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Performance of Si-based PV rooftop systems operated under distinct four seasons
Renewable Energy 81 (2015) 482e489
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance of Si-based PV rooftop systems operated under distinct four seasons Seung Yeop Myong*, You-Chul Park, Sang Won Jeon Energy R&D Center, KISCO, Miamri 1044, Jeungpyeong-eup, Jeungpyeong-gun, Choongcheongbuk-do, 368-906, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2014 Accepted 19 March 2015 Available online
We have investigated the electrical energy yield of hydrogenated amorphous silicon (a-Si:H) singlejunction and crystalline (c-Si) photovoltaic (PV) rooftop systems operated under distinct four seasons. The impact of the module type and installed tilt angle on the annual electrical energy yield has been monitored and then compared with the data predicted by the computer simulation. Despite a good temperature coefficient and less shading effect of a-Si:H single-junction modules, the energy output gain of the a-Si:H single-junction PV generator is only 2.7% compared to the c-Si PV generator installed using c-Si PV modules. It is inferred that a nominal rated power of the a-Si:H single-junction modules determined by an indoor light soaking test is not suitable for the design of PV systems operated under distinct four seasons. Thus, the nominal rated power of the a-Si:H single-junction PV modules should be determined through a proper outdoor exposure test considering thermal annealing and light soaking effects under various seasonal weather conditions. In addition, it is found that the performance of the Sibased PV rooftop systems operated under distinct four seasons could be improved by simply toggling the tilt angle considering the plane-of-array irradiance and snowfall effect. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Amorphous silicon photovoltaic generator Crystalline silicon photovoltaic generator Electrical energy yield Nominal rated power Tilt angle
1. Introduction Rapid global warming caused by excess emission of CO2 warns about the climate change. Sufficient supplies of clean energy are intimately linked with global stability, economic prosperity, and quality of life. Accordingly, clean renewable energy including solar, wind, and hydrogen energy becomes a prime issue. A photovoltaic (PV) system using solar light is a promising candidate among the renewable energy sources because the sun is our primary source of clean, abundant energy. Actually, there has been a drastic increase in the global PV market during the last two decades. In 2013, approximately 37 GWp PV systems were installed worldwide, leading to cumulative global installations of ~140 GWp. Recently, Northeast Asia having distinct four seasons (China, Japan, and Republic of Korea) has gained great attraction as an emerging PV market. In this region, it is believed that nuclear power plants are no longer stable and cheap energy source after Fukushima nuclear meltdowns. In Republic of Korea (South Korea), 295 MWp PV systems were installed in 2012. Hence, cumulative PV installations firstly reached 1 GWp, which corresponding to 2.7% of the total
* Corresponding author. Tel.: þ82 43 8389146; fax: þ82 43 8389144. E-mail address: [email protected] (S.Y. Myong). http://dx.doi.org/10.1016/j.renene.2015.03.055 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
renewable energy supply [1]. Moreover, the country recorded the highest annual PV installations of 330 MWp in 2013. The Korean government plans to increase the PV supply up to 4.1% of the total renewable energy by 2030 [2]. Currently, crystalline silicon (c-Si) PV systems using mono and multi c-Si PV modules share > 90% of the Korean PV market [3] mainly due to a lack of local manufacturers producing thin-film PV modules. KISCO is the first-ever thinfilm PV module manufacturer in this country, which started mass production of thin-film silicon (Si) PV modules in 2008. The thinfilm Si PV modules using hydrogenated amorphous Si (a-Si:H) absorbers have been considered as next-generation PV modules due to the various advantages of remarkably low consumption of the Si raw material, large-area deposition, low-temperature production, and low temperature coefficient. The thin-film Si PV technology also profits from the wide experience base of display industries [4]. However, the recent sharp drop in the module price orders costeffective PV modules. Thus, the company also started mass production of c-Si PV modules in 2011. In this work, authors have investigated the outdoor performance of self-produced a-Si:H single-junction and c-Si PV rooftop generators operated under distinct four seasons. Based on long-term field tests, the authors will provide the easy and economic way to improve the electricity generation considering the seasonal weather conditions. Furthermore, we will discuss the suitable rated power of the a-Si:H single-
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junction PV modules for use in regions with distinct four seasons since the outdoor performance of the metastable modules exposed to the various seasonal weather conditions is significantly different from their indoor performance. 2. Methodology For this research, a-Si:H single-junction and c-Si PV power plants with a capacity of over 100 kWp were built in Jeungpyeong, Republic of Korea. Fig. 1(a) shows the 132-kWp a-Si:H singlejunction PV rooftop system mounted with the tilt angle of 5 . This system was built in 2010. Fig. 1(b) shows the 379-kWp c-Si PV rooftop system built in 2012. The system has been operated by toggling the tilt angle manually in April and September between 10 and 35 . The module operating temperature and horizontal irradiance were offered via a monitoring facility. The electrical power-generation of the installed PV generators was checked every day. Jeungpyeong (36.80 N, 127.59 E) locating in central Korea (see Fig. 2) has distinct four seasons e warm and dry spring, hot and rainy (humid) summer, cool and clear blue sky in autumn, and cold and snowy winter. 3-kWp PV generators were also installed for the systematic investigation. The impact of the module type and installed tilt angle on the annual electrical energy yield were monitored and then compared with the data predicted by the simulated values using the commercially available software program PVsyst from the University of Geneva [8]. Fig. 3(a) shows a front image of a fabricated a-Si:H singlejunction PV module. 1.43 m2 p-i-n-type a-Si:H single-junction PV modules were produced using a fully automated in-line system with an annual capacity of 20 MWp. Textured SnO2:F (FTO)-coated soda-lime glasses were used as substrates. Every segment (cell) has the structure of glass/FTO/hydrogenated p-type amorphous siliconcarbide (p-a-SiC:H) window layer/p-type buffer layer (p-buffer)/ intrinsic a-Si:H (i-a-Si:H) absorber (250 nm)/hydrogenated n-type a-Si:H (n-a-Si:H)/boron (B)-doped zinc oxide (ZnO:B) back reflector/Al back contact. For the deposition of thin-film Si layers, large-area cluster-type radio-frequency plasma-enhanced chemical vapor deposition (PECVD) apparatus was used in order to minimize the dangling bonds [5] and residual impurity cross-contamination. To achieve a high deposition rate of 0.41 nm/s, the SiH4 concentration (¼ [SiH4]/([SiH4] þ [H2])) is varied from 4.8% to 7.7% without any turn-off of the plasma power [6]. The 20-nm-thick interfacial part of i-a-Si:H was deposited using the low SiH4 concentration to improve the p/i interface, whereas the 230-nm-thick bulk part of ia-Si:H was deposited using the high SiH4 concentration to obtain the high deposition speed. The detailed deposition conditions for the component layers were described in our previous reports [6,7]. The monolithic series integration of the a-Si:H single-junction PV modules was realized via the plurality of three parallel laser-
Fig. 2. Map of South Korea where the PV power generators were installed.
