- Email: [email protected]
Durability of solar energy materials
Renewable Energy 24 (2001) 597–607 www.elsevier.nl/locate/renene
Durability of solar energy materials M. Koehl
*
Fraunhofer Institut fu¨r Solare Energie Systeme, ISE, Oltmannstrabe 5, D-79100 Freiburg, Germany
Abstract A general test procedure for qualification of solar absorber surface durability has been developed based on the results of a comprehensive case study performed within the framework of the IEA Solar Heating and Cooling Programme Task X. It was assumed, in the development of the qualification procedure, that the intended use of the absorber surface to be qualified, was in single glazed flat plate solar collectors for domestic hot water production. The absorber surface should be considered qualified if it met the requirement of a design service life of 25 years with maximum loss in the optical performance of the absorber surface corresponding to a 5 % relative reduction in the performance of a solar domestic hot water system. The testing procedure, consisting of three kinds of constant load accelerated life-time tests, was limited to simulation of the following kinds of absorber surface degradation processes: a) high temperature degradation, e.g. oxidation, b) degradation by the action of moisture or condensed water on the absorber surface, e.g. hydration or hydrolysis. To quantify expected environmental stress on the absorber surface related to the environmental factors of interest, microclimate data, representing typical service conditions for absorbers in single-glazed flat plate collectors for domestic hot water production were used. 2001 Published by Elsevier Science Ltd. Keywords: Solar absorber surface; Durability; Test procedure; Qualification; Service life assessment
1. Introduction The performance of materials for solar energy conversion has reached a remarkably high level during the last decades. A well-defined state of performance testing was achieved, too. The confidence in the reliability of innovative materials and components, however, could still be improved. Standard tests are missing, since conventional corrosion and durability test procedures do not take into account the special
* Tel.: +49-761-4016682; fax: 49-761-4016681. E-mail address: [email protected] (M. Koehl). 0960-1481/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 0 4 6 - 5
598
M. Koehl / Renewable Energy 24 (2001) 597–607
requirements and loads for materials applied for solar energy conversion. A suitable methodology for a service life testing was developed in the framework of the Solar Heating and Cooling Programme of the International Energy Agency and applied to solar absorber coatings. The results are presented in this paper as an example of the way in which to develop a suitable durability test procedure for solar materials. This methodology is now transferred to other materials and components used in ventilated flat plate collectors. The heart of a solar collector (Fig. 1) is the absorber coating, because it is the most important part regarding the performance. It should be designed as an optical coating, which absorbs as much as possible from the incoming solar radiation (in the spectral wavelength range from 0.3 µm to 3 µm). Moreover, it should not lose energy by emitting thermal radiation in the infrared region according to Planck’s law. These losses depend on the surface temperature (in the power of four) and the spectrally depending emittance in the IR (3 µm–50 µm) of the coating system. An optimised coating, showing a high absorptance in the solar range and a low emittance in the thermal range, is called a spectrally selective solar absorber coating (see Fig. 2). A common way to achieve this is the deposition of a coating that absorbs the solar irradiation and transmits the thermal radiation, on a IR-reflecting metal substrate or metal coating. The absorber coating has to be relatively thin (between 0.15 µm and 1 µm). Therefore, its durability and the corrosion protection of the substrate is a very important issue. A large variety of coatings have been developed and new products produced with modern vapour deposition technologies are coming onto the market. Therefore, it was necessary to provide accelerated durability tests designed for these materials in their specific application to enable lifetime assessment during the development phase and not to prevent novel products by applying inadequate standard weathering tests. 2. Methodology In adopting commonly used methodologies for service life prediction the following major steps were addressed:
Fig. 1.
Schematic drawing of a flate plate solar collector.
M. Koehl / Renewable Energy 24 (2001) 597–607
599
Fig. 2. Spectra of a selective solar absorber coating, the solar irradiation and the Planck-function for 373 K.
1. 2. 3. 4. 5. 6. 7.
performance requirements stress analysis accelerated indoor testing failure analysis modelling for service life prediction validation of the model design of a standard test procedure
These steps are described in the following sections. Two types of absorber coating were used for a case study: 앫 electroplated black chromium on stainless steel and on electroplated nickel on copper sheets. These coatings consist of a mixture of metallic chromium and chromium oxide (Cr2O3) with a sub-micron surface roughness; 앫 nickel-pigmented anodised aluminium. Nickel is electroplated into the pores of the alumina. The pores are filled up only partly. The empty upper part of the pores forms an anti-reflective layer. These coatings were exposed outdoors in monitored simulated operation conditions and used as samples for the accelerated testing.
