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The solution of micro-inverter in photovoltaic building integration

Dec 18, 2020

The solution of micro-inverter in photovoltaic building integration

1. The difference between micro inverter and traditional inverter


For optimizing the efficiency and reliability of solar photovoltaic power generation systems, a relatively new approach is to use micro-inverters connected to each solar cell module. Equipping a separate micro-inverter for each solar cell allows the system to adapt to changing loads and weather conditions, thereby providing the best conversion efficiency for a single solar cell and the entire system.


The concept of micro inverter has a long history, but it did not attract people's attention at first. In recent years, with the development of solar power generation technology and technological progress, micro inverters are very attractive. Enphase of Petaluma, California, USA started commercial mass production of micro inverters in 2008, and achieved good sales results, making micro inverters more widely recognized, attracting many companies to join micro inverters. R&D ranks.


A photovoltaic power generation system with a micro-inverter structure can simplify wiring, which means lower installation costs, improve the efficiency of the solar power system, and shorten the time required to "recover" the initial investment.


Many domestic photovoltaic grid-connected inverter manufacturers are mainly engaged in the development of high-power centralized grid-connected inverter products. With the increasing popularity of the domestic and foreign micro-inverter market, many manufacturers have also begun to develop micro-inverter products. Involar New Energy Technology Co., Ltd. is the earliest company engaged in micro-inverter research in China. The company has been developing micro-inverter technology since the beginning of 2008. After nearly two years of hard work, it has completely mastered the development of micro-inverters. Core technology, and successfully released its first-generation product NAC250 in May 2010. At present, this micro-inverter product has been introduced to the market. The difference between the micro inverter and the traditional inverter is as follows:


1) The inverter has low input voltage and high output voltage. The output voltage range of a single solar cell module is generally 20-50V, and the peak voltage of the grid is about 311V (220VAC) or 156V (110VAC). Therefore, the output peak voltage of the micro inverter is much higher than the input voltage, which requires Micro-inverters adopt inverter topology with buck-boost conversion function; while centralized inverters are generally buck converters, which usually adopt bridge topology, and the peak value of the inverter output AC side voltage is lower than the input DC side voltage.


2) Low power. The power of a single solar cell module is generally 100W~300W. The micro-inverter is directly matched with a single solar battery module, and its power level is 100W~300W, while the traditional centralized inverter power passes through multiple solar battery modules. The series-parallel combination generates high enough power, and its power level is generally above 1kW.


2. Advantages of micro inverter


Common photovoltaic grid-connected power generation system structures include centralized, string, multi-string, and AC modular solutions. In centralized, string, and multi-string systems, there are series and parallel solar cell modules. Therefore, the maximum power point tracking of the system is for the entire series and parallel solar cell arrays. It cannot take into account each solar cell array in the system. The utilization rate of the solar cell array is low, the system's ability to resist local shadows is poor, and the flexibility of system expansion is insufficient. The photovoltaic grid-connected micro-inverter is connected to a single solar cell module, which can directly convert the direct current output by the solar cell module into alternating current and transmit it to the grid. It has the following advantages:


1) Ensure that each solar cell module runs at the maximum power point and has a strong ability to resist local shadows.


2) Integrating the micro-inverter and solar cell components can achieve modular design, plug-and-play and hot-swappable, and system expansion is simple and convenient.


3) Micro-inverters basically do not occupy installation space independently, and distributed installation is easy to configure, and can make full use of space and adapt to applications with different installation directions and angles.


4) The system has high redundancy and high reliability, and the failure of a single module will not affect the entire system.


