In the world where energy shortages and environmental pollution are increasingly serious, the full development and use of solar energy is one of the energy strategies that governments around the world are actively implementing. The application of solar LED lighting system meets the development trend of this strategic decision. However, the development of LED lighting systems has been largely affected by heat dissipation issues.
For LED lighting systems, LED can only convert a small part of the electrical energy into light energy during the work process, and most of the energy is converted into heat energy. With the increase of LED power, the heat generation increases. If the heat dissipation problem is not solved well, the heat is concentrated in a chip with a small size, which makes the internal temperature of the chip higher and higher. When the temperature rises, it will cause the following effects [1]: (1) The working voltage decreases; the light intensity decreases; the wavelength of light becomes longer. (2) Reducing the efficiency of the LED driver, damaging the lifetime of the magnetic element and output capacitor, and reducing the reliability of the LED driver. (3) Reduce the life of the LED and accelerate the LED light decay. The problem of heat dissipation in LED lighting systems has become a major obstacle to the development of this technology. Currently, the main methods used to solve the problem of heat dissipation in LED lighting systems include: adjusting the spacing of LEDs; reasonably increasing the distance between the LED and the metal core printed board; punching the hole; installing the fan. These methods are affected by many objective conditions in practical applications, and the heat dissipation effect is not ideal.
Semiconductor refrigeration, also known as thermoelectric cooling [2], is the use of Peltier effect of semiconductor materials. When the direct current passes through two different semiconductor materials connected in series, the two ends of the couple can absorb heat and release heat respectively, and the cooling can be achieved. It is a refrigeration technology that produces negative thermal resistance. Its characteristic is that it has no moving parts and its reliability is relatively high. The use of semiconductor cooling to solve the problem of heat dissipation in LED lighting systems has a high practical value.
1. The working principle of semiconductor refrigeration In 1934 the French Peltier found that when the current flows through the junction formed by two different conductors, there will be exothermic and endothermic phenomena, and the exothermic or endothermic heat is determined by the size of the current.
Q=aTI
In the above formula: Q is the heat release or endothermic power; a is the thermoelectric emf ratio; T is the cold junction temperature; I is the operating current.
Based on the Peltier effect principle, Peltier effect cooling is also called temperature difference cooling. The main principle of semiconductor refrigeration technology is based on the Peltier effect. Semiconductor refrigeration is based on the characteristics of thermoelectric effect technology, using a special semiconductor material thermopile to refrigerate, can directly convert electrical energy into heat, with high efficiency. At present, most of the semiconductor materials used in refrigerators are cerium-tellurium phosphide [3], and impurities are treated with special treatment to form N-type or P-type semiconductor temperature difference components. Its working characteristics are one side of cooling and one side of heat.
According to quantum theory, metal and semiconductor materials have different energy levels, different contact potential differences, and different load bodies. As shown in Fig. 1, the P-type and N-type semiconductors are connected by a metal plate, and the other end is constituted by a metal plate to form a circuit in the figure. When the key k is closed, the current in the figure passes through the PN junction. The Peltier cooling effect is formed at the upper end of the connection between the semiconductor and the metal plate, and the Peltier heating effect is formed at the lower end [4].
2. The design of semiconductor refrigeration system 2.1 The semiconductor refrigeration system is composed of semiconductor refrigeration system, the cooling plate adopts TEC1-12703 type thermoelectric cooling refrigeration assembly, according to the characteristics of the lighting system, select the organic glass with visibility, toughness, high temperature resistance and other characteristics As a refrigerator wall. In order to better solve the heat dissipation problem of solar LED lighting systems, the controller is used to effectively control the semiconductor refrigeration system.
