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Performance evaluation of micromorph based thin film photovoltaic modules in real operating conditions of composite climate
Energy xxx (2016) 1e12
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Performance evaluation of micromorph based thin film photovoltaic modules in real operating conditions of composite climate Rahul Rawat a, *, Ramayan Singh b, O.S. Sastry b, S.C. Kaushik a a b
Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India National Institute of Solar Energy, Ministry of New and Renewable Energy, Gwal Pahari, Gurgaon, Haryana 122003, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2016 Received in revised form 16 November 2016 Accepted 18 November 2016 Available online xxx
The micromorph thin-film photovoltaic (TFPV) technology uses tandem solar cell structure comprising of hydrogenated amorphous and microcrystalline p-i-n junction silicon cells which needs a long period of outdoor exposure for stabilization. In this paper, the performance analysis of grid interactive (GI) micromorph TFPV system has been assessed for a period of 166 days in real operating conditions on the basis of performance ratio (PR), thermal normalized PR (PRSTC), alternate reporting conditions (ARC), energetic and exergetic studies. A novel methodology for performance evaluation has been developed by utilizing per minute operating data collected from a micromorph based GI-TFPV system of 7.92 kWP rated stabilized capacity. The system is found to be operating in the range of 5.1 kWP to 5.6 kWP as compared to the 5.94 kWP stabilized rated capacity after removing the strings with mechanical damaged modules. The average PR and PRSTC of the system are found to be 0.83 and 0.89 respectively, with an average degradation rate of 1.53%/month and 1.22%/month respectively. The average exergetic, energetic and alternate current output efficiencies of the 7.92 kWP GI-TFPV system are found to be 7.38%, 6.83% and 6.69% respectively. The performance of inverters and effect of module breakage has also been evaluated. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Micromorph Tandem cell Thin film photovoltaics Performance Exergy Efficiency
1. Introduction The solar photovoltaic is coming out as one of the most promising alternatives to the conventional power generation technologies with considerable progress in installed capacity as well as industry oriented research in the field of modelling, design methodologies, size optimization techniques and reliability analyses [1]. The installation of PV systems has grown worldwide with compound annual growth rate of 40% from year 2000e2014. According to the photovoltaics' progress report by Fraunhofer Institute for Solar Energy Systems, the annual production of PV modules was about 47.5 GWP in year 2014, out of which about 43.1 GWP (90.73%) was well established and predominant silicon wafer based technology while the emerging TFPV technology accounts about 4.4 GWP (9.26%) [2]. Although the world solar market is dominated by silicon wafer based module technology, efforts are still going to improve performance and stability of the TFPV technology such as CIGS, CdTe and micromorph due to their higher yields and PR in specific environmental conditions. The TFPV modules, which are
* Corresponding author. E-mail address: [email protected] (R. Rawat).
fabricated by depositing a thin layer of PV material having a thickness in the range of mm, is competing the wafer based technology due to the high absorption coefficient of thin film material which reduces the material requirement and hence reduces the cost. The TFPV technologies are expected to have competitive efficiency, ease in producing large size modules, monolithic PV cell integration and low energy consumption in the manufacturing process which not only reduces the energy payback period but also decreases the manufacturing cost and greenhouse gas emissions [3]. Among TFPV technologies, the amorphous silicon (a-Si: H) is most popular and oldest, but it undergoes light induced degradation (LID) during stabilization because of Staebler-Wronski effect. Therefore, an alternative tandem cell structure of hydrogenated amorphous silicon (a-Si: H) based high band gap p-i-n junction semiconductor as top cell and hydrogenated microcrystalline silicon (mc-Si: H) based low band gap p-i-n junction semiconductor as bottom cell was suggested by Meier et al. (1994) in order to improve the stability which is named as micromorph technology [4]. The cells are deposited typically at 200 C-300 C temperature using plasma enhanced chemical vapour deposition, which is relatively very low as compared to crystalline Si. The spectral
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Nomenclature
j
A B E G GI H I kW kWh P PR Q sf T tf TFPV V y
Subscripts a Ambient AC Alternate current ARC Alternate reporting conditions C Convective d Diurnal f Final i Instant I Current INV Inverter M Module MP Maximum power point n Number of individual array p Peak P Power R Reflective RD Radiative ref Reference ROC Real operating conditions S Sun SKY Sky STC Standard test conditions T Tilted U Recoverable V Voltage
g dB ε
h l r s
Area Exergy Electrical Energy Irradiance Grid interactive Radiation Current kilo Watt kilo Watt hour Electrical Power Performance ratio Energy Spectral factor Temperature Thermal factor Thin film photovoltaic Voltage Normalized energy yield Temperature coefficient Exergy destruction Emissivity Energy efficiency Wavelength Reflectivity Stefan-Boltzmann constant
response and electrical properties of the mc-Si:H p-i-n junction cell approaches the monocrystalline p-n junction cell [5]. The module has amorphous and microcrystalline silicon multi-junction cell coated with EVA foil and sandwiched between front and rear thermally strengthened glass. The a-Si:H and mc-Si:H p-i-n junction diodes of micromorph cell have 1.7 eV and 1.1 eV band gap respectively, which is nearly ideal to absorb more solar radiation spectrum with the thickness range of about 0.2e0.25 mm and 1.6e2.2 mm respectively. The efficiency of the technology has been improved by using highly reflective transparent conductive oxide (TCO), metal contacts and intermediate reflectors for light trapping, and depositing n-doped layer of silicon oxide for increasing current generation [6]. The highest stabilized power conversion efficiency of large area micromorph module (1.43 m2) achieved so far is 12.34% by TEL Solar AG by using thicker intrinsic layer with optimized TCO layer, anti-reflective coating and doped layer [7]. Earlier in 2013, it was 10.7% for the same area module by the same manufacturer. The typical stabilization period of micromorph PV modules is 6e8 months with 8e20% LID of power [8]. Since, both the a-Si: H and mc-Si: H cells of the micromorph PV module have hydrogenated dangling bonds, which induces LID, therefore it is vital to carry out a detailed performance evaluation of the technology in real operating condition during stabilization in order to ensure the reliable lifetime operation. Tossa et al. (2016) have carried out performance analysis of monocrystalline, multicrystalline and micromorph based silicon PV modules in hot and harsh climate and found that micromorph is highest performing technology among the three with an average PR of 0.92 due to relatively low temperature coefficient and series resistance [9]. Torres-Ramírez et al. (2016) have studied the performance of micromorph TFPV technology on the basis of analytical modelling approaches in order to estimate the power and energy output of the technology [10]. Savvakis and Tsoutsos (2015) has presented a
Exergy efficiency
performance assessment of 2 years of operation of 2.18 kWP micromorph based GI-TFPV technology having an average energy efficiency of 7.65% and a PR of 0.85 [11]. In this paper, performance studies are carried out using 7.92 kWP micromorph based GI-TFPV system, having large area modules of 1.43 m2, in order to understand their stabilization process so as to estimate the reliable outdoor spatial life of the technology. 2. Performance evaluation Unlike wafer based PV technology, the micromorph thin film PV technology is neither proven nor established and moreover, it consists of a-Si: H which is prone to light induced degradation on the initial illumination of solar radiation. Hence, it is important to carry out performance evaluation and determine the degradation of emerging micromorph technology in real operating conditions. The outdoor experimental data have been filtered for improving the accuracy of the study and the required data have been analyzed in order assess the performance on the basis of energetic, exergetic, performance ratio, alternate reporting conditions and inverter performance. The detailed methodology used for the performance evaluation has been presented in flow chart as shown in Fig. 1 and discussed in the subsequent subsections. 2.1. Energetic and exergetic analysis The energetic analysis is based on the first law of thermodynamics, which deals with energy efficiency and losses of individual PV array as well as PV systems. The first law of thermodynamics provides quantitative analysis based on the energy balance in which efficiency is defined as the ratio of energy or power output to the input. The micromorph TFPV module has glass to glass
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Fig. 1. Methodology for the performance evaluation of the PV system.
assembly having the tandem cell sandwiched between TCO coated front glass and bottom glass. The energy balance of the
micromorph TFPV module has been illustrated in the layer diagram as shown in Fig. 2 and the energy balance of the overall system having four arrays is given by Eq. (1).