3. Photovoltaic building integration


Photovoltaic building integration is a new concept in the application of solar power generation. Simply put, the solar photovoltaic power generation array is installed on the outer surface of the building envelope to provide electricity. According to the different ways of combining photovoltaic arrays and buildings, photovoltaic building integration can be divided into two categories: one is the combination of photovoltaic arrays and buildings. The other is the integration of photovoltaic arrays and buildings. Such as photovoltaic tile roofs, photovoltaic curtain walls and photovoltaic roofs. In these two ways, the combination of photovoltaic array and building is a commonly used form, especially the combination with building roof. Because the combination of photovoltaic arrays and buildings does not occupy additional ground space, it is the best installation method for photovoltaic power generation systems widely used in cities, so it has attracted much attention. The integration of photovoltaic arrays and buildings is an advanced form of BIPV, which has higher requirements for photovoltaic modules. Photovoltaic modules must not only meet the functional requirements of photovoltaic power generation, but also take into account the basic functional requirements of the building. During the "Twelfth Five-Year Plan" period, 2,000 demonstration units of conservation-oriented public institutions will be created. In addition to public institutions, commercial institutions have a relatively high willingness to participate in energy conservation due to their large power consumption and financial advantages. They should also prioritize the development of an integrated photovoltaic building model. According to the different ways of combining photovoltaic arrays and buildings, the integration of solar photovoltaic buildings can be divided into two categories:


1) The combination of photovoltaic array and building. In this way, the photovoltaic array is attached to the building, and the building acts as a support for the photovoltaic array. The further combination of photovoltaic arrays and buildings is to integrate photovoltaic devices with building materials. Generally, coatings, decorative tiles or curtain wall glass are used on the outer surface of buildings to protect and decorate the building. If photovoltaic devices are used to replace part of building materials, that is, photovoltaic modules are used for building roofs, exterior walls and windows, which can be used as building materials as well as power generation, which is perfect. For a building with a frame structure, the entire enclosure structure can be made into a photovoltaic array, and appropriate photovoltaic modules can be selected to absorb both direct sunlight and reflected sunlight. At present, large-scale color photovoltaic modules have been developed, which can achieve the above objectives and make the appearance of the building more attractive.


2) Integration of photovoltaic arrays and buildings. In this way, photovoltaic modules appear in the form of a building material, and the photovoltaic array becomes an integral part of the building.


The photovoltaic system combined with the building can be used as an independent power source or in a grid-connected manner. When the system participates in the grid-connected, storage batteries are not required. But it needs to be connected to the grid, and grid-connected power generation is a new trend in photovoltaic applications today. Install the photovoltaic module on the roof or external wall of the building, and connect the output terminal to the public grid through the controller, which needs to supply power to the photovoltaic array and the grid in parallel to the user, which constitutes a grid-connected photovoltaic system. The photovoltaic building integrated system has the following advantages:


1) Green energy. The integration of solar and photovoltaic buildings produces green energy and uses solar power to generate electricity without polluting the environment. Solar energy is the cleanest and free of charge, and there will be no ecological side effects during the development and utilization process. It is also a kind of renewable energy, inexhaustible and inexhaustible.


2) Do not occupy land. Photovoltaic arrays are generally installed on idle roofs or external walls without additional land occupation, which is especially important for urban buildings with expensive land; summer is the peak season of electricity consumption, which also happens to be the period when the amount of sunlight is the largest and the photovoltaic system generates the most power , Can play a role in peak regulation of the power grid.


3) The integrated solar photovoltaic building technology adopts a grid-connected photovoltaic system, which does not need to be equipped with batteries, which saves investment and is not restricted by the state of charge of the batteries, and can make full use of the electricity generated by the photovoltaic system.


4) Play the role of building energy saving. The photovoltaic array absorbs solar energy and converts it into electricity, which greatly reduces the overall outdoor temperature, reduces the heat gain of the wall and the cooling load of the indoor air conditioning, so it can also play a role in building energy saving. Therefore, the development of solar photovoltaic building integration can "energy saving and emission reduction".


Although the integration of solar photovoltaic buildings has many advantages such as high efficiency, economy, and environmental protection, photovoltaic buildings have not yet entered the homes of ordinary people, and residential communities using this technology have not appeared. This is due to the following shortcomings of solar photovoltaic building integration:


1) Higher cost. The cost of building integrated solar photovoltaic buildings is relatively high. The integrated design and construction of buildings with photovoltaic power generation systems cost a lot, and the scientific research technology needs to be improved.


