Bifacial Photovoltaics: Technology, applications and economics
Bifacial photovoltaic (PV) modules are able to utilize light from both sides and can therefore significantly increase the electric yield of PV power plants, thus reducing the cost and improving profitability. Bifacial PV technology has a huge potential to reach a major market share, in particular when considering utility scale PV plants. Accordingly, bifacial PV is currently attracting increasing attention from involved engineers, scientists and investors. There is a lack of available, structured information about this topic. A book that focuses exclusively on bifacial PV thus meets an increasing need. Bifacial Photovoltaics: Technology, applications and economics provides an overview of the history, status and future of bifacial PV technology with a focus on crystalline silicon technology, covering the areas of cells, modules, and systems. In addition, topics like energy yield simulations and bankability are addressed. It is a must-read for researchers and manufacturers involved with cutting-edge photovoltaics.
Other keywords: levelized cost; PV systems; bankability; PV-generated electricity; design; environmental conditions; reliability; bifacial gain; geographic location; characterisation; energy yield prediction simulation models; PV technologies market introduction; bifacial solar cells
- Book DOI: 10.1049/PBPO107E
- Chapter DOI: 10.1049/PBPO107E
- ISBN: 9781785612749
- e-ISBN: 9781785612756
- Page count: 300
- Format: PDF
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Front Matter
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1 Introduction
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Photovoltaics (PV) is becoming, much faster than anybody would have expected, the most cost-effective electricity source on earth. Not only that it is clean, low maintenance, decentralized and scalable - in some cases, the costs per kWh in large systems are already even cheaper than coal energy. In India and other sunny countries, planned coal plants were even cancelled already in 2017 in favour of PV systems. In a couple of years, PV will become an unbeatable electricity source, as there is still huge potential for cost reductions - some of that e.g. offers making full use of bifaciality in many applications. In 2017, about 100 GWp additional PV module installations have been added to the existing ca. 300 GWp - much faster than anybody would have expected. The most optimistic scenarios forecasted a 100 GWp market in 2022 - which happened now 5 years earlier. In 2020 or 2021, we will have a total of 1 TWp installed PV systems worldwide. In this chapter, we sketch a complete picture of PVs status, explain the role of bifaciality and predict what the importance of bifacial PV in future PV systems in terms of reduction of electricity generation costs will be.
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2 Bifacial cells
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In 2017, the majority of the PV modules installed are based on p-type silicon solar cells that feature a full-area aluminum rear contact and back surface field (BSF). Because of their fully covered, opaque rear side, these cells are unable to convert the light that falls on the rear side of the solar cells into electricity. On the other hand, solar cells with a rear side that is only partially covered with metallization (the so-called bifacial solar cells) are able to simultaneously and efficiently convert light that illuminates the solar cell from the front side as well as from the rear side. In this chapter, a short review of the history, physics, characterization, as well as a description of the five most common cell architectures of n- and p-type bifacial solar cells is given.
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3 Bifacial modules: design options, characterisation and reliability
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Bifacial modules can be applied for large PV plants as well as for residential (flat white roof) and more specific BIPV (facade) applications and can also open up new PV application opportunities like in sound barriers or other vertical installations (fences, balconies).For bifacial PV plants, the objective is to exploit the main bifacial benefit which is a large reduction of LCOE (due to higher energy yield) with a minimal technical change or investment.
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4 Simulation models for energy yield prediction of bifacial systems
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In this chapter, an optical, electrical and thermal model have been presented as the basic elements of a simulation model for the energy yield prediction of bifacial models and systems. Keeping in mind that for each of these models, several different concepts are viable and are under investigation by various research groups around the world, the model as published in [8] has been presented as an example. Thereby, an optical model for the rear side irradiance of bifacial PV modules, both stand-alone as well as in-field installed, has been established and after its implementation as a software tool, simulations of the energy yield for different scenarios have been conducted. Given appropriate weather data, simulations can be carried out for various locations. In combination with an electrical model, such tools allow for the estimation of the bifacial gain, i.e. the additional yield compared to a standard PV module, for various installation parameters, such as the tilt angle, installation height, distance between module rows and constant ground albedo coefficient.
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5 Bifacial PV systems and yield data (bifacial gain)
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A major motivation for bifacial photovoltaics (PV) is an expected additional energy yield, compared to monofacial panels, due to the two-sided light sensitivity. The potential for an improved module power output and energy yield was repeatedly demonstrated by simulations [1-8], measurements on stand-alone modules [9-14] or installations [15-19] in various orientations. However, uncertainties concerning the actual output of projected systems still deter possible investors. Even in the PV community, the real quantitative benefit due to bifaciality and the best-suited technical concepts are still under discussion [20-22], as reflected by numerous publications dealing with these topics.
