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University of Ljubljana Faculty of Mechanical Engineering Department of Energy Engineering Laboratory for Heat and Power ENERGY CONVERSION SYSTEMS Study material Authors: dr. Boštjan Drobnič Rok Stropnik dr. Mitja Mori Ljubljana, march 2016 Energy Conversation Systems Study material 1 Energy conversation systems The concept of 'energy conversation systems’ covers a wide range of processes, machines, devices and systems that are running energy conversion, to change the energy from its naturally available primary forms in contemporary society the most useful forms, especially in electricity. Conversions can be roughly divided into 'hot' and 'cold'. In the first exploit internal, chemical or nuclear energy primary sources and through heat pretend firstly into mechanical work and continue in electrical power. A Basic laws of thermodynamics Processes for energy conversion may be very different, but all are limited to the basic laws of nature. Those that relate specifically to energy conversion are included in the basic laws of thermodynamics: Zeroth law of thermodynamics If two systems (A and B) are in thermal equilibrium independently with a third system they must be in thermal equilibrium with each other (A and C, B and C). This law helps define the notion of temperature. First law of thermodynamics (energy law) The increase in internal energy of a closed system is equal to the heat and work supplied to the system. Second law of thermodynamics (entropy law) Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time (Clausius statement). In every neighborhood of any state S of an adiabatically enclosed system there are states inaccessible from S (Principle of Carathéodory). It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects (Kelvin statement). Third law of thermodynamics The system is not possible cooled, in a finite number of steps, to absolute zero. B Working substance, state, transformation, process For thermodynamic process it’s necessary to have the working substance (water, air …). The working substance brings the energy different forms in the machines and devices, where the energy transformation taking place, and carried out. In doing so, the working substance change its thermodynamic properties. That can be adequately described by thermodynamic state variables such as pressure, temperature and density or volume, which the substance occupies. In addition to these, for the analysis of the energy transformation in the energy system are important two more state variables - enthalpy 2 2015/2016 Energy Conversation Systems Study material (the amount of energy carried by the working substance) and entropy (allowing the assessment of the reversibility of the processes ...). State variables, which will be used for the analysis of the thermodynamic processes related to producing (useful forms of) the energy are: variable symbol unit pressure p Pa, bar, mmHg, ... temperature T K, °C, F, ... specific volume v m3/kg, ... specific enthalpy h kJ/kg, ... specific entropy s kJ/kgK, ... Pressure (symbol p) is the force (F) applied perpendicular to the surface (S) of an object per unit area over which that force is distributed. The pressure at any given point of a non-moving (static) fluid is always perpendicular to the surface and a sign of pressure is chosen so that the direction is always positive in the fluid. Temperature is a physical property of matter that quantitatively express general concepts such as 'hot' and 'cold'. Subjects which have low temperatures are cool, different degrees of higher temperatures it is described as warm or hot. It can be measured with a thermometer or a calorimeter. It is a Means of determining the internal energy contained within the system. Formally the temperature is equal to the derivative of the entropy with respect to the internal energy. The specific volume of a substance is the ratio of the substance's volume to its mass. It is the reciprocal of density and an intrinsic property of matter as well. Specific volume is defined as the number of cubic meters occupied by one kilogram of a particular substance. Enthalpy is a measure of energy in a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. Enthalpy is defined as a state function that depends only on the prevailing equilibrium state identified by the variables internal energy, pressure, and volume. It is an extensive quantity. The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements at constant pressure, because it simplifies the description of energy transfer. Entropy is a thermodynamic property and it’s a measure of the energy not available to do work in the thermodynamic processes. Since the second law of thermodynamics entropy of an isolated system always increases or remains constant, the entropy is a measure of the tendency of processes that spontaneously flow in a specific direction (heat always flows from a higher to a lower temperature). Such processes reduce the order in the systems, so the entropy is also a measure of disorderly or randomness in a system. 3 Entropija je termodinamična lastnost, ki je med drugim merilo energije, ki ni na voljo za opravljanje dela v termodinamičnih procesih, kjer potekajo pretvorbe energije. Ker se po drugem zakonu termodinamike entropija izoliranega sistema vedno povečuje ali ostaja konstantna, je entropija tudi merilo težnje procesov, da spontano tečejo v točno določeni 2015/2016 smeri (toplota teče vedno od višje temperature proti nižji). Taki procesi zmanjšujejo urejenost sistemov, zato je entropija tudi merilo neurejenosti ali naključnosti v sistemih. Energy Conversation Systems Study material In the energy processes the state of the working fluid and thermodynamic state variables changes as a result of feeding or extraction of energy in the form of heat or mechanical work. Changing the state of the working substance is called thermodynamic process and is usually described with the initial state (point) and the final state, and additionally we can also describes the changes between the start and end points. In simplified cases, the thermodynamic process taking place in four typical ways in which one of the thermodynamic state variable remains unchanged: process Constant example variable Isobaric isochoric isothermal isentropic p v T S heating/cooling of substance at constant pressure heating/cooling of substance at constant volume compression/expansion of substance at constant temperature fast compression/expansion (temperature is amended) The sequence of the various thermodynamic processes, which brings the state of the working substance back to the initial state, called a thermodynamic cycle and is the basis for most technical processes, to convert different forms of energy through the heat into mechanical work. Examples include the production of mechanical work (which is often further converted into electricity) power plant, nuclear power plants, internal combustion engines, jet engines, ... 4 2015/2016 Energy Conversation Systems Study material The primary purpose of the thermodynamic cycle is the conversion of heat, which is relatively easy to obtain from the primary sources of energy into mechanical work or electricity, which is in practice more useful form of energy. In general, in the cycle process is supplied the energy in the form of heat (high temperature), and the mechanical part, then the heat is drawn off from the process (low temperature), and the mechanical work. In the cycle is more discharged mechanical work than supplied on the other hand we get less output heat than input. For a full thermodynamic cycle, as well as for individual parts of the process, subject to the second law of thermodynamics, or the law of conservation of energy. Two basic and most simple cycle processes, which are in practice the most widely used in the 'hot' energy transformations, are the gas (Joule or Brayton) and steam (Rankine) cycle process. On the previous figure shows two typical installations (systems of machines and devices), in which previous mentioned process takes place. On the simplified scheme of these systems are shown only four basic elements (input of mechanical work, heat input, output mechanical work (useful work), heat sink (unused heat). In engineering sciences we use agreed symbols to show the basic elements of the system. The individual elements of the gas and the steam thermodynamic process are labeled the same as the corresponding pictures above. 5 2015/2016 Energy Conversation Systems Study material To see the changing state properties of the working substance in the process we use the relevant diagrams. Typically used diagrams are T-s, h-s, h-s. T-s diagram is used in thermodynamics to visualize changes to temperature and specific entropy during a thermodynamic process or cycle. It is a useful and common tool, particularly because it helps to visualize the heat transfer during a process. For reversible (ideal) processes, the area under the T-s curve of a process is the heat transferred to the system during that process. H-s chart or Mollier diagram plots the total heat against entropy, describing the enthalpy of a thermodynamic system. A p-v diagram plots the change in the pressure with respect to the volume for some process or processes. Typically in thermodynamics, the set of processes forms a cycle, so that upon the completion of the cycle there has been no net change and the state of the system. 6 2015/2016