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The Photovoltaic Heat Island Effect: Larger solar power plants increase local temperatures (Open access: http://www.nature.com/articles/srep35070)

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While photovoltaic (PV) renewable energy production has surged, concerns remain about whether or not PV power plants induce a “heat island” (PVHI) 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 PVHI 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.
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Scientific RepoRts | 6:35070 | DOI: 10.1038/srep35070
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The Photovoltaic Heat Island
Eect: Larger solar power plants
increase local temperatures
Greg A. Barron-Gaord1,2, Rebecca L. Minor1,2, Nathan A. Allen3, Alex D. Cronin4,
Adria E. Brooks5 & Mitchell A. Pavao-Zuckerman6
While photovoltaic (PV) renewable energy production has surged, concerns remain about whether
or not PV power plants induce a “heat island” (PVHI) eect, much like the increase in ambient
temperatures relative to wildlands generates an Urban Heat Island eect in cities. Transitions to PV
plants alter the way that incoming energy is reected 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 eect, we examined the PVHI 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 eect 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 recent dec-
ades1–3. is proliferation in renewable energy portfolios and PV powerplants demonstrate an increase in the
acceptance and cost-eectiveness of this technology4,5. Corresponding with this upsurge in installation has been
an increase in the assessment of the impacts of utility-scale PV4,6–8, including those on the ecacy of PV to oset
energy needs9,10. A growing concern that remains understudied is whether or not PV installations cause a “heat
island” (PVHI) eect that warms surrounding areas, thereby potentially inuencing wildlife habitat, ecosystem
function in wildlands, and human health and even home values in residential areas11. As with the Urban Heat
Island (UHI) eect, large PV power plants induce a landscape change that reduces albedo so that the modied
landscape is darker and, therefore, less reective. Lowering the terrestrial albedo from ~20% in natural deserts12
to ~5% over PV panels13 alters the energy balance of absorption, storage, and release of short- and longwave
radiation14,15. However, several dierences between the UHI and potential PVHI eects confound a simple com-
parison and produce competing hypotheses about whether or not large-scale PV installations will create a heat
island eect. ese include: (i) PV installations shade a portion of the ground and therefore could reduce heat
absorption in surface soils16, (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 signicant during the day when PV modules
are oen 20 °C warmer than ambient temperatures, (iii) vegetation is usually removed from PV power plants,
reducing the amount of cooling due to transpiration14, (iv) electric power removes energy from PV power plants,
and (v) PV panels reect 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 eect 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
opposition11. Yet, there is a remarkable lack of data as to whether or not the PVHI eect is real or simply an issue
1School of Geography & Development, University of Arizona, Tucson, AZ, USA. 2Oce 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. 5Department
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: gregbg@email.arizona.edu)
Received: 26 May 2016
Accepted: 23 September 2016
Published: 13 October 2016
OPEN
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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 eect on the
local environment, depending on the eciency and placement of the PV panels17,18. 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 concentrated10,19. Most
previous research, then, is based on untested theory and numerical modeling. erefore, the potential for a PHVI
eect 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
reected back to the atmosphere or absorbed, stored, and later re-radiated in the form of latent or sensible heat
(Fig.1)20,21. 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). is heat-dissipating
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. is increased absorption, in turn, could
increase soil temperatures and lead to greater sensible heat eux from the soil in the form of radiation and con-
vection. Additionally, PV panel surfaces absorb more solar insolation due to a decreased albedo13,23,24. PV panels
will re-radiate most of this energy as longwave sensible heat and convert a lesser amount (~20%) 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. is increased absorption could lead to greater sensible heat eux from the soil
that may be trapped under the PV panels. A PVHI eect would be the result of a detectable increase in sensible
heat ux (atmospheric warming) resulting from an alteration in the balance of incoming and outgoing energy
uxes due to landscape transformation. Developing a full thermal model is challenging17,18,25, and there are large
uncertainties surrounding multiple terms including variations in albedo, cloud cover, seasonality in advection,
and panel eciency, which itself is dynamic and impacted by the local environment. ese uncertainties are
compounded by the lack of empirical data.
