Photovoltaic Sustainability: Environmental Impact and Optimal Site Selection
Once plagued by high unit costs, rendering large scale roll out infeasible, solar photovoltaic technology has finally begun to penetrate consumer and industrial power generation markets. For all types of renewable energy generation, it is important to understand the overall environmental impact and sustainability of the energy source, which can be analysed through the lens of an Energy Life Cycle Analysis and an optimal site selection methodology for such technology.
With more photovoltaic systems being installed on top of private homes, Tesla giga factories, and in solar farms, it is important to understand any potential long-term environmental impacts that large scale implementation of photovoltaic technology might have. Furthermore, installing solar panels is often a long-term commitment due to their 25–30-year life spans and warrants careful consideration when selecting where to set them up.
Energy Life Cycle Analysis
An Energy Life Cycle Analysis (ELCA) is one of several ways to analyse the environmental impact that a particular technology produces and allows for a quick comparison to other technology. It quantifies the Energy Pay Back Time (EPBT), the number of years taken for the system to produce the same amount of energy used in manufacturing, operating, and decommissioning the technology. The lifecycle of most technology can be divided into an upstream, core, and downstream phase shown in Figure 1.
Manufacturing crystalline photovoltaic panels is an energy intensive process which dominates the lifecycle analysis as it constitutes between 50–75% of the cumulative energy demand associated with photovoltaic technology, shown in Figure 2 [1][2]. Solar panels require extremely high purity silicon, colloquially known as ‘9N’ or up to 99.9999999% purity compared to the 99.99% purity required for electronics grade silicon [3]. To put this into context, consider an average suburban house that can accommodate 40 m² of solar panels, roughly half the size of a badminton court. The total energy required to purify the silicon, cut the silicon into wafers, process and assemble the cells is approximately 48 GJ, which is the same as amount of energy that an average UK household consumes in approximately 10 months [2][4]. Similarly, the entire manufacturing process would release approximately 279 kg of CO2 emissions, roughly the same carbon footprint as 3 direct flights from London to Frankfurt [5].
More recent ELCA studies on solar photovoltaics, that examine the energy consumption of the upstream, core, and downstream phases, quote Energy Pay Back Times in the range of 1.01–2.3 years [2][6]. In comparison, offshore and onshore wind energy usually have EPBTs of 5–11 months [7]. One way to lessen the environmental burden of implementing solar photovoltaic energy is to further develop recycling processes for the aluminium scrap, glass scrap, and silicon. This will be of increasing importance as more solar panels will be reaching the end of their 25–30-year lifespan in coming years.
The recycling process for solar panels can be divided into disassembly, delamination, and material separation. Presently, due to various manufacturer’s designs, solar panels require manual disassembly. As the technology matures and an optimal design has been determined, disassembly processes can be further streamlined. Delamination involves removing the glass coverings and Ethyl Vinyl Acrylate (EVA) from the silicon solar cells themselves shown in Figure 3, which protects the solar cells against air and moisture. A common method to achieve this is to heat the modules to temperatures of 450–600 °C allowing the EVA layer to breakdown via thermal decomposition, enabling the recovery of the glass and direct reuse of the silicon cells [11]. Presently, the only method to achieve material separation is through chemical etching which uses chemicals such as bromine gas and nitric, hydrofluoric, and ethanoic acid to further separate the silicon from the other metal and glass compounds [13]. The delamination and material separation steps produce harmful gases and liquid waste products respectively, and further research into alternative methods may be required to mitigate these by-products.
Optimal Site Selection
Manufacturing solar panels is a resource intensive process, and therefore it is imperative to ensure that solar panels are placed in optimal locations to maximize electricity production whilst minimizing environmental impact. A common method to identify these locations is to prepare climate and terrain maps, assign a weighting to each map layer, and then produce a composite map based on the different weights of each map layer.
Geographic Information System (GIS) software has been widely implemented to identify suitable preliminary locations for construction of wind farms, solar farms, and other power generation sources. Amongst numerous studies, the following factors were consistently considered when determining the optimal site selection: distance to roads, distance to transmission lines, amount of solar irradiation, slope of terrain, land use, temperature, and humidity [8][9].
The GIS software is used to prepare maps of these input parameters and score them between 0 and 1 through fuzzy classification techniques, usually where a score closer to 1 indicates a stronger suitability, illustrated in Figure 4. Based on the seven factors above, the ideal location would be one that is close to access roads and transmission lines so that transportation and connection costs are minimised. Favoured locations exhibit low sloping terrain such that construction costs are minimised, and low temperature and humidity levels but high levels of solar irradiation to maximize solar panel performance.
The simplest and easiest method to implement to determine the weighting of each factor, is the Additional Hierarchical Process (AHP) method which essentially compares the importance of one factor against another in a 7x7 matrix based on the factors above. Naturally, some factors are in competition with each other, as possible locations which are in close proximity to roads may be situated in urban or densely populated residential areas rendering the possibility of large solar farms unlikely. Similarly, mountainous regions are often such that the performance of solar photovoltaic panels is improved. However, these locations are usually far from power transmission lines which makes construction difficult. Ultimately, this algorithm considers these contrasting factors and produces a weighting for each factor between 0 and 1.
Once a satisfactory weighting for each map layer is obtained, a composite map can be constructed which will help readers to quickly visualize suitable locations based on the various influencing factors, shown in Figure 5, with scores closer to 1 indicating a stronger suitability [10].
Conclusion
Make no mistake, solar photovoltaic energy is a promising form of clean energy and will play an instrumental role in helping many countries reach net zero by 2050. However, it is important to understand the environmental impacts that are associated with its production and operation so that we can strategically deploy them in optimal locations to maximise their energy output. Development of economical recycling processes is necessary to keep pace with the increased uptake of solar photovoltaic energy and the looming expiry of existing solar panels.
While most homeowners may not have the luxury of choosing where to install solar panels, aside from their own rooftop or backyard, this is an important consideration for large solar farms or factories that wish to use solar energy to offset their energy consumption. Furthermore, with increased uptake in installing renewable energy generation sources, this will inevitably result in real-estate competition for construction of these projects. The prudent use of site selection studies to guide policy makers and corporations will play a big role as we rise to the challenge of ensuring a greener future.
References
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