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SCIENTIFIC REP RTS OPEN The Photovoltaic Heat Island Effect: Largersolar power plants increase local temperatures Received: 26 May 2016 Greg A. Barron-Gafford1’2, Rebecca L. Minor”2, Nathan A. Allen3, Alex D. Cronin4, Accepted: 23 September 2016 Adria E. Brooks5 & Mitchell A. Pavao-Zuckerman6 Published: 13 October 2016 While photovoltaic (PV) renewable energy production has surged, concerns remain about whether or not PV power plants induce a “heat island” (PVI-l I) effect, much like the increase in ambient temperatures relative to wildlands generates an Urban Heat Island effect in cities. Transitions to PV plants alter the way that incoming energy is reflected back to the atmosphere or absorbed, stored, and reradiated because PV plants change the albedo, vegetation, and structure of the terrain. Prior work on the PVHI has been mostly theoretical or based upon simulated models. Furthermore, past empirical work has been limited in scope to a single biome. Because there are still large uncertainties surrounding the potential for a PHVI effect, we examined the PVH I empirically with experiments that spanned three biomes. We found temperatures over a PV plant were regularly 3—4°C warmer than wildlands at night, which is in direct contrast to other studies based on models that suggested that PV systems should decrease ambient temperatures. Deducing the underlying cause and scale of the PVHI effect and identifying mitigation strategies are key in supporting decision-making regarding PV development, particularly in semiarid landscapes, which are among the most likely for large-scale PV installations. Electricity production from large-scale photovoltaic (PV) installations has increased exponentially in tecent dec ades’-3. This proliferation in renewable energy portfolios and PV powerplants demonstrate an increase in the acceptance and cost-effectiveness of this technology45. Corresponding with this upsurge in installation has been an increase in the assessment of the impacts of utility-scale PV40, including those on the efficacy of PV to offset energy needs9”0. A growing concern that remains understudied is whether or not PV installations cause a “heat island” (PVHI) effect that warms surrounding areas, thereby potentially influencing wildlife habitat, ecosystem function in wildlands, and human health and even home values in residential areas’1. As with the Urban Heat Island (UHI) effect, large PV power plants induce a landscape change that reduces albedo so that the modified landscape is darker and, therefore, less reflective. Lowering the terrestrial albedo from —20% in natural deserts’2 to over PV panels’3 alters the energy balance of absorption, storage, and release of short- and longwave radiation’4”5. However, several differences between the UHI and potential PVHI effects confound a simple com parison and produce competing hypotheses about whether or not large-scale PV installations will create a heat island effect. These include: PV installations shade a portion of the ground and therefore could reduce heat absorption in surface soils’6, (ii) PV panels are thin and have little heat capacity per unit area but PV modules emit thermal radiation both up and down, and this is particularly significant during the day when PV modules are often 20”C warmer than ambient temperatures, (Ui) vegetation is usually removed from PV power plants, reducing the amount of cooling due to transpiration’4, (iv) electric power removes energy from PV power plants, and PV panels reflect and absorb upwelling longwave radiation, and thus can prevent the soil from cooling as much as it might under a dark sky at night. Public concerns over a PVHI effect have, in some cases, led to resistance to large-scale solar development. By some estimates, nearly half of recently proposed energy projects have been delayed or abandoned due to local opposition’’. Yet, there is a remarkable lack of data as to whether or not the PVHI effect is real or simply an issue ‘School of Geography & Development, University of Arizona, Tucson, AZ, USA. 2Office of Research & Development; College of Science, Biosphere 2, University of Arizona, Tucson, AZ, USA. 3Nevada Center of Excellence, Desert Research Institute, Las Vegas, NV, USA. 4Department of Physics, University of Arizona, Tucson, AZ, USA. ‘Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI, USA. 6Department of Environmental Science & Technology, University of Maryland, College Park, MD, USA. Correspondence and requests for materials should be addressed to G.A.B.