PV Systems in Warm and Sunny Climates: Performance Assessment of Commercially Available Solar Photovoltaic Technologies Under Different Climatic Conditions in The Brazilian Energy Mix
PV Systems in Warm and Sunny Climates: Performance Assessment of Commercially Available Solar Photovoltaic Technologies Under Different Climatic Conditions in The Brazilian Energy Mix
PV Systems in Warm and Sunny Climates: Performance Assessment of Commercially Available Solar Photovoltaic Technologies Under Different Climatic Conditions in The Brazilian Energy Mix
Abstract — The performance assessment of seven different, penetration and is part of the strategic planning of the national
commercially-available PV technologies was carried out at eight electricity sector in Brazil.
different climates in Brazil. The eight identical, fully-monitored 70
kWp photovoltaic Evaluation Sites (ESs) have all electrical and II. EXPERIMENTAL SETUP AND METHODOLOGY
environmental parameters measured at one-second intervals. The
R&D project, funded by twelve Brazilian Electric Utilities in the Eight identical fully-monitored 70 kWp PV Evaluation Sites
scope of the Brazilian National Regulatory Agency’s R&D (ESs), each using seven different, commercially-available PV
Program, aims at providing performance information for Utilities technologies (single- and multi-crystalline silicon, two-axis-
to make informed decisions on which PV technology to adopt in
the forthcoming utility-scale solar energy auctions taking place in tracking 820 suns concentrated PV, and the thin-film PV
Brazil. The PV technologies evaluated are: thin-film amorphous technologies CdTe, CIGS, a-Si and a-Si/μc-Si), were installed
silicon (a-Si), microcrystalline silicon (a-Si/μc-Si), cadmium in different regions of Brazil between 2013 and 2014. Fig. 1
telluride (CdTe), copper indium gallium diselenide (CIGS), and shows the map of Brazil and the location of the eight ESs, while
bulk mono and multi-crystalline silicon (c-Si and m-Si), all at Fig. 2 shows an aerial view of one of the eight ES. The only
fixed, latitude tilt, as well as double-axis tracking, concentrated PV
(CPV) using InGap/GaAs/Ge at 820 suns concentration. difference between the eight ESs is the module tilt angle for the
Index Terms — cloud enhancement, PV performance, utility- fixed-tilt arrays at each site, which is equal to the local latitude
scale solar, O&M, PID. in each case. The ESs are constantly monitored, with irradiance,
temperature (ambient and back-of-module) and electrical
I. INTRODUCTION parameters measured and logged at one-second intervals. High-
quality, research-grade and state-of-the-art irradiance, electrical
With the declining costs of photovoltaics (PV), and the parameters and temperature measurement devices and
excellent solar energy resource availability in the country, the dataloggers were used. A more detailed description of the eight
Brazilian government and the electricity sector have started to locations, the experimental set up and equipment specifications
evaluate and consider PV as a serious potential contributor to were presented elsewhere [3].
the national electricity mix. Since the late 1990’s, Brazilian
electrical utilities are required by the National Electrical Energy
Regulatory Agency (ANEEL) to invest 1% of their operational
income on R&D. In 2011, ANEEL issued an R&D call
dedicated to utility-scale PV. The solar energy research group
at the Universidade Federal de Santa Catarina
(www.fotovoltaica.ufsc.br) has been actively investigating and
promoting PV in Brazil, and operates since 1997 the first grid-
connected, thin-film PV generator in the country [1]-[2].
This paper shows the results of this utility-scale R&D project
with the PV assessment of six fixed, flat-plate PV technologies,
as well as double-axis tracking concentrated PV (CPV)
installed at eight different sites, with distinct Brazilian climatic
conditions. The results also present some of the peculiarities
observed during the continuous and high-temporal-resolution
monitoring of PV generators at all these warm and sunny sites.
Cloud-edge and cloud-enhancement effects of solar irradiance
resulted in operational issues that were not previously described
in the literature. This information is of great scientific and Fig. 1. General location in the Brazilian territory of the eight
economic interest as the PV technology increases its identical R&D ESs, located between 3°S and 28°S, each with 70 kWp
of PV installed power evenly distributed among seven different
commercially-available PV technologies.
TABLE II
IRRADIANCE-WEIGHTED BACK-OF-MODULE TEMPERATURE
AVERAGE OF THE SIX FIXED PV MODULE TECHNOLOGIES AT
EACH OF THE EIGHT ESS DURING THE ANALYZED PERIOD.
Fig. 2. Aerial view of one of the eight identical R&D ESs, with 70
kWp of PV using seven different commercially-available PV For the presented data it can be observed that the thin-film a-
technologies.