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6 Impact of bifaciality on the levelized cost of PV-generated electricity
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The cost of electricity generated by photovoltaic (PV) systems is an important criteria that determines the competitiveness of PV in general compared to other - fossil and renewable - methods of electricity generation and that serves also to determine the best choice - from the economic point of view - in terms of PV module technology and system configuration for a given application and given specific geographical location. The reduction of the cost of PV-generated electricity is the driving force behind all research and development activities along the whole value chain of PV manufacturing, starting from the purification of the silicon feedstock and ending with the design and construction of PV systems and their components as well as of their efficient operation and maintenance (O&M). The levelized cost of electricity (LCOE) is a widely used metric that aims to include, on the one hand, the complete cost (e.g. in euro or USD) related to the construction and operation of a PV system and on the other hand, all factors that have an impact on the total electricity generated (in kWh) during the lifetime of the PV system. In the following, the concept of the LCOE and its application to PV will be shortly introduced and the impact of the use of bifacial - instead of monofacial - PV modules on LCOE will be discussed.
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7 Importance of bankability for market introduction of new PV technologies
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Whether a project is bankable or not, depends not only of the project, but depends also on the partners and how the partners are rated. The evaluation sheets and check lists of rating companies and banks have been introduced shortly within this chapter. New technologies with not existing deep experience, like bifacial energy gain, cannot get rated all its upside potential. For new technology a sound cooperation between technicians, project developers, engineers for simulation and banks are very important in order to make such a project feasible from the financing point of view. At the time of writing of the present book, bifacial system has a small market share and the value chain is not yet fully developed: energy yield simulation, demonstration plants and technical descriptions are not yet at a mature stage or no relevant track record exists. In the near future, bifacial PV systems are expected to gain more market share and, with improving track record, the technological risk perceived by banks and rating companies is expected to approach more and more the rating of comparable standard (monofacial) PV systems. The market is moving away from protected schemes like FiT to the common energy market. In the fierce competition, every financial and technical advantage plays an important role. Bifacial PV technology has some very strong advantages. It is sure that this technology will come in focus in the very next years.
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8 A "global" view on bifacial gain: dependence on geographic location and environmental conditions
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The simulation results presented in this chapter demonstrate how bifacial PV systems can bring a performance gain all around the globe, resolving a certain misbelief that they may only form a niche market restricted to specific locations or installations. Global predictions, based on irradiance, indicate bifacial gains in the order of 50% of the natural albedo for equator-facing systems with optimized tilt angle. In locations with low clearness index, the bifacial gain can even be higher. At system level the bifacial gain will be reduced by 20% to 50% relative to a free-standing panel, due to increased self-shading and to limitation of the diffuse irradiance on the rear by adjacent sheds. Even though the rear side irradiance of a bifacial module is inherently inhomogeneous, due to self-shading and external mounting structures, our simulations have shown that the RSD of the total irradiance does not exceed 5%, even at very high albedo, and will usually be much smaller. The simulation methods applied in this chapter and their ongoing integration into commercially available software tools, like PVsyst, will give system designers and installers more visibility on the configuration layout of a bifacial PV plant at their specific location, the associated bifacial gain and the effect on their return on investment.
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9 Summary and outlook
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The extremely hot summer in 2018 could have a similar consequence for the coal power plants as the Fukushima accident for nuclear power stations in 2011. The lack of cooling water had the consequence that many coal power plants had to be shut down during summer and more electricity from renewables was fed into the grid. Due to hotter and hotter summers the coal power plants have to be step by step replaced by renewables: this was also requested by the EU commission after the summer 2018. This will speed up PV installations in EU and world-wide again. We have summarised in our bifacial PV book that, in order to bring new photovoltaic (PV) technology into the market - even if it is only an evolutionary technology - much more has to be considered and worked on than just high power and low costs. A big challenge is how to make the technology bankable and how to reach and to convince the end customers. However, bankability still remains an issue. Therefore, setting standards, as well as create easy, understandable and comfortable simulations that are validated for their accuracy by a sufficient amount of case studies (field data), are very important issues. The chapters of this book describe step-by-step the technological, economical and commercial status of bifacial technology and sketch the future variety and fields of applications.
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Back Matter
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Supplementary material
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Errata Sheet for 'Bifacial Photovoltaics'
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There is an errata sheet for this book, containing corrected figures for chapter 6, Figures 4 and 5.
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