1. The current safety issues of photovoltaic power generation

With the decrease in system cost of solar photovoltaic power generation technology in recent years, the cost of electricity is much lower than the cost of conventional fossil fuel power generation, such as coal, oil, natural gas, etc., and also lower than the cost of nuclear power generation.

In the past, in order to reduce the system cost of photovoltaic power generation, photovoltaic companies and R & D institutions have invested the most energy to improve the photoelectric conversion efficiency and reduce the system cost. These two focuses have paid little attention to the diversity of photovoltaic application scenarios and potential threats caused by human activities.

In the past 20 years, photovoltaic power generation originated in the United States and Europe. The total installed capacity of the world has accounted for the global power generation. It seems that the development of the photovoltaic industry has entered a mature industrial stage, but it is still far from the general use of human production. There is a long distance. At the same time, the process of replacing renewable energy with fossil energy is still a long process. The energy revolution is not an overnight thing.

The photovoltaic power generation industry has gone through 20 years from the beginning to the industrialization, and has also experienced 15 years of journey in China. Although China has become the world's largest photovoltaic manufacturing and application country, photovoltaics have not undergone revolutionary changes on the application side. They still use series circuits to form higher DC voltages, and then conduct AC and DC through centralized inverters. Conversion, input AC power into the grid. A voltage of up to 1000V has been formed in the series circuit on the DC side. In recent years, in order to reduce the amount of cable on the DC side, it has begun to increase the DC voltage to a voltage level of 1500V.

Such a system principle has formed a fixed thinking format since the beginning of industrialization of photovoltaics. The biggest advantage of this technical route is that the voltage of a single cell formed by a photovoltaic PN junction is always only about 0.5 ~ 0.6V, and the current and voltage band The inverse ratio relationship determines that only a higher voltage can obtain a lower current, reduce the resistance loss of the system, improve system efficiency, and reduce system cost. This is the last 20 years, this is the consistent thinking and continues to develop to this day.

If this technical route selection is only used for ground power generation and existing roof support installation systems, it seems that the risk is less. But as more and more rooftop photovoltaic power plants are put into operation, people find that this technical route is facing more and more hidden safety hazards and challenges. In particular, on August 22, 2019, CCTV Financial Report: Wal-Mart supermarkets in the United States in the past 11 months have continuously ignited the photovoltaic power generation on the roof of seven shopping centers. Wal-Mart sued the United States Tesla SolarCity for all the removal of rooftop photovoltaic power generation and compensated for the losses. This incident triggered widespread attention to the safety of photovoltaic power generation.

In fact, in China, with the development of photovoltaic poverty alleviation and rural distributed development in recent years, rooftop photovoltaic power plant fire incidents have occurred from time to time, and have become an important factor threatening people's building safety. There is still no detailed statistics on the probability of fire of photovoltaic power plants, but the fact that the rooftop photovoltaic power generation technology route dominated by high-voltage DC has already constituted a potential safety risk to the building, and must be highly valued.

To this end, in 2014 and 2016, the European Union and the United States respectively passed legislation mandating that rooftop photovoltaic power generation facilities must be equipped with smart circuit breakers. The maximum voltages set are 60 and 80v, respectively. In 2020, China passed the "Technical Regulations for Building Photovoltaic Integration" and referred to the US standard for the 80V fault shutdown requirements. However, because it is not a mandatory standard, it has not attracted sufficient attention in China.

Foreword

CCTV Financial Report on August 22, 2019: In the past 11 months, Wal-Mart supermarkets in the United States have continuously ignited photovoltaic power generation on the roofs of seven shopping centers. Wal-Mart sued Tesla SolarCity in the United States for all rooftop photovoltaic power generation and compensated for losses. This incident triggered widespread attention to the safety of photovoltaic power generation.

 Photovoltaic roof

In fact, in China, with the development of photovoltaic poverty alleviation and rural distributed development in recent years, rooftop photovoltaic power plant fire incidents have occurred from time to time, and have become an important factor threatening people's building safety. There is still no detailed statistics on the probability of fire of photovoltaic power plants, but the fact that the rooftop photovoltaic power generation technology route dominated by high-voltage DC has already constituted a potential safety risk to the building, and must be highly valued.

To this end, in 2014 and 2016, the European Union and the United States respectively passed legislation mandating that rooftop photovoltaic power generation facilities must be equipped with smart circuit breakers. The highest voltages set respectively are 60v and 80v. In 2020, China passed the "Technical Regulations for Building Photovoltaic Integration" and referred to the US standard for the 80V fault shutdown requirements. However, because it is not a mandatory standard, it has not attracted sufficient attention in China.

