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Home My WebLink About2017 Solar Published ArticleAn Alaska case study: Solar photovoltaic technology in remote microgrids Erin Whitney, and Christopher Pike Citation: Journal of Renewable and Sustainable Energy 9, 061704 (2017); View online: https://doi.org/10.1063/1.4986577 View Table of Contents: http://aip.scitation.org/toc/rse/9/6 Published by the American Institute of Physics Articles you may be interested in Wind power project size and component costs: An Alaska case study Journal of Renewable and Sustainable Energy 9, 061703 (2017); 10.1063/1.4986579 Preface: Technology and cost reviews for renewable energy in Alaska: Sharing our experience and know-how Journal of Renewable and Sustainable Energy 9, 061501 (2017); 10.1063/1.5017516 An Alaska case study: Energy storage technologies Journal of Renewable and Sustainable Energy 9, 061708 (2017); 10.1063/1.4986580 An Alaska case study: Electrical transmission Journal of Renewable and Sustainable Energy 9, 061702 (2017); 10.1063/1.4986582 An Alaska case study: Diesel generator technologies Journal of Renewable and Sustainable Energy 9, 061701 (2017); 10.1063/1.4986585 An Alaska case study: Biomass technology Journal of Renewable and Sustainable Energy 9, 061705 (2017); 10.1063/1.4986578 An Alaska case study: Solar photovoltaic technology in remote microgrids Erin Whitney and Christopher Pike Alaska Center for Energy and Power, University of Alaska Fairbanks, P.O. Box 755910, Fairbanks, Alaska 99775-5910, USA (Received 6 June 2017; accepted 29 September 2017; published online 21 December 2017) Solar photovoltaic (PV) technology is a nascent but promising energy option in remote Alaskan microgrids and serves as an example for isolated electrical grids worldwide. This study examines community scale solar PV installations in Alaska, ranging in size from 2.2kW in Ambler to 50kW in Galena. Total installed costs arguably show a trend toward lower values with larger installation sizes although prices in Alaska are still signiﬁcantly higher than in the rest of the United States. Capacity factors range from 6% to 15%. However, it should be noted that some installation conﬁgurations, particularly in the northwestern part of the state, were installed with the goal of a broad production curve rather than maximum power production.Published by AIP Publishing.https://doi.org/10.1063/1.4986577 INTRODUCTION Signiﬁcant volatility in fuel prices in the past decade and concerns over energy security have thrust isolated grids such as those found in rural communities, island states, and remote military installations using conventional fossil fuel power generation into an energy crisis. Many of these remote locations are turning to renewable energy to reduce fuel consumption and costs and to ensure a more independent and reliable energy source. Over 200 remote communities in Alaska are largely dependent on diesel generators and arguably have the highest electric rates in the nation due to the logistics of importing fuel. These stand-alone village microgrids typically serve 300–450 people with average loads of 200kW (AEA, 2011). As such, the microgrids provide ideal laboratories to test and validate variable generation and load control strategies with broader applications to high-penetration renewable islanded systems globally. The renewable energy contribution in many small Alaska communities is already much higher proportionally than what utilities in larger grids nationally would even consider although they are on a trajectory that will require doing so in future operations. One of these renewable energy sources is solar photovoltaic (PV) power (Schwabe, 2016). Although Alaska’s high latitude creates large ﬂuctuations in sunlight throughout the year, com- puter simulations show the solar PV potential in Alaska to be on a par with or greater than that in Germany, the largest solar PV power market in the world (Wirth, 2015). Furthermore, Alaska’s cold temperatures increase system voltage, reduce electrical resistance, and yield higher-than-rated outputs associated with reﬂected light and albedo effects (Nelson, 2003 and Brennan et al., 2014). These factors, combined with declining module prices, are making solar PV technology more economical. Solar PV arrays have been installed in all areas of the state from the southwest to the Arctic, and low sun angles and long daylight hours represent opportu- nities to mount panels vertically on walls as well as on the east and west sides of buildings. This review of solar PV technology in Alaska is a result of Alaska Senate Bill (SB) 138. In this bill, the Alaska State Legislature created an uncodiﬁed section of law entitled: “Plan and