We addressed the paucity of direct quantication of a PVHI eect 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 dene a PVHI eect as the dierence in ambient air temperature
between the PV power plant and the desert landscape. Similarly, UHI is dened as the dierence in temperature
between the built environment and the desert. We reduced confounding eects 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 °C, indicating a PVHI eect. Temperature dierences between areas varied
signicantly depending on time of day and month of the year (Fig.2), but the PV installation was always greater
than or equal in temperature to other sites. As is the case with the UHI eect in dryland regions, the PVHI eect
delayed the cooling of ambient temperatures in the evening, yielding the most signicant dierence 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 °C. is PVHI eect was more signicant in terms of actual
degrees of warming (+ 3.5 °C) in warm months (Spring and Summer; Fig.3, right).
Figure 1. Illustration of midday energy exchange. Assuming equal rates of incoming energy from the sun, a
transition from (A) a vegetated ecosystem to (B) a photovoltaic (PV) power plant installation will signicantly
alter the energy ux dynamics of the area. Within natural ecosystems, vegetation reduces heat capture and
storage in soils (orange arrows), and inltrated water and vegetation release heat-dissipating latent energy uxes
in the transition of water-to-water vapor to the atmosphere through evapotranspiration (blue arrows). ese
latent heat uxes are dramatically reduced in typical PV installations, leading to greater sensible heat uxes (red
arrows). Energy re-radiation from PV panels (brown arrow) and energy transferred to electricity (purple arrow)
are also shown.
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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.1)15. Because PV installations
introduce shading with a material that, itself, should not store much incoming radiation, one might hypothesize
that the eect of a PVHI eect would be lesser than that of a UHI. Here, we found that the dierence 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.3). e PVHI eect 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. is more signicant warming under the PVHI than the UHI may be due
to heat trapping of re-radiated sensible heat ux under PV arrays at night. Daytime dierences from the natural
ecosystem were similar between the PV installation and urban parking lot areas, with the exception of the Spring
and Summer months, when the PVHI eect was signicantly 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 °C.
e results presented here demonstrate that the PVHI eect is real and can signicantly increase temperatures
over PV power plant installations relative to nearby wildlands. More detailed measurements of the underlying
causes of the PVHI eect, potential mitigation strategies, and the relative inuence of PVHI in the context of the
intrinsic carbon osets from the use of this renewable energy are needed. us, 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 eect results from the eective transition in how energy moves in and out of a PV
installation versus a natural ecosystem. However, measuring the individual components of an energy ux model
remains a necessary task. ese measurements are dicult and expensive but, nevertheless, are indispensable
in identifying the relative inuence of multiple potential drivers of the PVHI eect found here. Environmental
Figure 2. Average monthly ambient temperatures throughout a 24-hour period provide evidence of a
photovoltaic heat island (PVHI) eect.
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conditions that determine patterns of ecosystem carbon, energy, and water dynamics are driven by the means
through which incoming energy is reected or absorbed. Because we lack fundamental knowledge of the changes
in surface energy uxes and microclimates of ecosystems undergoing this land use change, we have little ability to
predict the implications in terms of carbon or water cycling4,8.
What are the physical implications of a PVHI, and how do they vary by region?
e size of an UHI is determined by properties of the city, including total population26–28, spatial extent, and the
geographic location of that city29–31. We should, similarly, consider the spatial scale and geographic position of
a PV installation when considering the presence and importance of the PVHI eect. Remote sensing could be
coupled with ground-based measurements to determine the lateral and vertical extent of the PVHI eect. We
could then determine if the size of the PVHI eect scales with some measure of the power plant (for example,
panel density or spatial footprint) and whether or not a PVHI eect reaches surrounding areas like wildlands and
neighborhoods. Given that dierent 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 dierent outcomes than demonstrated here. e
paucity in data on the physical eects 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
eects?
