Thermodynamics Notes26

Thermodynamics Notes26 - ME 311 FALL 2007 FINAL LECTURE...

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ME 311 THERMODYNAMICS S. Masutani FALL 2007 FINAL LECTURE CHAPTER 10 REFRIGERATION AND HEAT PUMPS The objective of refrigeration systems is to remove energy by means of heat transfer from an insulated space held at a temperature below ambient. This energy ultimately is exhausted to the environment. The objective of heat pumps is to add thermal energy into a space maintained at a temperature above ambient by means of a combination of energy extraction from the (cooler) environment and power input. Carnot refrigerator (or heat pump) T H T C W Q H Q C 4 3 2 1 S T T H T L Work Q C 1 2 : adiabatic compression 2 3 : isothermal heat rejection 3 4 : adiabatic expansion 4 1 : isothermal heat addition m W = enclosed area in the T-s diagram; to minimize m W , we would like to be small. C H T T Recall : the Clausius statement of the 2 nd Law declares that a continuously operating zero work refrigerator (or heat pump) is impossible. A refrigeration cycle is rated in terms of its coefficient or performance (COP): W Q COP C ref = heat extracted from cold space/work required 1
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 Carnot COP ref yields the best performance for given and and can be expressed as H T C T C H C C H C carnot T T T Q Q Q COP = = (>1) Real systems have . carnot COP COP < For a heat pump, the goal is to provide large amounts of for small W. Thus H Q W Q COP H HP and, for a Carnot heat pump, C H H C H H HP T T T Q Q Q COP = = (>1) Note: a Carnot refrigerator (or heat pump) is just a Carnot engine run in reverse
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This note was uploaded on 05/19/2008 for the course ME 311 taught by Professor Masutani during the Fall '07 term at Hawaii.

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Thermodynamics Notes26 - ME 311 FALL 2007 FINAL LECTURE...

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