Recommendations to the Legislature on Infrastructure Needed to Deliver Affordable Energy of the State to Areas That Do Not Have Direct Access to a [proposed] North Slope Natural Gas Pipeline.” To support the Alaska Energy Authority (AEA) in its development of 1941-7012/2017/9(6)/061704/9/$30.00 Published by AIP Publishing.9, 061704-1 JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 9, 061704 (2017) an Alaska Affordable Energy Strategy, the Alaska Center for Energy and Power (ACEP) con- tracted with AEA to document technology development needs speciﬁc to Alaska with regard to renewable and sustainable energy technologies. The intention was to determine what tar- geted, energy technology development solutions could be implemented in Alaska to make energy more affordable in the Alaska Affordable Energy Study area. While the focus was on technology research solutions, other factors such as logistics, labor, and training were also addressed. Drafts of technology reviews were vetted by expert roundtables in late February and early March 2016. These reviews are not meant to be exhaustive discussions of energy technologies in Alaska or proper designs for each technology, and they should not be used as guides for the choice and installation of speciﬁc systems. As such, not all possible issues with power production and each technology are addressed. Data for each technology were collected from surveys and pub- lically available databases. Only completed projects, or projects with clearly reported data, were included in each technology analysis. These distinctions and descriptions of data sources are included in each technology review. METHODS To obtain information regarding the current state of the solar industry in Alaska, we con- sulted installers, community development staff, and Alaska Energy Authority (AEA) staff. Many of the systems installed in communities around the state are currently being monitored, and data are available via online portals. Cost information is harder to acquire. For state- funded projects, cost information is available from the AEA, but few projects have been funded by the state. Cost information is sometimes available via community development staff. This case study covers community installations that range in size from 2.2kW in Ambler to 50kW in Galena. Signiﬁcant data collection is still needed for speciﬁc details such as module technology type, mounting types, and other characteristics that can help to further reﬁne analysis. DISCUSSION To illustrate the solar resource in Alaska, Fig.1 shows the expected average daily solar radiation levels for varying surface angles for different cities in Alaska relative to Seattle and Phoenix in the continental United States (ACEP, 2012). Anchorage (61 N) is located in south- central Alaska, Fairbanks (65 N) is in the interior, and Kotzebue (67 N) is in the far north. Figure 1(c)shows the amount of solar radiation that horizontal collectors would be expected to receive throughout the year, with the highest radiation levels in May and June. In the graph showing the collectors tilted at 15 steeper than the latitude angle and tilted vertically, a strong improvement in springtime performance is seen. The graphs in the ﬁgure show that Fairbanks and Kotzebue receive almost 5kWh/m 2 day in March and almost 6kWh/m 2 day in April. This early season performance improvement is attributed to more direct radiation from the low sun angles and high levels of reﬂected radiation from the snow-covered ground. Signiﬁcant space- heating demands also coincide with this springtime arctic solar resource. While solar PV power is not a viable year-round resource for Alaskan communities, it can be of use for seasonal applications and paired with other energy sources for winter energy demands. Figure 1 uses data from the National Renewable Energy Laboratory (NREL), collected between 1961 and 1990 and based on averaged values of radiation. It does not account for weather patterns and cloud cover. In the NREL model, the albedo of snow was taken into account to calculate reﬂected radiation (NREL, 1992). Surface albedo was adjusted depending on the presence of snow cover. If there was snow on the ground, the surface albedo was set to 0.6 (albedo for snow ranges from 0.35 for old snow to 0.95 for dry new snow). If no snow was indicated, the surface albedo was set to 0.2, a nominal value for green vegetation and some soil types (NREL, 1992). 