Photovoltaic power generation, as one of the main ways of utilizing and developing solar energy, can have a large or small system scale and is a technology that benefits long-term investment. Promoting and applying this technology to ordinary households, leveraging its advantages of on-site power generation, peak shaving and valley filling, reducing transmission losses, and on-site fault resolution, will better reflect the comprehensive economic benefits of photovoltaic power generation technology. Solar photovoltaic power generation systems are mainly divided into two types: independent and grid connected. The former is mainly used for independent power supply occasions such as mountainous areas with difficulty in power transmission and solar street lights, while the latter is mainly used for household residential buildings and large-scale power generation systems. If ordinary household photovoltaic grid connected power generation systems can be widely used, it will greatly alleviate the problem of tight energy demand in China.
Grid connected photovoltaic power generation is the process of converting the direct current emitted by the photovoltaic battery pack into alternating current of the same frequency and phase as the power grid, and connecting the AC side to the power grid. Grid-connected solar photovoltaic power generation systems can be divided into two types: with energy storage devices and without energy storage devices, with energy storage devices generally being batteries. The grid connected photovoltaic power generation system with energy storage devices has adjustability and can be connected to or removed from the power grid according to demand. If the power grid fails and cannot be supplied normally, it can serve as a backup power source. Photovoltaic grid connected power generation systems with energy storage devices are often placed on the roof of buildings; Grid connected power generation systems without energy storage devices do not have the function of adjustability and reserve power, and are usually installed on larger equipment devices. The main discussion is about grid connected household photovoltaic power generation systems with energy storage devices. This type of power generation system is more reliable and flexible, especially in nighttime and poor weather conditions, where power can be maintained by batteries.
1. Current research status of domestic and foreign topics
Household distributed photovoltaic power generation systems are a major avenue for the popularization and application of solar power generation. At present, domestic and foreign experts have also conducted a large amount of research on this topic. Introduced the driving factors of solar power generation both domestically and internationally, as well as the feasibility and policy drivers of system scale application. The research focus is mainly on the calculation and selection of parameters for various components of the system, including the power parameter matching of controllers and inverters, batteries, and photovoltaic modules, as well as the installation tilt angle of photovoltaic modules. This paper introduces a household solar power generation system that uses fuzzy control algorithm to achieve maximum power tracking point. We have conducted in-depth discussions on the charging and discharging schemes of batteries in photovoltaic systems. We mainly discussed the design scheme of grid connected inverters, using active inverter circuits to achieve it.
2. Structure of household distributed photovoltaic grid connected power generation system
Distributed photovoltaic power generation can be used as a distributed power source, which is a small-scale photovoltaic power generation system installed locally or near the user’s electric field to ensure the electricity consumption of specific users, while being compatible with the current distribution network. Household distributed photovoltaic grid connected power generation systems are the main trend of small-scale photovoltaic power generation systems. The household distributed photovoltaic grid connected power generation system mainly includes photovoltaic cell components, photovoltaic array brackets, grid connected inverters, photovoltaic controllers, batteries, multi-purpose meters, and AC loads. Its operating mode is that during good weather conditions during the day, the solar battery pack provides power to the grid connected inverter through a photovoltaic controller, and finally supplies AC loads. During the day, the electricity consumption is low, and excess energy can be stored in the battery pack or sold to the grid; When the weather conditions are relatively poor or at night, the battery can provide power to the grid connected inverter through the photovoltaic controller, and finally supply AC load. If the battery energy is insufficient to meet the AC load, it can be switched to the grid for power supply. The structural diagram of the household distributed grid connected photovoltaic power generation system is shown in Figure 1.
3. Design of household distributed photovoltaic power generation systems
3.1 Ordinary household electricity demand
An ordinary household electricity load can be summarized as: 4 20W lighting fixtures, working an average of 6 hours per day; A 180W television, working an average of 4 hours per day; 1 200W refrigerator with an average daily power consumption of 1kW. h; 1 600W rice cooker, with an average daily power consumption of 0.6kw. h; The daily average power consumption of other household electrical equipment is calculated as 1kW. h.