2.2 The composition and control principle of the semiconductor refrigeration controller According to semiconductor refrigeration theory, applying a DC voltage across a TEC (semiconductor refrigeration system) will generate a DC current, which will cause the TEC end to heat the other end of the cooling. We say that the end of the heat is the "hot end" and the end of the cooling is the "cold end." The polarity of the voltage across the TEC is reversed, the current will flow in reverse, and the "hot end" and the "cold end" will also be interchanged. TEC is a cold and heat source in semiconductor refrigeration applications. Its operation is reversible and can be used for cooling as well as for heating. In order to solve the actual situation of the solar LED lighting system heat dissipation problem, we choose high-integration high-performance single-chip ADUC824 as the control core, through the software programming to complete the control of the semiconductor refrigerator. ADUC824 is an 8051 core high-performance MCU introduced by AD company. It integrates two (21-bit + 16-bit) A/D, 12-bit D/A, FLASH, WDT, μP monitor, temperature sensor, SPI, and I2C bus interface. With rich resources integrated into one, the ADUC824 is small in size, low in power and capable of on-line programming and debugging, eliminating the need to develop devices. The use of the ADUC824 as the core of the semiconductor refrigeration controller improves design reliability while greatly simplifying circuit design. The semiconductor cooling power drive adopts an H-type (full-bridge) circuit, which can perform bidirectional current driving of the load under the single-supply power supply and complete the TEC cooling operation, thereby realizing the target control. Based on ADUC824 semiconductor cooling control block diagram shown in Figure 2 [1].
2.3 Design Model of Semiconductor Refrigeration System From the analysis of the previous part of the article, we can see that using direct current through a PN junction allows heat to be transferred from a high temperature object to a low temperature object, and changing the current flow can be easily converted to cooling and heating. The use of semiconductor refrigeration does not take into account the environmental pollution caused by refrigerant leakage, and the entire system has no welding lines. Figure 3 shows the model structure of a semiconductor refrigeration system. It consists of many N-type and P-type semiconducting particles arranged in line with each other, and the NPs are connected by a common conductor to form a complete line, usually a copper, aluminum or other metal conductor, and finally sandwiched between two ceramic plates. . After the DC power is turned on, the electrons start from the negative pole (-) and first pass through the P-type semiconductor, where they absorb heat, reach the N-type semiconductor, and release heat. Every time an NP module passes, heat is sent from one side to the other. On the other side, it causes a temperature difference, thereby forming hot and cold ends.
3. Analysis of Characteristics of Semiconductor Refrigeration System 3.1 Advantages of Semiconductor Refrigeration System:
(1) Small size, light weight, suitable for special cooling environments with small capacity and small size.
(2) No refrigerant is used, so there is no leakage and no pollution to the environment.
(3) No moving parts, so no noise, no wear, long life and high reliability.
(4) The parameters of the semiconductor refrigeration system are not affected by the spatial direction, ie are not affected by the gravitational field, and are widely used in the aerospace field.
(5) Fast acting speed, reliable operation, long service life, easy control, convenient adjustment, and the cooling capacity can be adjusted by adjusting the operating current. It is also possible to change the cooling or heating operating state by switching the direction of the current.
Based on the above characteristics, it can be applied to solve the heat radiation problem of the solar LED lighting system.
3.2 Operating characteristics of semiconductor refrigeration system The semiconductor refrigeration system consists of a thermopile, a cold-side heat exchanger, a hot-side heat exchanger, and a controller. The thermopile is a refrigeration device. Since a thermopile is made up of pairs of galvanic couples, each pair of galvanic couples is connected in series to the current, and each pair of galvanic couples is connected in parallel to the heat flow. Therefore, when analyzing the performance of a thermopile, it is sufficient to analyze the cooling performance of the galvanic couple. The cooling capacity, voltage, output power, and cooling coefficient of a couple are [2]:
4. The cooling effect of semiconductor refrigeration systems has caused an upsurge of semiconductor refrigeration as early as the 1950s. However, due to poor component performance at that time (ie, the coefficient of performance was too low), it had not been put into practical use [5]. Semiconductor refrigeration materials and processes are the key to determining the rise and fall of this technology, mainly to improve the coefficient of merit of semiconductor materials.
The coefficient of merit Z is a technical index [6] that is used to measure the refrigerating performance of semi-conducting materials. It determines the maximum temperature difference that a refrigeration element can achieve. The higher the coefficient of merit, the better the refrigeration performance and the higher the efficiency. The coefficient of merit is mainly determined by the parameters of the temperature difference electromotive force α of the semiconductor material, the total thermal conductivity k of the semiconductor material, and the resistivity r, and the formula is:
With the increase of carrier concentration, the thermoelectric emf α decreases, and the resistivity r decreases. The total thermal conductivity k and carrier concentration make Z reach the maximum. When the carrier concentration is close to 1019cm-3, the semiconductor material has the highest figure of merit.