Fig. 3. Front images of the fabricated PV modules: (a) the a-Si:H single-junction PV module and (b) the c-Si PV module.
scribed patterns. The cell width for the a-Si:H single-junction PV modules was selected as 9.8 mm in order to prevent an extremely high open-circuit voltage (Voc), which caused a considerable balance of system (BOS). The module encapsulation was performed by
Fig. 1. Pictures of the PV power plants: (a) 132-kWp a-Si:H single-junction PV rooftop systems mounted with the fixed tilt angle of 5 and (b) 379-kWp c-Si PV rooftop systems being operated by toggling the tilt angle between 10 and 35 .
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laminating the back sheet including an Al foil with an ethylenevinyl acetate (EVA) film. Fig. 3(b) exhibits a front image of a fabricated c-Si PV module. 1.63 m2 c-Si PV modules were produced using a semi-automated in-line system with the annual capacity of 65 MWp. 6-inch (156 156 mm2) p-type mono c-Si solar cells were interconnected using the three bus bar configuration in order to form a 6 10 matrix. The interconnected solar cells were encapsulated between a polyamide (PA)-based multilayer back sheet and low iron patterned tempered glass using EVA films. The photo currentevoltage (IeV) characteristics for the fabricated PV modules were measured under the standard test conditions (STC; 25 C, AM 1.5G, and 1000 W/m2) using a WACOM, WPSS-1.5 1.2e50 4 AM1.5 GF solar simulator. The a-Si:H singlejunction PV modules acquired the International Electrochemical Commission (IEC) 61646 module certification for thin-film terrestrial PV modules from UL, USA and TÜV, Germany, while the c-Si PV modules acquired IEC 61215 module certification from TÜV, Germany. 3. Results and discussion Fig. 4 depicts the photo IeV curves for the fabricated a-Si:H single-junction and c-Si PV modules. As shown in Fig. 4(a), the initial maximum power (Pmax) of the a-Si:H single-junction PV module is 123.0 W, which is corresponding to the initial conversion efficiency (h) of 8.6% (Voc ¼ 99.4 V, short-circuit current (Isc) ¼ 1.76 A, and fill factor (FF) ¼ 0.703). The indoor light soaking test was performed at 50 C using a class C solar simulator based on the IEC 61646 light soaking test. After light soaking, the a-Si:H single-junction PV module is stabilized to 101.3 W (Voc ¼ 96.4 V, Isc ¼ 1.71 A, and FF ¼ 0.613, and h ¼ 7.1%) with the light-induced degradation ratio (¼ Dh/initial h) of 17.6%. Thus, the nominal rated power of 100.0 W was selected for the a-Si:H single-junction PV modules fabricated under the same conditions. It has been reported that the deposition rate for a state-of-the-art i-a-Si:H absorber is 0.2e0.5 nm/s [9]. The deposition rate of the a-Si:H absorbers is the key factor governing the production throughput. However, the so-called “Staebler-Wronski effect”, which is the lightinduced degradation of a-Si:H-based films arising from the photocreation of dangling bonds accomplished by nonradiative recombination of photogenerated electronehole pairs [10], remains as a major technical challenge for mass production. To reduce the light-induced degradation of i-a-Si:H absorbers that
leads to the degradation of thin-film Si PV devices, there have been extensive investigations during the past three decades. The deposition of i-a-Si:H using H2-dilution of SiH4 is known as the best way to reduce the Staebler-Wronski effect, however it is impossible to eliminate the effect [11e18]. The stability of the aSi:H single-junction PV modules is mainly influenced by the thickness and film quality of the absorber. The higher deposition rate of i-a-Si:H has resulted in lower initial h and higher lightinduced degradation of a-Si:H PV devices, leading to lower stabilized h [19,20]. As commented in our previous reports [6,7], the two-step deposition method of i-a-Si:H had been developed by adjusting the deposition rate in order to meet the high throughput and high stabilized h simultaneously [12,21,22]. In addition, we reported that a-Si:H single-junction PV modules fabricated using the two-step deposition method showed a higher stability against indoor light soaking without deteriorating the production throughput compared to those fabricated using the one-step deposition method [6]. To the best of our knowledge, the electrical energy yield of large-area PV arrays comprised of a-Si:H single-junction PV modules fabricated using the two-step deposition method will be first-ever reported in this work. As shown in Fig. 4(b), the c-Si PV module has initial Pmax of 251.7 W, which is corresponding to initial h of 15.4% (Voc ¼ 37.5 V, Isc ¼ 8.58 A, and FF ¼ 0.776). Thus, the value of 250.0 W was selected as the nominal rated power for c-Si PV modules. Fig. 5 reveals the electrical energy yield of the Si-based PV rooftop systems under consideration. It is found that the electrical energy yield of the a-Si:H single-junction PV rooftop system installed with the fixed tilt angle of 5 is widely fluctuating depending on the seasons. Compared to the c-Si PV rooftop system, the a-Si:H single-junction PV rooftop system has higher yields in summer. However, it has poor yields in winter. Thus, the average electricity-generation time calculated for the a-Si:H single-junction PV rooftop system is just 3.17 h/day. On the contrary, the c-Si PV rooftop system operated by toggling the tilt angle shows the narrow distribution of the electrical energy yield together with the good average electricity-generation time of 3.69 h/day. As depicted in Fig. 6, the a-Si:H single-junction PV module exhibits high yield values in summer by fixing the tilt angle at 5 because of high plane-of-array irradiance. However, snow covering the modules severely disturbs the electricity-generation, leading to the disastrous energy harvest in winter (see Fig. 7). Consequently, the widely fluctuating electrical energy yield is achieved. The a-Si:H single-junction PV module mounted with the tilt angle of 30
Fig. 4. Photo IeV characteristics of the fabricated a-Si:H single-junction and c-Si PV modules: (a) initial and stabilized performances of the a-Si:H single-junction PV module and (b) the initial performance of the c-Si PV module.
S.Y. Myong et al. / Renewable Energy 81 (2015) 482e489
Fig. 5. Electrical energy yield of the PV rooftop systems. The solid lines are eye guides.
obtains the annual energy output gain of 9.2% compared to its counterpart mounted with the tilt angle of 5 . For the detailed comparison, three kinds of 3-kWp PV generators were installed at Jeungpyeong. Fig. 8 displays pictures of the installed 3-kWp PV generators. Each PV generator was mounted on an open rack supporting structure facing south with a fixed tilt angle in the maximum power point (MPP) state. The 3-kWp a-Si:H single-junction PV generator is comprised of 30 modules (6 strings composed of respective 5 serially connected modules), while the 3kWp c-Si PV generators are composed of 12 modules (1 string composed of 12 serially connected modules), respectively. Accordingly, the former generator needs 2.2 times larger ground than the latter generators. Fig. 9 shows monthly horizontal irradiance and module operating temperatures during the outdoor field test from July, 2012 to June, 2014. Monthly horizontal irradiance and mean ambient temperatures were informed by observed values at Cheongju (36.38 N, 127.26 E) synoptic station, which conducted by Korea Meteorological Administration (KMA) with an automated synoptic observing system (ASOS) (see Fig. 2) [23]. The 16 weather elements, including temperature and horizontal solar irradiance, were measured every hour. Annual horizontal irradiance of the site is 1299.1 kWh/m2. Spring (from March to May) is the highest season for solar irradiance. In particular, the maximum solar irradiance is
Fig. 6. Dependence of plane-of-array irradiance and electrical power generation of aSi:H PV modules on the tilt angle (field test during 2010).
