Energy Payback Time of a Rooftop Photovoltaic System in Greece

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS Energy Payback Time of a Rooftop Photovoltaic System in Greece To cite this article: E. Rachoutis and D. Koubogiannis 2016 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. Related content - Renewables: Light D Elliott - Thin film metallic glass as a diffusion barrier for copper indium gallium selenide solar cell on stainless steel substrate: A feasibility study Wahyu Diyatmika, Lingjun Xue, Tai-Nan Lin et al. - Photovoltaic application of Si nanoparticles fabricated by multihollow plasma discharge CVD: Dye and Si cosensitized solar cells Hyunwoong Seo, Daiki Ichida, Shinji Hashimoto et al. This content was downloaded from IP address on 26/12/2017 at 06:40

2 Energy Payback Time of a Rooftop Photovoltaic System in Greece E.Rachoutis 1, D. Koubogiannis 1* 1 Department of Energy Technology Engineering, Technological Educational Institute of Athens, Agiou Spyridonos Street, Egaleo, Athens, Greece * dkoubog@teiath.gr Abstract. Life Cycle Analysis (LCA) is an important tool to quantitatively assess energy consumption and environmental impact of any product. Current research related to energy consumption in buildings moves towards Nearly Zero Energy Building (NZEB). In such a building, an important issue concerns the energy production by renewable sources, including on-site production. The most feasible way to achieve renewable energy utilization in a building level in Greece is by using rooftop Photovoltaic (PV) systems, also promoted in the last decade by the national legislation concerning energy conservation measures. Apart from cost-related issues and payback times, Embodied Energy (EE) and Embodied CO 2 (ECO 2 ) emissions have also to be considered against the anticipated corresponding savings. Using a particular PV system as a case study, its basic constitutive materials are determined and their masses are calculated. Embodied energy values are estimated by using embodied energy coefficients available in the international literature. Considering a specific geographic location in Greece for the building on which the PV is installed, the annual energy generated by the system is estimated based on its performance data and curves. The Energy and CO 2 Payback Times (EPBT and CO 2 PBT) are estimated and assessed, as well as future work is suggested. 1. Introduction The rapid increase in the energy needs worldwide and the growing concern about global warming has led scientists during the last decades to search for new environmentally conscious electricity generation technologies. Given the unlimited supply of sunlight and the old idea of the conversion of sun power into electricity, one of the technologies that have met a rapid development is arguably the photovoltaic energy generation devices. Although such devices cause no greenhouse gas emissions and do not consume energy during their operation, they actually affect the environment and consume an amount of energy during their life. This energy and the related environmental impact concerns the export, transportation and processing of their constitutive raw materials, the manufacturing and construction of the devices, their transportation, distribution and installation to site, maintenance and replacement actions during their operational stage of life and finally their removal and disposal and/or recycling of materials. In order to completely assess the sustainability of such systems fairly and suggest possible improvements, the amounts of energy and emissions involved with them, which are known as Embodied Energy (EE) and Embodied CO 2 (ECO 2 ) emissions, have to be taken into account. Such an Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 analysis is usually realized in the context of a cradle-to-grave Life Cycle Assessment (LCA). In addition such data are valuable for the LCA of the whole building, since current research concerning energy consumption in buildings moves towards the Nearly Zero Energy Building (NZEB), in which an important issue is the energy production by renewable sources, including on-site production. The most feasible way to achieve renewable energy utilization in a building level in Greece is by using rooftop Photovoltaic (PV) systems. Such PV systems and motivation for their use have also been included in national legislation concerning energy conservation measures. A sufficient number of comprehensive studies which are concerned with the LCA of different PV systems and technologies can be found in the literature. Some representative works are presented below. In [1], the LCA of a poly-crystalline silicon PV system was presented and it was found that the gross energy required through the life cycle of the system was 1494 MJ/panel and the global warming potential 80 kgco 2 /panel. The Energy Pay Back Time (EPBT) of the system was found to be from 3.3 to 6.5 years depending on the geographic conditions. In [2], the greenhouse gas emissions of a rooftop multi-crystalline silicon PV under average Southern-Europe insolation (1700 kwh/m 2 /year) were calculated to be about grco 2 /kwh e. Moreover, in [3], the LCA of the components of a 3.5 MW multi-crystalline PV system was presented an embodied energy value of 542 MJ/m 2, an amount of greenhouse gas emissions of 29 kgco 2 /m 2 and an energy payback time of 0.37 years for average USA insolation and temperature conditions were estimated. In the present work, a typical commercial rooftop PV system is selected and installed to be considered in a particular location in Greece. Simulating its performance by means of freely available software, annual energy and corresponding emissions savings due to its operation are calculated. In addition, the amount of the initial embodied energy and emissions of this PV system are quantified and the Energy Pay Back Time (EPBT) and the CO 2 emissions Pay Back Time (CO 2 PBT) of the system are estimated and assessed. Comparisons with corresponding results from the literature are provided and future research is suggested. 2. Case study - Methodology In the present work, a typical commercial PV system was selected as a case study. This system was considered to be installed at a particular location in Greece. In order to calculate and assess the EPBT and CO 2 PBT quantities related to it, the following tasks had to be accomplished: Quantification of the performance of the PV system, i.e. calculation of the energy savings (E SAV ) and emissions (CO 2,SAV ) annual savings due to the utilization of the system. E SAV is calculated in the form of electrical energy (kwh e ). Estimation of the Embodied Energy (EE) and the Embodied CO 2 savings (ECO 2 ) of the PV system. EE is calculated in the form of primary energy (MJ p ). Calculation of the Energy Pay Back Time (EPBT), according to the formula EPBT[yr] = EE[MJ p ] / E SAV [MJ p /yr] (1) Calculation of the CO 2 emissions Pay Back Time (CO 2 PBT), according to the formula CO 2 PBT[yr] = ECO 2 [kgco 2 ] / CO 2,SAV [kgco 2 /yr] (2) It should be remarked that in order to apply equation (1), the energy savings E SAV have to be calculated in the form of primary energy (in MJ p ). The PV system that was selected as a case study herein consists of the following parts: Ten poly-si panels, each of them having a nominal power of 0.3 kw. Thus the nominal power of the whole system is 3 kw. These panels are accompanied by a 10-year guarantee and assurance that their efficiency will not be reduced below 90% during the first 10 years and 80% during the first 25 years of operation. An inverter with a nominal output power of 3 kw and a five-year guarantee. 2

