Steam Power Plant Cycles

Thermodynamics deals with the conversion of one form of energy to another. In a thermodynamic cycle a fluid is returned to its initial state after transfer of heat/work across the system boundary. Laws of thermodynamics form the basis on which the whole foundation of thermodynamics is developed. The first law of thermodynamics states that heat and work are mutually convertible, and the second law of Thermodynamics states that work must always be less than heat. From the entropy of a fluid we can assess the degree of conversion of heat into work. The Carnot Cycle lays the foundation of second law of thermodynamics. The Rankine cycle calculates the maximum possible work that can be developed by an engine using dry saturated steam between the pressure limits of boiler and condenser. With the help of reheat-regenerative cycle the efficiency of a plant can be enhanced substantially. Other cycles of interest are the Kalina cycle, binary, vapor cycle, etc.

Keywords

system; boundary; cycle; work; heat; entropy; efficiency; reheat; regenerative; Kalina; binary vapor

1.1 Introduction

The science of thermodynamics covers various concepts and laws describing the conversion of one form of energy to another, e.g., conversion of heat energy into mechanical energy as in a steam or gas turbine or conversion of chemical energy into heat energy as observed during the combustion of fuel. The science of thermodynamics also deals with the various systems that are put into service to perform such conversions. A system in thermodynamics refers to a definite quantity of matter bounded by a specified region (Figure 1.1), where the transfer and conversion of mass and energy take place. A boundary is a surface that separates the quantity of matter under investigation from its surroundings. While the region may not be fixed in either shape or volume, the boundary either may be a physical one, as the walls of a pressure vessel, or it could be an imaginary surface [1,2].

There are two types of thermodynamic systems: open and closed. In an open system, mass enters or leaves through the system boundary (Figure 1.2) as in case of “steam flow through a turbine.” In the closed system, mass remains completely within the system boundary throughout the period of thermodynamic study and observation, as in the event of “expansion or compression of steam in a reciprocating steam engine.” In this system, there is no interchange of matter between system and surroundings. It is to be noted that in both open and closed systems, heat/work may cross the system boundary [1–4].

Thermodynamics also deals with the relations between properties of a substance and quantities of “work” and “heat,” which cause a change of state. Properties of a substance that describe its condition or state are characterized as pressure (P/p), temperature (T), volume (V/v), etc. These properties are measurable and depend only on the thermodynamic state and thus do not change over a cycle. From a thermodynamic point of view, the state of a system at any given moment is determined by the values of its properties at that moment [1,2,5].

When a system passes through a series of states in a process or series of processes in such a way that the final state of the system becomes identical to its initial state in all respects and is capable of repeating indefinitely then the system has completed a cycle [2]. The same principle is also followed in a thermodynamic cycle in which a fluid is returned to its initial state after the transfer of heat/work across the system boundary irrespective of whether the system is open or closed. However, heat and work are not zero over a cycle, they are process dependent. If the cyclic process moves clockwise around the loop producing a net quantity of work from a supply of heat, it represents a heat engine, and “work” will be positive. If the cyclic process moves counterclockwise, during which the net work is done on the system while a net amount of heat is rejected, then it represents a heat pump, and “work” will be negative [2].

A reservoir is a source of heat or a heat sink so large that no temperature change takes place when heat is added or subtracted from it. When heat from a reservoir is transferred to a working fluid circulating within a thermodynamic cycle mechanical power is produced. Thermodynamic power cycles are the basis of the operation of heat engines, which supply most of the world’s electric power and run almost all motor vehicles. The most common power cycles used for internal combustion engines are the Otto cycle and the Diesel cycle. The cycle used for gas turbines is called the the Brayton cycle, and the cycle that supports study of steam turbines is called the Rankine cycle.

A thermodynamic cycle is ideally be made up of any three or more thermodynamic processes as follows:

Table 1.1 shows some examples of thermodynamic cycles.

In a simple steam power plant as the fluid circulates it passes through a continuous series of cyclic mechanical and thermodynamic states. Water enters a steam generator at a certain pressure and temperature and gets converted to steam, the high-pressure steam then enters a steam turbine and expands to a low pressure while passing through the turbine, the low pressure steam then gets condensed in a condenser, and the condensed water is recycled back to the boiler at original pressure and temperature (Figure 1.3) [6,7].

The performance of combined heat and power (CHP) generation steam power plants is determined by a term specific steam consumption (s.s.c.), which is defined as the mass flow of steam required per unit of power output (kg/kWh). The smaller the value of s.s.c., the bigger the plant size and vice-versa, e.g., for a CHP plant of 30 MW or lower capacity the value of s.s.c. is more than 4 kg/kWh, while for a utility plant of 250 MW or higher size the s.s.c. is close to or less than 3 kg/kWh. In utility power plants, however, the heat rate, defined as the energy required to be supplied to generate unit of power (kcal/kWh), is the best tool to determine the efficiency of the plant [7].

1.2 Laws of Thermodynamics

Before we further discuss thermodynamic cycles it is essential to cover the fundamental laws of thermodynamics on which science of thermodynamics is based. While studying the laws of thermodynamics it should also to be kept in mind that the science of thermodynamics deals with relations between heat and work only.

1.2.1 First law of thermodynamics

In a cyclic process, since the initial and final states are identical, the net quantity of heat delivered to the system is proportional to the net quantity of work done by the system. When heat and work are mutually convertible, we have the first law of thermodynamics. Mathematically speaking,

dQ∞∑dW (1.1)

where

From Eq. 1.1 we can see that the first law of thermodynamics is an expression of the principle of conservation of energy. With the help of this law it is possible to...