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Wize University Physics Textbook (Master) > Thermodynamics

Refrigerators and Heat Pumps

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Second Law of ThermodynamicsNext Section
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Heat Pumps

  • Heat pumps are similar to refrigerators but a heat pump can be used to either heat a system or cool down a system
  • The COP (coefficient of performance) for heat pumps is defined as:

COP=∣Qdesired∣∣W∣COP=\dfrac{\left|Q_{desired}\right|}{\left|W\right|}

Calculate the coefficient of performance (COP) for an air-to-air heat pump to maintain an inside temp of 70°F with an outside temperature of 30°F.


COP = (Thot / Thot - Tcold)

1. Convert the temperatures from Fahrenheit to Celsius:
Th = (70 - 32) x 5/9 = 21°C
Tc = (30 - 32) x 5/9 = -1°C.

2. Convert the Celsius temperatures to Kelvin by adding 273:
Th = 21°C + 273 = 294K
Tc = -1°C + 273 = 272K

3. Plug the temperatures into the formula:
COP = (294K / 294K - 272K) = 294 / 22 = 13.3
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A reversible heat pump is used in winter to maintain an internal of a house at 20°C. The average outdoor temperature is 2°C. The house loses loses heat at an estimated 82 KJ/hr. If the pump consumes 8 kW while operating, find:
a) how long the heat pump ran on that day
b) the cost to heat, using a cost of $0.085/kWhr for electricity,
Extra Practice
Thermodynamics: Refrigerators and Heat Pumps
A refrigerator is operated by a 1000 watt motor. If the “coefficient of performance” of this refrigerator is 5, what is the rate in which heat transfers to the room from the back pf the refrigerator? Imagine you put a plate of food with the total mass of 500g and a temperature of 50 °𝐶 inside the refrigerator. If the effective specific heat of the plate of food is 4000 Jkg⋅K\frac{J}{kg\cdot K}, how long does it take for the refrigerator to cool it down to 5 °𝐶?
Thermodynamics: Refrigerators and Heat Pumps
A heat engine with 55% efficiency is used as a refrigerator. If the reservoirs
remain the same, what is the coefficient of performance for this refrigerator?
If the heat absorbed by the refrigerator is equal to 5kJ, calculate the work
For heat engine cycle is
clockwise
while for heat pump cycle is
counterclockwise
A Carnot air conditioner takes energy from the thermal energy of a room at 70o F and transfers it to the outdoors which is at 96o F. For each joule of electric energy required to operate the air conditioner, how many joules are removed from the room?
heat pump refrigerators
A refrigerator has a coefficient of performance of 3.0.