Category Archives: SonSolar

Results and discussion

140

120

100

Ґ.

80

L-‘

:

60

Г’

40

20

4

/“

0

0

50

100 150 200 250

strain [%]

Fig. 3: Stress-strain plots of the investigated materials and laminates

Table 1 and Fig. 3 summarize the results of the tensile tests. For PET elastic modulus values of about 4000MPa were obtained. In contrast, the fluoropolymers PVF and PVDF exhibit significant lower elastic modulus values, ranging from 1500 to about 2000MPa. The lowest elastic modulus of about 600MPa was determined for the EVA-PVF-PET-PVF laminate, the elastic modulus is dominated by the EVA elastomer. The plastic modulus values are ranging from 10 to 76MPa. PVDF showed a negative slope of the stress-strain curve after the yield point; the plastic modulus E2 was set zero.

Table 1: Results of the tensile tests

thickness h

[p. m]

E1

[MPa]

E2

[MPa]

8y

[%]

PET

75

4242 ±116

32.8 ± 0.4

2.30 ± 0.07

PET-SiOx

100

3452 ± 199

75.9 ± 2.2

2.39 ± 0.13

PVF

45

2053 ± 120

19.1 ± 0.3

1.68 ± 0.09

PVDF

37

1464 ±134

0

3.46 ± 0.18

PVF-PET

138

2263 ± 140

19.5 ± 1

2.92 ± 0.14

EVA-PVF-PET-PVF

660

565 ± 30

11.9 ± 0.6

3.33 ± 0.21

PET

PET-SiOx

PVDF

————— PVF-PET

EVA-PVF-PET-PVF

Fig. 4 depicts the force versus displacement plots of the three thermoplastic laminates PVDF/PET, PVDF/SiOx-PET and PVF/PET-PVF. The curve of the PVDF/PET laminate without the silicon oxide barrier layer shows two levels of peel force. The high level is interpreted as a cohesive fracture in the peel arm and the low level can be attributed to adhesive fracture between the two peel arms (Moore and Williams, 2001). The peel force is independent of the configuration; the mean value is 9.1±0.3N. As to the peel angle ф of the PVDF/PET laminate significant differences between the two configurations were obtained. Configuration A (PVDF peel arm top) yielded an angle of 13.3±0.4°, whereas in configuration B (PET peel arm top) an angle of 11.9±0.6° was measured. This difference can be explained by gravity effects in combination with the low flexural stiffness of the laminate. Fig. 5 shows the curvature of the PVDF/PET laminate.

For the PVDF/SiOx-PET laminate with an 80 nm thick silicon oxide barrier layer no effect of the configuration on the peel angle was observed. For both configurations mean values of 12.7±0.4° were obtained. In contrast to the PVDF/PET laminate the peel force is significant higher (11.1±0.3N).

PVDF-PET———- PVDF-SiOx — PET———— PVF — PET — PVF

displacement [mm]

Fig. 4: Peel force versus displacement of the thermoplastic laminates investigated

Fig. 5: PVDF/PET specimen during peel test

The PVF/PET-PVF laminate did not show stable peeling, the PVF peel arm broke before peeling could be established at a maximum force of 8.6±1.4N. For the PVF/PET-PVF laminate it was not possible to do the adhesive fracture toughness evaluation.

Due to the differences in the peel angle ф the adhesive fracture energy (GA) values of the PVDF/PET laminate without silicon oxide barrier layer scattered significantly. GA values of 192±23J/m2 were obtained. Doing the evaluation for each configuration A and B, Ga values of 210±12J/m2 and 175±17J/m2 were calculated for configuration A (PVDF peel arm top) and B (PET peel arm top), respectively. For the PVDF/SiOx-PET laminate (with the silicon oxide barrier layer) an enhanced GA value of 284±20J/m2 was determined. In comparison to common thermoplastic laminates which are used for packaging purposes the tested laminates showed equal or more than 30% higher GA values (Moore and Williams, 2001).

The investigations on the two PVDF/PET laminates with and without SiOx barrier layer indicated that the energy based fracture mechanics approach is much more

sensitive to describe the delamination behaviour compared to the usually applied load based analysis.

Fig. 6: Peel force versus displacement of the PET/EVA laminate

Fig. 6 indicates that the adhesion between PET and EVA is much higher than between PET and the other materials (PVF, PVDF). A peel force P of 142±6N, a peel angle ф of 77.3±0.8° and an adhesive fracture energy of 7895±454J/m2 were determined. The obtained GA value is significant higher than any in the literature reported value of GA for plastics. Hence, for the investigated laminates a delamination within the backsheet material may be more likely than delamination between backsheet material and EVA solar cells encapsulation material.