600
M. Koehl / Renewable Energy 24 (2001) 597–607
3. Performance analysis The performance of a solar absorber coating is described by the solar absorptance and the thermal emittance e. These properties are evaluated by a weighted integration of the spectral hemispherical reflectance of the coatings:
冕 ⬁
aS⫽
r(l)·Sl dl Sl
(1)
0
冕 ⬁
e(T)⫽
r(l)·Pl(T) dl Pl(T)
(2)
0
where Sl is the spectral solar irradiance according to ASTM E 891 Table 1 and Pl(T) the Planck function for a black-body radiator at temperature T, which is set to 373 K in the following. The absorber is part of a domestic hot water system. The performance of such a system depends on aS and e which can be considered as independent variables and a number of other parameters, which are kept constant for comparison. The solar irradiation is taken into account by using a test reference year for a given location as input of a system simulation computer program. Two alternative systems were modelled with variable aS and 앫 a Canadian system running in Toronto for preheating at low temperature levels; 앫 a Swiss system running in Zu¨ rich at a high temperature level. The results were correlations between the heat gain and aS and e in the interesting ranges (0.8⬍aS⬍0.98 and 0.1⬍e⬍0.3). But the properties are weighted differently: at the lower temperatures the weight of e is only a quarter of aS, while it is about 0.5 at the higher temperature levels. Therefore, the impact of changes of aS and e ageing on the system performance P can be described as: ⌬PL⫽⫺⌬as⫹0.25⌬e
(3)
⌬PH⫽⫺⌬as⫹0.5⌬e
(4)
or with ⌬as=as(0)⫺as(t) and ⌬e=e(0)⫺e(t) (note that degradation decreases and increases). The degradation of these properties is a steady function over ageing time and no catastrophic failures (sudden death) are expected. Therefore, a degradation limit has to be defined. It was concluded that a maximum decrease of 5% of the solar heat again due to the degradation of the absorber coating could be accepted, yielding the performance criteria PC: PCL⫽⫺⌬as⫹0.25⌬eⱕ0.05
(5)
M. Koehl / Renewable Energy 24 (2001) 597–607
PCH⫽⫺⌬as⫹0.50⌬eⱕ0.05
601
(6)
4. Stress analysis The absorbers are irradiated by nearly the whole solar spectrum, since they are usually covered by highly transparent iron-free glazing which transmits UV as well. UV could be one stress factor. But most of the coatings have no sensitivity to UV, since they consist of inorganic materials. Therefore, UV was not considered. The coating is heated by the absorber light. The maximum temperature is reached when the heat is not removed by the heat transfer fluid (e.g. during the installation, vacancies, failures), the so-called stagnation conditions. Modern devices can reach about 200°C. The duration of the temperature load depends also on the varying solar irradiation. Therefore, statistics based on long-term monitoring of the surface temperature during operation and stagnation are needed. The frequency distribution (Fig. 3) shows the time periods spent by four different collectors (averaged) during 11 months of operation and 1 month of stagnation during summer in Switzerland (Fig. 3). Another important stress factor is moisture, since the collector cases are not tight, and the absorber temperature drops below the dew point in the case of radiative cooling during clear nights. Humidity sensors between absorber and glazing were used to determine the microclimate around the absorber by monitoring relative humidity and temperature for 1 year. Comparison with real-time degradation of materials showed that for this case all values above 99% RH contribute to the ‘time of wetness’, which is relevant for moisture degradation (Fig. 4). The impact of SO2 as atmospheric pollutant was investigated by the use of metal
Fig. 3.
Temperature load distribution.
602
M. Koehl / Renewable Energy 24 (2001) 597–607
Fig. 4.
Time of wetness during 1 year.
coupons of Cu, Fe, Zn in the microclimate. The corrosion rate was determined by EDX and gravimetrically and compared with those in the test chambers.
