Thermodynamics

Chapter 6 | 311Solution A heat pump maintains a house at a constant temperature. Therequired minimum power input to the heat pump is to be determined.Assumptions Steady operating conditions exist.Analysis The heat pump must supply heat to the house at a rate of Q H 135,000 kJ/h 37.5 kW. The power requirements are minimum when areversible heat pump is used to do the job. The COP of a reversible heatpump operating between the house and the outside air isCOP HP,rev 111 T L >T H 1 15 273 K2>121 273 K2 11.3Then the required power input to this reversible heat pump becomesW # net,in Q H 37.5 kW 3.32 kWCOP HP 11.3Discussion This reversible heat pump can meet the heating requirements ofthis house by consuming electric power at a rate of 3.32 kW only. If thishouse were to be heated by electric resistance heaters instead, the powerconsumption would jump up 11.3 times to 37.5 kW. This is because inresistance heaters the electric energy is converted to heat at a one-to-oneratio. With a heat pump, however, energy is absorbed from the outside andcarried to the inside using a refrigeration cycle that consumes only 3.32 kW.Notice that the heat pump does not create energy. It merely transports itfrom one medium (the cold outdoors) to another (the warm indoors).TOPIC OF SPECIAL INTEREST*Household RefrigeratorsRefrigerators to preserve perishable foods have long been one of the essentialappliances in a household. They have proven to be highly durable andreliable, providing satisfactory service for over 15 years. A typical householdrefrigerator is actually a combination refrigerator-freezer since it has afreezer compartment to make ice and to store frozen food.Today’s refrigerators use much less energy as a result of using smaller andhigher-efficiency motors and compressors, better insulation materials, largercoil surface areas, and better door seals (Fig. 6–54). At an average electricityrate of 8.3 cents per kWh, an average refrigerator costs about $72 a yearto run, which is half the annual operating cost of a refrigerator 25 years ago.Replacing a 25-year-old, 18-ft 3 refrigerator with a new energy-efficientmodel will save over 1000 kWh of electricity per year. For the environment,this means a reduction of over 1 ton of CO 2 , which causes global climatechange, and over 10 kg of SO 2 , which causes acid rain.Despite the improvements made in several areas during the past 100 yearsin household refrigerators, the basic vapor-compression refrigeration cyclehas remained unchanged. The alternative absorption refrigeration andthermoelectric refrigeration systems are currently more expensive and lessBetter doorsealsRefrigeratorBetter insulationmaterialsMore efficient motorsand compressorsFIGURE 6–54Today’s refrigerators are much moreefficient because of the improvementsin technology and manufacturing.*This section can be skipped without a loss in continuity.