Design of solar powered refrigeration system based on ATMEGA16

At present, most refrigeration equipment is driven by electricity. Traditional refrigeration equipment not only consumes a lot of electricity, but also pollutes the environment because of the use of refrigerants such as Freon. Therefore, energy saving and environmental protection in refrigeration have become the focus of attention, and people are looking for refrigeration methods that are powered by clean energy and do not use traditional refrigerants such as Freon. The refrigeration system studied in this paper uses solar photovoltaic cells to provide driving energy and semiconductor refrigeration chips as cold sources. It is a new type of refrigeration method that is energy-saving and environmentally friendly.

Semiconductor refrigeration chips are also called electronic refrigeration chips, which perform refrigeration based on the Peltier effect principle. Semiconductor refrigeration chips do not require refrigerants, have no pollution sources, have no vibration or noise when working, and have a long life. As a current transducer chip, it can achieve high-precision temperature control by controlling the input current. Semiconductor refrigeration has been widely used in aerospace, medical technology, bioengineering and other fields.

1 Refrigeration system design

1.1 Calculation of refrigeration power

The matching of parameters of each part of the system depends on the refrigeration capacity required by the system, so the calculation of refrigeration capacity is the premise of design. In this paper, the refrigeration environment is a closed cylindrical granary. Since the top layer of the granary is prone to heat accumulation when the outside temperature is high, in order to maintain the purpose of storing grain in a low temperature or quasi-low temperature environment, it is necessary to refrigerate the air layer above the grain stack line in the granary. According to the basic principles of heat transfer, the cooling load of the granary can be calculated.

The cooling capacity requirement of the air in the granary:
Q1=ρVC(T0-T1) (1)
There is heat transfer between the top air layer and the sides, roof and grain in the granary. After time τ, the cold air diffuses outward:
Q2=KS(T2-T3) (2)
The total cooling load of the granary:
Q=Q1+Q2 (3)
Wherein, ρ is the density of the air in the granary; V is the volume of air; C is the specific heat of air; T0 is the initial temperature of the air in the granary; T1 is the cooling target temperature; K is the equivalent heat transfer coefficient, in W/K; S is the effective heat transfer area; T2 and T3 are the temperatures inside and outside the granary that change with time, in K.