061704-2 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) Total installed costs Total installed costs in $/W plotted as a function of installation size show a trend toward lower costs with larger installation sizes, as seen in Fig.2. In this case, total installed costs are the sum of labor, parts and materials, and shipping. In Alaska, the 6.7kW installation in Galena ($3.19/W) and the 18kW installation in Fort Yukon ($3.89/W) were accomplished with creative means to cut costs. In Fort Yukon, these means included volunteer labor and a shipping deal. For a number of other installations, ﬁgures are based on verbal estimates from batched pur- chases and are not public record. The inconsistency of information is indicative of the nascent solar PV industry in Alaska. In general, however, prices in Alaska are still higher than prices in the contiguous United States. According to the Lawrence Berkeley National Laboratory (LBNL) report, “Tracking the Sun VII” (Barbose et al., 2014), in the Lower 48, “Installed FIG. 1. Expected average daily solar radiation levels for varying surface angles by city in Alaska, compared with Seattle and Phoenix. 061704-3 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) prices exhibit signiﬁcant economies of scale, with a median installed price of $4.8/W ($4800/ kW) for systems 2kW completed in 2013, compared to $3.1/W ($3100/kW) for commercial systems >1000kW” (p. 2). Operation and maintenance (O&M) costs The cost calculation for operation and maintenance (O&M) of a PV system is an area of increasing interest. Most systems around the United States have been installed within the last 8years, and limited O&M cost data exist (Enbar et al., 2015). In Alaska, most grid-tied PV sys- tems have been installed for less than 5years. According to the Electric Power Research Institute, O&M costs include scheduled maintenance and cleaning, unscheduled maintenance, and inverter replacement reserves, with costs up to $47/kW/yr for non-tracking systems (Enbar and Key, 2010). The O&M ﬁgures from a report by Black and Veatch (2012)and by the LBNL (Bolinger et al., 2015) are $20–$50/kW/yr for non-tracking PV systems. Obviously, this range is wide due to limited data and the short amount of time that grid-tied PV systems have been installed. In addition, industry’s best practices are just beginning to emerge. The Cold Climate Housing Research Center (CCHRC) has some of the oldest grid-tied solar installations in Alaska; it maintains three pole-mounted PV systems on two-axis tracking systems with a total installed size of 8kW. A relay has needed replacement, but otherwise very little maintenance has been required. According to staff at the CCHRC, 4h of maintenance are devoted to the systems per year (2h twice each year). Assuming $60/h, yearly maintenance costs equal $30/kW/yr, without taking into account inverter replacement. The trackers are locked at a ﬁxed angle of 80 degrees azimuth facing due south between November and February, when solar insolation is at a minimum and temperatures are coldest; they are set to track the rest of the year. One aspect of O&M in Alaska that deserves special mention is that of snow clearing. A study by students at the University of Alaska Fairbanks involved simulating the cost and beneﬁt of clearing snow from a hypothetical 1MW solar installation that faced south at a panel angle of 70 . The study plainly demonstrated that the cost savings from increased generation of elec- tricity due to snow having been cleared from the panels did not justify the cost of labor to per- form the task of clearing snow. This study was performed in Fairbanks, where winds are light and extended cold temperatures cause snow that occurs in fall and winter to remain on the ground into springtime. The results would likely be the same, if not more exaggerated, in Western Alaska, where high winds blow and mid-winter warm-ups melt snow from roofs (Vilagi and Brown, 2015). During discussions with a number of individuals involved in the solar industry in Alaska, it was generally agreed that O&M costs might be approximately $100 per installed kW of PV power on the high side [Most solar systems within Alaska have been installed in the last FIG. 2. Total installed costs ($/kW) as a function of installation size (kW) show a trend towards lower costs with larger installation sizes. 