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Fig. 7. Picture of the a-Si:H single-junction PV rooftop system covered by snow (taken in December, 2013).
inspected in May. During the season, the monthly mean value of the daily highest module operating temperature elevates month after month. Summer (from June to August) is the season when monthly accumulated solar irradiance is considerably high except a typical rainy season in Northeast Asia. Summer is the highest season for the monthly mean value of the daily highest module operating temperature (~50 C), while the monthly maximum of the daily highest module operating temperature is saturated at 60 C. The saturated maximum module operating temperature can be ascribed to the open rock supporting structure during the outdoor field test. The PV module operating temperature can be influenced by the ambient temperature, wind speed, solar irradiance, and module supporting structure [24]. In autumn (from September to November), monthly accumulated solar irradiance reduces month after month. In particular, there is a considerable drop in solar irradiance in November due to low solar altitude at meridian passage and a lot of foggy days. During the autumn season, all the module operating temperatures gradually decrease month after month. Winter (from December to February) is the worst season for the electrical energy generation using PV systems due to the lowest solar altitude at meridian passage and snowfall [25]. In January, the monthly mean value of the daily highest module operating temperature has the lowest value. The monthly mean ambient temperature has a similar pattern to the monthly minimum value of the daily highest module operating temperature. Fig. 10 represents simulated and measured electrical energy yield values for the 3-kWp PV generators under consideration. The computer simulations were performed by designing the gridconnected PV systems using specific values of the PV modules and arrays. Fig. 10(a) exhibits the results for the 3-kWp PV generator comprised of the fabricated a-Si:H single-junction PV modules mounted with the fixed tilt angle of 35 , while Fig. 10(b) shows the results for the 3-kWp c-Si PV generators mounted with the fixed tilt angle of 10 and 35 . In general, the outdoor electricity-generation of the PV generators is affected by the module temperature, planeof-array irradiance [26], spectral distribution [26,27], temperature coefficient [28], transmittance of aged PV glasses [29] and aged EVA films [30], dust effect [31e33], and partial shading effect on PV modules [34,35]. The superstrate-type a-Si:H single-junction PV modules may be free from the effect of the aged EVA films, however it is also significantly impacted by the Staebler-Wronski effect [27,36]. It takes about two years until the performance of a-Si:H single-junction PV modules becomes stable in the outdoor environment [37,38]. All the PV generators exhibit the highest electrical
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Fig. 8. Pictures of the installed 3-kWp a-Si:H single-junction and c-Si PV generators: (a) the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 , (b) the c-Si PV generator installed with the fixed tilt angle of 10 , and (c) the c-Si PV generator installed with the fixed tilt angle of 35 .
Fig. 9. Outdoor field test conditions for the 3-kWp PV generators: (a) annual horizontal irradiance and (b) the monthly mean ambient temperature were displayed with the maximum, minimum, and mean of the daily highest operating temperatures for a c-Si PV module. The solid and dashed lines are eye guides.
energy yield in spring by virtue of the maximum solar irradiance and moderate module operating temperatures. Excepting the aforementioned rainy and foggy seasons, the a-Si:H single-junction PV generator produces electricity well in summer and autumn thanks to high solar irradiance. Moreover, the a-Si:H single-
junction PV generator obviously performs better than the c-Si PV generators during the seasons. The indoor temperature coefficient of the a-Si:H single-junction PV modules measured by UL is as low as 0.20%/ C. In addition, it is reported that the outdoor temperature coefficient of a-Si:H single-junction PV modules is rather
Fig. 10. Annual electrical energy yield of the 3-kWp PV generators: (a) the simulated and measured data for the a-Si:H single-junction PV rooftop system installed with the fixed tilt angle of 35 . The simulated data using the corrected rated power of 92.2 W are also revealed. (b) The simulated and measured data for the c-Si PV rooftop system installed with the fixed angle of 10 and 35 . The solid lines are eye guides.
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positive (~0.35%/ C) [38]. On the contrary, the outdoor temperature coefficient of the c-Si PV modules measured by TÜV is 0.43%/ C, which is severely worse than that of the a-Si:H single-junction PV modules. Therefore, the a-Si:H single-junction PV generator produces electricity more effectively at higher ambient temperatures compared to the c-Si PV generators. In winter, the lowest solar altitude at meridian passage and snow covering the PV modules lead all the PV generators to the respective lowest performance. The negative impact of snowfall is more critical to the PV generator installed with the lower tilt angle because snow can be maintained for a longer time without any mandatory removal. Compared to the c-Si PV generator installed with the fixed tilt angle of 35 , the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 produces less electricity in winter. It can be attributed to the module operating temperatures lower than room temperature. It is also found that the seasonal variation of the electrical energy yield of the a-Si:H PV generator is higher than that of the c-Si PV generator. The wider seasonal fluctuation of the electrical energy yield for the a-Si:H single-junction PV generator is mainly due to the effects of thermal annealing and light soaking, which are deeply related to the Staebler-Wronski effect of the a-Si:H single-junction PV modules [38,39]. The c-Si PV generator installed with the fixed tilt angle of 10 produces more electricity from May to August compared to the c-Si PV generator installed with the fixed tilt angle of 35 . It is mainly due to higher plane-of-array irradiance during the period caused by the lower tilt angle (see Fig. 6). However, the opposite trend of plane-of-array irradiance is inspected during other months. Thus, the lower tilt angle is beneficial to the PV generators from May to August. In PVsyst, the IeV characteristics of the PV modules are simulated using the one-diode model given by Refs. [40,41]:
IðVÞ ¼ Iph I0 e
VþIRs q nkTN cs
V þ IRs 1 ; Rsh
(1)
where I and V are respective current and voltage of the PV module, Iph is the photocurrent, I0 is the diode reverse saturation current, Rs is the series resistance, Rsh is the shunt resistance, q is the electron charge, n is the diode ideality factor, T is the module operation temperature in K, k is the Boltzmann constant, and Ncs is the number of cells in series in the PV module, respectively. In this model, Iph is varied with measured irradiance G in W/m2 and T as follows:
Iph ðG; TÞ ¼
G h I 1000 ph;
STC
i þ gðT 298Þ ;
(2)
where Iph, STC is the photocurrent under STC and g is the temperature coefficient of Isc, respectively. In addition, I0 is assumed to be dependent on T as follows:
I0 ðTÞ ¼ I0;STC
T 298
3
e
qEg nk
1 1 298 T
;
(3)
