Language
Country/Area
No result found
Why can't heat be perfectly converted? Understand the key role of entropy generation in heat engines!
In our daily lives, the use of heat energy is common and important. Whether it is simple cooking or large-scale power plants, the conversion of heat energy exists without exception. However, why we cannot achieve perfect conversion of heat energy? What is the reason behind this? This involves an important concept of entropy - entropy generation (or entropy generation).
Entropy generation is the amount of entropy produced in a thermal process and is used to evaluate the efficiency of the process.
The historical context of entropy
The concept of entropy can be traced back to 1824, when the physicist Carnot realized the importance of avoiding irreversible processes. In 1865, Rudolf Clausius expanded on his 1854 preliminary research on "uncompensated transitions" (unkompensierte Verwandlungen) and provided a preliminary formula for the generation of entropy. According to his research, the amount of entropy generated can be expressed by the following formula:
N = S - S0 - ∫ dQ/T
Where S is the entropy of the final state, and S0 is the entropy of the initial state. According to Clausius, if the process is reversible, then N = 0; if it is irreversible, then N > 0.
The first and second laws of thermodynamics
The laws of thermodynamics are primarily used to describe the behavior of well-defined systems, such as the transfer of heat and mass within and outside boundaries in a nonadiabatic open system. The generation of entropy (usually represented by the symbol Si) is one of the core elements of the second law. It tells us that in any natural process, the rate of change of entropy must be positive or zero. This is an important law of nature.
The generation of entropy is an inevitable phenomenon in every process in nature, and its rate is always positive or zero.
Examples of irreversible processes
The generation of entropy mainly occurs in irreversible processes. Some important irreversible processes include heat flow through thermal resistance, fluid through flow resistance, Joule heating effect, friction between solid surfaces, and fluid viscosity within the system. These processes will generate a certain amount of entropy, which reduces the conversion efficiency of thermal energy.
Efficiency of heat engines and refrigerators
Most heat engines and refrigerators are usually closed loop systems. In the steady state, the internal energy and entropy of the motor will return to the starting values after completing a cycle, so the first and second laws of thermodynamics can be simplified. It is against this background that we can gain an in-depth understanding of the working principles of heat engines and refrigerators.
For a heat engine, the basic form of its working principle is QH - Qa - P = 0, and the entropy relationship is QH/TH - Qa/Ta + Si = 0.
The essence of these equations is to reveal how a heat engine uses heat energy to produce power, however, the maximum efficiency achieved under ideal conditions is only when entropy is zero.
Conclusion: Reflection on the limits of thermal energy utilization
With the deepening understanding of the principles of thermodynamics, the concept of entropy has undoubtedly become the key to understanding the thermal energy conversion process, whether for engineering applications or scientific research. However, can we overcome the limitations of entropy generation and allow thermal energy to be continuously and efficiently converted?
