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Inverse Thermodynamics: Why can't heat flow automatically to high-temperature areas?
In the basic principles of thermodynamics, we often mention an eternal concept: heat cannot automatically flow from a low-temperature area to a high-temperature area. The fundamental reason for this phenomenon comes from the second law of thermodynamics, which we often call the irreversible process of thermodynamics. In simple terms, this law states that in order for heat to move to a higher temperature, external energy or work must be provided.
The natural flow of heat is from high temperature areas to low temperature areas, and this phenomenon is ubiquitous in nature.
In our daily lives, from the operation of refrigerators to the use of air conditioners, they all rely on the movement of heat. But when we imagine heating a cold room in winter, how does it work? In these systems, we must rely on some mechanical device, such as a heat pump or cooling system, to forcefully move heat from a low-temperature area to a high-temperature area.
Basic concepts of thermodynamic cycles
The operating principles in heat pumps and cooling systems are closely related to the thermodynamic cycle. According to the theoretical model of thermodynamics, these systems can be described as thermodynamic cycles, including vapor compression cycle, vapor absorption cycle, and gas cycle.
Vapor compression cycle
Vapor compression cycles are the most common form of cooling and heating applications today. During this process, the refrigerant enters the compressor as low-pressure, low-temperature vapor. After being compressed, it becomes a gas with high pressure and high temperature, and then enters the condenser to release heat and transform into a liquid state. Next, the low-pressure liquid passes through the expansion valve to reduce its pressure, and then enters the evaporator to absorb heat, ultimately forming an operating cycle.
In an ideal vapor compression cycle, the refrigerant absorbs heat from the evaporator and releases heat from the condenser to achieve heating or cooling.
Vapor absorption cycle
Another form of cycle is the vapor absorption cycle. Although its performance is generally not as good as the vapor compression cycle, it can still play a role in certain needs, especially when heat sources are more readily available than electricity, such as industrial waste heat or solar energy. etc. situation. This cycle uses heat energy to vaporize and release the refrigerant by mixing refrigerant and absorbent.
Gas circulation
Compared to these types of cycles, the gas cycle revolves around gas without phase change. This process is often used in certain applications, such as compressed air systems commonly found on aircraft, because these systems can directly use the compressed air generated by the engine for cooling and ventilation.
The concept of reverse Carnot cycle
The reverse Carnot cycle is an ideal theoretical model that can be used to describe equipment operating as a cooler or heat pump. This cycle includes four processes: the refrigerant from a low-temperature source absorbs heat, is then compressed without transferring heat to the outside world, then releases heat at a high temperature, and finally reduces the pressure back to its original state to start the cycle again.
The movement of heat must rely on external work so that heat can flow from a low-temperature area to a high-temperature area. This process shows the characteristics and limitations of thermodynamics.
Performance indicators and conclusions
Rather than cooling or heating mechanisms alone, the efficiency of chiller and heat pump systems can be referenced in terms of a performance index (COP), which reflects the energy efficiency of the system. In many cases, these systems can operate at high efficiency, but under extreme conditions, performance can be compromised.
Perhaps, when we rely on these technological products to enjoy a convenient and comfortable life, people can't help but reflect: How can we use these thermodynamic principles more effectively to reduce energy waste and promote sustainable development?
