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Energy and Buildings 69 (2014) 426–433

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me pa g e: www.elsev ier .com/ locate /enbui ld

valuation of the photovoltaic generation potential and real-timenalysis of the photovoltaic panel operation on a building facade inouthern Brazil

oão Vicente Akwa ∗, Odorico Konrad, Gustavo Vinícius Kaufmann,ezar Augusto Machado

enter of Science and Technology, UNIVATES University Center, Lajeado, Rio Grande do Sul, Brazil

r t i c l e i n f o

rticle history:eceived 3 July 2013eceived in revised form9 September 2013ccepted 1 November 2013

a b s t r a c t

Solar photovoltaic systems are an alternative to the current model of power generation, supplying cleanenergy with little environmental impact and no significant losses associated with distribution networks.This study aims to obtain data on electricity generation in real time for a photovoltaic panel installed inthe city of Lajeado, Rio Grande do Sul, Brazil, comparing the electric power generation of the photovoltaicpanel with the solar radiation data of the city, obtained with the use of a pyranometer. Data from incident

eywords:enewable energynergy generation potentialolar photovoltaiclectric power generation

solar radiation, measured in the city, on the period of 2007–2012 were used. Comparisons with data frompower generation of the photovoltaic panel and assessment of the solar potential were performed. Thephotovoltaic panel has an area of 16.5 m2 and was installed on the campus of UNIVATES University Center,arranged so that it faces the true north and tilted at an angle of 24◦, for better utilization of solar radiationincident along the year. At the end of this phase of the study, it was obtained an average power generation

iency

of 11.0 kWh/day and effic

. Introduction

The continued population growth and energy consumption on alobal scale, associated with the finite nature of fossil fuels and thenvironmental impacts generated by its use decreases the reliabil-ty of the current energy model. In Brazil, the main feature of thelectrical system is the use of large plants that centralize the gen-ration of electricity, and transport this amount of energy throughxtensive transmission lines and distribution. In contrast to thisodel, arises the distributed electric power generation, in which

he generators are located close to consumers reducing the envi-onmental impact and the losses occurring in the transport of thenergy [1–5].

In the year of 2012, as reported in the national energy balancef 2013 [6], 455.6 TWh of all electricity generated in the country592.8 TWh) was obtained by hydroelectric plants. It is a formidableontribution to the electric power generation, much higher than theydropower share that occurs in most countries. Even with large

lectric power generation from hydroelectric plants, the presentnstalled capacity is 0.084 TW that represents only 32.4% of theotential for hydroelectric generation in Brazil, which is equal to

∗ Corresponding author at: Rua Avelino Tallini, 171, Lajeado, RS 95.900-000, Brazil.el.: +55 51 3714 7000x5201.

E-mail addresses: jvakwa@univates.br, joaoakwa@gmail.com (J.V. Akwa).

378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.enbuild.2013.11.007

of the modules in the order of 12.6%.© 2013 Elsevier B.V. All rights reserved.

0.260 TW. However, the largest share of favorable sites for hydro-electric plants is located in environmental conservation areas, awayfrom urban centers. This scenery also promotes the construction oflow-power hydroelectric plants. To provide this much electricityto the final consumer, the country needs to maintain under sat-isfactory operation, one of the largest interconnected systems fortransmission and distribution of electricity in the world, resultingin increased cost to the kWh generated. Another factor that causesvulnerability for the system is the dependence on favorable climaticfactors, such as rainfall regularity [6–8].

In Brazil, the largest contribution for electricity generation in2012 was hydropower (76.9%), followed by gas (7.9%), biomass(6.8%), petroleum products (3.3%), nuclear (2.7%), coal and oilproducts (1.6%) and wind power (0.9%) [6]. The diversificationof energy sources and the use of distributed generation withrenewable energy could be an alternative to strengthening thecountry’s energy system. The use of decentralized power gener-ation decreases the costs with transmission and distribution ofenergy and reduces the strain on the national interconnected sys-tem. The generation of electricity by photovoltaic panels integratedinto buildings fits into this context [8]. Although the installed capac-ity of photovoltaic generation is negligible among other forms of

generation, Brazil has a huge solar energy potential because its ter-ritory, covering more than 8.5 million km2, has most of its landbetween latitudes 5◦ N and 33◦ S, on the tropical zone. Rüther [1]compares the potential of solar photovoltaic generation in Brazil

d Buildings 69 (2014) 426–433 427

atr4cth

lw[taseg

pesce

edtifopsivw

aataif

2

2

bvaTypthPtGfa

(vdl

Fig. 1. Photovoltaic module.

