A Study on Prevention of Fire Proliferation in Building-Type Solar Modules

Abstract

To prevent the vertical spread of fire from flammable components in Building-Integrated Photovoltaic (BIPV) modules during building fires, we applied a fire-resistant (FR) coating technology to the surface of BIPV modules, which are commonly used in Zero Energy Buildings (ZEBs). By applying an acrylic FR coating to the BIPV module, we quantitatively evaluated the influence of heat damage before and after the FR coating, the average propagation rate of flames, and the module failure time in a combustion environment. The results demonstrate that the flame-blocking function and fire diffusion prevention effect of the FR coating are excellent in all combustion environments. Particularly, flame damage is minimized under the condition of an FR coating with a thickness of at least 50 μm. The current work suggests fire resistance mechanisms in various combustion environments and provides the applicability of FR coating technology on BIPV modules for fire non-proliferation.

1. Introduction

The frequency of fire incidents in high-rise buildings has been increasing in recent years. A significant case occurred on 8 October 2020, when a fire broke out on the third-floor terrace of the Samhwan Art Nouveau apartment in Ulsan, resulting in 26 people being hospitalized [1]. Strong winds caused the fire to spread rapidly through the building’s exterior insulation material, eventually engulfing the entire exterior wall of the 33-story building. Although large aerial ladder trucks were deployed for firefighting, they could only reach the lower floors. While firefighters contained the fire on the lower exterior walls, the chimney effect caused it to continue spreading upward, resulting in significant damage.

A similar incident occurred at the Wooshin Golden Suite in Haeundae, Busan. The fire, which started on the fourth floor, rapidly spread upward through the combustible aluminum composite panel cladding due to the chimney effect. Despite deploying numerous firefighting equipment and helicopters, emergency responders could not contain the rapidly ascending flames [2]. The building used aluminum composite panels consisting of 5 mm aluminum plates with a polyethylene (PE) synthetic resin core. This cladding material has a heat value equivalent to 3.8 L of gasoline per square meter of polyethylene. With one liter of gasoline having a heat value of 8300 kcal, this amounts to 31,540 kcal. Although PE insulation is a highly flammable polymer with a limited oxygen index (LOI) of 18 and does not readily burn even at 240 °C, the use of combustible adhesive to bond the aluminum plates and PE, combined with an air insulation layer between the composite panel and inner concrete wall, created a conduit for oxygen supply that facilitated the fire’s spread (see Figure 1, [3]).

To address these safety concerns, the Korean government is implementing the ‘Fire Resistance Performance and Fire Spread Prevention Structure of Building Finishing Materials’ standard to enhance the fire safety of building exterior finishing materials. Materials must satisfy performance criteria through testing according to the Korean Industrial Standard ‘KS F 8414 (Fire Safety Performance Test Method for Building Exterior Finishing Systems)’.

The majority of solar modules currently in use employ the GTB (Glass-to-Backsheet) method, valued for its cost-effectiveness and lightweight properties. This method utilizes a five-layer structure (tempered glass–encapsulant–solar cell–encapsulant–backsheet), with three layers containing combustible polymer materials. The sealant and backsheet consist of low-melting-point polymer organic compounds that can accelerate flame spread during fires [4,5,6]. The sealant, composed primarily of Ethylene/vinyl acetate copolymer (flash point: 350 °C), and the backsheet, made of Polyethylene terephthalate (flash point: 480 °C), represent significant fire safety concerns. Given their thermal decomposition characteristics and combustibility, conducting robust fire safety research is crucial for building-integrated photovoltaic installations.

This study proposes the application of fire-resistant (FR) coating technology, commonly used in building structural materials, to solar modules. Our approach aims to enable the safe use of GTB-type solar modules in buildings while maintaining the material properties of the combustible polymer components, such as sealants and backsheets. Through combustion testing, we quantitatively evaluated the fire spread rate and the extent of module damage caused by flames in solar modules.

