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Different Input and Output Devices

Most of the energy sources involve the conversion of thermal energy to work. 
The device which can convert thermal energy to work is called as heat engine. 
It produces the mechanical work by utilising a part of thermal energy which can be generated by combustion of fuel or in nuclear reactors. 
Some devices can convert the thermal energy to directly an electrical output. 
The thermodynamic laws show the relation between heat and mechanical or electrical or energy. 
No doubt these laws are applicable only for thermal equilibrium not for any rapid changes.

Carnot’s theorem: the most efficient engine possible and is stated in the theorem as well. 
Of all the heat engines working between two given temperatures, none is more efficient than a Carnot engine
We can actually prove Carnot’s theorem on the basis of Clausius statement of the second law of thermodynamics
The first law proves the conservation of energy and can be written as  
Δ Q = Δ U + Δ W
Where Δ Q = Input of heat energy to the system
 Δ U = internal energy change
Δ W = Output in the form of system’s work done

In other words; if an engine produce an output in the form of work by using thermal energy, the net output of work will be equal to net input of thermal energy. Therefore there is no restriction on the amount of thermal energy that can be converted into output like work. 
But according to second law of thermodynamic the amount of work obtained from heat engine which is working between high and low temperature, is limited by the temperature if source (high temperature T1) and sink (low temperature T2).
A reversible theoretical device which is called as Carnot engine, gives the maximum work output for a given thermal input. It operates through cycle of isothermal and adibatic processes. 
Complete engine works in four steps;
•    Imagine the gas and cylinder kept first on the source of heat 
•    Thereafter over a insulator or we can say there is no heat exchanged
•    Within a heat sink or we can say that heat gets removed
•    And at the end we put it back on the insulator

As first law of thermodynamic states that there is no loss or gain of energy, hence the work output of Carnot engine should be equals to the difference between the heat absorbed from the source and passed to the sink. 
The input of heat and output work leads to the definition of efficiency of an ideal Carnot heat engine which is given by;
Efficiency  = Woutput x  100 %

Since, Woutput = Q1 − Q2 and Winput = Q1 
            So   Efficiency= (Q1 − Q2)  / Q1x 100 %

Though the Carnot engine is a imagination only and there is no heat engine which can operate like Carnot cycle as there will be some loss of energy in form of friction, but still it helps to provide idea for increasing the efficiency of practical engines.

The Carnot Cycle: this consist of four operations and that too in succession and they are as follows:
(a)    Isothermal expansion at higher temperature
(b)    Adiabatic expansion between high temperature and low temperature
(c)    Isothermal compression at constant lower temperature
(d)    Adiabatic compression between lower and higher temperatures

When we cross check the efficiency of any engine we find that it depends only on temperatures of source and sink and would be independent of all other factors.
Moreover, all these reversible heat engines which work between same temperatures are actually equally efficient and under no circumstances a heat engine could be more efficient than Carnots.
As we could see on kelvin scale, the temperature could never be negative and when the higher and lower temperatures are found to be finite the efficiency of heat engine is found to be lesser than unity or we could say that the entire heat could never be converted into useful work.
Let us take a summary of the following:
Efficiency of Carnot’s engine is not dependent on the nature of the working substance usually a gas.
Efficiency of Carnot’s engine would depend upon the heat source and sink’s temperature.
Efficiciency of Carnot’s cycle would be 100% if and only if T1 = infinity or T2 = zero K and as these are not possible or could be attained, so the Carnot’s heat engine based on reversible cycle cannot have 100% efficiency.

Instances where the heat sources and sink are at same temperature (T2 = T1) the efficiency of this heat engine would become zero. 
When a refrigerator is working within a closed room with its door closed the refrigerator would reject heat from inside into this room continuously and this will in turn increase the heat of the room slowly and steadily. 
In case the door of the refrigerator is left open the heat rejected by the refrigerator would be more than it takes from the room. 
So in the second instance the temperature of room will indeed increase but in a slower rate than the first instance.
If the heat rejected by the refrigerator could be thrown out directly then the room will cool down gradually and this is the principle of an airconditioner.