The development of technology and the significant consumption of energy are eroding the environment that humans rely on for survival. In recent years, a series of environmental crises such as geological subsidence and severe haze weather have severely compressed the air, soil, and other living spaces closely related to life that humans rely on for survival. Limited energy is also facing danger with the development of industry. In order to better protect the living environment and make contributions to human continuous development by utilizing energy, humans have effectively utilized various energy resources stored in nature based on their own science and technology. Among them, solar energy, as an emerging renewable energy source, has been developed and utilized by humans. Solar photovoltaic power generation and solar photothermal power generation are the two main aspects of solar energy utilization. Solar photovoltaic power generation is currently the most widely used technology due to its modular components, convenient installation, simple maintenance, and flexible electricity usage. Solar thermal power generation adopts a concentrated heat collection system, which gathers sunlight and heats the medium to achieve traditional steam power generation equipment. Solar power generation for energy conservation and environmental protection has received strong support and widespread promotion from the government.
1.Principles and Design of Solar Power Generation
1.1 Principles of solar power generation
Solar power generation mainly utilizes the potential difference formed by P-N junctions under the irradiation of the sun to form electrical energy. There are two ways to generate electricity: one is the light heat electricity conversion method; Another method is the direct conversion of light to electricity. The light heat electricity conversion method generates electricity by utilizing the thermal energy generated by solar radiation. This power generation method receives sunlight through a collector, and the energy (thermal energy) in the sunlight is converted into working fluid steam through specialized equipment. The steam turbine can operate under huge steam pressure to generate electricity, but this power generation method is extremely expensive, about 5-10 times that of ordinary thermal power plants, and is currently rarely popularized, Especially, individuals cannot afford the high cost of power generation investment. The direct conversion method of light to electricity is a power generation mode that utilizes the core equipment – solar cell components to receive sunlight, and utilizes the photoelectric effect to directly convert light energy into electrical energy. Currently undergoing professional testing, solar cells can still operate normally after 25 years of power generation. Compared with other batteries, one investment can be used for a long time, and it will not cause environmental pollution compared to limited resource thermal power generation.
The principle of photovoltaic conversion in solar cells is that when sunlight shines on the surface of the solar cell, the silicon ions on the surface of the cell absorb a portion of the sunlight and then transmit it to the silicon atoms, continuously spreading around, causing holes to move and gather towards the P region through the PN junction. The electrons and holes gathered on both sides of the P-N junction form a potential difference. When the circuit is connected, a certain amount of current and electrical power will be formed, This process is the process of converting light energy into electrical energy.
1.2 Design of Off grid Solar Power Generation System
There are two main ways to use solar power generation in practical life. One is grid connected power generation, which mainly connects the generated electricity (mainly AC) to the national grid. When the power generation is large, the excess electricity can be transmitted to the national grid. If the power generation is not enough due to weather conditions, it can be purchased from the national grid; Another type of power generation is the off grid solar power generation system, which belongs to a type of power generation that is not connected to the national grid and achieves independent power generation, which is a type of solar power generation that is spontaneously self used.
The article mainly focuses on the design of an off grid solar power generation system. The components of this power generation system are mainly composed of solar panels, batteries, controllers, inverters, DC loads, and AC loads, forming an independent power generation system. The functional block diagram is shown in the figure.
1.2.1 Solar cell modules
The solar cell module is the core component that can generate electricity, and is also the most valuable part of the solar power generation system. Its function is to convert the light intensity radiated by the sun to electricity (which changes with the intensity of the sun) or send it to the battery for storage, in order to drive the DC load to work. Solar panels, through the photoelectric effect, receive sunlight when it strikes a semiconductor and convert it into the required direct current.
