Life Cycle Assessment of tandem LSC-Si devices
Introduction
The continuous development of photovoltaic (PV) technologies focuses on reducing costs and enhancing efficiencies and stability for solar cells and modules [1]. Silicon (Si) remains the dominant commercial PV technology (above 90% share) in the industry [2], but alternative materials and structures are being reported. PV single-junction solar cell limiting power conversion efficiency (PCE), even with improvements, is approximately 30% (depending on specific assumptions) [3]. Tandem technology makes use of sub-cells optimized for each part of the spectrum, connected either electrically in series (two-terminal) or kept electrically separate (four-terminal), achieving higher efficiencies when compared with single junction solar cells [4,5]. Researchers have explored different organic and inorganic materials for tandem solar cells to produce higher PCEs [6].
Recent advances in luminescent solar concentrators (LSC), for example, present a method to capture diffuse sunlight and maintain a relatively low manufacturing cost [7], offering a way to optimise the value of tandem solar cells. The theoretical efficiency limits of an LSC that is optically coupled to a flat-plate Si cell (LSC/Si tandem) were explored for a four-terminal tandem structure. This structure exhibits strong power conversion efficiencies for potential high-performance PV [8].
An LSC structure consists of a polymer waveguide with embedded luminophores (photoactive particles), coupled to a PV collecting material [9,10]. Incident light is absorbed and re-radiated at down-shifted wavelengths by the luminophores – termed photoluminescence (PL) – within the polymeric layer. For additional light trapping, common LSC structures optically encase the luminophoric waveguide within filters that are designed to reflect wavelengths of light near the PL center [6]. For such an LSC/Si tandem module, the design necessitates integration of various optical and electrical components, as shown in Fig. 1.
The top filter transmits short-wavelength (300–550 nm) and long-wavelength (700–1100 nm) light for luminophore and subcell absorption, respectively. The LSC structure employs a one-dimensional photonic crystal (PC) structure as a top, wavelength-selective filter, using alternating layers of low/high refractive index material to achieve the notch-filter reflectance shape. In this analysis we assess two different configurations of LSC cell, which are differentiated by the top filter layers (dielectrics and polymers).
The dielectric PC consists of SiO2 (low index, ƞavg ≈ 1.45 visible) and TiO2 (high index, ƞavg ≈ 2.5 visible) and has approximately 100 layers each of varying thickness on the order of 50 nm. Such a filter can be constructed via sputter deposition and, in this study, is called top filter “a”.
The polymeric PC uses poly(methyl-methacrylate) (PMA) (low index, ƞavg ≈ 1.48 visible) and poly(styrene sulfide) (PSS) (high index, ƞavg ≈ 1.65 visible) and has approximately 1000 layers each of varying thickness on the order of 50 nm. Polymeric filters such as this can be fabricated using extrusion and roll-to-roll fashion. In this study the polymeric PC is called top filter “b”.
Together with the development of advanced technologies and processes there should also be a concern for the environmental impacts. Manufacturing a solar cell involves complex process steps and the use of diverse materials. Each process or resource has its peculiarities and can come from many different sources, involving a specific series of inputs (material or energy which enters a unit process [11]) and outputs (material or energy which leaves a unit process [11]). Each one of these processes and the use of every material has potential environmental impacts. Several tools and methodologies can be used to calculate those effects. Life cycle assessment (LCA) is a method used to evaluate potential environmental impacts associated with the production, use and disposal phases of a product during its lifetime. The depth of detail of an LCA study varies with each case and depends on the goal, scope definition and the assumptions made [11].
This LCA study can assist in sustainable technology development by focusing on the life cycle environmental consequences of a fresh technology that is still in the early stages of development. The advance of PV technologies and structures require comprehensive LCA studies to guide manufactures and policy makers to the best environmental choice of technology such as identifying opportunities to improve the environmental aspects of products at various points in their lifecycle, decision-making in industry, governmental or non-governmental organisations, selection of relevant indicators of environmental performance and marketing [11].
This report explores the potential environmental impacts from a four-terminal tandem LSC/Si solar cell [12], considering a standard commercially available passivated emitter rear contact (PERC) Si subcell. To date, the environmental analysis of this structure has not been reported to the best of the authors’ knowledge. Besides, as important outcome from this study is the publication of useful inventories for LCA studies, which are continuously changing for innovative Si technologies. There may be found one or more production steps that dominate the environmental cost of a product, as has been found for Si PV modules and for all the Si-based tandem modules studied to date [[13], [14], [15], [16]]. If so, LCA can identify the step(s) and illuminate the areas where attention should be paid to alleviate the environmental costs.
LCA is a reliable method to calculate and analyse environmental impacts and support claims. It provides designers, regulators and engineers with valuable evidence that help them to make decisions about each life stage of materials and products. The importance of conducting an LCA study in new technologies, such as LSC/Si tandem solar modules, is to identify the environmental hot spots in this new device in order to improve its environmental performance before it can be industrialised. LCA is also important during the development of new products when environmental footprint is important to the future marketing or cost structure of this product.
Section snippets
Methods
The goal of this LCA is to assess global warming potential (GWP), human toxicity potential - cancer effects (HTP-CE), human toxicity potential – non-cancer effects (HTP-nCE), freshwater eutrophication potential (FEuP), freshwater ecotoxicity potential (FEcP) and abiotic depletion potential (ADP), comparing LSC/Si tandem and Si solar modules. The reference and characterisation factor for each environmental impact category is shown in Table 1.
Results and Discussion
The environmental impacts from the PV technologies analysed are calculated based on the LCA methodology described previously and the collected inventory (Tables SI-1 and SI-2 in the SI).
Fig. 3 presents the results for GWP, HTP-CE, HTP-nCE, FEuP, FEcP and ADP of LSC/Si tandem compared to Si (PERC) solar modules.
The analysis of the results (Fig. 3) shows that the tandem technology's efficiency improvement influences the environmental outputs positively, due to the lower energy usage to produce
Conclusions
An LCA of LSC/Si (PERC) tandem solar modules was conducted to guide researchers, manufacturers and policymakers to the possible environmental impacts related to this new technology. A set of six environmental impacts, considered by the authors to be the most relevant for the studied product and materials, were analysed: GWP, HTP-CE and HTP-nCE, FEuP, FEcP and ADP. The results demonstrate that the increase in the efficiency of the cells and, consequently, of the modules caused by the use of
Acknowledgements
MML and RC acknowledge the support of the Australian Government through the Australian Renewable Energy Agency (ARENA, Grant SRI001). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. Additionally, the first author would like to acknowledge Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for her scholarship.
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