2) High cost. The cost of solar power generation is high. The cost of solar power generation is 2.5 yuan per kWh, which is double the cost of conventional power generation at 1 yuan per kWh.


3) Unstable. Solar photovoltaic power generation is unstable, greatly affected by the weather, and fluctuating. This is because the sun is not available 24 hours a day, so how to solve the volatility of solar photovoltaic power generation and how to store electricity is also an urgent problem to be solved.


4. Micro inverter solution


In the BIPV system, the installation of solar cell modules first involves the installation angle and direction of the solar cell module. The installation angle is the inclination of the solar cell module. The choice of the inclination angle is directly related to the solar cell module. Power generation efficiency. For the same solar cell module, the amount of radiation received by choosing different installation angles is different. Due to the problem of the orientation of each wall, the installation angle and direction of solar cell modules at different installation positions cannot be exactly the same, which determines Its power generation efficiency and instantaneous power cannot be guaranteed to be completely consistent.


Another key problem that needs to be solved in the BIPV system is the problem of shadow occlusion. There are many reasons for shadows. The generation of shadows can be random or systematic. The shadows mainly come from the surrounding buildings, the occlusion of trees, the mutual occlusion between various solar cell modules, clouds, etc. The output characteristics of the solar cell module determine that after being partially blocked or shaded, its power generation efficiency will be greatly reduced, which will have a significant impact on the power generation of the entire system.


In order to maximize the power generation efficiency of the BIPV system, in addition to planning and designing as much as possible during installation, it is also necessary to adopt a suitable photovoltaic power generation system structure. Figure 1 is a schematic diagram of the electrical structure commonly used in the current BIPV system.



In Figure 1, the centralized system first connects a large number of solar cell modules in series or parallel according to the designed voltage and power level, and then converts the DC power output by the solar cell array into AC through a centralized inverter Electric energy; string and multi-string systems connect multiple solar cell modules in series to form a solar cell module string. After each string is boosted by a DC-DC converter, it outputs AC power through an inverter. In the above three systems, there are solar cell modules in series or parallel. The maximum power point of the system is tracked for the entire string. Therefore, there is no guarantee that each module will run at the maximum power point, nor can each solar cell module be obtained. On the other hand, due to the different installation directions and angles of the solar cell modules on the building surface, the power generation efficiency of each solar cell module is different from each other. The use of centralized maximum power point tracking will greatly reduce the power generation efficiency of the system ; When part of the solar cell components are blocked, the power generation efficiency of the entire system will be seriously reduced, greatly reducing the energy conversion efficiency of the system, and may even form hot spots, resulting in system damage.


Micro-inverter technology proposes to integrate the inverter directly with a single solar cell module, and equip each solar cell module with an inverter module with AC-DC conversion function and maximum power point tracking function. Electrical energy is directly converted into AC electrical energy for use by AC loads or transmitted to the grid.


The application of micro-inverters to BIPV systems can fully adapt to the application requirements of building integrated photovoltaic power generation systems, adapt to different installation angles and orientations of solar cell modules, avoid the impact of local shadows on the system power generation efficiency, and maximize the power generation efficiency of the BIPV system . The structure of the building photovoltaic power generation system using micro inverters is shown in Figure 2.



In Figure 2, the micro-inverter is directly connected to the solar cell module, and the electric energy generated by the solar cell module is directly transmitted to the grid or used by the local load. Multiple micro-inverters are directly connected to the grid in parallel, and each micro-inverter There is no mutual influence with the solar cell components, and the failure of a single module will not affect the entire system.


Combining the micro-inverter technology with the power line carrier communication technology, the output power and status information of each micro-inverter and solar cell components can be collected through the AC bus of the grid, which is very convenient to realize the monitoring of the entire system without additional The communication line does not have any burden on the system connection, which greatly simplifies the system structure.