061704-4 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) 5 years, and little maintenance has been needed. The ﬁgure of $100/kW was reached after dis- cussions with Ingemar Mathiasson (Northwest Arctic Borough), Robert Bensin (Bering Straits Development Company), Jeremy Osborne (Yuut Elitnuarviat), and David Pelunis-Messier (Tanana Chiefs Conference)]. Note that many of the PV arrays installed around the state have not needed any maintenance since installation. Given all the documents reviewed to date, for PV systems less than 20kW in Alaska, O&M likely ranges from $50/kW/yr on the road sys- tem or in hub communities to $100/kW/yr in more remote areas. Operation and maintenance costs are not completely dependent on the system size; they are also a function of the level of local expertise available for repairs, the cost of travel to and from the site, occasional cleaning and inspection, unscheduled warranty work, and inverter replacement reserves. Expected life Most installers assume a system life of 25years although they are useful to consider expected lifetimes of individual components. Panels are typically warrantied for 10years on materials and 25years for power output, and inverters can be warrantied from 10 to 20years. No failure has been reported to date. Capacity factors and diesel offset Capacity factor is a function of weather, system design, system installation location, angle, and azimuth. It is a unitless ratio of the average power generated, divided by the rated peak power. Note that many of the systems installed in the Northwest Arctic Borough were installed in a semicircular fashion, with the goal of a broad production curve rather than maximum power production at midday. More systems are installed around the state than the ones reported here; however, insufﬁcient data were available to obtain capacity factor information on the sys- tems not listed. In Table I, the diesel offset was calculated by dividing the community diesel power plant efﬁciency (found in reports by the AEA on power cost equalization) by the system’s annual solar production to obtain gallons of diesel offset by the solar PV installation. While additional factors contribute to the amount of diesel fuel offset by a renewable energy system, this method provides a rough approximation. Levelized cost per kW The simple levelized cost of renewable energy (cents/kWh) was calculated at 70.5 cents/ kWh based on the following inputs into the National Renewable Energy Laboratory (NREL) levelized cost of electricity (LCOE) calculator: Period: 25years Discount rate: 3% Capital cost (average): $8000/kW Capacity factor (average): 9% Fixed O&M Cost: $100/kW/yr Variable O&M cost: none Heat rate: none Fuel cost: none Considering the capacity factors for installations in Alaska, the LCOE ranges from 42.3 to 105.8 cents/kWh over a capacity factor range of 6%–16%, all other variables remaining con- stant. Similarly, factoring in the range of capital costs for installations in Alaska, which are assumed to be equal to the total installed costs for our purposes, since solar PV costs are pre- dominantly capital costs, the LCOE ranges from $0.40–$1.22/kWh over a capital cost range of $3190–$13300/kW. 061704-5 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) Conditions for the greatest efficiency Photovoltaics work best under clear, cold, and sunny conditions. Photovoltaic panels are more efﬁcient and produce more power at colder temperatures, and high springtime snow albedo can reﬂect more solar radiation towards steeply angled panels. These cold, clear condi- tions and long days with high albedo ground cover usually make April the highest production solar month in most locations around Alaska. At cold temperatures, short-circuit current decreases slightly, while open-current voltage increases rapidly (LG Solar, 2017). For example, power output at 25 C can be approximately 25% higher than output at the standard test condition cell temperature of 25 C, given the same irradiance (LG Solar, 2017). Note that this temperature dependence has been best characterized at temperatures higher than standard test conditions and that this temperature-power correlation needs further independent research and ﬁeld characterization in Alaska’s below-freezing environments. Cost curve over time The cost curve for using solar PV technology in Alaska over time is virtually impossible to establish given that installations in the state are fairly recent and that there are inconsistencies in data and differences in the installation approach (i.e., some installations are bid out, some use volunteer labor, some ﬁnd ways to cover shipping, etc.). As a point of reference, we can look to national trends showing a steady decline in cost over the last two decades from LBNL’s publication “Tracking the Sun VIII” (Barbose and Darghouth, 2015), where the follow- ing is reported: TABLE I. Capacity factors and diesel offsets for selected solar installations in Alaska. Village Rated size (kW) PV capacity factor (%) 2013 community diesel efficiency (kWh/gal)a Average daily solar performance since installation (kWh) Annual diesel offset (gal) Ambler 8.4 9 14.1 17.5 453 Ambler IRA 2.2 12 14.1 6.1 157 Kobuk 7.4 6 14.3 10.8 275 BSNC 9 16.2 37.3 840 Shungnak 7.5 7 14.3 12.4 316 Noorvik 12 6 12.4 17.6 518 Noatak 11.3 8 14.1 21.1 546 Deering 11.1 10 13.6 26.9 721 Selawik 9.7 