5. Accelerated indoor testing The stress factors were applied in constant load tests separately (as far as possible). The tests were interrupted for measurements of the optical properties after given time periods in order to assess the progress of degradation. The temperature tests were carried out in circulating air ovens, in the temperature range between 300°C and 600°C because radiative heating is problematic with spectrally selective coatings. The moisture tests were performed in climatic cabinets set to 95% RH at different levels between 17°C and 90°C. The samples were mounted electrically isolated on cooled sample holders with a tilt angle of 45°. The sample temperature was kept 5 K below the cabinet temperature providing permanent condensation on the sample surface. The degradation of nickel-pigmented anodised aluminium by moisture is shown in Figs. 5 and 6 as an example. The sulphur dioxide tests were based on ISO 10062 ‘Corrosion tests in artificial atmosphere at very low concentrations of polluting gases’. The SO2 concentration was kept at 1 ppm in N2 while the temperature was 20°C, 50°C or 70°C at 95% RH. Detailed results can be found in Ref. 4.
6. Failure analysis Infrared spectroscopy (Fig. 5), EDX and Auger-electron-spectroscopy were used for failure analysis. The high temperature degradation was caused by oxidation pro-
M. Koehl / Renewable Energy 24 (2001) 597–607
603
Fig. 5. Changes of the spectral reflectance of nickel-pigmented anodised aluminium due to moisture degradation.
Fig. 6. Changes of the performance criterion of nickel-pigmented anodised aluminium during condensation tests at various constant temperatures.
cesses. The metallic part of the coatings decreased and the absorptance accordingly. Black chrome is not sensitive to moisture, but the anodised aluminium showed a severe degradation due to formation of aluminium hydroxide because of the open pores. In the presence of sulphur dioxide another degradation mode could be observed. Electrochemical corrosion of the nickel results in the formation of nickel sulphate.
604
M. Koehl / Renewable Energy 24 (2001) 597–607
7. Modelling for service life prediction All described degradation modes are first order chemical processes combined with diffusion processes. Their kinetics are temperature-dependent according to Arrhenius’s law. In most cases one process was dominating. Therefore, it should be possible to describe the acceleration by temperature enhancement with an Arrhenius factor containing only one unknown parameter: the activation energy for the degradation process. The acceleration factor a (T1, T2) is the ratio of the time periods which are needed to yield the same degradation at different temperature levels
冉
t1 ⌽ a(T1,T2)⫽ ⫽exp ⫺ (T −1 ⫺T −1 2 ) t2 R 1
冊
(7)
where R is the Boltzmann constant. The activation energy f can be determined from the slope of the function a(T) versus 1/T, the so-called Arrhenius plot, if all data points can be fitted by a straight line. In this case the assumption of one dominating process is verified. In the other cases steps may indicate phase transitions in the investigated temperature range or changes of the slope show different degradation processes and prevent the application of this model. Monotonous non-linear curves recommend the application of more sophisticated models, like Eyring, which can be found in the literature. The comparison with real ageing fluctuating loads is difficult because of this nonlinear temperature dependence which depends on the degradation process and the materials. A practical way is the integration of the load profiles (Figs. 3 and 4) by transforming all temperature intervals to a given reference temperature using Eq. (7):
冕
Tmax
tref(Tref)⫽
Tmin
冉 冉 冊 冊
⌽ 1 1 t(r)exp ⫺ ·dT ⫺ R T Tref
(8)
tref is the time period at temperature Tref yielding the same degradation as the fluctuating loads together for the first process with the activation energy f. The effective mean temperature is defined as the constant temperature resulting in the same degradation in the same time period as the fluctuating loads. Teff can be calculated by use of Eqs. (7) and (8) for any range of activation energies for a given load profile. Eq. (7) enables the calculation of a corresponding accelerated ageing test at any test temperature yielding the same degradation as the real operation for a known activation energy f providing the validity of the assumptions. The test results can be transformed to a master degradation progress at a given temperature (Fig. 7), if the activation energy was determined. The transformation of the effective mean temperature calculated from a load profile which was normalised to the design service life time (25 years for solar absorber coatings) to the same reference temperature indicates the life time. If the performance criterion is reached earlier, the tested coating is not durable enough.
M. Koehl / Renewable Energy 24 (2001) 597–607
605
Fig. 7. Time-transformed data from Fig. 6.
8. Validation of the model A direct validation of this life time estimation is usually not possible, if the estimated life time exceeds the average duration of a research project. In our case study the shortest life time was about 11 years. During the project we could monitor the loads and measure the degradation of the coatings for 3 years. Changes in the performance up to 4% could be detected. A comparison with coatings which have been in operation for more than 10 years showed clear similarities to the ageing phenomena seen after accelerated tests at the performance limit [1] (Fig. 8).