According to the thermoelectric cooling principle of semiconductor cooling film, the characteristic parameters of semiconductor cooling element can be calculated according to the measured temperature, voltage and current: Where, α is the Seebeck coefficient of the cooling element, unit is V/K; I is the working current of the semiconductor cooling film, unit is A; Th and Tc are the temperature of the hot end and cold end of the cooling film, unit is K; R is the resistance of the cooling film, unit is Ω; Kt is the total thermal conductivity of the cooling film, unit is W/K. Through formulas (1), (2) and (3), the cooling load of the system, that is, the cooling capacity required by the system, can be estimated. Combined with formulas (4), (5), (6) and (7), the cooling capacity and input power of the system can be optimized and analyzed to determine the power of the power supply so that the cooling efficiency of the system can be maximized, thereby achieving a reasonable match of the key device parameters of the system (power of photovoltaic cells, capacity of batteries, cooling power of cooling film and input power). 1.2 Overall structure of the system In this design, solar semiconductor cooling is to generate electricity to drive semiconductor cooling film through the photoelectric conversion of photovoltaic panels. The advantage of this method is that it is relatively easy to control and has low cost. The intensity of solar energy is affected by many factors and cannot be maintained constant. In order to achieve stable and efficient operation between power supply and load and improve the power supply quality, it is necessary to design a reliable and efficient solar power controller. The solar semiconductor refrigeration system consists of solar cell group, battery, controller, semiconductor refrigeration sheet, sensor, drive circuit, sampling circuit and display circuit. Its structure is shown in Figure 1. 2 System Hardware Design The hardware circuit of the controller is mainly composed of microprocessor and its peripheral refrigeration drive circuit, temperature detection and current sampling circuit. 2.1 ATMEGA16 microprocessor The AVR series ATMEGA16 microprocessor is selected as the core control processing unit. The ATMEGA16 microcontroller is a high-performance and low-power 8-bit processor in the AVR series. It has rich internal resources. It integrates 8-way 10-bit analog-to-digital converter (ADC) with optional programmable gain and its unique pulse width modulation output PWM function. ATMEGA16 has the advantages of high reliability, good real-time performance, strong anti-interference ability and low cost. 2.2 Semiconductor Refrigeration Drive Circuit The model of the selected refrigeration chip is TEC1-12706, with a maximum operating current of 6 A and an operating voltage of 12 V. The semiconductor refrigeration chip needs to use DC current to realize operation, which can be used for both cooling and heating. By changing the polarity of the DC current, it is determined whether to realize cooling or heating on the same refrigeration chip. The temperature range is from positive temperature 80℃ to negative temperature 55℃. When the semiconductor refrigeration chip is working, the PWM function of the ATMEGA16 chip is used to control it. The on and off of the Darlington tube BD243 is controlled by the optical coupler to achieve the control of the input voltage of the refrigeration chip, and then control the working temperature of its cold end. The working drive circuit of the semiconductor refrigeration chip is shown in Figure 2, where RL is the semiconductor refrigeration chip. In the actual refrigeration process, in order to ensure the refrigeration efficiency, the working current of the refrigeration chip is required to be in the order of amperes. The BD243 in the circuit can provide a maximum collector current of 6 A, which meets the working requirements of the refrigeration chip. The 4700μF capacitor smoothes the input voltage of the refrigeration chip so that the ripple factor is less than 10% to ensure the refrigeration condition. [page] The heat dissipation of the hot end of semiconductor refrigeration can reduce the heat transfer from the hot end to the cold end by lowering the temperature of the hot end. Therefore, the heat dissipation of the hot end is very important. Reducing the temperature difference between the hot and cold ends has become an important factor in improving the performance of thermoelectric refrigeration. In this design, the heat sink plus air forced convection heat dissipation is used to dissipate the heat of the semiconductor refrigeration. After repeated experiments, it has been proved that this method has a good heat dissipation effect. 2.3 Current sampling circuit In order to prevent the load current from being too high, it is necessary to detect the current passing through the load. The current signal is sampled by the constantan wire resistor. The voltage signal sampled by the constantan wire resistor is amplified by LM258 and input to the analog-to-digital converter port of ATMEGA16 for A/D conversion. As shown in Figure 3. 2.4 Temperature detection circuit The temperature sensor uses DS18B20. Compared with traditional thermistors, DS18B20 can directly read the measured temperature and can realize 9-12-bit digital value reading mode through simple programming according to actual requirements. 9-bit and 12-bit digital quantities can be completed in 93.75ms and 750ms respectively, and the information read from or written into DS18B20 only requires one interface line (single bus interface) to read and write, and the single bus itself can also supply power to the connected DS18B20. Its temperature detection circuit is shown in Figure 4. 3 System software design The system software design process is shown in Figure 5. After the system is initialized, the internal A/D converter is started to sample the battery voltage VBAT. If the battery voltage is less than the normal voltage VC, the charging program is entered; if the battery voltage is normal, the temperature Ta of the cooling target is sampled. If the temperature is higher than the preset temperature Th, the cooling program is started. After the system enters the charging program, the light intensity of the solar cell is detected and judged. If the light intensity is low, the system goes into sleep; if the light intensity is higher than a certain value, the battery is charged in segments and the battery voltage state is judged. When the battery voltage rises to the normal voltage, the charging is completed. After starting the refrigeration program, the deviation between the current temperature Ta and Th and the deviation change rate signal are determined, and the PWM pulse width signal of the refrigeration drive circuit is adjusted through PID control to control the semiconductor refrigeration. When the temperature is not higher than the preset temperature Th, the refrigeration ends and returns.

4 Conclusion

The experiment shows that the refrigeration system has a simple structure, stable performance and good refrigeration effect. It can be widely used in industrial storage and daily refrigeration and preservation. The system adopts single-chip microcomputer control technology to realize semiconductor refrigeration based on solar power supply, showing certain advantages in low energy consumption and environmental protection.