11 13.9 25 656 Yuut Elitnaurviat (Bethel) 10 14 13.7 33.6 895 Kaltag 9.6 9 13 21.7 609 Galena 6.7 12 13.1 18.6 518 Ruby Washeteria 5.4 10 13.4 12.8 348 Ruby Health Clinic 5.5 8 13.4 10.8 294 Manley 6 9 12.5 12.3 359 Nenana 4.4 12 GVEA b 12.5 CCHRCc 8 15 GVEA 29.7 aFrom the Alaska energy data gateway. bNenana is on the Golden Valley Electric Association (GVEA) grid, which receives power from a number of generation sources including hydro, coal, natural gas, fuel oil, and wind. Due to this variety, no diesel efﬁciency is given, and no diesel offset is calculated. cThe CCHRC has 3 tracking PV systems. The performances of these systems were averaged to determine capacity factors and summed to calculate the average daily performance. 061704-6 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) Starting in 2009, installed prices resumed their descent and have fallen steeply and steadily since, with average annual declines of 13%–18% per year across the three customer segments. These recent price declines are the result of reductions in global PV module prices, as well as declines in other hardware costs and ‘soft’ costs. Within the last year of the analysis period, from 2013 to 2014, median installed prices fell by $0.4/W (9%) for residential systems, by $0.4/W (10%) for non-residential systems <500kW, and by $0.7/W (21%) for non-residential systems <500kW (Barbose, 2015). Anecdotal evidence suggests that solar module prices and equipment prices have dropped in Alaska, as they have in Lower 48. The costs of shipping and installation remain higher than in the rest of the nation. Cost data Cost data for solar installations in rural Alaska (Table II) are difﬁcult to obtain. Often the contractor bids on a job as a lump sum, and separating labor from equipment and materials is difﬁcult to do accurately. Of note, the 6.7kW installation in Galena ($3.19/W) and the 18kW installation in Fort Yukon ($3.89/W) were accomplished with creative means to cut costs. In Fort Yukon, these means included volunteer labor and a shipping deal. For a number of instal- lations, ﬁgures are based on verbal estimates from batched purchases and are not public record. Transportation Further data collection is needed for this category. Technology trends In Alaska, options in solar PV systems include micro-invertors, which are attached to each panel and prevent an entire string of panels from going ofﬂine if just one panel is damaged. To date, solar PV systems in Alaska have comprised only mono-crystalline and poly-crystalline sil- icon modules. Module costs continue to drop, and efﬁciencies continue to increase, especially for non-silicon technologies. Other technologies may lend advantages for use in Alaska. Finally, concentrated solar PV technology is a candidate for generating heat as well as electric- ity but may not be suitable for Alaska. Storage systems Currently, energy storage is not a signiﬁcant component of solar PV systems in Alaska. An off- grid utility-scale example outside Alaska that may provide guidance in this direction is the 600kWh Absorbent Glass Mat battery bank in the Star Island solar installation in Maine. In addition, Tesla’s 7kWh Powerwall batteries may provide promising storage solutions for smaller installations. Refurbishment/upgrade market In the broader solar PV market, systems are generally replaced rather than upgraded. Both used and surplus panels are available. However, purchasing used panels introduces the possibil- ity that the panels may not work properly. Surplus panels are usually older models that the manufacturer sells at a greatly discounted rate. Because these panels are older, they may not be quite as efﬁcient as brand new panels but can still be a reasonable value. Realized cost savings Cost savings from integrating renewable power are difﬁcult to gauge due to technical and incentive impacts at the entire power systems level. At the technical level, for example, the effects of diminished losses of secondary services such as recovered waste heat and reductions in fuel efﬁciency are hard to gauge, as they depend not only on average reductions in load but also on speciﬁc operating schemes regarding mini- mum allowable load on diesels and on spinning reserve kept. 061704-7 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) CONCLUSIONS Solar photovoltaic (PV) technology is a nascent but promising energy option in remote Alaskan microgrids and serves as an example for isolated electrical grids worldwide. This study examines community-scale solar PV installations in Alaska, ranging in size from 2.2kW in Ambler to 50kW in Galena. Total installed costs arguably show a trend toward lower values with larger installation sizes although prices in Alaska