Category Archives: EuroSun2008-5

AN EXPERIMENTAL ANALYZE OF A SOLAR COLLECTOR

G. Zorer Gedik1* and A. Koyun2

1Yildiz Technical University, Architecture Faculty Istanbul /Turkiye
2Yildiz Technical University, Mechanical Engineering Fac., Istanbul /Turkiye,
Corresponding Author, gzorer@hotmail. com

Abstract

In this study the experimental results of a solar energy collector which is installed to a south classroom window of a school in Istanbul are presented. The collector unit had been tested experimentally and numerical to determine its thermal performance before its integration into the south window. In this paper, the experimental analyze will be given in detail. The collector is tested using infrared radiation lamps in the laboratory. The collector reaction to change in the value of heat transfer is measured. Air temperatures and velocities are measured at the bottom (air entry) and top (air exit) of the collector. Moreover, performance of the collector geometry is analyzed using Computing Fluid Dynamics (CFD). The results of the measurements and theoretical analysis are compared. The back face of the collector is insulated to generate effective convective air flow on the basis of the test results. Then thermal efficiency measurements were executed in the classroom and the efficiency curves displayed and evaluated. A computer system with a software was designed to obtain air temperatures, velocities and solar radiation data in the classroom.

Keywords: Solar heating, solar collector, experimental, collector geometry.

1. Introduction

A research project supported by TUBITAK (The Scientific and Technical Research Council of TUrkiye) is designed to improve the thermal efficiency of classroom design through the use of solar energy. [1] This paper presents a part of the results of the project. A solar energy collector was attached to a south classroom window of a school in Istanbul which has existing large classroom windows. The main components of the solar air collector are the glazing, the air space between the glazing and the collector plate, the aluminum collector profiles with air chambers and the insulated backing of the collector. (Figure 1) The function of the glazing is to admit as much solar radiation as possible and to restrict the transmission of heat radiation back through the glazing so that the optimum greenhouse effect results. Hence, the existing single glazed in front of the collector in the classroom changed the special glazing has ninety-two percent transmittance of light. [2]

The wavelength selective coating is applied to the front face of the collector profiles that have a low emmisivity of energy in the infrared wavelengths. Since infrared energy flow makes up a large part of the total energy loss of the system through the glazing, selective surfaces is quite effective in improving performance. [3]

image001
image002

Figure 1:The detail of solar collector. Figure 2. The profiles of solar collector.

Input-Output Controller (IOC)

The Input-Output Controller is a simulation based failure detection method available on the market since 2007. The first variant of the method monitors only the energy yields in the solar circuit. Furthermore, two temperature values in the storage are used as input for the simulation. A second approach also includes the buffer storage discharging. The IOC compares the daily measured and expected energy yields in the solar loop. The standard uncertainty (o) of the IOC-procedure, including measurements and yield calculation, is about 7 % (o). If the difference between measured and simulated yield is larger than 20 % (3 o) a fault is detected. This leads to a 99 % reliability for a correct fault prediction. Below a yield of 1.5 kWh/m2d the uncertainty margins are higher. There is a failure tree to establish if the fault occurred inside or outside the solar loop, and if it is for example the control or the solar station which causes the problems. The IOC is sold for 1190 € inclusive temperature and irradiance sensors, but without volume flow measurements. To be able to check the performance from home an extra data logger is necessary [9; 10].

3.3. Kassel University method (KU)

At Kassel University a failure detection method was developed, that combines a static algorithm based function control with dynamic simulation based failure detection [11]. The method consists of three steps. In the first step it is checked if too much data is missing due to data gaps and sensor defects. A minimum of 95 % of data points should be available to continue with the failure detection. In a second step a plausibility check is carried out, in which the correct operating of individual components is checked, similar to the approach used in [3]. The third step is a simulation based step in which the system is modelled with TRNSYS. Measured and simulated energy gains are compared at the heat exchanger for charging and or discharging the storage unit. If the difference is larger than the uncertainty margins on both sides an error is reported [12; 11].

Several failures were detected and partially identified. These were for example air in the collector field and a calcified heat exchanger. This approach is being further developed.

3.4. Guaranteed Solar Results (GRS)

In Guaranteed Result of Solar Thermal Systems the energy yield is guaranteed by the seller/builder of the system. Sophisticated measurement equipment is installed and monitors the system, costs for the measurement equipment and one year of operation are in the range of 10 k€. Daily averaged and monthly measured values are sent. Measured yearly energy yields are compared to simulations with f — chart, a simple simulation program, although also other simulation programs could be used. A comparison on a shorter basis is not possible, due to limitation of the simulation program. Large failures on a yearly basis can be detected; however failure analysis is not possible [13; 14].

