dependent on the specific location, the tilt angle of the ... irradiance at these two locations should bracket many ..... [6] www.pv.kaneka.co.jp/why/index.html.
PERFORMANCE ASSESSMENT OF PV MODULES – RELATIONSHIP BETWEEN STC RATING AND FIELD PERFORMANCE 1
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Marko Topic , Kristijan Brecl and James Sites Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia 2 Department of Physics, Colorado State University, Fort Collins, CO 80523 USA 1
ABSTRACT PV module field performance can be assessed by annual effective efficiency and module performance ratio. The module specific data may be represented with normalized conversion efficiency as a function of irradiance at the average daylight ambient temperature. We demonstrate how to calculate annual performance ratio and present results for six PV modules at two locations and different tilts. Results are only weakly dependent on the specific location, the tilt angle of the module or normal variations in the spectral distribution. They depend on average annual daylight temperature, but only very weakly on the temperature distribution. The two key module parameters that control the effective efficiency are series resistance and leakage conductance. Effect of their variation is quantitatively determined. Discussion on different approaches for PV module energy rating is given. INTRODUCTION A single defined test, such as standard test conditions (STC) with one-sun irradiance (1 kW/m2, AM1.5 spectrum) and cell temperature (Tj) of 25 ºC, is not the appropriate protocol for rating and comparing PV modules under field conditions. Some groups have suggested inclusion of spectral change effects in energy rating of modules by Location specific data: • Irradiance – G (W/m2) • Solar Energy – SE (kWh/m2) • Tamb (ºC) • wind, orientation of wind, humidity, …
PV module specific data: • STC: Pmax, Vmpp, Impp, Voc, Isc, FF, hSTC • a, b , d , NOCT, hNOCT • dTj/dG
rating them for typical days [1,2]. Such approach treats temperature and irradiance variations including spectral changes on a daily basis accurately, but does not render accurate energy rating over a year. Other groups studied spectral change and incidence angle effects [3,4], but the effects on a yearly basis were found to be in the relative range of less than few percent. Some manufacturers of thin film PV modules [5-7] have reported that when comparisons of the total amount of power produced over a year are made, their product performs better in the field than is inferred from performance measurements made under STC. To predict the effective efficiency averaged over a typical year of operation with precision an average over time-dependent irradiance, temperature, and solar-angle distributions at the installation site is required (Fig. 1). In the paper, a straightforward procedure to evaluate PV modules’ field performance by means of intensity dependent normalized conversion efficiency is reported, resulting in an annual effective efficiency and performance ratio of a PV module. We demonstrate the assessment procedure on hourly irradiation and temperature data from two specific locations: Golden, Colorado, in the western USA, and Ljubljana, Slovenia, in central Europe. The irradiance at these two locations should bracket many locations. We have done this comparison for several PV modules currently being marketed. We will demonstrate how changes in the module’s effective series resistance and leakage conductance impact its effective efficiency in the field and discuss the approaches for PV module energy rating. PROCEDURE TO ASSESS FIELD PERFORMANCE BY NORMALIZED EFFICIENCY VS. IRRADIANCE Annual effective efficiency heff can be calculated as a ratio of total available electrical energy generated in a year divided by the annual solar energy:
PV module YIELD – YM (kWh/yr/kWp) @ location (SE, Tamb) PV module Performance Ratio - PRM
ò h (G,T
amb
heff =
Effective Conversion Efficiency - hEFF
Fig. 1 Input data needed to assess the outdoor performance. Location specific data are time-dependent.