where I0, STC is the diode reverse saturation current under STC and Eg is the bandgap energy of the cell material, respectively. In the Version of 6.32, PVsyst provides a tool to adjust the free parameter Rs to fit to the module power depending on G. In contrast, both the c-Si and a-Si:H single-junction PV modules indicate the exponential decay of Rsh as a function of G fitted by this equation:
Rsh ðGÞ ¼ Rsh ð1000Þ þ ðRsh ð0Þ Rsh ð1000ÞÞe
d
G 1000
;
(4)
487
where d is the fitting parameter of the exponential decay. In the case of the a-Si:H single-junction PV modules, the StaeblerWronski effect should be taken into account for the simulation. Thus, the recombination current in i-a-Si:H (Irec) is subtracted from the photocurrent. Irec is assumed to be proportional to the electric field in i-a-Si:H as follows [40]:
Irec ¼
Iph d2i ; meff ðVbi e V IRs Þ
(5)
where di is the thickness of i-a-Si:H, meff is the effective diffusion length of photogenerated carriers, and Vbi is the built-in potential, respectively. In addition, the spectral correction for the a-Si:H single-junction PV modules is performed because Eg of i-a-Si:H (~1.7 eV) is higher than that of c-Si (1.12 eV) [4]. From Fig. 10(a) and (b), all the simulated data of the a-Si:H single-junction and c-Si PV generators show the considerably similar trend to the corresponding measured data. However, as summarized in Table 1, the measured annual electrical energy output of the a-Si:H singlejunction PV generator is 4.6% higher than its simulated annual electrical energy output. Besides, both the c-Si PV generators produced 7.4% higher electrical energy compared to the respective prediction. The highest deviation between the simulated and measured values is inspected during the summer season because the simulation using PVsyst underestimates the electrical energy output especially at the high module operating temperatures [41]. High irradiance values during the summer season also contribute to the underestimations. Furthermore, the severe deviations for the cSi PV modules originates from their lower temperature coefficient than the a-Si:H single-junction PV modules. As evaluated in Table 1, the average electricity-generation time values for the a-Si:H singlejunction PV generator installed with the fixed tilt angle of 35 , c-Si PV generator installed with the fixed tilt angle of 35 , and c-Si PV generator installed with the fixed tilt angle of 10 are 3.68, 3.58, and 3.36 h/day, respectively. Compared to the c-Si PV generator installed with the fixed tilt angle of 10 , the c-Si PV generator installed with the fixed tilt angle of 35 achieves the drastic energy output gain of 6.7%. From the simulation, the energy output of the c-Si PV generator could be improved by 4.3% by maintaining the tilt angle at 10 from May to August and at 35 during other months. Actually, the energy output is improved by 1.7% by toggling the tilt angle. Based on the result of Fig. 6, the energy output of the a-Si:H PV generator could also be improved by the simple toggling operation of the tilt angle in April and September. Meanwhile, the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 obtains the energy output gain of 2.7% compared to the c-Si PV generator installed with the same tilt angle. This value is less than the simulated energy output gain of 5.8% caused by the lower temperature coefficient and less shading effect of the a-Si:H singlejunction PV generator. From our previous reports [6,7], the lightinduced degradation of a-Si:H single-junction PV modules against the outdoor exposure test would be higher than the light-induced degradation against indoor light soaking depending on the weather conditions such as temperatures and irradiance. Therefore, we demonstrated that the nominal rated power of the a-Si:H singlejunction PV modules should be determined based on the outdoor exposure test with accumulated solar irradiance of over 380 kWh/ m2. Actually, the fabricated a-Si:H single-junction PV module under consideration exhibited stabilized Pmax of 92.2 W through the longterm outdoor exposure test from summer to winter [6]. Thus, the simulated curve for the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 is obtained by assuming the nominal rated power of 92.2 W instead of 100 W (see Fig. 10(a) and Table 1). As a result, the reasonable energy output gain of 2.0% is
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Table 1 Summary of annual electrical energy produced by 3-kWp PV generators. Modules
Tilt angle ( )
Data type
Electrical energy (kWh)
Generation time (hours/day)
a-Si:H
35 35 35
4025.0 3847.4 3716.3
3.68 3.51 3.51
c-Si
35 35 10 10 Toggling Toggling
Measured Simulated Simulated using the corrected rated power Measured Simulated Measured Simulated Measured Simulated
3920.5 3649.5 3674.5 3419.7 3987.0 3807.1
3.58 3.33 3.36 3.12 3.64 3.48
achieved compared to the simulated output gain of the c-Si PV generator installed with the fixed tilt angle of 35 . Therefore, the nominal rated power of the a-Si:H single-junction PV modules should be determined based on the long-term outdoor exposure test for the operation under the distinct four seasons. Finally, we will comment on expensive BOS for thin-film Si PV systems. Due to lower h, an a-Si:H single-junction PV array needs more ground and connecting wires compared to a c-Si PV array with an identical capacity. Also, more string boxes and inverters are essential for the a-Si:H single-junction PV array because of higher Voc. It should be noted that PV inverters with a transformer must be implemented to the thin-film Si PV array for long-term use. This in turn increases BOS. Therefore, thin-film Si PV modules should be designed with a low Voc value and a proper nominal rated power determined through an outdoor exposure test. Furthermore, costeffective, high-efficient thin-film Si PV modules should be developed using effective light trapping and innovative multi-junction schemes. 4. Conclusion We investigated the electricity-generation of a-Si:H singlejunction and c-Si PV rooftop systems operated under the distinct four seasons. The influence of the module type and installed tilt angle on the annual electrical energy yield of 3-kWp PV generators were systematically analyzed by comparing the simulated data with the measured data. It was found that all the simulated data exhibited considerably analogous trends to the measured data. The developed high production throughput, high efficient a-Si:H singlejunction PV modules using the two-step deposition for i-a-Si:H with the high deposition rate of 0.41 nm/s were installed. Nonetheless, the energy output gain of the 3-kWp a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 is only 2.7% compared to the 3-kWp mono c-Si PV generator installed with the fixed tilt angle of 35 . It is inferred that the determination of the nominal rated power of the a-Si:H single-junction PV modules based on the light soaking test of IEC 61646 standards leads to the overestimated value under the distinct four seasons. To highlight the merit of the lower temperature coefficient and less shading effect, the nominal rated power of the a-Si:H-based PV modules should be determined through the proper outdoor exposure test with accumulated solar irradiance of over 380 kWh/m2. It was also observed that the Si-based PV rooftop systems installed with the low tilt angle caused the disappointing consequence of energy harvesting in winter due to the lowest solar altitude at meridian passage and snowfall. However, the performance of the Si-based PV rooftop systems could be improved by the simple toggling operation of the tilt angle between 10 and 35 considering plane-ofarray irradiance and snowfall effect.
