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System description
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Latitude: 47°60′ N Longitude: 19°35′ E Nearest town / city: G6c6ll6(28 kmfer from Budepest) ________________________ |
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Table 1. System hardware configuration
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The PV system planned to be developed is part of the PV Enlargement project. The system in G6d6lo is structured to 3 man blocks of PV pends. One subsystem is penned by using pieces of ASE-100 type moduls (RWE Solar Gmbh), and two identical subsystems using 77 pieces of DS40 (Dunasolar Rt) type moduls each. The total power of the system is 9,6 kWp. The principal elements of the systems are presented in Table 1.
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After the selection of the system elements the electronic connection plan of the system, together with the data logging properties and measurement points were elaborated. The detailed plan can be seen in the Fig. 1. |
Coefficient of Determination R2
The ratio
R2 = SSr / SSt
is called the coefficient of determination and is used to judge the adequacy of a regression model. This value is referred to as the amount of variability in the data explained or accounted for by the regression model (Montgomery, Runger, 1998).
To allow further study to be carried out in determining thermal interface resistance a guarded heat flow meter was designed and manufactured to the ASTM 5470 standard.
The Design of Experiments was carried out on the small-scale PV laminators’ process parameters. The selected factors studied were Post Curing Time, TPC, EVA Molten Time TEVA and the Temperature, Temp. Using data acquisition and Minitab the order in which each factor most affected Thermal Interface Resistances in the experiments was:
1. Temperature — Temp
2. Post Curing Time — TPC
3. EVA Molten Time — TEVA
From analysis of variance at a 95% confidence interval it was found that Temp, TPC, TEVA and the interaction TEVA* Temp had a significant effect on TIR.
SHAPE * MERGEFORMAT
Regression analysis was used to explore the relationship between a dependent variable and one or more independent variables (which are also called predictor or explanatory variables). In the experiment the dependant variable is Thermal Interface Resistances and the independent variables are the post curing time, the EVA molten time and the temperature of the laminator. The following regression equation was found:
TIR = 0.84 — 0.101 Tpc + 0.0021 Teva + 0.0245 Temp (R2=0.56)
Where: TIR = the response; Thermal Interface Resistances (K/W)
Tpc = Factor A; Post curing time (mins)
TEVA = Factor B; EVA Molten time (mins)
Temp = Factor C; Temperature (°С)
The accuracy of this equation was calculated to be 56%. In other words 56% of the variability in TIR can be explained by the factors Tpc, TEVA and Temp. The remaining 44% of variability is due to other unexplained external factors, which have not been included in the experiment.
□ >A Guarded Heat Flow Meter was designed and manufactured to the ASTM
5470 standard. This meter allows the measure for thermal transmission properties of thin thermally conductive solid electrical insulation materials to be successively carried out.
□ >From analysis of variance at a 95% confidence interval it was found that
Temp, TPC, TEVA and the interaction TEVA * Temp had a significant effect on TIR of the laminates. An R2 value of only 0.56 indicates that other factors need to be considered to fully explain for the variation in measured TIR.
Using data acquisition and Minitab the order in which each factor most affected Thermal Interface Resistances in the experiments was:
1. Temperature — Temp
2. Post Curing Time — TPC
3. EVA Molten — TEVA
ASTM D5470-95, 1995,“Standard test methods for thermal transmission properties of thin thermally conductive solid electrical materials”, PA 194-2959.
Montgomery D. C. et al, 1997, “Design and Analysis of Experiments”, John Wiley & Sons, Inc.
Peace G. S., 1993, “Taguchi Methods: A Hands-On Approach”, Addison — Wesley Publishing Company, Inc.
Ranjit, R., 1996, “A primer on the Taguchi Method”, Chapman and Hall.
Matching of aesthetical options with the user context light spectrum
In comparing the aesthetical options the PV performance in a certain user-context will be an important point of consideration. It will be clear that coloured PV cells illuminated with light which has its spectral peaks outside the sensitive colour transmission window of the PV cell coating is in fact illuminated by the wrong spectral distribution. This will for sure impair the function of the PV cell
For example a blue coated PV cell will reflect mainly the blue component of the light spectrum. In case this PV cell is illuminated by light of an artificial light environment with a strong blue sensitivity, the result would not yield a good combination. On the other hand, a red coated PV cell would absorb mainly the blue component and is therefore preferred.
Also it will be evident that bending a PV cell might reduce the performance of that PV cells. Apart from the non uniform light incidence and the inevitability of shadows, the bending itself might have some negative effects.
Preliminary experiments done on ribbon flexible PV cells; show that by multiple bending only a small (less than 10%) reduction in performance was detected [Boschloo and Hagfeld, 2003]. These experiments need to be extended further.
If for example a mono crystalline Silicon PV cell is bent, this bending introduces a horizontal and a vertical strain in the PV cell, parallel and perpendicular to its surface. Therefore one can expect in such bent Silicon PV cells performance degradation due to the so-called Piezo-Resistance effect. Namely: the change in resistance proportional to the induced strain. This increase in resistance will have a negative impact on the ‘Fill Factor’ of the PV cell and as a result on its conversion efficiency. The overall impact and loss of PV cell performance as direct result of aesthetics is still under investigation and will be reported elsewhere
• The concept of mature design provides a selection method for designing PV powered products.