Fig. 8. FT-IR spectra of degraded absorbers coated with nickel-plated anodised aluminium (non-aged samples show no absorption bands above 3 µm).
606
M. Koehl / Renewable Energy 24 (2001) 597–607
Fig. 9. Testing time periods for accelerated temperature and condensation constant loads corresponding to 25 years life time as a function of the activation energies.
9. Design of a standard test procedure The respective accelerated tests for a service life of 25 years were calculated on the basis of the Arrhenius-model for the degradation kinetics of the load distributions shown in Figs. 3 and 4 (Fig. 9). A qualification test procedure was developed (ISO CD 12 952.2) and tested [2] which abandons the determination of process parameters like the activation energy, in order to get a quick and not too costly life-test. The procedure is based on an Arrhenius model with rough estimation, whether the activation energy is high enough to ensure a sufficient durability of the absorber coating at natural working conditions in flat plate solar collectors [3] (Fig. 10).
Fig. 10.
Schematic diagram of the qualification test procedures proposed to ISO.
M. Koehl / Renewable Energy 24 (2001) 597–607
607
Acknowledgements The author would like to express his thanks to all the participating material experts in Task X Solar Materials Research and Development and the Solar Thermal Materials Working Group of the IEA Solar Heating and Cooling Programme.
References [1] Carlsson B, Mo¨ ller K, Frei U, Brunold S, Ko¨ hl M. Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems. Solar Energy Mater Solar Cells 2000;61:223–38. [2] Brunold S, Frei U, Carlsson B, Mo¨ ller K, Ko¨ hl M. Round robin on accelerated life testing of solar absorber surface durability. Solar Energy Mater Solar Cells 2000;61:239–53. [3] Carlsson B, Mo¨ ller K, Ko¨ hl M, Frei U, Brunold S. Qualification test procedure for solar absorber surface durability. Solar Energy Mater Solar Cells 2000;61:255–75. [4] Carlsson B, Frei U, Ko¨ hl M, Mo¨ ller K. Accelerated life testing of solar energy materials. Report of Task X Solar Materials Research and Development. International Energy Agency Solar Heating and Cooling Programme, 1994.
Durability of solar energy materials M. Koehl
*
Fraunhofer Institut fu¨r Solare Energie Systeme, ISE, Oltmannstrabe 5, D-79100 Freiburg, Germany
Abstract A general test procedure for qualification of solar absorber surface durability has been developed based on the results of a comprehensive case study performed within the framework of the IEA Solar Heating and Cooling Programme Task X. It was assumed, in the development of the qualification procedure, that the intended use of the absorber surface to be qualified, was in single glazed flat plate solar collectors for domestic hot water production. The absorber surface should be considered qualified if it met the requirement of a design service life of 25 years with maximum loss in the optical performance of the absorber surface corresponding to a 5 % relative reduction in the performance of a solar domestic hot water system. The testing procedure, consisting of three kinds of constant load accelerated life-time tests, was limited to simulation of the following kinds of absorber surface degradation processes: a) high temperature degradation, e.g. oxidation, b) degradation by the action of moisture or condensed water on the absorber surface, e.g. hydration or hydrolysis. To quantify expected environmental stress on the absorber surface related to the environmental factors of interest, microclimate data, representing typical service conditions for absorbers in single-glazed flat plate collectors for domestic hot water production were used. 2001 Published by Elsevier Science Ltd. Keywords: Solar absorber surface; Durability; Test procedure; Qualification; Service life assessment
1. Introduction The performance of materials for solar energy conversion has reached a remarkably high level during the last decades. A well-defined state of performance testing was achieved, too. The confidence in the reliability of innovative materials and components, however, could still be improved. Standard tests are missing, since conventional corrosion and durability test procedures do not take into account the special
* Tel.: +49-761-4016682; fax: 49-761-4016681. E-mail address: [email protected] (M. Koehl). 0960-1481/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 0 4 6 - 5
598
M. Koehl / Renewable Energy 24 (2001) 597–607
