Among all cyclic thermodynamic processes, the Carnot cycle has a special theoretical significance and practical application. Often it is called unsurpassed, great, ideal, etc. And for many, it generally seems to be something mysterious and incomprehensible. However, if all the accents are correctly placed, then all the simplicity, genius and beauty of this invention, which was discovered by the French scientist and engineer Sadi Carnot, will instantly open up. And it will become clear that there is nothing supernatural in the process he proposed, but only the most effective use of some laws of nature.
So what is the famous and mysterious Carnot cycle really? It can be defined as a quasi-static process based on bringing a thermodynamic system into thermal contact with a pair of thermostatic tanks having constant and stable temperature values. Whereinit is assumed that the temperature of the first (heater) exceeds that of the second (refrigerator). The Carnot cycle consists in the fact that first a thermodynamic system, initially having a certain thermal value, comes into contact with a heater. Then, by an infinitely slow decrease in pressure, a quasi-static expansion is caused in it, accompanied by the borrowing of heat from the heater and resistance to external pressure.
After that, the system is isolated, which again causes a quasi-static adiabatic expansion in it until its temperature reaches that of the refrigerator. With this type of expansion, a certain work of resistance to external pressure is also performed by the thermodynamic system. In this state, the system is brought into contact with the refrigerator, while by continuously increasing the pressure it is compressed to a certain point, as a result of which it then gives the second reservoir the thermal energy borrowed from the heater in full. The Carnot cycle is unique in that it is not accompanied by any heat loss. Theoretically, such a scheme can be called a perpetual motion machine. This is because the thermal efficiency of the Carnot cycle, depending solely on the temperatures of the tank pair, will always be the highest possible. However, no one has yet managed to create a machine whose thermal efficiency would exceed thirty percent of that allowed by the cyclic process of Sadi Carnot.
And this process is called ideal because itmuch better than other cycles is able to convert heat into useful work. On the other hand, due to the difficulties in organizing and carrying out isothermal processes, its application in real engines is extremely difficult. For maximum heat transfer efficiency, such a machine must be completely isolated from the external environment, which in reality is almost impossible.
The reverse Carnot cycle underlies the principle of operation of a heat pump, which, unlike a refrigerator, must give as much energy as possible to some hot object, such as a heating system. Some of the heat is borrowed from the environment, which has a lower temperature, the rest of the required energy is released during the performance of mechanical work, such as a compressor.
