Risk Analysis and Prevention of Fire Spread in Single-layer Roof Photovoltaic System

（JUAL Roofing Technology（苏州）有限公司，江苏苏州，215000）

With the rapid development of photovoltaic technology, more and more photovoltaic systems have been widely used in building roofs, especially industrial plants with large available areas, which have become the preferred carriers for industrial and commercial distributed photovoltaic power plants in recent years. During the power generation period of photovoltaic modules, the internal parts of the modules cannot be completely shut down, coupled with the more complex electrical structure and material characteristics, there is a certain risk of fire in photovoltaic systems. In the event of a fire, the rapid spread of fire between PV modules and roof structures may pose a serious threat to plant operations and personnel safety.

1. Causes of fire in roof photovoltaic power station The main causes of fire in roof photovoltaic systems can be summarized into the following three categories:

A. electrical factors Loose wiring: loose connectors or cables, resulting in poor contact, the formation of arcs, causing fire.

Short circuit: aging or improper installation of electrical components may cause short circuits in circuits, generate high temperatures and cause fires.

DC arc: solar power generation system DC voltage is high, if there is a break or poor contact, easy to produce arc.

B. component factors Hot spot effect:

When the photovoltaic module is partially blocked or damaged, the heat is concentrated in a specific area, which may cause the module to overheat or even burn.

C. external factor Poor grounding: unreasonable grounding system design, resulting in static accumulation or leakage accident. Extreme weather: natural disasters such as lightning strikes, storms, hail, etc. may cause component or system damage and cause fires. Foreign matter interference: such as dust deposit, fallen leaves, bird nest and other blocking component surface or compression cable, causing local overheating.

2. Analysis of Combustion Energy of Each Material in Single-layer Roof Photovoltaic System The energy released by combustion in single-layer roof photovoltaic systems can be estimated by estimating the composition of each material and its calorific value.

The main materials of photovoltaic modules include photovoltaic glass (70-75%, non-flammable), encapsulating material EVA/POE (10-15%, flammable, calorific value about 30MJ/kg), silicon wafer (5-8%, non-flammable), backsheet PET/PVDF (5-10%, flammable, calorific value about 30MJ/KG), metal conductor (aluminum frame, wiring, etc., 3-5%, non-flammable) and junction box (1-2%, flammable, calorific value about 30MJ/kg). The main materials on the single-layer roof include waterproof coiled material (TPO/PVC, combustible, calorific value 40/17MJ/kg), thermal insulation material (rock wool non-combustible, XPS combustible, calorific value about 40 MJ/kg) and vapor barrier layer (PE, combustible, calorific value about 40MJ/kg).

Take the common PV module size 2278× 1134mm in China as an example, its weight is about 27kg, in which the combustible material accounts for 25%(i.e. 6.75kg), and the energy released by combustion is 6.75kg× 30MJ/kg=202.5MJ; take the TPO waterproofing membrane + rock wool insulation material as an example, the weight of TPO waterproofing membrane is estimated as 1.5kg/m², the weight of PE vapor barrier layer is 0.4kg/m², i.e. the total combustible material is about 4.9kg, and the energy released by combustion is 4.9kg× 40MJ/kg= 196MJ