-G. (email: gregbgemail.arizona.edu) FFiC REPORTS 16:350701 DOl: 10.1038/srep35O7O ---PAGE BREAK--- www nature com/scientsficreportsl Figure 1. Illustration of midday energy exchange. Assuming equal rates of incoming energy from the sun, a transition from a vegetated ecosystem to a photovoltaic (PV) power plant installation will significantly alter the energy flux dynamics of the area. \‘Vithin natural ecosystems, vegetation reduces heat capture and storage in soils (orange arrows), and infiltrated water and vegetation release heat-dissipating latent energy fluxes in the transition of water-to-water vapor to the atmosphere through evapotranspiration (blue arrows). These latent heat fluxes are dramatically reduced in typical PV installations, leading to greater sensible heat fluxes (red arrows). Energy re-radiation from PV panels (brown arrow) and energy transferred to electricity (purple arrow) are also shown. associated with perceptions of environmental change caused by the installations that lead to “not in my back yard” (NIMBY) thinking. Some models have suggested that PV systems can actually cause a cooling effect on the local environment, depending on the efficiency and placement of the PV panels’7°. But these studies are limited in their applicability when evaluating large-scale PV installations because they consider changes in albedo and energy exchange within an urban environment (rather than a natural ecosystem) or in European locations that are not representative of semiarid energy dynamics where large-scale PV installations are concentrated’°”. Most previous research, then, is based on untested theory and numerical modeling. Therefore, the potential for a PHVI effect must be examined with empirical data obtained through rigorous experimental terms. The significance of a PVHI effect depends on energy balance. Incoming solar energy typically is either reflected back to the atmosphere or absorbed, stored, and later re-radiated in the form of lateot or sensible heat (Fig. 1 Within natural ecosystems, vegetation reduces heat gain and storage in soils by creating surface shad ing, though the degree of shading varies among plant types22. Energy absorbed by vegetation and surface soils can be released as latent heat in the transition of liquid water to water vapor to the atmosphere through evapotranspi ration — the combined water loss from soils (evaporation) and vegetation (transpiration). This heat-dissipatiog latent energy exchange is dramatically reduced in a typical PV installation (Fig. 1 transition from A-to-B), poten tially leading to greater heat absorption by soils in PV installations. This increased absorption, in turn, could increase soil temperatures and lead to greater sensible heat efflux from the soil in the form of radiation and con vection. Additionally, PV panel surfaces absorb more solar insolation due to a decreased albedo’32321. PV panels will re-radiate most of this energy as longwave sensible heat and convert a lesser amount of this energy into usable electricity. PV panels also allow some light energy to pass, which, again, in unvegetated soils will lead to greater heat absorption. This increased absorption could lead to greater sensible heat efflux from the soil that may be trapped under the PV panels. A PVHI effect would be the result of a detectable increase in sensible heat flux (atmospheric warming) resulting from an alteration in the balance of incoming and outgoing energy flaxes due to landscape transformation. Developing a full thermal model is challenging’7’825, and there are large uncertainties surrounding multiple terms including variations in albedo, cloud cover, seasonality in advection, and panel efficiency, which itself is dynamic and impacted by the local environment. These uncertainties are compounded by the lack of empirical data. We addressed the paucity of direct quantification of a PVHI effect by simultaneously monitoring three sites that represent a natural desert ecosystem, the traditional built environment (parking lot surrounded by com mercial buildings), and a PV power plant. We define a PVHI effect as the difference in ambient air temperature between the PV power plant and the desert landscape. Similarly, UHI is defined as the difference in temperature between the built environment and the desert. We reduced confounding effects of variability in local incoming energy, temperature, and precipitation by utilizing sites contained within a 1 km area. At each site, we monitored air temperature continuously for over one year using