are still signiﬁcantly higher than in the rest of the United States. Capacity factors range from 6% to 15%. However, it should be noted that some installation conﬁgurations, particularly in the northwestern part of the state, were installed with the goal of a broad production curve rather than maximum power production. ACKNOWLEDGMENTS The authors wish to thank the Alaska Energy Authority for its funding to support this project. Many people have contributed information and insight to this study. For their review and TABLE II. Cost data for selected solar installations in Alaska. Installed cost by major components a Location System size (kW) Installation date (month/year) Hardware ($) Support structure Labor/travel ($) Shipping ($) Cost/Watt ($) Total cost ($) Installed systems. Costs were based on percentages of estimated total system cost b Ambler 8.4 3/2013 41250 Included in hardware 11250 22500.00 8.93 75000 Ambler IRA 2.2 3/2013 13750 Included in hardware 3750 7500.00 11.36 25000 Kobuk 7.4 3/2013 41250 Included in hardware 11250 22500.00 10.14 75000 Shungnak 7.5 10/2013 41250 Included in hardware 11250 22500.00 10.00 75000 Noorvik 12 10/2013 41250 Included in hardware 11250 22500.00 6.25 75000 Noatak 11.3 11/2013 41250 Included in hardware 11250 22500.00 6.64 75000 Deering 11.1 11/2013 41250 Included in hardware 11250 22500.00 6.76 75000 Kotzebue-1 10.5 10/2014 45650 Included in hardware 12450 24900.00 7.90 83000 Kotzebue-2 10.5 11/2014 45650 Included in hardware 12450 24900.00 7.90 83000 Selawik 9.7 11/2014 45650 Included in hardware 12450 24900.00 8.56 83000 Kiana 10.5 8/2015 45650 Included in hardware 12450 24900.00 7.90 83000 Buckland 10.5 2015 45650 Included in hardware 12450 24900.00 7.90 83000 Kivalina 10.5 2015 45650 Included in hardware 12450 24900.00 7.90 83000 Installed systems with detailed cost records Eagle 24 7/2015 115552 Included in hardware 94632 10.88 261000 Kaltag 9.6 2012 78657 Included in hardware 15946 6465.00 13.33 128000 Galena 6.7 11/2012 14400 2000 5000 City covered shipping cost 3.19 21400 Fort Yukon 18 7/2015 45000 Included in hardware 20000 5000.00 3.89 70000 Galena 50 Dec 2015 estimate only Lumped together in bid 30000.00c 4.07 203613 aSystems in Ambler, Kobuk, Shungnak, Noorvik, Noatak, Deering, Kotzebue, Selawik, Kiana, Buckland, and Kivalina (shaded in green) were installed by Bering Straits Development Company through coordination with the Northwest Arctic Borough. Costs for these systems were difﬁcult to separate from the main lump sum bid. Based on input from Rob Bensin, costs were separated using 30% for logistics, 15% for labor, and the remainder for racking, hardware, and materials. Systems in Eagle and Kaltag were installed by the utilities using funding from the Renewable Energy Fund. Systems in Galena and Fort Yukon were installed with assistance from the Tanana Chiefs Conference. bSystems were bid as a group (Bensin, 2016). cThis system was only bid and not installed. Per price quote, “Heavy equipment to be provided for trenching/anchors/material handling.” In addition, shipping was not included but was estimated after discussions with the energy manager at Tanana Chiefs Conference. Shipping is estimated here at $30000 per Dave Pelunis-Messier, based on other similar systems in the Interior. 061704-8 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017) comments, we wish to thank Rob Bensin, Energy Efﬁciency and Renewable Energy Division Manager at Bering Straits Development Company; Ingemar Mathiasson, Energy Manager for the Northwest Arctic Borough, Paul Schwabe of the National Renewable Energy Laboratory; Dave Messier, Rural Energy Coordinator for the Tanana Chiefs Conference; Bob Deering of the United State Forest Service; Bruno Grunau of the Cold Climate Housing Research Center; Alan Mitchell of Analysis North; and Dave Lockard, Sam Tappen, and Neil McMahon of the Alaska Energy Authority. ACEP (Alaska Center for Energy and Power), “An investigation of solar thermal technology in arctic environments: A pro- ject by Kotzebue Electric Association,” Denali Commission—Emerging Energy Technology Grant Report (2012). AEA (Alaska Energy Authority),Renewable Energy Atlas of Alaska (AEA, 2011). 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Schwabe, P., “Solar energy prospecting in remote Alaska: An economic comparison of electricity generation costs between solar photovoltaics and diesel fuel expenditures,” NREL Report No. 65834 (2016). Vilagi, A. and Brown, P.,Effects of Snowfall on Solar Power Generation (University of Alaska Fairbanks, 2015). Wirth, H.,Recent Facts about Photovoltaics in Germany (Fraunhofer ISE, 2015). 061704-9 E. Whitney and C. Pike J. Renewable Sustainable Energy 9, 061704 (2017)