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Sol Energy Mater Sol Cells 2011;95:373e6. [25] Andrews RW, Pollard A, Pearce JM. The effect of snowfall on solar photovoltaic performance. Sol Energy 2013;92:84e97. [26] Andrews RW, Pearce JM. The effect of spectral albedo on amorphous silicon and crystalline silicon solar photovoltaic device performance. Sol Energy 2013;91:233e51. [27] Yamamoto K, Nakajima A, Yoshimi M, Sawada T, Fukuda S, Suezaki T, et al. A thin-film silicon solar cell and module. Prog Photovolt Res Appl 2005;13: 489e94. [28] Sriprapha K, Yunaz IA, Myong SY, Yamada A, Konagai M. Temperature dependence of Si-based thin film solar cells fabricated on the phase of amorphous to microcrystalline silicon transition. Jpn J Appl Phys 2007;46: 7212e4. [29] Noh HN, Myong SY. Antireflective coating using A WO3-TiO2 nanopaticle photocatalytic composition for high efficiency thin-film Si photovoltaic modules. Sol Energy Mater Sol Cells 2014;121:108e13. [30] Parretta A, Bombace M, Graditi G, Schioppo R. Optical degradation of longterm, field-aged c-Si photovoltaic modules. Sol Energy Mater Sol Cells 2005;86:349e64. [31] Appels R, Lefevre B, Herteleer B, Goverde H, Beerten A, Passen R, et al. The effect of soiling on photovoltaic modules. Sol Energy 2013;96:283e91. n R, Luque I. Dust effects on PV array performance: in-field [32] Lorenzo E, Moreto observations with non-uniform patterns. Prog Photovolt Res Appl 2014;22: 666e70.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance of Si-based PV rooftop systems operated under distinct four seasons Seung Yeop Myong*, You-Chul Park, Sang Won Jeon Energy R&D Center, KISCO, Miamri 1044, Jeungpyeong-eup, Jeungpyeong-gun, Choongcheongbuk-do, 368-906, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2014 Accepted 19 March 2015 Available online
We have investigated the electrical energy yield of hydrogenated amorphous silicon (a-Si:H) singlejunction and crystalline (c-Si) photovoltaic (PV) rooftop systems operated under distinct four seasons. The impact of the module type and installed tilt angle on the annual electrical energy yield has been monitored and then compared with the data predicted by the computer simulation. Despite a good temperature coefficient and less shading effect of a-Si:H single-junction modules, the energy output gain of the a-Si:H single-junction PV generator is only 2.7% compared to the c-Si PV generator installed using c-Si PV modules. It is inferred that a nominal rated power of the a-Si:H single-junction modules determined by an indoor light soaking test is not suitable for the design of PV systems operated under distinct four seasons. Thus, the nominal rated power of the a-Si:H single-junction PV modules should be determined through a proper outdoor exposure test considering thermal annealing and light soaking effects under various seasonal weather conditions. In addition, it is found that the performance of the Sibased PV rooftop systems operated under distinct four seasons could be improved by simply toggling the tilt angle considering the plane-of-array irradiance and snowfall effect. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Amorphous silicon photovoltaic generator Crystalline silicon photovoltaic generator Electrical energy yield Nominal rated power Tilt angle
1. Introduction Rapid global warming caused by excess emission of CO2 warns about the climate change. Sufficient supplies of clean energy are intimately linked with global stability, economic prosperity, and quality of life. Accordingly, clean renewable energy including solar, wind, and hydrogen energy becomes a prime issue. A photovoltaic (PV) system using solar light is a promising candidate among the renewable energy sources because the sun is our primary source of clean, abundant energy. Actually, there has been a drastic increase in the global PV market during the last two decades. In 2013, approximately 37 GWp PV systems were installed worldwide, leading to cumulative global installations of ~140 GWp. Recently, Northeast Asia having distinct four seasons (China, Japan, and Republic of Korea) has gained great attraction as an emerging PV market. In this region, it is believed that nuclear power plants are no longer stable and cheap energy source after Fukushima nuclear meltdowns. In Republic of Korea (South Korea), 295 MWp PV systems were installed in 2012. Hence, cumulative PV installations firstly reached 1 GWp, which corresponding to 2.7% of the total
* Corresponding author. Tel.: þ82 43 8389146; fax: þ82 43 8389144. E-mail address: [email protected] (S.Y. Myong). http://dx.doi.org/10.1016/j.renene.2015.03.055 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
renewable energy supply [1]. Moreover, the country recorded the highest annual PV installations of 330 MWp in 2013. The Korean government plans to increase the PV supply up to 4.1% of the total renewable energy by 2030 [2]. Currently, crystalline silicon (c-Si) PV systems using mono and multi c-Si PV modules share > 90% of the Korean PV market [3] mainly due to a lack of local manufacturers producing thin-film PV modules. KISCO is the first-ever thinfilm PV module manufacturer in this country, which started mass production of thin-film silicon (Si) PV modules in 2008. The thinfilm Si PV modules using hydrogenated amorphous Si (a-Si:H) absorbers have been considered as next-generation PV modules due to the various advantages of remarkably low consumption of the Si raw material, large-area deposition, low-temperature production, and low temperature coefficient. The thin-film Si PV technology also profits from the wide experience base of display industries [4]. However, the recent sharp drop in the module price orders costeffective PV modules. Thus, the company also started mass production of c-Si PV modules in 2011. In this work, authors have investigated the outdoor performance of self-produced a-Si:H single-junction and c-Si PV rooftop generators operated under distinct four seasons. Based on long-term field tests, the authors will provide the easy and economic way to improve the electricity generation considering the seasonal weather conditions. Furthermore, we will discuss the suitable rated power of the a-Si:H single-
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junction PV modules for use in regions with distinct four seasons since the outdoor performance of the metastable modules exposed to the various seasonal weather conditions is significantly different from their indoor performance. 2. Methodology For this research, a-Si:H single-junction and c-Si PV power plants with a capacity of over 100 kWp were built in Jeungpyeong, Republic of Korea. Fig. 1(a) shows the 132-kWp a-Si:H singlejunction PV rooftop system mounted with the tilt angle of 5 . This system was built in 2010. Fig. 1(b) shows the 379-kWp c-Si PV rooftop system built in 2012. The system has been operated by toggling the tilt angle manually in April and September between 10 and 35 . The module operating temperature and horizontal irradiance were offered via a monitoring facility. The electrical power-generation of the installed PV generators was checked every day. Jeungpyeong (36.80 N, 127.59 E) locating in central Korea (see Fig. 2) has distinct four seasons e warm and dry spring, hot and rainy (humid) summer, cool and clear blue sky in autumn, and cold and snowy winter. 3-kWp PV generators were also installed for the systematic investigation. The impact of the module type and installed tilt angle on the annual electrical energy yield were monitored and then compared with the data predicted by the simulated values using the commercially available software program PVsyst from the University of Geneva [8]. Fig. 3(a) shows a front image of a fabricated a-Si:H singlejunction PV module. 1.43 m2 p-i-n-type a-Si:H single-junction PV modules were produced using a fully automated in-line system with an annual capacity of 20 MWp. Textured SnO2:F (FTO)-coated soda-lime glasses were used as substrates. Every segment (cell) has the structure of glass/FTO/hydrogenated p-type amorphous siliconcarbide (p-a-SiC:H) window layer/p-type buffer layer (p-buffer)/ intrinsic a-Si:H (i-a-Si:H) absorber (250 nm)/hydrogenated n-type a-Si:H (n-a-Si:H)/boron (B)-doped zinc oxide (ZnO:B) back reflector/Al back contact. For the deposition of thin-film Si layers, large-area cluster-type radio-frequency plasma-enhanced chemical vapor deposition (PECVD) apparatus was used in order to minimize the dangling bonds [5] and residual impurity cross-contamination. To achieve a high deposition rate of 0.41 nm/s, the SiH4 concentration (¼ [SiH4]/([SiH4] þ [H2])) is varied from 4.8% to 7.7% without any turn-off of the plasma power [6]. The 20-nm-thick interfacial part of i-a-Si:H was deposited using the low SiH4 concentration to improve the p/i interface, whereas the 230-nm-thick bulk part of ia-Si:H was deposited using the high SiH4 concentration to obtain the high deposition speed. The detailed deposition conditions for the component layers were described in our previous reports [6,7]. The monolithic series integration of the a-Si:H single-junction PV modules was realized via the plurality of three parallel laser-
Fig. 2. Map of South Korea where the PV power generators were installed.
