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A test procedure for extruded polymeric solar thermal absorbers
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 92 (2008) 445–452 www.elsevier.com/locate/solmat
A test procedure for extruded polymeric solar thermal absorbers A. Olivaresa,, J. Rekstada, M. Meira, S. Kahlenb, G. Wallnerc a
Department of Physics, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway b Polymer Competence Center Leoben GmbH, ParkstraX e 11, A-8700 Leoben, Austria c University of Leoben, Franz-Josef Strasse 18, A-8700 Leoben, Austria Received 20 July 2007; accepted 29 October 2007 Available online 18 December 2007
Abstract An indentation test is proposed to study the degradation of extruded polymeric solar absorbers. The thermal degradation caused by accelerated aging is investigated. The results are compared with the thermal and mechanical impacts during the operation of the solar collector. r 2007 Elsevier B.V. All rights reserved. Keywords: Absorber; Polymer; Indentation test; Thermal degradation; Solar collector
1. Introduction Polymeric solar absorbers are considered as a promising alternative to conventional flat plate collectors for reducing the cost of solar thermal energy [1]. Unglazed solar collectors are widely used for heating of swimming pools [2]. The use of glazing improves the performance of polymeric absorbers and makes these suitable for space heating and domestic hot water preparation. The challenge is to find polymeric materials, which withstand the high operational and stagnation temperatures expected for glazed solar collectors. In particular the overheating periods during the service life will define the demands on a suitable polymeric material for solar collectors [3]. Stagnation conditions may change the structural and morphological parameters of polymeric materials and thus will have an impact on application relevant properties such as mechanical strength, dimensional stability or optical properties. Polymer degradation may deteriorate different properties of the solar absorber during its service life. Rather sensitive parameters for polymer degradation are the ultimate mechanical properties deformation at break and mechanical strength [4]. The most critical operational demand of a Corresponding author. Tel.: +47 2285 6461; fax: +47 2285 6422
E-mail address: [email protected] (A. Olivares). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.10.006
solar absorber is to avoid leaks of the heat carrier. Thus appropriate tools to evaluate these aspects are needed for a successful development of polymeric solar absorbers. Standard tests used for characterization of the polymeric materials before processing give limited information about the service life of the absorber because the processing from granules to the final product may change the properties of the polymeric material [5]. Hence, this work focused in the development of a test that can be applied directly on the absorber sheet. In this way the effect of the processing during the absorber production can be studied, however test on bulk material (polymer films) are currently carried out in collaboration with the Polymer Competence Center Leoben [4]. Using standard tests after the processing could be an option, but is hampered by the complex geometry of the absorber. Tensile tests require strong mechanical fixations, which are inappropriate for the geometry of the polymeric absorber. Hardness tests do not give information on the influence of the absorber geometry, which may play a major role. Impact tests (Charpy, IZOD) are principally designed for defined specimens according to ASTM/ISO and will be difficult to apply for the present absorber geometry. Hence a suitable test to evaluate the mechanical properties of the polymeric absorber requires that it can be applied directly on the absorber after the processing.
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A. Olivares et al. / Solar Energy Materials & Solar Cells 92 (2008) 445–452
It should give information on the strength and mechanical stability of the material as well as on the influence of the absorber geometry. The present work proposes the use of an indentation test as a tool for the evaluation of extruded polymeric absorbers. Such a test has not been reported previously, but the use of mechanical tests to characterize polymer degradation is a rather meaningful and sensitive approach [4,6,7]. 2. Absorber and functional demands The absorber is an extruded triple-wall sheet made of the material NORYLs Resin EN150SP [8]. The material is an amorphous polyphenylene/polystyrene (PPE/PS) blend, which was designed for the application in glazed solar collectors. NORYL blends have good temperature stability and low water absorption, 0.14% at 23 1C [9]. The absorber sheet’s outer dimensions are 10 mm 540 mm and the modules are available in different standard lengths (2, 3, 4 and 6 m). The absorber is designed and manufactured by Solarnor AS [10]. The absorber’s cross-section is shown in Fig. 1. The triple-wall sheet consists of three parallel plates connected by a series of inner walls that form two layers of intrinsic channels. The absorber is part of a drain-back collector system. The channels of the front layer transport the heat carrier (water) when the solar collector system is in operation, while the channels in the back layer remain filled with air and water vapour. During system standstill the heat carrier drains to the heat store and all channels are filled with air. Both ends of the absorber sheet are closed with end-caps with connections for the heat carrier’s forward- and return flow. The solar collector system is a vented system and the operational pressure in the solar loop is slightly below the atmospheric pressure. The hydrostatic pressure, which is caused by the water column inside the channels of the absorber sheet, is estimated: In the bottom of the absorber it can reach up to 50 kPa, which corresponds to an absorber of 5 m length filled with water and placed in vertical position. These conditions are illustrated by simulations with the software COMSOL Multiphysicss
Fig. 1. Cross-section of the polymeric extruded absorber showing the two layers of intrinsic channels and the bottom end-cap.
