Heat Pump Or Refrigerator

Since, we know that thermodynamics is concerned with the conversion of heat into work and work into heat. That's why we need a machine that converts heat into work and work into heat. "A heat engine is a machine that converts heat into work whereas a heat pump or refrigerator is a machine that converts work into heat". Here, we use both the laws of thermodynamics (1st law and 2nd law) to understand the working operation of a Heat-Pump/Refrigerator. The first law of thermodynamics tells us about the conservation of energy while the second law of thermodynamics tells us about the direction of energy transfer. 

What Is Heat Pump ???

In a heat engine, we saw that heat transfer takes place from a higher temperature body to a lower temperature body and the device or machine produces some useful work. And that’s obviously right. 

But what if we want to transfer the heat in the reverse direction ,i.e, from lower temperature body to higher temperature body???

Heat transfer never takes place from lower temperature to higher temperature on it’s own. In this case, a new device is used to do so, and this device is known as a "Heat Pump".

!!!!A Heat Pump Is Basically A Heat Engine That Runs In The Opposite Direction!!!!

.........."A heat pump is a machine that operates between two reservoirs in which one acts as a heat source while the other acts as a heat sink. These reservoirs are capable of absorbing or rejecting an unlimited amount of heat without a change in temperature. Its function is to completely convert the work into heat using external work"..........

In simple words, A heat pump is a thermodynamic system that operates in a cycle and removes heat from a low temperature reservoir and delivers it to a high temperature reservoir using external energy in the form of work. Here,

      Heat Sink works as ➝ Surroundings

      Heat Source works as ➝ System

Heat Pump Working

Heat pump operation;

1. By using external work, it extracts heat from a low temperature reservoir.

2. Transfer the total amount of heat or energy (extracted heat+work) to a high temperature reservoir.

We can understand this through a diagram;

As shown in above diagram, heat pump extracts Q₂ amount of heat from the lower temperature body and delivers Q₁ amount of heat to the higher temperature body. But this heat transfer cannot take place on it’s own. For this, we have to supply some work to this device in order to transfer the heat from lower temperature reservoir to higher temperature reservoir. And the amount of input work required is;

                        ➩ [ W = Q₁ – Q₂ ]

The unit of this work done is Joule (J). This is how the heat pump works. Here,"Heat transferred to a high temperature reservoir is a useful quantity".

Coefficient Of Performance

It is an important factor that describes how much work is converted into heat. Coefficient of performance is defined as the ratio of heat transferred to a high temperature reservoir and the work input. It is denoted as 'COP'. 

According to the above figure, the total energy transfer to a high temperature reservoir (Q₁) is the summation of external work input (W) and extracted heat from a low temperature reservoir (Q₂).

      ➩ total energy transfer, Q₁ ＝ (W＋Q₂)

 Thus;  

If Q₂ becomes zero then the coefficient of performance of the heat pump will be 100%.

REFRIGERATION

The operation of refrigeration is similar to that of a heat pump. Refrigeration also turns work into heat. But its purpose is different. The refrigerator operates in a cycle. 

It is defined as, "A refrigerator is a device that continuously takes heat from a low temperature reservoir with the help of an external function and transfers it to a high temperature reservoir. Its main purpose is to refrigerant a system with the help of external work". 

These reservoirs are capable of absorbing or rejecting an unlimited amount of heat without a change in temperature. 

Here,

      Heat Sink works as ➝ System

      Heat Source works as ➝ Surrounding

Coefficient Of Performance

It is an important factor that describes how much work is converted into heat. Coefficient of performance for refrigeration is defined as the ratio of heat extracted from low temperature reservoir to the work input. It is denoted as '(COP)ʀ'.  

According to the above figure, heat extracted from a heat sink (or low temperature reservoir) is 'Q₂' and the amount of work input is 'W'. Thus,

Here,"Heat extracted from a low temperature reservoir is a useful quantity".

!!!!The Performance Of A Heat Engine Is Expressed In Terms Of Energy Conversion Efficiency Whereas The Performance Of A Heat Pump Or Refrigerator Is Expressed As A Coefficient Of Performance Rather Than Thermal Efficiency!!!!

Difference Between Heat Pump And Refrigerator

A device that absorbs energy in the form of heat from a low-temperature reservoir when work is done on it and it releases energy as heat to higher-temperature reservoir, is called a "Heat-Pump/Refrigerator". 

The difference between heat pump and  refrigerator is described below in point format one by one;

(1) If the purpose of a device is to maintain the reservoir at a low temperature by absorbing energy in the form of heat from the reservoir then it is called a "Refrigerator". Usually the refrigerator discharges energy in the form of heat to the ambient atmosphere. Thus, in case of refrigeration, the surroundings (or ambient atmosphere) are considered as the high temperature reservoir.

         If the purpose of an device is to maintain the reservoir at a high temperature by discharging energy in the form of heat to the reservoir then it is called a "Heat Pump". In the case of heat pumps, usually the surrounding (or ambient environment) is used as a low-temperature reservoir from which energy is absorbed as heat.

(2) Heat pumps and refrigerators have different coefficients of performance because of their different objectives. 

Coefficient of performance for Refrigeration;                       ➩ { (COP)R ＝ Q₂/W }

Coefficient of performance for Heat pump;

               ➩ { (COP)HP ＝ Q₁/W }

(3) Heat pumps and refrigerators are essentially the same type of devices but their objectives are different,

               Heat pump used for  ➝   "HEATING"

             Refrigerators used for  ➝  "COOLING"

(4) The primary purpose of refrigeration is to remove heat from a low-temperature reservoir while the primary purpose of a heat pump is to generate heat (or mechanical energy) by the pump. 

(5) When the heat pump and refrigerator are operating between the same high and low temperature reservoir, the COP of the heat pump is always higher than the COP of the refrigeration. Thus, 

(6) When the heat engine and refrigerator are operating between the same high and low temperature reservoir then the relation between 'η' and '(COP)ʀ' is given by;

Hope you have found this article helpful!!

Do you have suggestions? Please write in comment box!!

Feel free to comment if you have any queries!!