Who "stole" 6% of the power station's electricity?

introduction

"For the same power station, the difference in the comprehensive power generation of different brands of inverters in one year is more than 6%", said the person in charge of a power station in Northwest China. This situation has happened to more than one power station. What is surprising is that the conversion efficiency of inverters produced by most manufacturers is between 98% and 99%, with a difference of only a few tenths of a percent. But why is the difference in power generation so large? Have we overlooked other factors that cannot be measured or predicted? Today, I will unveil this mystery from a technical perspective!

Who stole 6% of my electricity generation?

As the bridge of the photovoltaic system, the inverter accounts for a small proportion of the entire system cost, but its role cannot be ignored. The inverter plays a decisive role in how much power the system generates. Although there are many manufacturers in China currently making inverters, the conversion efficiency of most products is between 98% and 99%, and the comprehensive power generation of inverters of different brands varies dramatically.

"The same 100MW power station uses inverter products from two manufacturers. According to one year's data, the power generation difference is more than 6%. Seeing this result, I feel shocked." The person in charge of a power station in the northwest said, and also expressed greater concern that "this is still the first few years of the initial operation of the power station, and the future is even more difficult to predict." Take a power station in Qinghai, northwest China as an example. According to the power generation data of the past year, the power generation difference of inverters from different manufacturers is 6.71%. If 1MW generates 1.6 million kWh per year, 0.9 yuan/kWh, this price difference is about 160*6.71%*0.9=96,600 yuan per year, and the price difference of a 100MW power station is 9.66 million yuan per year.

Converted to per watt, it is equivalent to a difference of 9.66 cents per watt in one year. If calculated based on the inverter price difference of 0.3 cents per watt, the extra investment can be recovered in 3 to 4 months.

Comprehensive evaluation of the inverter's true efficiency during its life cycle

At present, we are used to evaluating inverters based on conversion efficiency, because this efficiency is also the easiest to measure. However, the evaluation of a product should be a comprehensive evaluation of the entire life cycle of the product. Through years of industry experience, here is a comprehensive evaluation formula for the inverter's entire life cycle:

The real efficiency of the inverter = inverter conversion efficiency * dynamic MPPT efficiency * availability * real life rate (real life rate = actual service life / nominal life of the inverter).

From the above formula, we can see that when everyone's conversion efficiency is almost the same, the dynamic MPPT efficiency, product availability and the actual life of the inverter will also have a huge impact on the benefits. However, these items are difficult to measure and evaluate.

Dynamic MPPT efficiency is the maximum power point tracking efficiency in the system, and this efficiency is difficult to measure on site. Because a power station will be in a dynamic state of change with changes in light conditions, voltage, and temperature, it is not easy to find the dynamic maximum power point. At present, each manufacturer has different algorithms, but the effectiveness still depends on the performance of the product and the company's own long-term experience accumulation.

The availability of the inverter reflects the actual power generation time compared with the expected normal operation under the normal operation of the power station system. Through a large number of power station analyses, the main factors affecting the availability of products are overheating shutdown, poor grid adaptability, overheating capacity reduction, high failure rate, untimely maintenance, etc. Therefore, the availability rate not only refers to the inverter itself, but also to the service system. For example, the service response is slow. Excellent companies can handle the problem in half a day or even at night, but some companies need a week to recover, which also leads to low availability.

The last influencing factor is the actual life rate of the product, that is, the actual service life/nominal life of the inverter. At present, most manufacturers in the industry claim 25 years, but the actual life span is ultimately how many years, which is generally impossible for customers to measure, and this mainly depends on the industrial design of the product itself and the environment in which it is used. Some products have design defects, such as the heat dissipation efficiency of the shell, the exhaust strength of the fan, the rationality of the air duct design, and the temperature resistance of the internal components themselves. Of course, it is also related to the design and use. For example, if you choose an outdoor product, due to the large temperature change rate, strong ultraviolet rays will affect the aging of the equipment. The industry's debate on whether to have fans and fuses is also based on the life of the inverter.

Another extreme case is that if the inverter manufacturer ceases to exist after a few years, and other manufacturers are unable to repair problems with their products, then the only option is to replace them with new ones, and the life of the original products will also end. At this time, the life rate will drop significantly.

in conclusion

From the above analysis, we can see that when purchasing an inverter, a comprehensive and comprehensive evaluation should be conducted on the actual efficiency of the entire life cycle: inverter conversion efficiency, dynamic MPPT efficiency, availability and actual life rate. At the same time, it should be ensured that the product supplier has accumulated several years of data in the photovoltaic field, has high customer recognition, can accurately diagnose and promptly resolve problems that arise during operation and maintenance, and has the ability to continuously innovate and provide services during the 25-year equipment life cycle.