Fig. 2. Simulation of the stress distribution in a section of the absorber. The maximum stress in the extruded sheet is of the order of 1.6 MPa. The total absorber is assumed to be 5 m long, placed vertically and filled with water.
[11] for a section in the bottom of the absorber (see Fig. 2). The simulation was conducted with a simplified geometry of the extruded absorber to reduce the computational effort. A section of 10 mm length and 8 intrinsic channels in width were studied (instead of the 99 channels of the real extruded sheet); the rest of the section’s dimensions are equal to the extruded absorber shown in Fig. 1. The stress distribution is caused by 50 kPa of internal pressure. A maximum stress of 1.6 MPa is located at the round sides of the absorber. Such stress levels are usually lower than the internal stresses induced by processing via the melt and cooling the component. Hence, the calculated stress due to hydrostatic pressure is relatively low and not considered as a significant factor to cause a failure of the absorber sheet. Other impacts will also generate stress. The temperature of the absorber can under winter conditions be 20 1C and during summer at stagnation close to 150 1C. The temperature gradient along the sheet is an additional stress factor. If only a part of the absorber surface is irradiated due to shadow effect, the temperature gradient in the transition zone and also the stress can be significant. Manufacturing, transport and installation of the absorbers will also generate stress. Some of the stress situations occur only at room temperature, while other stress situations happen simultaneously with the presence of other stress factors. If the absorber is permanently filled with a heat carrier at a certain pressure, loads due to the hydraulic pressure and high temperatures appear simultaneously, in a drain-back concept the hydraulic pressure is present only at relatively low temperatures. Such design aspects are important to consider in relation to the choice of polymer material in the absorber sheet. A full analysis of the various stress situations that occurs during the service of the absorber in the drain-back concept investigated here gives a maximum stress of approx. 5 MPa.
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Additionally, thermal stagnation leads to high temperatures, which accelerate the degradation of the absorber. For typical collector installations under Norwegian climate (401 tilt angle, latitude 601) maximum temperatures up to 145 1C were measured [12]. As a result of this accelerated aging, the material looses ductility and becomes brittle, increasing the risk of having a fracture or leak during operation. How fast the polymeric material deteriorates its mechanical properties after the absorber extrusion is a major factor to determine the service life of the solar absorber. Hence the development of a method to study the mechanical properties of the extruded sheet is the first step to estimate its service life. 3. Instrumentation and experimental procedure The test instrumentation consists of an indenter (a piece of metal with a known geometry), which is forced into the test specimen by means of a hydraulic bench press. A S-type load cell from Tedea Huntleigh, Model 616 [13], measures the load during the indentation and a position transducer from Novotech, Series KL-1000 [14], measures the indenter’s penetration depth into the specimen. The indentation speed is controlled manually and is in
Fig. 4. Dimensions and geometries of the two indenters.
the range of 15 mm/min. The experimental set-up is shown in Fig. 3. The signals of the load cell and the position transducer are converted from analogue to digital by a data acquisition device (DAQ) from National Instruments, model PCI 6013 [15]. The software application LabVIEW (National Instruments) is used for programming the data logging procedures. LabView provides a graphical user interface, handles communication with the DAQ devices, file I/O, and displays measured data, etc. All data are saved in a computer. Two indenters are used to study the extruded absorber. Both indenters are made in brass and are referred to as prismatic and squared indenter, respectively (see Fig. 4). The indenters are used in different orientations, which are referred to as indenter modes (Fig. 5):
Fig. 3. The hydraulic bench press used for testing of the sheets.