• Some novel concepts and options of integrating PV cells directly on curved structural elements of consumer products have been introduced. These presented application examples demonstrate clearly the potential synergy that could be obtained by integrating solar cells on some structural elements of the products.
• Synergy becomes even more apparent by taking into account that another function can be realised at the same time such as ergonomics and not at the least aesthetics.
• Due to its flexibility in design, this integration approach yields also some incentive for innovative industrial design.
• Standardization worldwide is a must in order to come to a mature design of PV powered products.
5. Acknowledgments
This research is funded by NWO, the Netherlands Organization for Scientific Research.
References
• Arthur J. R., Robert K. Graupner, Tyrus K. Monson, James A. van Vechten and Ernest G. Wolf; US Patent no. 5672214 and no 5415700, Oregon, 1997.
• Beers S. van, Zonne-mobiliteit DCMR, Milieudienst Rijnmond, graduation master thesis, Design for Sustainability program, Delft University of Technology, 2002
• Boschloo G. and Hagfeldt A., Flexible Solar Cells, Angstrom Solar Centre, Private conversation ISES Solar Congress 2003, Goteborg, Sweden June 14 — 19, 2003.
• BP Solar ; http://www. bpsolar. com/ContentDetails. cfm? page=59,2004
• CET SOLAR Products; 2004 http://www. unlimited- power. co. uk/Solar Powered Products UK. html
• Curvet; 2004, http://www. curvet. it
• Glunz, S. and G. Willeke, Scientists make progress towards "paper thin” 20 % efficient crystalline Silicon Cell, Fraunhover Istitut Freiburg ISE, http://www. solarbuzz. com/News/NewsEUTE10.htm; December 9- 2003
• Husqvarna solar lawn mower; http://international. husqvarna. com/
• Pellegrino M., Flaminio G., Leanza G., Privato C. And A. Scognamiglio; Aesthetical Appeal of BIPV or Electrical Performance, Conference: PV in Europe, from PV Technology to Energy Solutions, Rome Italia, 2002.
• Photon Technologies; 2004; http://members. aol. com/photontek/photon/photon2a. html
• Poelman W. A., Onbedorven jonge ontwerpers; Product, Vol. 8 no. 1, 2000
• Poelman W. A and S. Y.Kan: Semiconductor on Insulator Enamel techniques, Registered Invention, Zeist, 2000.
• Kan S. Y., Reduction of hidden power consumption in consumer products; An application example of the Smart Photovoltaic Battery, Proceedings of EuroSun 2002, Bologna, Italia; June, 23-26 June, ISES ITALIA, Roma, 2002.
• Kan S. Y., PV Powered Mobility and Mobile / Wireless Product Design, Proceedings of the ISES Solar World Congress 2003, Gotenborg, Sweden; June 14 — 19 2003, ISES Sweden 2003.
• Kan S. Y., Silvester S. and H. Brezet; Design applications of combined photovoltaic and energy storage units as energy supplies in mobile / wireless products, Proceedings of the TMCE 2004, Lausanne, Switzerland, April 12-16, 2004, Millpress, Rotterdam 2004.
• Luther J., Photovoltaic electricity generation — Status and perspectives, Proceedings of the ISES Solar World Congress 2003, Gotenborg, Sweden June 14 — 19 2003, ISES Sweden.
• Solar Products; http://solar-world. com/home garden. htm , 2004.
• Solar Car roof: www. sunovation. de, 2004.
• Solaron CIS PV cells; www. solaron. de , 2004
• Verkuijl M., Verlichting op zonne-energie, graduation master thesis, Design for Sustainability program, Delft University of Technology, 2001
• Weitjens B., Met de zon in de rug, graduation master thesis, Design for Sustainability program, Delft University of Technology, 2003
Compositional analysis, XPS depth profile
Fig.3a shows XPS depth profile of a ZnO/ZnS/Silica bilayer, where the ZnS thin film has been obtained from the zinc acetate dehydrated and thiourea initial solution. In this figure is being represented the atomic concentration as a function of the sputtering time. It should be noted that the oxygen content in the bilayer decreases after 8 minutes of sputter, when the approximate depth is 24 nm (4 KeV Ar+) because it starts to appear sulphur, O1s and S2p peak are located at 530.04 eV and 161.0 eV respectively. The concentration of both elements change slowly inside the films and it is not until the 21 (1.5 KeV Ar+) minutes of bombardment and a estimated depth of 35.5 nm, when oxygen and sulphur present similar percentages, 24.35% oxygen located at 530.25 eV and 23.23% sulphur located at 162.06 eV. After this sputter time both elements follow the same trend, O1s profile decreases and S2p profile increases. This result may suggest the diffusion of ZnO in the ZnS film during the deposition of the outer layer.