Acknowledgements

The research work of this paper was performed within project S.9 at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the Kplus — Program of the Austrian Ministry of Traffic, Innovation and Technology with the contributions by the University of Leoben, Graz University of Technology, Johannes Kepler University Linz, Joanneum Research ForschungsgmbH and Upper Austrian Research GmbH. The PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria.

Improved irradiance calculation

The incoming radiation at the site of the PV system has to be known to simulate the expected energy yield. This information is derived currently by measurements of the geostationary weather satellite Meteosat-7 (It will be changed to Meteosat-8 in the future.). These data have a high temporal resolution and a better spatial coverage than meteorological ground stations. The Meteosat images are received half-hourly operational at the University of Oldenburg. The Heliosat method (Hammer, 1999) is used to determine ground irradiance for a pixel with a resolution of 3 km x 5 km above the site of a PV system. Validations with ground stations showed that the error of this method can reach up to 30% for hourly values and lies between 5 and 10 % for monthly irradiance values (Hammer, 1999). These errors are strongly dependent on the weather situation (i. e. variability) and on the sun elevation angle. Under cloudy sky conditions the error increases while only small errors occur on days with clear skies. Larger errors also occur for low solar elevation angles and result in more deviations during wintertime. The quality assessment on these derived irradiance data and its dependencies is rather complex. Further details and explanations on the quality assessment will be published soon.

One further development to improve the irradiance calculation and to reduce the error is the introduction of a new model to calculate diffuse irradiance (Mueller et al, 2004).

Another way for improving the accuracy is the combination of the satellite data with ground measurements of irradiance with the geostatistical method kriging-of-the-differences. The error can be reduced this way to 3-4% for monthly values and also for hourly data (Betcke and Beyer, 2004).

These errors within the derived irradiance data have to be considered in the error detection routine. The exact knowledge of these errors and its dependencies is an absolutely necessary prerequisite for a variable definition of allowed differences between the expected and the real energy yield and therefore, for the exact determination of the occured malfunction.

Selection of the production factors

Before conducting the experiment, the knowledge of the product/process under investigation is of prime importance for identifying the factors likely to influence the outcome. In order to compile a comprehensive list of factors, the input to the experiment is generally obtained from all the people involved in the project with expert knowledge. In this case the main source of such knowledge was Dr Daniel Nuh of EETS, who designed and built the laminator used for module production.

The typical lamination cycle for a fast cure Solar EVA is illustrated in figure 4. First the Solar laminate is assembled from its individual layers and placed face down on the hot plate. The lid of the laminator is closed and the plate is held at a constant temperature. Under vacuum, the EVA is now subjected to a set time, TEVA. At point A, atmospheric air is allowed into the top of the vacuum divider sheet, which applies a pressure on the laminate pressing it down onto the hot plate. Here the laminate is left for a further set time, TPC. Finally the vacuum is switched off and the laminate removed for cooling.

The different variables or factors associated with the Lamination process are:

* Temperature, 0C — Operating Temperature

* Pressure, Torr — Vacuum pressure

* TEVA — Solar EVA Molten Time, mins.

* TPC — Post Curing Time, mins.

The pressure is a fixed factor throughout the cycle therefore it is excluded from the Design of Experiments. Therefore the Factors to be studied are:

* Temperature, 0C — Operating Temperature

* TEVA — Solar EVA Molten Time, mins.

*

.

— £

A к Ї

_ Tetra

‘ — у ^ "

Тре

— 1 :

Temperature, Deg C

Pressure, Torr

140

80

60

40

2:0

4 6 8 10 12 14 16 18 20 22 24 26 28 30

Lamination Time, minutes.

Platen temperature, Deg C.

Pressure bottom & top chambers, Torr.

Pressure top chamber, Torr. Pressure bottom chamber. Torr.

Figure 4: Typical lamination cycle for fast cure solar EVA

TPC — Post Curing Time, mins.

Comparing mature and immature design of sustainable products by its functionality and use

In comparing mature and immature design of sustainable products, appearance will evidently play a role as is their user friendliness, their usability and last but not least the way of PV modules implementation. In practice it turns out there will be a compromise between the appearance, its functionality, usability and utility. Where to draw the precise line will be a matter that can change at each case.

Up to now in PV powered products the main concern has just been the PV performance. However there is also a growing preference for additional comfort, convenience of use in other words improved user friendliness, or good ergonomically designed products and appealing designs are in demand. Depending on the user context as a result, this will have a direct impact on the shape and colour of the PV cells to be used.