4 Two aluminum bases, one for four and one for six PV panels, respectively. Two types of cables, namely 10m of solar cable for the DC part of the system and 20m of power supply cable for the AC part. The efficiency of such a PV system is generally influenced by the following parameters: The latitude of the place where it is installed (the lower the latitude, the higher the intensity of the solar radiation and the higher the efficiency of the panel). The sun height (higher production of electricity during summer than winter period). The orientation of the PV panel (south for the north hemisphere). The slope of the panel (ranging from 20 o to 40 o, depending on the location). The panel temperature (the lower the temperature, the higher the efficiency). The above PV system was considered to be installed at a particular location in Greece, namely on the roof of an apartment building at Piraeus, where latitude is 37 o 57'35" north, longitude is 23 37'38" east and elevation is about 34m above sea level. 3. Performance of the PV system In order to estimate the performance of the PV system, the free Software PVGIS [4] was utilized. Some data were required by the software for the simulation of the present case study. The data values used in the present study are provided for the sake of completeness below: The geographical data mentioned in the previous section. Built-in PVGIS database for the solar radiation, according based on the particular geographical data. Nominal power of the PV system: 3 kw. Losses due to temperature and low irradiance: 10.2% (using local ambient temperature). Loss due to angular reflectance effects: 2.7%. Other losses (concerning cables, inverter etc): 14.0%. Combined PV system losses: 24.8%. Figure 1 shows a screen printout of the PVGIS software during simulation. In particular, PV geographical information and performance data for the case under consideration are depicted. Figure 2 presents data for the PV system concerning the average daily (E d ) and the average monthly (E m ) electricity production (in kwh), as well as the average daily (H d ) and the average monthly (H m ) specific global irradiation (in kwh/m 2 ) received by the modules of the given system. In the same figure, E m and H m have also been plot per month of the year. According to the simulation results shown in Figure 2 (left), it was estimated that the annual energy production by the selected PV system in the particular location is 4010 kwh e /year, i.e. E SAV =4010 kwh e /yr. This has to be transformed into primary energy in MJ p /yr. According to the national regulation in Greece, the conversion factor that is used (in energy performance of buildings) to calculate primary energy from final energy use is 2.9 for electricity. Thus, E SAV = 4010 kwh e /yr * 3.6 MJ e / kwh e * 2.9 MJ p /MJ e = MJ p /yr, i.e. E SAV = MJ p /yr. Similarly, due to the fact that the conversion factor to calculate CO 2 emissions from final energy use is kgco 2 /kwh e for electricity, CO 2,SAV = kgco 2 /kwh e * 4010 kwh e /yr = kgco 2 /yr, i.e. CO 2,SAV = kgco 2 /yr. 3

