Heat Engines
Heat Engines
1. The Energy Content of Fuels How heat is derived from fuels? For example, we may consider the burning process for heptane C7H16, colorless liquid constituent of gasoline CH16+1102→7CO2+8H2O+1.15×10° calories per100gCnH Carbon dioxide and water are the only material products of the reaction, and the energy liberated is in the form of heat The number at the right in the formula is the heat of combustion for heptane. Every fuel has a tabulated value for this quantity The heat of combustion is the definite maximum amount of energy available from a fuel which cannot be exceeded
How heat is derived from fuels? For example, we may consider the burning process for heptane, C7H16, colorless liquid constituent of gasoline. C7H16 + 11O2 → 7CO2 + 8H2O + 1.15x106 calories per 100g C7H16 Carbon dioxide and water are the only material products of the reaction, and the energy liberated is in the form of heat. The number at the right in the formula is the heat of combustion for heptane. Every fuel has a tabulated value for this quantity. The heat of combustion is the definite maximum amount of energy available from a fuel, which cannot be exceeded. 1. The Energy Content of Fuels
1. The Energy Content of Fuels Two basic purposes to obtain fossil fuels: to provide direct heating and lighting, and to power heat engines Direct heat, light Fossil fuels Electrical energy Heat engine Mechanical Transportation. Figure 3.1 The general pathways by which we utilize energy from fossil fuels
1. The Energy Content of Fuels Two basic purposes to obtain fossil fuels: to provide direct heating and lighting , and to power heat engines Figure 3.1 The general pathways by which we utilize energy from fossil fuels
2. The Mechanical Equivalent of Heat Unit for heat energy: 1 Btu (raise the temperature of one pound of water by one degree Fahrenheit) Unit for mechanical energy: 1 foot-pound (raise one pound of water one foot higher) Which one is larger? 1 Btu =778 foot-pound You can lifting a one-pound weight 778 feet into the air with the energy released by the burning of only one match Capture the heat energy of the fuel and turn it into mechanical energy The possibility of easing human labor by utilizing heat sources has been the driving force behind a long history of development of what we now call heat engines
2. The Mechanical Equivalent of Heat Which one is larger? 1 Btu = 778 foot-pound You can lifting a one-pound weight 778 feet into the air with the energy released by the burning of only one match. Capture the heat energy of the fuel and turn it into mechanical energy. The possibility of easing human labor by utilizing heat sources has been the driving force behind a long history of development of what we now call heat engines. Unit for heat energy: 1Btu (raise the temperature of one pound of water by one degree Fahrenheit) Unit for mechanical energy: 1 foot-pound (raise one pound of water one foot higher)
3. The Thermodynamic of Heat engines A heat engine is any device that can take energy from a warm source and convert a fraction of this heat energy to mechanical energy Heat source hot Q1 Figure 3.2 a thermodynamic diagram of a heat engine operating between a heat source and heat sink Work output at a lower temperature The work HEAT ENGINE Aw= Cnot-Qcold) output must equal the difference between the heat energy extracted from the source and that rejected to the sink- the Principle of Energy Heat sink Conservation old
3. The Thermodynamic of Heat Engines A heat engine is any device that can take energy from a warm source and convert a fraction of this heat energy to mechanical energy. Figure 3.2 A thermodynamic diagram of a heat engine operating between a heat source and heat sink at a lower temperature. The work output must equal the difference between the heat energy extracted from the source and that rejected to the sink – the Principle of Energy Conservation
Not all of the heat energy taken from the source is being used to performed useful work. Some fraction of the heat energy must always be rejected, at a temperature cooler than that of the warm source to the environment The second law of thermodynamics Kelvin statement It is impossible to convert heat completely into works in a cyclic process. Clausius statement Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature In a system a process that occurs will tend to increase the total entropy of the universe work done Efficiency <100% energy put into the system
Not all of the heat energy taken from the source is being used to performed useful work. Some fraction of the heat energy must always be rejected, at a temperature cooler than that of the warm source, to the environment. – The second law of thermodynamics Kelvin statement: It is impossible to convert heat completely into works in a cyclic process. Clausius statement: Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature. In a system, a process that occurs will tend to increase the total entropy of the universe. Efficiency = work done energy put into the system < 100%
work done Efficiency energy put into the system Qhat-Q Efficiency )×100% For an ideal engine, the ratio of two energy terms is identical to the ratio of two temperature terms Carnot cold cold /I hot where the temperatures are given on the absolute(Kelvin scale Efficiency 7)×100% cold It is remarkable that this efficiency( Carnot) depends only on the temperatures of the two reservoirs between which the heat engine operates
Efficiency = work done energy put into the system Efficiency = Qhot - Qcold Qhot = (1 - ) x 100% Qcold Qhot For an ideal engine, the ratio of two energy terms is identical to the ratio of two temperature terms: Qcold / Qhot = Tcold / Thot where the temperatures are given on the absolute (Kelvin) scale. Tcold Efficiency = (1 - ) x 100% Thot It is remarkable that this efficiency (Carnot) depends only on the temperatures of the two reservoirs between which the heat engine operates. Carnot
A generalized thermodynamic cycle A Carnot cycle taking place between a hot reservoir at temperature TH and a cold reservoir at temperature te /= QHQC Efficiency =(1-=)X100% S
A Carnot cycle taking place between a hot reservoir at temperature TH and a cold reservoir at temperature TC . A generalized thermodynamic cycle Tcold Efficiency = (1 - ) x 100% Thot
Example: For a coal-fire electric power plant, T hot(the boiler temperature)would be 825 K, and Tcold(the cooling tower)would be about 300 K. This leads to Efficiency=(1-300/825)X100%=(1-0.36)×100%=64% In this case, 36% of the heat energy from the energy of the fuel must be wasted by rejecting it through the cooling tower to the surrounding atmosphere To make the efficiency as high as possible it would be desirable to increase T hot and decrease The limit on Thot is imposed by the materials from which the boilers can be constructed and the limit on Tcold is imposed by the availability in nature of large sinks at sufficiently low temperature
Example: For a coal-fire electric power plant, Thot (the boiler temperature) would be 825 K, and Tcold (the cooling tower) would be about 300 K. This leads to Efficiency = (1 – 300 / 825) x 100% = (1 – 0.36) x 100% = 64% • In this case, 36% of the heat energy from the energy of the fuel must be wasted by rejecting it through the cooling tower to the surrounding atmosphere. • To make the efficiency as high as possible, it would be desirable to increase Thot and decrease Tcold. • The limit on Thot is imposed by the materials from which the boilers can be constructed and the limit on Tcold is imposed by the availability in nature of large sinks at sufficiently low temperature
4. Generation of Electricity In 1831, in London, Michael Faraday(1791-1876)discovered electromagnetic induction -one of the greatest discoveries of all time Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field The discovery made the generation and transmission of electricity possible and quickly lead to the invention of electric generators This is all very interesting, but of what possible use are these toys? I cannot say what use they may be, but I can confidently predict that one day you will be able to tax them “ Of what use is a newborn baby?
4. Generation of Electricity In 1831, in London, Michael Faraday (1791-1876) discovered electromagnetic induction – one of the greatest discoveries of all time. Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field. The discovery made the generation and transmission of electricity possible, and quickly lead to the invention of electric generators. "This is all very interesting, but of what possible use are these toys?" "I cannot say what use they may be, but I can confidently predict that one day you will be able to tax them." “ Of what use is a newborn baby?