2. Materials and Methods

2.1. PV Module Specimens and FR Coating

The feasibility of applying the FR coating to commercial solar panels was evaluated. A FRAFLOR solar module with a 1.6 W output was purchased. The FR coating, formulated with an acrylic base, was custom-developed by Samhwa Paint Research Institute. The coating was prepared by combining aluminum hydroxide (4 g), ammonium polyphosphate (APP, 4 g), paraffin chloride (7 g), melamine (1.4 g), xylene (19 g), 2-ethylhexyl methacrylate-styrene polymer (1.4 g), toluene (3 g), and aromatic hard naphtha solvent (0.1 g). The selection of materials, such as aluminum hydroxide, APP, and melamine, was based on their established roles as flame retardants and gas-expanding agents [7,8]. APP releases phosphoric acid to form a char layer, while melamine contributes to gas expansion, improving the heat-shielding effect [8,9,10,11,12]. Detailed ingredient information is restricted from disclosure as it is a trade secret of Samhwa Paint. Figure 2 is a photograph of the FR coating layer formed under the back sheet of the solar panel. A flame-retardant composition mixture was uniformly distributed on the surface of the rear back sheet of the solar panel using a brush. The FR coating was applied in multiple layers using a paint roller. Each layer was dried for 20 min, with a coating thickness of 25 μm per application. The coating thickness was measured using PCE-CT 80, and the average thickness from five repeated measurements was used. Four specimens were used in the combustion test. The coating thickness was chosen to evaluate the gradual effect of the FR coating on flame suppression and thermal protection, with 50 μm as a baseline and up to 200 μm to observe the performance improvement with increasing thickness. The specimen without the FR coating is marked ‘Raw’, and specimens coated with 50 μm are marked as ‘FR-1’. ‘FR-2’ is a specimen coated with the FR coating agent having a thickness of 100 μm, and ‘FR-3’ is a specimen coated with 200 μm.

These specimens were thoroughly dried for more than 24 h, and the FR coating layer was stably cured. The specifications and analysis equipment of the FR coating specimen used in the experiment are as follows.

• Power of the solar module: 1.6 W (270 mA, 6 V).
• Module dimensions: 110 × 110 mm², thickness 2.8 mm.
• FR coating thickness: 0 μm (Raw-1-3), 50 μm (FR-1), 100 μm (FR-2), 200 μm (FR-3).
• Microstructure analysis: DMi8 optical microscope (Leica Microsystems, Wetzlar, Germany); Merlin Compact scanning electron microscope with EDS (Carl Zeiss AG, Oberkochen, Germany).

2.2. Combustion Test

As shown in Figure 3, burner tests were conducted to directly irradiate the flame using a 1.5 kW burner on the rear surface of the solar panel. The burner was installed to ensure that the flame was incident vertically onto the module, and a flame was applied to each specimen for 30 s. In order to analyze the chemical reaction of the FR coating layer by flame and whether the heat-shielding layer could be properly formed, the prepared samples were adjusted to 700–800 °C with a burner using butane as fuel and air as a supporting gas. Then, the flame was irradiated for 30 to 40 s at 10 cm intervals on the back of the solar panel to analyze the thermal damage.

3. Results

3.1. FR Coating Reaction Mechanism

The chemical reaction of the FR coating and its mechanisms that can effectively suppress the fire propagation of solar panels in a combustion environment are as follows. When the FR-coated solar module is exposed to flames, the FR coating layer expands to form porous ceramic multilayers including a carbonized char layer and an oxide layer on the surface, thereby forming a heat-shielding layer having extremely low thermal conductivity. Figure 4 shows the conceptual diagram of the reaction mechanism of the FR coating layer applied to the solar panel.

• 1st step (FR coating): application of FR coating on the bottom of the back sheet of PV panel.
• 2nd step (chemical reaction): melting of FR coating by flame and initiation of chemical reaction.
• 3rd step (char expansion): release of N₂ gas from melamine and formation of porous char.
• 4th step (surface oxidation): stabilization of porous char and thermal barrier oxidation layer.

As shown in Figure 4a, solar panels are installed at a 10 cm air cavity interval from concrete outside the building. In step 1, the FR coating is applied to the backside of the PV panel. The FR coating layer is disposed to face the concrete surface of the building. In step 2, when a fire occurs in the building, the flame propagating along the outer wall first contacts the FR coating layer. The temperature of the FR coating layer rises due to the flame, and, upon reaching approximately 250 °C, APP begins to decompose, triggering the gas expansion reaction from melamine.

The FR coating layer includes a halogen component such as chlorine and fluorine, which may stabilize radicals generated from combustion gases during fire to minimize combustion. In the event of fire, combustible polymer materials (back sheets, junction boxes, etc.) of the PV module generate OH⁻ and H⁺ radicals during combustion and thus produce continuous heat. On the other hand, the halogen-based flame retardant includes F⁻ and Cl⁻, which are weak in activity, replace OH⁻ and H⁺ radicals, and interfere with combustion. In particular, APP is composed of polyphosphate chains formed by the condensation of ammonium ions (NH₄⁺) and phosphate molecules (H₃PO₄). It decomposes at about 250 °C to release phosphoric acid. When combined with polyol, this phosphoric acid forms a char that physically serves as a protective layer [11,12]. The APP reaction for char formation is below.