4 QS ¼ ð4 QR Þ þ QRD1 þ QRD2 þ QRD3 þ QRD4 þ QC1 þ QC2 þ QC3 þ QC4 þ QU1 þ QU2 þ QU3 þ QU4 þ PMP1 þ PMP2 þ PMP3 þ PMP4 (1) The exergetic analysis is based on the second law of thermodynamics, which deals with quality of energy along with the quantity and states that there is always a degradation of energy during any irreversible energy conversion process [12]. The exergy is also known as available energy to do maximum possible work with respect to the surrounding. As it is a qualitative analysis, it highlights the actual losses and identifies the site for improvement in the system [13]. The irreversibility during the irreversible energy conversion process is described by the Gouy-Stodola law which quantified it as the product of entropy generation (Sgen) and environment temperature.
Irr ¼ Ta Sgen
Fig. 2. Energy balance of the micromorph TFPV module.
(2)
The exergetic analysis of a PV system can be classified into internal and external exergy [14,15]. The internal exergy of a PV module belongs to the chemical exergy balance while the external exergy belongs to the thermodynamic exergy balance with respect to the environment. The external exergy is also known as
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thermodynamic or physical exergy which can be determined by positive balance and anti-balance [16]. In this paper, the exergetic efficiency and exergy losses of micromorph TFPV technology has been assessed by using positive exergy balance given by Eq. (3) [17]. The total exergy of solar irradiance received over the surface area of 4 arrays of the TFPV system is equal to the thermal exergy lost due to reflection, radiation and convection; recoverable thermal exergy stored in the modules; electrical exergy output; and exergy destruction [18,19].
þ BC3 þ BC4 þ BU1 þ BU2 þ BU3 þ BU4 þ PMP1 þ PMP2 þ PMP3 þ PMP4 þ dB1 þ dB2 þ dB3 þ dB4 (3) where the exergy of solar irradiance received over the surface area (A ¼ 25.74 m2) of PV modules is calculated by using correlation given by Petela (1964) for radiative heat transfer from sun temperature to earth's ambient temperature [20].
3
hi ¼
PMPn ¼ 4 A GT n¼1
4 4 3 ; A 3TMn þ TSKY 4TSKY TMn
P
Ta ; BCn ¼ h A ðTMn Ta Þ 1 TMn
(7)
Ta ; BUn ¼ QUn 1 TMn
(8)
hd ¼
t
P t
vBn ¼ BS ðBR þ BRDn þ BCn þ BUn þ PMPn Þ;
(10)
(13)
! (14)
ð4 QS Þ
The exergy efficiency or second law efficiency of any system is defined as the ratio of exergy output to the exergy input. The instant exergy efficiency of each array and overall PV system is defined as the ratio of electrical exergy output for a given time to the exergy of input solar irradiance for the same time as given by Eq. (15) and Eq. (16) respectively.
ji ¼
PMPn BS
(15)
P4
n¼1 PMPn 4 BS
ji ¼
(16)
The diurnal exergy efficiency of the array and overall PV system is the ratio of electricity generated during a day to the exergy of solar radiation received during the same day as given by Eq. (17) and Eq. (18) respectively.
P
PMPn
t
jd ¼
t
(17)
BS
P P4
n¼1
P t
where PMP is the power output at maximum power point (MPP) and the system is assured to be operated at MPP by maximum power point techniques which uses electronic power tracking algorithms [21]. The exergy destruction from each array has been calculated from back calculation as given by Eq. (10).
(12)
n¼1 PMPn
jdn ¼ tP
(9)
n¼1 PMPn 4 QS
QS
P P4
where h is the convective heat transfer coefficient given by h ¼ 5:7 þ 3:8v and v is the wind speed. The recoverable heat loss or the heat stored in the module which causes power as well as module degradation is given by
QUn ¼ QS ðQR þ QRDn þ QCn þ PMPn Þ
P4
PMPn
t
(6)
Here, the subscript ‘n’ represents the number of individual arrays. The exergy lost due to convective heat transfer and recoverable thermal exergy of each array has been determined by Eq. (7) and Eq. (8) respectively.
(11)
The diurnal energy efficiency of a PV system is the ratio of total energy output during a day to the total radiation received over the surface during the same period of time. The diurnal efficiency of the array and the overall system is given by Eq. (13) and Eq. (14) respectively.
(5)
The exergy lost due to radiative heat transfer from module to sky is given by Eq. (6) which includes emissivity (ε) of the PV module, the Stefan-Boltzmann constant (s ¼ 5:67 108 Wm2 K 4 ), module temperature (TM) and sky temperature (TSKY).
s
PMPn P ¼ MPn A GT QS
(4)
The exergy lost due to reflection is determined by the reflectivity (r) of the module.
BRDn ¼ ε
hi ¼
hdn ¼ tP
"
BR ¼ r BS ;
The instant energy efficiency of individual array and overall system for DC output is given by Eq. (11) and Eq. (12) respectively, which is the ratio of DC output to the input irradiance for a particular time instant [22].
P4
4 BS ¼ ð4 BR Þ þ BRD1 þ BRD2 þ BRD3 þ BRD4 þ BC1 þ BC2
4 # 1 Ta 4 Ta A GT ; BS ¼ 1 þ 3 3 TS TS
2.2. Energetic and exergetic efficiency
! PMPn
ð4 BS Þ
(18)
The instant and diurnal AC efficiency of the overall system has been calculated by the ratio of AC power output to the input solar irradiance over the surface area of PV modules as given by Eq. (19) and Eq. (20) respectively.
hACi ¼
PAC PAC ¼ 4 A GT 4 QS
(19)
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P
hACd ¼ P t
t
ðPAC Þ (20)
ð4 QS Þ
2.3. Performance ratio In general, the ratio of normalized array yield (Yf) during a fixed time interval to the reference yield (Yref) during same interval is known as performance ratio (PR) [23,24]. The array yield is defined as the final energy yield normalized with rated power while the reference yield is insolation normalized with rated radiation. It is the most widely used performance indicator of PV systems in ROC which is derived from efficiency and can also be defined as the ratio of efficiency in ROC to the nominal efficiency in reference condition.
Yf E=PSTC E=HT h ¼ ¼ ¼ ROC Yref HT =GSTC PSTC =GSTC href
PR ¼
(21)
The PR for a particular instant of time or power output can be calculated by the ratio of final power output to the reference power output divided by the ratio of irradiance over the PV surface to the reference irradiance as given by Eq. (22).
PRi ¼
PMP =PSTC P =G h ¼ MP T ¼ ROC GT =GSTC PSTC =GSTC href
(22)
The PR provides the fraction of the output of a system in real operating conditions with respect to the output at STC normalized input solar radiation. It provides actual performance of the PV system, however, the PR of two different months or season cannot be compared because of different module temperature and spectrum. Therefore, the conventional PR has been normalized to PR at STC (PRSTC) with module temperature using thermal factor (tf) and spectrum using spectrum factor (sf) [25]. Ideally, the PRSTC should be equal to 1.
tf ¼
1 1 þ gP ðTM TSTC Þ
(23)
where, tf is thermal factor, TM is module temperature and gP is the temperature coefficient of power at maximum power point of PV module (gP ¼ 0:33%).
using Eq. (26) to Eq. (28) in order to carry out time elapsed analysis and determine the deviation of the parameter from the initial measured value and rated values [27]. The voltage and current at MPP measured for irradiance more than 600 W/m2 has been converted to STC in order to determine the power at STC. For further analysis, the measured data in the irradiance range of 150e250 W/ m2, 350e450 W/m2, 550e650 W/m2, 750e850 W/m2 and 950e1050 W/m2 has been translated to voltage, current and power at ARC for 200 W/m2, 400 W/m2, 600 W/m2, 800 W/m2 and 1000 W/m2 respectively and 25 C module temperature [28].