aspirated temperature probes 2.5 m above the soil surface. Average annual temperature was 22.7 + 0.5°C in the PV installation, while the nearby desert ecosystem was only 20.3 + 0.5 indicating a PVHI effect. Temperature differences between areas varied significantly depending on time of day and month of the year (Fig. but the PV installation was always greater than or equal in temperature to other sites. As is the case with the UHI effect in dryland regions, the PVHt effect delayed the cooling of ambient temperatures in the evening, yielding the most significant difference in overnight temperatures across all seasons. Annual average midnight temperatures were 19.3 + 0.6°C in the PV installation, while the nearby desert ecosystem was only 15.8 + 0.6 This PVHI effect was more significant in terms of actual degrees of warming in warm months (Spring and Summer; Fig. 3, right). S(JE NTIFIC REPORTS I 6:350701 DOl: 1O.1038/srep35O7O ---PAGE BREAK--- 25 20 15 10 30 0 25 U) D 20 CD U) 15 a. 2 ci) 35 C C) 30 .0 2 25 20 30 25 20 Is 10 Hour of day Figure 2. Average ambient temperatures throughout a 24-hour period provide evidence of a photovoltaic heat island (PVHI) effect. In both PVHI and UHI scenarios, the greater amount of exposed ground surfaces compared to natural sys tems absorbs a larger proportion of high-energy, shortwave solar radiation during the day. Combined with min imal rates of heat-dissipating transpiration from vegetation, a proportionally higher amount of stored energy is reradiated as longwave radiation during the night in the form of sensible heat (Fig. Because PV installations introduce shading with a material that, itself, should not store much incoming radiation, one might hypothesize that the effect of a PVHI effect would be lesser than that of a UHI. Here, we found that the difference in evening ambient air temperature was consistently greater between the PV installation and the desert site than between the parking lot (UHI) and the desert site (Fig. The PVHI effect caused ambient temperature to regularly approach or be in excess of 4°C warmer than the natural desert in the evenings, essentially doubling the temperature increase due to UHI measured here. This more significant warming under the PVHI than the UHI may be due to heat trapping of re-radiated sensible heat flux under PV arrays at night. Daytime differences from the natural ecosystem were similar between the FV installation and urban parking lot areas, with the exception of the Spring and Summer munths, when the PVHI effect was significantly greater than UHI in the day. During these warm seasons, average midnight temperatures were 25.5 + 0.5°C in the PV installation and 23.2 + 0.5°C in the parking lot, while the nearby desert ecosystem was only 21.4 + 0.5 The results presented here demonstrate that the PVHI effect is real and can significantly increase temperatures over PV power plant installations relative to nearby wildlands. More detailed measurements of the underlying causes of the PVHI effect, potential mitigation strategies, and the relative influence of PVHI in the context of the intrinsic carbon offsets from the use of this renewable energy are needed. Thus, we raise several new questions and highlight critical unknowns requiring future research. What is the physical basis of land transformations that might cause a PVHI? We hypothesize that the PVHI effect results from the effective transition in how energy moves in and out of a PV installation versus a natural ecosystem. However, measuring the individual components of an energy flux model remains a necessary task. These measurements are difficult and expensive but, nevertheless, are indispensable in identifying the relative influence of multiple potential drivers of the PVHI effect found here. Environmental 0 4 5 12 16 20 0 4 5 12 16 20 0 4 5 12 16 20 SCNN NC REPORTS 16:350701 DOl: 10.1038/srep3SO7O ---PAGE BREAK--- 2 —4 -I -4 -2 T - 4 2 0 Figure 3. (Left) Average levels of Photovoltaic Heat Islanding (ambient temperature difference between PV installation and desert) and Urban Heat Islanding (ambient temperature difference between the urban parking lot and the desert). (Right) Average night and day temperatures for four seasonal periods, illustrating a significant PVHI effect across all seasons, with the greatest influence on ambient temperatures at night. conditions that determine patterns of ecosystem carbon, energy, and water dynamics are driven by the means through which incoming energy is reflected or absorbed. Because we lack fundamental knowledge of the changes