Fig. 3. Front images of the fabricated PV modules: (a) the a-Si:H single-junction PV module and (b) the c-Si PV module.
scribed patterns. The cell width for the a-Si:H single-junction PV modules was selected as 9.8 mm in order to prevent an extremely high open-circuit voltage (Voc), which caused a considerable balance of system (BOS). The module encapsulation was performed by
Fig. 1. Pictures of the PV power plants: (a) 132-kWp a-Si:H single-junction PV rooftop systems mounted with the fixed tilt angle of 5 and (b) 379-kWp c-Si PV rooftop systems being operated by toggling the tilt angle between 10 and 35 .
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laminating the back sheet including an Al foil with an ethylenevinyl acetate (EVA) film. Fig. 3(b) exhibits a front image of a fabricated c-Si PV module. 1.63 m2 c-Si PV modules were produced using a semi-automated in-line system with the annual capacity of 65 MWp. 6-inch (156 156 mm2) p-type mono c-Si solar cells were interconnected using the three bus bar configuration in order to form a 6 10 matrix. The interconnected solar cells were encapsulated between a polyamide (PA)-based multilayer back sheet and low iron patterned tempered glass using EVA films. The photo currentevoltage (IeV) characteristics for the fabricated PV modules were measured under the standard test conditions (STC; 25 C, AM 1.5G, and 1000 W/m2) using a WACOM, WPSS-1.5 1.2e50 4 AM1.5 GF solar simulator. The a-Si:H singlejunction PV modules acquired the International Electrochemical Commission (IEC) 61646 module certification for thin-film terrestrial PV modules from UL, USA and TÜV, Germany, while the c-Si PV modules acquired IEC 61215 module certification from TÜV, Germany. 3. Results and discussion Fig. 4 depicts the photo IeV curves for the fabricated a-Si:H single-junction and c-Si PV modules. As shown in Fig. 4(a), the initial maximum power (Pmax) of the a-Si:H single-junction PV module is 123.0 W, which is corresponding to the initial conversion efficiency (h) of 8.6% (Voc ¼ 99.4 V, short-circuit current (Isc) ¼ 1.76 A, and fill factor (FF) ¼ 0.703). The indoor light soaking test was performed at 50 C using a class C solar simulator based on the IEC 61646 light soaking test. After light soaking, the a-Si:H single-junction PV module is stabilized to 101.3 W (Voc ¼ 96.4 V, Isc ¼ 1.71 A, and FF ¼ 0.613, and h ¼ 7.1%) with the light-induced degradation ratio (¼ Dh/initial h) of 17.6%. Thus, the nominal rated power of 100.0 W was selected for the a-Si:H single-junction PV modules fabricated under the same conditions. It has been reported that the deposition rate for a state-of-the-art i-a-Si:H absorber is 0.2e0.5 nm/s [9]. The deposition rate of the a-Si:H absorbers is the key factor governing the production throughput. However, the so-called “Staebler-Wronski effect”, which is the lightinduced degradation of a-Si:H-based films arising from the photocreation of dangling bonds accomplished by nonradiative recombination of photogenerated electronehole pairs [10], remains as a major technical challenge for mass production. To reduce the light-induced degradation of i-a-Si:H absorbers that
leads to the degradation of thin-film Si PV devices, there have been extensive investigations during the past three decades. The deposition of i-a-Si:H using H2-dilution of SiH4 is known as the best way to reduce the Staebler-Wronski effect, however it is impossible to eliminate the effect [11e18]. The stability of the aSi:H single-junction PV modules is mainly influenced by the thickness and film quality of the absorber. The higher deposition rate of i-a-Si:H has resulted in lower initial h and higher lightinduced degradation of a-Si:H PV devices, leading to lower stabilized h [19,20]. As commented in our previous reports [6,7], the two-step deposition method of i-a-Si:H had been developed by adjusting the deposition rate in order to meet the high throughput and high stabilized h simultaneously [12,21,22]. In addition, we reported that a-Si:H single-junction PV modules fabricated using the two-step deposition method showed a higher stability against indoor light soaking without deteriorating the production throughput compared to those fabricated using the one-step deposition method [6]. To the best of our knowledge, the electrical energy yield of large-area PV arrays comprised of a-Si:H single-junction PV modules fabricated using the two-step deposition method will be first-ever reported in this work. As shown in Fig. 4(b), the c-Si PV module has initial Pmax of 251.7 W, which is corresponding to initial h of 15.4% (Voc ¼ 37.5 V, Isc ¼ 8.58 A, and FF ¼ 0.776). Thus, the value of 250.0 W was selected as the nominal rated power for c-Si PV modules. Fig. 5 reveals the electrical energy yield of the Si-based PV rooftop systems under consideration. It is found that the electrical energy yield of the a-Si:H single-junction PV rooftop system installed with the fixed tilt angle of 5 is widely fluctuating depending on the seasons. Compared to the c-Si PV rooftop system, the a-Si:H single-junction PV rooftop system has higher yields in summer. However, it has poor yields in winter. Thus, the average electricity-generation time calculated for the a-Si:H single-junction PV rooftop system is just 3.17 h/day. On the contrary, the c-Si PV rooftop system operated by toggling the tilt angle shows the narrow distribution of the electrical energy yield together with the good average electricity-generation time of 3.69 h/day. As depicted in Fig. 6, the a-Si:H single-junction PV module exhibits high yield values in summer by fixing the tilt angle at 5 because of high plane-of-array irradiance. However, snow covering the modules severely disturbs the electricity-generation, leading to the disastrous energy harvest in winter (see Fig. 7). Consequently, the widely fluctuating electrical energy yield is achieved. The a-Si:H single-junction PV module mounted with the tilt angle of 30
Fig. 4. Photo IeV characteristics of the fabricated a-Si:H single-junction and c-Si PV modules: (a) initial and stabilized performances of the a-Si:H single-junction PV module and (b) the initial performance of the c-Si PV module.