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In mode-1, the squared indenter is forced into the extruded sheet with two faces parallel to the intrinsic channels. In mode-2 the prismatic indenter is forced into the extruded sheet with the contact edge parallel to and exactly in the middle between two walls of the intrinsic channels. In mode-3 the prismatic indenter is forced into the extruded sheet with the contact edge perpendicular to the intrinsic channels.
The specimens for the test are produced by cutting the absorber in segments of 87 mm 72 mm. The label indicates the zone of the absorber from where the specimen comes from. Fig. 6 shows the labelling nomenclature, numbers are related to the extrusion direction of the sheet and letters to the absorber’s cross-section. The results are plotted in a load–penetration depth curve as shown in Fig. 7. The limit of the curve at 2000 N is due to the nominal limit of the load cell. The uncertainties of the measured quantities caused by the instruments have been determined empirically using statistical methods [16]. The uncertainties of the measured quantities were determined to be 70.04 mm for the penetration depth and 75 N for the load. Fig. 7 shows typical test results for the three modes proposed for the indentation test. The initial deformation is characterized by a linear region of the
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Fig. 7. Typical load–penetration depth measurements of the structured absorber sheet for the three different indenter modes (see Fig. 5).
Fig. 5. Indenter modes: mode-1: squared indenter oriented with two faces parallel to the intrinsic channels; mode-2: prismatic indenter oriented with the contact edge parallel to the intrinsic channels; mode-3: prismatic indenter oriented with the contact edge perpendicular to the inner channels. Fig. 8. Simulation of mode-1: the stress distribution indicates that most of the deformation occurs in the internal walls direct under the squared indenter.
4. Simulation of the stress distribution for different indenter modes
Fig. 6. The complete absorber is cut in segments of 87 72 mm2, which are labelled as it is shown in the figure. The numbers are related to the extrusion direction and the letters to the absorber’s cross-section.
curves, which indicates the viscoelastic deformation of the specimen. The end of the viscoelastic region is visualized by a change in the curve’s linearity that indicates the viscoelastic limit. Beyond this point the specimen is under plastic deformation.
As mentioned in Section 2 the distribution of stress on the various parts of the absorber sheet can be studied by means of simulations. The indenter geometry and the orientation relative to the specimen produce different stress distributions on the internal structure of the structured sheet. The results may be relevant depending on which structure is considered critical for the application as absorber in a solar collector. In mode-1 the highest stress is located in the internal walls of the specimen, as illustrated in the computer simulation in Fig. 8. The thickness of the surface (1 mm) is small compared to the height of the internal walls (10 mm), this causes that most of the specimen’s deformation is
ARTICLE IN PRESS A. Olivares et al. / Solar Energy Materials & Solar Cells 92 (2008) 445–452
Fig. 9. Simulation of mode-3: the stress distribution indicates that most of the deformation occurs in the internal walls in a zone of 8 mm directly under the prismatic indenter.
Fig. 10. Simulation of mode-2: the stress distribution indicates that the deformation happens in the top surface as much as in the internal walls closest to the prismatic indenter.
