Applications of Thermodynamics: Heat Pumps and Refrigerators Quiz

PDF page 705: (credit: Id1337x, Wikimedia Commons) Heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. They are heat engines run backward. We say backward, rather than reverse, because except for Carnot engines, all heat engines, though they can be run backward, cannot truly be reversed. Heat transfer occurs from a cold reservoir and into a hot one. This requires work input , which is also converted to heat transfer. Thus the heat transfer to the hot reservoir is

PDF page 705: (credit: Id1337x, Wikimedia Commons) Heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. They are heat engines run backward. We say backward, rather than reverse, because except for Carnot engines, all heat engines, though they can be run backward, cannot truly be reversed. Heat transfer occurs from a cold reservoir and into a hot one. This requires work input , which is also converted to heat transfer. Thus the heat transfer to the hot reservoir is

PDF page 705: (credit: Id1337x, Wikimedia Commons) Heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. They are heat engines run backward. We say backward, rather than reverse, because except for Carnot engines, all heat engines, though they can be run backward, cannot truly be reversed. Heat transfer occurs from a cold reservoir and into a hot one. This requires work input , which is also converted to heat transfer. Thus the heat transfer to the hot reservoir is

PDF page 705: sign.) A heat pump’s mission is for heat transfer to occur into a warm environment, such as a home in the winter. The mission of air conditioners and refrigerators is for heat transfer to occur from a cool environment, such as chilling a room or keeping food at lower temperatures than the environment. (Actually, a heat pump can be used

PDF page 706: FIGURE 15.27 A simple heat pump has four basic components: (1) condenser, (2) expansion valve, (3) evaporator, and (4) compressor. In

PDF page 706: the heating mode, heat transfer occurs to the working fluid in the evaporator (3) from the colder outdoor air, turning it into a gas. The electrically driven compressor (4) increases the temperature and pressure of the gas and forces it into the condenser coils (1) inside the heated space. Because the temperature of the gas is higher than the temperature in the room, heat transfer from the gas to the room

PDF page 706: the heating mode, heat transfer occurs to the working fluid in the evaporator (3) from the colder outdoor air, turning it into a gas. The electrically driven compressor (4) increases the temperature and pressure of the gas and forces it into the condenser coils (1) inside the heated space. Because the temperature of the gas is higher than the temperature in the room, heat transfer from the gas to the room

PDF page 707: Since the efficiency of a heat engine is , we see that , an important and interesting fact. First, since the efficiency of any heat engine is less than 1, it means that is always greater than 1—that is, a heat pump always has more heat transfer than work put into it. Second, it means that heat pumps work best when temperature differences are small. The efficiency of a perfect, or Carnot, engine is ; thus, the smaller the temperature difference, the smaller the efficiency and the greater the (because ). In other words, heat pumps do not work as well in very cold climates as they do in more moderate climates. Friction and other irreversible processes reduce heat engine efficiency, but they do not benefit the operation of a heat pump—instead, they reduce the work input by converting part of it to heat transfer back into the cold reservoir before it gets into the heat pump.

PDF page 707: Since the efficiency of a heat engine is , we see that , an important and interesting fact. First, since the efficiency of any heat engine is less than 1, it means that is always greater than 1—that is, a heat pump always has more heat transfer than work put into it. Second, it means that heat pumps work best when temperature differences are small. The efficiency of a perfect, or Carnot, engine is ; thus, the smaller the temperature difference, the smaller the efficiency and the greater the (because ). In other words, heat pumps do not work as well in very cold climates as they do in more moderate climates. Friction and other irreversible processes reduce heat engine efficiency, but they do not benefit the operation of a heat pump—instead, they reduce the work input by converting part of it to heat transfer back into the cold reservoir before it gets into the heat pump.

PDF page 707: Since the efficiency of a heat engine is , we see that , an important and interesting fact. First, since the efficiency of any heat engine is less than 1, it means that is always greater than 1—that is, a heat pump always has more heat transfer than work put into it. Second, it means that heat pumps work best when temperature differences are small. The efficiency of a perfect, or Carnot, engine is ; thus, the smaller the temperature difference, the smaller the efficiency and the greater the (because ). In other words, heat pumps do not work as well in very cold climates as they do in more moderate climates. Friction and other irreversible processes reduce heat engine efficiency, but they do not benefit the operation of a heat pump—instead, they reduce the work input by converting part of it to heat transfer back into the cold reservoir before it gets into the heat pump.

PDF page 707: goes into heat transfer before it gets into the heat engine, thereby reducing its coefficient of performance . In this figure, represents the portion of that goes into the heat pump, while the remainder of is lost in the form of frictional heat to the cold reservoir. If all of had gone into the heat pump, then would have been greater. The best heat pump uses adiabatic and isothermal processes, since, in theory, there would be no dissipative processes to reduce the heat transfer to the hot reservoir.

PDF page 708: so that 15.42 Discussion This result means that the heat transfer by the heat pump is 5.30 times as much as the work put into it. It would cost 5.30 times as much for the same heat transfer by an electric room heater as it does for that produced by this heat pump. This is not a violation of conservation of energy. Cold ambient air provides 4.3 J per 1 J of work from the electrical outlet.