3.2 Selection of photovoltaic controller
The photovoltaic controller should be a bidirectional AC charger, which can store the energy emitted by the solar photovoltaic panel in the battery and release the battery energy to the household AC load or power grid. Due to the daily average power consumption of the system being 3.68kW. h, based on the local average sunshine time of 5 hours, it can be concluded that the hourly average power of solar photovoltaic panels is:
According to the conversion efficiency of solar panels to sunlight of 90%, and the conversion efficiency of controllers and inverters of 75%, the power of solar panels is obtained as follows:
Due to the series connection of the battery and based on the above calculation structure, the author chose a 48V/60A controller, model LB01.
3.3 Grid connected inverter
The design of this project is a household grid connected solar power generation system with a photovoltaic cell capacity of 3.68kW. The technical key is to design a grid connected inverter that matches the 3.68kW capacity. The design of transformers and flat reactors in the grid connected inverter is also a major challenge. The grid connected inverter designed for this project adopts an active inverter, which connects the AC side of the inverter to the power grid. The basic circuit model used is a full wave inverter circuit. This inverter circuit has a simple structure and is easy to implement. The difficulty lies in designing synchronous trigger pulses with a phase difference of 180 °. Because the thyristor in the inverter circuit of this project requires high-power trigger pulses and has very high synchronization requirements, while the commonly used single junction transistor trigger circuit has low driving power and poor synchronization of the sawtooth wave trigger pulse circuit, the key technical problem to be solved is to design a trigger pulse circuit with good synchronization effect and high driving power. In order to improve the maximum power output of photovoltaic panels, an ideal MPPT maximum power point tracking control based on load impedance regulation is adopted. The MPPT maximum power tracking control of photovoltaic cells is a method of adjusting the output voltage by changing the duty cycle of the switching power supply waveform, and supplying the energy generated by solar photovoltaic cells to the load in the form of maximum power. The focus of this project is to solve the technical problem of insufficient photovoltaic power generation due to adverse weather conditions, which requires automatic switching to the power grid for power supply. The method used is to collect the power output of the photovoltaic panel in real-time. When the output power of the photovoltaic panel cannot meet the electricity demand for household use, the inverter will be stopped and the power grid will be used for power supply.
This project also needs to be able to measure the electricity sold back to the grid and obtained from the grid through solar power generation. The solution is to use a bidirectional energy meter for measurement. The single-phase full wave active inverter circuit is shown in Figure 2. Figure 3 shows the voltage waveform of point A in a single-phase full wave active inverter circuit.
3.4 Capacity design of battery
The formula for calculating the capacity of a battery is:
In the formula: QL is the total daily load of all electrical equipment, W.h; D is the number of days of energy storage; F is the battery capacity correction factor, usually taken as 1.2; K is the power loss including the inverter, taken as 0.8-1; D is the discharge depth of the battery, usually taken as 0.5. In this article, the discharge depth is taken as 80%, and the loss rate is taken as 0.8. Based on the above electricity consumption and energy storage duration, the battery capacity can be calculated as CW=11.04kW. h. If a single lead-acid battery with a nominal voltage of 12V is selected and connected in series to form a 48V battery pack, the capacity of the battery can be calculated based on the number of battery pack capacity (A.h)=required W.h and the voltage of the battery pack:
3.5 Calculation of solar cell capacity
The calculation formula for the capacity of solar cell modules is as follows:
In the formula, Tmax is the average daily maximum sunshine, Ks is all external factors such as dust on the battery panel, power attenuation of the array itself, line loss, etc., usually ranging from 1.6 to 2. This system considers the air in Chongqing area to be good, and the impact factor on the solar panel is small. If Ks is taken as 1.8, the total capacity of the solar cell module is:
4. Conclusion
This article mainly introduces the main structure and working mode of the household distributed photovoltaic grid connected power generation system, outlines the selection of photovoltaic controllers in the system, adopts single-phase active inverter grid connected mode, calculates the capacity of the battery and the capacity of the solar cell. This system provides a solution for the promotion and application of distributed photovoltaic power generation, and has certain application value.