5 Figure 1. Screen printout of PVGIS concerning geographical information and performance data for the case under consideration. Fixed system: inclination=31, orientation=0 (Optimum at given orientation) Month E d E m H d H m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual average Annual total Figure 2. Left: Average daily (E d ) and average monthly electricity production (E m ) (in kwh), average daily sum (H d ) and average monthly (H m ) specific global irradiation received by the modules of the given system (in kwh/m 2 ). Right: Monthly evolution of E m and H m. 4

6 4. Estimates for the embodied energy and CO 2 of the PV system In this section, the concepts of Embodied Energy (EE) and Embodied CO 2 (ECO 2 ) emissions are briefly presented. Then a reference is made to the methods of embodied energy calculation. Finally estimates for the EE and ECO 2 of the PV system under consideration are provided. 4.1 The concepts of embodied energy and CO 2 Three main stages can be distinguished during the life time of a product; the initial stage concerning its production, i.e. the period of time from the beginning of its construction till the moment it becomes operational, the period of time that the product is operational and the final stage of its life concerning its withdrawal and removal of its materials. The amounts of energy involved in the above stages sum up to the so called Embodied Energy (EE) of the product. In particular, the energy consumed during the first stage is the Initial EE (IEE). The energy consumed during the operational stage is the Recurrent EE (REE). Similarly let the energy concerning its withdrawal named after Withdrawal EE (WEE). Referring to a PV system, the IEE consists of two parts. The first one is the energy consumed directly (IEE D ) for actions related directly to site like transportation and installation. The second one is the energy consumed indirectly (IEE I ), upstream these activities for the manufacturing of the PV system, as well as for the production of all the materials required for it. REE concerns the energy to be consumed for the construction/materials/processes/equipment related to maintenance/replacement actions during the operational period of the PV system (e.g. for the replacement of the inverter). Finally WEE concerns energy related to withdrawal and disassembly actions. Like IEE, REE and WEE consist of two parts, one concerning direct and one concerning indirect energy consumption. Again the direct parts refer to energy consumed on site by equipment, tools, etc or to transport materials and products to the site, while the indirect parts refer to energy consumed to acquire process and manufacture the materials involved at the corresponding stage. In the light of the above, the total EE of the product can be written as EE = IEE D + IEE I + REE + WEE, where the first term concerns on site actions for the PV system, the second term concerns energy consumed upstream this activity, while the rest terms concern energy to be consumed downstream (with respect to time). A companion concept concerning the environmental impact of Embodied Energy is that of the embodied carbon emissions or Embodied CO 2 (ECO 2 ). This is accordingly defined for each of the EE terms mentioned above, to be the corresponding amount of CO 2 emissions to the atmosphere. In this context, ECO 2 of a product is defined to be the CO 2 quantity corresponding to its EE. 4.2 Methods of embodied energy calculation According to the literature [5], the two basic categories of methods for calculating EE rely either on the Process Analysis or the Input-Output (IO) Analysis. Each of them has its own pros and cons [5], so Hybrid Analysis methods have been proposed in order to combine the advantages of both approaches and facilitate more comprehensive and accurate analysis. Furthermore, hybrid analysis has been proposed in the forms of process-based and IO-based hybrid analysis [5]. Unfortunately, independent the method of analysis used, EE databases suffer from problems of variation and incompatibility [6]. In previous works by the second author, the focus was placed on the Initial indirect EE (IEE I ) of the materials and equipment comprising typical Hellenic dwellings. For example in [7] the EE of the Electro-Mechanical Installations items were estimated for typical Hellenic dwellings, while in [8] similar work has been conducted for the corresponding Civil Engineering (building construction) materials. In these works, a three-step methodology was followed. According to this, the first step is a material analysis, i.e. the breakdown of the product under consideration to its constitutive single materials. The calculation of the corresponding masses was the second step, while at a third step, EE and ECO 2 values were obtained by multiplying mass values by the corresponding EE and ECO 2 coefficients of the materials. Although, these coefficients are nationally dependent parameters, available values from the open literature were used [9], since there is lack of a comprehensive Hellenic 5

