technical performance evaluation of stand-alone photovoltaic array for outdoor field conditions of...
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Applied Energy 92 (2012) 644–652
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Applied Energy
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Technical performance evaluation of stand-alone photovoltaic array for outdoorfield conditions of New Delhi
Rakhi Sharma ⇑, G.N. TiwariCentre for Energy Studies (CES), Indian Institute of Technology Delhi, Haus Khas, New Delhi 110 016, India
a r t i c l e i n f o
Article history:Received 12 October 2010Received in revised form 23 May 2011Accepted 20 June 2011Available online 15 September 2011
Keywords:Power conversion efficiencyPV arrayElectrical energy outputPV operating temperature
0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.06.033
⇑ Corresponding author. Tel.: +91 9868548535; faxE-mail address: [email protected] (R. S
a b s t r a c t
In this communication, an attempt has been made to investigate the performance assessment of a solarphotovoltaic (PV) array system based on electrical energy output and power conversion efficiency. Sim-plified mathematical expressions for evaluating performance indices using experimental observations forentire PV array and its individual component subarrays on daily, monthly and annual basis have alsobeen developed. Experiments have been carried out on two individual 1.2 kWp and 1.12 kWp componentsubarrays of 2.32 kWp stand-alone PV array system for climatic condition of New Delhi (latitude:28�350N, longitude: 77�120E and an altitude of 216 m above mean sea level). Individual performancesof both component subarrays were evaluated and its effect on the actual performance of entire PV arrayhas been presented. Numerical computation was carried out for a typical clear day in the month of July2010. It was found from experimental results that daily power conversion efficiency of entire PV arrayand its component subarrays1 and 2 were 6.24%, 9.5% and 3.9% respectively. For more effective perfor-mance assessment of PV array/subarrays, on field experimental performance results have been comparedwith the rated (max.) results estimated at STC and also with the maximum performance results estimatedfor actual climatic conditions as obtained during experimentation.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Renewable technologies are substantially safer offering a solu-tion to meet present increasing demand of electrical power andmany environmental and social problems associated with fossiland nuclear fuels. People pay more and more attention to thehigh-quality and renewable solar energy, therefore, testing andpredicting PV arrays in order to put solar energy into full use be-come a focus [1]. A PV array [2,3] consists of a collection of solarcells connected in series and/or parallel. Each of these cells is basi-cally a p–n diode that can convert the light energy into electricalenergy. The parameters of PV arrays provided by manufacturersare given in the standard testing conditions (ISTC = 1000 W/m2,TSTC = 25 �C). These parameters at STC do not actually reflect thecharacteristics of PV arrays in actual application conditions dueto change in temperature and solar illumination around PV array.The electrical output of PV arrays vary with environment remark-ably, the output power of PV arrays change with different illumina-tion and temperature [4]. These conditions are not always typicalof how PV modules and array operate in the field, and actual per-formance is usually 85–90% of the standard test condition [5,6].
ll rights reserved.
: +91 11 26591251.harma).
Models that relate the PV performance to irradiance and tempera-ture are well developed [7–10]. When the solar radiation increases,the short circuit current, maximum power and conversion effi-ciency will increase [11].
In addition, according to the demand of output power PV cells ormodules are assembled in different series parallel combinations.Considering the year’s use of PV arrays, various losses in arrayand the changing working condition, the parameters of PV arraysand its performance indices cannot hold the line. Therefore relyingsolely on the standard parameters of PV arrays, PV system designwill always be difficult to achieve the desired effect. Researchand development work on the photovoltaic technology is continu-ously going on for many years. Many innovative systems and prod-ucts have been put forward and their quality evaluated byacademics and professionals. Some studies can be found in litera-tures for on field performance evaluation of standalone PV arraysystem and most of them are either needed detailed data andcomplex to use or usually restricted to economic performance eval-uation [12–18]. Standard Test Condition data can lead to an overes-timation of the production. Reliable knowledge on the performanceof different photovoltaic generators under actual operatingconditions is essential for correct product selection and accurateprediction of their electricity production [19,20].
