The selection of photovoltaic modules and inverters for distributed photovoltaic power generation projects is crucial for the feasibility study and preliminary design success of the project.
Nowadays, the use of solar power generation to provide distributed power supply to various factories, enterprises, office buildings, and residential buildings has become very mature and popular. For example, providing electricity to factories in the park can be achieved. Due to the high power consumption in the factory area, almost all of the electricity can be used for self use. Therefore, a distributed power generation model of self use surplus electricity can be adopted. After the completion of a distributed photovoltaic power generation project, it can effectively alleviate the power supply pressure of some factories in the park, Effectively alleviating the supply-demand contradiction of local power grids, optimizing the system power structure, reducing environmental pressure, promoting sustainable economic development in the region, and contributing to energy conservation and emission reduction in the region, in line with the national work philosophy of distributed photovoltaic power generation.
1.Recent Status of Photovoltaic Power Generation
Distributed photovoltaic power generation projects consist of photovoltaic modules, grid connected inverters, metering devices, and distribution systems. Solar energy is converted into direct current through photovoltaic modules, which are then converted into sinusoidal currents of the same frequency and phase as the grid through grid connected inverters. A portion of the energy is supplied to the local load, while the remaining power is fed into the grid. In the feasibility study and preliminary design of distributed photovoltaic power generation projects, the selection of photovoltaic modules and inverter configuration is an important link.
Photovoltaic modules are composed of several individual solar cells connected in series and parallel, and tightly packaged. Now, traditional polycrystalline silicon and monocrystalline silicon modules have gradually developed into thin film power generation, color steel tile power generation, and other methods. The photovoltaic conversion efficiency of silicon materials has also been continuously improved, and the conversion efficiency has increased from 14% to over 20% in the past five years. It is expected that the photovoltaic conversion efficiency of silicon materials will further improve in the future. In addition, the price of photovoltaic modules is significantly decreasing, from 4 yuan/Wp to 2 yuan/Wp in the past five years. Currently, there is still room and trend for decline. It can be foreseen that photovoltaic power generation is about to enter the era of affordable grid access, and the distributed photovoltaic stage will become increasingly large, which is very beneficial for human future development.
2.Selection of photovoltaic modules
At present, the most common photovoltaic modules are monocrystalline silicon photovoltaic modules and polycrystalline silicon photovoltaic modules. The production process of polycrystalline silicon photovoltaic modules is similar to that of monocrystalline silicon photovoltaic modules. The efficiency of polycrystalline silicon photovoltaic modules can reach 16.5% to 18%, and the conversion efficiency of monocrystalline silicon solar modules is generally 17% to 19%, slightly lower than that of monocrystalline silicon photovoltaic modules, but the cost of monocrystalline silicon is slightly higher than that of polycrystalline silicon modules. At present, crystal silicon solar modules produced by mainstream domestic manufacturers are used in large-scale grid connected photovoltaic power generation systems, with most of their specifications ranging from 270Wp to 360Wp. The mainstream products of 60 domestically produced polycrystalline silicon battery modules are 270Wp and 275Wp, while the mainstream products of monocrystalline silicon battery modules are 285Wp, 290Wp, and 295Wp; The mainstream products for 72 polycrystalline silicon battery modules are 300Wp-310Wp, while the mainstream products for 72 monocrystalline silicon battery modules are 350Wp and 360Wp. Based on actual project experience, single crystal silicon modules have higher power generation efficiency and smaller efficiency attenuation, so single crystal silicon modules are generally selected.
3.Inverter selection
The commonly used solar energy inverter methods currently include centralized inverter, series inverter, multi series inverter, and component inverter (micro inverter). At a close initial investment cost, for distributed photovoltaic power generation projects, the string inverter solution has a unique advantage over centralized inverters and micro inverters, effectively solving the problems encountered by centralized solutions. For rooftop distributed photovoltaic power generation projects, string inverters should be selected to avoid safety hazards caused by DC combiner boxes and shorten the length of DC circuits, Reduce the risks brought by DC circuits. At present, there are two mainstream inverters in China: 60kW and 50kW. The cost of 60kW inverters is slightly higher than that of 50kW inverters, but their relative inverter efficiency is slightly better.
4.Selection of photovoltaic modules and inverters
The following is a practical application case to illustrate how to conduct analysis and design for the selection of photovoltaic modules and inverters.
