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Thursday, 4 August 2016

WHAT IS HEAT PUMP IN THERMODYNAMICS?

We were discussing the concept of first law of thermodynamics for an open system or control volume in our previous post. We have seen the fundamentals of a heat engine and also the procedure for calculating the efficiency of a heat engine in the field of thermal engineering.

Let us go ahead to discuss the concept of working of a heat pump in thermodynamics. We will see here the fundamentals of a heat pump and also we will calculate here the performance parameter of a heat pump i.e. coefficient of performance of a heat pump.

So let us first see here the concept of a heat pump

Heat pump is basically defined as a device which will be operated in a thermodynamic cyclic process and will transfer the heat from a cold object or lower temperature object to a high temperature object by taking work energy from surrounding.

As we know that heat can never be transferred from a lower temperature body to a high temperature body in nature but we must note it here that it will be possible if a heat pump is used. Following figure, displayed here, indicates the fundamentals of a heat Pump.
Heat Pump
As we can see above in figure, a heat pump will have three important terms and these are as mentioned below
  • Thermal reservoir at higher temperature
  • Working media or substance
  • Thermal reservoir at lower temperature
According to the second law of thermodynamics, heat energy will always flow from higher temperature body to lower temperature body  and we can also write here that heat will never be transferred from lower temperature body to high temperature body until unless there is no work provided from the surrounding.

As we are observing in above figure, heat pump is taking heat QC from cold body and delivering the heat QH to higher temperature body by absorbing work energy W from surrounding.
Therefore we can summaries here that work is done here on the system by the surrounding. Heat will be extracted by the heat pump from lower temperature body and will be rejected to a higher temperature body.

Co-efficient of performance of a heat pump (COP)

Let us determine the co-efficient of performance of a heat pump, before going for calculation we need to know the each term used here and these are as mentioned below.

Temperature of hot thermal reservoir is TH
Temperature of cold thermal reservoir is TC
Work energy provided by the surrounding on the system is W and it could also be called as input work energy.
Heat energy at output of the heat pump is QH
Heat energy at input of the heat pump is QC.

Coefficient of performance (COP) is basically a parameter in order to determine the performance of a heat pump. Coefficient of performance (COP) is basically defined as the ratio of desired output to required work input and we can write here as mentioned below.

COP = Desired output/Required work input
 
Heat Pump: COP
We will come again with another important topic i.e. Reversible process in thermodynamics in our next post.
Do you have suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. Nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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