In this paper to study the on field technical performance of PVarray/subarrays, performance indices of a photovoltaic (PV) system
Nomenclature
A area (m2)Dn number of clear days in nth monthEPV array total daily hourly electrical energy output of PV array
(kW h/day)Esubarray1 total daily hourly electrical energy output from
subarray1 (kW h/day)Esubarray2 total daily hourly electrical energy output from
subarray2 (kW h/day)ðEPV arrayÞn average of daily hourly PV array electrical energy
output measured for No. of typical days of nth month(kW h/day)
hEFS equivalent hours of full sunshine (hours)Isc short circuit current (A)It1 average of daily hourly measured solar radiations over
the PV subarray1 area (W/m2)It2 average of hourly measured solar radiations over the PV
subarray2 area (W/m2)ðIt1Þn average of total daily hourly measured solar radiations
for No. of typical days of nth month over the PVsubarray1, (W/m2)
ðIt2Þn average of total daily hourly measured solar radiationsfor No. of typical days of nth month over the PVsubarray2 (W/m2)
Ip peak intensity with value of 1000 W/m2
Iave average of incident solar radiation (W/m2)kWp kilo Watt peak, max. or peak power at STCMs no. of modules in series in a subarrayMp no. of modules in parallel in a subarrayT temperature (�C)Pm peak power of array measured in standard test
conditions (Wp)Voc open circuit voltage (Volt)
Greek symbolgsubarray1 power conversion efficiency of subarray1 for a typical
day (%)gsubarray2 power conversion efficiency of subarray2 for a typical
day (%)
gPV array actual electrical efficiency or power conversionefficiency of entire PV array consists of number ofsubarrays (%)
ðgPV arrayÞannual actual electrical efficiency or power conversionefficiency of entire PV array calculated on annual basisusing monthly observation (%)
gSTC actual electrical efficiency or Power conversionefficiency at STC (%)
b array efficiency coefficient (%)nmax maximum daily energy output of PV array (kW h/day)nreceived total energy received by flat unit area during a day
(kW h/day)
AbbreviationFF fill factor (dimensionless)SAPV stand-alone photovoltaicCEL company of PV module (35Wp) (36 circular shape solar
cells in a module)Siemens company of PV module (75Wp) (36 pseudo square
shaped solar cells in a module)OT operating temperature (�C)STC standard test conditionUAO unit array output (Wh/Wp/day)IIT Indian Institute of Technology
Subscriptn nth month where n = 1, 2, . . . , 12s modules in seriesp modules in parallel1 for subarray12 for subarray2oc open circuitsc short circuitm moduleave average
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such as electrical energy output and actual electrical efficiency orpower conversion efficiency of PV array and its component subar-rays have been calculated from on field data after conducting theexperiment. Further these actual results are compared with nomi-nal rated performance results, which can be useful for any possibleimprovements. For comparison nominal rated performance resultshave been corrected or calculated for same environmental condi-tions of PV operating temperature and incident solar intensity asobtained during experimentation. For existing PV array/subarrays,to evaluate approximate value of on field performance on the daily,monthly and annual basis useful simplified mathematical modelhave also been developed using experimental parameters.
2. General PV terms
Stand-alonesystem
An energy generating system that worksentirely on its own
Solar cell
A basic PV device that covert sunlight intodirect-current (dc) electricityPV module
An appropriately interconnected combinationof solar cells with two output terminalsPV array
An installation of appropriatelyinterconnected combination of PV modules/panelsPV subarray
An array can be divided into number ofsubarrays for engineering convenienceBalance of system(BOS)
Components of a photovoltaic system otherthan the photovoltaic array. BOS mainlycomprises electronic components, cabling,support structures and, if applicable,electricity storage devices
Photovoltaic (PV)
System A complete set of components forconverting sunlight into electricity by thephotovoltaic process, including the array andbalance of system componentsShort circuitcurrent (Isc)
The maximum current delivered by a solarcell to short circuited terminals (zeroresistance), which is directly proportional tothe incident solar intensity and cell surfacearea
Open circuitvoltage (Voc)
The maximum voltage produced by a solarcell under open circuit conditions (withoutany connected load)
AC Loads
ChargeController20A,48V
Bat
tery
Ban
k 36
0Ah,
48V
PV Subarray 2 CEL Make (1.12 kWp)
Charge Controller 20A, 48V
Inve
rter
3k
VA
PV Subarray 1 Siemens Make
(1.2 kWp)
Fig. 1. 2.32 kWp stand-alone PV system for mud house at IIT Delhi, India.
Fig. 2. PV subarray1: 1.2 kWp (Siemens make).
Table 1PV module technical specifications.
PV array (2.32 kWp) PV subarray1 (1.2 kWp) PV subarray2 (1.12 kWp)
PV modules make Siemens CELModule details Peak wattage – 75 Wp Peak wattage – 35 Wp
Isc – 4.8 A Isc – 2.35 AVoc – 21.7 V Voc – 20.5 VIrated – 4.4 A Irated – 2.1 AVrated – 17 V Vrated – 16.5 V
Module area 0.605 m2 0.4 m2
646 R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652
3. System description and experimental instrumentation
3.1. Design and installation of 2.32 kWp stand-alone PV system
Fig. 1 presents design and installation of 2.32 kWp PV system formud House at IIT Delhi, India. This stand-alone PV system of2.32 kWp, is equipped with, two subarrays of rating 1.2 kWp and1.12 kWp each as shown in Figs. 2 and 3 respectively. These subar-rays; PV subarray1 and PV subarray2 consist of 16 modules(Siemens make, 15 years old) of 75 Wp each and 32 modules (CELmake, 25 years old) of 35 Wp each respectively. The modules arecomprised of 36 cells per module of monocrystalline silicon.
