What Is Thermodynamic Analysis?

Thermodynamic analysis usually includes three aspects: (1) determining the law of working fluid state changes and corresponding state parameters in the process; (2) determining the quantitative relationship of energy conversion in the process; (3) revealing the irreversible degree in the process and reflecting Completeness of energy conversion and utilization.

Thermodynamics is a discipline that studies the nature of a material system in equilibrium and establishes a balanced relationship of energy in thermal phenomena, and interacts with the outside world when the state changes. Engineering thermodynamics is a macro theory of thermal phenomena. The research method is macro. It takes the first law of thermodynamics, the second law of thermodynamics, and the third law of thermodynamics as the basis of reasoning. Macro parameters such as specific volume and specific volume, and overall behaviors such as heating, cooling, expansion, and contraction are studied for macro phenomena and thermal processes. Three commonly used thermodynamic analysis methods are enthalpy analysis, entropy analysis, and exergy analysis. Through various thermodynamic analysis methods to analyze the energy conversion process and improve the installation and replacement equipment, it can use energy more reasonably, which is of great significance to the development of human society.

Thermodynamic analysis usually includes three aspects: (1) determining the law of working fluid state changes and corresponding state parameters in the process; (2) determining the quantitative relationship of energy conversion in the process; (3) revealing the irreversible degree in the process and reflecting Completeness of energy conversion and utilization.

The specific steps are: (1) delineate the system according to the specific situation; (2) determine the specific law of the state change in the process according to the process characteristics; (3) use the chart to illustrate the thermodynamic process; (4) according to the appropriate thermodynamic law , List the balance equations, and solve unknowns. ^[1]

Currently, the most used energy is chemical energy of fuel. Through combustion, the chemical energy of the fuel is converted into thermal energy, which is then converted into mechanical energy or electrical energy for people to use. The development and utilization of energy sources, on the one hand, provide the necessary energy for the development of human society, on the other hand, the large-scale exploitation and use of non-renewable fossil fuels eventually lead to the depletion of resources and serious environmental pollution problems throughout the world. Moreover, the efficiency of converting thermal energy into mechanical energy through thermal power devices is low, and even the most advanced large-scale steam power devices today have a thermal efficiency of just over 40%. Therefore, the development and utilization of new energy, renewable energy and improving the efficiency of energy conversion devices are currently effective methods to solve energy and environmental problems. Therefore, fuel cell power generation technology has gradually attracted widespread attention due to its high efficiency and environmental protection.

The first concept introduced was the reversible battery. Reversible batteries are a very important concept because only reversible batteries can perform rigorous thermodynamic analysis. Reversible batteries must meet two conditions:

(1) The electrode reaction must be reversible, that is, when the direction of the current changes, the electrode reaction proceeds in reverse. Take the copper-zinc battery as an example, the external electromotive force E is connected with the copper-zinc battery. If the copper-zinc battery has an electromotive force E> E, the copper-zinc battery is discharged to the outside. Charging. From the above discussion, we can see that the electrode reaction of copper-zinc batteries is reversible.

(2) The current through the battery should be infinitely small, that is, it works under infinitely close to equilibrium conditions. A battery meeting the above two conditions is a reversible battery, and the electrodes constituting the reversible battery are all reversible electrodes. Actually not all batteries are reversible. For example, a battery constructed by inserting metallic copper and zinc into an HCl solution is not a reversible battery.

Another concept is introduced, that is, Gibbs function, also known as Gibbs free energy. Gibbs free energy definition: G = H-TS. G is a physical quantity obtained by combining H, T, and S, and it has the dimension of energy. Since H, S, and T are all state functions, G must also be a state function. When the system changes, G also changes. The change value G, called the Gibbs free energy change of the system, depends only on the starting and ending states of the change, and has nothing to do with the path of change: G = Gending -G starting

According to the definition of Gibbs free energy, it can be inferred that when the system changes from state 1 to state 2, the system's Gibbs free energy becomes:

G = G2-G1 = H- (TS)

For the reaction under isothermal conditions, if T1 = T2 = T, then G = G2-G1 = HT S. (1)

Under isothermal and isostatic conditions, G is equal to the non-volume work of the reversible process. For fuel cells, non-volume work is electrical work, so for reversible batteries, G = W _r '.

The electrochemical reaction of the fuel cell is performed under isothermal conditions, which reduces the increase in entropy during the reaction. It is precisely because of this isothermal reaction, the exergy of the reactants is not consumed to increase the product temperature, but is instead It is more converted into electrical energy, and the irreversible loss in the reaction process is greatly reduced compared with the combustion process, thereby improving the thermodynamic efficiency. ^[2]

During the process of deformation and failure of rocks, materials and energy are constantly exchanged with the outside world, and the thermodynamic state of rocks is constantly changing accordingly. According to the theory of non-equilibrium thermodynamics, the characteristics of energy dissipation and energy release in the process of rock deformation and failure are explained theoretically. During the deformation and failure of rocks, the heat supply and the changes in the shape and position of the rock volume elements serve as energy sources for plastic hardening and micro-defect formation in the rock volume elements, leading to the continuous evolution of micro-defects dispersed in the rock, from disorderly distribution. It gradually develops in an orderly manner, forming macro-cracks, and eventually macro-cracks converge in a certain direction to form large cracks, leading to overall instability (catastrophic). From a mechanical point of view, it is actually a process from local dissipation to local destruction and finally to overall catastrophe. From the perspective of thermodynamics, the process of deformation (destruction, destruction, and catastrophe) of rock (rock mass) is an irreversible process of energy dissipation, including energy dissipation and energy release. The relationship between the two: the overall catastrophe of the rock mass is essentially the entire process of energy dissipation and energy release, while the catastrophe is based on energy release as the main driving force.

