A heat engine running backward is called a refrigerator if

Chapter 21, Problem 21.77

(choose chapter or problem)

A heat engine running backward is called a refrigerator if its purpose is to extract heat from a cold reservoir. The same engine running backward is called a heat pump if its purpose is to exhaust warm air into the hot reservoir. Heat pumps are widely used for home heating. You can think of a heat pump as a refrigerator that is cooling the already cold outdoors and, with its exhaust heat QH, warming the indoors. Perhaps this seems a little silly, but consider the following. Electricity can be directly used to heat a home by passing an electric current through a heating coil. This is a direct, 100% conversion of work to heat. That is, 15 kW of electric power (generated by doing work at the rate of 15 kJ/s at the power plant) produces heat energy inside the home at a rate of 15 kJ/s. Suppose that the neighbors home has a heat pump with a coefficient of performance of 5.0, a realistic value. Note that what you get with a heat pump is heat delivered, QH, so a heat pumps coefficient of performance is defined as K = QH/Win. a. How much electric power (in kW) does the heat pump use to deliver 15 kJ/s of heat energy to the house? b. An average price for electricity is about 40 MJ per dollar. A furnace or heat pump will run typically 250 hours per month during the winter. What does one months heating cost in the home with a 15 kW electric heater and in the home of the neighbor who uses a heat pump?