1.2.2 Solar controller
The direct current from solar cells is unstable because the sun’s irradiation intensity varies at each time period, resulting in different diffusion rates of electrons and holes, resulting in different direct currents. If this direct current is directly applied to daily life, the direct current load in daily life will be damaged. Therefore, it is necessary to use a controller to stabilize the direct current generated by solar cells, The controller plays a stabilizing role in the solar power generation system, while also charging the battery. During the charging process, it also plays a role in overcharging protection and discharging protection. Therefore, the solar controller is used to convert unstable DC electrical control into a stable DC power of a certain value, and transmit the stable DC power to the DC load in daily life. Due to the fact that off grid solar power generation systems belong to spontaneous self use power generation systems, they cannot generate electricity at night or on rainy days. Therefore, it is necessary to save the electricity that is usually generated by the sun. For special situations, batteries are required. The principle for selecting the size of batteries is as follows:
Among them, the actual capacity of P load, T discharge time, Vf temperature coefficient (the influence of temperature on the battery, taken as 1.1-1.2), Cf capacity compensation coefficient (taken as 1.1), Lf aging coefficient (taken as 1.0-1.1), UN voltage, and Ef maximum discharge depth.
For batteries, the larger the capacity, the better. The more electricity they store, but according to the actual situation, if the battery is too large, it will waste the investment cost of the power station economically. In terms of floor space, the larger the capacity, the larger the volume of the battery, and the larger the floor area, which is a waste of resources.
1.2.3 Solar inverters
In daily life, there are not only DC loads, but also AC loads. Therefore, a device that can convert DC electricity into AC electricity is needed. The function of a solar inverter is to use a full bridge circuit composed of MOS field-effect switching tubes, and then use various intermediate links such as SPWM controller, DC conversion circuit, feedback circuit, LC oscillation, output circuit, etc. to obtain the required AC electricity, According to the different requirements, it can be converted into different AC power (usually 220V 50Hz AC power required for daily life). This system is mainly designed for off grid solar power generation systems, therefore, when selecting inverters, off grid inverters should be selected.
2. Maintenance of power generation system
The ability of the power generation system to generate electricity normally can only be said to have achieved basic functions, and whether this power generation system can work for a long time is a more important part. Therefore, how to maintain the power generation system to work normally and achieve the optimal long-term power generation is another crucial link.
(1) In terms of appearance, the lighting surface of the component that receives sunlight and converts light energy into electrical energy must be clean and free of dust. Regular cleaning should be done with a brush (choose a soft bristle brush) and clean and gentle water. During the cleaning process, the force should not be too strong. If there are difficult to remove objects or debris, it is prohibited to use hard objects such as knives to scratch the lighting surface of the component, The method of slowly wiping or not damaging the daylighting surface should be chosen to remove stains. Regularly check whether the bracket is firm, the contact surface is good, the frame is broken, and there are obstructions. Under the condition of basically the same solar radiation intensity, regularly use a DC clamp type ammeter to measure the input current of each photovoltaic module string, and the deviation value should not exceed 5%.
(2) The indoor area where batteries are placed should be dry, ventilated, and well lit, but not directly exposed; No fireworks allowed; Maintenance personnel should wear protective equipment and use insulated equipment to prevent personal electric shock, short circuit, or open circuit of the battery; Do not disassemble the battery safety valve without permission.
(3) Inverters and controllers are very important equipment in this power generation system. Any changes in the parameters of these devices will directly harm the output, which directly threatens the connected load, such as supplying power to households. If the inverter or controller is damaged or the parameters change, it will directly damage household appliances and bring losses to users. Therefore, it is necessary to maintain the integrity of the inverter structure and electrical connections, and avoid rust, dust accumulation, and other phenomena. It is necessary to have a good heat dissipation environment and not block the filter screen of the air exchange port.
The heat sink of some components or modules such as reactors and transformers in the inverter should be able to start or stop automatically based on temperature. If there is abnormal noise or vibration, it should be promptly powered off for inspection; Regularly check the wiring of inverters and controllers for looseness, and check whether the temperature of the DC bus capacitor in the inverter is too high or exceeds the usage limit; For inverters with the same capacity during the same period, whether there is a phenomenon such as low power displayed by the inverter and whether the generated power is close. If such a situation is found, the cause should be checked and repaired in a timely manner.
(4) The components are connected through photovoltaic cables, and in order to ensure their normal operation, the connecting cables should be securely tied without looseness or damage; At the same time, the cable identification plate should not be damaged or missing; The places where cables are buried should have intact markings and ensure that there is no excavation on the ground near the path; No stacking of heavy objects, building materials, or temporary facilities, and no discharge of corrosive substances.
(5) The grounding resistance of the lightning protection grounding system should be measured at least once a year, and the aging of lightning arrester components should be checked with a tester.