Inherent occupational health hazards in the production of solar grade silicon

Highlights

• Occupational health in the production of silicon polycrystalline is evaluated.

• A systematic evaluation of the Process Route Healthiness Index Methodology is used.

• Approaches to upgrade occupational health problems are presented.

• Key factors that influence the selection of the process are identified.

Abstract

Solar energy has become one of the most developed renewable energy sources in recent years. As with any energy source or product, there are health risks associated with the manufacturing of solar cells. And even though the photovoltaic industry uses far lesser amounts of toxic and flammable substances than many other industries, the use of hazardous chemicals can present occupational and environmental hazards. One of the most important aspects in the selection of new processes lies in the protection of workers’ health. Health risks can be reduced if a process is chosen properly and in preliminary phases. Since we have found that it is necessary to carry out an evaluation of the health risks to workers in the production of polycrystalline silicon for the manufacturing of photovoltaic cells, in this work we will use the Process Route Healthiness Index to quantify the health risk that each silicon production process represents (the higher the index, the higher the hazards). The polycrystalline silicon production processes evaluated with the healthiness index are: Siemens Process, Intensified Fluidized Bed Reactor Union Carbide Process, and Hybrid Process. Our results show that the Siemens Process is the healthiest process, but with the Process Route Healthiness Index values are closer to the Hybrid Process. Apart from this, a guide to the assessment of inherent occupational health hazards in Si_SG production processes was also developed, which provides results alike those to the PHRI methodology.

Introduction

In recent years, the energy industry has paid special attention to productivity improvement, to waste reduction and to quality control, all in the areas of research, development, and manufacturing. This is due not only to the consideration of cost reduction, but also to the awareness of sustainability increase in the manufacturing process (Cave and Edwards, 1997). Although it is known that the processes of obtaining non-renewable energy impacts the environment in various ways. The processes of non-renewable energy production by their nature turn out to be potentially dangerous for human and environmental health (Owusu and Asumadu-Sarkodie, 2016).

To achieve this, there are two approaches to make these processes healthier, safer and more environmentally friendly, called internal and external means (Hassim and Edwards, 2006). However, the use of internal media, commonly known as an inherent approach, turns out to be better, since it is based on the fundamental properties of the process, on the nature of the chemicals required by the process and on the conditions of the process (Adu et al., 2008; Warnasooriya and Gunasekera, 2017). If, in the inherent approach a chemical does not exist, it does not represent any danger. Therefore, the inherent approach requires less protection systems, which will make them more manageable (Edwards and Lawrence, 1993).

However, there are not many studies that assess the principles of inherent occupational health hazards in energy production processes from renewable sources. It is believed that renewable energy and its obtaining process turn out to be harmless. In spite of this, each one of the parameters or principles of health hazards has to be evaluated in order to compare and to decide which process is more appropriate under this approach.

Inside the renewable energies, the energy from the sun is the most abundant. It is estimated that it could cover around 35% of the total energy that the United States will require by 2050 (Fthenakis et al., 2009). Presently, research on the potential of solar energy continues on the economic, social and technical aspects, as well as being compared to the potential of fossil fuels. Contrary to fossil fuels, solar energy is based on cost per kilowatt and in recent years, the United States, China and countries in the European Union, have implemented initiatives to reduce the cost of solar energy per watt. In some cases, as in a project developed by First Solarse, it has managed to reduce the cost as far as one U.S. dollar per watt (United States Department of Energy, 2012).

Renewable sources have been steadily pairing up to fossil fuels in economic value; and, despite the idea that these are “clean resources”, they also represent a continuous struggle with the environmental and health risks that they themselves may cause. Solar industry is no exception. Nowadays, the massive production of solar panels has resulted in a problem that needs special attention due to the use of toxic compounds that are harmful for both humans and the environment.

Despite the aforementioned, there exist evidence that solar panel production is much safer for the environment and workers than fossil fuel energy production (Galland, 2012). However, this raises the question to the evaluation problem in health and environmental aspects in solar panel production. Even if the photovoltaic industry uses far fewer amounts of toxic and flammable substances than many other industries, the use of hazardous chemicals can represent occupational and environmental hazards. Nowadays, there are reports that consider health, environmental impact and industrial hygiene in the photovoltaic industry (Briggs and Owens, 1980; Taylor, 2010; Fthenakis and Moskowitz, 2000). These reports display discussions about aspects among the various technologies of photovoltaic cells production: monocrystalline and polycrystalline silicon cells, gallium arsenide cells, cadmium sulfide cells. However, none of these reports show in detail the health aspects that represent each of the processes for raw material production in the manufacture of cells.

There is a great array of materials for solar panel production, the leading technologies at a commercial level are silicon-based, whether it be monocrystalline or polycrystalline (Briggs and Owens, 1980). In 2010, silicon represented 88% in all the photovoltaic cells (Price et al., 2010). A key point in the manufacture of silicon based solar cells is the acquisition of raw material. The literature shows two industrial consolidated processes for the acquisition of silicon polycrystalline, the first one is the Siemens Process, which is the most widely used (Bye and Ceccaroli, 2014). The second one is the Fluidized Bed Reactor (FBR) from Union Carbide (Erickson and Wagner, 1952). Moreover, Ramírez-Márquez et al. (2018) proposed an improved FBR process, called Hybrid, which conceptually results in higher production of silicon polycrystalline, in addition to being suitable in economic, safety and environmental aspects (Ramírez-Márquez et al., 2019).

Even though in the work by Ramírez-Márquez et al. (2019) aspects such as economy, environmental impact and safety are addressed, it is important to make a detailed study of the evaluation of inherent occupational health hazards of the three processes; this, due to the nature of said processes, since these represent a real potential hazard to the operator's health, and they require the use of raw materials (in liquid, solid and gas state) with inherent toxicological properties which can represent a health risk (Warnasooriya and Gunasekera, 2017).

That is why a polycrystalline silicon production health risk evaluation must be a determining factor for selecting the best route. Although there is research that evaluates the inherent occupational health hazards issues in the early stages of design and help to choose the appropriate process route (Koller et al., 2000; Adu et al., 2008; Sugiyama, 2007).

In this work, we use the methodology of inherent occupational health hazards of Hassim and Edwards (2006) to assess the occupational health problems related in production of silicon polycrystalline in the three processes mentioned above. The Hassim and Edwards methodology (2006) is used because the technique takes into account both the hazard from the chemicals present, and the potential damage caused by the exposure of workers to chemicals. Assessing occupational health in all processes is of great importance since workers are exposed to dangerous chemical substances which can cause chronic diseases in the long run. With this in mind, it is necessary to identify hazardous substances and how to detect which parts of the processes cause the most damage in order to make improvements and prevent any type of incidents.