An inverter, storage batteries, charging regulator are otherimportant components of the 2.32 kWp photovoltaic system. Thephotovoltaic modules are mounted on a fixed metal supportingstructure. On the basis of latitude of place (New Delhi) and for
Fig. 3. PV subarray2: 1.1
receiving the maximum solar radiation inclination of the frame ismaintained at around 45�. In order to supply the power generatedfrom the 2.23 kWp system for the uses in the mud house located atsolar energy park of IIT Delhi, a connection was made between theinverter and the general switching board of the mud house makinguse of electric cable. This photovoltaic system provides the neces-sary input energy for lighting tube lights, running a ceiling fan,computer of mud house, lighting CFL lamps for streetlight, and alsorunning submersible water pump. The power supply from the bat-teries is drawn only during night period and when the power deliv-ered by array is less than the power required by loads.
3.2. Design specification of photovoltaic modules/subarrays/array
A solar photovoltaic system is an integrated assembly of mod-ules and other components, designated to convert solar energy intoelectrical energy. Group of suitably connected modules is com-bined and interconnected to form PV array; PV array may consistof no. of subarrays for engineering convenience.
2 kWp (CEL make).
Table 2Design specifications/ratings for component subarrays of 2.32 kWp PV array.
Subarray detail PV subarray1 PV subarray2
Number of PV modules 16 32Each module –75 Wp
(Siemens make)Each module – 35 Wp
(CEL make)Number of series and
parallel modulesFour parallel stringswith four seriesmodules in each
Eight parallel stringswith four series modulesin each
Maximum output rating 1.2 kWp 1.12 kWp
Short circuit current 19.2 A 18.8 AOpen circuit voltage 86.8 V 82 VMaximum rated current 17.6 A 16.8 AMaximum rated voltage 68 V 66 V
Table 3Measured values of equivalent hours of full sunshine (EHFS), average PV operatingtemperature (OT) for a typical day in July 2010.
Size of PV array (kWp) Equivalent hours of fullsunshine (hEFS) (h)
PV operatingtemperature (OT) (�C)
PV array: 2.32 3.8 38.6Subarray1: 1.2 (Siemens) 3.7 38.4Subarray2: 1.12 (CEL) 3.9 38.9
R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652 647
The rating of PV modules and array with component subarraysare shown in Tables 1 and 2. The output characteristics of Table 2were calculated from the one in Table 1. The cell temperature coef-ficient b has been considered 0.45%/�C for monocrystalline silicon[21].
3.3. Experimental instrumentation and observations
Block diagram of existing experimental setup is shown in Fig. 1and data were sampled every hour during whole day during exper-imentation. Table 3 shows experimental data collected on a typicalclear day of July, 2010 at solar house of IIT in New Delhi (India).Daily hourly observations of solar radiation on both subarrays,ambient air temperature, PV operating temperature, short circuitcurrent for subarray1 and subarray2, battery voltage, open circuitvoltage for both subarrays were being measured during experi-mentation with the help of portable calibrated solar mete (leastcount 10 W/m2 and accuracy of ±2% of measured solar radiationreading), calibrated mercury in glass thermometer (least count1 �C with accuracy ±10% of reading (or ±0.1 �C),digital infrared laserthermometer (least count 0.1 with accuracy ±1%) and portable dig-ital clamp meter or tong meter (least count 0.01 with ±1% accu-racy) respectively.
4. Simplified methodology for experimental calculations
The daily output of a solar array depends on solar radiation andsolar cell temperature. By continuous monitoring of short circuitcurrent (Isc) and open circuit voltage (Voc) of array, it is possibleto immediately detect the performance of PV array system. Thevariation in plane of array irradiance is directly proportional toshort circuit current obtained from PV array. The open circuit volt-age, however, depends logarithmically on light intensity [22]. Thesmall variation in (Voc) during the day is due to temperaturechanges and cloud cover. By monitoring these two parameters con-tinuously, module/array performance and also degradation or fail-ure can be readily detected [23].
Performance indices such as electrical energy output, electricalefficiency of PV array/subarrays can be experimentally calculatedfor given PV array/subarray system with the help of developedexpressions given below in this section. These expressions can also
be applied to calculate daily, monthly and yearly performance indi-ces of any kind of PV array/subarray system, using experimentallymeasured parameters.
