Refrigerators & Heat Pumps
Heat transfers spontaneously from an object with a high temperature to an object with a lower temperature.
The opposite process is not possible naturally, which requires the use of refrigerators.
A Heat Pump is a Refrigerator in reverse.
The only difference is the desired output.
Refrigerators purpose it to deliver a 'cool' output.
Heat Pumps purpose is to deliver a 'warm' output.
Coefficient of Performance COP = Efficiency of how well a refrigerator or a heat pump performs.
Both these processes require a work input, Wnet,in.
The efficiency of the heat pump / refrigerator can be expressed as:
By using the conservation of energy principle, the energy balance for each system can be represented as:
COPR can be greater than 1, which means that:
"The amount of heat removed from the refrigerated space is greater than the work input."
For fixed values of QL and QH:
This means the COP of a heat pump is always greater than 1, since COP for a refrigerator is greater than 1.
So, at worst, a heat pump will simply supply as much energy as it electrically consumes.
Questions
A refrigerator electrically consumes 4kW of power to cool its contents. If the refrigerator removes heat at a rate of 720kJ/hr. Determine:
The coefficient of performance of the refrigerator. What does your answer represent in terms of how much energy it removes?
The rate at which the hot air is rejected into the surrounding environment.
The interior of car has a temperature of 25oC, and rejects heat at a rate of 1,000kJ/hr to its outside surroundings at 0oC. If the heating system has a coefficient of performance of 3.0, what is the power consumed by the heating system?
Answers are in .pdf format below.
