Modern physics tells us that the nature of energy is related to the motion of related objects or particles and their relative positions. The change of energy between particles can be manifested in various forms such as the form of mechanical function, the flow of electric current, the transfer of heat energy, the change of the internal state of the object, and the distribution of electromagnetic oscillation. Energy can neither be created nor disappear, energy can only be transformed from one form to another. This is the so-called first law of thermodynamics, the law of conservation of energy.
The second law of thermodynamics tells us that the transfer of energy can only be spontaneously produced in a constant direction, and continues until a new balance is achieved: that is, hotter objects automatically transfer heat to colder objects, Until the two are isothermal. After any kind of initial energy is used (transferred), it will be converted from one form to another-another form. Moreover, during the transfer process, only part of the energy can be converted into work that can be used effectively. The extent to which energy may be effectively used in the conversion process, or the effect; generally we use efficiency to evaluate. The so-called efficiency (η) refers to the ratio of the effective energy effect (L) to the total energy input (Q), that is, η=L/Q.
The second law of thermodynamics also proves that any thermal energy conversion process is accompanied by losses. Therefore, the energy conversion efficiency cannot reach 100% at any time; that is to say, only part of the heat in the energy conversion (or transfer) process can be effectively used, which has the characteristics of unequal value.
Heat is a special form of energy transfer, and any form of energy can be converted into heat by itself. But on the contrary, heat cannot be converted into other forms of energy by itself after the transfer process is over without any changes in the shape of the surrounding objects. That is, there is irreversibility in the process of “heat-energy” conversion.
In the long scientific explorations of mankind, people have also found that the process of using energy is often accompanied by the generation of heat. And between two objects with different temperatures, it will inevitably be transferred spontaneously, naturally, and not according to people’s will, and heat will be transferred from the high temperature area to the low temperature area. The transfer of heat can be direct, indirect, or even across spatial distances. Scientists have summarized and concluded that in the universe as we know it, there are only three ways of heat transfer that can be followed: conduction, convection and radiation.
The so-called conduction refers to a substance (in) between two different temperature fields, and its heat will be spontaneously transferred from the high (temperature) end to the low (temperature) end through molecular vibration or through free electron activity. The physical phenomenon of thermal energy. For example: the process of connecting a solid metal rod between the hot end and the cold end that can directly transfer the heat from the hot end and heat the cold end is called “conduction” of heat. In theory, all substances can conduct heat, but the thermal conductivity of various substances is different. The ability of a substance to conduct heat is related to its molecular structure, material density, dry humidity, pressure state, and temperature of the object itself. Natural physical factors are related. According to the thermal conductivity of each substance under normal conditions, we can classify it into several categories such as thermal conductive materials, thermal insulation materials and thermal insulation materials.
Convection refers specifically to the uneven heating of the fluid (water, oil, gas), resulting in different formation densities (specific gravity), resulting in changes in the relative positions of heat-carrying molecules in the fluid, and the free movement of fluid molecules or between fluids. The physical process of directional macro-circulation flow, and then (realize) the heat exchange. For example: the water in the steamer sitting on the stove, the heated water close to the bottom of the steamer will rise due to the increase in temperature and the decrease of its specific gravity; at the same time, the cold water in the upper part of the steamer will drop due to its relatively high specific gravity. Shen, under the continuous heating of the stove, the water in the steamer undergoes repeated natural convection heat exchange, and finally reaches boiling and vaporization. Among them, the convection phenomenon caused by the change in the density (specific gravity) of the fluid after the fluid is heated is called “natural circulation”; relatively speaking, the mechanical dynamic pressure is applied to the fluid to force the fluid to produce a macroscopic power cycle flow and heat exchange. Phenomenon, we call it “forced circulation”. In addition, the process of heat transfer between solids and fluids in direct contact is collectively referred to as “heat exchange”; among them, the convective heat transfer process in which the fluid transfers heat to the solid is called “condensation”; the solid transfers heat energy The phenomenon of heat release to the fluid is called “heat dissipation”. Compared with the fluid, in the process of heat collection or the process of absorbing the solid device to transfer heat, the physical process of the fluid’s volume expansion and the decrease of the specific gravity is called It is “evaporation”; and along with the heat released to the solid container, the physical phenomenon that the volume of the fluid shrinks and the specific gravity increases, we call it “condensation”.
