Thermodynamics II Engines and Refrigerators HEAT ENGINE REFRIGERATOR

Thermodynamics II

Engines and Refrigerators HEAT ENGINE REFRIGERATOR TH TH QH system QH W QC TC system taken in closed cycle Usystem = 0 l therefore, net heat absorbed = work done QH - QC = W (engine) QC - QH = -W (refrigerator) energy into green blob = energy leaving green blob l 11

Heat Engine: Efficiency The objective: turn heat from hot reservoir into work The cost: “waste heat” 1 st Law: QH -QC = W HEAT ENGINE TH QH efficiency e W/QH = (QH-QC)/QH W QC TC = 1 -QC/QH 13

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Refrigerator: Coefficient of Performance REFRIGERATOR The objective: remove heat from cold reservoir The cost: work TH QH 1 st Law: QH = W + QC coeff of performance Kr QC/W = QC/(QH - QC) W QC TC 22

Carnot Cycle l Idealized Heat Engine èNo Friction è S = Q/T = 0 èReversible Process » Isothermal Expansion » Adiabatic Expansion » Isothermal Compression » Adiabatic Compression 32

Engines and the 2 nd Law The objective: turn heat from hot reservoir into work The cost: “waste heat” HEAT ENGINE TH QH 1 st Law: QH -QC = W efficiency e W/QH = 1 -QC/QH S = QC/TC - QH/TH 0 S = 0 for Carnot Therefore, QC/QH TC/ TH QC/QH = TC/ TH for Carnot Therefore e = 1 - QC/QH 1 - TC/ TH e = 1 - TC/ TH for Carnot e = 1 is forbidden! e largest if TC << TH W QC TC 36

Summary l First Law of thermodynamics: Energy Conservation èQ = U + W l Heat Engines èEfficiency = = 1 -QC/QH l Refrigerators èCoefficient of Performance = QC/(QH - QC) S = Q/T l Entropy l 2 nd Law: Entropy always increases! Carnot Cycle: Reversible, Maximum Efficiency e = 1 – Tc/Th l 50