VMP;ARC ¼ VMP þ gV ðTARC TM Þ IMP;ARC ¼ IMP
GARC GT
(26)
þ gI ðTARC TM Þ
PMP;ARC ¼ VMP;ARC IMP;ARC
(27) (28)
where temperature coefficient of voltage gV and current gI are 0.37% and þ0.08% respectively. 2.5. Inverter performance The PV system has two inverters in order to convert DC power to 3 phase AC power and their performance has been assessed on the basis DC to AC conversion efficiency, phase difference and frequency of output AC power for each inverter. The power factor (cos F) should be equal to one and the frequency should be 50 Hz for each phase of the power output. The occurrence magnitude of deviation of these parameters from the ideal conditions has been assessed. The instant and diurnal DC to AC efficiency conversion efficiency of the inverter is defined as the ratio of AC power output to the DC power input as given by Eq. (29) and Eq. (30) respectively.
hINVi ¼ P2
PAC
n¼1
P
hINVd ¼
t
P P2 t
(29)
PMPn PAC
n¼1
!
(30)
PMPn
Z
Z
xSTC ðlÞ$SRðlÞdl sf ¼
5
Z
Z
xSTC ðlÞdl
xðlÞ$SRðlÞdl
3. System description
(24)
xðlÞdl
where sf is spectrum factor, xSTC is standard spectrum at STC, x is the solar spectrum of the location, SR is the relative spectral response of the PV technology and l is the wavelength (nm). However, the spectrum factor of micromorph PV technology is 1 with a variation less than 0.003, therefore it can be ignored [26]. The PRSTC of micromorph technology can be determined by Eq. (25).
PRSTC ¼ PR tf sf zPR tf
(25)
2.4. Alternative reporting conditions The electrical parameters measured at real operating conditions have been translated to a common alternative reporting condition
The micromorph based GI-TFPV system has been installed by TEL Solar AG at National Institute of Solar Energy, Gurgaon Gwal Pahari, Gurgaon, India, which has geographical co-ordinates 28.42 N latitude and 77.15 E longitude. The modules are mounted due south with tilt angle of 25 as shown in Fig. 3. The system having initial rated power of 8.64 kWP comprises of 8 strings having total 72 modules of initial 120 WP and stabilized 110 WP rated power each. Although, the stabilized rated power of each module is 110 WP, the initial rated power is used in this study for several calculations because the measurement data used for analysis is of an initial 6 months of the outdoor exposure. Each string has 9 series connected modules and 2 strings are connected in parallel combination making an array. The system consists of 4 such arrays out of which array 1 and 2 are supplying power to inverter A and array 3 and 4 to inverter B simultaneously as shown in Fig. 4. The inverters A and B are three phase inverter which is converting DC to 3 phase AC at 50 Hz frequency to supply power to
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Fig. 3. Photograph of the micromorph based TFPV system.
Fig. 4. Layout diagram of the micromorph based TFPV system.
the grid. The technical specifications of PV module and inverter are given in Table 1. The per minute operating data of several days during the initial 6 months have been recorded and filtered for the clear sky day. The clear sky day data of the day having clearness index above 0.5 during the 166 days of outdoor exposure have been used for assessment in order to improve the accuracy of the investigation. The environmental conditions of the clear sky days have been presented in Table 2. The effect of module breakage on the performance of the string has been assessed using per minute data of 166th day. The separate data extracted from inverters has been used for calculating monthly energy yield for the entire year.
4. Results and discussion The diurnal energy generated by each string of the system shows that the string 1.2 and string 3.2 are under performing as compared to others as shown in Fig. 5. It has been found by visual investigation that there are broken modules in the string 1.2 and 3.2 which reduces the power generation of the strings. Since, the power degradation in these strings is due to mechanical damage and does not impart in the performance and degradation of the technology. Therefore, the data of these strings are removed and not used in the further analysis making the rated capacity of the system to be 6.48 kW initially and 5.94 kW after stabilization. However, the data of the entire system are used in the section of inverter performance because both the inverters are connected to one of the
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Table 1 Technical specifications of the PV module and inverter. Parameter
Technical specifications Initial
PV Module Technology
Stabilized
Micromorph (a-Si:H/mcSi:H) mm-Si plus 110 120 W 110 W 67 V 64 1.84 A 1.76 88 V 87 2.11 A 1.98 2 1.43 m
Model Power (PM) Voltage at MPP (VM) Current at MPP (IM) Open circuit voltage (VOC) Short circuit current (ISC) Area Inverter Model Rated Power (at 230 V, 50 Hz) Maximum DC Power Maximum input voltage Maximum input current (For string 1/String 2) Maximum output current
Sunny Tripower 8000 TL 8000 W 8200 W 1000 V 15 A/10 A 11.6 A
Fig. 6. Diurnal PR and PRSTC of each string of the micromorph PV system.
Table 2 Environmental conditions of the location. Day Radiation (kWh/ m2/day)
Ambient temperature ( C)
Relative humidity (%)
Wind speed (m/s)
Average
Average
Average
0 8 16 30 36 48 60 80 103 120 140 166
33.82 32.61 39.04 32.56 36.44 37.40 42.05 34.12 32.22 33.50 33.13 34.65
30.09 30.96 15.87 41.96 27.14 30.62 19.37 62.15 77.39 58.13 45.03 41.87
1.02 1.52 1.67 1.78 2.21 1.29 1.67 1.85 1.02 1.85 1.56 1.52
5.21 6.06 5.28 6.40 6.24 5.69 6.09 4.69 4.44 6.61 6.21 5.94
two low performing strings. Fig. 7. Diurnal PR and PRSTC of the overall micromorph PV system.
4.1. Performance ratio The average diurnal PR and PRSTC of all the strings for up to 166
Fig. 5. Diurnal energy generation of each string of the system.
days of outdoor exposure are determined to be 0.83 and 0.89 respectively, and diurnal PR and PRSTC of 8 strings are shown in Fig. 6. The calculations have been performed on the basis of initial rated power of the modules i.e. 120 WP. The peak PR and PRSTC are found to be 0.88 and 0.94 respectively, with an average degradation rate of 1.53%/month and 1.22%/month respectively, if degradation is scaled linearly. The difference between PR and PRSTC varies from 0.05 to 0.06 which depends on the module temperature. The average diurnal PR and PRSTC of the overall system are found to be 0.83 and 0.89 respectively, which are varying from 0.76 to 0.86 and 0.82 to 0.92 respectively, as shown in Fig. 7. If the stabilized rated power, i.e. 110 WP is used, the average diurnal PR and PRSTC of the overall system are found to be 0.90 and 0.97 respectively. Therefore, the system is found to be operated at high PR with respect to the stabilized rated power, which shows that the system is operating near to the stabilized rated capacity and curve shows that it is further decreasing with time. The percentage frequency of instant PR of overall system determined for the first and final day after 166 days of outdoor exposure shows a shift towards left with an increased spread as shown in Fig. 8. The system is found to be operated in the PR range of 0.82e0.85 during the 64.9% time of the initial day and 0.76 to 0.79 during the 65.1% time of the final day. The full width at half
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Fig. 8. Frequency plot of instant PR of initial and final day of outdoor measurement.
maximum (FWHM) Gaussian function shows that the center of the fit shifted from 0.83 to 0.74 whereas the FWHM width and area of the fit increases from 0.022 to 0.031 and 0.56 to 0.65 respectively. 4.2. Alternate reporting conditions
Fig. 9. Frequency plot of PM of overall system at STC.
The system is initially rated as 6.48 kWP and stabilized at 5.94 kWP power capacity at STC. However, the system is found to be operated in the range of 5.1 kWP to 5.6 kWP at STC for about 92% of the total measured data over 600 W/m2 as shown in Fig. 9. The weighted arithmetic mean of the operation of the system at STC with frequency is 5.32 kWP which is about 82% and 90% of the initial and rated capacity respectively. The system is providing about 1.16 kWP and 0.62 kWP lower than the initial and stabilized rated power capacity respectively. The measured values of PM have been translated to ARC for the 200 W/m2, 400 W/m2, 600 W/m2 and 800 W/m2 irradiance levels respectively. The daily average of the instant PM of the overall system with initial (dots) and stabilized (dashes) rated power at the
Fig. 10. Daily average power of overall system at different ARC.