1-4244-0016-3/06/$20.00 ©2006 IEEE
) × G × dt
year
ò
(1)
G × dt
year
2
where G is irradiance (W/m ), Tamb is ambient temperature
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Normalized Conversion Efficiency (%)
1.1
(°C). Formally, the spectral profile and incidence angles for different irradiance conditions should also be included, and the integral done over wavelength as well. However, using a single typical irradiance spectrum produces on a yearly basis an error in the relative range of less than few percent. We also find that integration using the actual temperature variations is found to be well approximated by an average daylight ambient temperature (Tamb AVG daylight). We refer to the effective-efficiency to STC-efficiency ratio as the PV module’s annual performance ratio (PRM), which can also be expressed as: PRM =
heff YM × G0 = hSTC SE
(2)
Tamb= 20 oC 1.0 0.9 0.8 0.7
SM110 (c-Si) ASE-160 (EFG-Si) WS75 (CIGS) ST40 (CIGS) FS55 (CdTe) MA100 (a-Si)
0.6 0.5 0.0
0.2
0.4
0.6
0.8
1.0
1.2
2
Irradiance (kW/m )
h(G(i ),Tamb AVG daylight ) hSTC
i
PRM = å i
×
DSE(i)
h (G( i ),Tamb AVG daylight ) hSTC
(3)
1000 W/m2 ×
DSE(i)
(4)
SE
The module specific data is represented with normalized conversion efficiency as a function of intensity at the average daylight ambient temperature (normalized to the given STC conversion efficiency - see Eqs. 3 and 4). Normalized conversion efficiency vs. irradiance at specific Tamb can be either measured or simulated by proper model with input parameters. We took a one-diode model with effective series resistance Rs and shunt conductance Gsh on the solar cell level. The input parameters were deduced from current-voltage characteristics at different irradiances and temperatures published by manufacturers. Figure 2 shows simulated normalized h vs. G curves for modules from several manufacturers and technologies (monocrystalline silicon – Shell Solar SM110 [8] and multi/ribbon-crystalline silicon – RWE Schott Solar ASE160 [9], CIGS – Wurth Solar WS75 [10] and Shell Solar ST40 [11], CdTe – First Solar FS55 [12], and amorphous silicon (a-Si) – Mitsubishi Heavy Ind. MA100 [13]) deduced from their datasheets. In each case, the maximum efficiency is in the vicinity of one-half sun, but
there are differences in how the efficiency falls off for high and low illumination intensity. Modules with larger series resistance had a more negative slope at higher intensities, which lowered the one-sun efficiency compared to that near one-half sun. Those with larger leakage had a greater efficiency decrease at lower irradiance. The simulation of the a-Si and CdTe modules was slightly more complicated than the others, because the effective Gsh was significantly smaller at lower irradiance (modeled as Gsh increasing linearly with G). Figure 3 shows the profile of annual irradiance histogram, based on hourly averaged irradiance, from two locations (Golden and Ljubljana). The vertical scale (DSE) 80 Golden Ljubljana
2
YM = å
Figure 2. Irradiance-dependent normalized efficiency curves for several modules (normalized to individual STC efficiency).
Share of Annual SE (kWh/m )
where SE is the annual solar energy per area (denominator of Eq. 1), YM is the PV module's annual yield (kWh/yr/kWp) and G0 is the STC irradiance (1000 W/m2, AM1.5). YM is commonly used for comparisons, although it is bound to the location specific SE. PRM should be more useful than YM, because it normalizes YM to the given location-specific SE and thus becomes a more universal parameter for comparison of PV module field performance. The calculation can be done simply by weighting module specific data with location specific data as follows (in Eqs. 3 and 4):
60
40
20
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
2
Irradiance (kW/m )
Figure 3. Distribution of annual solar energy on a horizontal surface (j = 0°) for two locations: Golden and 2 Ljubljana. Irradiance is divided into 25 W/m increments.
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RESULTS Table 1 shows calculated annual performance ratio for the six PV modules (SM110, ASE-160, WS75, ST40, FS55 and MA100) for Golden and Ljubljana at different tilts (j). Despite a significant difference in illumination conditions at both locations, however, the values of PRM for each module show little variation between these two locations. The comparison between 0º-tilt and 30º-tiltvalues of PRM also show little variation, which results in almost no dependency over a broad range of tilt angles.