A similar plot was obtained for a ZnO/ZnS/Silica bilayer (Fig 3b), where in this case the ZnS thin film has been prepared from zinc chloride and thiourea solution. The ZnO thin film is overcome after 6 minutes of Ar+ (4 KeV) sputtering, when 18.0 nm is the approximate depth, because 7.85% of sulphur is detected at 162.47 eV, O1s peak is located at 530.41 eV in 39.81%, the Zn2p peak has been used as a reference located at
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Approximate depth (nm) 15.0 30.0 32.5 35.0 41.5 47.5 |
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Approximate depth (nm) 15.0 20.0 22.5 25.0 27.5 42.5 |
1022 eV. The depth profile follows showing an oxygen decrease and a sulphur increase. After 16 minutes of sputtering (1.5 KeV Ar+) when the it has been sputtered 23 nm, it is found a similar concentration of ZnO and ZnS because there are a 23.69% oxygen detected at a binding energy of 529.91 eV and a 24.66% sulphur at the energy of 162.27 eV, 12.5 nm before than in the sample represented in Fig.3a, where it is also represented the approximate depth of the sputtering time, it has been stimated using the rates showed in the experimental method. These fact shows that the ZnS thin film deposited using zinc chloride and thiourea initial solution has a smoother surface, it is more dense and thicker.
Figure 3a.- XPS depth profile of a ZnO/ZnS/Silica bilayer where the ZnS thin film has been obtained using zinc acetate dehydrated and thiourea. Figure 3b.- XPS depth profile of a ZnO/ZnS/Silica bilayer where the ZnS thin film has been obtained using zinc chloride and thiourea.
The ZnO and ZnS thin films and the bilayers ZnO/ZnS obtained by CSP present similar optical, structural and morphological properties to the films obtained from high vacuum methods. In general, the films present a smooth and homogenous surface without hollow or crack, and they show a good transmittance (80%-85%). However, for ZnS films, the initial precursor, ZnCl2 or acetate Zn, have influence in physical properties (optical, structural and morphological) of the films and, in general, the ZnS films obtained from ZnCl2 present better optical properties than the films obtained from acetate.
Neither carbon nor others by-products which could change the refractive index of bilayer have been found in any interface ZnO/ZnS, independent of the Zn precursor; however, there are some differences between the bilayer with the ZnS film obtained from ZnCl2 or Zn acetate. The bilayer with the ZnS film obtained from ZnCl2 show a depth
profile with a stronger decrease of oxygen and an increase of sulphur than the other interface; moreover, the sulphur appear at lower depth and the oxygen and sulphur concentrations become equals at lower depth. We can put these difference to the films obtained with ZnCl2 are smoother, denser and thicker than the other ZnS films.
The authors are grateful to the European Union and CICYT (Spain) (grant MAT2000- 1505-CO2-O2) and the Junta de Andalucia trough the research group FQM-192. The bursary holders, M. C. Lopez wish to thank to the MEC (Spain) and the CSIC (Spain).
[1] . High-efficiency AlGaAs/GaAs Tandem solar cells, Ken Takamash, Shigeki Yamada, Tsunhero unno.
[2] . IEEE Transaction on electron devices, Vol 44, n°9 September 1997
[3] . Journal of electronic materials, Vol 29 n°7, 2000
[4] . Solar Energy materials and solar cells 52 (1998) 79-93
[5] . Surface and coatings technology 106 (1998) 117-120
[6] . Solar energy materials & Solar cells 64 (2000) 393-404
[7] . Materials Chemistry and physics 68 (2001) 175-179
[8] . Thin solid films 398-399 (2001) 24-28
[9] . Applied surface science 136 (1998) 131-136
[10] . Sensor and actuators A67 (1998) 68-72
[11] . Thin solid films 379 (2000) 199-201
[12] . Materials Letter 55 (2002) 67-72
[13] . Journal of crystal growth, 247, (2003) 497-504
[14] . Thin Solid Films 426 (2003) 68-77.
[15] . Swanepool R. J. Physics E.1983; 16;1213
[16] . Shirley DA: Phys Rev. B1972;5;4709
[17] . Physical Electronics, 6509 Flying Cloud Drive, Eden Prairie, Mn 55344, USA
[18] . Materials Research Bulletin, Vol 32, n°12 pp 1631-1636, 1997
[19] . Thin Solid Films 403-404 (2002) 102-106
[20] . Matterials Chemistry and Physics 9463 (2002) 1-8
[21] . Materials Chemistry and Physics 78 (2002) 373-379
[22] . Journal of Applied Physics, Vol 90, N°5, 1 September 2001
[23] . Thin Solid Films 431-432 (2003) 242-248
Reduction of breakage losses in silicon-cell processing — Investigations towards an optimization of manufacturing processes
Joachim Beinert, Rainer Kubler, Horst Kordisch, Laszlo Konczol, Gunter Kleer, Fraunhofer-Institut fur Werkstoffmechanik, Freiburg, Germany
Silicon solar cells are industrially produced from thin silicon wafers. Currently the thickness of these wafers is in the range of 300 pm. Silicon is a relatively expensive material. About the half of the production costs of a silicon-cell module is due to the silicon wafers. Silicon is also brittle and fragile. During the manufacturing of wafers, cells and modules, the silicon disks are subjected to numerous processes implying substantial mechanical or thermal loadings such as sawing, etching, doping, printing, firing and handling procedures. Therefore significant economic losses can occur due to fracturing of the brittle silicon. Savings concerning this expensive material, therefore, entail a great potential for cost reductions.