Fig. 2. Modules installation.

J.V. Akwa et al. / Energy an

nd Germany, the country with the largest installed capacity forhis source of energy. He concluded, from maps of incident solaradiation, that solar radiation in the sunniest region of Germany is0% lower than in the less sunny region of Brazil, proving the greatapacity of solar photovoltaic power generation in Brazil. Accordingo the solarimetric atlas [9], the Brazilian annual average for globalorizontal solar radiation varies from 3.89 to 5.56 kWh/m2/day.

In 2011, a few projects of photovoltaic plants, totaling 804 MW,ocated primarily in the northeastern region of Brazil, were under-

ay, but without any specific date for installation and completion10]. In 2012, a normative resolution of the National Agency of Elec-ric Energy was created to regulate the general conditions for theccess of distributed generation into the electric power distributionystem, as well as compensation procedures for those generatingnergy [8,11]. This may give new perspectives to decentralizedeneration through photovoltaic panels in the country.

The direct generation of electricity from sunlight through thehotovoltaic effect, presents itself as one of the best ways of gen-rating electrical power [12]. Thus, the inclusion of photovoltaicystems in the national energy supply, in a complementary way,ould bring great benefits to the energy sector, as well as to theconomic, environmental and social development [8,12,13].

Considering the importance that solar energy can have on thenergy sector in Brazil, this research work aims to obtain real-timeata for a photovoltaic electricity generating system installed inhe municipality of Lajeado, Rio Grande do Sul, southern Brazil,n order to study the viability of this source of solar energy. Datarom incident solar radiation, measured in the city, in the periodf 2007–2012, were used in comparisons with data from electricower generation of a photovoltaic panel. An assessment of theolar potential was also performed. The study, however, is notntended to assert that the engineering solution of solar photo-oltaics, linked to the self-generation of energy, is the best or onlyay to introduce in practice.

The study is justified by the need to evaluate the performance of solar photovoltaic panel if it were installed in the city of Lajeadond the possibility to estimate the benefits that it could bring botho the people and to the environment. There is also a lack of databout potential of solar electricity generation in the region. Thisnformation is only available to cities distanced more than 80 kmrom the study site [14].

. The photovoltaic panel

.1. Installation of the photovoltaic panel

The photovoltaic panel used in this study was placed on aalcony of the Center of Science and Technology of UNIVATES Uni-ersity Center in the city of Lajeado, as shown in Figs. 1–3. It wasrranged so that it faces the true north and tilted at an angle of 24◦.he commercial software PVsyst, which is used for sizing and anal-sis of photovoltaic systems, was used to define the tilt angle of theanel. The software presented this tilt value as the most suitable forhe location where the solar panel was installed, rendering a moreomogeneous incident radiation throughout the year. According toereira and Oliveira [15], the tilt of solar modules should be definedaking into account the solar angle elevation throughout the year.iven the difficulty in changing the inclination of the absorbing sur-

ace throughout the year, the angle was chosen by the type of use,s detailed in Table 1.

As shown in Table 1, the recommended tilt angle is 24◦

29◦ − 5◦ = 24◦) for the desired need in this research study. The samealue is displayed by the software PVsyst, therefore it is a reliableata. The effect of varying the tilt angle of the panel was also ana-

yzed, using data on solar radiation, measured over five years on the

Fig. 3. Solar panel operation.

campus of UNIVATES. The results and conclusions of this analysisare discussed in Section 4.4.