The coefficient of merit Z of a semiconductor material is a function that changes with temperature. Therefore, when selecting a semiconductor material, it is required not only that the coefficient of merit is as large as possible, but also that the coefficient of merit changes little in the use temperature range, and can always be Maintain a high value and meet the requirements of mechanical strength, thermal shock resistance, weldability, and material source and cost. Use cost-effective semiconductor materials as much as possible to improve cooling capacity.
5. Simulation experiment equipment mainly used: semiconductor refrigeration system, solar LED lighting system, controller, insulation board, temperature sensor, temperature acquisition equipment, computer, thermal silica gel and so on.
Experimental procedure and method: The cold end of the semiconductor refrigeration system is installed in the solar LED lighting system, and the hot end is placed outside the lighting system so that it can be in direct contact with the external environment. Then place a temperature sensor inside the lighting system. The controller and temperature sampling instrument can obtain the temperature inside the lighting system in real time through the temperature sensor. Finally, the lighting system installed with the semiconductor cooling system and the temperature sensor is sealed so that it will not be affected by the outside temperature. Figure 4 shows the system diagram of this simulation experiment.
The lighting system was allowed to operate for 30 minutes. The internal temperature was measured to be 69.3°C. The semiconductor refrigeration system was started to work. After 15 minutes of cooling, the temperature inside the lighting system was found to have dropped to 39°C. Experiments show that the semiconductor refrigeration system can well solve the heat dissipation problem of solar LED lighting systems.
6. Conclusion In the past few decades, great progress has been made in the research of semi-conductor refrigeration materials and their devices. The commercialization of this technology has been the subject of common discussions in the world. In order to produce a good-quality semiconductor refrigeration module, the cooling material must have a high coefficient of merit (Z). The world's higher Z-value semiconductor refrigeration material is Bi2Te3 alloy. Recently, in the field of semiconductor refrigeration, there has been an upsurge of research on two new types of semiconductor refrigeration materials and devices in the world, and certain progress has been made to commercialize this technology.
The author of this article innovates: At present, the development of solar LED lighting systems is largely affected by heat dissipation issues. Applying semiconductor cooling technology to solve this problem is an original new idea. After theoretical demonstration and many experiments, the application of this technology will become more and more mature.
For LED lighting systems, LED can only convert a small part of the electrical energy into light energy during the work process, and most of the energy is converted into heat energy. With the increase of LED power, the heat generation increases. If the heat dissipation problem is not solved well, the heat is concentrated in a chip with a small size, which makes the internal temperature of the chip higher and higher. When the temperature rises, it will cause the following effects [1]: (1) The working voltage decreases; the light intensity decreases; the wavelength of light becomes longer. (2) Reducing the efficiency of the LED driver, damaging the lifetime of the magnetic element and output capacitor, and reducing the reliability of the LED driver. (3) Reduce the life of the LED and accelerate the LED light decay. The problem of heat dissipation in LED lighting systems has become a major obstacle to the development of this technology. Currently, the main methods used to solve the problem of heat dissipation in LED lighting systems include: adjusting the spacing of LEDs; reasonably increasing the distance between the LED and the metal core printed board; punching the hole; installing the fan. These methods are affected by many objective conditions in practical applications, and the heat dissipation effect is not ideal.
Semiconductor refrigeration, also known as thermoelectric cooling [2], is the use of Peltier effect of semiconductor materials. When the direct current passes through two different semiconductor materials connected in series, the two ends of the couple can absorb heat and release heat respectively, and the cooling can be achieved. It is a refrigeration technology that produces negative thermal resistance. Its characteristic is that it has no moving parts and its reliability is relatively high. The use of semiconductor cooling to solve the problem of heat dissipation in LED lighting systems has a high practical value.
1. The working principle of semiconductor refrigeration In 1934 the French Peltier found that when the current flows through the junction formed by two different conductors, there will be exothermic and endothermic phenomena, and the exothermic or endothermic heat is determined by the size of the current.
Q=aTI
In the above formula: Q is the heat release or endothermic power; a is the thermoelectric emf ratio; T is the cold junction temperature; I is the operating current.