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Fig. 11. Energy and exergy efficiency of individual arrays of the PV system.
4 irradiance levels and module temperature 25 C is shown in Fig. 10. The results show that system is performing better at the lower irradiance level as compared to higher irradiances. The power at irradiance level 400 W/m2, 600 W/m2 and 800 W/m2 are following the same trend with the increasing deviation from rated values. The power at 200 W/m2 irradiance lies between the initial and stabilized rated values up to 100 days and then decreases with the exposure, however the deviation is less as compared to the rate at higher irradiances.
4.3. Energy and exergy analysis The energy and exergy efficiency of each array follows the pattern of PR as shown in Fig. 11. The energy and exergy efficiencies of the system vary from 6.12% to 7.30% and 6.63%e7.89% respectively, with an overall average of 6.91% and 7.48% respectively. The energy and exergy efficiency of array 3 is lowest among all the arrays. The exergy destruction of the all the arrays is found to be approximate to each other at an overall average 80% and increased linearly with incident solar exergy having a slope of þ0.79 with
Fig. 12. Frequency plot of Energy efficiency for first and last day of exposure.
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Fig. 13. Monthly energy yield of the micromorph TFPV system. (*Plant was shut down for 10 days).
Fig. 15. Frequency plot of efficiency of the inverter A and Inverter B.
Fig. 14. Energy, exergy and AC efficiency of overall PV system. Fig. 16. AC frequency of individual phase of both the inverters.
0.99 R2 values of regression analysis. The exergy destruction of all the arrays remains almost constant with time while the convective and radiative heat losses of the system found to be increased with time. However, the exergy of convective and radiative losses are very less i.e. in the range of 0.001%e0.002% and 0.0003%e0.0007% respectively of the total exergy received over the array. The stored recoverable exergy of the array is about 2.4%e4.2% of the total received exergy which can be utilized in thermal processes. The frequency plot of energy efficiency of initial and final day (166th day) of outdoor exposure for per minute index shows a shift towards the left. The energy efficiency of the system is found to be in the range of 6.6%e7.2% initially and 5.8%e6.6% after 166 days of outdoor exposure with a shift of the peak from 7% to 6.4% as shown in Fig. 12. The illustration shows negligible variation in the spread of FWHM Gaussian fit. 4.4. Inverter performance The monthly energy yield of the inverter A and inverter B is shown in Fig. 13. The total energy generation of inverter A and inverter B are found to be 5048 kWh and 5060 kWh respectively
out of which the system was shut down for total 20 days (10 days each in January and February). Each inverter in the system is connected to a PV plant of 3.96 kW rated capacities and location has more than 300 days of sunshine. As a most probable approach, if we suppose 300 days of sunshine in a year with 4.5 h per day of 1 kW/ m2, a PV plant of 3.96 kW rated capacities would theoretically generate 5346 kWh annually. The energy, exergy and AC efficiency of the overall PV system are shown in Fig. 14 with respect to the days of outdoor exposure. The average energy, exergy and AC efficiencies of the system are calculated to be 6.83%, 7.38% and 6.69% respectively. The overall deviation of the efficiencies of the final day with respect to the initial day is 11.41%, 11.39% and 9.91% respectively. However, the deviation is not linear with time and the efficiencies of the system shows a continuous decreasing trend after 100 days. The frequency plot instant efficiency of the two inverters is shown in Fig. 15. Both the inverters are found to be operated primarily above 96% efficiency with the majorly at 99%. However, the spread of inverter A is from 98% to 100%, while inverter B is from 97% to 99%. The AC frequency of each phase of both the three phase
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11
inverter is shown in Fig. 16 which shows that both the inverter provides AC power output at the frequency of 50 Hz ± 0.2. The power factor (cos f) of all the phase of AC power output of both the inverter is found to be 0.99 to 1 for AC power more than 300 W of the respective phase or irradiance more than 200 W/m2 as shown in Fig. 17.
4.5. Effect of module breakage
Fig. 17. Power factor of each phase with respect to the AC power output.
Fig. 18. Current generation of each string in the array 1 and array 3 during 166th day of outdoor exposure.
The large area modules without frame are difficult to handle during installation and prone to the breakage. Sometimes, the microscopic cracks occurred in the module during handling and installation which causes the module to be broken up on thermal and mechanical stress after installation. The system with the broken module not only reduces the power generation, but also raises safety issues especially during rainy season. The string 1.2 and string 3.2 of array 1 and array 3 respectively, have a module with breakage. The array 1 generates 6.33% less energy as compared to the array 2 of inverter A while the array 3 generates 5.04% less energy as compared to the array 4 of inverter B on the 166th day of outdoor exposure. It is to be noted that the voltage of all the arrays is approximately equal to each other and the breakage of the module is not affecting the string voltage. The breakage of the module has decreased the current output of the respective string which is represented in Fig. 18 having per minute current generation of string 1.1, string 1.2, string 3.1 and string 3.2 on 166th day of outdoor exposure. The DC current from the string with module breakage and string without module breakage of array 1 and array 3 with respect to the irradiance are shown in Fig. 19 (a) and (b) respectively. The slope of string with and without breakage of array 1 is determined to be 0.00153 and 0.00177 respectively, whereas it is found to be 0.00148 and 0.00166 for string with and without breakage of array 3 using linear regression. It shows that the magnitude of effect of module breakage increases with irradiance. The strings are connected in parallel connection, therefore the current output of both the strings sums up to produce a total array current and avoids the mismatch loss between the two strings. However, the current output of a string with module breakage includes mismatch loss between the series connected modules.
Fig. 19. Effect of module breakage on string current of (a) Array 1 (b) Array 3 with respect to the irradiance.
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5. Conclusions The GI-TFPV system of 7.92 kWP stabilized rated capacity having 72 large area micromorph based TFPV modules has been evaluated in order to determine its performance during stabilization for a period 166 days in real operating conditions. The string 1.2 and string 3.2 are the lowest performing strings due to the cracked modules. After removing the date of two strings, the GI-TFPV system of 5.94 kWP stabilized rated capacity has been analyzed. The major conclusions of the investigation are: The average diurnal PR and PRSTC of the system are calculated to be 0.83 and 0.89 respectively, with respect to the initial rated capacity while 0.90 and 0.97 respectively with respect to the stabilized rated capacity. The degradation rate of PR and PRSTC of the strings without any cracked modules are found to be about 1.53%/month and 1.22%/ month respectively. The system is found to be operated at an average 5.32 kWP capacity as compared to the 6.48 kWP and 5.94 kWP of initial and stabilized rated capacity. The deviation of power at the ARC from rated power capacity increases with the irradiance, therefore the system is performing comparatively better at lower irradiance. The average energy and exergy efficiencies of the system are determined to be 6.91% and 7.48% respectively, with an exergy destruction of approximately 80%. The average energy, exergy and AC efficiencies of entire GI-TFPV system of 7.92kWP at the input and output points of inverters are calculated to be 6.83%, 7.38% and 6.69% respectively. The energy efficiency of array with and without breakage of the module on 166th day are found to be 5.9% and 6.43% respectively. The arrays having broken PV modules generated 6.33% and 5.04% less energy as compared to the respective arrays without the broken PV modules of same inverter.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17] [18]
On the basis of assessment, it can be concluded that the technology is operating a little under the stabilized rated capacity, however the negative curve shows it has not been stabilized even after 166 days. For further analysis, the I-V characteristic of each PV module, visual inspection, thermal imaging and electroluminescence imaging of electrically degraded and broken module will be carried out after 1 year of outdoor exposure in order to assess the mismatch losses in the strings, power degradation rate of the module and reliability of the technology.
[19]
[20] [21]
[22]
Acknowledgement [23]
The authors would like to acknowledge the support from Indian Institute of Technology Delhi (IIT Delhi) for fellowship and National Institute of Solar Energy (NISE) for data. Data analysis part of the work is carried out under the Solar Energy Research Institute for India and the United States (SERIIUS) project.