The installation of the photovoltaic panel occurred in August2012. The design of the electrical system and the panel support

428 J.V. Akwa et al. / Energy and Buildings 69 (2014) 426–433

Table 1Tilt of the solar panel as required in installation [15].

Use Method for obtaining an tilt angle

Summer (hotels of the season) Subtract 15◦ from the angle of local latitudeWinter (heating) Sum up 15◦ from the angle of local latitudeAnnual (domestic or other

non-seasonal)Subtract 5◦ from the angle of local latitude

Table 2Electrical parameters of the used photovoltaic array [20].

Electrical parameters Values at standardtest conditions

Values at nominaloperating cell

Power output 0.230 kW 0.167 kWVoltage at maximum power 29.5 V 26.6 V

scT2

2s

maaetacnpmd

dpwa2

ctfcp

3a

d

Ishr

∑31j=

Current at maximum power 7.80 A 6.29 AOpen-circuit voltage 37.0 V 33.8 VShort-circuit current 8.40 A 6.81 A

tructure were included in this process. The DC/AC inverter, used toonnect the photovoltaic panel to the electric grid, was the Sununo-L 2K, from the brand SAJ. This equipment has a rated power of

kW, works with 220 V on alternating voltage and current of 11 A.

.2. Technical features of the photovoltaic panel examined in thistudy

In this study, ten photovoltaic arrays of model YL230P-29b,anufactured by YINGLI SOLAR, were used in the tests. Each

rray contains 60 multicrystalline silicon solar cells, connected in series–parallel combination, which produces an output powerqual to 230 W/array, under standard test conditions. The standardest conditions are characterized by photovoltaic array cell temper-ture of 25 ◦C, irradiance of 1 kW/m2 and spectral content of lightorresponding to air mass equal to 1.5. These ten arrays were con-ected in series, providing an output power of 2.3 kW for a totalanel area of 16.5 m2 and efficiency equal to 14.1% [20]. The esti-ate of electric power generated in one month is 250–300 kWh,

epending on the local weather conditions.Table 2 shows the electrical parameters at standard test con-

itions and nominal operating cell temperature of the usedhotovoltaic array. At nominal operating cell temperature the arrayorks under conditions of open-circuit module operation temper-

ture at irradiance equal to 0.8 kW/m2, ambient temperature of0 ◦C, and wind speed equal to 1 m/s.

After installation and testing phase, the photovoltaic panel wasonnected to an electrical meter that recorded the amount of elec-ricity produced and an inverter, which converted the electricityrom direct current to alternating current. The whole system wasonnected to the electric power system of the building, where theower generated is sent to the public grid.

. Collection and processing of data from global radiationnd day length

The weather station DAVIS Vantage Pro 2 was used to obtainaily data on global horizontal solar radiation and hours of daylight.

HJanuary =

[∑31j=1Hj/31

]2007

+[∑31

j=1Hj/31]

2008+

[

t was located on the campus of UNIVATES, free of obstacles andhadows, at 85 m of altitude in relation to sea level. This stationas been operating since 2003 and data provided are reliable to aadius of 30 km. Radiation data is collected every half hour. Fig. 4

Fig. 4. Weather station DAVIS Vantage PRO 2.

shows the station in operation and its console. The station has aphotovoltaic pyranometer for the measurement of global horizon-tal solar radiation. The accuracy in measuring this parameter is 5%of the measured value.

The photovoltaic panel was installed on the Center of Scienceand Technology of UNIVATES University Center, on the geographiccoordinates 29◦26′54′′ S and 51◦56′42′′ W. The weather station isinstalled at 1300 m west from this site. As this weather station pro-vides reliability on measurement, valid within a radius of 30 km, theradiation data and hours of daylight, collected by it, are assumed tobe valid when compared with the electricity production by photo-voltaic panels.

