16-01-2013, 02:14 PM
A PROJECT REPORT ON ENERGY AND EXERGY ANALYSIS OF A COMBINED CYCLE POWER PLANT
EXERGY ANALYSIS.docx (Size: 868.84 KB / Downloads: 39)
ABSTRACT
The second law efficiency is the governing factor in the design and operation of a thermodynamic cycle. The exergy analysis paints the real picture of the system as it qualitatively and quantitatively reflects the energy flow through it. The following project assesses the impact of varying operating parameters of pressure and temperature on the thermal and exergy efficiency of combined cycle power plant. The exergy content of the working fluid always gets destroyed during the heat transfer process because of finite temperature difference between the source and the working fluid. The relationship between the various parameters and the second law efficiency is derived and substantiated by including graphs However since a near idealized case is presented many assumptions are made while examining the effects.
INTRODUCTION
The study of energy flow lies at the heart of any physical process that occurs in the universe. The universe consists of matter and energy which interacts with each other in order to sustain the balance. This very balance is the reason for life on earth and hence the study of energy flow became the quest for mankind since the dawn of civilization.
The study of energy and the interaction between various forms of energy is called thermodynamics. The word “thermodynamics “comes from Greek language “therme” (heat) and “dynamics” (force) which basically means converting heat into work. But the word takes a completely new dimension in today’s world which includes all kinds of energy interactions. The application of principles of thermodynamics in everyday life forms the branch of thermal engineering.
ENERGY
The concept of energy was first introduced in mechanics by Newton when he hypothesized about kinetic and potential energies. However, the emergence of energy as a unifying concept in physics was not adopted until the middle of the 19th century and was considered one of the major scientific achievements in that century. Energy is a scalar quantity that can not be observed directly but can be recorded and evaluated by indirect measurements. The absolute value of energy of system is difficult to measure, whereas its energy change is rather easy to calculate. In our life the examples for energy are endless. The sun is the major source of the earth's energy. It emits a spectrum of energy that travels across space as electromagnetic radiation. Energy is also associated with the structure of matter and can be released by chemical and atomic reactions. Throughout history, the emergence of civilizations has been characterized by the discovery and effective application of energy to society's needs.
FORMS OF ENERGY
Energy manifests itself in many forms, which are either internal or transient, and energy can be converted from one form to another. In thermodynamic analysis, the forms of energy can be classified into two groups:
• The macroscopic forms of energy are those where a system possesses as a whole with respect to some outside reference frame such as kinetic and potential energies. For example, the macroscopic energy of an up moving object changes with velocity and elevation. The macroscopic energy of a system is related to motion and the influence of some external effects such as gravity, magnetism, electricity and surface tension. The energy that a system possesses as a result of its motion relative to some reference frame is called kinetic energy. The energy that a system has as a result of its elevation in a gravitational field is called potential energy. Kinetic energy refers to the energy possessed by the system because of its overall motion, either translational or rotational. The word "overall" is italicized because the kinetic energy to which we refer is the kinetic energy of the entire system, not the kinetic energy of the molecules in the system. If the system is a gas, the kinetic energy is the energy due to the macroscopic flow of the gas, not the motion of individual molecules.
FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics [FLT] states that the energy of a closed system is conserved. Energy can be neither created nor destroyed; it just changes form. The FLT defines internal energy as a state function and provides a formal statement of the conservation of energy. To change the energy of a closed system, energy must be transferred to or from the system. Heat and work are the only two mechanisms by which energy can be transferred to or from a control mass. Work performed on a body is, by definition an energy transfer to the body that is due to a change of the external parameters of the body (such as the volume, magnetization, center of mass position in a gravitational field etc.). Heat is the energy transferred to the body in any other way .However; it provides no information about the direction in which processes can spontaneously occur, that is, the reversibility aspects of thermodynamic processes. For example, it cannot say how cells can perform work while existing in an isothermal environment.
SECOND LAW OF THERMODYNAMICS
The FLT gives no information about direction of a process; it merely states that when one form of energy is converted into another, identical quantities of energy are involved regardless of feasibility of the process. In this regard, events could be envisioned that would not violate the FLT, e.g., transfer of a certain quantity of heat from a low temperature body to a high-temperature body, without expenditure of work. However, the reality shows that this is impossible and FLT becomes inadequate in gauging the complete energy transfer. Furthermore, experiments indicated that when energy in the form of heat is transferred to a system, only a portion of heat can be converted into work. The SLT establishes the difference in quality between different forms of energy and explains why some processes can spontaneously occur, whereas other can not. It indicated a trend of change and is usually expressed as an inequality. The SLT defines the fundamental physical quantity entropy as a randomized energy state unavailable for direct conversion to work. It also states that all spontaneous processes, both physical and chemical, proceed to maximize entropy, that is, to become more randomized and to convert energy into a less available form. A direct consequence of fundamental importance is the implication that at thermodynamic equilibrium the entropy of a system is at a relative maximum; that is, no further increase in disorder is possible without changing by some external means (such as adding heat) the thermodynamic state of the system. A basic corollary of the SLT is the statement that the sum of the entropy changes of a system and that of its surroundings must always be positive, that is, the universe