4.1. Electrical energy output of PV array
The electrical power output is the product of the voltage andcurrent. Total energy output of entire PV array will be the sum ofthe output from subarrays. From experiment open circuit voltage(Voc) and short circuit current (Isc) of PV subarrays are measuredhourly since morning to evening during a whole day, then totaldaily hourly electrical energy output from subarray1 and subar-ray2 can be calculated. The total daily hourly energy output of en-tire PV array consisting two subarrys is expressed by the followingequation:
EPV array ¼X
hourly
ðFF � Voc � IscÞsubarray1 þX
hourly
ðFF � Voc � IscÞsubarray2
ð1Þ
For PV array consisting n subarrays the total daily hourly energyoutput can be expressed by the following equation:
EPV array ¼X
hourly
ðFF � Voc � IscÞsubarray1 þX
hourly
ðFF � Voc � IscÞsubarray2
þ � � � þX
hourly
ðFF � Voc � IscÞsubarray n ð2Þ
For calculating the approximate monthly electrical energy out-put, average daily electrical energy output for a particular month ismultiplied with the recorded number of clear days in that month.Net approximate annual energy output can be calculated by addingmonthly electrical energy output over a year. Eq. (3) express netannual electrical energy output from PV array.
ðEPV arrayÞannual ¼X12
n¼1
ðEPV arrayÞn � Dn� �
ð3Þ
where ðEPV arrayÞn is average daily electrical energy output of PV arrayfor nth month and is obtained by taking average of total daily elec-trical energy output measured for n no. of typical days of that nthmonth, Dn is number of clear days in nth month. Similarly annualelectrical energy output from PV subarray1 and subarray2 can beexpressed.
4.2. Power conversion efficiency or actual electrical efficiency of PVarray
Daily power conversion efficiency of subarray1 for a typical daycan be calculated by taking the ratio of output energy of PV subar-ray1 and incident solar energy to subarray1, similarly daily powerconversion efficiency of subarray2 can be calculated by taking theratio of output energy of PV subarray2 and incident solar energy tosubarray2.
Now the daily actual electrical efficiency or Power conversionefficiency of entire PV array is the ratio of total electrical outputof PV subarray1 and subarray2 and total input incident solar en-ergy on PV subarray1 and subarray2. This Eq. (4) is used to calcu-late daily power conversion efficiency for a typical day.
gPV array ¼P
hourlyðFF �Voc � IscÞsubarray1 þP
hourlyðFF �Voc � IscÞsubarray2PhourlyðI1 �Asubarray1Þ þ
PhourlyðI2 �Asubarray2Þ
h ið4Þ
where I1 is the average of hourly measured solar radiations over theof PV subarray1 area, Asubarray1. Here, Asubarray1 is area of PV mod-ule � no. of PV modules in subarray1. I2 is the average hourly mea-sured solar intensity over the of PV subarray2 of area Asubarray2. Here,Asubarray2 is Area of PV module � no. of PV modules in subarray2.
648 R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652
For entire PV array consist of n number subarrays actual electri-cal efficiency or power conversion efficiency can also be expressedby the following equation:
gPV array ¼P
hourlyðFF � Voc � IscÞsubarray1 þP
hourlyðFF � Voc � IscÞsubarray2 þ � � � þP
hourlyðFF � Voc � IscÞsubarraynPhourlyðI1 � Asubarray1Þ þ
PhourlyðI2 � Asubarray2Þ þ � � �
PhourlyðIn � AsubarraynÞ
" # ð5Þ
If experimental data is obtained for n no. of typical clear days ofeach month in a year then more approximate power conversionefficiency of PV array on the basis of monthly experimental datacan be calculated by taking the ratio of net annual energy outputfrom PV array and total input incident solar energy on PV subar-ray1 and subarray2 throughout a year. No. of clear days in eachmonth is recorded.
So, more approximate power conversion efficiency of a PV arrayon the annual basis by using measured monthly observations hasbeen developed as follows:
ðgPV arrayÞannual ¼P12
n¼1 EPV array� �
n�Dn� �
P12n¼1ðIt1Þn�Dn�Asubarray1
� �þP12
n¼1ðIt2Þn�Dn�Asubarray2
� �h ið6Þ
where ðIt1Þn is the average of total daily hourly measured solar radi-ations for any no. of typical days during nth month over the of PVsubarray1 of area Asubarray1. ðIt2Þn is the average of hourly measuredsolar radiations for any no. of typical days in nth month over theof PV subarray2 of area, Asubarray2.
5. Estimation of nominal rated daily performance indices
5.1. Estimation of rated daily electrical energy output
Estimation of PV energy output, power conversion efficiency ofPV array and its component subarrays at nominal rating are usefulin studying the PV performance and possible improvements.