requirements and loads for materials applied for solar energy conversion. A suitable methodology for a service life testing was developed in the framework of the Solar Heating and Cooling Programme of the International Energy Agency and applied to solar absorber coatings. The results are presented in this paper as an example of the way in which to develop a suitable durability test procedure for solar materials. This methodology is now transferred to other materials and components used in ventilated flat plate collectors. The heart of a solar collector (Fig. 1) is the absorber coating, because it is the most important part regarding the performance. It should be designed as an optical coating, which absorbs as much as possible from the incoming solar radiation (in the spectral wavelength range from 0.3 µm to 3 µm). Moreover, it should not lose energy by emitting thermal radiation in the infrared region according to Planck’s law. These losses depend on the surface temperature (in the power of four) and the spectrally depending emittance in the IR (3 µm–50 µm) of the coating system. An optimised coating, showing a high absorptance in the solar range and a low emittance in the thermal range, is called a spectrally selective solar absorber coating (see Fig. 2). A common way to achieve this is the deposition of a coating that absorbs the solar irradiation and transmits the thermal radiation, on a IR-reflecting metal substrate or metal coating. The absorber coating has to be relatively thin (between 0.15 µm and 1 µm). Therefore, its durability and the corrosion protection of the substrate is a very important issue. A large variety of coatings have been developed and new products produced with modern vapour deposition technologies are coming onto the market. Therefore, it was necessary to provide accelerated durability tests designed for these materials in their specific application to enable lifetime assessment during the development phase and not to prevent novel products by applying inadequate standard weathering tests. 2. Methodology In adopting commonly used methodologies for service life prediction the following major steps were addressed:
Fig. 1.
Schematic drawing of a flate plate solar collector.
M. Koehl / Renewable Energy 24 (2001) 597–607
599
Fig. 2. Spectra of a selective solar absorber coating, the solar irradiation and the Planck-function for 373 K.
1. 2. 3. 4. 5. 6. 7.
performance requirements stress analysis accelerated indoor testing failure analysis modelling for service life prediction validation of the model design of a standard test procedure
These steps are described in the following sections. Two types of absorber coating were used for a case study: 앫 electroplated black chromium on stainless steel and on electroplated nickel on copper sheets. These coatings consist of a mixture of metallic chromium and chromium oxide (Cr2O3) with a sub-micron surface roughness; 앫 nickel-pigmented anodised aluminium. Nickel is electroplated into the pores of the alumina. The pores are filled up only partly. The empty upper part of the pores forms an anti-reflective layer. These coatings were exposed outdoors in monitored simulated operation conditions and used as samples for the accelerated testing.
600
M. Koehl / Renewable Energy 24 (2001) 597–607
3. Performance analysis The performance of a solar absorber coating is described by the solar absorptance and the thermal emittance e. These properties are evaluated by a weighted integration of the spectral hemispherical reflectance of the coatings:
冕 ⬁
aS⫽
r(l)·Sl dl Sl
(1)
0
冕 ⬁
e(T)⫽
r(l)·Pl(T) dl Pl(T)
(2)
0
where Sl is the spectral solar irradiance according to ASTM E 891 Table 1 and Pl(T) the Planck function for a black-body radiator at temperature T, which is set to 373 K in the following. The absorber is part of a domestic hot water system. The performance of such a system depends on aS and e which can be considered as independent variables and a number of other parameters, which are kept constant for comparison. The solar irradiation is taken into account by using a test reference year for a given location as input of a system simulation computer program. Two alternative systems were modelled with variable aS and 앫 a Canadian system running in Toronto for preheating at low temperature levels; 앫 a Swiss system running in Zu¨ rich at a high temperature level. The results were correlations between the heat gain and aS and e in the interesting ranges (0.8⬍aS⬍0.98 and 0.1⬍e⬍0.3). But the properties are weighted differently: at the lower temperatures the weight of e is only a quarter of aS, while it is about 0.5 at the higher temperature levels. Therefore, the impact of changes of aS and e ageing on the system performance P can be described as: ⌬PL⫽⫺⌬as⫹0.25⌬e