Conclusion: Summary and Future Perspectives

A complete research and testing laboratory for scientific research in the field of solar energy utilization was built near Tripoli, Libya, for the CSES of the National Office for Research and Development by the general contractor, Bavaria Engineering GmbH, together with several project partners. TUV Rheinland Immissionsschutz und Energiesysteme GmbH, which recommended PSE AG as provider of the indoor/outdoor test stand for solar thermal collector testing, was project partner especially for the solar thermal test systems.

This paper describes the concept and realization of the test stand for solar thermal collectors. PSE AG and Fraunhofer ISE were able to use their years of experience with indoor and outdoor test stands and to develop the technology further.

PSE AG’s next project consists of the installation of a complete indoor test stand for solar thermal collectors at the French national institute for solar energy, INES, in the fall of 2008.

image174

Figure 5: Rendering of PSE s next generation of indoor test stands

Numerous innovations will also be realized in this test stand. Besides motor-driven lamp positioning, all test-stand-specific parameters will be centrally controlled and recorded so that tests can be easily documented and reproduced.

We are glad to be able to use our expertise for the realization of test stands for solar thermal collectors.

References

[1] Zahler, C.; Luginsland, F.; Haberle, A; Rommel, M.; Koschikowski, J.;

Fertigung und Installation eines Sonnensimulators fur das GREEN Labor an der Pontificia Universidade Catolica de Minas Gerais in Brasilien,

OTTI: 15. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2005 pp 462-467

[2] Haberle, A.; Berger, M.; Luginsland, F.; Zahler, C.; Rommel, M.; Baitsch, M.;Henning, H.-M.; Linear konzentrierender Fresnel-Kollektor fur Prozesswarmeanwendungen,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 185-190

[3] Haberle, A.; Luginsland, F.; Zahler, C.; Topor, A.; Reetz, C.; Apian-Bennewitz, P.;

Ein praziser undpreiswerter Sonnenstandssensor,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 144-149

Vertical fin ray trace analysis

The optical efficiency based on a surface reflectance measurements is 0.94, the gap between reflective surface and the absorber fin is 4 mm and the absorptance is 0.95. The first gap loss (green rays) is detected at an incident angle of 44 degrees which is depicted as a decrease in the optical efficiency seen in Fig. 6.

In Fig. 6 gap losses separate into roughly two ranges. A flat response occurs between 80 and 100 degrees and an abrupt efficiency drop occurs at 44 degrees. To show that the gap loss is the cause

of the optical efficiency drops, another simulation is run in which there is no gap loss. See Fig. 7. The graph depicts a rounded distribution with no abrupt jump in efficiency at any nominal angle.

To understand the nature of gap loss, the gap loss is plotted over the incident angles in Fig. 8. The gap losses are separated into roughly two levels.

image020

Fig. 12: Rays Striking the Horizontal Fin ICPC at a Nominal Angle of 150 Degrees.

image021

Fig. 13: Comparing Energy Efficiencies for Different Reflectances (Horizontal Fin ICPC).

Подпись: Fig. 11: Optical Efficiency (Horizontal Fin) from Incident Angles of 30 to 150. Подпись: Fig. 14: Comparing Efficiencies for Four Gaps of 0, 4, 6, and 10 mm (Horizontal Fin ICPC). Подпись: Fig.15: Laser and Sensor Assembly The reflectivity is now changed to 0.7 to achieve an optical efficiency curve that has a shape that is more of a dished appearance around the 90 degree incident angle. See Fig. 9.