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Fig. 11. Cross-section during an indentation measurement in mode-2 (virgin absorber sheet).
penetration depth as for mode-1 and mode-3. This is evident from Fig. 7. Mode-2 also presents a larger plastic deformation of the specimen than the two previous cases. Fig. 11 shows the deformation of the specimen during testing. When the load is increased the internal walls of the specimen present buckling rather than compression, which accounts for parts of the changes of the slope in the plastic region of the curve (Fig. 7) (mode-2). Hence, we estimate that in mode-2 the deformations of the surface and the inner walls account for 40% and 60% of the total deformation, respectively (valid in the viscoelastic range of the curve). The surface deformation in mode-1 and mode-3 can be neglected. Indenter mode-2 is preferred to study the present polymeric absorber sheet because the surface strength is critical in preventing leaks during the operation of the absorber. 5. Accuracy of the experimental results
located on the internal walls. The computer simulation in Fig. 9 shows that mode-3 presents a similar situation as for mode-1, but for mode-3 the resistant area is reduced due to the sharp contact edge of the prismatic indenter. Most of the stress is located in a zone of 8 mm under the indenter. The resistant area in mode-3 is smaller than in mode-1, which accounts for the smaller slope in the elastic region of the load–penetration depth curve in Fig. 7. The analysis of mode-2 presents a more complex situation compared to the two previous cases. As Fig. 10 shows, the walls of the internal channels are in compression, and the specimen’s surface directly under the contact edge of the indenter bends. The total deformation is then the result of the compression of the internal walls and the deflection of the surface. The stress on the intrinsic walls and the surface is higher than for mode-1 and mode-3. Thus the slope of the load–penetration curves for mode-2 is not expected to reach as high load values for the same
Two conditions are necessary to secure reproducible results with the indentation test. The tested specimens should come from comparable zones of the extruded absorber and the indentation speed should not have large fluctuations. Specimens labelled as 1D, 2D (Fig. 6) are likely to show good agreement in the results because the extrusion direction of the absorber gives them similar properties. As Fig. 12 exhibits, the typical spread between specimens from comparable zones is small. Still the extrusion process introduces a variation of 75% relative to a homogenous absorber. As revealed in Fig. 13, specimens labelled as A, B, C are likely to show larger differences in the load–penetration depth curves. The absorber sheet is designed with internal channels in the centre with thinner walls than the internal channels at the sides. Hence, specimens labelled with
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Fig. 12. The typical variation between segments from similar zones of the extruded sheet is in the range of 75% (mode-2).
Fig. 14. Typical variations due to the speed of indentation (mode-3).
was found between the test performed with the hydraulic bench press (‘‘hand press’’) and the Instron machine (‘‘Instron 10 mm/min’’) (Fig. 14). The different slopes in the linear part of the curves are presumably due to specimens from different extrusions, which were investigated in Leoben and Oslo, thus the material of the extruded sheet was not exactly the same. The discontinuities in the curve of the hydraulic bench press (at approximately 2.1 mm penetration depth in Fig. 14) are due to the operator reaching the maximum range of the pump lever in the bench press, which causes a stop of the hydraulic flow for a few seconds until the start position of the lever is reached again. Considering the results shown in this section, it is fair to conclude that the hydraulic bench press is reliable enough to perform the test and reproducible results can be achieved with the proposed method if the conditions described above are taken into account. Fig. 13. Typical variation between segments from the different zones of the extruded sheet (mode-2).
different letters have larger variation of the ultimate properties and the indentation test results are difficult to compare. To study the effect of the variation in the indentation speed, the measurements with an Instron 4505 Universal Testing Machine were compared with the present results from the hydraulic bench press [17]. The indentation speed of the Instron testing machine can be accurately controlled over a wide range; testing speeds of 1, 10 and 100 mm/min were applied for comparison. The indentation speed of the hydraulic bench press is manually controlled by the operator and is in the range of 15 mm/min (see Fig. 3). Both tests were performed in mode-3. A good agreement
6. Thermal degradation The specimens are exposed to thermal degradation using an oven (type: TERMAKS Series TS8000) with stabilized airflow to control the exposure temperature. The specimens are exposed to constant temperature for different periods of times and then tested to study the reduction in their mechanical properties. Compared to the real condition of the polymeric absorber during operation, this procedure produces an accelerated aging in the specimens. Fig. 15 shows four specimens that have been exposed to 120 1C in the oven for different periods of time. The fifth specimen labelled Sunlab has been obtained from a solar collector exposed to outdoor weather conditions during a summer–winter period under thermal stagnation (208 days since April without water circulation). Measurements indicates that in stagnation conditions the polymeric
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Fig. 16. Typical failure in the plastic region of the load–penetration depth curve. Fig. 15. Different times of exposure to 120 1C compared with a specimen from a collector exposed to 208 days of outdoor weather condition (Oslo climate).