in surface energy fluxes and microclimates of ecosystems undergoing this land use change, we have little ability to predict the implications in terms of carbon or water cycling1’8. What are the physical implications of a PVHI, and how do they vary by region? The size of an UHI is determined by properties of the city, including total population26-28, spatial extent, and the geographic location of that city2931. We should, similarly, consider the spatial scale and geographic position of a PV installation when considering the presence and importance of the PVHI effect. Remote sensing could be coupled with ground-based measurements to determine the lateral and vertical extent of the PVHI effect. We could then determine if the size of the PVHI effect scales with some measure of the power plant (for example, panel density or spatial footprint) and whether or not a PVHI effect reaches surrounding areas like wildlands and neighborhoods. Given that different regions around the globe each have distinct background levels of vegetative ground cover and thermodynamic patterns of latent and sensible heat exchange, it is possible that a transition from a natural wildland to a typical PV power plant will have different outcomes than demonstrated here. The paucity in data on the physical effects of this important and growing land use and land cover change warrants more studies from representative ecosystems. What are the human implications of a PVHI, and how might we mitigate these effects? \Arith the growing popularity of renewable energy production, the boundaries between residential areas and larger-scale PV installations are decreasing. In fact, closer proximity with residential areas is leading to increased calls for zoning and city planning codes for larger PV installations3233, and PVHt-based concerns over potential reductions in real estate value or health issues tied to Human Thermal Comfort (HTC)34. Mitigation of a PVHI effect through targeted revegetation could have synergistic effects in easing ecosystem degradation associated with development of a utility scale PV site and increasing the collective ecosystem services associated with an area4. But what are the best mitigation measures? What tradeoffs exist in terms of various means of revegetating degraded PV installations? Can other albedo modifications be used to moderate the severity of the PVHI? 6 Winter January February March Urban parking lot PhOtOVOltC installation - - April May June September ci) ci,ta) cc, Il) V in in a) S0 ci) D in a)0. Sa) in a) S (U a) C) ci) 0 0 4 2 — I July October August November Surnrci I C VC cci (cia)I C) cci 0>0 0 0 0 it)0) in in inC0inin U) Cl) December Fall 12 16 20 16 20 2 16 20 Hour of day Midnight Noon SEIEN TIFC REPORTS 6:35070 I DOl: 10.1038/srep3SOlO ---PAGE BREAK--- I— Figure 4. Experimental sites. Monitoring a natural semiarid desert ecosystem, solar (PV) photovoltaic installation, and an “urban” parking lot — the typical source of urban heat islanding — within a 1 km2 area enabled relative control for the incoming solar energy, allowing us to quantify variation in the localized temperature of these three environments over a year-long time period. The Google Earth image shows the University of Arizona’s Science and Technology Park’s Solar Zone. To fully contextualize these findings in terms of global warming, one needs to consider the relative signifi cance of the (globally averaged) decrease in albedo due to PV power plants and their associated warming from the PVHI against the carbon dioxide emission reductions associated with PV power plants. The data presented here represents the first experimental and empirical examination of the presence of a heat island effect associated with PV power plants. An integrated approach to the physical and social dimensions of the PVHI is key in supporting decision-making regarding PV development. Methods Site Description. We simultaneously monitored a suite of sites that represent the traditional built urban environment (a parking lot) and the transformation from a natural system (undeveloped desert) to a 1 MW PV power plant (Fig. 4; Map data: Google). To minimize confounding effects of variability in local incoming energy, temperature, and precipitation, we identified sites within a 1 km area. All sites were within the boundaries of the University of Arizona Science and Technology Park Solar Zone (32.0921 50°N, 11 O.808764”W; elevation: 888 m ASL). Within a 20Dm diameter of the semiarid desert site’s environmental monitoring station, the area is composed of a sparse mix