PV module SM110 (c-Si) ASE-160 (EFG-Si) WS75 (CIGS) ST40 (CIGS) FS55 (CdTe) MA100 (a-Si)
Golden (j = 0°) PRM 0.94 0.91 0.93 0.94 0.98 0.95
Ljubljana (j = 0°) PRM 0.94 0.90 0.92 0.95 0.98 0.94
Ljubljana (j = 30°) PRM 0.94 0.91 0.93 0.94 0.98 0.95
0.96
0.92
0.88
2
Leakage Conductance (mS/cm )
is the contribution of solar irradiation each year at an irradiance interval between G and G+DG (DG = 25 W/m2), and the horizontal axis is the irradiance G. Thus, the total annual solar energy per unit area (SE) for each location in kWh/m 2/yr is the sum of the individual contributions. The profile on a horizontal surface for Golden, which is 2 generally sunnier, sums to 1660 kWh/m /yr, while the 2 cloudier location in Slovenia yields 1100 kWh/m /yr.
3 0.96
0.92
0.88
2
1.00 0.92
0.96
1 WS75 PRM = 0.93 0.92
ST40 PRM = 0.95
1.00 1.04
0.96
0 0
1
2
3
4
5
2
Series Resistance (Wcm )
Figure 4. Contour plot of CIGS module performance ratio for Ljubljana (j = 30°) as a function of effective Rs and Gsh. Filled circles represent Rs and Gsh values of two commercial CIGS PV modules. of contour plot we can derive the sensitivity of performance ratio on Rs (+2.3 %/Wcm2) and on Gsh (-1.9 %/mScm-2). The same sensitivity holds for the yield. The other circle overlaid on Fig. 4 designates the deduced values of Rs, Gsh, and calculated PRM for the CIGS module ST40 (Rs = 2.3 Wcm2 and Gsh = 1 mScm -2). A clear message from Fig. 5 is that the performance ratio may vary not only among modules from different manufacturers using the same technology, but also among modules from the same production line.
Table 1. Calculated annual performance ratio for several modules. DISCUSSION To improve the effective efficiency and the performance ratio four parameters: dTj /dG, d, Rs and Gsh should be as small as possible. There is little potential, however, to improve dTj /dG, since all flat-plate modules are dark bodies (good absorbers) and solar cells are generally sealed under glass. Similarly, d is determined by the technology employed, predominately by the band-gap of absorber layer. Hence, the module parameters that design engineering is likely to improve are the effective series resistance Rs and the effective leakage conductance Gsh. Figure 4, which is based on input parameters of the CIGS module WS75, shows a contour plot how changes in Rs and Gsh would impact the performance ratio. The assumed irradiance conditions were for Ljubljana and with tilt of 30°. In the ideal case (Rs = Gsh = 0), the performance ratio of CIGS PV module is 0.92. A slightly higher value (0.93) is achieved for CIGS module based on WS75 data reported by the manufacturer (denoted with a circle at Rs = 1.2 Wcm2 and Gsh = 1 mScm -2). In this area
Comparisons of module efficiencies can vary by the order of 10% depending on the conditions under which modules are evaluated. The question is whether there is a simple protocol that predicts the effective module efficiency averaged over a typical year of operation. To do so with precision requires an average over the illumination, temperature, and solar-angle distributions at the installation site. We have shown that this average is relatively insensitive to the details of these distributions, and hence evaluation at one site gives a reasonable effective efficiency for most other locations. The STC module rating at one sun and 25 ºC module temperature implicitly assumes that all module efficiencies have the same relative intensity dependence. Although temperature tends to increase with illumination intensity at approximately the same rate for all modules [14], the fractional decrease of efficiency with temperature (d) can vary significantly among modules. Furthermore, the impact of series resistance will lower the STC rating of some modules more than others.