With 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 installations32,33, and PVHI-based concerns over potential
reductions in real estate value or health issues tied to Human ermal Comfort (HTC)34. Mitigation of a PVHI
eect through targeted revegetation could have synergistic eects 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 tradeos exist in terms of various means of revegetating
degraded PV installations? Can other albedo modications be used to moderate the severity of the PVHI?
Figure 3. (Le) Average monthly levels of Photovoltaic Heat Islanding (ambient temperature dierence
between PV installation and desert) and Urban Heat Islanding (ambient temperature dierence between
the urban parking lot and the desert). (Right) Average night and day temperatures for four seasonal periods,
illustrating a signicant PVHI eect across all seasons, with the greatest inuence on ambient temperatures at
night.
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To fully contextualize these ndings in terms of global warming, one needs to consider the relative signi-
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. e data presented here
represents the rst experimental and empirical examination of the presence of a heat island eect 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 eects of variability in local incoming
energy, temperature, and precipitation, we identied sites within a 1 km area. All sites were within the boundaries
of the University of Arizona Science and Technology Park Solar Zone (32.092150°N, 110.808764°W; elevation:
888 m ASL). Within a 200 m diameter of the semiarid desert site’s environmental monitoring station, the area is
composed of a sparse mix of semiarid grasses (Sporobolus wrightii, Eragrostis lehmanniana, and Muhlenbergia
porteri), cacti (Opuntia spp. and Ferocactus spp.), and occasional woody shrubs including creosote bush (Larrea
tridentata), whitethorn acacia (Acacia constricta), and velvet mesquite (Prosopis velutina). e remaining area is
bare soil. ese species commonly co-occur on low elevation desert bajadas, creosote bush ats, and semiarid
grasslands. e 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 (°C) was measured with a
shaded, aspirated temperature probe 2.5 m above the soil surface (Vaisala HMP60, 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 Scientic, Logan, Utah or Microstation, Onset, Bourne, MA). Data from this
Figure 4. Experimental sites. Monitoring a (1) natural semiarid desert ecosystem, (2) solar (PV)
photovoltaic installation, and (3) 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. e Google Earth
image shows the University of Arizona’s Science and Technology Park’s Solar Zone.
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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 datalogger.
Statistical analysis. Monthly 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) eect was calculated as dierences
in these hourly averages between the PV site and the natural desert site, and estimates of Urban Heat Island
(UHI) eect was calculated as dierences 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, dierences
in temperatures among the three measurement sites and to test for signicance of heat islanding at these times.
Comparisons among the sites were made using Tukey’s honestly signicant dierence (HSD) test35. Standard
errors to calculate HSD were made using pooled midnight and noon values across seasonal periods of winter
(January-March), spring (April-June), summer (July-September), and fall (October-December). Seasonal anal-
yses allowed us to identify variation throughout a yearlong period and relate patterns of PVHI or UHI eects
with seasons of high or low average temperature to examine correlations between background environmental
parameters and localized heat islanding.
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Acknowledgements
e authors thank Ken Marcus for access to the University of Arizona Solar Zone and the Science and Technology
Park and to Tucson Electric Power for access to their PV installation. This research was supported by the
University of Arizona Institute of the Environment and the Oce of Research & Development through the TRIF-
funded Water, Environmental and Energy Solutions initiative.
Author Contributions
G.A.B.-G., R.L.M. and N.A.A. established research sites and installed monitoring equipment. G.A.B.-G. directed
research and R.L.M. conducted most site maintenance. G.A.B.-G., N.A.A., A.D.C. and M.A.P.-Z. led eorts to
secure funding for the research. All authors discussed the results and contributed to the manuscript.
Additional Information
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Barron-Gaord, G. A. et al. e Photovoltaic Heat Island Eect: Larger solar power
plants increase local temperatures. Sci. Rep. 6, 35070; doi: 10.1038/srep35070 (2016).
is work is licensed under a Creative Commons Attribution 4.0 International License. e images
or other third party material in this article are included in the article’s Creative Commons license,
unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license,
users will need to obtain permission from the license holder to reproduce the material. To view a copy of this
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© e Author(s) 2016
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