In sustainable product design, the actual colour of the PV cells will become a less important factor since a dominant colour will on the long term limit the time a product is in vogue. This is for example demonstrated in the new PV roof projects in which the PV modules on the roof are built without being conspicuous, just resembling normal tiles. The main restriction is that the overall performance of the PV cells is not frustrated by its colour (see section 3.3)

1.1.3 Convenience of use, and appearance

Figure 3b: PV powered torch with curved PV surface

Fiaure 3a: PV Dowered torch

Convenience of use is demonstrated for instance for a product which has to fit nicely into the palm of your hand and therefore should not have sharp edges. In figure 3a [Solar — Home,2004] it can be seen that the rectangular and flat shaped PV module on the PV torch hampers the proper use mainly due to its sharp edges. In addition it is not aesthetically designed. This design ‘craves’ for PV panels with curved surfaces, which follow the cylindrical shape of the battery holder as can be seen in Figure 3b.

1.1.4 PV implementation and appearance

Figure 4b: PV (street) Lamp in use

Figure 4a: PV powered lamp

Comparing the PV powered lamp presented in Figure 4a with another lamp presented in Figure 4b namely a PV (streetlight) lamp designed in the framework of a master thesis project [Verkuijl 2000] one can observe the following differences:

In the first case, the PV module is just an add-on unit not well integrated into the overall design. In the second case the curved PV panels will function as Solar Cell on one side and simultaneously as light reflector of the generated light on the other side. A sound integration of the PV panel into the overall design is demonstrated. As a result of this integrated design, synergy between the energy conversion function and the energy use (light) is obtained. The energy storage was done at lamp location, introducing a mobile design concept. In addition the use of curved PV panels enhanced the aesthetics since this street lamp resembles a flying object. The latter design is more mature than the former one.

In both torch and lamp examples above the need of curved PV panels is evident. However one should be cautious to generalize such need to all PV powered products. A design rule could be that curved PV cells are to be used if and only if this feature introduces a new added value.

AR glass performance for PV applications

1.1 Evaluation strategy

Any attempt to measure slight differences in efficiencies (in the percent range) on complete modules is likely to be successful only if a large number of modules are monitored over long period of times. This is because it is far from trivial to prepare modules with similar initial efficiency (current mismatch between the cells, serial connections) and because it is not easy to measure complete modules before and after lamination. Ab-initio simulations of the optical properties of complete systems are also highly difficult, since several unknown parameters would have to be taken into account (surface structure of the cells, internal reflection at the backside of the cells, internal reflection inside the glass). Therefore we adopted the following experimental strategy to be able to predict as accurately as possible the efficiency gain in STC conditions (which will give the nominal gain in Wp of the module) and the total yearly energy yield (which is the quantity of interest for the operator of a PV system):

Selection of 24 standard multi-crystalline silicon solar cells with SiN antireflection layer, front screen-printed Ag and back Al contacts. Prior to measuring, all cells are tabbed with tinned-copper strings (normally used for cells interconnection)

• Measurement of the efficiency and of the short-circuit current Isc under STC (AM1.5g, 25°C), measurement of the spectral response and of the angular response of the cells. On this base, two similar groups of 11 cells are prepared. 2 cells are kept for calibration purpose.

• Individual encapsulation of each solar cell in mini-modules, in a standard Glass/EVA/Cell/EVA/Tedlar structure. The EVA foils are 1 mm thick and a curing temperature of 157°C was applied for 11 minutes. 11 Cells are prepared with the AR glass, 11 without.

• Repetition of the measurements with the mini-modules

• Preparation and outdoor monitoring of 2 modules (18 W each) with and without AR layer to evaluate temperature effects given by the aR layer

• Simulation of the expected yearly energy yields using a specially developed software (Real Reporting Conditions Rating [10]) using a meteorological database and taking into accounts all the parameters measured on the mini-modules.

System set-up

In many remote places like farms, households which are far away to the grid there is great need for electricity. Photovoltaic system offers possibilities to satisfying this need and they provide a free and clean energy source from the sun and the other hand resolve a social problem in this remote area. The investment cost to connecting this region where the households are dispersed to the log distance from each other, in especially in the mountain region is very high for example 10.000-12.500 Euro/km. Starting point in this study was the distance from the grid connection is 4-5 km. It would mean over 50.000 Euro investment in case of grid development. In this context for the electrification of the area one way is the stand alone PV system what can be extended with another energy sources and the second is the electrification by micro grid that is indicated for villages with 100-300 households. The present paper designs and simulated a stand-alone PV system developed for this remote area. The figure 4 shows the principal design of a photovoltaic system with alternating current output.