A wafer breaks, when the applied tensile stress exceeds the strength of the wafer. The strength of a wafer is predominantly controlled by the conditions regarding micro-damages in its surface whereas bulk material properties or structural parameters are of minor significance, as is shown in a previous paper [1]. From these elementary facts it can be concluded, that for an optimisation of manufacturing processes with respect to breakage loss reduction two aspects must be considered: The prevention of formation or growth of micro-damages as well as the decrease of wafer impacting stresses.
For the optimization of processes an integral concept was developed and applied in industrial manufacturing lines of three silicon-cell producers. This concept combines different approaches:
— Strength analysis to study the influence of particular process steps onto the mechanical strength of the wafers and cells
— Damage analysis to investigate the formation and evolution of damages and fractures
— Process analysis to determine the mechanical or thermo-mechanical loading of the wafers in fracture relevant manufacturing processes
— Test methods to develop practicable methods for strength characterisation as well as for damage detection which are capable to eliminate damaged wafers early in the value-added chain.
Results of Measurements
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5 ——— |
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4 |
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3 |
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2 |
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160 — ^ 140 — 120 ■ 100 — 80 — 60 40 20 0 |
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Pmax Curve і——————- 1—————— 1— 20 30 40 Voltage [V] |
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20 30 Voltage [V] |
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0 |
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10 |
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50 |
The U-characteristics, Pmax curves and the irradiance data provide the background for selection of valid data used to determine the efficiency of the PV module with and without antireflection treated front glass. All Ul-characteristics and Pmax curves are manually studied to assure they do not vary from the shapes of figure 3, which give an example of a valid U-characteristic and the corresponding Pmax curve. For each measurement under cloudless conditions the solar irradiance on the PV module vary less than 1%. For each measurement under cloudy conditions the solar irradiance vary less than 3%. The duration of a measurement is 30-90 s. If the specified irradiance conditions are not fulfilled, an influence on the shapes is seen, for which reason the results are disregarded.
Figure 3. UI characteristic and the corresponding Pmax curve. Measured on a cloudless day with an incidence angle of 0° and a total irradiance on the PV module of 960 W/m2. PV module with normal front glass.
Table 1. Measured data for the PV module with the normal front glass for cloudless conditions. |
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Table 2. Measured data for the PV module with the normal front glass for cloudy |
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conditions. |
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Table 3. Measured data for the PV module with the antireflection treated front glass for |
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cloudless conditions. |
Values of Pmax are extracted from the valid data sets for both the cloudless and the overcast weather conditions. Table 1, 2, 3 and 4 show the measured data. The efficiency is based on the total solar cell area.
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Diffuse radiation on PV module |
Outdoor temperature °C |
Pmax W/m2 |
Efficiency |
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64 |
18.7 |
9.5 |
14.8 |
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78 |
18.7 |
11.2 |
14.3 |
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80 |
18.7 |
11.5 |
14.4 |
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153 |
15.0 |
23.1 |
15.1 |
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Table 4. Measured data for the PV module with the antireflection treated front glass for cloudy conditions. |
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I Without AR front glass "И" With AR front glass I |
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I Without AR front glass "И" With AR front glass! |
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Angle of incidence [°] Angle of incidence [°] |
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15 14.5 14 13.5 13 12.5 12 |
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0 15 30 45 60 75 |
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Angle of incidence [°] |
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11 1 10.5 10 |
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Angle of incidence [°] I Without AR front glass With AR front glass! |
Figure 4 shows for cloudless periods the measured values for Pmax, totai and the total efficiency as well as the total irradiance and the ratio of diffuse radiation on the PV module both for the PV module with the normal front glass and with the antireflection treated front glass. The figures give an overview of how the different conditions affect the efficiency. Pmax, total is clearly depending on the total irradiance, whereas the efficiency is depending on a number of conditions, whose different influence is not clear from the figures.
Figure 5 shows for cloudy periods the measured efficiency as a function of the diffuse irradiance on the PV module both for the PV module with the normal front glass and with the antireflection treated front glass. The efficiency for diffuse radiation, ndiffuse is found by:
P
max, diffuse
ndiffuse= .
G diffuse
The PV module utilizes the diffuse radiation the most when equipped with an antireflection treated front glass.
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Diffuse irradiance [W/m2] |
Without AR front glass * With AR front glass Figure 5. Efficiency as a function of the diffuse irradiance on the PV module.
Despite the short test period, it was not possible to obtain exactly the same weather conditions, i. e. temperature, diffuse and direct irradiance. For instance, the outdoor temperature was higher during the tests of the PV module with the antireflection treated front glass than during the tests of the PV module with the normal front glass.
The measurements for the PV module with the antireflection treated front glass are therefore modified so that the efficiency for the PV module is found at an outdoor temperature equal to the outdoor temperature during the test of the PV module with the normal front glass.