[24]
[25]
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Performance evaluation of micromorph based thin film photovoltaic modules in real operating conditions of composite climate Rahul Rawat a, *, Ramayan Singh b, O.S. Sastry b, S.C. Kaushik a a b
Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India National Institute of Solar Energy, Ministry of New and Renewable Energy, Gwal Pahari, Gurgaon, Haryana 122003, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2016 Received in revised form 16 November 2016 Accepted 18 November 2016 Available online xxx
The micromorph thin-film photovoltaic (TFPV) technology uses tandem solar cell structure comprising of hydrogenated amorphous and microcrystalline p-i-n junction silicon cells which needs a long period of outdoor exposure for stabilization. In this paper, the performance analysis of grid interactive (GI) micromorph TFPV system has been assessed for a period of 166 days in real operating conditions on the basis of performance ratio (PR), thermal normalized PR (PRSTC), alternate reporting conditions (ARC), energetic and exergetic studies. A novel methodology for performance evaluation has been developed by utilizing per minute operating data collected from a micromorph based GI-TFPV system of 7.92 kWP rated stabilized capacity. The system is found to be operating in the range of 5.1 kWP to 5.6 kWP as compared to the 5.94 kWP stabilized rated capacity after removing the strings with mechanical damaged modules. The average PR and PRSTC of the system are found to be 0.83 and 0.89 respectively, with an average degradation rate of 1.53%/month and 1.22%/month respectively. The average exergetic, energetic and alternate current output efficiencies of the 7.92 kWP GI-TFPV system are found to be 7.38%, 6.83% and 6.69% respectively. The performance of inverters and effect of module breakage has also been evaluated. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Micromorph Tandem cell Thin film photovoltaics Performance Exergy Efficiency
1. Introduction The solar photovoltaic is coming out as one of the most promising alternatives to the conventional power generation technologies with considerable progress in installed capacity as well as industry oriented research in the field of modelling, design methodologies, size optimization techniques and reliability analyses [1]. The installation of PV systems has grown worldwide with compound annual growth rate of 40% from year 2000e2014. According to the photovoltaics' progress report by Fraunhofer Institute for Solar Energy Systems, the annual production of PV modules was about 47.5 GWP in year 2014, out of which about 43.1 GWP (90.73%) was well established and predominant silicon wafer based technology while the emerging TFPV technology accounts about 4.4 GWP (9.26%) [2]. Although the world solar market is dominated by silicon wafer based module technology, efforts are still going to improve performance and stability of the TFPV technology such as CIGS, CdTe and micromorph due to their higher yields and PR in specific environmental conditions. The TFPV modules, which are
* Corresponding author. E-mail address: [email protected] (R. Rawat).
fabricated by depositing a thin layer of PV material having a thickness in the range of mm, is competing the wafer based technology due to the high absorption coefficient of thin film material which reduces the material requirement and hence reduces the cost. The TFPV technologies are expected to have competitive efficiency, ease in producing large size modules, monolithic PV cell integration and low energy consumption in the manufacturing process which not only reduces the energy payback period but also decreases the manufacturing cost and greenhouse gas emissions [3]. Among TFPV technologies, the amorphous silicon (a-Si: H) is most popular and oldest, but it undergoes light induced degradation (LID) during stabilization because of Staebler-Wronski effect. Therefore, an alternative tandem cell structure of hydrogenated amorphous silicon (a-Si: H) based high band gap p-i-n junction semiconductor as top cell and hydrogenated microcrystalline silicon (mc-Si: H) based low band gap p-i-n junction semiconductor as bottom cell was suggested by Meier et al. (1994) in order to improve the stability which is named as micromorph technology [4]. The cells are deposited typically at 200 C-300 C temperature using plasma enhanced chemical vapour deposition, which is relatively very low as compared to crystalline Si. The spectral
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Nomenclature
j
A B E G GI H I kW kWh P PR Q sf T tf TFPV V y
Subscripts a Ambient AC Alternate current ARC Alternate reporting conditions C Convective d Diurnal f Final i Instant I Current INV Inverter M Module MP Maximum power point n Number of individual array p Peak P Power R Reflective RD Radiative ref Reference ROC Real operating conditions S Sun SKY Sky STC Standard test conditions T Tilted U Recoverable V Voltage
g dB ε
h l r s
Area Exergy Electrical Energy Irradiance Grid interactive Radiation Current kilo Watt kilo Watt hour Electrical Power Performance ratio Energy Spectral factor Temperature Thermal factor Thin film photovoltaic Voltage Normalized energy yield Temperature coefficient Exergy destruction Emissivity Energy efficiency Wavelength Reflectivity Stefan-Boltzmann constant
response and electrical properties of the mc-Si:H p-i-n junction cell approaches the monocrystalline p-n junction cell [5]. The module has amorphous and microcrystalline silicon multi-junction cell coated with EVA foil and sandwiched between front and rear thermally strengthened glass. The a-Si:H and mc-Si:H p-i-n junction diodes of micromorph cell have 1.7 eV and 1.1 eV band gap respectively, which is nearly ideal to absorb more solar radiation spectrum with the thickness range of about 0.2e0.25 mm and 1.6e2.2 mm respectively. The efficiency of the technology has been improved by using highly reflective transparent conductive oxide (TCO), metal contacts and intermediate reflectors for light trapping, and depositing n-doped layer of silicon oxide for increasing current generation [6]. The highest stabilized power conversion efficiency of large area micromorph module (1.43 m2) achieved so far is 12.34% by TEL Solar AG by using thicker intrinsic layer with optimized TCO layer, anti-reflective coating and doped layer [7]. Earlier in 2013, it was 10.7% for the same area module by the same manufacturer. The typical stabilization period of micromorph PV modules is 6e8 months with 8e20% LID of power [8]. Since, both the a-Si: H and mc-Si: H cells of the micromorph PV module have hydrogenated dangling bonds, which induces LID, therefore it is vital to carry out a detailed performance evaluation of the technology in real operating condition during stabilization in order to ensure the reliable lifetime operation. Tossa et al. (2016) have carried out performance analysis of monocrystalline, multicrystalline and micromorph based silicon PV modules in hot and harsh climate and found that micromorph is highest performing technology among the three with an average PR of 0.92 due to relatively low temperature coefficient and series resistance [9]. Torres-Ramírez et al. (2016) have studied the performance of micromorph TFPV technology on the basis of analytical modelling approaches in order to estimate the power and energy output of the technology [10]. Savvakis and Tsoutsos (2015) has presented a
Exergy efficiency
performance assessment of 2 years of operation of 2.18 kWP micromorph based GI-TFPV technology having an average energy efficiency of 7.65% and a PR of 0.85 [11]. In this paper, performance studies are carried out using 7.92 kWP micromorph based GI-TFPV system, having large area modules of 1.43 m2, in order to understand their stabilization process so as to estimate the reliable outdoor spatial life of the technology. 2. Performance evaluation Unlike wafer based PV technology, the micromorph thin film PV technology is neither proven nor established and moreover, it consists of a-Si: H which is prone to light induced degradation on the initial illumination of solar radiation. Hence, it is important to carry out performance evaluation and determine the degradation of emerging micromorph technology in real operating conditions. The outdoor experimental data have been filtered for improving the accuracy of the study and the required data have been analyzed in order assess the performance on the basis of energetic, exergetic, performance ratio, alternate reporting conditions and inverter performance. The detailed methodology used for the performance evaluation has been presented in flow chart as shown in Fig. 1 and discussed in the subsequent subsections. 2.1. Energetic and exergetic analysis The energetic analysis is based on the first law of thermodynamics, which deals with energy efficiency and losses of individual PV array as well as PV systems. The first law of thermodynamics provides quantitative analysis based on the energy balance in which efficiency is defined as the ratio of energy or power output to the input. The micromorph TFPV module has glass to glass
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3
Fig. 1. Methodology for the performance evaluation of the PV system.
assembly having the tandem cell sandwiched between TCO coated front glass and bottom glass. The energy balance of the
micromorph TFPV module has been illustrated in the layer diagram as shown in Fig. 2 and the energy balance of the overall system having four arrays is given by Eq. (1).