Si technology resulted in a superior PR than the other
technologies for most of the ESs. This behavior is due to the
IV. RESULTS AND DISCUSSION: PV PERFORMANCE lower temperature coefficient of this technology, which results
ASSESSMENT in good performance in warm climates [10]. A bluer spectral
content of sunlight in the region might also have been beneficial
In a warm and sunny environment, temperature is the main
for these blue-biased PV devices. The analyzed period was after
cause of energy losses in a PV system. Table I shows the
stabilization of the Staebler-Wronski effect (SWE), which
temperature coefficient of maximum power point for the PV
affects the output performance of thin-film a-Si PV modules
module technologies used in this study and Table II presents the
strongly during the first year of outdoor operation and stabilizes
irradiance-weighted back-of-module average temperature of
after some 1000 kWh/m2 of sunlight exposure [11].
the six fixed PV module technologies at each of the eight ESs
For c-Si and m-Si technologies, a good performance is
during the analyzed period.
observed in most of the ESs, with both technologies presenting
The AC Performance Ratio (PR) assessment of the PV
very similar PR. However, a few ESs have presented a marked
technologies installed in the ESs is presented in Fig. 3. The
discrepancy between the two technologies. Field evaluations
analyzed period includes only simultaneous valid data for all
revealed intense PID degradation in coastal areas with high
the PV technologies on each ES; this is the reason why some
relative temperate and humidity at such sites. Fig. 4 shows the
sites have only a few months of available data.
effect of PID in two p-type multicrystalline silicon strings from
the Capivari de Baixo ES (28°S, 49°W), located on the coast of
southern Brazil.
(b)
(c)
Fig. 4. PID effect on two p-type multicrystalline strings from the
Capivari de Baixo ES (28°S, 49°W), located on the coast of Southern
Brazil. (a) Shows the mapping of the two strings connected to
independent MPPT channels: string M (outlined in blue, with 13
modules) and string P (outlined in orange, with 16 modules), with
Fig. 3. AC Performance Ratio (PR) of the PV technologies installed positive modules in green and negative modules in red. (b) Shows the
at the eight ESs in different climates in Brazil. EL images made for all the m-Si modules within the two strings. (c)
Shows the measured power, obtained using an I-V curve tracer,
Fig. 4 (a) shows the mapping of the two strings: string M relative to the healthiest module on the string. The modules shaded in
outlined in blue, and string P outlined in orange, with 13 and 16 red and black in the pictures are modules that were damaged and were
modules, respectively. In the image, modules in green belong thus disregarded in the study.
to the positive end of the string, while modules in red belong to
the negative end. Fig. 4 (b) shows the electroluminescence (EL) For the CdTe technology, important advances have been
images made for the same two strings. Finally, Fig. 4 (c) shows made by the manufacturer First Solar since the installation of
the measured power relative to the healthiest module on the these systems [13], and the output characteristics of Series 2
string, obtained from individually measured I-V curves. The modules installed at our eight ESs were considerably improved
modules marked in black on images (a) and (c), and shaded and in the current Series 4 product. AC performance ratios 2-3%
outlined in red on image (b), are modules that were blown away higher than multicrystalline silicon have been reported in the
from the metallic structure during an extreme meteorological literature for Northern Brazil in a similar R&D project using
event that hit the region on October 16th, 2016 with strong CdTe Series 3 modules from the same manufacturer [14].
winds (up to 200 km/h gusts), prior to the EL imaging and the At the end of the project, all installed double-axis tracking
measuring of the modules I-V curves. Many modules were concentrated PV (CPV) were inoperative or intermittent. In
severely damaged, but not all of them could be replaced, and view of the faced problems, it was not possible to assess the
therefore were put back on the structure but not connected back long-term evaluation of this PV technology. Despite the state of
to the strings they belonged to, hence should be disregarded in maturity of CPV systems, until the deployment of this project
this analysis. In Fig. 4 (b), it can be easily seen that the modules (2013-2016) no such system had been installed in Brazil. The
located towards the negative end of the strings present more equipment was supplied by the company for the first time to the
darkened cells on the EL image and, therefore, presented a Southern hemisphere, which required changes in system
lower peak power when measured with the I-V curve tracer, as control and hardware, which were not sufficient for the
seen in Fig. 4 (c). acceptable operation of the systems.
Fig. 5 shows the individually measured I-V curves for each A detailed energy loss analysis was also carried out through
of the modules on the two strings shown in Fig. 4. A reduction simulations regarding the Itiquira ES (17°S, 54°W), using PV
in shunt resistance can be seen, as well as a decrease in Voc, sizing software PVsyst 6.72 with default parameters values,
reflecting the junction to be less capable of separating holes and with exception of soiling losses, which were assessed on site
electrons [12]. through the measurement of I-V curves before and after
For the a-Si/μc-Si and CIGS technologies, a slight negative cleaning the PV modules. The simulation was performed using
discrepancy between the measured power and the power on-site measured GHI (secondary standard pyranometer) and
declared by the manufacturer was observed at the ambient temperature. The total measured loss and the simulated
commissioning of these technologies. losses are presented in Fig. 6 and in Table III a summary of the
results obtained in this part of the study is shown.