Figure 4. Principal design of a PV system with alternating current output

The PV system presented in figure 4 consists of 12 photovoltaic module (1) with a peak power 40Wp each and the type is Kyocera KC 40. The charge regulator (2) is C 40 Trace and the used inverter (3) is Piccolo 21. The produced energy is stored in a battery bank (4) and the type Rex Homas. The monitoring and the data logging are realised by the ENERPAC-data logging unit. At our latitude an energy supply based exclusively on photovoltaic requires large photovolatic generators due to the fluctuation is solar radiation. The same is true of photovolatic system that has to have great availability. Hence, a mixture of generator types is generally combined to from hybrid system. Combining PV generator and motor other wind generators ensures the same power security as in a public grid. In this way was design the system the component structure especially the battery bank and the monitoring equipment permit this enlargement of the system using another classical and RES sources, like wind hydro and fuel cells. In the future if the user want completely independent of fuel supplies an electrolyser and a hydrogen storage system can be integrated.

Parameter Studies — Variation of “к” and “n”

Till now we assumed active materials for our simulations which are already in use for the design of organic solar cells. Now, how would it affect the behavior of an organic

solar cell’s light absorption, if we expanded the range of optical parameters. Respectively, what kind of materials would be desirable for optimal power conversion and what optical parameter would provide the theoretical optimum of light absorption efficiency. Considering this we always kept in mind the practical relevance of used optical parameters concerning the producibility of the material. For each layer we have to determine the thickness di, the refractive index ni and the index of extinction Ki. That

leads to a parameter space with the dimension 3i for our optimization algorithms in the general case. For the four-layer-solar-cells we start with a 12-dimensional parameter space. Due to constant parameters of the ITO and the PEDOT layer the processing times for the optimization of the remaining six parameters could be kept in operable

limits. We figured out the optimum of the 6-dimensional hyper surface which provided the maximum of Aeff with the optimized parameters di, ni and к for the active layers. In order to gain a better understanding of the usefulness of the optimized parameters we varied both of them at a time in certain ranges while the others were fixed according to the optimized Aeff.

Fig. 7: Effective absorption in dependence on the refractive indices of the p-layer (n3) and the n-layer (n4) of a photovoltaic device

The figures 7-9 show Aett in dependence on the optical parameters of the active layers.

Fig. 8: Effective absorption in dependence on the index of extinction of the p-layer (кз) and the refractive index of the n-layer (n4) of a photovoltaic device

These graphs are intended to give a concept where to go searching for new solar cells’ materials. Appropriate optical parameters are not the only criterion for the improvement of solar cells materials but it will help us to estimate the energy absorption potential of new materials in a very straight way.

Results and Conclusions

Table 1 Energy produce indication for alternative scenarios

Installation

scenarios

Energy production from the PV system (kWh/year)

Minimum

Maximum

Average

Scenario 1

467,528

521,016

494,272

Scenario 2

475,061

531,324

503,192

Scenario 3

476,385

532,857

504,621

Table 1 shows the results for different installation scenarios. It can be seen that for all scenario, the energy production from the PV system is depended on the installation conditions. The scenario 3 shows the higher energy out put but when compared with other scenario it is a very small percentage. Scenario 3 is 0.28% (1,429 kWh/year) and 2.05% (10,349 kWh/year) higher than scenario 2 and 1 respectively.

This value only shows the pre-feasibility technical study of the project. It needs more detailed study such as financial study in further works. Also, for the details of engineering work from the actual site is a very important issue as well.

Acknowledgements

We thank Tesco Lotus, Ek-Chai Distribution System Co., Ltd., Thailand for their financial support. We are also grateful to several people for contributing invaluable input and advice during the preparation of this paper.

References

Bakos, G. C., Soursos, M., Tsagas, N. F., 2003. Technoeconomic Assessment of a Building-Integrated PV System for Electrical Energy Saving in Residential Sector. Energy and Building 35. 757-762.

IEA, 2001. PV System Installation and Grid-Interconnection Guidelines in Selected IEA countries. Report IEA PVPS T5-04:2001

IEA, 2002. Reliability Study of Grid Connected PV Systems: Field Experience and Recommended Design Practice. Report IEA-PVPS T7-08: 2002 PVS 2000 User Manual. PVS for Windows: Simulation and Sizing of Photovoltaic Systems. Econzept Energieplanung GmbH Freiburg Germany.