Thermodynamic principles of energy conversion Md. Mizanur Rahman School of Mechanical Engineering Universiti Teknologi Malaysia 1 Introduction • Mechanical and electrical power developed from the combustion of fossil fuels or the fission of nuclear fuel or renewable sources • This released energy is never lost but is transformed into other forms • This conservation of energy is explicitly expressed by the first law of thermodynamics • The work of an engine cannot be the same as the energy available in the fuel sources • Second law of thermodynamics is to meet to happen the process 2 Forms of energy • Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system. • Mechanical Energy • Kinetic energy, KE: The energy that a system possesses as a result of its motion relative to some reference frame, KE=1/2mV2. • Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational field, PE=mgh • For a moving body, E=PE+KE remains constant 3 Internal energy, U: The sum of all the microscopic forms of energy. • For gases, molecules are so widely separated in space that they may be considered to be moving independently of each other, each possessing a distinct total energy. • In the case of liquids or solids, each molecule is under the influence of forces exerted by nearby molecules • Changes in internal energy are measurable by changes in temperature, pressure, and density. 4 • Chemical energy: The internal energy associated with the atomic bonds in a molecule. • Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself. Thermal = Sensible + Latent Internal = Sensible + Latent + Chemical + Nuclear 5 Energy form examples • In a gasoline engine, the combustion of the fuel–air mixture involves U, PdV and Echem • In a steam and gas turbine, only H and KE change • In a nuclear power plant fuel rod, U and Enuc are involved • In a magnetic cryogenic refrigerator, U and Emag are important. 6 The various forms of energy that can be possessed by a material body can be added together to define a total energy, to which we give the symbol E, Total energy per unit mass 7 Work and Heat interactions • Work Interaction W = pΔV Heat Interaction Q = mCpΔT 8 THE FIRST LAW OF THERMODYNAMICS • Energy can be neither created nor destroyed during a process; it can only change forms. • The First Law: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process. • The change of energy in the system dE equals the heat dQ transferred to the system minus the work dW done by the system In a cyclic process, Ei=Ef I.e. Net heat and work are equal 9 A process must satisfy the first law to occur, however, satisfying the first law alone does not ensure that the process will actually take place. Processes proceed in a certain direction and not in the reverse direction In a process for which dQ = 0, which is called an adiabatic process, the entropy may remain the same or increase but may never decrease. An adiabatic process for which the entropy increases is an irreversible process. 10 The Carnot cycle is composed of four reversible processes—two reversible isothermal and two reversible adiabatic process Net work done during Carnot cycle In a P-V diagram the area under the process curve represents the boundary work for quasi-equilibrium (internally reversible) processes, The area under curve 1-2-3 is the work done by the gas during the expansion The area under curve 3-4-1 is the work done on the gas during the compression. The area enclosed by the path of the cycle (area 1-2-3-4-1) is the difference between these two and represents the net work done during the cycle. 12 REFRIGERATORS AND HEAT PUMPS Heat is transferred in the direction of decreasing temperature, that is, from high-temperature mediums to low temperature ones. The reverse process, however, cannot occur by itself. The transfer of heat from a low-temperature medium to a high-temperature one requires special devices called refrigerators and heat pump . 13 Coefficient of Performance The COP of a refrigerator can be expressed as 14 Rankine Cycle: The Ideal Cycle for Vapour Power Cycles . Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser. The cycle that results is the Rankine cycle, which is the ideal cycle for vapor power plants. The ideal Rankine cycle does not involve any internal irreversibilities. 1-2 Isentropic expansion in a turbine 2-3 Constant pressure heat rejection in a condenser 3-4 Isentropic compression in a pump 4-5-1 Constant pressure heat addition in a boiler The simple ideal Rankine cycle 15 The simple ideal Rankine cycle 16 Energy Analysis of Basic Rankine Cycle (ideal) . The steam flows round the cycle and each process is analyzed using steady flow energy equation. Using energy balance for a steady flow system . For single stream (one-inlet-one-exit) systems, mass flow rate remains constant. If kinetic and potential energy are negligible, the energy equation becomes 17 Types of Gas Turbine Cycles There are two types of gas turbine cycle; Brayton/Joule cycle and Atkinson cycle Brayton Cycle . Heat added and rejected is at constant pressure 18 Brayton Cycle: Ideal Cycle for Gas Turbine Cycle . Gas turbines usually operate on an open cycle. Air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. .The high-temperature gases then enter the turbine where they expand to atmospheric pressure while producing power output. .Some of the output power is used to drive the compressor. .The exhaust gases leaving the turbine are thrown out (not re- circulated), causing the cycle to be classified as an open cycle. 19 Brayton Cycle - Closed Cycle Model .The open gas-turbine cycle can be modelled as a closed cycle, using the air-standard assumptions .The compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat addition process from an external source. .The exhaust process is replaced by a constant-pressure heat rejection process to the ambient air. 20 The Brayton Cycle Process .The ideal cycle that the working fluid undergoes in the closed loop is the Brayton cycle. It is made up of four internally reversible processes: . 1-2 Isentropic compression; . 2-3 Constant-pressure heat addition; . 3-4 Isentropic expansion; . 4-1 Constant-pressure heat rejection. .The T-s and P-v diagrams of an ideal Brayton cycle are shown beside. .Note: All four processes of the Brayton cycle are executed in steady-flow devices thus, they should be analyzed as steady-flow processes. 21 Otto and Diesel Cycle 22 Stirling cycle The ideal Otto and Diesel cycles are not totally reversible because they involve heat transfer through a finite temperature difference The irreversibility renders the thermal efficiency of these cycles less than that of a Carnot engine operating within the same limits of temperature. The Stirling cycle has two isentropic processes featured in the Carnot cycle 1-2 Isothermal heat addition (expansion) 2-3 Isochoric heat removal (constant volume) 3-4 Isothermal heat removal (compression) 4-1 Isochoric heat addition (constant volume) 23 Ericsson Cycle 1-2 Isothermal heat addition (expansion) 2-3 Isobaric heat removal 3-4 Isothermal heat removal (compression) 4-1 Isobaric heat addition The Ericsson cycle is often compared with the Stirling cycle, since the engine designs based on these respective cycles are both external combustion engines with regenerators. The most well-known ideal cycle is the Carnot cycle, although a useful Carnot engine is not known to have been invented. The theoretical efficiencies for both, Ericsson and Stirling cycles acting in the same limits are equal to the Carnot Efficiency for same limits. 24 FUEL CELLS • We see several different systems for converting the energy of fuel to mechanical energy by utilizing direct combustion of the fuel with air, each based upon an equivalent thermodynamic cycle. • In these systems, a steady flow of fuel and air is supplied to the “heat engine,” within which the fuel is burned, giving rise to a stream of combustion products that are vented to the atmosphere. • The thermal efficiency of these cycles, which is the ratio of the mechanical work produced to the heating value of the fuel, is usually in the range of 25% to 50%. 25 FUEL CELLS • This efficiency is limited by the combustion properties of the fuel and mechanical limitations of the various engines. • Is there a more efficient way to convert fuel energy to work? The second law of thermodynamics places an upper limit on the amount of work that can be generated in an exothermic chemical reaction, such as that involved in oxidizing a fuel in air. 26 • A fuel cell is an electrochemical device that converts chemical energy from a fuel into electrical energy without any moving parts • Fuel cells are operationally equivalent to a battery, but the reactants or fuel in a fuel cell can be replaced unlike a standard disposable or rechargeable battery 27 28 29 Group discussion Discuss with your friends about the following issues (5 minutes) 1. An modern engine can have more efficiency than a Carnot engine operating between same temperature limits? 2.