3.1. Statistical treatment of the data

For the statistical treatment of the data obtained from theweather station, data from five years of incident solar radiation,from 2007 to 2011 (period without interruption in the measure-ments), and hours of daylight were converted to daily data, andprocessed into monthly averages. As the pyranometer recordsinstantaneous radiation data on a horizontal plane, G, every 30 min,the global radiation accumulated during a day, H, is obtainedthrough integration over the correspondent time, according to Eq.(1) [17]. Monthly averages for horizontal global radiation, H, areobtained with the arithmetic mean of daily data for radiation. Thehistorical average for a month is obtained by averaging the dataover the five years of measurement, as exemplified for January inEq. (2). The pyranometer was used to record the hours of daylightand arithmetic means were utilized to obtain the historic averageof hours of daylight for each month of a year.

H =∫

G dt ∼=48∑i=1

Gi · �t =48∑i=1

Gi

(kWm2

)0.5 (h) (1)

1Hj/31]

2009+

[∑31j=1Hj/31

]2010

+[∑31

j=1Hj/31]

20115

(2)

In addition to the radiation data of the five years mentioned,

the radiation measured on the same day of the photovoltaic paneloperation was recorded, for the purpose of comparing it with theelectric power generated. The instantaneous data of radiation werealso transformed into daily data with the use of the Eq. (1).

J.V. Akwa et al. / Energy and Buildings 69 (2014) 426–433 429

3

rootod

ofhrafssbt�th

H

R

ieHpdvosid

s ≤ 81.4◦ and KT < 0.715

◦

Table 3Annual radiation recorded on five years of measurements.

Year Annual radiation on a horizontal plane (kWh/m2)

2007 1476.642008 1595.262009 1468.09

five years of measurement can be seen in Fig. 6, which presentsthe daily average horizontal radiation, in kWh/m2/day, in relationwith time. It is also shown the average behavior of this parameter,

Fig. 5. Isotropic sky model [17].

.2. Calculation of radiation on the tilted plane

The isotropic sky model developed by Liu and Jordan [16] andefined by Duffie and Beckman [17] was used to convert the valuesf solar radiation on a horizontal plane into values of solar radiationn a tilted plane (tilt of 24◦). As shown in Fig. 5, this model conceiveshat the solar radiation on a tilted plane is composed of three parcelsf radiation: beam from the sun, isotropic diffuse radiation from skyome, and reflected radiation from the ground.

The global solar radiation on a tilted plane, Hˇ, in kW/m2, isbtained by Eq. (3), in which H is the global solar radiation thatocuses on a horizontal surface, Hd is the diffuse solar radiation on aorizontal surface, Rb represents the geometric factor (ratio of beamadiation on tilted surface to that on horizontal surface); is the tiltngle of the panel; � indicates the percentage of radiation reflectedrom the ground. � reaches values as high as 0.9 for grounds withnow or white sand. There is a lot of vegetation and dark soil in thetudied area, so a value of 0.2 was assigned to �. Rb can be calculatedy Eq. (4), where � is the angle of incidence, or the angle betweenhe beam radiation on a surface and the normal to that surface; andz is the zenith angle, which is the angle between the vertical andhe line to the sun, or the angle of incidence of beam radiation on aorizontal surface.

ˇ = H(

1 − Hd

H

)Rb + Hd

(1 + cos ˇ

2

)+ H�

(1 − cos ˇ

2

)(3)

b = cos �

cos �z(4)

The ratio Hd/H in Eq. (3) is a function of KT, the day’s clearnessndex. (H/H0) is the ratio of daily total radiation, H, to the dailyxtraterrestrial radiation, H0. Eq. (5) expresses the correlations tod/H, where ωs is the sunset hour angle, which is the angular dis-lacement of the sun (at sunset) east or west of the local meridianue to rotation of the earth on its axis at 15◦ per hour, with negativealues for the morning and positive values for the afternoon. H0 isbtained by integrating Eq. (6) over the period from sunrise to sun-et. In the Eq. (6), G0 is the instantaneous extraterrestrial radiationncident on the plane normal, in kW/m2, to the radiation on the nthay of the year and Gsc is the solar constant, equal to 1.367 kW/m2.⎧⎪⎪⎪⎪⎨

