![]() When you burn fuel to keep warm, you pay for all of it. You only pay for W, and you get an additional heat transfer of Q c from the outside at no cost in many cases, at least twice as much energy is transferred to the heated space as is used to run the heat pump. Heat transfer is from the outside air, even at a temperature below freezing, to the indoor space. The great advantage of using a heat pump to keep your home warm, rather than just burning fuel, is that a heat pump supplies Q h = Q c + W. There is heat transfer Q c into the system from a cold reservoir along path DC, and heat transfer Q h out of the system into a hot reservoir along path BA. The area inside the loop is negative, meaning there is a net work input. (b) diagram for a Carnot cycle similar to that in Figure 3 but reversed, following path ADCBA. The directions of W, Q h, and Q c are opposite what they would be in a heat engine. (a) Schematic diagram showing heat transfer from a cold reservoir to a warm reservoir with a heat pump. The one shown here is based on a Carnot (reversible) engine. ![]() Heat pumps, air conditioners, and refrigerators are heat engines operated backward. In this section we will concentrate on its heating mode.)įigure 2. It is essentially an air conditioner and a heating unit all in one. (Actually, a heat pump can be used both to heat and cool a space. ![]() The mission of air conditioners and refrigerators is for heat transfer Q c to occur from a cool environment, such as chilling a room or keeping food at lower temperatures than the environment. (Note that Q h, Q c, and W are positive, with their directions indicated on schematics rather than by sign.) A heat pump’s mission is for heat transfer Q h to occur into a warm environment, such as a home in the winter. Thus the heat transfer to the hot reservoir is Q h = Q c + W. This requires work input W, which is also converted to heat transfer. Heat transfer occurs from a cold reservoir Q c and into a hot one. 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 pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. Most people don’t realize they are also sharing their homes with a heat pump. Almost every home contains a refrigerator. ![]()
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