Based on the Peltier effect principle, Peltier effect cooling is also called temperature difference cooling. The main principle of semiconductor refrigeration technology is based on the Peltier effect. Semiconductor refrigeration is based on the characteristics of thermoelectric effect technology, using a special semiconductor material thermopile to refrigerate, can directly convert electrical energy into heat, with high efficiency. At present, most of the semiconductor materials used in refrigerators are cerium-tellurium phosphide [3], and impurities are treated with special treatment to form N-type or P-type semiconductor temperature difference components. Its working characteristics are one side of cooling and one side of heat.
According to quantum theory, metal and semiconductor materials have different energy levels, different contact potential differences, and different load bodies. As shown in Fig. 1, the P-type and N-type semiconductors are connected by a metal plate, and the other end is constituted by a metal plate to form a circuit in the figure. When the key k is closed, the current in the figure passes through the PN junction. The Peltier cooling effect is formed at the upper end of the connection between the semiconductor and the metal plate, and the Peltier heating effect is formed at the lower end [4].
2. The design of semiconductor refrigeration system 2.1 The semiconductor refrigeration system is composed of semiconductor refrigeration system, the cooling plate adopts TEC1-12703 type thermoelectric cooling refrigeration assembly, according to the characteristics of the lighting system, select the organic glass with visibility, toughness, high temperature resistance and other characteristics As a refrigerator wall. In order to better solve the heat dissipation problem of solar LED lighting systems, the controller is used to effectively control the semiconductor refrigeration system.
2.2 The composition and control principle of the semiconductor refrigeration controller According to semiconductor refrigeration theory, applying a DC voltage across a TEC (semiconductor refrigeration system) will generate a DC current, which will cause the TEC end to heat the other end of the cooling. We say that the end of the heat is the "hot end" and the end of the cooling is the "cold end." The polarity of the voltage across the TEC is reversed, the current will flow in reverse, and the "hot end" and the "cold end" will also be interchanged. TEC is a cold and heat source in semiconductor refrigeration applications. Its operation is reversible and can be used for cooling as well as for heating. In order to solve the actual situation of the solar LED lighting system heat dissipation problem, we choose high-integration high-performance single-chip ADUC824 as the control core, through the software programming to complete the control of the semiconductor refrigerator. ADUC824 is an 8051 core high-performance MCU introduced by AD company. It integrates two (21-bit + 16-bit) A/D, 12-bit D/A, FLASH, WDT, μP monitor, temperature sensor, SPI, and I2C bus interface. With rich resources integrated into one, the ADUC824 is small in size, low in power and capable of on-line programming and debugging, eliminating the need to develop devices. The use of the ADUC824 as the core of the semiconductor refrigeration controller improves design reliability while greatly simplifying circuit design. The semiconductor cooling power drive adopts an H-type (full-bridge) circuit, which can perform bidirectional current driving of the load under the single-supply power supply and complete the TEC cooling operation, thereby realizing the target control. Based on ADUC824 semiconductor cooling control block diagram shown in Figure 2 [1].
2.3 Design Model of Semiconductor Refrigeration System From the analysis of the previous part of the article, we can see that using direct current through a PN junction allows heat to be transferred from a high temperature object to a low temperature object, and changing the current flow can be easily converted to cooling and heating. The use of semiconductor refrigeration does not take into account the environmental pollution caused by refrigerant leakage, and the entire system has no welding lines. Figure 3 shows the model structure of a semiconductor refrigeration system. It consists of many N-type and P-type semiconducting particles arranged in line with each other, and the NPs are connected by a common conductor to form a complete line, usually a copper, aluminum or other metal conductor, and finally sandwiched between two ceramic plates. . After the DC power is turned on, the electrons start from the negative pole (-) and first pass through the P-type semiconductor, where they absorb heat, reach the N-type semiconductor, and release heat. Every time an NP module passes, heat is sent from one side to the other. On the other side, it causes a temperature difference, thereby forming hot and cold ends.
3. Analysis of Characteristics of Semiconductor Refrigeration System 3.1 Advantages of Semiconductor Refrigeration System:
(1) Small size, light weight, suitable for special cooling environments with small capacity and small size.
(2) No refrigerant is used, so there is no leakage and no pollution to the environment.
(3) No moving parts, so no noise, no wear, long life and high reliability.
(4) The parameters of the semiconductor refrigeration system are not affected by the spatial direction, ie are not affected by the gravitational field, and are widely used in the aerospace field.