of semiarid grasses (Sporn bolos wrightii, Eragrostis lehnianniana, and Muhienbergia porteri), cacti (Opnntia spp. and Ferocactus spp.), and occasional woody shrubs including creosote bush (Larrea tridentata), whitethorn acacia (Acacia constrieta), and velvet mesquite (Prosopis velntina). The remaining area is bare soil. These species commonly co-occur on low elevation desert bajadas, creosote bush flats, and semiarid grasslands. The photovoltaic installation was put in place in early 2011, three full years prior when we initiated monitoring at the site. We maintained the measurement installations for one full year to capture seasonal var iation due to sun angle and extremes associated with hot and cold periods. Panels rest on a single-axis tracker system that pivot east-to-west throughout the day. A parking lot with associated building served as our “urban” site and is of comparable spatial scale as our PV site. Monitoring Equipment & Variables Monitored. Ambient air temperature was measured with a shaded, aspirated temperature probe 2.5 m above the soil surface (Vaisala HMP6O, Vaisala, Helsinki, Finland in the desert and Microdaq U23, Onset, Bourne, MA in the parking lot). Temperature probes were cross-validated for precision (closeness of temperature readings across all probes) at the onset of the experiment. Measurements of temperature were recorded at 30-minute intervals throughout a 24-hour day. Data were recorded on a data-logger (CR1000, Campbell Scientific, Logan, Utah or Microstation, Onset, Bourne, MA). Data from this TiFC REPORT S 16:350701 DOl: 10.1038/srep3SO7O ---PAGE BREAK--- / / /7 instrument array is shown for a yearlong period from April 2014 through March 2015. Data from the parking lot was lost for September 2014 because of power supply issues with the datalogget. Statistical analysis. averages of hourly (on-the-hour) data were used to compare across the nat ural semiarid desert, urban, and PV sites. A Photovoltaic Heat Island (PVHI) effect was calculated as differences in these hourly averages between the PV site and the natural desert site, and estimates of Urban Heat Island (UHf) effect was calculated as differences in hourly averages between the urban parking lot site and the natural desert site. We used midnight and noon values to examine maximum and minimum, respectively, differences in temperatures among the three measurement sites and to test for significance of heat islanding at these times. 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Deserts are spacious, relatively flat, rich in silicon — the raw material for the semiconductors from which solar cells are made and never short of sunlight. In fact, the 10 largest solar plants around the world are all in deserts or dry regions. Researchers imagtto it might be possible to transform the world’s largest desert, the Sahara, into a giant solar farm, capable of meeting four times the world’s current energy demand. Blueprints have been drawn up for projects in Tunisia and Morocco that would supply electricity for millions of households in Europe. While the black surfaces of solar panels absorb most of the sunlight that reaches them, only a fraction (around 15 percent) of that incoming energy gets converted to electricity. The rest is returned to the environment as heat. The panels are usually much darker than the ground they cover, so a vast expanse of solar cells will absorb a lot of additional energy and emit it as heat, affecting the climate. If these effects were only local, they might not matter in a sparsely populated and barren desert. But the scale of the installations that would be needed to make a dent in the world’s fossil energy demand would be vast, covering thousands of square kilometers. Heat re-emitted from an area this size will be redistributed by the flow of air in the atmosphere, having regional and even global effects on the climate. jgyao Lu Researcher in Physical Geography Lund University Benjamin Smith Director of Research Hawkesbury Institute for the Environment, Western Sydney University L Solar panels in Sahara could boost renewable energy but damage the global climate,!! Image courtesy of The Conversation. ---PAGE BREAK--- Clockwise from top left: Bhadla solarpark, India; Desert Sublight solar farm, U.S.; Hainanzhou solar park, China and Ouarzazate solar park, Morocco. //lmage courtesy of The Conversation A greener Sahara A 2018 study used a climate model to simulate the effects of lower albedo on the land surface of deserts caused by installing massive solar farms. Albedo is a measure of