1 − 0.2727KT + 2.4495K2T − 11.9514K3

T + 9.3879K4T for ω

Hd

H= ⎪⎪⎪⎪⎩

0.143 for ωs ≤ 8

1 + 0.2832KT − 2.5557K2T + 0.8448K3

T for ωs >

0.175 for ωs > 8

2010 1467.532011 1504.60Average 1502.07

G0 = Gsc

(1 + 0.033 cos

360n

365

)cos �z (6)

With the ratio of the horizontal radiation to the tilted radiation,the data on electricity generation of the photovoltaic panel can becompared with the solar energy available. With these values, theperformance of the panel in its operation can also be calculated.This procedure is automatically performed with the aid of the opensource software RADIASOL, developed by the Solar Energy Labora-tory of Federal University of Rio Grande do Sul, which automatesthe manual calculations regarding the model described in the pre-vious equations. The software allows the entry of data measured inmeteorological stations. It also offers a database of meteorologicalstations located in nearby cities [14].

4. Results and discussion

4.1. Radiation data from the five years of measurements

The solar radiation that falls on the surface of the Earth consistsof a beam part and a portion of diffuse radiation, caused by theradiation scattering in the atmosphere [16,17]. The global radiationthat can reach the photovoltaic panel must be quantified in orderto assess the potential of solar energy. Both parcels of the radiation(direct and diffuse) are considered to collaborate for photovoltaicgeneration. In order to get the potential of photovoltaic solar energyof the city of Lajeado, data were acquired from horizontal globalsolar radiation, in unit kW/m2, through measurements taken every30 min by pyranometer installed at the Hydrometeorological Infor-mation Center of UNIVATES.

The period of data collection through the pyranometer was fromJanuary 2007 to January 2012. Since the pyranometer provides dataevery 30 min, it was calculated the integration of the data obtainedby the pyranometer for each day, in a way to obtain the averagedaily radiation data (in kWh/day and kWh/m2/day) for each monthof the year. This methodology is described by of Eqs. (1) and (2).

Table 3 shows the annual radiation recorded on five years ofmeasurements. These data express the solar energy available toeach analyzed year, per square meter, on a horizontal plane. Theaverage solar energy available per year, calculated based on thearithmetic mean of measurements of the five years, has a valueof 1502.42 kWh/m2 for a standard deviation equal to 3.5% of thisvalue. 1.78 kWh/m2, referring to February 29, were subtracted fromthe amount of energy of 2008 to calculate the average. The way inwhich the available radiation on a horizontal plane varied over the

1.4 and KT ≥0.715

81.4◦ and KT < 0.722

1.4◦ and KT ≥0.722

(5)

430 J.V. Akwa et al. / Energy and Buildings 69 (2014) 426–433

recorded on five years of measurements.

ciae

4

euir

spofptdlts

ozvsma5

TA

Fig. 6. Average horizontal radiation

alculated with the arithmetic mean. The radiation values recordedn this study indicate that the levels of radiation in the region aredequate for the type of power generation studied and the solarnergy potential does not show great irregularities over the time.

.2. Incident radiation and day length

In Table 4, daily average horizontal global solar radiation, forach month of the year, are shown. These values were obtainedsing the methodology described by of Eq. (2). The data presented

n this table can be better analyzed when expressed in the graphicalepresentations of Figs. 7 and 8.

Analyzing the Figs. 7 and 8, it is observed that the values mea-ured by the weather station are in agreement with the valuesrovided by LABSOL/UFRGS [14], by interpolation with the use ofpen source software RADIASOL, representing that they are validor the proposed situation. The open source software RADIASOLrovides values for solar radiation from some meteorological sta-ions data in Brazil [14]. The software allows interpolation to theata of the nearest weather stations to obtain radiation data at

ocations where measurements are not conducted. The interpola-ion used was performed with the values of the nearest weathertations, distant about 80 and 120 km from the study site.