7 database. Since the coefficients used by [9] rely on process analysis, such an approach suffers from truncation errors [10]; the latter are related to the definition of the method boundary. So, although not being a rigorous approach, it provides the capability to obtain important practical results for initial guidance. 4.3 Calculation of the PV system EE and ECO 2 A typical PV panel consists of the following materials: Aluminum Frame, which is responsible for the stability of the whole construction. Solar Glass, for the protection of the solar cells. Polymer Material EVA (Ethylane Vinyl Acetate), placed in the front and back part of the panel, for the encapsulation of the solar cells. Solar Cells, constructed by a semiconductor like silicon, for the absorption of the photons. Back sheet. Junction Box. Apart from the panel, the other important components of a PV system are: The base, which is responsible for the proper support of the PV panel with the proper slope. The inverter to convert the DC output of the PV panels into AC, so that the connection with the electrical grid can be possible. The cables for the connection between the solar panels and the DC part of the inverter, as well as the AC part of the inverter with the electrical grid. In [11], a detailed Life Cycle Inventory of the PV system under consideration has been conducted. In what follows, the main results are presented. The EE and ECO 2 coefficients [9] that were utilized are presented in Table 1. Due to the fact that the ECO 2 of silicon was not available, the value kgco 2 /kg from the literature [12] was used. Table 1 EE and ECO 2 coefficients from [9] utilized in the present work. Material EE [MJ/kg] ECO 2 [kgco 2 /kg] Αluminium Cermics Copper Plastic Iron PVC Rubber Stainless Steel Silicon Glass EVA Table 2 summarizes results for the whole set of materials used in the PV system. The results concern the mass, EE and ECO 2 contribution of each material to the system, both in terms of absolute value and percentage. The latter, namely the percentage contributions of each material in terms of mass, EE and ECO 2 are schematically shown in Figure 3 (left). It is obvious that glass and aluminum are the dominant materials in terms of mass, while silicon is ranked third. However, in terms of EE and ECO 2, silicon has the greater impact due to the high corresponding coefficients, while glass is ranked third due to low such values. Aluminum is at the second position both in terms of mass and EE / ECO 2, contributing about one third of the total PV system values. The contribution of these three materials (glass, aluminum, silicon) is summed up to about 90% for mass, 95% for EE and 96% for ECO 2. 6

8 In Figure 3 (right) the percentage contribution of the PV system basic items to the total mass, EE and ECO 2 values are depicted. According to this, among the basic items of the PV system, the PV panels are dominant in mass and exhibit the greater energy and environmental impact. Table 2 Mass, EE and ECO 2 contribution of each material to the whole PV system, both in terms of absolute value and percentage. Material Mass [kg] EE [MJ] ECO 2 [kgco 2 ] Mass [%] EE [MJ] ECO 2 [%] Solar Glass Aluminum Silicon Copper EVA PVC Galvanized Steel Plastic Rubber Iron Ceramics Total Figure 3. Left: Percentage contribution of each material in the PV system in terms of mass, EE and ECO 2. Right: Percentage contribution of the PV system basic items to the total mass, EE and ECO Results and discussion Energy production by renewable energy technologies is an environmentally friendly procedure, as CO 2 and other GHGs are not emitted and energy is not used during the stage of operation. However, such technologies like PV panels do emit CO 2 and other gasses and consume energy during the stage of their construction, transportation, collection of the required raw materials, etc. This is exactly what the concepts of the Energy Pay-back Time and Carbon Dioxide Pay-back Time represent. Based on the results obtained in sections 3 and 4 and according to equations (1) and (2), these two quantities for the case under consideration give the following values: EPBT = ( MJ p ) / ( MJ p /yr) = 1.4 yr = 16.8 months 7

9 CO 2 PBT = (2610 kgco 2 ) / ( kgco 2 /yr) = 0.7 yr = 8.4 months The payback times obtained above are judged to be rather low. In order to assess them, they were compared with corresponding values from similar studies in the literature [13], [14]. Table 3 presents this comparison, as well as the location of the place where each study has been conducted and the corresponding annual received energy intensity. According to it, EPBT is lower in Southern Europe compared to that of Switzerland (Central Europe). This is logical, since at lower latitudes (Southern Europe) the solar radiation is greater, so the denominator at equation (1) becomes greater and EPBT is reduced. Moreover, the present result is lower than that of [14], although the locations exhibit rather similar received energy values (actually, the denominator is slightly greater due to the higher received energy of 1780 instead of 1700 kwh/m 2 /yr). What is important is that in the present study an underestimated EE value at the numerator of equation (1) has been used. This is due to the fact that only material production and not manufacturing EE has been taken into account (due to lack of information), as well as energy consumed for transportation and installation of the PV system to site (i.e. direct energy) has not been considered. Table 3 Comparison of EPBT with corresponding values from similar studies. Reference Location Energy received [kwh/m 2 /yr] EPBT [yrs] [13] Switzerland [14] South Europe Present Greece Assuming that for the PV system under consideration producing 4010 kwh e /yr, the efficiency of the PV panels remains unaffected, the following environmental impact can be estimated: 32.5 gco 2 /kwh e for 20-years operation, 26 for 25-years operation and 21.7 for 30-years operation. The latter value is compared in Table 4 with corresponding values from similar studies (also concerning 30-years operation). As it can be seen, the emissions per unit of annual energy production are underestimated. This is attributed to the same reasons explained before. Table 4 CO 2 emissions per unit of annual energy production. Reference [2] [15] [16] Present [gco 2 /kwh e ] ~22 5. Conclusions Future research A rooftop photovoltaic system was studied. Assuming the location of installation and using open simulation software, the annual energy production was calculated. In addition, the energy embodied in the system was estimated and the energy payback time was evaluated. Similarly, the corresponding CO 2 payback time was quantified. Both values were underestimated compared to the literature. This was attributed to the greater energy production in Greece with respect to locations used in the literature, but mainly to the underestimation of the embodied energy. Among the basic issues that are missing in the present procedure is to take into account the energy required for the manufacturing of the PV system, not only the energy related to the production of its constitutive materials. As a continuation to this research, it is planned to estimate the manufacturing energy consumed in the production line of a typical photovoltaic system, make a thorough literature review on the topic to perform comparisons and study how energy payback time is affected by different installation sites in Greece. The aim in the long-term is to obtain in a more reliable way representative values of the embodied energy and emissions concerning PV systems operating in Greece (for example in order to use them in LCA studies of Near Zero Energy Buildings). Acknowledgement The authors would like to acknowledge company Aleo Solar Greece and especially Mr Panagiotis 8