ðgstcÞPV array ¼½ðFF � Vocm �Ms � Iscm �MpÞsubarray1 þ ðFF � Vocm �Ms � Iscm �MpÞsubarray2�
½ðIp � Asubarray1Þ þ ðIp � Asubarray2Þ�ð10Þ
In a simplified way approximate maximum daily energy outputfrom PV array can be calculated mathematically by multiplying thepeak power of PV panel with equivalent hours of full sunshine(hEFS) as expressed in Eq. (7). Peak power (Pm) from array isconsidered at standard test conditions (STC) as given bymanufacturer.
n max ¼ Pm � hEFS ð7Þ
Peak power ðPmÞ of array ¼ Peak power ðPmÞ of module
� No: of modules in a array ð8Þ
Similarly maximum daily output energy from PV subarrays canbe also be calculated by multiplying the peak power of PV subarraywith equivalent hours of full sunshine (hEFS). Standard test condi-tions (STC) can be specified by 100 mW/cm2 (=1000 W/m2) solarflux conforming to the standard reference AM 1.5 G spectrum,and temperature 298.16 K (25 �C). The use of this flux value is veryconvenient, as the efficiency in percent is numerically equal to thepower output in mW/cm2 [24].
5.1.1. Measurement of equivalent hours of full sunshine (hEFS)Equivalent hours of full sunshine are defined by no. of hours of
incident radiation at a place, if intensity of radiation is kept con-
stant at its peak value of 1 kW/m2, that gives the same energy re-ceived from sunrise to sundown.
Equivalent hours of full sunshine (hEFS) for particular day can beobtained by the curve, which shows the hourly variation of solarintensity over PV surface for whole day. Integration of area underthe curve gives total solar energy received by the unit area on thatday.
Suppose, integration of area under the curve of typical daily var-iation of incident solar radiation intensity on a flat unit area surfaceis expressed by N kW h/m2, then this can further be expressed asconstant peak value of solar radiation of 1 kW/m2 incident onreceiving surface for N hours, then hEFS will be equal to N hours.The expression is given by Eq. (9).
Total solar energy received by flat unit area of array (kW h/m2) =peak solar intensity
(1 kW/m2) � hEFS (hours)
nreceived ¼ Ipeak � hEFS ð9Þ
5.2. Actual electrical efficiency or power conversion efficiency of PVarray at standard test conditions (STC)
Power conversion efficiency at STC can be calculated forcomponent subarrays1 and subarray2 individually. An expres-sion for calculating PV array power conversion efficiency atSTC has been developed as Eq. (10), when array consists oftwo subarrays
where Ms is no. of modules in series in subarray, Mp is no. of mod-ules in parallel in subarray, Vocm is open circuit voltage of module,Iscm is short circuit current of module, Ms represents no. of modulesin series in a subarray and Mp represents no. of parallel strings ofseries connected modules in a subarray. FF is fill factor, Ip peakintensity with value of 1000 W/m2. All these parameters are mea-sured at STC and provided by manufacturer specifications. Samedeveloped formula can be modified for n number of subarrays ofany given PV array.
5.3. Temperature effect on nominal rated performance indices of PVarray
The daily output of a solar array depends on solar radiation andPV operating temperature. Rise in the PV operating temperaturereduces array peak energy output and PV electrical efficiency mea-sured at STC.
It is clear that actual evaluation of PV array performance foroutdoor field conditions needs to be consider PV operating temper-ature of given location in order to translate the performance of PV
8am
9am
10am
11am
12no
on
1pm
2pm
3pm
4pm
5pm
Ave
rage
Sol
ar
Inte
nsity
(I ave)
,W/m
2
Iave
0100200300400500600700
R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652 649
arrays from the standard rating temperature of 25 �C to the arrayperformance at actual PV operating temperature.
PV operating temperature can be calculated using measuredambient temperature at given location and incident solar intensityon PV array [25,26]. Consequence of including the effects of PVoperating temperature in the PV electrical energy output and elec-trical efficiency are presented by Eqs. (11) and (14) respectively.
5.3.1. Estimation of maximum electrical energy output with PVoperating temperature effect
The power output of a PV module depends linearly on the oper-ating temperature, decreasing with TOT. Effects of PV operatingtemperature on PV electrical energy output can be expressed bythe following equation [27]:
nPV ¼ Pm � hEFS � ½1� bðTOT � TSTCÞ� ð11Þ
With necessary correction applied to hEFS, the result would bethe unit array output (UAO), in units of watt hour per peak wattper day. UAO is a preferred parameter for the sizing exercise incomparison to parameters such as hEFS or global radiation. UAO isgiven by the following equation:
UAO ¼ hEFS � ½1� bðTOT � TSTCÞ� ð12Þ
Thus Eq. (11) can be modified for temperature corrected PVelectrical energy output as follows:
nPV ¼ Pm � UAO ð13Þ
TimeFig. 4. Hourly variation of solar intensity (Iave) on PV array.
