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REVERSED CARNOT HEAT ENGINE CYCLE

We have already discussed the concept of “Carnot cycle and its efficiency” and we have also seen the concept of “Carnot theorem and its explanation” in our previous post. 

Today we are interested to see the basic and fundamental concept of reversed Carnot heat engine cycle or reversed Carnot cycle with the help of this post.

As we have already discussed that Carnot cycle is a reversible cycle and each process of Carnot cycle will be reversible process and therefore we can consider that each process of Carnot cycle is reversed and cycle is being operated in reverse direction.

We must note it here that if a reversible process is reversed then all types of energy, such as heat energy or work energy, associated with the process will have same magnitude but the direction of energy interaction will be reversed.

Let us see here first reversed Carnot heat engine cycle

Following figure, displayed here, indicates the reversed Carnot heat engine cycle. There are four processes in reversed Carnot heat engine cycle as shown in figure, now we will see here the each process of reversed Carnot heat engine cycle.
Reversed Carnot heat engine cycle

Process 1 to 2: Adiabatic compression process

Process 1 to 2 will be the adiabatic compression process or isentropic compression process. Working fluid will be compressed here during the adiabatic compression process or isentropic compression process 1 to 2 by receiving work energy W from surrounding.

Work energy will be added to the system from surrounding or we can say that work will be done on the system by the surrounding during this process. Therefore input energy during this adiabatic compression process or isentropic compression process will be work energy.

During this adiabatic compression process, pressure and temperature of working fluid will be increased but there will not be any heat interaction between system and surrounding during this process 1 to 2. Volume of working fluid will be reduced during this process.

As shown in figure, temperature of the working fluid is increased from T1 to T2 during this adiabatic compression process or isentropic compression process 1 to2.

Process 2 to 3: Isothermal compression process

Process 2 to 3 will be the isothermal compression process and therefore working fluid will be compressed here by keeping temperature constant. 

As we know that during the compression of the working fluid, temperature of working fluid will be increased but as we need to maintain the temperature constant and therefore we will use one cold thermal reservoir so that heat will be rejected to the cold thermal reservoir and temperature of the working fluid remain constant.

Therefore, heat energy Q1 will be rejected during this process at constant high temperature.
So, during this process 2 to 3, temperature of working fluid will remain constant and heat energy Q1 will be rejected by the system to the surrounding during this process.

Process 3 to 4: Adiabatic expansion process

Process 3 to 4 will be the adiabatic expansion process or isentropic expansion process. During this adiabatic expansion process or isentropic expansion process, pressure and temperature of working fluid will be reduced while volume of working fluid will be increased during this process.

There will not be any heat interaction between system and surrounding during this adiabatic expansion process or isentropic expansion process.
Temperature of working fluid will be reduced from T2 to T1 as shown in figure.

Process 4 to 1: Isothermal expansion process

Process 4 to 1 will be the isothermal expansion process and therefore working fluid will be expanded here by keeping temperature constant. Pressure of working fluid will be reduced and volume of working fluid will be increased during this isothermal expansion process.

As we know that temperature of working fluid will fall during the isothermal expansion process and therefore we have used one hot thermal reservoir in order to keep the temperature of working fluid constant because this process is one isothermal expansion process and hence we will have to maintain the temperature of working fluid constant.

Heat energy Q2 will be added here to the system from surrounding at lower temperature or we can say that during this process, heat energy Q2 will be absorbed by the working fluid at low temperature T1 from the space being cooled.

We have secured one reversible cycle 1-2-3-4-1 and this reversible cycle will be termed as reversed Carnot heat engine cycle.

A reversed Carnot heat engine is also termed as refrigerating machine. It will receive heat energy from a low temperature region and will deliver the heat energy to a high temperature region by securing the work energy from surrounding.

Removal action of heat energy from a cold region will reduce the temperature of that cold region below the temperature of surrounding and that is the basic concept of refrigeration and we will discuss this i.e. basic concept of refrigeration later in detail.

Coefficient of performance of a refrigerator (C.O.P)

C.O.P = Heat absorbed / Work supplied
C.O.P = Heat absorbed / [Heat rejected - Heat absorbed]
C.O.P = Q2 / [Q1 – Q2]

C.O.P = T2 / [T1 – T2]


We will see another topic i.e. "What are the corollaries of Carnot Theorem? in our next post in the category of thermal engineering.
Do you have suggestion? Please write in comment box

Reference:

Engineering thermodynamics by P. K. Nag
Image courtesy: Google

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