4 QS ¼ ð4 QR Þ þ QRD1 þ QRD2 þ QRD3 þ QRD4 þ QC1 þ QC2 þ QC3 þ QC4 þ QU1 þ QU2 þ QU3 þ QU4 þ PMP1 þ PMP2 þ PMP3 þ PMP4 (1) The exergetic analysis is based on the second law of thermodynamics, which deals with quality of energy along with the quantity and states that there is always a degradation of energy during any irreversible energy conversion process [12]. The exergy is also known as available energy to do maximum possible work with respect to the surrounding. As it is a qualitative analysis, it highlights the actual losses and identifies the site for improvement in the system [13]. The irreversibility during the irreversible energy conversion process is described by the Gouy-Stodola law which quantified it as the product of entropy generation (Sgen) and environment temperature.
Irr ¼ Ta Sgen
Fig. 2. Energy balance of the micromorph TFPV module.
(2)
The exergetic analysis of a PV system can be classified into internal and external exergy [14,15]. The internal exergy of a PV module belongs to the chemical exergy balance while the external exergy belongs to the thermodynamic exergy balance with respect to the environment. The external exergy is also known as
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thermodynamic or physical exergy which can be determined by positive balance and anti-balance [16]. In this paper, the exergetic efficiency and exergy losses of micromorph TFPV technology has been assessed by using positive exergy balance given by Eq. (3) [17]. The total exergy of solar irradiance received over the surface area of 4 arrays of the TFPV system is equal to the thermal exergy lost due to reflection, radiation and convection; recoverable thermal exergy stored in the modules; electrical exergy output; and exergy destruction [18,19].
þ BC3 þ BC4 þ BU1 þ BU2 þ BU3 þ BU4 þ PMP1 þ PMP2 þ PMP3 þ PMP4 þ dB1 þ dB2 þ dB3 þ dB4 (3) where the exergy of solar irradiance received over the surface area (A ¼ 25.74 m2) of PV modules is calculated by using correlation given by Petela (1964) for radiative heat transfer from sun temperature to earth's ambient temperature [20].
3
hi ¼
PMPn ¼ 4 A GT n¼1
4 4 3 ; A 3TMn þ TSKY 4TSKY TMn
P
Ta ; BCn ¼ h A ðTMn Ta Þ 1 TMn
(7)
Ta ; BUn ¼ QUn 1 TMn
(8)
hd ¼
t
P t
vBn ¼ BS ðBR þ BRDn þ BCn þ BUn þ PMPn Þ;
(10)
(13)
! (14)
ð4 QS Þ
The exergy efficiency or second law efficiency of any system is defined as the ratio of exergy output to the exergy input. The instant exergy efficiency of each array and overall PV system is defined as the ratio of electrical exergy output for a given time to the exergy of input solar irradiance for the same time as given by Eq. (15) and Eq. (16) respectively.
ji ¼
PMPn BS
(15)
P4
n¼1 PMPn 4 BS
ji ¼
(16)
The diurnal exergy efficiency of the array and overall PV system is the ratio of electricity generated during a day to the exergy of solar radiation received during the same day as given by Eq. (17) and Eq. (18) respectively.
P
PMPn
t
jd ¼
t
(17)
BS
P P4
n¼1
P t
where PMP is the power output at maximum power point (MPP) and the system is assured to be operated at MPP by maximum power point techniques which uses electronic power tracking algorithms [21]. The exergy destruction from each array has been calculated from back calculation as given by Eq. (10).
(12)
n¼1 PMPn
jdn ¼ tP
(9)
n¼1 PMPn 4 QS
QS
P P4
where h is the convective heat transfer coefficient given by h ¼ 5:7 þ 3:8v and v is the wind speed. The recoverable heat loss or the heat stored in the module which causes power as well as module degradation is given by
QUn ¼ QS ðQR þ QRDn þ QCn þ PMPn Þ
P4
PMPn
t
(6)
Here, the subscript ‘n’ represents the number of individual arrays. The exergy lost due to convective heat transfer and recoverable thermal exergy of each array has been determined by Eq. (7) and Eq. (8) respectively.
(11)
The diurnal energy efficiency of a PV system is the ratio of total energy output during a day to the total radiation received over the surface during the same period of time. The diurnal efficiency of the array and the overall system is given by Eq. (13) and Eq. (14) respectively.
(5)
The exergy lost due to radiative heat transfer from module to sky is given by Eq. (6) which includes emissivity (ε) of the PV module, the Stefan-Boltzmann constant (s ¼ 5:67 108 Wm2 K 4 ), module temperature (TM) and sky temperature (TSKY).
s
PMPn P ¼ MPn A GT QS
(4)
The exergy lost due to reflection is determined by the reflectivity (r) of the module.
BRDn ¼ ε
hi ¼
hdn ¼ tP
"
BR ¼ r BS ;
The instant energy efficiency of individual array and overall system for DC output is given by Eq. (11) and Eq. (12) respectively, which is the ratio of DC output to the input irradiance for a particular time instant [22].
P4
4 BS ¼ ð4 BR Þ þ BRD1 þ BRD2 þ BRD3 þ BRD4 þ BC1 þ BC2
4 # 1 Ta 4 Ta A GT ; BS ¼ 1 þ 3 3 TS TS
2.2. Energetic and exergetic efficiency
! PMPn
ð4 BS Þ
(18)
The instant and diurnal AC efficiency of the overall system has been calculated by the ratio of AC power output to the input solar irradiance over the surface area of PV modules as given by Eq. (19) and Eq. (20) respectively.
hACi ¼
PAC PAC ¼ 4 A GT 4 QS
(19)
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P
hACd ¼ P t
t
ðPAC Þ (20)
ð4 QS Þ
2.3. Performance ratio In general, the ratio of normalized array yield (Yf) during a fixed time interval to the reference yield (Yref) during same interval is known as performance ratio (PR) [23,24]. The array yield is defined as the final energy yield normalized with rated power while the reference yield is insolation normalized with rated radiation. It is the most widely used performance indicator of PV systems in ROC which is derived from efficiency and can also be defined as the ratio of efficiency in ROC to the nominal efficiency in reference condition.
Yf E=PSTC E=HT h ¼ ¼ ¼ ROC Yref HT =GSTC PSTC =GSTC href
PR ¼
(21)
The PR for a particular instant of time or power output can be calculated by the ratio of final power output to the reference power output divided by the ratio of irradiance over the PV surface to the reference irradiance as given by Eq. (22).
PRi ¼
PMP =PSTC P =G h ¼ MP T ¼ ROC GT =GSTC PSTC =GSTC href
(22)
The PR provides the fraction of the output of a system in real operating conditions with respect to the output at STC normalized input solar radiation. It provides actual performance of the PV system, however, the PR of two different months or season cannot be compared because of different module temperature and spectrum. Therefore, the conventional PR has been normalized to PR at STC (PRSTC) with module temperature using thermal factor (tf) and spectrum using spectrum factor (sf) [25]. Ideally, the PRSTC should be equal to 1.
tf ¼
1 1 þ gP ðTM TSTC Þ
(23)
where, tf is thermal factor, TM is module temperature and gP is the temperature coefficient of power at maximum power point of PV module (gP ¼ 0:33%).
using Eq. (26) to Eq. (28) in order to carry out time elapsed analysis and determine the deviation of the parameter from the initial measured value and rated values [27]. The voltage and current at MPP measured for irradiance more than 600 W/m2 has been converted to STC in order to determine the power at STC. For further analysis, the measured data in the irradiance range of 150e250 W/ m2, 350e450 W/m2, 550e650 W/m2, 750e850 W/m2 and 950e1050 W/m2 has been translated to voltage, current and power at ARC for 200 W/m2, 400 W/m2, 600 W/m2, 800 W/m2 and 1000 W/m2 respectively and 25 C module temperature [28].