S.Y. Myong et al. / Renewable Energy 81 (2015) 482e489
Fig. 5. Electrical energy yield of the PV rooftop systems. The solid lines are eye guides.
obtains the annual energy output gain of 9.2% compared to its counterpart mounted with the tilt angle of 5 . For the detailed comparison, three kinds of 3-kWp PV generators were installed at Jeungpyeong. Fig. 8 displays pictures of the installed 3-kWp PV generators. Each PV generator was mounted on an open rack supporting structure facing south with a fixed tilt angle in the maximum power point (MPP) state. The 3-kWp a-Si:H single-junction PV generator is comprised of 30 modules (6 strings composed of respective 5 serially connected modules), while the 3kWp c-Si PV generators are composed of 12 modules (1 string composed of 12 serially connected modules), respectively. Accordingly, the former generator needs 2.2 times larger ground than the latter generators. Fig. 9 shows monthly horizontal irradiance and module operating temperatures during the outdoor field test from July, 2012 to June, 2014. Monthly horizontal irradiance and mean ambient temperatures were informed by observed values at Cheongju (36.38 N, 127.26 E) synoptic station, which conducted by Korea Meteorological Administration (KMA) with an automated synoptic observing system (ASOS) (see Fig. 2) [23]. The 16 weather elements, including temperature and horizontal solar irradiance, were measured every hour. Annual horizontal irradiance of the site is 1299.1 kWh/m2. Spring (from March to May) is the highest season for solar irradiance. In particular, the maximum solar irradiance is
Fig. 6. Dependence of plane-of-array irradiance and electrical power generation of aSi:H PV modules on the tilt angle (field test during 2010).
485
Fig. 7. Picture of the a-Si:H single-junction PV rooftop system covered by snow (taken in December, 2013).
inspected in May. During the season, the monthly mean value of the daily highest module operating temperature elevates month after month. Summer (from June to August) is the season when monthly accumulated solar irradiance is considerably high except a typical rainy season in Northeast Asia. Summer is the highest season for the monthly mean value of the daily highest module operating temperature (~50 C), while the monthly maximum of the daily highest module operating temperature is saturated at 60 C. The saturated maximum module operating temperature can be ascribed to the open rock supporting structure during the outdoor field test. The PV module operating temperature can be influenced by the ambient temperature, wind speed, solar irradiance, and module supporting structure [24]. In autumn (from September to November), monthly accumulated solar irradiance reduces month after month. In particular, there is a considerable drop in solar irradiance in November due to low solar altitude at meridian passage and a lot of foggy days. During the autumn season, all the module operating temperatures gradually decrease month after month. Winter (from December to February) is the worst season for the electrical energy generation using PV systems due to the lowest solar altitude at meridian passage and snowfall [25]. In January, the monthly mean value of the daily highest module operating temperature has the lowest value. The monthly mean ambient temperature has a similar pattern to the monthly minimum value of the daily highest module operating temperature. Fig. 10 represents simulated and measured electrical energy yield values for the 3-kWp PV generators under consideration. The computer simulations were performed by designing the gridconnected PV systems using specific values of the PV modules and arrays. Fig. 10(a) exhibits the results for the 3-kWp PV generator comprised of the fabricated a-Si:H single-junction PV modules mounted with the fixed tilt angle of 35 , while Fig. 10(b) shows the results for the 3-kWp c-Si PV generators mounted with the fixed tilt angle of 10 and 35 . In general, the outdoor electricity-generation of the PV generators is affected by the module temperature, planeof-array irradiance [26], spectral distribution [26,27], temperature coefficient [28], transmittance of aged PV glasses [29] and aged EVA films [30], dust effect [31e33], and partial shading effect on PV modules [34,35]. The superstrate-type a-Si:H single-junction PV modules may be free from the effect of the aged EVA films, however it is also significantly impacted by the Staebler-Wronski effect [27,36]. It takes about two years until the performance of a-Si:H single-junction PV modules becomes stable in the outdoor environment [37,38]. All the PV generators exhibit the highest electrical
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Fig. 8. Pictures of the installed 3-kWp a-Si:H single-junction and c-Si PV generators: (a) the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 , (b) the c-Si PV generator installed with the fixed tilt angle of 10 , and (c) the c-Si PV generator installed with the fixed tilt angle of 35 .
Fig. 9. Outdoor field test conditions for the 3-kWp PV generators: (a) annual horizontal irradiance and (b) the monthly mean ambient temperature were displayed with the maximum, minimum, and mean of the daily highest operating temperatures for a c-Si PV module. The solid and dashed lines are eye guides.
energy yield in spring by virtue of the maximum solar irradiance and moderate module operating temperatures. Excepting the aforementioned rainy and foggy seasons, the a-Si:H single-junction PV generator produces electricity well in summer and autumn thanks to high solar irradiance. Moreover, the a-Si:H single-
junction PV generator obviously performs better than the c-Si PV generators during the seasons. The indoor temperature coefficient of the a-Si:H single-junction PV modules measured by UL is as low as 0.20%/ C. In addition, it is reported that the outdoor temperature coefficient of a-Si:H single-junction PV modules is rather
Fig. 10. Annual electrical energy yield of the 3-kWp PV generators: (a) the simulated and measured data for the a-Si:H single-junction PV rooftop system installed with the fixed tilt angle of 35 . The simulated data using the corrected rated power of 92.2 W are also revealed. (b) The simulated and measured data for the c-Si PV rooftop system installed with the fixed angle of 10 and 35 . The solid lines are eye guides.
S.Y. Myong et al. / Renewable Energy 81 (2015) 482e489
positive (~0.35%/ C) [38]. On the contrary, the outdoor temperature coefficient of the c-Si PV modules measured by TÜV is 0.43%/ C, which is severely worse than that of the a-Si:H single-junction PV modules. Therefore, the a-Si:H single-junction PV generator produces electricity more effectively at higher ambient temperatures compared to the c-Si PV generators. In winter, the lowest solar altitude at meridian passage and snow covering the PV modules lead all the PV generators to the respective lowest performance. The negative impact of snowfall is more critical to the PV generator installed with the lower tilt angle because snow can be maintained for a longer time without any mandatory removal. Compared to the c-Si PV generator installed with the fixed tilt angle of 35 , the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 produces less electricity in winter. It can be attributed to the module operating temperatures lower than room temperature. It is also found that the seasonal variation of the electrical energy yield of the a-Si:H PV generator is higher than that of the c-Si PV generator. The wider seasonal fluctuation of the electrical energy yield for the a-Si:H single-junction PV generator is mainly due to the effects of thermal annealing and light soaking, which are deeply related to the Staebler-Wronski effect of the a-Si:H single-junction PV modules [38,39]. The c-Si PV generator installed with the fixed tilt angle of 10 produces more electricity from May to August compared to the c-Si PV generator installed with the fixed tilt angle of 35 . It is mainly due to higher plane-of-array irradiance during the period caused by the lower tilt angle (see Fig. 6). However, the opposite trend of plane-of-array irradiance is inspected during other months. Thus, the lower tilt angle is beneficial to the PV generators from May to August. In PVsyst, the IeV characteristics of the PV modules are simulated using the one-diode model given by Refs. [40,41]:
IðVÞ ¼ Iph I0 e
VþIRs q nkTN cs
V þ IRs 1 ; Rsh
(1)