A theoretical heat balance for the solar cells is used to modify the measurements. The heat balance is depending on the outdoor temperature, Tout, the incidence angle, the direct and diffuse irradiance on the PV module as well as knowledge of the solar transmittance for the glass. The heat balance is used to determine the solar cell temperature. As specified for the panel used, it is assumed that the power of the PV module is decreased with 0.43% per K temperature increase of the solar cell [6].
~•—Without AR front glass * ‘ With AR front glass
Figure 6. Efficiency for the PV module for diffuse radiation as a function of the irradiance on the PV module at an outdoor temperature of 13.7°C.
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Figure 6 shows the efficiency of the PV module for diffuse radiation at an outdoor temperature of 13.7°C both for the PV module with the normal front glass and with the antireflection treated front glass. |
With the same outdoor temperature the efficiency of the PV module for diffuse radiation can be improved by up to 1 %-point, corresponding to a relative improvement of 7%..
Even under completely cloudless conditions a part of the irradiance on the PV module is diffuse. To find the efficiency for the direct radiation, the measurements under cloudless conditions must be corrected for diffuse radiation. Pmax consists of a contribution from both direct and diffuse radiation, respectively Pmax, direct and Pmax, diffuse. Pmax, diffuse is calculated based on the efficiency expressions for diffuse radiation and extracted from the total Pmax, hence obtaining P max, direct. The efficiency for direct, ndirect, is found by:
The efficiency, ndirect, is found for different incidence angles, see figure 7. The efficiency of the two modules follow the same pattern, though the efficiency of the module with the antireflection treated front glass is higher than the efficiency of the module with the normal front glass. When the incidence angle reaches 60°, the efficiency of the module with the normal front glass decreases significantly. The decrease in the efficiency of the module with the antireflection treated front glass is delayed, due to the fact that the efficiency of this module is relatively high for diffuse radiation and due to the fact that the difference in transmittance between the two glasses is relatively high for large incidence angles.
0 15 30 45 60 75 |
Angle of incidence [°]
Without AR front glass With AR front glass Figure 7. Efficiencies for direct irradiance as a function of the incidence angle.
To provide a better basis for comparing the efficiencies, the calculated efficiencies of the PV module with the antireflection treated front glass for direct radiation are shown in figure 8 along with the efficiency of the PV module with the normal front glass at the same weather conditions as used for the tests of the PV module with the normal front glass.
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15 30 45 60 75 |
15
13.5
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07 |
13
12.5 12
11.5 11
10.5 10
0
Angle of incidence [°]
Without AR front glass * With AR front glass
Figure 8. Efficiencies for direct irradiance on the PV module as a function of the incidence angle at an outdoor temperature of 7.6°C.
Figure 8 shows that at the same outdoor temperature the efficiency of the PV module for direct radiation can be improved by making use of an antireflection treated front glass by about 0.4 %-point, corresponding to a relative improvement of 3% for incidence angles
smaller than 60°. For higher incidence angles the advantage of the antireflection treatment is higher.
It is estimated that the power from the PV module is measured with accuracy well below 1%, and that the efficiency of the PV module is measured with a relative accuracy of about 2%. The measuring accuracy is estimated to be sufficient to elucidate the relative small efficiency differences between the PV module with and without the antireflection treated outer glass surface.
However, the comparison of the efficiencies of the PV module with the normal glass and with the antireflection treated glass was quite difficult, since the weather conditions during the tests were not the same. The outdoor temperature, the total irradiance on the PV module and the ratio of diffuse radiation on the PV module varied from test to test. The measured power and efficiency of the PV module was therefore, based on traditional heat transfer theory, modified to a power and efficiency for fixed weather conditions in order to make a fair comparison possible.
It is recommended to carry out more measurements in order to elucidate if the above — mentioned modification is done in the right way. The measurements should focus on elucidating the influence of the outdoor temperature on the PV module efficiency.
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07 |
The efficiency of the PV module can be improved by making use of a front glass with an antireflection treated outer surface, both for diffuse and direct radiation on the PV module.
With the same weather conditions the efficiency of the PV module for diffuse radiation can be improved by up to 1 %-point, corresponding to a relative improvement of 7%.
With the same weather conditions the efficiency of the PV module for direct radiation can be improved by 0.4 %-point, corresponding to a relative improvement of 3% for incidence angles smaller than 60°. For higher incidence angles the advantage of the antireflection treatment is higher.
It is recommended, based on measured efficiencies and weather data of Test Reference Years to calculate the yearly energy production of the PV module with the normal front glass and with the antireflection treated front glass.
Optimum energy yield of a PV system
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Impacts on the solar yield |
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07 |
High quality components and a professional installation are the essential item for a long — lasting operation of a PV system. How much solar electricity it will produce furthermore depends on the interaction of the single components, on the structural preconditions or design demands and on the specific location. Particularly shading — coming from trees or neighbouring buildings, but also from projecting roof structures, chimneys or lightning protection systems on the building itself — has distinct negative effects on the yield. Furthermore high operating temperatures of the modules, e. g. with poorly ventilated installation, result in reduced yields. Significant yield losses due to operational defects or even total failure can be prevented with automatic monitoring and continuous maintenance.