Initial impacts of the implementation of the regulations on solar collectors

2.1. The education and training for project design professionals

Portuguese Civil Engineers are the legal responsible professionals for elaborate the water supply design projects. Until now they had to deal with simple equipments of hot water production but now they need to calculate a more complex and integrated system. After consulting a sufficient number of senior project design Civil Engineers they have responded that they are not very comfortable with this new technology and with all that is related with mechanical equipments. Traditional projects in the pass never forced them to know more about equipment subjects. Buildings are until now predominantly constructive but they are aware that intelligent and automatic buildings are a future inevitable reality. Despite there are some standard systems that function like a kit module, it is not sufficient to respond efficiently to the building design demand. In some cases, they are letting the design of solar collector system to be developed later by the installation firms with negative consequences on final building quality. Thus, it is absolutely essential that official institutions promote and stimulate even more training for these professionals. The process is been slow but some are making efforts to get training. Also, recent graduate Civil Engineers, who have just finishing theirs courses, faced almost the same problem mainly because many of the Institutions of Higher Education in Portugal still do not provide the required competencies on these subjects. In most of the courses there is still not an adequate and integrated group of curricular subjects that goes deep in these matters, providing the minimal competencies to accomplish sufficient professional practice in this area. Therefore, it would be necessary to make a great effort to introduce on the curricular course structures even more contents on those matters. And we can not forget other important aspect, as the firm’s know-how is still not very high there is always a tendency to charge more for the installation and maintenance operations and give not so qualified engineering consultant. Now, these firms must be prepared to correspond to a more informed attitude from design professionals.

Engineering and set-up of a new testing facility for solar thermal systems

At the Fraunhofer ISE a testing facility for solar domestic hot water systems (thermosiphon systems, systems with forced circulation, integral collector-storage systems) and heat storages is already in operation since 1997.

Now a new testing facility has been engineered to satisfy the growing demand for the tests on solar domestic hot water systems according EN 12976 [1]. This testing stand provides the possibility to perform tests of four different systems at the same time and independently.

The testing facility and its sensors fulfils very high testing and measuring requirements. The boundary conditions during testing can be handled in a far better way than in the previous test stand. This concerns, for example, the cold water inlet temperature during the tests and the ambient temperature of the storage tank.

The testing facility is designed for tests on complete solar heating systems and heat storages.

Model description

1.1. Validated models

Validated models are used for the parameter analyses [4]. The validation was carried out in 2007 with measurement data from the four different evacuated tubular solar collectors tested in a test facility at the Technical University of Denmark. In Table 1 the values used and determined in the validation process are shown.

Table 1. Parameters for the four solar collectors.

Parameter

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Collector type

Total gross area of the collectors [m2]

2.07

2.07

3.53

3.53

Transparent area of the collectors [m2]

1.38

1.38

2.25

2.25

Number of tubes [-]

8

8

20

20

Glass tube radius [m]

0.05

0.05

0.035

0.035

Tube centre distance [m]

0.120

0.120

0.09026

0.09026

Collector panel tilt [°]

67

67

67

67

Collector azimuth [°]

0

0

0

0

Incidence angle modifier for the diffuse radiation [-]

0.9

0.9

0.9

0.9

Effective transmittance — absorptance product, [-]

0.83

0.83

0.83

0.83

Number of discretizations for the absorber [-]

41

41

33

15

Angle dependence of tau-alpha product based on tangent equation [-]

3.8

3.8

3.8

3.8

Heat capacity of the fluid in the manifold tube [kJ/kgK]

3.8

3.8

3.8

3.8

Heat pipe length [m]

1.73

1.73

1.61

1.61

Absorber radius [m] / Absorber width [m]

0.041

0.085

0.027

0.056

Angle of strip [°]

164

164

Thickness of strip [mm]

0.47

0.47

0.60

0.60

Thermal conductivity of strip [W/mK]

238

238

238

238

Density of strip [kg/m3]

2700

2700

2700

2700

Heat capacity of strip [kJ/kgK]

0.896

0.896

0.896

0.896

Lowest evaporation temperature [°C]

15

15

15

15

Manifold heat exchange capacity rate [W/K]

10

10

10

10

Mass of the fluid inside the heat pipe [kg]

0.0038

0.0038

0.0038

0.0038

Effective heat capacity of the collector [kJ/K]

4.04

3.41

6.31

4.94

Validation of the mathematical model

For the validation of the developed mathematical model, the results of the DST tests of one thermosiphon system product line are used. Systems of this product line with different collector area and store volume were tested according to the DST method at the three laboratories CSTB, INETI and ITW. The results obtained from these tests were compared with the results that are obtained with the mathematical model. In Table 4, the test results based on the DST-test for the location of Athens and a hot water demand of 200 l/d are listed in the column fsol, DST.

