operation of steam plants and transforms heat energy into mechanical work and finally, electricity. It consists of four main processes:It consists of four main processes:

1.Heat Addition (Process 1-2)

The process starts with the production of heat in boiler at the constant pressure working fluid (typically water). This process we will be drawing line 1-2 on T-s diagram, where the cooling is taking place, resulting to saturated steam generation.

2.Expansion (Process 2-3)

Heat and pressure generated within a steam turbine execute work by expanding and converting some of its kinetic energy into mechanical work. This (1) expansion, which (2) graphically represented by the line 2-3 on the T-s diagram, shows how volume changes with temperature. The steam pressure and temperature decrease because the steam is expressing. The expanding steam is causing the spin of the blades, and as a result, rotational energy is generated.

3.Heat Rejection (Process 3-4)

Following passing the turbine, steam flows into an apparatus called a condenser, where it is cooled off and condensed back into liquid. It is the partial evaporation of water or the process of air conditioning that transmits heat to the medium (that is, the cooling fluid) at the same constant pressure represented by line 3-4 on the T-s diagram. During the process of the release of the heat energy in this latent form along with the transition from the steam to liquid state heat energy will be transferred to the cooling medium.

4.Compression (Process 4-1)

By this way every drop of the water is condensed and turned into a constant pressure water which is then fed back to the boiler. The phenomenon is termed as compression and represented on the T-s diagram by the line 4-1. The water is pressurized to boiler pressure, and it is ready to undergo both heat addition and circulation, resulting in a repetition of the cycle.

Working principle of Rankine cycle

The Rankine cycle converts heat into work. A schematic of the four steps of the Rankine cycle—boiler, turbine, condenser, and pump—is shown in Figure 1.This section explains the roles of these steps.

When burning fuel is used as the source of our energy, there is no naturally occurring motion—we have to convert energy from some other form into kinetic energy. A nozzle is a device that accelerates a fluid. Nozzles are used in rockets to propel gases downward at a high velocity, causing the rocket to move upward due to the conservation of momentum.
As a more modest example, garden hoses have nozzle attachments that produce faster-moving streams of water. (You may have used your thumb as a sort of nozzle to influence the velocity of water coming from a hose.) The kinetic energy in the exiting stream has to “come from” someplace, and in fact, it comes from the fluid itself.
Figure 2 shows a schematic of a nozzle, where steam enters at P 5 5 bar and T 5 450°C with negligible velocity and leaves at P 5 1 bar and T 5 228°C with a velocity of 669 m/s. The decreased temperature and increased velocity of the exiting fluid represent a conversion of the steam’s internal energy into kinetic energy. (Example 4-6 demonstrates why the exiting stream has T 5 228°C and v 5 669 m/s, specifically.)

Figure 2

A turbine (Figure 3) is an integrated device that in effect combines nozzles with windmills. Consequently, a turbine is a machine that converts internal energy into shaft work. The details of how nozzles and turbine blades are designed are beyond the scope of this book; however, it must be noted that significant pressure drops occur in a turbine, so the entering vapor or gas must be at a high pressure.Furthermore, internal energy increases with increasing temperature, and at a given set of conditions, vapors are higher in internal energy than liquids. Thus, in the Rankine cycle, the stream entering the turbine is a high-temperature, high-pressure vapor. In principle, the vapor can be any compound, but it is typically steam, which is what we will assume here.

Figure 3

Where does the high-temperature, high-pressure steam come from? It takes a great deal of shaft work to compress gases to higher pressures, because the changes in volume accompanying pressure changes are so large. It would be self-defeating to use up a lot of work to compress steam to a high pressure just so this steam could be fed to a turbine to produce work.
However, the volume of a liquid is much smaller than the volume of an equivalent mass of vapor. As a result, compressing a liquid requires significantly less work (often 2 to 3 orders of magnitude less) than compressing an equivalent mass of a vapor to the same pressure. Consequently, in the Rankine cycle, a pump is used to compress liquid water to high pressure. This liquid water then enters a boiler.
Figure 4 shows a schematic of a boiler in which fuel is burned in a furnace. The liquid water enters, travels through coils inside the furnace, and emerges as water vapor. In effect,

The internal energy in the fuel is converted into heat through a combustion reaction.
The heat is transferred to the water and increases the internal energy of the water.
The increase in internal energy causes the water to increase in temperature, boil into vapor, and possibly increase in temperature further after the phase change is complete.

