Energy Payback Time Report Final

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Energy Payback Time When assessing the sustainability of photovoltaic modules, one of the factors examined by investigators is the Energy Payback Time (EPBT). The usefulness of this specific metric is embedded in its ability to compare the total amount of energy used during the technology’s manufacturing and installation processes versus the amount of energy returned by the module while active.1 In fact, EPBT is defined as the simple ratio of total energy input over the total energy output per year. The result of such a quotient is the amount of years necessary for the module to completely recover the quantity of energy used to make it operational.2 The calculation for the EPBT of a certain photovoltaic involves first establishing certain parameters. These include, but are not limited to: the module efficiency, the irradiation level or amount of available sunlight and the lifespan of the equipment.1 The first technology discussed is mono-crystalline silicon. Although this photovoltaic is not one of the selections our consulting company wishes to evaluate, it is important to establish it as a control given that it is one of the most produced solar energy cells today. In its EPBT calculation, the following considerations were used: a lifetime of 30 years, an efficiency of 14%, and an irradiation level of 1700 kWh/m2/year. Bear in mind that the irradiation level used corresponds to the geographical location of Southern Europe, which in fact is extremely close to the world average irradiation level. In addition to that, two other important assumptions were utilized. The first was that the installation consisted of a grid-connected rooftop system, where the panels are connected to a utility grid and thus do not use batteries. The other was a performance ratio (PR) of 0.75. Recall that a performance ratio describes how efficiently a photovoltaic plant delivers energy to the grid after accounting for energy losses.3 After considering all these limitations, the EPBT obtained was 2.68 years, meaning that only after 2.68 years would the module have captured the same amount of energy used to produce it in the first place. 4 The second photovoltaic we shall assess is the CIGS technology. Before stating that the result of each EPBT calculation for the three different technologies are analogous, one must verify that the parameters selected are the same or at least similar to one another. Some of the assumptions made include a performance ratio of 0.75, an irradiation level of 1700 kWh/m2/year and an efficiency of 10.5%. Incorrectly, the study fails to specify the assumed module’s. Nevertheless, it is safe to speculate that a lifespan of around 25 years was utilized, given that it is the average industrial value for such technology. Lastly, one must note that the type of installation assumed for this specific calculation was the frameless rooftop module. Given these constraints, the EPBT computed was of 1.4 years. 5 The third module we shall examine is the organic photovoltaic (OPV). Because of the relative scarcity of information regarding OPVs, the calculation done for this particular equipment was derived from a process denominated ProcessOne, one of the first reported and most thorough industrial manufacturing processes for OPVs available. Similar to silicon and CIGS, the irradiation parameter used was of 1700 kWh/m2/year. However, the efficiency and lifetime assumptions, as expected, are not identical, but have instead been modified to 5% and 15 years respectively. Note that because OPVs are a relatively new technology, data regarding their performance in an industrial setting is based on models. It is believed that industrially an OPV can effortlessly reach an average efficiency of 5% and that was the efficiency the consulting group decided to use given the

nature of the project proposal presented. The last main assumption made was a performance ratio of 0.80. Thus, the EPBT value obtained was of 0.81 years. 6 Although at first the EPBT metric seems to be quite flawless providing a semi-complete assessment of the sustainability of each technology, a couple of points must be made regarding its faults. First, as evidenced above, a myriad of assumptions are made. Although these are not made thoughtlessly, it is important to bear in mind that they are still assumptions that might not reflect truth. These suppositions consequently lead to the problem of inaccuracy, making the EPBT value a metric that naturally lacks exactness and thus must always be used with caution. Furthermore, some of the studies performed in order to compute EPBTs fail to divulge important information. As evidenced with CIGS, the investigator failed to disclose the lifespan of the module making it difficult for one to assess the comparability of the three different studies. While the silicon and OPV calculations used efficiencies considered industrial averages, if the study done on CIGS utilized a value that cannot be easily reproduced in large-scale manufacturing processes, its EPBT result becomes impractical for our purposes. Along the same lines, some studies used models in order to predict parameters. Such was the case with the efficiency of OPVs. While the efficiency used for the silicon calculation was based off typical industrial considerations, the lack of such information for OPVs forced the examiner to utilize a model in order to speculate a likely industrial efficiency. Another flaw is that studies will sometimes use different parameters and because converting these to the same terms is quite complex, the EPBT values are ideally not analogous despite their proximity. For instance, the PR used in silicon and CIGS photovoltaics was of 0.75, while OPV followed a 0.80 ratio. The result of such ambiguity revolving EPBT calculations can be evidenced in the fact that there is a variety of publications showing different EPBT values even though many use similar conditions and assumptions. As a result, our consulting group reached the decision that despite the theoretical strength of the EPBT metric, its lack of precision forces one to lower its weight in a Pugh matrix analysis. Westin, Per-Oskar. "Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology ." Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology . (2011): 18. Web. 15 Oct. 2012. <http://uu.divaportal.org/smash/record.jsf?pid=diva2:389744>. 1

U.S. Department of Energy - Office of Energy Efficiency and Renewable Energy. National Renewable Energy Laboratory (NREL). PV FAQs. Washington, D.C.: , 2004. Web. <http://www.nrel.gov/docs/fy04osti/35489.pdf>. 2

"Performance Ratio: Quality Factor for PV Plant." SMA Solar Technology AG. N.p.. Web. 15 Oct 2012. <http://files.sma.de/dl/7680/Perfratio-UEN100810.pdf>. 3

Alsema, Erik, and Mariska Wild-Scholten. "13th CIRP Intern. Conf. on Life Cycle Engineering." 13th CIRP Intern. Conf. on Life Cycle Engineering. (2006): n. page. Web. 16 Oct. 2012. <ftp://ftp.ecn.nl/pub/www/library/report/2006/rx06041.pdf>. 4

Mariska, de Wild-Scholten. "Energy Payback Times of PV Modules and Systems." Workshop Photovoltaik-Modultechnik. Energy Research Centre of the Netherlands. Germany, Köln. 26 2009. <http://www.apollon-eu.org/Assets/20091218Energier%C3%BCcklaufzeiten%20f%C3%BCr%20PV-Module%20und%20Systeme%20%20deWild%20-%20final.pdf.>. 5

Garcia-Valverde, Rafael, Antonio Urbina, and Frederik Krebs. "Solar Energy Materials and Solar Cells."Solar Energy Materials and Solar Cells. 95.5 (2011): 1293-1302. Print. <http://www.sciencedirect.com/science/article/pii/S0927024810004770>. 6

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