010
2030
4050
8:00
am
9:00
am
10:0
0am
11:0
0am
12:0
0noo
n
1:00
pm
2:00
pm
3:00
pm
4:00
pm
5:00
pmTime
Ave
. Tem
pera
ture
Tamb PV Operating Temperature
Fig. 5. Hourly variation of ambient temperature and PV operating temperature forarray.
80Voc1(Subarray1) Voc2(Subarray2)
5.3.2. Estimation of power conversion efficiency or actual electricalefficiency with PV operating temperature effect
To show the importance and consequence of including the ef-fects of PV operating temperature in the PV electrical efficiency atraditional linear expression for temperature corrected PV electri-cal efficiency gOT is given by [28–31]
gOT ¼ gSTC ½1� bðTOT � TSTC � ð14Þ
where gSTC is the PV electrical efficiency at STC, b is array efficiencycoefficient, TSTC is reference temperature at STC for PV electrical effi-ciency, TOT is the average PV operating temperature.
5.4. Production factor (PF)
One of the performance indices for evaluating PV array perfor-mance is production factor and can be defined by ratio of actual ar-ray yield obtained from experimental results and potential arrayyield obtained at nominal rating and operating temperature at aparticular location. Expression for production factor is given bythe following equation:
PF ¼ EPV array
nPVð15Þ
010203040506070
8:00
am
9:00
am
10:0
0am
11:0
0am
12:0
0noo
n
1:00
pm
2:00
pm
3:00
pm
4:00
pm
5:00
pm
Time
Ope
n C
ircui
t Vol
tage
Voc
(V)
Fig. 6a. Hourly variation of open circuit voltage (Voc).
6. Experimental results and discussion
PV array experimental field observations generally use the tra-ditional method like measurement of Voc, Isc, Tamb, It for calculatingelectrical efficiency and energy output. The experimental data for atypical clear day in July 2010 have been used for the calculations ofvarious performance indicators of 2.32 kWp PV array and its com-ponent subarray1: 1.2 kWp and subarray2: 1.12 kWp.
These experimental observations of PV system parameters wereplotted graphically as shown in Figs. 4–8.
Average hourly variation of solar intensity (Iave) on given PV ar-ray located at Solar energy park, IIT Delhi for typical clear day isshown in Fig. 4. Here solar intensity on entire PV array at any
particular time has been calculated by taking average of measuredintensities on subarray1 and subarray2 at that same time. Integra-tion of area under the curve (Fig. 4) gives total solar energy re-ceived by the unit area on that day and this was used tocalculate Equivalent hours of full sunshine (hEFS).
Fig. 5 exhibits the hourly variation of ambient temperature andoperating temperature for entire PV array for a typical day ofexperimentation at IIT Delhi. These both subarrays have been lo-cated side by side. Maximum PV operating temperature of PV arraywas found 43.3 �C at 12:00 noon when ambient temperature wasmeasured 31.0 �C and solar radiation was 590 W/m2. PV arrayoperating temperature depends on ambient temperature and solarintensity on PV array. Table 3 depicts the value of measured
8am
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1pm
2pm
3pm
4pm
5pm
Time
Shor
t Circ
uit C
urre
nt,
Isc
(Am
p)Isc1(Subarray1)
Isc2(Subarray2)
0
2
4
6
8
10
12
Fig. 6b. Hourly variation of short circuit current (Isc).
8am
9am
10am
11am
12no
on
1pm
2pm
3pm
4pm
5pm
Time
Hou
rly E
lect
rical
Pow
er O
utpu
t (W
)
P1(Subarray1)
P2(Subarray2)
P(t) Array output
0100200300400500600700800900
Fig. 7. Hourly variation of electrical power output for 2.32 kWp PV array and itssubarray1, subarray2.
8am
9am
10am
11am
12no
on
1pm
2pm
3pm
4pm
Time
Hou
rly E
lecr
tical
Effi
cien
cy in
%
elect.eff.subarray1elect.eff.subarray2elect.eff.array2.32kwp
0
2
4
6
8
10
12
Fig. 8. Hourly variation of power conversion efficiency or actual electrical efficiencyfor 2.32 kWp PV array and its subarray1, subarray2.
650 R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652
Equivalent hours of full sunshine (hEFS) and PV operating tempera-ture (OT) of PV array for a typical day. High value of PV operatingtemperature causes reduction in electrical efficiency of PV array.