10 Fragkos for providing practical information and valuable data. References [1] Stoppato, A., Life cycle assessment of photovoltaic electricity generation, Energy, 33(2): , [2] Fthenakis, V. and Alsema, E., Photovoltaics Energy Payback Times, Greenhouse Gas Emissions and External Costs: 2004-early 2005 Status, Progress in Photovoltaics: Research and Applications, 14(3): , [3] Mason, J.E., Fthenakis, V.M., Hansen, T. and Kim H.C., Energy Pay-Back and Life Cycle CO2 Emissions of the BOS in an Optimized 3.5 MW PV Installation, Progress in Photovoltaics: Research and Applications, 14: , [4] [5] Dixit, M.K. A Framework for an Improved Input-output-based Hybrid Method for Embodied Energy Calculation. Proc. 51st ASC Annual International Conference hosted by Texas A&M University in College Station, Texas April 22-25, 2015; [6] Dixit, M.K., Fernandez-Solis, J.L., Lavy, S., Culp, C.H., Need for an embodied energy measurement protocol for buildings: A review Paper. Renewable and Sustainable Energy Reviews, 16(6): , [7] Koubogiannis, D.G. and Balaras, C.A. Embodied Energy in Electro-Mechanical Installations of Hellenic Dwellings. Proc. International Conference Energy in Buildings, Athens: ASHRAE Hellenic Chapter and Technical Chamber of Greece; [8] Syngros G. Embodied energy and embodied CO 2 estimation of building constructive materials in typical Hellenic dwellings, MSc Diploma Thesis, MSc in Energy, TEI Athens and Heriot- Watt University, [9] Hammond, G.P. and Jones, C.I., Inventory of Carbon and Energy (ICE) Version 1.6a. Sustainable Energy Research Team, Department of Mechanical Engineering, University of Bath; [10] Lenzen, M., Errors in conventional and input-output based life-cycle inventories. Journal of Industrial Ecology, 4: , [11] Rachoutis, E., Life Cycle Assessment of a Rooftop Photovoltaic System, Diploma Thesis (in Greek), Energy Technology Engineering Department, Technological Educational Institute of Athens, Greece, [12] Kemmoku, Y., Ishikawa, K., Nakagawa, S., Kawamoto, T. and Sakakibara, T., Life Cycle CO 2 emissions of Photovoltaic/Wind/Diesel Generating System, Electrical Engineering in Japan, 138(2):14-23, [13] Alsema, E.A. and de Wild-Scholten, M.J. Environmental impacts of crystalline silicon photovoltaic module production, LCE2006, 13th CIRP International Conference on Life Cycle Engineering, Leuven, Belgium, [14] Jungbluth, N. Tuchschmid, M. And de Wild-Scholten, M., Life Cycle Assessment of Photovoltaics: Update of ecoinvent data v2.0, ESU-services Ltd. Working Paper, download from: [15] Tripanagnostopoulos, Y., Souliotis, M., Battisti, R. and Corrado, A. Energy, Cost and LCA Results of PV and Hybrid PV/T Solar Systems, Prog. Photovolt: Res. Appl., 13: , [16] National Renewable Energy Laboratory (NREL), USA, Life Cycle Greenhouse Gas Emissions from Solar Photovoltaics 9