In step 3, the expansion of char formed by APP decomposition is activated. As the internal temperature of the FR coating layer increases to about 350 °C, APP (67 wt% nitrogen content) releases a significant amount of expandable gases (N₂, CO₂), facilitating the expansion and foaming of the carbonized layer. Nitrogen and carbon dioxide gas released from the decomposition of melamine expand the char into a porous form, thereby forming a porous heat-shielding layer that effectively protects the module from high-temperature flames [11,12,13,14]. The blowing agent melamine is below.

Therefore, the flame-retardant structure for solar panels forms an FR coating layer with increased adhesion properties to the back sheet by applying a flame-retardant composition to one side of the solar panel. The FR coating layer can be placed at regular intervals toward the outer wall of the building to maintain a side reaction, but, in case of fire, it is possible to minimize damage caused by fire in solar panel installations by suppressing heat shielding and smoke diffusion by directly contacting flames along the outer wall of the building.

3.2. Combustion Test Results

In the combustion test, the solar panel was mounted so that the flame was vertically incident, and a burner was used to create a high-temperature flame atmosphere of 700–800 °C. The flame was exposed to the solar panel for 30 to 40 s at a distance of 10 cm. The results of measuring the thermal damage area ratio of the solar panel and the expansion thickness of the FR coatings after the experiment are presented in Figure 5.

In the solar module Raw that is not provided with the FR coating layer, the rear damage area range reached 94%, and the front surface was also damaged by 92%. The back sheet was completely damaged in the area where the flame directly contacted. On the other hand, in the case of FR-1 to FR-3, in which the FR coating layer was formed, the rear damage area range was reduced to 29% only, with a thickness of 50 μm (FR-1), and the front damage range was only 27%, indicating that the FR coating layer exhibits very remarkable flame-blocking and suppressing effects.

Meanwhile, it was found that when the coating thickness of the FR coating layer increases, the char foaming thickness increased, and, at the same time, the protection function against flames was also improved. In this experiment, the thickness of porous char layers expanded from a minimum of 30 to a maximum of 100 times. The thickness of the FR coating layer is considered to be an important factor in protecting the module from flames. In the 100μm thickness (FR-2) condition, the damage area range had an average reduction effect of 21%. The average damage rate in the FR-3 specimen, which increased the FR coating thickness four times compared to FR-1, was only 17%. Although the thickness of the char expansion layer was not linearly proportional, the thermal damage of the modules could be further reduced with increasing the FR coating thickness. Therefore, it is concluded that the expanded protective layer reduces thermal conductivity and limits oxygen penetration, thereby enhancing flame suppression efficiency.

The changes in microstructure caused by flame diffusion after flame exposure of the solar panel, damage time of the solar cell, and expansion of the FR coating were analyzed. Figure 6 shows the average velocity of flame diffusion during irradiating flames on the rear of the solar panel. The flame propagated rapidly at an average rate of 1.3 cm/s in the solar panel (Raw). However, in case of the solar panels (FR-1, FR-2, and FR-3) with the FR coating layer, the average propagation rate of the flame was found to be 0.3 cm/s, 0.2 cm/s, and 0.2 cm/s for each specimen, respectively. In the FR-coated solar module, the flame diffusion rate was similar, exhibiting 0.3 cm/s or less, but a considerably lower value compared to specimen Raw. The results suggest that the diffusion of the flame could be sufficiently suppressed by a FR coating layer of at least 0.05 mm thick.

We further evaluated the time to EVA damage as a sealing material and peeling properties between the materials inside the solar panel, when the flame continuously irradiated on the back of the solar panel for more than 40 s. In the solar panel Raw, the solar cell was exposed to the outside by damage to the sealing material, and electricity production was stopped due to damage to the electric circuits such as bus bars and grid lines. In addition, all the materials in the module were separated due to a weakening of adhesive force. The weakening of the adhesive force might cause secondary accidents such as a crash of tempered glass, which would give rise to physical damage, which would give rise to physical damage. The solar panel (Raw) without the FR coating layer was completely destroyed in only 15 s, while the solar modules (FR-1, FR-2, FR-3) maintained a stable state without significant damage even over 40 s. It was revealed that safety in the solar panel should be secured by the application of FR coating.