Figs. 6a and 6b shows the hourly variation of Voc and Isc for sub-array1 and subarray2 of the 2.32 kWp monocrystalline silicon PVarray. The variation in the short-circuit current is attributed to
the variation in the solar irradiance because Isc is directly propor-tional to incoming light intensity. The relatively smaller variationin Voc during the day is mainly due to temperature changes andcloud cover. By continuous monitoring of Isc and Voc of a module/array, it is possible to immediately detect any degradation in mod-ule/array performance or failure can be readily detected [8].
Fig. 7 shows individual subarray output of subarray1 of Siemensmakes (1.2 kWp) and subarray2 of CEL make (1.12 kWp). Experi-mentally calculated output of subarray1 of Siemens makes is quitehigh as compared to subarray2 of CEL make. Entire PV array outputis the sum of both subarrays output. From the experiment for atypical day maximum electrical power output of subarray1 of Sie-mens make has been observed 514.641 W at 12:00 noon and elec-trical power output of subarray2 of CEL make has been obtained274.982 W at 12:00 noon. Total electrical energy output of subar-ray1 (Siemens) and subarray2 (CEL) were experimentally calcu-lated 3.406 kW h/day and 1.974 kW h/day respectively. Electricalenergy output of entire SAPV array of 2.32 kWp was calculated5.38 kW h/day by using Eq. (1). These experimentally calculatedperformance values already including the effect of PV operatingtemperature during the day of experimentation as hourly mea-sured parameters Voc and Isc are temperature dependent. For sim-plification in experimental calculations fill factor (FF) has beenassumed of value 0.72 as obtained from rated values.
Fig. 8 shows hourly variation of actual electrical efficiency orpower conversion efficiency of 2.32 kWp PV array and its subarray1,subarray2. It is observed that hourly electrical efficiency of PV sub-array2 (CEL make) is lower than hourly electrical efficiency of PVsubarray1 (Siemens make). This is mainly due to the degradationlosses in CEL modules of subarray2 and yellowing of its moduleswith high installation age. Maximum actual electrical efficiency ofsubarray1 (Siemens make) and subarray2 (CEL make) has been cal-culated from observation 7.13% and 4.34% respectively at 2:00 pm.Maximum electrical efficiency of entire PV array of 2.32 kWp hasbeen calculated 11.238%. Average daily power conversion efficiencyof subarray1 (Siemens) and subarray2 (CEL) of PV array were calcu-lated 9.5% and 3.9% respectively and average daily power conver-sion efficiency of entire SAPV array of 2.32 kWp was calculated6.24% by using Eq. (4). Although the measured actual electrical effi-ciency of subarray1 of PV array is obtained high but reduction inelectrical efficiency of entire PV array is due to low value of mea-sured electrical efficiency of subarray2. These measured dailypower conversion efficiency or electrical efficiency includes theeffect of PV operating temperature and other degradation losses.Production factor of 2.32 kWp PV array and its component subar-ray1 (Siemens) and subarray2 (CEL) were calculated 0.65, 0.82and 0.48 respectively by using Eq. (15).
6.1. Assessment by result comparison
The comparison of actual on field results of performance indiceswith nominal rated (max.) results, computed by using manufac-turer specifications helps to assess actual on field performance ofPV array/subarrays.
For actual effective result assessment of PV array/subarraysnominal rated performance indices, which are obtained at STC byusing Eqs. (9) and (10) for daily maximum energy output and dailymaximum power conversion efficiency respectively, have beencorrected for typical day PV operating temperature obtained atparticular location. Temperature corrected maximum electrical en-ergy output and temperature corrected maximum PV electricalefficiency for PV array/subarrays are evaluated by using Eqs. (11)and (14) respectively.
The details of obtained results for actual on field experimentalperformance indices of PV array and its component subarrays withstandard nominal rated performance indices and temperature
Table 4Electrical energy output in kWh/day of PV array/subarray for a typical day of New Delhi.
Size of PV array (kWp) Experimentally calculated actualelectrical energy output underoutdoor field conditions (kW h/day)
Estimated maximumenergy output atSTC (kW h/day)
Estimated maximum electrical energyoutput, with PV operating temperatureeffect (kW h/day)
Subarray1: 1.2 (Siemens) 3.406 4.440 4.172Subarray2: 1.12 (CEL) 1.974 4.368 4.094Entire PV array: 2.32 5.380 8.816 8.276
9.5
3.9
6.24
10.35
12.5
8.759.72
11.75
8.2
PV Array Subarray1 Subarray2 Act
ual E
lect
rical
Effi
cien
cy in
%
Calculated Actual Elec. Efficiency(Experimental) Estimated Max.Electrical Efficiency at STCEstimated Max.Electrical Efficiency with Temp. Correction
02468
101214
Fig. 9. Daily power conversion efficiency or actual electrical efficiency of PV arrayand its subarrays.