(3)
⌬PH⫽⫺⌬as⫹0.5⌬e
(4)
or with ⌬as=as(0)⫺as(t) and ⌬e=e(0)⫺e(t) (note that degradation decreases and increases). The degradation of these properties is a steady function over ageing time and no catastrophic failures (sudden death) are expected. Therefore, a degradation limit has to be defined. It was concluded that a maximum decrease of 5% of the solar heat again due to the degradation of the absorber coating could be accepted, yielding the performance criteria PC: PCL⫽⫺⌬as⫹0.25⌬eⱕ0.05
(5)
M. Koehl / Renewable Energy 24 (2001) 597–607
PCH⫽⫺⌬as⫹0.50⌬eⱕ0.05
601
(6)
4. Stress analysis The absorbers are irradiated by nearly the whole solar spectrum, since they are usually covered by highly transparent iron-free glazing which transmits UV as well. UV could be one stress factor. But most of the coatings have no sensitivity to UV, since they consist of inorganic materials. Therefore, UV was not considered. The coating is heated by the absorber light. The maximum temperature is reached when the heat is not removed by the heat transfer fluid (e.g. during the installation, vacancies, failures), the so-called stagnation conditions. Modern devices can reach about 200°C. The duration of the temperature load depends also on the varying solar irradiation. Therefore, statistics based on long-term monitoring of the surface temperature during operation and stagnation are needed. The frequency distribution (Fig. 3) shows the time periods spent by four different collectors (averaged) during 11 months of operation and 1 month of stagnation during summer in Switzerland (Fig. 3). Another important stress factor is moisture, since the collector cases are not tight, and the absorber temperature drops below the dew point in the case of radiative cooling during clear nights. Humidity sensors between absorber and glazing were used to determine the microclimate around the absorber by monitoring relative humidity and temperature for 1 year. Comparison with real-time degradation of materials showed that for this case all values above 99% RH contribute to the ‘time of wetness’, which is relevant for moisture degradation (Fig. 4). The impact of SO2 as atmospheric pollutant was investigated by the use of metal
Fig. 3.
Temperature load distribution.
602
M. Koehl / Renewable Energy 24 (2001) 597–607
Fig. 4.
Time of wetness during 1 year.
coupons of Cu, Fe, Zn in the microclimate. The corrosion rate was determined by EDX and gravimetrically and compared with those in the test chambers.
5. Accelerated indoor testing The stress factors were applied in constant load tests separately (as far as possible). The tests were interrupted for measurements of the optical properties after given time periods in order to assess the progress of degradation. The temperature tests were carried out in circulating air ovens, in the temperature range between 300°C and 600°C because radiative heating is problematic with spectrally selective coatings. The moisture tests were performed in climatic cabinets set to 95% RH at different levels between 17°C and 90°C. The samples were mounted electrically isolated on cooled sample holders with a tilt angle of 45°. The sample temperature was kept 5 K below the cabinet temperature providing permanent condensation on the sample surface. The degradation of nickel-pigmented anodised aluminium by moisture is shown in Figs. 5 and 6 as an example. The sulphur dioxide tests were based on ISO 10062 ‘Corrosion tests in artificial atmosphere at very low concentrations of polluting gases’. The SO2 concentration was kept at 1 ppm in N2 while the temperature was 20°C, 50°C or 70°C at 95% RH. Detailed results can be found in Ref. 4.
6. Failure analysis Infrared spectroscopy (Fig. 5), EDX and Auger-electron-spectroscopy were used for failure analysis. The high temperature degradation was caused by oxidation pro-
M. Koehl / Renewable Energy 24 (2001) 597–607
603
Fig. 5. Changes of the spectral reflectance of nickel-pigmented anodised aluminium due to moisture degradation.
Fig. 6. Changes of the performance criterion of nickel-pigmented anodised aluminium during condensation tests at various constant temperatures.
cesses. The metallic part of the coatings decreased and the absorptance accordingly. Black chrome is not sensitive to moisture, but the anodised aluminium showed a severe degradation due to formation of aluminium hydroxide because of the open pores. In the presence of sulphur dioxide another degradation mode could be observed. Electrochemical corrosion of the nickel results in the formation of nickel sulphate.