If conduction is the process of transferring heat energy in solid matter, then radiation refers to the physical phenomenon in which heat energy propagates in a vacuum of non-material. To be precise, the process by which an object emits energy in the form of electric avoidance can be called “radiation”. Among them, the radiation sent out due to its own thermal energy is called “thermal radiation”. The most typical view of thermal energy transfer across time and space in a non-contact state is the sun’s thermal energy lead. The sun, 150 million kilometers away, can radiate the huge energy generated inside the sun to the entire universe through electromagnetic waves. In its orbit around the sun, the earth can obtain 1/2.2 billionth of the sun’s total radiant energy per second, which is about 1.73x1014W (equivalent to 5 million tons of standard coal) solar radiant energy. Some people estimate that its radiation amount is more than 20,000 times the total amount of available energy developed and utilized by humans in various ways in the world. In other words, as long as we have a way to effectively collect one ten-thousandth of the energy radiated by the sun to the surface of the earth, we can meet the energy needs of the global human society for normal production and life.
Finally, let us use a few more memorable visual language to express the core content of the heat energy (conversion) transfer mechanism: as long as there is a temperature difference, positive energy heat transfer will inevitably occur.
Efficiency: There is a loss in every link in the transfer of heat energy.
Heat conduction: Only by intimate contact can you perceive the inner warmth.
Convective heat transfer: The heat wave pushes the sprouting of the undercurrent.
Radiation heat transfer: across time and space, embrace cutting with warmth.
Use the theory of heat transfer to explain the above three heat transfer methods. Theoretically, different heat transfer methods have essential differences in the heat conversion mechanism. However, the various forms of heat transfer that occur in nature and actual production and life, and the three heat transfer methods with different heat transfer mechanisms seldom exist alone. They often take one form of heat transfer as the main body, accompanied by The other, or the simultaneous occurrence of two heat transfer methods. For example: when a flat-plate solar collector collects solar radiant energy, the heat of the plate core is mainly from solar radiant energy; but there is radiation heat exchange between the glass cover and the plate core of the hot box type heat collector. At the same time, convection heat exchange also exists; conduction heat exchange between the heat collection tube and the fins, and at the same time through the heat collection tube heat dissipation method, the solar radiation heat collected by the plate core is transferred to the water in the heat collection tube ; Then the heat-carrying medium-water, oil or air, transfers the heat to the heat preservation hot water storage tank through temperature difference convection circulation (or forced circulation). The frame and the bottom plate not only participate in the convective heat exchange in the thermal box of the collector, but also conduct heat exchange between each other, and cause the convective heat loss and radiant heat loss of the entire system to the outside. According to the theory of heat transfer, for flat-plate collectors, in order to improve the light-to-heat conversion efficiency of the collector, it is necessary to greatly improve the thermal insulation performance of the entire flat plate, and the convective heat exchange between the glass cover and the plate core must be greatly improved. The loss is reduced as much as possible. The four corners and two-sided joints between the glass cover and the frame and the bottom plate and the frame are not tightly sealed, causing water leakage, air leakage, and heat insulation problems to be completely resolved, as well as the frame (around) and the bottom plate lining Thermal insulation materials, seals, thermal insulation, airtight thermal insulation, heat aging characteristics of its own materials, solar reflectance, transmittance, heat capacity and other physical properties of the glass cover have been comprehensively improved and perfected. Only in this way can it be possible to reduce the heat loss coefficient of the hot box flat plate collector as a whole, to obtain a higher system heat collection efficiency, or to obtain more heat output.
Heat transfer theory is widely used in all aspects of solar thermal utilization, from the selection of materials, the design and manufacture of equipment, the selection and application of heat transfer media and insulation materials, to the system engineering design and engineering assembly of the entire application facility. The links are inseparable from the scientific guidance of the basic theories and practical experience of heat transfer. Those who are interested in the development and utilization of solar energy should regard the above-mentioned basic theories as professional guides for new energy businesses, and strictly implement them in engineering practice, in order to ensure the quality of products and the reputation of manufacturers.
The nature and human society’s utilization of solar-thermal radiant energy can generally be realized through three forms or three types of technical means: light-heat conversion, light-electric conversion and light-chemical conversion. But no matter what kind of utilization, there is a common problem: the efficient collection of solar radiant energy and the high-density, long-term storage and utilization across time and space.