Based on the values measured by the weather station, the valuef 4.11 kWh/m2/day was obtained for the average annual hori-ontal solar radiation. This value differs by 1.6% compared to thealue obtained by interpolation with data from nearby weather

tations. The value is also in agreement with the Brazilian solari-etric atlas [9] which mentions that, in the country, the annual

verage for global horizontal solar radiation varies from 3.89 to.56 kWh/m2/day.

able 4veraged horizontal global solar radiation, for each month of the year.

Month Horizontal global radiation per unit of area obtained byinterpolation from the nearest weather stations(kWh/m2/day) [14]

January 5.58

February 5.25

March 4.64

April 3.74

May 2.79

June 2.36July 2.55

August 3.05

September 3.81

October 4.61

November 5.63

December 6.13

Fig. 7. Graphical representation of the horizontal solar radiation per unit of area.

Using data provided by the pyranometer, the sum of hours ofdaily global radiation was performed. The monthly average of thisdata was also made to determine whether the site is suitable forthe installation of photovoltaic systems or not. Table 5 presents themonthly average of hours of daylight (hours of global radiation),while Fig. 9 shows its graphical representation.

As shown in Table 5, the months with the greatest day lengthare January, February, November and December, all above the13 h daily. In the other hand, the months of May, June and Julyare the ones with less hours of daylight, with averages around

Horizontal global radiation perunit of area obtained bymeasurements (kWh/m2/day)

Horizontal global radiation onarea equivalent to panel area(kWh/day)

5.67 93.605.13 84.604.65 76.703.67 60.502.67 44.002.27 37.402.42 40.002.97 49.003.70 61.004.42 73.005.60 92.406.18 101.9

J.V. Akwa et al. / Energy and Buildings 69 (2014) 426–433 431

Fig. 8. Horizontal solar radiation over an area equivalent to the area of the panel.

Table 5Hours of global radiation per day.

Month Daylength (h)

January 14.0February 13.3March 12.5April 11.6May 10.8June 10.4July 10.6August 11.1September 11.9October 12.9

1lTAot

4

gpf

ttn

Fig. 9. Graphical representation of the global radiation hours per day.

TI

November 13.8December 14.2

0 h/day. According to Rüther [1] these amounts of hours of day-ight are attractive for investments in photovoltaic generation.hese amounts also differ from those presented in the Solarimetrictlas of Brazil [9], because it only considered the direct radiationn the earth’s surface, when in the present study it was performedhe sum of the beam and diffuse radiation.

.3. Electric power generation from the photovoltaic panel

The photovoltaic panel started its operation on August 16, 2012,enerating electricity from the solar radiation incident on the tiltedlane. In August, electric power generation data were obtainedrom the 16th to 24th, as shown in Fig. 10.

The electric power generation during the period aforemen-ioned maintained an average value equal to 11 kWh/day. Betweenhe 25th and 31st of August, the solar panel was under mainte-ance and therefore it was not possible to record the amount of

able 6ncident radiation and electric power generation.

Day Measured horizontalradiation (kWh)

Averaged horizontalradiation (kWh)

Averaged tiltradiation (kW

8/16 52.17 48.84 59.90

8/17 75.72 48.84 59.90

8/18 68.73 48.84 59.90

8/19 63.60 48.84 59.90

8/20 70.36 48.84 59.90

8/21 69.95 48.84 59.90

8/22 80.14 48.84 59.90

8/23 82.91 48.84 59.90

8/24 76.59 48.84 59.90

Fig. 10. Graphical representation of the electric power generated on August 2012.

electricity generated. Fig. 11 shows the data of electric power gen-eration in September, in which it can be observed the variationof electric power generation due to adverse climate of the month(cloudy and rainy). In September, the most productive days in termsof power generation were the 14th, 22nd and 28th, generatingmore than 14 kWh/day each. The monthly average generation was

8 kWh, within the expected for a rainy month in the region wherephotovoltaic panel was installed [18,19].

edh)

Estimated tiltedradiation (kWh)

Electric powergeneration (kWh)

Estimatedefficiency (%)

63.98 7.500 11.7292.87 12.60 13.5784.29 10.80 12.8178.00 9.500 12.1886.29 10.30 11.9485.79 11.30 13.1798.29 12.90 13.12

101.7 13.00 12.7893.93 11.40 12.14

432 J.V. Akwa et al. / Energy and Buildings 69 (2014) 426–433

coitptsttgdi

gdrg

4p

pnosmiUloa(p

t2tfioife

Fig. 11. Electric power generated on September 2012 and rainfall.