A distributed photovoltaic power generation project is constructed using the roofs of four buildings, namely Factory A, Factory B, Factory C, and Factory C, of a company in Huizhou City, Guangdong Province. The total roof area is approximately 16000m2. After on-site investigation, the project can have two grid connection points. After load analysis and preliminary arrangement design of photovoltaic modules, it is preliminarily determined that each connected photovoltaic capacity is 600kWp, with a total of 1200kWp. The photovoltaic power generation project adopts spontaneous self use, The remaining part runs in online mode. Based on the preliminary judgment of the maximum capacity above, analyze and list the selection of polycrystalline silicon and monocrystalline silicon components of the same brand.
| Project | Unit | Polycrystalline silicon -290Wp | Single crystal silicon 290Wp |
| Regulations on attenuation rate of power generation calculation | The attenuation rate in the first year is 2.5%, with an average annual attenuation of 0.7% in the 24th year and a total attenuation of 19.3% in the 25th year | The attenuation rate in the first year is 3%, with an average annual attenuation of 0.55% in the 24th year and a total attenuation of 16.2% in the 25th year | |
| Number of components | Block | 4200 | 4200 |
| Unit price | Yuan/Wp | 2.50 | 2.54 |
As shown in Table , the above comparison shows that the efficiency of the single crystal silicon 290Wp module is higher than that of the polycrystalline silicon 290Wp module, and the actual price difference is not significant. Therefore, the single crystal silicon 290Wp module is chosen. As one of the key equipment for converting direct current to alternating current in photovoltaic power generation systems, component selection plays an important role in the conversion efficiency and reliability of the power generation system. Based on the requirements of the Southern Power Grid’s “Technical Standards for Grid Connection of Photovoltaic Power Generation” and other relevant specifications, and the selection of inverter capacity based on the capacity of photovoltaic modules, the following ideas can be considered:
(1) After being equipped with an inverter, the three-phase voltage imbalance at the grid connection point shall not exceed the value specified in the “Power Quality – Permissible Unbalance of Three Phase Voltage”, usually 1.3%.
(2) The output power factor of the inverter device selected for the project should be as close as possible to 1, so that the configured reactive power compensation device can meet the requirements of the power grid for reactive power, improve voltage quality, and reduce line losses.
(3) After being equipped with an inverter, it is necessary to meet practical requirements and standards in terms of voltage deviation, frequency, harmonics, and power factor when connected to the grid.
(4) The total harmonic distortion rate of the harmonic current generated by the inverter must be controlled within 3%, in accordance with the regulations of “Power Quality Public Grid Harmonics”.
(5) The inverter requires a warranty of 10 years, must meet the system’s PID resistance performance, and should also have reverse charging repair function.
(6) The higher the conversion efficiency of the inverter, the higher the system efficiency of the photovoltaic power generation system. Therefore, when a single unit has the same rated capacity, it is recommended to choose an inverter with high conversion efficiency or directly choose an inverter with high rated capacity to achieve higher efficiency.
(7) The inverter conversion efficiency includes maximum efficiency and Chinese efficiency. China’s efficiency is a weighting of the efficiency of different power points, which better reflects the comprehensive efficiency characteristics of inverters. The output power of photovoltaic power generation systems is constantly changing with the intensity of solar radiation, so the efficiency in China is more practical than the maximum efficiency.
(8) Choose an inverter with a wide range of DC input voltage to absorb maximum sunlight.
(9) The output power of solar cell modules varies at any time and has nonlinear characteristics, so the inverter selected should have maximum power point tracking function.
(10) After being equipped with an inverter, the overall economic efficiency of the project is better.
Under certain external conditions, single crystal silicon 290Wp modules, single crystal silicon 295Wp modules, 60kW inverters, and 50kW inverters will be selected and compared for the full life cycle economic calculation of the project, in order to comprehensively consider the selection of photovoltaic modules and inverters. This economic calculation mainly saves on supporting structures, DC cables and combiner boxes, construction and installation in the photovoltaic field area, and inverter selection.
Therefore, overall, in this project, the use of single crystal silicon 295Wp components and 60kW inverter options resulted in the best project benefits.
5.Conclusion
The selection of photovoltaic modules and inverters for distributed photovoltaic power generation projects requires comprehensive consideration of basic technology and economic analysis. On the basis of meeting the requirements of efficient power generation, the optimal combination of economic allocation is selected.