Conception or the DGS hallmark
If a PV system is carried out with the DGS hallmark the involved companies commit themselves to meet certain quality standards voluntarily. These standards have been set in close discussion with planning bureaus, installation companies and component producers. The DGS Berlin Brandenburg as independent expert sees the planning documents in the initial stages to check the keeping of the quality standards and to discover week points which can lead to yield reductions during the future operation. Thus the DGS can give tailor-made recommendations to optimise the system even before the start of construction. The set goal is always to reach the greatest possible yield for the particular installation.
This means that for example a fagade integrated system, which will see less sunshine due to the perpendicular installation and inevitably will show less yield than an ideally tilted roof installation, nevertheless can be labelled with the DGS hallmark. Unavoidable shadings (especially with building integrated systems) are not necessarily an obstacle, if the losses in yield are minimised by a well thought-out arrangement and interconnection of the modules. And, of course, a PV system installed in sunny southern regions and generating more than 1000 kWh per kWp and year does not innately rank before a system in Northern Germany with maybe only 800 kWh/kWp annually.
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07 |
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Shade caused by close objects may reduce the energy yield significantly |
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Comparison of shading effects on thin-film and on crystalline silicon PV modules |
Attachment of thin-film PV modules taking account of shadow
The DGS hallmark is modularly structured and divided into five components. As required the owner or operator can take on certain tasks himself or commission the DGS. Module 1,
2 and 5 are carried out by the DGS in any case.
Module 1: Quality checkup according to the DGS hallmark standards
From the planning documents the DGS evaluates the PV system and awards the DGS hallmark, if the demanded aims and criteria are achieved. This applies to systems which are still being planned as well as to already built ones. An on-site visit is also possible. The standards start with the selected components. Apart from the usual certificates and guaranteed quality the most important feature is the power tolerance of the modules.
Some manufacturers deliver modules with up to 17 % less than nominal power. With the professional planning and installation all technical regulations and standards (first of all national and European standards, IEC) must be observed. Emphasis is placed on optimum configuration and dimensioning of the components. A full documentation of the installation is compulsory as well as an acceptance report. Last but not least after putting into service the system must be monitored and serviced within a maintenance contract so that a trouble-free operation is ensured.
Quality standards of the DGS hallmark for large grid-connected PV systems |
Module 2: Forecast of yield
Based on long-term weather data from the national meteorological service (monthly average of the horizontal global irradiation at the exact location) the DGS predicts the energy yield of the PV system using several acknowledged simulation programmes. This expertise includes a shading analysis. The results of the simulations are technically adjusted. In this way loss factors and influences which are not integrated the programmes are taken into account empirically. Possible degradation effects of the modules within a period of up to 25 years are taken into consideration. Thus all factors described above which may influence the yield are comprised in the DGS expertise. Moreover the minimum yield in kWh / a and in kWh / (kWp ■ a) for the solar guarantee is determined which includes a safety margin of 5 % below the predicted energy yield.
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Facade integration, no ventilation |
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Roof integration, no ventilation On / in facade, poor ventilation On / in facade, good ventilation On / in roof, poor ventilation On / in roof, good ventilation On roof with large gap Completely freestanding |
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■ Temperature increase ■ Reduction in the energy yield |
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55 К |
□
n
□
(/j
я
ssss
Temperature increase and reduction in the annual energy yield with various types of mounting. Data: Photovoltaic supplied small systems and devices, Fraunhofer ISE
Module 3: Guaranteed energy yield
If e. g. a 300 kWp PV system yields only 700 instead of the predicted 850 kWh / kWp per year this results in a loss of feed-in compensation amounting to more than 20.000 Euros per year. In order to ensure the refinancing of the PV system in this case, the owner or operator concludes a solar guarantee contract with the vendor (planning or installation company). In this contract the minimum yield from the DGS yield forecast is fixed as guaranteed yield and a compensation payment is arranged in case the system generates less energy. As many vendors of large pV systems are swamped with accruing reserves for substantial guarantee payments they are well advised to contract an insurance on their part to cover the financial risk. Such a yield insurance for solar power systems associated with the DGS hallmark is offered by the Mannheimer insurance company. This is not a part of the DGS hallmark, whereas any legally binding solar guarantee contract signed by the operator / owner, planning / installation company and the DGS is obligatory.
Module 4: Monitoring
All relevant measurement values and operating data must be recorded and analysed. The regular or an automatic monitoring must ensure an immediate error message as soon as noticeable faults occur and result in prompt trouble-shooting. At least once a month the yields are analysed taking into account the actual irradiation. If the system is not equipped with an own irradiance sensor, weather records of the nearest meteorological station are to be used instead. The meter readings are monthly forwarded to the DGS for permanent function and guarantee control. On request the DGS can perform the complete monitoring.
Module 5: Examination of energy yield and guarantee
In order to find out whether there is a claim for compensation payment the DGS checks the results of every month and year of operation. The guaranteed yield is adjusted and corrected by simulating again with the actual weather conditions. All external influences which are not at the responsibility of the vendor and were not foreseeable in the planning process are taken into account: the deviation of the actual irradiation from the average of many years, shading that has occurred afterwards and operational interruptions e. g. caused by the grid. At the end of a year of operation the DGS compiles an annual report. There the measured yield is compared with the corrected guaranteed yield. Any less generated kilowatt hour has to be compensated by the vendor or his insurance company. The DGS calculates the amount of the payment according to the guarantee contract.