In the same table, the results obtained with the mathematical model are depicted. The parameters of the different systems that have been entered in the program as “system tested” are defined in the line: “Input value fsol, DsT“. With the extrapolation tool DHWScale, the values fsol, calc are calculated for the remaining systems of the product line. In addition, the discrepancy between the calculated values of the solar fraction and the solar fractions obtained by the DST-method (Afsol = fSOl, DST — fsOi, caic ) and the relative error (srel) between both values is displayed. The relative error between the results obtained for fsol from the DST test and the calculation procedure is calculated by the following equation (5),

sol, DST

For the validation, three trials have been performed:

In the first trial (Trial 1) a thermosiphon system with Ac = 3.48 m2 and Vsto = 0.18 m3 (System no. 1) has been tested with the DST-method and a solar fraction of fsol, DST = 0.74 has been obtained. These values are now entered into the DHWScale program to extrapolate the results to systems of the same product line but different in size. For the systems 2 to 7 of the same product line the values of fsol can be found in the column “Trial 1, fsol, calc1”.

In a second trial it was assumed that a thermosiphon system with Ac = 3.96 m2 and Vst0 = 0.15 m3 (System no. 4) has been tested with the DST-method and a solar fraction of fsolDst = 0.70 has been obtained. Again, these values are entered in the DHWScale program. The results obtained for the other systems of the product line by means of extrapolation are listed in the column “Trial 2, fsol, calc1”.

For the third trial it is assumed that two thermosiphon systems (system 3 and system 7) have been tested with a DST-test. The parameters of both systems have been entered into the program as input data in order to obtain the solar fractions for the 5 remaining systems of the product line.

Table 4. Comparison of DST-results and the results from the mathematical model

DST

Trial 1

Trial 2

Trial 3

No

Ac

Vsto

fsol. DST

fsol, calc,1

A fsol / (ЄГЄі) [%]

fsol, calc,2

Afsol / (єГЄі) [%]

fsol, calc,3

Afsol / (єГЄі) [%]

System tested with DST

і

4

1 and 4

Input value fsol, DST

Ac = 3.48 Vsto = 0.18

fsol, DST = 0• 74

Ac = 3.96 Vsto = 0.15

fsol, DST = 0.70

Ac = 3.48; 3.96 Vsto = 0.18; 0.15

fsol, DST = 0. 74; 0.70

1

3.48

0.18

0.74

0.714

2.6 (3.5)

0.680

6.0 (8.1)

0.703

3.7 (5.0)

2

5.22

0.30

0.82

0.816

0.4 (0.5)

0.777

4.3 (5.2)

0.803

1.7 (2.1)

3

3.96

0.18

0.72

0.744

2.4 (3.3)

0.708

1.2 (1.7)

0.731

1.1 (1.5)

4

3.96

0.15

0.70

0.741

4.1( 5.9)

0.705

0.5 (0.7)

0.727

2.7 (3.9)

5

3.96

0.30

0.75

0.750

0.0 (0.0)

0.713

3.7 (4.9)

0.742

0.8 (1.1)

6

1.86

0.30

0.61

0.582

2.8 (4.6)

0.550

6.0 (9.8)

0.583

2.7 (4.4)

7

3.72

0.30

0.77

0.735

3.5 (4.5)

0.688

8.2 (10.6)

0.727

4.3 (5.6)

8

5.58

0.30

0.82

0.830

1.0 (1.2)

0.791

2.9 (3.7)

0.816

0.4 (0.5)

As can be seen from Table 4, also for the system tested with DST, there is a discrepancy between the solar fraction obtained with the DST-method and the solar fraction obtained with the mathematical model. This results from the fact that there is no equation fsol = f (A, Vto) which exactly matches the default value from the DST-method.

The maximum relative error for Trial 1 is 5.9 % , 10.6 % for the second trial and 5.6 % for the third trial. The mean error, defined with

є1 +є2 +… + Є8
8

is 2.25 % (Trial 1), 5.9 % (Trial 2) and 3.0 % (Trial3). For the other hot water demands of 110 l/d and 300 l/d comparable results are obtained.

3. Conclusion

An extrapolation procedure including a mathematical model has been developed which can be used for determination of the solar fraction of systems which are part of a product line. By means of an extrapolation procedure, input values for only one or a few systems have to be determined by physical testing. The developed procedure has been implemented in an EXCEL based software tool named DHWScale.

