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?