(5) Fast acting speed, reliable operation, long service life, easy control, convenient adjustment, and the cooling capacity can be adjusted by adjusting the operating current. It is also possible to change the cooling or heating operating state by switching the direction of the current.
Based on the above characteristics, it can be applied to solve the heat radiation problem of the solar LED lighting system.
3.2 Operating characteristics of semiconductor refrigeration system The semiconductor refrigeration system consists of a thermopile, a cold-side heat exchanger, a hot-side heat exchanger, and a controller. The thermopile is a refrigeration device. Since a thermopile is made up of pairs of galvanic couples, each pair of galvanic couples is connected in series to the current, and each pair of galvanic couples is connected in parallel to the heat flow. Therefore, when analyzing the performance of a thermopile, it is sufficient to analyze the cooling performance of the galvanic couple. The cooling capacity, voltage, output power, and cooling coefficient of a couple are [2]:
4. The cooling effect of semiconductor refrigeration systems has caused an upsurge of semiconductor refrigeration as early as the 1950s. However, due to poor component performance at that time (ie, the coefficient of performance was too low), it had not been put into practical use [5]. Semiconductor refrigeration materials and processes are the key to determining the rise and fall of this technology, mainly to improve the coefficient of merit of semiconductor materials.
The coefficient of merit Z is a technical index [6] that is used to measure the refrigerating performance of semi-conducting materials. It determines the maximum temperature difference that a refrigeration element can achieve. The higher the coefficient of merit, the better the refrigeration performance and the higher the efficiency. The coefficient of merit is mainly determined by the parameters of the temperature difference electromotive force α of the semiconductor material, the total thermal conductivity k of the semiconductor material, and the resistivity r, and the formula is:
With the increase of carrier concentration, the thermoelectric emf α decreases, and the resistivity r decreases. The total thermal conductivity k and carrier concentration make Z reach the maximum. When the carrier concentration is close to 1019cm-3, the semiconductor material has the highest figure of merit.
The coefficient of merit Z of a semiconductor material is a function that changes with temperature. Therefore, when selecting a semiconductor material, it is required not only that the coefficient of merit is as large as possible, but also that the coefficient of merit changes little in the use temperature range, and can always be Maintain a high value and meet the requirements of mechanical strength, thermal shock resistance, weldability, and material source and cost. Use cost-effective semiconductor materials as much as possible to improve cooling capacity.
5. Simulation experiment equipment mainly used: semiconductor refrigeration system, solar LED lighting system, controller, insulation board, temperature sensor, temperature acquisition equipment, computer, thermal silica gel and so on.
Experimental procedure and method: The cold end of the semiconductor refrigeration system is installed in the solar LED lighting system, and the hot end is placed outside the lighting system so that it can be in direct contact with the external environment. Then place a temperature sensor inside the lighting system. The controller and temperature sampling instrument can obtain the temperature inside the lighting system in real time through the temperature sensor. Finally, the lighting system installed with the semiconductor cooling system and the temperature sensor is sealed so that it will not be affected by the outside temperature. Figure 4 shows the system diagram of this simulation experiment.
The lighting system was allowed to operate for 30 minutes. The internal temperature was measured to be 69.3°C. The semiconductor refrigeration system was started to work. After 15 minutes of cooling, the temperature inside the lighting system was found to have dropped to 39°C. Experiments show that the semiconductor refrigeration system can well solve the heat dissipation problem of solar LED lighting systems.
6. Conclusion In the past few decades, great progress has been made in the research of semi-conductor refrigeration materials and their devices. The commercialization of this technology has been the subject of common discussions in the world. In order to produce a good-quality semiconductor refrigeration module, the cooling material must have a high coefficient of merit (Z). The world's higher Z-value semiconductor refrigeration material is Bi2Te3 alloy. Recently, in the field of semiconductor refrigeration, there has been an upsurge of research on two new types of semiconductor refrigeration materials and devices in the world, and certain progress has been made to commercialize this technology.
The author of this article innovates: At present, the development of solar LED lighting systems is largely affected by heat dissipation issues. Applying semiconductor cooling technology to solve this problem is an original new idea. After theoretical demonstration and many experiments, the application of this technology will become more and more mature.
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