INVESTIGATION OF THE EFFECTS OF PHOTOVOLTAIC (PV) SYSTEM COMPONENT AGING ON FIRE PROPERTIES FOR RESIDENTIAL ROOFTOP APPLICATIONS
By:
Nur Aliah Fatin Mohd Nizam Ong, Safety Engineering Interest Group (SEIG), Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia.
Mohd Zahirasri Mohd Tohir, Safety Engineering Interest Group (SEIG), Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia
INTRODUCTION
According to the International Energy Agency, worldwide energy demand is expected to rise significantly at a rate of 2.1% per year to the year 2040, in line with ever-increasing population growth and rapid industrial development [1]. This increasing demand necessitates higher electricity generation due to swift economic expansion and recent growth in developed and developing countries [2,3]. Presently, electricity is generated using conventional thermal power generation such as coal-fired, oil-fired or gas-fired power plants that convert the respective energy sources into electrical energy. Nevertheless, the burning of these energy sources also emits greenhouses gases which contributes to global environmental pollution and climate change issues [4]. Moreover, ever since the energy crisis in the early 1970s there has been an interest in exploring and venturing into renewable energy to meet this growing energy demand. Just like conventional power plants, renewable energy also derives its energy from natural sources. Therefore, renewable energy is preferred over fossil fuels because the sources are inexhaustible and do not run out over time, thus making it a more reliable source [1,5]. By 2050, renewable energy will be one of the major contributors to global energy consumption as projected by International Energy Outlook [6]. The abundance of solar energy is one of the best options to be used as a renewable energy source [7]. One of the leading-edge technologies that utilises the solar power is photovoltaic (PV) technology.
PV technology is a system used to harness the thermal radiation from the sun and convert it into electricity. PV systems are a relatively young technology and classified as an active solar technology which uses mechanical and electrical equipment in converting those sun’s energy. In 1954, researchers at the Bell Laboratories had successfully invented the first industrial PV application [8]. The team demonstrated their solar panel by using it to power up a small toy Ferris wheel and a radio transmitter [8]. These first silicon solar cells (also known as photovoltaic cells) reached about 6% efficiency in solar-electrical energy conversion, which had been acknowledged as a massive improvement in comparison to any previous solar cells [8]. Since then, PV technology has continued to evolve with regard to conversion efficiency and system reliability. Nowadays, this state-of-the-art technology provides a sustainable solution for the planet to reduce the worldwide greenhouse effect due to the burning of fossil fuels from conventional thermal power plants [9].
PV systems can be categorised into two types – off-grid and grid-connected. Generally, a PV system consists of PV panels and other mechanical and electrical components, for instance, PV cables, DC/AC inverters, connectors and isolators. However, the main difference between standalone off-grid PV systems and grid-connected PV systems is that off-grid PV systems have additional components, namely battery banks and charge controllers. In an off-grid system, the solar panels will convert the sun’s energy and store the energy for use at night or on cloudy days when the sunlight intensity is low. In contrast to the grid-connected system, the main grid will supply electricity to the household during any cloudy days and at night. Current PV power systems come in a variety of sizes based on the utilisation which can range from powering up any portable system, up to households or even for a large-scale utility application. Nowadays, the rooftop grid-connected PV systems have become the popular choice for residential building as the setup does not require additional land area and traditional electricity bills can be offset. Figure 1 shows the typical layout of a grid-connected PV system for rooftop application.
Figure 1. General layout of grid connected PV system according to MS 1837 which has been outlined by Department of Standard Malaysia together with SIRIM Malaysia [10]
FIRE HAZARD
In recent years, growth in installation of PV systems has occurred, which can be seen with the increasing amount of generation capacity. Although the initial installation of a PV system involves significant costs, in the long run, the operating and maintenance costs for a PV system are relatively low. While advances in PV technology have offered many benefits for energy generation, this young technology also raises concerns about fire safety and is often seen as a potential fire hazard [11]. Ideally, PV systems are a relatively safe and reliable technology [12]. The results of the ‘1,000 and 100,000 roofs program’ conducted in Germany concluded that PV system failures are scarce and mostly related to the PV equipment itself [13]. However, PV systems are often installed with very little consideration given to fire safety. As PV systems are part of the electrical family, they are also subjected to the typical types of electrical fire ignition [14].