Data on electricity generation from the photovoltaic panel wereompared with rainfall data obtained by a meteorological stationwned by the company CERTEL ENERGIA [18]. This station is locatedn the hydroelectric plant of Boa Vista, road RST 453, km 47.8, inhe neighboring city of Estrela, 8 km far from the local where theanel was installed (29◦28′21′′ S–51◦52′05′′ W). It was concluded,hat the days when there was little electricity generation were theame days when rainfall occurred in the region. In this situationhe diffuse solar radiation was the main responsible for the elec-ric power generation. The comparison between the electric powerenerated and rainfall is also presented in Fig. 11. The need to useata from another meteorological station was due to maintenance

n the station of UNIVATES.As shown in Fig. 11, it is visualized that the days of lower power

eneration coincided with the highest rainfall days, due to lowirect incident radiation on the photovoltaic panel. The graphic rep-esentation also reinforces the concept that, although lower, powereneration can still occur while there is diffuse solar radiation.

.4. Amount of solar energy incident on the plane of thehotovoltaic panel and effect of varying tilt angle of the panel

As the pyranometer captures solar radiation on the horizontallane and the photovoltaic panel is tilted at an angle of 24◦, it isecessary to calculate the amount of solar radiation on the planef the panel to correct data correlation. For this purpose, the openource software RADIASOL was used with the intent to facilitateanual calculations required under the isotropic sky model used

n this study, described in Section 3.2, based on the Eqs. (3)–(6).sing this software it was possible to enter the information col-

ected by the pyranometer, obtaining the incident global radiationn the desired angle. The software calculates the statistical aver-ge of a tilted plane incident radiation based on the input datameasured data), geometric data and geographical position of theanel.

Fig. 12 shows the monthly average data of incident radiation onhe horizontal plane collected by the pyranometer in the period of007–2012 (at tilt angle equal to 0◦) and the amount of incidentilted radiation to different tilt angles of the panel. Analyzing thegure, it can be observed the effect that occurs when the inclination

f a plane is changed. The optimum fixed tilt angle for a full years one that, using the average of the daily average radiation valuesor each month, causes the panel to receive the greatest amount ofnergy. For the city of Lajeado, as shown in Fig. 12, angles between

Fig. 12. Global radiation incident on the horizontal and tilted plane.

20◦ and 30◦ provide the largest amount of solar energy during theyear compared with other values. Based on these data, it is verifiedthat the tilt angle of 24◦ is appropriate. The difference betweenthis angle and approximate angles is small in respect to energyobtained. For tilt angles near zero (horizontal plane) and anglesnear 90◦, the obtained average solar energy reduces significantly.

Due to change in solar elevation throughout the year, a steepangle of the panel in the winter provides a more radiation onthe surface, maximizing the power generation. For the summer,small tilt angles provide increased power generation. However, fora panel with a fixed inclination throughout the year, an interme-diate value must be chosen to homogenize the power generationthroughout the year. According to the analysis, this angle is 24◦.

In the case of a tilt angle at 0◦, the annual average solar radiationequals to 67.7 kWh/day on the panel area, while in the case of a tiltangle at 24◦ it is obtained a value of 73.8 kWh/day for this sameparameter. When it is used an excessively higher inclination, forexample 70◦, the amount of solar radiation obtained is reduced to58.6 kWh/day. Therefore, the chosen tilt angle is adequate for therequired purpose.

4.5. Comparative between solar radiation and power generation

On all equipments used for converting one form of energy intoother energy type, there is not a conversion efficiency of 100%,because in the process always exists losses, according to the SecondLaw of Thermodynamics. Furthermore, the electric power gener-ated by the photovoltaic panel varies depending on the levels ofincident direct solar radiation and levels of dust and clouds thatinterfere on diffuse solar radiation.