As the model is based on second order polynomials, results can be obtained within seconds. The mathematical model has the advantage that only a few system tests are necessary to determine the solar fraction for a whole product line. This approach can offer the possibility to reduce the time and cost necessary to obtain Solar Keymark certification of factory made solar domestic hot water systems. The validation of the program with one thermosiphon product line tested by DST showed promising results. Depending on the desired accuracy already one system test may be sufficient (relative error of about 11 %). If two system tests are performed the relative error drops to 6 %. However, up to-date only DST-results for one product line are available. For a more profound assessment of the developed procedure more product lines have to be tested with the DST-method.

In principle it is possible to extend the DHWScale program towards other system designs such as forced circulated DHW systems with or without integrated auxiliary heating. First experiences have been gained with forced circulated DHW systems but a validation is necessary before any reliable statement about the accuracy of the results can be made. It is also expected that for other system types further parameters have to be taken into account such as the influence of the auxiliary heated volume. This will envoke a huge number of additional TRNSYS simulations.

The DHWscale program is a first approach for the determination of system test results by means of an extrapolation procedure. In principle the methodology of this approach can be extended to additional parameters (e. g. locations, loads) and other system concepts.

References

/1/ H. Druck, S. Fischer, H. Muller-Steinhagen,

Solar Keymark Testing of Solar Thermal Products; Proceedings of ISES 2007 Solar World Congress, September 18 to 21, 2007, Beijing, China, ISBN 978-7-302-16146-2, Tsinghua University Press, Beijing and Springer-Verlag GmbH Berlin Heidelberg, CD: ISBN 978-7-89486-623-3

/2/ DIN EN 12976, „Thermal solar systems and components — Factory made systems — Part 2: Test methods”

/3/ ISO 9459-5:1995, “Solar Heating — Domestic water heating systems — Part 2: Outdoor test methods for system performance characterisation and yearly performance prediction of solar system.

/4/ Research and experimental Validation on the DST Performance test Method for solar Domestic Water Heaters, Final Report, Contract No. SMT4-CT96-2067

Evaluation of Optical Properties

For each exposed sample a transmission spectrum was measured; in the beginning utilizing a manually operated Zeiss-Spectrometer and since 1995 with computerized Fourier Transform Spectrometers (Bruker, IFS 66), all equipped with integrating spheres. These spectra were used for the calculation of the solar transmittances based on an AM 1.5 solar spectrum (ISO 9845). In order

to approve the comparability of the different instruments, measurements performed on unexposed reference plates in the year 1985 were compared with re-measurements of the same samples in 2005. The comparison of 36 measurements resulted in a deviation of only 0.2 ± 0.5 % (relative) confirming the good comparability of these measurements. To investigate the effect of soiling half of each sample was cleaned with mild soap and a soft sponge to be measured separately. Some of the samples were additionally cleaned with ethanol to get a better differentiation of soiling and degradation effects.

2. Results

An overview of the losses in solar transmittance of the tested materials after 20 years of exposure is given in Fig. 3. The losses reported after 20 years are lower than after 10 years of exposure because of heavy rainfalls in the run-up to the sample collection in 2005, which led to non­negligible cleaning effects. Multi-skin sheets and sinuous plates are not considered in this study. For these samples additional losses were observed as a consequence of the special form and not as a material property. Detailed analysis of the weathering properties of the different materials are presented as follows.

Table 3. Comparison of the losses in transmittance in Davos and Rapperswil after 20 years of exposure. For a better comparability multi-skin-sheets and sinuous plates made of PC and PMMA are not considered.

Material

#

Davos; losses in % (rel.)

After

Total

cleaning

Gain from cleaning in % (rel.)

Rapperswil; losses in % (rel.)

After

Total

cleaning

Gain from cleaning

Low Fe glass

8

-0.2-4.5

-0.6-2.9

0.7-2.1

6.3-17.3

0.1-8.8

4.8-8.3

Float glass

8

0.7-2.7

0.0-2.0

0.0-1.1

7.7-12.1

0.7-6.7

4.2-10.3

PMMA plate

6

-0.2-1.4

-1.2-0.5

0.7-1.2

7.3-8.6

0.6-1.7

7.0-8.3

PC plate

5

3.3-7.7

0.7-10.5

-2.8-2.3

6.7-15.0

2.5-9.8

4.2-8.0

ETFE

3

15.7-18.7

14.1-18.3

-2.5-4.5

13.7-24.8

-0.3-12.9

3.3-14.1

FEP

2

9.0

0.0

9.0

9.2-15.3

-0.2-0.0

9.2-15.1

PVF

1

-1.0

-2.6

1.6

12.9

-2.0

15.0

UP

3

26.8-41.7

36.7-43.2

PET, PVC and PC films

6

destroyed

destroyed

destroyed

destroyed

destroyed

destroyed

2.1. Glass

Modern collector glazings are mainly made of low Fe glasses because of their good durability and high transmittance. The influence of a structured surface for reflex reduction (prism-like structure in the dimensions of millimetre fractions) on the soiling property of such glasses was investigated in this study. In Fig. 1 the losses in transmittance of samples with a structured surface are compared to the losses of the samples with a smooth surface. No difference according to their total transmittance losses could be identified between these two groups; the structure did not lead to increased soiling.