The common primary ignition scenarios reported within internal PV systems are:
- DC arc fault and overheating due to error or poor mechanical installations [14-18]
- DC arc fault and overheating due to low quality of PV components [15,19]
- DC arc fault and overheating due to aging or degradation of PV components which leads to spontaneous ignition [14,15,18]
- Localised overheating due to hot spot effects [14,15,18,20,21]
- Defective or damage system components or product failure leading to localised overheating [15,16]
These common primary ignition scenarios show that the causes of fire in PV systems can be classified into DC arc fault and localised overheating of PV components. In comparison to AC arcing, DC arc faults are more hazardous as the voltage continues to remain once the arcing is established. Statistics recorded in the USA, Germany and Italy show that a large number of DC arc fault events in PV systems have led to fire and significant damage [14,22]. When a solar panel catches fire, it does not just result in the reduction of power generation but also emissions of toxic gas (e.g. HF and HCl), property damage, injuries and even death [15,17]. In 2009, a fire occurred on the membrane rooftop of a retail store in California, USA damaging 1826 PV modules [11]. In the same year, another 15 events of solar PV module related fire were incidents recorded in the Netherlands [15]. In 2012, fire associated with solar panels occurred in a warehouse in Goch, Germany burning approximately 4000 m2 of roof area [23]. Another large fire incident occurred in 2013 at a food warehouse in New Jersey, USA where the roof was covered by 7000 PV modules [23]. Unfortunately, the building and contents were completely destroyed in the fire [23]. According to an investigation conducted by the BRE National Solar Center in 2018, 13 of the 80 reported PV fire incidents resulted in casualties (10 injuries/psychological trauma and 3 fatalities) [17]. The Netherlands began an investigation in 2018 into a fire incident involving PV panels on the roof with the aim of clarifying whether solar panels were responsible following the recent rise in rooftop fire incidents [24]. In 2019, the Japanese government warned against the fire risk from rooftop installed PV systems due to the upsurge of fire incidents logged from 2008 to 2017 [25]. Therefore, it is recommended that the design stage of a PV system should also be extended beyond the efficiency and reliability by considering fire safety aspects as well [14].
WAY FORWARD
Other than installation errors, the usage of low quality and aging PV components also substantially contributed to the occurrence of DC arc faults as well as overheating incidents. Solar panels are usually installed with the intention of the system having a service life of 20 to 30 years. However, the PV panels and other PV components are constantly exposed to extreme weather, especially in certain countries where the climate is hot and humid, such as in Asia. According to Manzini et al., in 2015 there were no standards specifically focused on the fire behaviour of PV modules [26]. Hence, it is crucial to investigate the fire hazard of both new and aged PV modules, as well as the other PV components, by thermally characterising the component’s materials to evaluate the potential fire danger so as to help improve the fire safety associated with PV systems. Figure 2 shows several examples of damage to PV components due to fire.
Figure 2. (A) Burnt-out roof with parts of the PV system intact [17] (B) Remains of a DC isolator being disassembled and inspected in the laboratory [17] (C) Remains of a DC connector ablated by arcing and burnt insulations [17] (D) Damaged connector with contacts still intact and engaged [17]
Currently, a research team from the Safety Engineering Interest Group at the Universiti Putra Malaysia is at the initial stages of a project that is investigating fire safety of PV systems with a particular emphasis on aged systems and components. By investigating the thermal properties of the material, additional safety elements can be considered in the design phase to reduce the frequency and severity of fire incidents caused by PV electrical systems installed on residential rooftops. Accurate predictions of fire may enable the design of appropriate fire safety systems. Besides that, the results can be utilised for more sophisticated computational assessments to simulate real-scale fire scenarios. Lessons learnt from past fire incidents involving PV systems, will provide valuable information and data to develop fire safety strategies for PV systems that are based on real-world fire incidents.
REFERENCES
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