Efficiency of Heat Engines Thermodynamics Efficiency of Heat Engines

The Second Law of Thermodynamics In an ideal cyclic process, internal energy is conserved Net work = net heat flow Wnet = Qnet = Qh – Qc Qh is the heat absorbed by the heat engine or refrigerator or whatever Qc is the heat given up to the surroundings

The Second Law of Thermodynamics Based on this, a heat engine could do more work on a hot day than it could a cold day and could always perform work as long as the engine’s temperature was less than the surrounding air temperature This situation never happens

The Second Law of Thermodynamics The second law of thermodynamics – no cyclic process that converts heat entirely into work is possible Some energy must always be transferred to the surroundings as heat

Thermodynamic Efficiency Efficiency is a rating of how well an engine works What percentage of heat absorbed is converted to useful work Efficiency = net work done by engine / energy added as heat Efficiency = (energy added as heat – energy removed as heat) / energy added as heat Efficiency = 1 – energy removed as heat / energy added as heat Efficiency = 1 – temperature of cold reservoir / temperature of hot reservoir eff = Wnet/Qh = (Qh-Qc)/Qh = 1-Qc/Qh = 1- (Tcold / Thot) You can multiply by 100 to get a percentage

Laws of Thermodynamics 0th - You must play the game 1st – You can’t win, you can only break even 2nd – You can’t break even 3rd – You can’t quit the game