Hidden worries of current photovoltaic technology applied to rooftop power generation

Most of the current photovoltaic string technology routes are mainly composed of more than 20 modules, forming a string, forming a 1000V, 1500V level DC high-voltage line into the DC combiner box, and then into the centralized inverter or string inverter for DC AC inverter, and then the AC power is merged into the power network.

In the course of a string, the voltages of the components are superimposed to form a superimposed DC-side high voltage. In order to reduce the process loss of the string and avoid the short board effect of the barrel, it is required that the working conditions (operating parameters of power generation) of the components in each string be as consistent as possible.

Ground photovoltaic power plants should be site-leveled so that each string component is installed in a bracket array as much as possible. At the same time, daily maintenance must be done to check for hot spots, string current deviation, and dust cleaning to ensure the string Consistency of working conditions of internal components.

However, distributed power generation application scenarios, especially the photovoltaic power generation systems (BIPV and BAPV) installed on the surface of the building, will have more uncertainties, and it is difficult to maintain and clean up the dust, and the consistency of the working conditions of the components in the string is difficult to guarantee. Therefore, it will amplify the occurrence of isolated or small probability events existing in the operation of the ground power station.

The main technical difficulties and safety hazards of conventional photovoltaic module technology applied to building roof and facade power generation include:

1. Hot spot effect:

If any battery in a series-connected branch battery is shaded, it will be used as a load to consume the energy generated by other solar cells with light, and the shaded solar cell assembly will generate heat at this time. This effect can severely damage the solar cell and directly lead to failure or fire. The structural design of traditional photovoltaic module technology has such natural defects.

2. PID effect:

Also known as potential-induced attenuation, it is the packaging material of the battery component and the material of its upper and lower surfaces. Ion migration occurs between the battery sheet and its grounded metal frame under the action of high voltage, which causes the phenomenon of component performance attenuation. The PID effect is the main cause of long-term module attenuation or even severe degradation. The resulting module power attenuation sometimes exceeds 50%; the root cause of this hazard is that the existing photovoltaic modules are connected in series when forming a power generation network. Form a single component voltage of more than 20 to 30 times, such as 1000V DC voltage.

3. High voltage DC arc drawing:

Traditional photovoltaic modules are connected in series to form high voltage on the DC side. Currently, two voltage levels of 1000V and 1500V are used. In the application scenario of building photovoltaic power generation, the voltage is in close contact with the photovoltaic panel, which poses a great security threat to human activities. High-voltage direct current is also easy to cause arc fires, which accounts for about 45% of the roof distributed photovoltaic power generation fires.

4. Weather resistance:

The traditional photovoltaic module is made of organic materials on the back. Under the environment of high temperature and high humidity on the roof, the weather resistance performance is greatly reduced, and the service life is lower than that of the ground power station scenario.

5. Independent operation problem:

The traditional photovoltaic modules are connected in series, so that each photovoltaic module cannot run independently. Once one of the modules has a problem, the energy in the string will be gathered to the problematic module, which is very likely to cause a fire risk.

6. Fire protection issues:

The back of traditional photovoltaic module products are organic materials or glass, which can not meet the requirements of building fire protection A2; the inside of BIPV products must be fireproof and flame retardant materials to ensure compliance with fire protection standards.

Article 3.2.16 of "Code for Fire Protection Design of Buildings GB50016" (2018) clearly stipulates that "non-combustible materials shall be used for the roof slabs of first- and second-class fire-resistant plants (warehouses).

7. Bearing problem:

The structural mechanical design of traditional component products can only guarantee a certain wind pressure, and never consider human activities on the component surface. However, BIPV products, as building materials, must consider the maintenance and walking activities of people.

8. Risk of cracking:

Cracks are some tiny cracks that are not visible to the naked eye. Due to the characteristics of its own crystal structure, crystalline silicon cells are easy to crack. Cracks can be said to be a common defect of the cell itself. It will cause part of the battery chip or even the whole chip to fail. Because traditional products do not consider human activities on the surface of components, there is no design control measures for the cracking problem. Because BIPV is affected by human daily activities, BIPV using crystalline silicon cells must consider the risks caused by cracking.

9. Electrical safety:

As a building material, BIPV products are significantly different from traditional component products. As a building material, they are closely connected with human life and are inseparable. Electrical safety has become a major consideration. Any BIPV product with a safe voltage higher than human needs will pose a threat to the safety of human life. The true BIPV products must first be thoroughly guarded against electrical voltage safety. Safe low-voltage products are undoubtedly the best solution.

10. Construction versatility:

As a building material product, BIPV products should be fully connected with the maintenance results of existing buildings, and be as universal as possible to achieve large-scale use of existing buildings.

The deterioration of the above-mentioned problems led to the failure of components, and even fire. It is not difficult to explain the reason why the rooftop photovoltaic power generation fire probability is much larger than the ground power station.