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also needed, but performance ratio results are only weakly dependent on the specific location, the tilt angle of the module or normal variations in the spectral distribution. They depend on average annual daylight temperature, but only very weakly on the temperature distribution. The two key module parameters that control the effective efficiency are the series resistance and leakage conductance, and the quantitative relationships derived should give some guidance on the cost-effectiveness of possible modifications to the production process. ACKNOWLEDGEMENT The authors would like to thank M. Gloeckler (CSU) and F. Smole (ULj) for valuable discussions. This work was carried out in the frame of US-Slovenian bilateral project BI-US/05-06/012 and research program P2-0197. Figure 5. Normalized conversion efficiency of SM110 and its three specific data from datasheet (performance at STC, NOCT and low irradiance). An additional complication is the effective leakage of a module, which reduces the module efficiency at lower illumination. However, the a-Si and CdTe modules that tend to have a large leakage coefficient at one sun also tend to have a smaller one under low illumination, and hence this problem is often at least partially mitigated. The normal-operating-condition (NOCT) rating, based on ambient temperature (20 ºC), compensates for variations in the module temperature coefficient and evaluates the efficiency at 0.8 sun (Fig. 5), closer to the average operational irradiance. This only partially addresses the series-resistance issue. In addition, the distribution of annual solar irradiation tends to peak closer to 0.5 sun. So one possibility is to simply redefine NOCT to be at 0.5 sun. The solution we recommend is to measure efficiency at multiple illuminations (0.1- or 0.2-sun intervals) under constant ambient temperature. An average of these values, weighted by a standardized solar-radiation distribution, should give an effective efficiency consistent with operation of the module in the field. CONCLUSIONS PV module performance assessment can be estimated by annual effective efficiency and module performance ratio, which both weight the module specific data by location specific data. The module specific data may be represented with normalized conversion efficiency as a function of irradiance at the average daylight ambient temperature. We demonstrated how to calculate annual outdoor performance ratio of six PV modules for two locations and different tilts. A distribution of annual solar energy as a function of irradiance for a specific location is
REFERENCES [1] Wohlgemuth J, Posbic J. Energy ratings for PV modules. Proceedings of 14th European Photovoltaic Solar Energy Conference, Barcelona, 1997; 313-316. [2] Marion B. A method for modeling the current-voltage curve of a PV module for outdoor conditions, Progress in Photovoltaics 10, 2002; 205-214. [3] Bücher K. Site dependence of the energy collection of PV modules, Solar Energy Materials and Solar Cells 47, 1997; 85-94. [4] Paretta A, Sarno A, Vicari L. Effects of solar irradiation conditions on the outdoor performance of photovoltaic modules, Optics Communications 153, 1998; 153-163. [5] Dimmler B, Powalla M, Schaeffler R. CIS solar modules: Pilot production at Wuerth Solar. Proceedings of 31st IEEE Photovoltaic Specialist Conference, Florida, 2005; 189-194. [6] www.pv.kaneka.co.jp/why/index.html [7]www.mhi.co.jp/power/e_a-si/contents/performance.html [8] http://sunwize.com/info_center/pdfs/shell_SM110-24P. pdf [9] http://www.rweschottsolar.com/en/files/media/2005/1/ 632405377225625000uaedax45mariqsn4ohk2nq55.pdf [10] http://www.wuerth-elektronik.de/we_web/frames.php? parLANG=EN&parKAT=157&parDOCID=1002935 [11] http://www.shell.com/home/ExitPage?URL=http:// www.shell.com/static/shellsolar/downloads/products/us/Sh ellST40_US.pdf&Source=CON&SourcesiteId=shellsolar [12] http://www.firstsolar.com/pdf/PD-5-401%20EU%20 Module%20Datasheet.pdf [13] http://www.mhi.co.jp/tech/pdf/e404/e404258.pdf [14] Tang Y, TamizhMani G, Ji L, Osterwald C. Outdoor energy rating of photovoltaic modules. Proceedings of 20th European Photovoltaic Solar Energy Conference, Barcelona, 2005; in print.
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