Energy conservation during rock deformation and destruction: During rock deformation and destruction, the energy provided by the environment includes work done by external forces and thermal energy brought by ambient temperature. According to the first law of thermodynamics, these energy will be converted into kinetic energy, potential energy and internal energy of the rock. May wish to abstract the rock as a continuous medium composed of a series of volume elements, each volume element contains a specific mineral or mineral-like composition and other rock organization. When the volume element is small enough, the local equilibrium assumption of non-equilibrium thermodynamics is satisfied.

Under the action of external load, the rock volume element will deform. When the time step examined is small, this deformation can be regarded as a small deformation. In general, the analysis of the deformation and failure process of rocks does not consider the influence of physical strength. The change of the internal energy of the rock volume element depends on 4 parts. One is the volume deformation caused by the hydrostatic stress, the other is the shape change caused by the stress bias, the third is the position rotation caused by the stress bias, and the fourth is the heat. Caused by flow. Except for heat flow, the other three parts are related to internal force, which can be collectively referred to as the increase of internal force energy, and the heat flow part can be referred to as the increase of thermal energy Q. Therefore, the internal energy increase u in the rock volume element per unit time is equal to the sum of the internal force energy increase and the thermal energy increase Q. It can be seen that during the deformation and failure of rock, the change of internal energy of rock mainly depends on the change of internal force and the flow of heat. Because the contribution of stress deviance is non-divergent, it shows that the internal energy of the volume element is not a conserved quantity. That is to say, due to the effect of the stress bias, the change in the internal energy of the volume element is not necessarily obtained only by the external environment, but may also be generated by the internal energy source of the volume element. Therefore, the energy conservation of rock during deformation and failure is a dynamic process, which is represented by the transformation and balance of external mechanical energy, thermal energy, and internal energy of rock. ^[2]

There are thermodynamic phenomena everywhere in life. The nature and principle of thermodynamic phenomena also come from life. In fact, we can often see many phenomena related to thermodynamics around us, but we often do not use the knowledge we have learned to connect and analyze it well. For example, the air conditioners, water heaters, pumps, pressure cookers, etc. used at home are examples that we can easily see around us.

The pressure cooker is not unfamiliar to everyone. There are still many households that use it for cooking. Let's find out some principles when it works. At the beginning, the pressure cooker mainly contains water and the corresponding food. The main change process is the change of water and water vapor. The water in the sealed pressure cooker is slowly heated to the saturated water under the corresponding temperature and pressure. This process is the preheating stage. The heat absorbed is called liquid heat. Continue heating in the state of wet steam. In this process, the corresponding temperature and pressure are unchanged, called saturation pressure and saturation temperature, until the water in the autoclave is heated to dry saturated steam. This process is called the gasification phase. The heat absorbed It is called latent heat of vaporization. Continue to heat the dry saturated steam to reach the superheated state, that is, get superheated steam. This is mainly the change of water and water vapor inside the autoclave. Then analyze the working condition of the pressure cooker gas valve from the internal pressure and external pressure of the passage. Each pressure cooker has a corresponding working pressure and operating value of the pressure relief valve. Generally, the operating pressure of the pressure cooker is about 80kpa, and the operating value of the pressure relief valve is about 120kpa. When it is in equilibrium with the pressure of the internal gas, the pressure valve will be intermittently pushed up. When the internal and external pressure difference changes, the internal high-temperature and high-pressure steam is continuously sent out. The specific volume of the steam inside will increase with time The food inside is mainly cooked and cooked by the action of high temperature and high pressure steam for a certain period of time.

Air conditioners are also cooling and heating heaters everywhere in our lives. It mainly uses the working medium to absorb heat and release heat during compression and expansion to achieve cooling and heating. Air-conditioning refrigeration principle: After the air conditioner is powered on, the low-pressure steam of the refrigerant in the refrigeration system is sucked into the compressor and compressed into high-pressure steam and then discharged to the condenser. At the same time, the outdoor air drawn by the axial fan flows through the condenser, taking away the heat released by the refrigerant, and condensing the high-pressure refrigerant vapor into a high-pressure liquid. The high-pressure liquid is sprayed into the evaporator after passing through the filter and the throttling mechanism, and is evaporated at the corresponding low pressure to absorb the surrounding heat. At the same time, the cross-flow fan keeps the air continuously entering the fins of the evaporator for heat exchange, and sends the cooled air to the room. In this way, the indoor air is continuously circulated to reduce the temperature. Working principle of heating: Heat pump heating uses the compression condenser of the refrigeration system to heat the indoor air. When the air conditioner works in refrigeration, the low-pressure refrigerant liquid evaporates and absorbs heat in the evaporator, and the high-temperature and high-pressure refrigerant releases heat and condenses in the condenser. Heat pump heating uses electromagnetic reversal to reverse the position of the suction and exhaust pipes of the refrigeration system. Originally, the indoor coil of the cooling working evaporator became a condenser during heating. In this way, the refrigeration system absorbs heat outdoors and releases heat to the indoor, thereby achieving the purpose of heating. ^[2]