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Household photovoltaic capacity ratio design solution

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Household photovoltaic capacity ratio design solution

In the design of the photovoltaic power station system, the ratio of the installed capacity of the photovoltaic module to the rated capacity of the inverter is the capacity

The ratio is a very important design parameter. In the "Efficiency Standards for Photovoltaic Power Generation System" released in 2012, the capacity ratio is designed according to 1:1, but due to the influence of light conditions and temperature, most of the time the photovoltaic modules cannot reach the nominal power, and the inverter basically They are all running at less than full load, and most of the time are in the stage of wasting capacity.

In the standard released at the end of October 2020, the capacity ratio of photovoltaic power plants is fully liberalized, and the maximum ratio of modules and inverters reaches 1.8:1. The new standards will greatly increase the demand for domestic modules and inverters. It can also reduce the cost of electricity and accelerate the arrival of the era of photovoltaic parity.

This article will take the distributed photovoltaic system in Shandong as an example, and analyze it from the perspective of the actual output power of photovoltaic modules, the proportion of losses caused by over-provisioning, and economics.

The trend of component overprovisioning

At present, the average over-allocation of photovoltaic power plants in the world is between 120% and 140%. The main reason for over-allocation is that photovoltaic modules cannot reach the ideal peak power during actual operation. The influencing factors include:

• Insufficient radiation intensity (winter)

• The ambient temperature is too high

• Stain and dust masking

• Component orientation is not optimal throughout the day (tracking brackets are less of a factor)

• Module decay: 3% in the first year, 0.7% per year thereafter

• Matching loss within and between strings

In recent years, the over-allocation ratio of photovoltaic systems has shown an increasing trend. In addition to the system loss, the further decline in component prices in recent years and the improvement of inverter technology have led to an increase in the number of strings that can be connected, making over-configuration more and more economical. In addition, over-provisioning of components can also reduce the cost of electricity, thereby increasing the internal rate of return of the project, so the anti-risk ability of project investment is improved.

In addition, high-power photovoltaic modules have become the main trend in the development of the photovoltaic industry at this stage, which further increases the possibility of over-provisioning of modules and the improvement of household photovoltaic installed capacity.

Based on the above factors, over-allocation has become a trend in photovoltaic project design.

Power generation and cost analysis

Taking the 6kW household photovoltaic power station invested by the owner as an example, LONGi 540W modules commonly used in the distributed market are selected. It is estimated that an average of 20 kilowatt-hours of electricity can be generated every day, and the annual power generation will be about 7,300 kilowatt-hours.

According to the electrical parameters of the components, the working current at the maximum working point is 13A. Choose the mainstream inverter Goodwe GW6000-DNS-30 on the market. The maximum input current of this inverter is 16A, which can adapt to the current market. high current components. Taking the average annual total radiation of light resources in Yantai City, Shandong Province for 30 years as a reference, various systems with different overproportion ratios were analyzed.

2.1 System efficiency

On the one hand, over-provisioning increases the power generation, but on the other hand, due to the increase in the number of components on the DC side, the matching loss of the components in the string and the loss of the DC line increase. Therefore, there is an optimal capacity ratio, so that the system’s Maximum efficiency. After PVsyst simulation, the system efficiency under different capacity ratios of the 6kVA system can be obtained. As shown in the table below, when the capacity ratio is about 1.1, the system efficiency reaches the maximum, which also means that the utilization rate of the components is the highest at this time.

System efficiency and annual power generation with different capacity ratios

2.2 Power Generation and Revenue

According to the system efficiency under different overproportion ratios and the theoretical attenuation rate of components in 20 years, the annual power generation under different capacity ratios can be obtained. According to the on-grid electricity price of 0.395 yuan/kWh (Shandong desulfurization coal benchmark electricity price), the annual electricity sales income is calculated. The calculation results are shown in the table above.

2.3 Cost Analysis

The cost is more concerned by users of household photovoltaic projects. Among them, photovoltaic modules and inverters are the main equipment materials, and other auxiliary materials such as photovoltaic brackets, protection equipment and cables, as well as installation related costs required for project construction, etc. In addition, users also need to consider the cost of maintaining photovoltaic power plants. The average maintenance cost accounts for about 1% to 3% of the total investment cost. In the total cost, photovoltaic modules account for about 50% to 60%. Based on the above cost expenditure items, the current household photovoltaic cost unit price is roughly shown in the following table:

Estimated Costs of Residential Photovoltaic Systems

Because of the different ratios of overprovisioning, the system cost will also change, including components, brackets, DC cables, and installation fees. According to the above table, the cost of different over-allocation ratios can be calculated, as shown in the figure below.

System cost, benefit and efficiency under different over-allocation ratios

Benefit Analysis of Added Costs

From the above analysis, it can be seen that although the annual power generation and income will increase with the increase of the over-allocation ratio, the investment cost will also increase accordingly. In addition, the above table shows that the system efficiency exceeds 1.1 times Best when paired. Therefore, from a technical point of view, 1.1 times overweight is the best.

However, from the perspective of investors, it is not enough to consider the design of photovoltaic systems from the technical level. It is also necessary to analyze the impact of over-allocation on investment income from the economic point of view.

The 20-year kWh cost of the system and the pre-tax internal rate of return can be calculated according to the investment cost and power generation income under the above-mentioned different capacity ratios.

Cost of electricity and internal rate of return under different over-provisioning ratios

It can be seen from the figure above that when the capacity ratio is small, the power generation and revenue of the system increase with the increase of the capacity ratio. At this time, the increased revenue can cover the extra cost due to over-provisioning. When the capacity ratio is too large, the internal rate of return of the system gradually decreases due to factors such as the gradual increase of the power limit of the added part and the increase of line loss. When the capacity matching ratio is 1.5, the internal rate of return IRR of the system investment is the largest. Therefore, from an economic point of view, 1.5:1 is the optimal capacity ratio for this system.

Through the same method as above, the optimal capacity ratio of the system under different capacities is calculated from the economic point of view, and the results are as follows:

epilogue

By using the data of solar energy resources in Yantai area, under the conditions of different capacity ratios, the power of the photovoltaic module output after loss and reaching the inverter is calculated. The results show that in Yantai City with better sunlight resources, for a 6kVA system, When the capacity ratio is 1.1, the system loss is the smallest, and the component utilization rate is the highest at this time. However, from an economic point of view, when the capacity ratio is 1.5, the photovoltaic project has the highest income. When designing a photovoltaic system, not only the utilization rate of components under technical factors must be considered, but economical efficiency is the key to project design. Through economical calculation, the 8kW system has the best economy when 1.3 is over-provisioned, the 10kW system has the best economy when 1.2 is over-provisioned, and the 15kW system has the best economy when 1.2 is over-provisioned.

When the same method uses the economical calculation of the capacity ratio in the industry and commerce, due to the reduction of the cost per watt of the system, the economical optimal capacity ratio will be higher. In addition, due to market reasons, the cost of photovoltaic systems will also vary greatly, which will also greatly affect the calculation of the optimal capacity ratio. This is also the fundamental reason why various countries have relaxed the restrictions on the design capacity ratio of photovoltaic systems.