Thus, low-pressure liquid water enters a series of three steps (pump, boiler, and turbine, see Figure 1-5) and leaves as low-pressure steam.This device can be run by continuously taking in fresh water and expelling steam but that is inefficient and impractical.For example, consider one of the early applications of the Rankine cycle:the steam engine for trains.One cannot reasonably stop a train every ten miles to take on fresh water.Instead the steam leaving the turbine goes to another unit where it gives off heat to the surroundings, condenses into liquid, and then goes to the pump.
This allows the Rankine cycle to operate at a steady state. A Steady-State process is a continuous process in which all parameters are constant with respect to time. For example, if the material leaving the condenser is at T 5 100°C,then a minute, an hour, or a day from now, it will still be at T 5 100°C if the process is operating at a steady state.
Thus, the Rankine cycle is a continuously operating, closed loop in which water sequentially circulates through the four steps pump, boiler, turbine, condenser thereby converting heat into shaft work. We can now make several observations about chemical engineering and thermodynamics, and how these observations are expressed in the Rankine cycle.

Energy can be regarded as a conserved quantity. It cannot be created or destroyed, but it can be converted from one form into another. This rule of experience is called the first law of thermodynamics.
An integral part of engineering practice is the design of processes that accomplish an objective. In the Rankine cycle, the objective is the conversion of heat into work.
Burning fuels is essentially conversion of the internal energy of the fuel into heat. The heat can subsequently be converted into work using the Rankine cycle.
A chemical process typically is comprised of several steps called unit operations each of which accomplishes a specific task. The Rankine cycle includes four-unit operations: a pump, a turbine, and two heat exchangers (a boiler and a condenser).

Other examples of unit operations familiar to chemical engineers include reactors, compressors, and various separation processes such as flash drums (in which liquid is partially vaporized due to a decrease in pressure), distillation columns (which separate compounds through differences in their boiling points),absorbers (which use liquid solvents to dissolve gases), extractors (which use two immiscible liquid phases where solutes are purified through their attraction to one or the other of these phases), and strippers (in which gas is bubbled through liquids,evaporates volatile components of the liquid, and leaves non-volatiles behind).

Many chemical processes are designed to operate continuously—at a steady state. Electrical generation, for example, needs to operate continuously, as there is constant demand.

Efficiency Factors and Improvements

The efficiency at which the Rankine cycle operates is affected by several aspects among them the design of the turbine, the boiler efficiency, the condenser performance, and the working fluid properties. Efficiencies and elaborations in these areas can reach the level of the whole system and diminish the environmental consequences.Some key advancements include.

Superheating: Meanwhile, the steam increases the degree of superheat, going beyond its saturation point so that additional heat energy can be added to the cycle, and, therefore, turbine efficiency can be improved.
Reheat Cycle: Reheat cycle involves steam extraction from the turbine partially expanded steam, re-heating in the boiler, and subsequently your steam expanded further in second stage of turbine. This minimizes moisture content and enhances turbine performance efficiency.
Combined Cycle: Employing a Rankine cycle along with the conventional Brayton cycle (gas turbine cycle) in a combined cycle power plant is more efficient than the former one as the heat in the gas turbine is transferred to the Rankine cycle resulting in additional steam.

Applications and Future Outlook

In today's power plant industry, the Rankine cycle plays an important role in electricity generation, using this cycle in such plants as coal-fired and natural gas, as well as nuclear power plants. Its rich versatility and sense of adaptability makes it the basis of the contemporary power generation and it ensures reliable and scalable method of revening heat into electrical energy.
In the last decade, there has been a great focus on improving power generation efficiency, as well as sustainability of the Rankine cycle, which largely has been achieved by development of turbine components and technics of heat recovery. Research and development continue to be in top gear in a bid to involve renewable energy sources including solar power and geothermal energy in the Rankine cycle mechanism which will in turn contribute to an energy mix and cleaner future.

Conclusion

The Rankine cycle is a key idea in thermodynamics and electricity production, powering steam power plants and playing a major role in worldwide power generation. The critical role of meeting the world's energy demands while focusing on sustainability and environmental stewardship is underscored by its principles, components, efficiency factors, and potential for improvement. As technology advances, the Rankine cycle remains important in the search for efficient, reliable, and environmentally friendly power generation options.