how well surfaces reflect sunlight. Sand, for example, is much more reflective than a solar panel and so has a higher albedo. The model revealed that when the size of the solar farm reaches 20 percent of the total area of the Sahara, it triggers a feedback loop. Heat emitted by the darker solar panels (compared to the highly reflective desert soil) creates a steep temperature difference between the land and the surrounding oceans that ultimately lowers surface air pressure and causes moist air to rise and condense into raindrops. With more monsoon rainfall, plants grow and the desert reflects less of the sun’s energy, because vegetation absorbs light better than sand and soil. With more plants present, more water is evaporated, creating a more humid environment that causes vegetation to spread. “The model revealed that when the size of the solar farm reaches 20% ofthe total area of the Sahara, it triggers a feedback loop.” This scenario might seem fanciful, but studies suggest that a similar feedback loop kept much of the Sahara green during the African Humid Period, which only ended 5,000 years ago. So, a giant solar farm could generate ample energy to meet global demand and simultaneously turn one of the most hostile environments on Earth into a habitable oasis. Sounds perfect, right? Not quite. In a recent study, we used an advanced Earth system model to closely examine how Saharan solar farms interact with the climate. Our model takes into account the complex feedbacks between the interacting spheres of the world’s climate — the atmosphere, the ocean and the land and its ecosystems. It showed there could be unintended effects in remote parts of the land and ocean that offset any regional benefits over the Sahara itself. Drought in the Amazon, cyclones in Vietnam Covering 20 percent of the Sahara with solar farms raises local temperatures in the desert by 1.5 degrees Celsius, according to our model. At 50 percent coverage, the temperature increase is 2.5 degrees Celsius. This warming is eventually spread around the globe by atmosphere and ocean movement, raising the world’s average temperature by 0.16 degrees Celsius for 20 percent coverage, and 0.39 degrees Celsius for 50 percent coverage. The global temperature shift is not uniform, though — the polar regions would warm more than the tropics, increasing sea ice loss in the Arctic. This could further accelerate warming, as melting sea ice exposes dark water which absorbs much more solar energy. ---PAGE BREAK--- This massive new heat source in the Sahara reorganizes global air and ocean circulation, affecting precipitation patterns around the world. The narrow band of heavy rainfall in the tropics, which accounts for more than 30 percent of global precipitation and supports the rainforests of the Amazon and Congo Basin, shifts northward in our simulations. For the Amazon region, this causes droughts as less moisture arrives from the ocean. Roughly the same amount of additional rainfall that falls over the Sahara due to the surface-darkening effects of solar panels is lost from the Amazon. The model also predicts more frequent tropical cyclones hitting North American and East Asian coasts. 20% cover’agdtemperature change - . . . 50% coverage temperature change ramfati and Wind change — . - Global temperature, rainfall and sur Inc o wind chnrigc’s in simulations with 20 percent and 50 percent solar panel coverage ofSahara. Lu et al. (2.02fl.// Image courtesy of The Conversation Some important processes are still missing from our model, such as dust blown from large deserts. Saharan dust, carried on the wind, is a vital source of nutrients for the Amazon and the Atlantic Ocean. So a greener Sahara could have an even bigger global effect than our simulations suggested. We are only beginning to understand the potential consequences of establishing massive solar farms in the world’s deserts. Solutions such as this may help society transition from fossil energy, but Earth system studies such as ours underscore the importance of considering the numerous coupled responses of the atmosphere, oceans and land surface when examining their benefits and risks. This article is republished from The Conversation under a Creative Commons license. ALSO ON GREENBIZ a month ago 1 comment a month ago 1 comment 22 days ago• 1 comr A new guidebook aims to Local lenders should not Decarbonizing help employees become lose sight of the funding that transportation is es climate activists. Can it already exists to meet climate goals I ramfair ana wInd change More on this topic