The DGS hallmark is a comprehensive instrument from one source for quality assurance with large grid-connected PV systems. On the one hand all yield influencing quality features from the development phase to the long-term operation of the system are allowed for in the yield forecast. On the other hand the results of the quality check enable the buyer to value the predicted energy yield and the guaranteed yield. For not every PV system with an annual yield of 850 kWh / kWp is well designed. Just as a system yielding only 650 kWh / kWp does not need to be poor quality, depending on the external circumstances at the certain installation site. This includes climatic conditions or for example the special features of building integration. Building integration can cause yield reductions due to shading and or temperature rises of the modules which in general are justified thanks to other advantages for the building. Optimisation under the given situation is always possible and represents the most important prereqisite for the DGS hallmark. The aligned monitoring reacts immediately to inconsistent results and prevents major losses due to faults.
As a result the DGS hallmark provides cooperation the involved planning company, installation firm and producers of the components and is aimed to mutual advantages: the buyer (owner or operator) is sure of optimum system with maximum possible yield and profit. The integrated yield guarantee accounts for a safe investment. Whereas the vendors (planner or installer, producer) benefit from business confidence due to the neutral quality label and, of course, from customer satisfaction. In the end high-quality and reliable photovoltaic projects are a benefit for the whole PV industry and trade.
[1] Haselhuhn R., Hemmerle C., Berger F.: Leitfaden Photovoltaische Anlagen fur Elektriker, Dachdecker, Fachplaner, Architekten und Bauherren, Berlin, editor: DGS, 2nd edition, Sept. 2002, ISBN 3-9805738-3-4. Reference book and guide for basic information, planning, installation and operation of PV systems; also available in English, Spanish, Italian and Portuguese
[2] Spitzmuller P.: Gutesiegel fur grofte solarthermische Anlagen im mehrgeschossigen Wohnungsbau, proceedings of the 10. Symposium Thermische Solarenergie,
OTTI Kolleg, May 2000
[3] Haselhuhn R.: Das DGS-Gutesiegel fur PV-Anlagen, Berlin, editor: DGS, proccedings of the 13. Internationales Sonnenforum, Sept. 2002, ISBN 3-9805738-5-0
Sizing of stand-alone photovoltaic system
The size of PV-system (Fig 1) is a general concept including the sizing of PV-array and the accumulator. A useful definition of such dimensions relates to the load: In daily basis, the PV-array capacity, (CA) is defined as the ratio between average PV-array energy production and the average load energy demand. The storage capacity, (CS) is defined as the maximum energy that can be taken out from accumulator divided by the average energy demand [10, 11, 12], so:
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(1) |
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Where APV is the PV-array area, qPV is the PV-array efficiency, H is the average daily irradiation on the PV-array, L is the average daily energy consumption, CS is the storage capacity and CU is the useful accumulator capacity. Note that CA depends on the meteorological conditions of the location. That means that the same PV-array for the same load can be ‘large’ in one site and ‘small’ in another site with lower solar radiation. The task of sizing a PV-system consists of finding the better trade-off between cost and reliability. Very often, the reliability is a priori requirement from the user, and the PV engineer problem is formulated as follows: which pair of CA and CS values leads to a given LLP value at the minimum cost? The sizing of the PV systems requires the knowledge of one of the components of solar radiation known as daily of global solar radiation data measured by meteorological stations. However, these data are not always available because of the few weather stations in Algeria. Because of that, these were collected from data measurement system using a network configuration [13] and Markov Transition Matrices (MTM) approach [14]. As an example, figure 2 shows the daily values of global solar radiation data for some sites. We calculate the various coefficients (CA, CS) corresponding at 200 sites. For this, we used the numerical method [10] Figure 3 presents the iso — reliability curves for some sites. Next, we calculate the optimal couples (CAOP, CSOP) based on analytical cost. In this case database of optimal sizing coefficients is formed corresponding to 200 |
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PV Generator |
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Inverter |
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Fig. 1 Diagram block of simplified stand-alone |
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200 Day |
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Lattude:23°N Longitude:4°W |
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200 Day |
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Fig. 2. Daily values of inclined global solar radiation data for samples sites |
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CaJVvML and Cs=Cf- |
sites. Table 1 shows the evolution the optimal sizing coefficients used in this study, for 4 sites.
0 50 100 0 50 100 |
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Fig.3 iso-reliability curves Table 1 Optimal sizing coefficients |
According to this table we note that the sites located in north have the higher coefficients values of to those the south.