VMP;ARC ¼ VMP þ gV ðTARC TM Þ IMP;ARC ¼ IMP
GARC GT
(26)
þ gI ðTARC TM Þ
PMP;ARC ¼ VMP;ARC IMP;ARC
(27) (28)
where temperature coefficient of voltage gV and current gI are 0.37% and þ0.08% respectively. 2.5. Inverter performance The PV system has two inverters in order to convert DC power to 3 phase AC power and their performance has been assessed on the basis DC to AC conversion efficiency, phase difference and frequency of output AC power for each inverter. The power factor (cos F) should be equal to one and the frequency should be 50 Hz for each phase of the power output. The occurrence magnitude of deviation of these parameters from the ideal conditions has been assessed. The instant and diurnal DC to AC efficiency conversion efficiency of the inverter is defined as the ratio of AC power output to the DC power input as given by Eq. (29) and Eq. (30) respectively.
hINVi ¼ P2
PAC
n¼1
P
hINVd ¼
t
P P2 t
(29)
PMPn PAC
n¼1
!
(30)
PMPn
Z
Z
xSTC ðlÞ$SRðlÞdl sf ¼
5
Z
Z
xSTC ðlÞdl
xðlÞ$SRðlÞdl
3. System description
(24)
xðlÞdl
where sf is spectrum factor, xSTC is standard spectrum at STC, x is the solar spectrum of the location, SR is the relative spectral response of the PV technology and l is the wavelength (nm). However, the spectrum factor of micromorph PV technology is 1 with a variation less than 0.003, therefore it can be ignored [26]. The PRSTC of micromorph technology can be determined by Eq. (25).
PRSTC ¼ PR tf sf zPR tf
(25)
2.4. Alternative reporting conditions The electrical parameters measured at real operating conditions have been translated to a common alternative reporting condition
The micromorph based GI-TFPV system has been installed by TEL Solar AG at National Institute of Solar Energy, Gurgaon Gwal Pahari, Gurgaon, India, which has geographical co-ordinates 28.42 N latitude and 77.15 E longitude. The modules are mounted due south with tilt angle of 25 as shown in Fig. 3. The system having initial rated power of 8.64 kWP comprises of 8 strings having total 72 modules of initial 120 WP and stabilized 110 WP rated power each. Although, the stabilized rated power of each module is 110 WP, the initial rated power is used in this study for several calculations because the measurement data used for analysis is of an initial 6 months of the outdoor exposure. Each string has 9 series connected modules and 2 strings are connected in parallel combination making an array. The system consists of 4 such arrays out of which array 1 and 2 are supplying power to inverter A and array 3 and 4 to inverter B simultaneously as shown in Fig. 4. The inverters A and B are three phase inverter which is converting DC to 3 phase AC at 50 Hz frequency to supply power to
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Fig. 3. Photograph of the micromorph based TFPV system.
Fig. 4. Layout diagram of the micromorph based TFPV system.
the grid. The technical specifications of PV module and inverter are given in Table 1. The per minute operating data of several days during the initial 6 months have been recorded and filtered for the clear sky day. The clear sky day data of the day having clearness index above 0.5 during the 166 days of outdoor exposure have been used for assessment in order to improve the accuracy of the investigation. The environmental conditions of the clear sky days have been presented in Table 2. The effect of module breakage on the performance of the string has been assessed using per minute data of 166th day. The separate data extracted from inverters has been used for calculating monthly energy yield for the entire year.
4. Results and discussion The diurnal energy generated by each string of the system shows that the string 1.2 and string 3.2 are under performing as compared to others as shown in Fig. 5. It has been found by visual investigation that there are broken modules in the string 1.2 and 3.2 which reduces the power generation of the strings. Since, the power degradation in these strings is due to mechanical damage and does not impart in the performance and degradation of the technology. Therefore, the data of these strings are removed and not used in the further analysis making the rated capacity of the system to be 6.48 kW initially and 5.94 kW after stabilization. However, the data of the entire system are used in the section of inverter performance because both the inverters are connected to one of the
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Table 1 Technical specifications of the PV module and inverter. Parameter
Technical specifications Initial
PV Module Technology
Stabilized
Micromorph (a-Si:H/mcSi:H) mm-Si plus 110 120 W 110 W 67 V 64 1.84 A 1.76 88 V 87 2.11 A 1.98 2 1.43 m
Model Power (PM) Voltage at MPP (VM) Current at MPP (IM) Open circuit voltage (VOC) Short circuit current (ISC) Area Inverter Model Rated Power (at 230 V, 50 Hz) Maximum DC Power Maximum input voltage Maximum input current (For string 1/String 2) Maximum output current
Sunny Tripower 8000 TL 8000 W 8200 W 1000 V 15 A/10 A 11.6 A
Fig. 6. Diurnal PR and PRSTC of each string of the micromorph PV system.
Table 2 Environmental conditions of the location. Day Radiation (kWh/ m2/day)
Ambient temperature ( C)
Relative humidity (%)
Wind speed (m/s)
Average
Average
Average
0 8 16 30 36 48 60 80 103 120 140 166
33.82 32.61 39.04 32.56 36.44 37.40 42.05 34.12 32.22 33.50 33.13 34.65
30.09 30.96 15.87 41.96 27.14 30.62 19.37 62.15 77.39 58.13 45.03 41.87
1.02 1.52 1.67 1.78 2.21 1.29 1.67 1.85 1.02 1.85 1.56 1.52
5.21 6.06 5.28 6.40 6.24 5.69 6.09 4.69 4.44 6.61 6.21 5.94
two low performing strings. Fig. 7. Diurnal PR and PRSTC of the overall micromorph PV system.
4.1. Performance ratio The average diurnal PR and PRSTC of all the strings for up to 166
Fig. 5. Diurnal energy generation of each string of the system.
days of outdoor exposure are determined to be 0.83 and 0.89 respectively, and diurnal PR and PRSTC of 8 strings are shown in Fig. 6. The calculations have been performed on the basis of initial rated power of the modules i.e. 120 WP. The peak PR and PRSTC are found to be 0.88 and 0.94 respectively, with an average degradation rate of 1.53%/month and 1.22%/month respectively, if degradation is scaled linearly. The difference between PR and PRSTC varies from 0.05 to 0.06 which depends on the module temperature. The average diurnal PR and PRSTC of the overall system are found to be 0.83 and 0.89 respectively, which are varying from 0.76 to 0.86 and 0.82 to 0.92 respectively, as shown in Fig. 7. If the stabilized rated power, i.e. 110 WP is used, the average diurnal PR and PRSTC of the overall system are found to be 0.90 and 0.97 respectively. Therefore, the system is found to be operated at high PR with respect to the stabilized rated power, which shows that the system is operating near to the stabilized rated capacity and curve shows that it is further decreasing with time. The percentage frequency of instant PR of overall system determined for the first and final day after 166 days of outdoor exposure shows a shift towards left with an increased spread as shown in Fig. 8. The system is found to be operated in the PR range of 0.82e0.85 during the 64.9% time of the initial day and 0.76 to 0.79 during the 65.1% time of the final day. The full width at half
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Fig. 8. Frequency plot of instant PR of initial and final day of outdoor measurement.
maximum (FWHM) Gaussian function shows that the center of the fit shifted from 0.83 to 0.74 whereas the FWHM width and area of the fit increases from 0.022 to 0.031 and 0.56 to 0.65 respectively. 4.2. Alternate reporting conditions
Fig. 9. Frequency plot of PM of overall system at STC.
The system is initially rated as 6.48 kWP and stabilized at 5.94 kWP power capacity at STC. However, the system is found to be operated in the range of 5.1 kWP to 5.6 kWP at STC for about 92% of the total measured data over 600 W/m2 as shown in Fig. 9. The weighted arithmetic mean of the operation of the system at STC with frequency is 5.32 kWP which is about 82% and 90% of the initial and rated capacity respectively. The system is providing about 1.16 kWP and 0.62 kWP lower than the initial and stabilized rated power capacity respectively. The measured values of PM have been translated to ARC for the 200 W/m2, 400 W/m2, 600 W/m2 and 800 W/m2 irradiance levels respectively. The daily average of the instant PM of the overall system with initial (dots) and stabilized (dashes) rated power at the
Fig. 10. Daily average power of overall system at different ARC.