Figure 7 shows the morphologies of the coating layer in which the combustion chemical reaction proceeded after irradiating the FR-coated solar panel with flames. In Figure 7a, the coating layer exhibits a uniform and smooth surface before flame exposure. When the temperature rises above 140 °C upon flame contact in Figure 7b, the APP component begins to melt, initiating thermal decomposition. In Figure 7c, the refractory coating layer expands significantly as bubbling occurs, driven by the release of gases such as nitrogen (N₂) and carbon dioxide (CO₂) from the thermal decomposition of expansion agents like melamine. The thermal decomposition of melamine complements APP’s char formation by releasing expandable gases, which create and stabilize the porous char layer, resulting in a synergistic effect on improving the heat-shielding and flame-retardant performance.

Finally, in Figure 7d, the thickness of the coating expands to approximately a maximum of 100 times its original value, forming a porous carbonized char layer internally and a mixture of carbonized and oxide layers externally. The porous structure acts as a thermal barrier by trapping gases within the charred layer, which significantly reduces heat transfer through convection and conduction [15]. The sponge-like porous morphology contains randomly distributed pores with an average diameter of 240 μm, which contributes to the thermal protection properties of the FR-coated layer.

Figure 8 shows the chemical composition analysis of the FR-2 coating layer before and after the combustion test using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis. It is observed that chemical components such as binders, flame retardants, and foams are evenly distributed in the FR coating layer. It contains elements such as phosphorus 14.43%, titanium 11.60%, aluminum 2.79%, nitrogen 9.43%, and chlorine 7.19%, which are the main components of flame retardants and binders. When a flame is irradiated on this coating layer, it undergoes bubbling, expansion, and foaming processes sequentially. After the combustion test, a lot of large and small pores were observed. The chemical composition changes were analyzed, focusing on pores with a diameter of 400 μm. The results demonstrate that the sulfur and phosphorus components decomposed and were substantially reduced due to the combustion reaction. The aluminum component was 3.94% and increased by 33% compared to before foaming (Al 2.97%). It is estimated that the component ratio increased by forming Al₂O₃ (alumina, 4Al + 3O₂ → 2Al₂O₃). Alumina’s low thermal conductivity and stability under high temperatures not only create an effective barrier against heat transfer and flame proliferation but also synergize with other components in the FR coating, such as char-forming agents and gas-releasing materials, enhancing overall fire resistance. The presence of metal oxides like Al₂O₃ reinforces the porous char layer, improving structural stability and further limiting oxygen penetration, which collectively contribute to the superior flame-retardant performance [16,17,18]. The formation of alumina inorganic oxides on the porous char carbonized layer plays a pivotal role in a heat-shielding function that acts as a PV panel protection against flames and blocks oxygen from being effectively introduced from the outside [18,19].

In summary, simply applying FR coating to the BIPV module can improve flame retardancy and stable heat-shielding performance. With only 50 μm of FR coating, the damage to the module caused by the flame was reduced to about 1/3, and the flame propagation rate was reduced to 1/4. In addition, increasing the thickness of the FR coating further improved the fire-resistance performance. In addition, a multi-stage chemical reaction mechanism applicable to BIPV modules was proposed. This reaction involves FR coating melting, expansion gas generation, porous char formation, and surface oxidation.

4. Conclusions

This study investigated the feasibility of applying the FR coating technology to mitigate fire proliferation and damage in building-type solar modules. An acrylic fire-resistance paint was applied to a commercial solar module, and a 700–800 °C combustion gas was applied to quantitatively evaluate the fire suppression effect. The results obtained are summarized as follows:

(1) The chemical reaction of the FR coating is temperature-dependent and acts as a heat shield through a multi-stage combustion reaction mechanism.

(2) The FR coating layer containing APP and melamine forms a char carbide by decomposition of APP and releases a large amount of expandable gas N₂ from the melamine component, forming a porous heat-shielding layer.

(3) The heat-shielding coating layer decreased the flame diffusion rate on the PV module by 25%. In the module without the FR coating, the diffusion rate of the flame was 1.3 cm/s, but, in the FR-coated module, the diffusion rate of the flame was reduced to 0.3 cm/s.

(4) In a combustion environment, the FR coating layer exhibited substantial effects on a repression of peeling between module materials and a delay in expansion of the encapsulant. In a module without an FR coating, the module was damaged by expansion of the encapsulant within 15 s, while the module including the FR coating layer maintained a stable state for more than 40 s.