Eight samples of iron containing float glass were included in this test. No clear difference in terms of losses in transmittance could be identified between samples with the tin-rich float side or the tin — poor side exposed (i. e. oriented to the ambient). For both glass types losses in transmittance in the range of 10 % (relative) were observed in Rapperswil after 20 years of exposure (without cleaning). After ten years of exposure the losses were even higher due to less rainfall in the time before sample collection. At the rural site of Davos only small losses in the range of some percent were observed over the whole test period. As shown in Figure 2 on the example of float glass, the cleaning with mild soap lowered the losses to about half of the initial value. A further amelioration was achieved by the cleaning with ethanol. A slight increase in transmission that was observed around the wavelength of 1 pm was probably caused by a photo induced oxidation of Fe2+ impurities [2].

For all flat PMMA samples good weathering properties were observed; the losses in transmittance were in the range of the losses of glass or even slightly better. Especially the cleaning with ethanol had a stronger effect compared to glass; see Fig. 2 and Fig 3. For the 20 year old samples cleaned with ethanol, close to no significant losses in transmittance were measured. On the other hand a loss in transmittance in the range of two percent (relative to the initial transmittance) was observed

for the 20 year old glass samples from Rapperswil.

Rapperswil

 

Davos

 

0 Uncleand x Soap О Ethanol

 

0

 

40 days 1 3 10 20 years

 

40 days 1 3 10 20 years

 

image106

image107

Fig. 3. Soiling and cleanability of PMMA; mean value and standard deviation of the six tested flat PMMA

types.

In the unexposed state the UV-absorbing effect of some protective additives was observed for all PMMA types but with different efficiencies (the full lines in Fig.4). For four of the six types this UV-blocking property remained over the whole 20 years of exposure, similar to the sample declared as ‘strong UV-absorbing’ in Fig. 4. They proved to be suitable for the use as UV-blocking layer to protect other materials. For the other two PMMA-types a continuous gain in transmittance in the UV-range was observed over the duration of exposure. This deterioration of the UV — blocking mechanism did not cause any other visible material degradation. For these samples even a slight gain in solar transmittance was observed due to the gain in the UV-region. But it is known from artificial weathering of PMMA that some photo-degradation in the form of chain braking exists and affects the mechanical properties [3].

PMMA no declaration concerning UV-absorption

image108

 

image109

0.3 0.35 0.4 0.45 0.5

Wavelenght in micrometer

 

Fig. 4. Changes in UV-blocking for different PMMA samples.

The two tested PMMA sinuous plates with glass fibre reinforcement suffered from large losses in transmittance caused by fissures in the polymer matrix similar to the UP samples (see 4.5. UP).

2.2. PC

For all tested PC-types material degradation was observed including yellowing, surface roughening and even biological infestation for some samples exposed in Rapperswil. PC degradation is caused by photo-Fries and photo-oxidation processes caused by UV-irradiation. These reactions are influenced by additional parameters as humidity and temperature resulting in a roughened layer of degraded material of variable thickness at the sample surface [4]. In this case of open exposure a macroscopic removal of material from the surface was observed and quantified. After 20 years the

loss in plate thickness was for all PC-types and at both exposure sites in the range 0.1 mm. In Fig.

Подпись: PC, Davos Fig. 5. Erosive material removal in terms of losses in the plate thickness of a PC sample exposed in Davos.