where I and V are respective current and voltage of the PV module, Iph is the photocurrent, I0 is the diode reverse saturation current, Rs is the series resistance, Rsh is the shunt resistance, q is the electron charge, n is the diode ideality factor, T is the module operation temperature in K, k is the Boltzmann constant, and Ncs is the number of cells in series in the PV module, respectively. In this model, Iph is varied with measured irradiance G in W/m2 and T as follows:
Iph ðG; TÞ ¼
G h I 1000 ph;
STC
i þ gðT 298Þ ;
(2)
where Iph, STC is the photocurrent under STC and g is the temperature coefficient of Isc, respectively. In addition, I0 is assumed to be dependent on T as follows:
I0 ðTÞ ¼ I0;STC
T 298
3
e
qEg nk
1 1 298 T
;
(3)
where I0, STC is the diode reverse saturation current under STC and Eg is the bandgap energy of the cell material, respectively. In the Version of 6.32, PVsyst provides a tool to adjust the free parameter Rs to fit to the module power depending on G. In contrast, both the c-Si and a-Si:H single-junction PV modules indicate the exponential decay of Rsh as a function of G fitted by this equation:
Rsh ðGÞ ¼ Rsh ð1000Þ þ ðRsh ð0Þ Rsh ð1000ÞÞe
d
G 1000
;
(4)
487
where d is the fitting parameter of the exponential decay. In the case of the a-Si:H single-junction PV modules, the StaeblerWronski effect should be taken into account for the simulation. Thus, the recombination current in i-a-Si:H (Irec) is subtracted from the photocurrent. Irec is assumed to be proportional to the electric field in i-a-Si:H as follows [40]:
Irec ¼
Iph d2i ; meff ðVbi e V IRs Þ
(5)
where di is the thickness of i-a-Si:H, meff is the effective diffusion length of photogenerated carriers, and Vbi is the built-in potential, respectively. In addition, the spectral correction for the a-Si:H single-junction PV modules is performed because Eg of i-a-Si:H (~1.7 eV) is higher than that of c-Si (1.12 eV) [4]. From Fig. 10(a) and (b), all the simulated data of the a-Si:H single-junction and c-Si PV generators show the considerably similar trend to the corresponding measured data. However, as summarized in Table 1, the measured annual electrical energy output of the a-Si:H singlejunction PV generator is 4.6% higher than its simulated annual electrical energy output. Besides, both the c-Si PV generators produced 7.4% higher electrical energy compared to the respective prediction. The highest deviation between the simulated and measured values is inspected during the summer season because the simulation using PVsyst underestimates the electrical energy output especially at the high module operating temperatures [41]. High irradiance values during the summer season also contribute to the underestimations. Furthermore, the severe deviations for the cSi PV modules originates from their lower temperature coefficient than the a-Si:H single-junction PV modules. As evaluated in Table 1, the average electricity-generation time values for the a-Si:H singlejunction PV generator installed with the fixed tilt angle of 35 , c-Si PV generator installed with the fixed tilt angle of 35 , and c-Si PV generator installed with the fixed tilt angle of 10 are 3.68, 3.58, and 3.36 h/day, respectively. Compared to the c-Si PV generator installed with the fixed tilt angle of 10 , the c-Si PV generator installed with the fixed tilt angle of 35 achieves the drastic energy output gain of 6.7%. From the simulation, the energy output of the c-Si PV generator could be improved by 4.3% by maintaining the tilt angle at 10 from May to August and at 35 during other months. Actually, the energy output is improved by 1.7% by toggling the tilt angle. Based on the result of Fig. 6, the energy output of the a-Si:H PV generator could also be improved by the simple toggling operation of the tilt angle in April and September. Meanwhile, the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 obtains the energy output gain of 2.7% compared to the c-Si PV generator installed with the same tilt angle. This value is less than the simulated energy output gain of 5.8% caused by the lower temperature coefficient and less shading effect of the a-Si:H singlejunction PV generator. From our previous reports [6,7], the lightinduced degradation of a-Si:H single-junction PV modules against the outdoor exposure test would be higher than the light-induced degradation against indoor light soaking depending on the weather conditions such as temperatures and irradiance. Therefore, we demonstrated that the nominal rated power of the a-Si:H singlejunction PV modules should be determined based on the outdoor exposure test with accumulated solar irradiance of over 380 kWh/ m2. Actually, the fabricated a-Si:H single-junction PV module under consideration exhibited stabilized Pmax of 92.2 W through the longterm outdoor exposure test from summer to winter [6]. Thus, the simulated curve for the a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 is obtained by assuming the nominal rated power of 92.2 W instead of 100 W (see Fig. 10(a) and Table 1). As a result, the reasonable energy output gain of 2.0% is
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Table 1 Summary of annual electrical energy produced by 3-kWp PV generators. Modules
Tilt angle ( )
Data type
Electrical energy (kWh)
Generation time (hours/day)
a-Si:H
35 35 35
4025.0 3847.4 3716.3
3.68 3.51 3.51
c-Si
35 35 10 10 Toggling Toggling
Measured Simulated Simulated using the corrected rated power Measured Simulated Measured Simulated Measured Simulated
3920.5 3649.5 3674.5 3419.7 3987.0 3807.1
3.58 3.33 3.36 3.12 3.64 3.48
achieved compared to the simulated output gain of the c-Si PV generator installed with the fixed tilt angle of 35 . Therefore, the nominal rated power of the a-Si:H single-junction PV modules should be determined based on the long-term outdoor exposure test for the operation under the distinct four seasons. Finally, we will comment on expensive BOS for thin-film Si PV systems. Due to lower h, an a-Si:H single-junction PV array needs more ground and connecting wires compared to a c-Si PV array with an identical capacity. Also, more string boxes and inverters are essential for the a-Si:H single-junction PV array because of higher Voc. It should be noted that PV inverters with a transformer must be implemented to the thin-film Si PV array for long-term use. This in turn increases BOS. Therefore, thin-film Si PV modules should be designed with a low Voc value and a proper nominal rated power determined through an outdoor exposure test. Furthermore, costeffective, high-efficient thin-film Si PV modules should be developed using effective light trapping and innovative multi-junction schemes. 4. Conclusion We investigated the electricity-generation of a-Si:H singlejunction and c-Si PV rooftop systems operated under the distinct four seasons. The influence of the module type and installed tilt angle on the annual electrical energy yield of 3-kWp PV generators were systematically analyzed by comparing the simulated data with the measured data. It was found that all the simulated data exhibited considerably analogous trends to the measured data. The developed high production throughput, high efficient a-Si:H singlejunction PV modules using the two-step deposition for i-a-Si:H with the high deposition rate of 0.41 nm/s were installed. Nonetheless, the energy output gain of the 3-kWp a-Si:H single-junction PV generator installed with the fixed tilt angle of 35 is only 2.7% compared to the 3-kWp mono c-Si PV generator installed with the fixed tilt angle of 35 . It is inferred that the determination of the nominal rated power of the a-Si:H single-junction PV modules based on the light soaking test of IEC 61646 standards leads to the overestimated value under the distinct four seasons. To highlight the merit of the lower temperature coefficient and less shading effect, the nominal rated power of the a-Si:H-based PV modules should be determined through the proper outdoor exposure test with accumulated solar irradiance of over 380 kWh/m2. It was also observed that the Si-based PV rooftop systems installed with the low tilt angle caused the disappointing consequence of energy harvesting in winter due to the lowest solar altitude at meridian passage and snowfall. However, the performance of the Si-based PV rooftop systems could be improved by the simple toggling operation of the tilt angle between 10 and 35 considering plane-ofarray irradiance and snowfall effect.
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