Judah Levine, 3070L11 1http://physics.colorado.edu/phys3070http://physics.colorado.edu/phys3070Judah LevineJudah LevineJILA SJILA [email protected]@colorado.edu303 492303 492--7785 (27785 (2--7785)7785)M,F 11M,F 11--Noon, W 1Noon, W 1--3 pm3 pmalso by appointmentalso by appointmentJudah Levine, 3070L11 2Exam next Wednesday in classThrough Carnot efficiencyOpen book + 1 page of notes +CalculatorCAPA + Clicker questionsConversion factors in bookOr givenJudah Levine, 3070L11 3Model heat engineModel heat enginegasForce > 0Outside air, temperature TLQH, Temperature THJudah Levine, 3070L11 4How well can we do?How well can we do?HLTTinputoutputefficiency −== 1Goal: minimize TL and maximize THTL=0 C = 273 K TH= 1100F = 600C = 873K%7069.08732731 ≈=−=eVery hard to do much better than thisTypical heat engine has thermal efficiency of 35% (about half)Judah Levine, 3070L11 5Where does the energy go?Where does the energy go?First, the usual suspects:Friction here, friction there, Mechanical losses, …Judah Levine, 3070L11 6Where does the energy go Where does the energy go ……Suppose TL= 400K and TH= 800K%)50(5.080040011 ==−=−=HLTTefficiencyJudah Levine, 3070L11 7Where does the energy go Where does the energy go ……Suppose TL= 400K and TH= 800K%)50(5.080040011 ==−=−=HLTTefficiencyIf you could do only one thing to improve the efficiency, should you:A: Raise the input temperature by 100K to 900KB: Lower the output temperature by 100K to 300KC: It doesn’t matter which one you doJudah Levine, 3070L11 8Optimum strategyOptimum strategy%)50(5.080040011 ==−=−=HLTTefficiency%)63(63.080030011 ==−=−=HLTTefficiency%)56(56.090040011 ==−=−=HLTTefficiencyLowering the output temperature is MUCH more effectiveJudah Levine, 3070L11 9Model heat engineModel heat enginegasForce > 0Outside airWhen does the force go to 0?QHJudah Levine, 3070L11 10Model heat engineModel heat enginegasForce > 0Outside airWhen does the force go to 0?Gas inside has cooled to outside temp.QHJudah Levine, 3070L11 11The problemThe problemThe problem with a real engine is that the exhaustIs HOT.A true Carnot engine would require no cooling.Judah Levine, 3070L11 12Extracting Extracting ALLALLthe heatthe heatTradeoff between efficiency and size and powerHigh efficiency implies:Large sizeLow speedJudah Levine, 3070L11 13Why is the exhaust hot?Why is the exhaust hot?dx1 dx2 gasForce > 0Outside air, temperature TLXJudah Levine, 3070L11 14Output as function of position05001000150020002500300035000 2 4 6 8 10 12X positionWork Outputkpv =5/7Judah Levine, 3070L11 15Judah Levine, 3070L11 16Travel of piston is about diameter of flywheelJudah Levine, 3070L11 17Making a virtue out of necessityMaking a virtue out of necessityQQUse exhaust for something elseUse exhaust for something else––Heater in car or busHeater in car or bus––Selling steam in New York CitySelling steam in New York CityQQCogenerationCogeneration––Generate electricityGenerate electricity––Use exhaust heat for heating and hot waterUse exhaust heat for heating and hot waterJudah Levine, 3070L11 18University cogeneration systemUniversity cogeneration systemQQProvide electricity and heat to all Provide electricity and heat to all campus buildingscampus buildingsQQGuest speaker:Guest speaker:––Mr. Paul Caldara, Univ. Mr. Paul Caldara, Univ. FacFac. Man..
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