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Fig. 11. Energy and exergy efficiency of individual arrays of the PV system.
4 irradiance levels and module temperature 25 C is shown in Fig. 10. The results show that system is performing better at the lower irradiance level as compared to higher irradiances. The power at irradiance level 400 W/m2, 600 W/m2 and 800 W/m2 are following the same trend with the increasing deviation from rated values. The power at 200 W/m2 irradiance lies between the initial and stabilized rated values up to 100 days and then decreases with the exposure, however the deviation is less as compared to the rate at higher irradiances.
4.3. Energy and exergy analysis The energy and exergy efficiency of each array follows the pattern of PR as shown in Fig. 11. The energy and exergy efficiencies of the system vary from 6.12% to 7.30% and 6.63%e7.89% respectively, with an overall average of 6.91% and 7.48% respectively. The energy and exergy efficiency of array 3 is lowest among all the arrays. The exergy destruction of the all the arrays is found to be approximate to each other at an overall average 80% and increased linearly with incident solar exergy having a slope of þ0.79 with
Fig. 12. Frequency plot of Energy efficiency for first and last day of exposure.
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Fig. 13. Monthly energy yield of the micromorph TFPV system. (*Plant was shut down for 10 days).
Fig. 15. Frequency plot of efficiency of the inverter A and Inverter B.
Fig. 14. Energy, exergy and AC efficiency of overall PV system. Fig. 16. AC frequency of individual phase of both the inverters.
0.99 R2 values of regression analysis. The exergy destruction of all the arrays remains almost constant with time while the convective and radiative heat losses of the system found to be increased with time. However, the exergy of convective and radiative losses are very less i.e. in the range of 0.001%e0.002% and 0.0003%e0.0007% respectively of the total exergy received over the array. The stored recoverable exergy of the array is about 2.4%e4.2% of the total received exergy which can be utilized in thermal processes. The frequency plot of energy efficiency of initial and final day (166th day) of outdoor exposure for per minute index shows a shift towards the left. The energy efficiency of the system is found to be in the range of 6.6%e7.2% initially and 5.8%e6.6% after 166 days of outdoor exposure with a shift of the peak from 7% to 6.4% as shown in Fig. 12. The illustration shows negligible variation in the spread of FWHM Gaussian fit. 4.4. Inverter performance The monthly energy yield of the inverter A and inverter B is shown in Fig. 13. The total energy generation of inverter A and inverter B are found to be 5048 kWh and 5060 kWh respectively
out of which the system was shut down for total 20 days (10 days each in January and February). Each inverter in the system is connected to a PV plant of 3.96 kW rated capacities and location has more than 300 days of sunshine. As a most probable approach, if we suppose 300 days of sunshine in a year with 4.5 h per day of 1 kW/ m2, a PV plant of 3.96 kW rated capacities would theoretically generate 5346 kWh annually. The energy, exergy and AC efficiency of the overall PV system are shown in Fig. 14 with respect to the days of outdoor exposure. The average energy, exergy and AC efficiencies of the system are calculated to be 6.83%, 7.38% and 6.69% respectively. The overall deviation of the efficiencies of the final day with respect to the initial day is 11.41%, 11.39% and 9.91% respectively. However, the deviation is not linear with time and the efficiencies of the system shows a continuous decreasing trend after 100 days. The frequency plot instant efficiency of the two inverters is shown in Fig. 15. Both the inverters are found to be operated primarily above 96% efficiency with the majorly at 99%. However, the spread of inverter A is from 98% to 100%, while inverter B is from 97% to 99%. The AC frequency of each phase of both the three phase
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inverter is shown in Fig. 16 which shows that both the inverter provides AC power output at the frequency of 50 Hz ± 0.2. The power factor (cos f) of all the phase of AC power output of both the inverter is found to be 0.99 to 1 for AC power more than 300 W of the respective phase or irradiance more than 200 W/m2 as shown in Fig. 17.
4.5. Effect of module breakage
Fig. 17. Power factor of each phase with respect to the AC power output.
Fig. 18. Current generation of each string in the array 1 and array 3 during 166th day of outdoor exposure.
The large area modules without frame are difficult to handle during installation and prone to the breakage. Sometimes, the microscopic cracks occurred in the module during handling and installation which causes the module to be broken up on thermal and mechanical stress after installation. The system with the broken module not only reduces the power generation, but also raises safety issues especially during rainy season. The string 1.2 and string 3.2 of array 1 and array 3 respectively, have a module with breakage. The array 1 generates 6.33% less energy as compared to the array 2 of inverter A while the array 3 generates 5.04% less energy as compared to the array 4 of inverter B on the 166th day of outdoor exposure. It is to be noted that the voltage of all the arrays is approximately equal to each other and the breakage of the module is not affecting the string voltage. The breakage of the module has decreased the current output of the respective string which is represented in Fig. 18 having per minute current generation of string 1.1, string 1.2, string 3.1 and string 3.2 on 166th day of outdoor exposure. The DC current from the string with module breakage and string without module breakage of array 1 and array 3 with respect to the irradiance are shown in Fig. 19 (a) and (b) respectively. The slope of string with and without breakage of array 1 is determined to be 0.00153 and 0.00177 respectively, whereas it is found to be 0.00148 and 0.00166 for string with and without breakage of array 3 using linear regression. It shows that the magnitude of effect of module breakage increases with irradiance. The strings are connected in parallel connection, therefore the current output of both the strings sums up to produce a total array current and avoids the mismatch loss between the two strings. However, the current output of a string with module breakage includes mismatch loss between the series connected modules.
Fig. 19. Effect of module breakage on string current of (a) Array 1 (b) Array 3 with respect to the irradiance.
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5. Conclusions The GI-TFPV system of 7.92 kWP stabilized rated capacity having 72 large area micromorph based TFPV modules has been evaluated in order to determine its performance during stabilization for a period 166 days in real operating conditions. The string 1.2 and string 3.2 are the lowest performing strings due to the cracked modules. After removing the date of two strings, the GI-TFPV system of 5.94 kWP stabilized rated capacity has been analyzed. The major conclusions of the investigation are: The average diurnal PR and PRSTC of the system are calculated to be 0.83 and 0.89 respectively, with respect to the initial rated capacity while 0.90 and 0.97 respectively with respect to the stabilized rated capacity. The degradation rate of PR and PRSTC of the strings without any cracked modules are found to be about 1.53%/month and 1.22%/ month respectively. The system is found to be operated at an average 5.32 kWP capacity as compared to the 6.48 kWP and 5.94 kWP of initial and stabilized rated capacity. The deviation of power at the ARC from rated power capacity increases with the irradiance, therefore the system is performing comparatively better at lower irradiance. The average energy and exergy efficiencies of the system are determined to be 6.91% and 7.48% respectively, with an exergy destruction of approximately 80%. The average energy, exergy and AC efficiencies of entire GI-TFPV system of 7.92kWP at the input and output points of inverters are calculated to be 6.83%, 7.38% and 6.69% respectively. The energy efficiency of array with and without breakage of the module on 166th day are found to be 5.9% and 6.43% respectively. The arrays having broken PV modules generated 6.33% and 5.04% less energy as compared to the respective arrays without the broken PV modules of same inverter.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17] [18]
On the basis of assessment, it can be concluded that the technology is operating a little under the stabilized rated capacity, however the negative curve shows it has not been stabilized even after 166 days. For further analysis, the I-V characteristic of each PV module, visual inspection, thermal imaging and electroluminescence imaging of electrically degraded and broken module will be carried out after 1 year of outdoor exposure in order to assess the mismatch losses in the strings, power degradation rate of the module and reliability of the technology.
[19]
[20] [21]
[22]
Acknowledgement [23]
The authors would like to acknowledge the support from Indian Institute of Technology Delhi (IIT Delhi) for fellowship and National Institute of Solar Energy (NISE) for data. Data analysis part of the work is carried out under the Solar Energy Research Institute for India and the United States (SERIIUS) project.
[24]
[25]
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