604
M. Koehl / Renewable Energy 24 (2001) 597–607
7. Modelling for service life prediction All described degradation modes are first order chemical processes combined with diffusion processes. Their kinetics are temperature-dependent according to Arrhenius’s law. In most cases one process was dominating. Therefore, it should be possible to describe the acceleration by temperature enhancement with an Arrhenius factor containing only one unknown parameter: the activation energy for the degradation process. The acceleration factor a (T1, T2) is the ratio of the time periods which are needed to yield the same degradation at different temperature levels
冉
t1 ⌽ a(T1,T2)⫽ ⫽exp ⫺ (T −1 ⫺T −1 2 ) t2 R 1
冊
(7)
where R is the Boltzmann constant. The activation energy f can be determined from the slope of the function a(T) versus 1/T, the so-called Arrhenius plot, if all data points can be fitted by a straight line. In this case the assumption of one dominating process is verified. In the other cases steps may indicate phase transitions in the investigated temperature range or changes of the slope show different degradation processes and prevent the application of this model. Monotonous non-linear curves recommend the application of more sophisticated models, like Eyring, which can be found in the literature. The comparison with real ageing fluctuating loads is difficult because of this nonlinear temperature dependence which depends on the degradation process and the materials. A practical way is the integration of the load profiles (Figs. 3 and 4) by transforming all temperature intervals to a given reference temperature using Eq. (7):
冕
Tmax
tref(Tref)⫽
Tmin
冉 冉 冊 冊
⌽ 1 1 t(r)exp ⫺ ·dT ⫺ R T Tref
(8)
tref is the time period at temperature Tref yielding the same degradation as the fluctuating loads together for the first process with the activation energy f. The effective mean temperature is defined as the constant temperature resulting in the same degradation in the same time period as the fluctuating loads. Teff can be calculated by use of Eqs. (7) and (8) for any range of activation energies for a given load profile. Eq. (7) enables the calculation of a corresponding accelerated ageing test at any test temperature yielding the same degradation as the real operation for a known activation energy f providing the validity of the assumptions. The test results can be transformed to a master degradation progress at a given temperature (Fig. 7), if the activation energy was determined. The transformation of the effective mean temperature calculated from a load profile which was normalised to the design service life time (25 years for solar absorber coatings) to the same reference temperature indicates the life time. If the performance criterion is reached earlier, the tested coating is not durable enough.
M. Koehl / Renewable Energy 24 (2001) 597–607
605
Fig. 7. Time-transformed data from Fig. 6.
8. Validation of the model A direct validation of this life time estimation is usually not possible, if the estimated life time exceeds the average duration of a research project. In our case study the shortest life time was about 11 years. During the project we could monitor the loads and measure the degradation of the coatings for 3 years. Changes in the performance up to 4% could be detected. A comparison with coatings which have been in operation for more than 10 years showed clear similarities to the ageing phenomena seen after accelerated tests at the performance limit [1] (Fig. 8).
Fig. 8. FT-IR spectra of degraded absorbers coated with nickel-plated anodised aluminium (non-aged samples show no absorption bands above 3 µm).
606
M. Koehl / Renewable Energy 24 (2001) 597–607
Fig. 9. Testing time periods for accelerated temperature and condensation constant loads corresponding to 25 years life time as a function of the activation energies.
9. Design of a standard test procedure The respective accelerated tests for a service life of 25 years were calculated on the basis of the Arrhenius-model for the degradation kinetics of the load distributions shown in Figs. 3 and 4 (Fig. 9). A qualification test procedure was developed (ISO CD 12 952.2) and tested [2] which abandons the determination of process parameters like the activation energy, in order to get a quick and not too costly life-test. The procedure is based on an Arrhenius model with rough estimation, whether the activation energy is high enough to ensure a sufficient durability of the absorber coating at natural working conditions in flat plate solar collectors [3] (Fig. 10).
Fig. 10.
Schematic diagram of the qualification test procedures proposed to ISO.
M. Koehl / Renewable Energy 24 (2001) 597–607
607
Acknowledgements The author would like to express his thanks to all the participating material experts in Task X Solar Materials Research and Development and the Solar Thermal Materials Working Group of the IEA Solar Heating and Cooling Programme.
References [1] Carlsson B, Mo¨ ller K, Frei U, Brunold S, Ko¨ hl M. Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems. Solar Energy Mater Solar Cells 2000;61:223–38. [2] Brunold S, Frei U, Carlsson B, Mo¨ ller K, Ko¨ hl M. Round robin on accelerated life testing of solar absorber surface durability. Solar Energy Mater Solar Cells 2000;61:239–53. [3] Carlsson B, Mo¨ ller K, Ko¨ hl M, Frei U, Brunold S. Qualification test procedure for solar absorber surface durability. Solar Energy Mater Solar Cells 2000;61:255–75. [4] Carlsson B, Frei U, Ko¨ hl M, Mo¨ ller K. Accelerated life testing of solar energy materials. Report of Task X Solar Materials Research and Development. International Energy Agency Solar Heating and Cooling Programme, 1994.