During the study period, the daily solar radiation on the hor-izontal plane had an average value equal to 71 kWh/day on thearea of the panel, reaching peaks of 82.9 kWh/day, as shown inTable 6. Based on the values collected by the pyranometer on theperiod of 2007–2012, the average of the values for solar energyin August was performed using the Eqs. (1) and (2), and the valueof 48.84 kWh/day was obtained. The value of 59.9 kWh/day wasobtained as average on tilted solar radiation for this month, withthe use of free use software RADIASOL [14], to facilitate manualcalculation, in Eqs. (1)–(6), of the isotropic model sky.

It is calculated that the radiation data on the tilted surface is 1.23times higher than the data corresponding to the horizontal radia-tion (Eq. (4)), and it is estimated that the daily radiation incident on

the tilted surface also retains this ratio. This estimation was usedto accomplish the calculations, otherwise the pyranometer wouldhave to be inclined at the same angle of the panel for the instan-taneous tilted radiation data to be collected. The data estimation

J.V. Akwa et al. / Energy and Buil

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Fig. 13. Incident solar radiation and electric power generated.

or incident solar radiation on the tilted plane can be comparedith the daily electric power generated, thus the efficiency of thehotovoltaic panel can be estimated.

The comparative between the averages for horizontal and tiltedncident solar radiation in August can be better visualized in theraphical representation of Fig. 13. The comparative between thestimated daily solar radiation on a tilted plane and the electricower generated by the photovoltaic panel can also be better visu-lized in Fig. 13.

As can be seen in Fig. 13, the incident solar radiation is aimiting factor of the photovoltaic electric power generation, so thathe moments of highest solar radiation incidence are coincidingith those moments of higher power generation. The genera-

ion of electricity has remained with an average value equal to1 kWh/day, operating without large variations. The efficiency ofhe photovoltaic panel to convert solar energy into electricity waspproximately 12.6%.

In Fig. 13, the measured data on radiation are higher than theverage data for the month, calculated by the equations of Section. This is because the average is based on five years of measurementhistorical average) and the data are relating to true days of August,012. As the measurements occurred in the second half of August,

t is natural for them to appear above the average of the month,ince the locality is in the southern hemisphere and the hours ofaylight continuously increase with proximity to the spring.

. Conclusion

The study aimed to install a photovoltaic panel in the city ofajeado, Rio Grande do Sul, Brazil, in order to obtain real-time dataf power generation. The study is pioneer in this respect, as pre-iously there were only estimates for solar power generation inhe region. It has never been performed a monitoring of incidentolar radiation in real-time and its comparison with solar powereneration in the region.

The value of 4.11 kWh/m2/day was obtained for the averagennual solar radiation incident on a horizontal plane. This solar

otential is suitable for electricity generation from photovoltaicanels set on a proper tilt. Besides being suitable for electric powereneration, the values for solar energy potential do not presentreat irregularities over the time.

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dings 69 (2014) 426–433 433

As an important factor that can influence the power generation,it is emphasized the inclination of the panel, which should be at anangle equal to 24◦. Based on data obtained for average global solarradiation for the period from 2007 to 2012, it is observed that thetilt at an angle of 24◦ homogenizes the indices of incident radia-tion on the panel during the year. In the summer months there isa decrease in solar radiation incidence from 2% to 5%, but in thewinter there is an increase in incident solar radiation ranging from20% to 35%, proving that the tilt of photovoltaic panels is a feasibleimprovement, with virtually no cost, which positively influencesthe generation of energy in the months with lower solar incidence.

It can be stated that the study has reached its objectives. Onlythe installation of the photovoltaic system itself has been a richsource of data regarding electric power generation in real-time.This information until then was lacking, and now it can be usedfor the region of the municipality of Lajeado. Based on the resultsobtained in this study, it was concluded that a photovoltaic systeminstalled in the aforementioned region produces sufficient power,and can be used in providing electricity to homes and businesses,incorporated in their facades or roofs.

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