Thermodynamics and Thermochemistry

Energy, entropy and exergy concepts come from thermodynamics and are applicable to all fields of science and engineering. Therefore, this article intends to provide background for better understanding of these concepts and their differences among various classes of life support systems with a diverse coverage and a study of these connections and a comprehensive and critical view on the most recent studies on this topic is presented. It also covers the basic principles, general definitions and practical applications and implications in various fields and its characteristic has been discussed and different forms of exergy have been derived. The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. Exergy is naturally related to the concept of quality of energy. Therefore, exergy analysis has been widely applied in parallel with energy analysis in order to find the most rational use of energy. Also a brief comparison between energy and exergy analysis has been done. Finally, conclusions regarding the usability of the exergy method as a tool to promote a more efficient use of available energy sources are also derived. Figure 1. Intersection between the domains of energy, exergy and entropy. [2]

2-1C The radiator should be analyzed as an open system since mass is crossing the boundaries of the system. 2-2C A can of soft drink should be analyzed as a closed system since no mass is crossing the boundaries of the system. 2-3C Intensive properties do not depend on the size (extent) of the system but extensive properties do. State, Process, Forms of Energy 2-4C In electric heaters, electrical energy is converted to sensible internal energy. 2-5C The forms of energy involved are electrical energy and sensible internal energy. Electrical energy is converted to sensible internal energy, which is transferred to the water as heat. 2-6C The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopic forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames. 2-7C The sum of all forms of the energy a system possesses is called total energy. In the absence of magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal energies. 2-8C The internal energy of a system is made up of sensible, latent, chemical and nuclear energies. The sensible internal energy is due to translational, rotational, and vibrational effects. 2-9C Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life. 2-10C For a system to be in thermodynamic equilibrium, the temperature has to be the same throughout but the pressure does not. However, there should be no unbalanced pressure forces present. The increasing pressure with depth in a fluid, for example, should be balanced by increasing weight. 2-11C A process during which a system remains almost in equilibrium at all times is called a quasi-equilibrium process. Many engineering processes can be approximated as being quasi-equilibrium. The work output of a device is maximum and the work input to a device is minimum when quasi-equilibrium processes are used instead of nonquasi-equilibrium processes. 2-12C A process during which the temperature remains constant is called isothermal; a process during which the pressure remains constant is called isobaric; and a process during which the volume remains constant is called isochoric. 2-13C The state of a simple compressible system is completely specified by two independent, intensive properties. 2-14C Yes, because temperature and pressure are two independent properties and the air in an isolated room is a simple compressible system. 2-15C A process is said to be steady-flow if it involves no changes with time anywhere within the system or at the system boundaries.

Thermodynamics: Thermodynamics (Greek: thermos = heat and dynamic = change) is the study of the conversion of energy between heat and other forms, mechanical in particular. All those problems that are related to the inter-conversion of heat energy and work done are studied in thermodynamics. In thermodynamics, we discuss different cycles such as Carnot cycle, Rankine cycle, Otto cycle, diesel cycle, refrigerator, compressors, turbines and air conditioners. Thermal equilibrium and Temperature: The central concept of thermodynamics is temperature. Temperature is familiar to us all as the measure of the hotness or coldness of objects. We shall learn afterwards that temperature is a measure of the average internal molecular kinetic energy of an object. It is observed that a higher temperature object which is in contact with a lower temperature object will transfer heat to the lower temperature object. The objects will approach the same temperature, and in the absence of loss to other objects, they will then maintain a constant temperature. They are then said to be in thermal equilibrium. Thermal equilibrium is the subject of the Zeroth Law of Thermodynamics. Temperature and Heat: If you take a can of cola from the refrigerator and leave it on the kitchen table, its temperature will rise-rapidly at first but then more slowly – until the temperature of the cola equals that of the room(the two are then in thermal equilibrium). In generalizing this situation, we describe the cola or coffee as a system (with temperature T S) and the relevant part of the kitchen as the environment (with temperature T E) of that system. Our observation is that if T S is not equal to T E , then T S will change until the two temperatures are equal and thus thermal equilibrium is reached. Such a change in temperature is due to the transfer of energy between the thermal energy of the system and the system's environment. It may be mentioned that thermal energy is an internal energy that consists of the kinetic and potential energies associated with the random motions of the atoms , molecules and other microscopic bodies within an object. The transferred energy is called heat and is symbolized Q. Heat is positive when energy is transferred to a system's thermal energy from its environment (we say that heat is absorbed). Heat is negative when energy is transferred from a system's thermal energy to to its environment (we say that heat is released or lost). We are then led to this definition of heat: " Heat is the energy that is transferred between a system and its environment because of a temperature difference that exists between them. " Recall that energy can also transferred between a system and its environment as work W via a force acting on a system. Heat and work, unlike temperature, pressure, and volume, are not intrinsic properties of a system. They have meaning only as they describe the transfer of energy into or out of a system. Let us now look into the Molecular Theory of Matter for an explanation of heat and temperature. Molecular Theory of Matter states that matter is made up of tiny particles called molecules. These particles are in constant motion within the bounds of the material. Since the relationship between kinetic energy of an object and its velocity is: KE = ½ mv 2 , which means that the more energy an object has, the faster it is traveling (or vice versa). Thus, when you provide extra energy to an object, you cause its molecules to speed up. Those molecules, in turn, can cause other molecules to speed up. The sum effect of the speed or energy of these molecules is PAGE 14