5 the loss in plate thickness as a function of the exposure time is shown for one sample, which was exposed in Davos. This figure shows that the material removal started shortly after three years of exposure, which coincides with a lifetime estimation of three years determined by Ram et al. [5]. The exact loss in transmittance due to material degradation could not be determined as soiling can not be cleaned without irritating the weathered surface. A good estimation can be taken from the total losses in Davos, where for other materials as glass or PMMA the contribution of soiling to the total losses in solar transmittance was small. Despite of a visible yellowing the solar transmittance only decreased by 3.3-7.7 % of which a small part is still caused by soiling. For the tested PC-types the cleaning effect of heavy rain in the period before of the 20 years sample collection was more pronounced than for other materials. An elevated gain in transmittance between 10 and 20 years of exposure is caused by the fact that not only soiling but also peaces of weathered polymer were removed from the surface by rainfall. A regain in transmittance for long exposure times was also observed in artificial weathering tests by Tjandraatmadja et al. [4] who identified photo-bleaching of the yellowed layer to be the cause of this effect. The PC films (thickness 0.375 mm) were mechanically destroyed as a consequence of material degradation.

4.4. Fluoropolymers

All fluoropolymers suffered from unexpected high losses in transmittance. In the case of PVF and FEP these losses were only caused by soiling; by cleaning with ethanol the initial transmittance could be recovered even after 20 years of exposure, see Fig. 6. The losses from soiling were in the range of ten percent or more, even in Davos where comparable losses of glass or PMMA were in the range of only one to two percent. For collectors with FEP or PVF glazing a regular cleaning is important to reduce losses in efficiency caused by soiling. For ETFE the effect of cleaning was small (exception: sample No. 1 from Rapperswil). The losses of these samples are caused by persistent soiling or actual material degradation. In contrary to FEP and PVF, this polymer contains unfluorinated ethylene which could serve as point of attack for degradation mechanisms.

Fig. 6. Overview of the transmittances of all tested fluoropolymer sheets. The values of the ethanol cleaned
20 years old samples are compared to the values of the different soiled samples.

4.5. UP

All tested UP samples were reinforced with glass fibres. Similar to the reinforced PMMA plates, these samples suffered from high losses in transmittance. Fissures arise from dilatation differences of the UP matrix and the glass fibres due to high temperature changes or water accumulation [6]. Near the surface the UP matrix broke and the fibres poked out of the surface. This roughened surface lead to increased accumulation of soil. On the samples which were exposed in Rapperswil additional biological infestation was observed after 10 and 20 years of exposure.

Situation of European standards and quality assurance today

Table 1 gives an overview over the important currently valid European and International Standards which should ensure the quality of solar thermal collectors and PV-modules articulated in application area, standard and short description of their content.

Table 1. European and International standards to ensure the quality of solar thermal collectors and PV-modules.

Application Area

Standard

Short description

Solar thermal energy

EN12975-1,2:2006

European standard:

efficiency and durability test of solar

thermal collectors

AS/NZS 2712:2007

Australian standard:

Efficiency and durability test of solar thermal collectors

Photovoltaics

IEC 61215: 2005-4

International standard: Efficiency and durability test of PV-modules

ASTM E 1038-05

International standard:

Standard test method for determining resistance of photovoltaic modules to hail by impact with propelled ice balls

Tests of solar thermal collectors and PV-modules according to the valid standards and regulations by independent laboratories should guarantee the quality standard related to the state of the technology, mainly to ensure the continuous growth in order to make a contribution to the sustainable energy supply. Furthermore such tests should ensure the continuous development and should sharpen up the transparency of the European and International market for the consumer. Essential conditions to reach these aims are the general performance of the mandatory tests of all solar thermal and PV-modules in the run up to the market entrance. Furthermore useful, which means to the respective state of the technology and the environmental conditions well adjusted requirements within the different standards. The necessary permanent amendment of the standards is not always able to fulfill these requirements, because the process of the amendment always takes a long time and furthermore is subjected to totally different conflicts of interests between manufacturers, certifiers and political framing conditions.

Under a closer consideration e. g. of the development of the European standard EN12975-1.2:2006 we will see, that the reliability test to check the impact resistance is, unlikely to older versions, no longer an obligatory test, even though severe hailstorms in Europe in recent years definitely increased. Concerning this matter the amendment of the standard does not reflect the requirements resulting from real environmental conditions. Also the appliance of other EU-Standards which harmonize the building shell e. g. for roof lights, for the quality assurance of solar energy systems, is not simply possible. By the reason of different requirements concerning the functionality, formulations like “the choice of the used materials should take into account the risk of hailstorms” are not transferable. For this account it is also not possible to transfer the results from other studies performed up to now which describe the impact resistance against hailstorms of building. Additionally, today we have to think about the standardisation and adaption of testing procedures for a wider distribution of different technologies in the field of solar energy systems.