Storage capacity(Cs) Storage capacity(Cs)
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Calculation resultsFor the model being presented the calculated results were obtained at the following conditions: Operational substance CF3I, pressure = 25 mmHg. Interior radius of the cell = 0.65 cm. The distance between the inlet and outlet orifices for gas pumping through the cell (the length of the active medium) = 67 cm. Resonator: R1 = 0.998; R2 = 0.95; (flat mirrors ); losses for one trip = 2%. The initial pumping velocity for the operational medium through the cell = 1920 cm/sec. Initial temperature of the operational medium = 310 K. Possibility of photo-dissociation у = I * a = 0.145 , where I — pumping intensity for the wavelength of 254 nm (photon/cm2*sec); a absorption cross-section of CF3I at 254 nm wavelength (cm2), which corresponds to the intensity of 190 solar constants. The pumping is considered to be axial-symmetric increasing to the cell center reverse proportional to the radius. Above all, the diffusion of the excited iodine atoms to the cell walls as also being considered. Calculation results: Laser generation power 183 mW; Final pumping velocity 2270 cm/sec; Final temperature of the operational medium 364 K. Fig. 3 presents the calculated distribution of generation power over the length of the operational medium and along the cell radius.
Both calculation and the experiment yielded that the effectiveness of C3F7I is lower as opposed to CF3I. From the viewpoint of the present model this can be due to different broadening ratios of generation line at the relatively low pumping levels. SILICON HETEROJUNCTION CELLS: A BREAKTHROUGH IN PHOTOVOLTAICSJulio Carabe, CIEMAT. Avda. Complutense, 22. E-28040 Madrid, Spain Francesco Roca, ENEA. Localita Granatello. 80055 Portici (Naples) Italy. Photovoltaics are facing the challenge to find new approaches to make solar cells competitive with respect to more conventional electricity sources. Crystalline-silicon (c-Si) wafer-based technology must evolve towards lower costs by implementing new material — fabrication processes and making wafers thinner. On the other hand, silicon grown directly on low cost substrates is forced to progress in the direction of improving the optoelectronic properties of the material and its growth rate and consistently of making the cell active — layer thicker. The key silicon material in next-generation PV is characterised by a medium crystallinity, a medium active-layer thickness and a high fabrication rate. The efforts towards this material are generating new approaches involving the combination of wafer and thin-film technologies. The characteristics of new materials impose new limitations, thus requiring innovative solutions. Silicon-heterojunction (SHJ) cells, basically made of a crystalline-silicon wafer or ribbon absorber and one or two thin-film-silicon emitter(s), represent a promising option. Its features are: a simple low temperature fabrication process, an important cost-reduction capability, high efficiencies and a high potential for improvements. Furthermore SHJ technology has proven to provide excellent surface-passivation approaches. Particularly remarkable is the work done by Sanyo, who have reported 21% efficiency on a cell of this kind (so-called HIT®) and have attracted much attention. This new product has allowed to the Japanese company to reach a market share of 6% of all PV sales in the world. A number of research groups and companies are working hard on SHJ cells in Europe. Their individual results are promising and reveal an excellent scientific level. The need to address the fragmentation of European R&D in this field must however be recognised by creating a permanent structure ensuring the harmonisation of the whole R&D on SHJ cells. Photovoltaics are dominated by silicon. About 84% of the world PV market share corresponds to mono — and multicrystalline-silicon wafer technology. Only 0.4% is covered by thin-film chalcogenides, basically CdTe. The remaining 15.6% is shared by various silicon-based technologies, such as ribbon silicon, amorphous silicon and silicon heterojunction cells1. This primacy is very likely to remain for at least the next ten years. The cause of this predominant position is the combination of a number of factors, such as the maturity of silicon PV technologies, the good and well-known optoelectronic properties of the material, its availability, lack of toxicity, cost, chemical stability, etc. Europe is a leader in the production of crystalline ingots and wafers for PV applications. At present, the cost of materials and processes used in c-Si PV applications is high. Every effort done to reducing the costs of the materials involved in the process without detrimental effects on conversion efficiency is considered of vital importance. In the search for breakthroughs, the most promising initiatives for the reduction of costs in the production of c-Si solar cells are addressed towards replacing the present raw material by thinner, less-crystalline silicon (EFG, string ribbon silicon, Silicon Film®, dendritic-web silicon, etc.). If these new materials are to be used, a number of issues become essential, such as low — temperature approaches for junction formation and cost-effective processes for passivation. On the other hand, thin-film silicon research is evolving towards the preparation of thicker, more crystalline films (nano — and microcrystalline silicon) where the stability and high deposition rate of the material and the quality of the interfaces are key factors. It seems that the two main silicon technologies are mutually approaching. This is particularly apparent in the Sanyo development of hybrid wafer — and thin-film approaches, such as the silicon heterojunction (sHj) solar cell. Here the advantages of both technologies mutually interact for the development of low-cost, stable high-efficiency solar cells one of whose types is the well-known 21%-efficiency HIT® cell developed by Sanyo2. The fundamental approach is the replacement of the more conventional emitter thermal diffusion by a low-temperature deposition process. Key features of SHJ technology are: a very simple fabrication process, an important cost-reduction capability, relatively high efficiencies, and a high potential for significant improvements. Of particular relevance is the suitability of the approach to process low-cost wafers or ribbons without degradation of their transport properties (given the low temperatures involved, around 250°C), and relatively thin substrates, since the control of emitter thickness is extremely good. |