Table 5Unit array output (UAO) for PV array/subarray for a typical day of New Delhi.
Size of PV array Experimentallycalculated unitarray outputunder outdoor fieldconditions(Wh/Wp/day)
Estimated maximumvalue of unit arrayoutput (UAO)(Wh/Wp/day)
PV array: 2.32 kWp 2.293 3.567Subarray1: 1.2 kWp (Siemens) 2.838 3.476Subarray2: 1.12 kWp (CEL) 1.762 3.656
R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652 651
corrected nominal rated performance indices obtained for a typicalday have been presented in Table 4 for daily electrical energy out-put. Similarly Fig. 9 clearly indicates the comparative performancerepresentation of daily power conversion efficiency of existing PVarray/subarrays.
Table 5 shows the experimentally calculated unit array output(UAO) in Wh/Wp/day under outdoor field conditions in compari-son to estimated maximum value of unit array output (UAO) usingEq. (13). Actual on field calculated low value UAO of subarray2draws special attention. From the present array analysis perfor-mance indices such as calculated daily electrical energy output ofPV array, daily power conversion efficiency, UAO and productionfactor for subarray2 of 1.12 kWp (CEL) were obtained quite less(i) compare to subarray1 (Siemens) performance indices and (ii)compare to estimated temperature corrected maximum perfor-mance results of same subarray2. It is clear that reduction in over-all performance of entire PV array is mainly due to the underperformance of subarray2.
7. Conclusions and recommendations
On the basis of present study and experimental results, the fol-lowing conclusions have been drawn:
� Maximum electrical energy output with PV operating temper-ature effect has been estimated 8.276 kW h/day for entirePV array, where 4.172 kW h/day (50.41%) is contributed by
subarray1 (Siemens make) and 4.094 kW h/day (49.46%) iscontributed by subarray2 (CEL make). Whereas in actual onfield operating conditions out of total experimentally calcu-lated electrical energy generation(5.38 kW h/day), 3.406 kWh/day(63.19%) of total output energy is contributed by subar-ray1(Siemens) and 1.974 kW h/day(36.69%) is contributed bysubarray2 (CEL).� For existing PV array system maximum daily power conversion
efficiency with PV operating temperature effect has been esti-mated 11.5% for subarray1, 8.2% for subarray2 and 9.72% forentire PV array consisting of both subarrays in operation.Whereas, experimentally measured daily power conversionefficiency has been calculated 9.5% for subarray1, 3.9% for sub-array2 and 6.24% for entire PV array.� Actual on field PV array/subarrays performance has been com-
pared with rated performance estimated for same climatic con-ditions as obtained during specific day of field experimentation.As a result from the experimental performance assessment, onfield actual PV array/subarrays performance have been foundalmost 82% for subarray1: 1.2 kWp (Siemens make), 48% forsubarray2: 1.12 kWp (CEL make) and 65% for entire PV arrayof 2.32 kWp (Siemens and CEL make) in compare to temperaturecorrected estimated maximum(rated) performance of respec-tive array/subarrays.� From the on field experimental observation results it is clear
that the performance indices calculated for subarray1: 1.2kWp(Siemens) were quite considerable, but significant atten-tion must be given to the subarray2. The poor performance ofsubarray2 (CEL) is mainly due to PV cell/module degradationlosses, that also include significant role of extended outdoorexposure periods as there is significant gap between the instal-lation age of both subarrays.� Extended outdoor exposure periods also cause module degrada-
tion and can decrease the module performance as much as 50%[32]. It is clear from the results that the poor performance ofsubarray2 in turn significantly affecting the entire PV array byreducing its overall technical performance. For reliable outdoorPV operation all PV module manufacturing companies shouldprovide the performance degradation rate parameter as perspecific location conditions during its life time as a qualityindicator.� Simplified mathematical expressions have been developed for
evaluating performance indices of PV array/subarrays in actualfield conditions and explained experimental methodology of per-formance assessment is applicable to any other kind of PV arraysystem also, which is having two or more than two subarrays.
Relying solely on standard parameters of PV arrays, PV systemdesign will always be difficult to achieve the desire effect. Thisstudy necessarily helps to the research and development of PV sys-tem. Outdoor field performance data, on the other hand, can behelpful in deriving recommendations for improving PV generatorsand to assist PV component manufacturers, plant designers,installers and operators in their efforts to realize successful PVsystems.
652 R. Sharma, G.N. Tiwari / Applied Energy 92 (2012) 644–652
Acknowledgements
The authors are grateful to Ministry of Human Resource andDevelopment, Government of India for financial support to carryout research work at IIT Delhi. Authors would like to acknowledgethe valuable suggestions for improvement from the reviewers andProf. J. Yan (Editor in-chief).
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