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Review p V diagram of Ideal Gas p 1 4 2 3 Ta Tb Vb Isotherms Va Point on pV diagram state Every state satisfies Path Line states connected with a process Isotherms T T0 constant state with the same temp 0 Process with changing T must V passing through different isotherms 1 PHYS1003 ENERGY AND RELATED ENVIRONMENTAL ISSUES Outline Thermodynamics of Heat Pumps Applications of heat pumps in daily appliances Heat Pump for Home Heating LECTURE 7 Heat Pump in Daily Life 2 HEAT ENGINES HEAT IN WORK OUT WORK Heat engine Lecture 8 11 FOSSIL FUELS 3 HEAT ENGINES AND HEAT PUMPS HEAT IN WORK OUT WORK IN HEAT OUT APPLICATION Transportation Electricity generation APPLICATION Home heating cooling Refrigeration 5 HEAT ENGINES AND PUMPS HEAT IN WORK OUT WORK IN HEAT OUT APPLICATION Home heating cooling Refrigeration 6 HEAT ENGINES AND PUMPS HEAT IN WORK OUT WORK IN HEAT OUT HEAT ENGINE W 0 Q 0 put heat in get work out HEAT PUMP W 0 Q 0 put work in get heat out p a b p a b Wby c d Won c d runs clockwise V V runs counter clockwise Compression Expansion W Area enclosed in the loop 7 HEAT PUMPS THERMODYNAMICS HEAT ENGINE RUNNING BACKWARDS Hot entity AC outdoor air Heating indoor air heat energy delivered Qh heat energy extracted Cold entity AC refrigerator Heating outdoor air Th Qc Tc HEATING MODE Home heating Work delivered W Qh Qc Qh and Qc are magnitudes COOLING MODE Home cooling and refrigeration 8 HEAT PUMPS THERMODYNAMICS HOW EFFICIENT ARE THEY EFFECTIVENESS described by COP the ratio of useful heat movement per work input Th Qh Qc Tc Work delivered W Qh Qc COOLING MODE COPcool HEATING MODE COPheat Qc W Qh W heat extracted work done heat delivered work done 99 HEAT PUMPS THERMODYNAMICS For Heat Pump HOW EFFICIENT ARE THEY Work delivered W Qh Qc 0 COOLING MODE COPcool Qc W Qc Qh Qc Tc Th Tc HEATING MODE COPheat Qh W Qh Th Th Tc COP 1 AND BECOMES LARGE FOR SMALL T Qh Qc Refrigerators work best in cold environment heat pumps in hot environment 10 Th Qh Qc Tc HEAT PUMPS THERMODYNAMICS For Heat Pump HOW EFFICIENT ARE THEY Work delivered W Qh Qc Th Qh Qc Tc 0 COOLING MODE COPcool Qc W Qc Qh Qc Tc Th Tc HEATING MODE COPheat Qh W Qh Th Qh Qc Th Tc COMPARISON HEAT ENGINES THERMAL EFFICIENCY 1 11 HEAT PUMPS THERMODYNAMICS For Heat Pump HOW EFFICIENT ARE THEY delivered work W Qh Qc Th Qh Qc Tc 0 COOLING MODE COPcool Qc W Qc Qh Qc Tc Th Tc HEATING MODE COPheat Qh W Qh Th Qh Qc Th Tc EXAMPLE Heat pump in heating mode maintaining 20oC in a house at 5oC outside COPheat Th Th Tc 293 25 11 72 12 HEAT PUMP CYCLE EXAMPLE COOLING MODE 1 CONCEPT Pump heat from inside to outside To T3 Ti Heat intake T1 To Heat release 2 3 4 Ti GOAL Build process that realizes this temperature difference by using external work IDEALIZED SIMPLIFIED PROCESS 1 2 Isochoric cooling W 0 Q 0 2 3 Adiabatic expansion W 0 Q 0 3 4 Isochoric heating W 0 Q 0 4 1 Adiabatic compression W 0 Q 0 13 HEAT PUMP CYCLE EXAMPLE COOLING MODE 1 CONCEPT Pump heat from inside to outside To Ti To T3 Ti Heat intake T1 To Heat release 2 3 4 Ti EXAMPLE How much work is required to extract Q 10kJ when Ti 20 C and To 35 C cool 1 1 1 10 kJ 1 512J LARGE Q SMALL W 14 308 293 HEAT PUMP CYCLE THE REALITY VAPOR COMPRESSION REFRIGERATION MOST COMMON REAL HEAT PUMP AC refrigerators industrial applications building cooling 1 2 4 3 CONCEPT Based on phase change of a refrigerant Operating along the liquid gas phase boundary 15 pliquidvapourliquid vapourVpTcThliquidevaporationcondensation 2 1 3 4 compressionthrottlingvapourliquid vapourV Where are the heat pumps 16 Where are the heat pumps Dehumidifier AC which exhausts hot air inside the same room 17 Living in Cold Place Traditional way Heater Energy Sources Fuel Electricity Modern way Energy source Electricity Heat in Surrounding Free 18 HEAT PUMP CYCLE EXAMPLE HEATING MODE CONCEPT Pump heat from outside to inside To Ti SOLUTION Just put the house to the other side of the cycle or Reverse the cycle direction Ti To EXAMPLE How much work is required to provide Q 10kJ when Ti 20 C and To 10 C heat 1 1 1 10 kJ 1 341J LARGE Q SMALL W 19 283 293 AIR SOURCED HEAT PUMP EXCHANGING HEAT WITH OUTSIDE AIR 20 GROUND SOURCED HEAT PUMP EXCHANGING HEAT WITH MOTHER EARTH IDEA Use ground heat as heat source ground temperature 10 m below surface not affected by seasons 35oC 10oC 20oC COP typically ranges from 3 5 influenced by ground temperature ADVANTAGE cooling in summer and heating in winter DISADVANTAGE high capital cost for equipment and drilling 10 100 m deep holes Constantly 12oC 21 27oC 22 Question of the Day Th 33oC Consider a person who emits 100W of heat in a house The temperature outside is on average 33oC He turns on the AC to keep the indoor temperature at Tc 19oC for 8 hours a day 100J sec Tc Question a If the AC is running a Carnot cycle find the COP of the AC operates at i TC 19oC and ii TC 25oC b Estimate how much energy in percentage he can save if he sets the indoor temperature to 25oC instead Energy saved in Energy spent at 19oC Energy spent at 25oC Energy spent at 19oC Hints 1 The answer actually depends on how efficient the AC is but we can assume the AC is the best AC which runs in Carnot Cycle 2 Assume that electricity power convert 100 to power in the compressor of the AC 23 Feedback Anonymous Poll Tell me what you think about this lecture Thank you 24


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hkust PHYS 1003 - Heat Engines and Heat Pumps

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