absorber can reach maximum temperatures of 145 1C [12], however in operative conditions stagnation is present in the absorber for short periods of time when the water flow of the system is interrupted. Several results as the ones shown in Fig. 15 indicate a significant effect of the temperature and the exposure time on the load–penetration curves. As a general conclusion, for a constant temperature, longer exposure times lead to a reduction of the maximum indentation load that the specimen can withstand during the test. A mathematical model for different temperature/time conditions may be used to predict how long time it will take to reduce the maximum indentation load to a level considered critical for the absorber operation. Using this model and the absorber temperature profile for one year of operation as a base for the temperature/time conditions during service life, it is possible to calculate after how may years the absorber will reach a level of degradation critical for its operation. Further details about the methods for estimation of the absorber service life will be discussed in a second paper [18]. Short exposure times at high temperatures mainly affect the plastic region of the curves. The typical failure observed in these cases is a permanent deflection on the surface and a fracture in the specimen’s internal walls, as seen in Fig. 16. This kind of failure is typically represented as a gradual reduction of the slope in the plastic region of the load–penetration curves (Fig. 15) (312 h). The results are consistent with the observation of severe reduction in the load at break after thermal exposure shown stress/strain measurements by Kahlen et al. [19]. If the exposure time is sufficiently long, the typical failure observed on the specimen is a break in the surface with almost no deformation on the specimen’s internal walls (Fig. 17). In the load–penetration curve, this is
Fig. 17. Typical failure in the viscoelastic region of the load–penetration depth curve.
represented as a sharp break in the curve’s continuity in the viscoelastic region (Fig. 15) (1000 h). The failures shown in Figs. 16 and 17 suggest a change in the properties of the material as consequence of the thermal degradation. These results show that the reduction of the ductility of the polymer leads to a brittle sheet. The reduction of the material’s ductility also occurs under operational conditions as the Sunlab specimen in Fig. 15 reveals. However, due to the cycles of low and high temperature much longer exposition times are required to achieve the same level of degradation. 7. Results and conclusions The mechanical integrity of the polymeric absorber has been identified as a significant factor to determine its
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service life. Mechanical failure occurs when the absorber structure cannot withstand the stress related with the conditions of operation. Studies carried out on this particular absorber show that the stress levels during operation are relatively low (5 MPa). Experiments carried out during boiling of the heat carrier fluid inside the absorber have shown no significant increase in the stress levels, since the solar collector is implemented as a none pressurized system, however it may be a significant factor to be considered in pressurized systems. Cycles of temperature changes during day and night may also introduce a significant mechanical impact due to the cyclic stress associated with the thermal dilatation. In the specific case of NORYL the materials have been reported to stand a cyclic stress of 20 MPa during 500,000 cycles [8], thus this is not considered a main concern for this particular absorber. Further studies in the stress conditions during the absorber’s operation (including internal pressure test of the absorber and endcaps) have been carried out in collaboration with the Polymer Competence Center Leoben. Results of these investigations will be presented in the future by the corresponding authors. The criteria for the mechanical failure of the absorber has been defined using the load of indentation. Comparison of the stress levels during the absorber operation and stress levels in the specimens during the indentation test have been used with this purpose. For this polymeric absorber in particular, a limit of 160 N is proposed as a critical indentation load to assure that the absorber can withstand the stress conditions during operation without mechanical failure. Thermal aging has been identified as the main cause for deterioration of the polymeric absorber. Long exposure time to high temperatures produce a transition in the material’s properties from ductile to brittle. The phenomenon is critical for the absorber service life, since a brittle material means higher risk for leaks in the absorber due to the low ductility to absorb mechanical impacts. Hence a pressurized system is not recommended in this kind of absorber since higher stress levels may lead to earlier failures during the absorber’s service life. 8. Summary A simple indentation test has been developed to measure the mechanical properties of a structured polymeric sheet used as absorber in a glazed solar collector. The test results
are shown to be reproducible and characterize the stability and strength of the absorber sheet. Different indentation modes were demonstrated in order to determine a critical failure of the polymeric absorber. Mode-2 has been chosen as the best method, since it tests the stability and strength of the internal structure and the surface of the specimen. The thermal degradation impact in the absorber was characterized by the maximum load reached by the specimens during the indentation test. Higher temperatures and longer exposure times systematically lead to lower indentation loads in the test results. Hence characterizing the thermal degradation of the absorber with the indentation test gives the possibility for developing a model to estimate the absorber service life, a subject that will be discussed in a second paper [18]. References [1] P.T. Tsilingiris, Energy Convers. Manage. 40 (1999) 1237–1250. [2] W. Weiss, I. Bergmann, G. Faninger, Solar heat worldwide, IEA Solar Heating and Cooling Program, April 2007. [3] J.H. Davidson, S.C. Mantell, G.J. Jorgensen, Adv. Sol. Energy 15 (2003) 149. [4] G.M. Wallner, C. Weigl, R. Leitgeb, R.W. Lang, Polym. Degrad. Stability 85 (2004) 1065. [5] E.G. El’darov, F.V. Mamedov, V.M. Goldberg, G.E. Zaikov, Polym. Degrad. Stability 51 (1996) 271. [6] O. Guseva, A. Lichtblau, Polym. Test. 24 (2005) 718. [7] J. Kim, W.I. Lee, S.W. Tsai, Compos. Part B: Eng. 33 (2002) 531. [8] General Electric Company, GE Advanced Materials Plastics, NORYLs Resin EN150SP Data sheet, 2005. [9] General Electric Plastics, NORYLs profile-Modified PPOs Resins brochure, 1999. [10] Solarnor AS, Oslo, Norway /http://www.solarnor.comS. [11] COMSOL Inc., Burlington, US /http://www.comsol.comS. [12] J. Gjessing, Ventilering som metode for a˚ redusere stagnasjonstemperatur i solfangere, Master Thesis, Department of Physics, University of Oslo, 2006. [13] Vishay Americas, Shelton, US /http://www.vishay.comS. [14] Novotechnik, Ostfildern, Germany /http://www.novotechnik.deS. [15] National Instruments Corp., Austin, US /http://www.ni.comS. [16] S. Rabinovicˇ, Measurement errors and uncertainties, second ed., AIP Press, 1999. [17] S. Kahlen, G. Wallner, 2006. Aging and aging mechanisms of functional polymers for optical applications. Investigation on absorber materials, Interim Report, IR-S.9-01, University of Leoben, Austria. [18] A. Olivares, J. Rekstad, M. Meir, Modeling thermal degradation in extruded polymeric solar absorbers, Sol. Energy Mater. Sol. Cells, to be submitted. [19] S. Kahlen, G. Wallner, J. Fischer, M.G. Meir, J. Rekstad, Basic characterisation of Polymeric materials for solar collector absorbers, in: Proceedings of the NORTHSUN 2005 Conference, Vilnius, Lithuania, pp. 32–40.
Solar Energy Materials & Solar Cells 92 (2008) 445–452 www.elsevier.com/locate/solmat
A test procedure for extruded polymeric solar thermal absorbers A. Olivaresa,, J. Rekstada, M. Meira, S. Kahlenb, G. Wallnerc a
Department of Physics, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway b Polymer Competence Center Leoben GmbH, ParkstraX e 11, A-8700 Leoben, Austria c University of Leoben, Franz-Josef Strasse 18, A-8700 Leoben, Austria Received 20 July 2007; accepted 29 October 2007 Available online 18 December 2007
Abstract An indentation test is proposed to study the degradation of extruded polymeric solar absorbers. The thermal degradation caused by accelerated aging is investigated. The results are compared with the thermal and mechanical impacts during the operation of the solar collector. r 2007 Elsevier B.V. All rights reserved. Keywords: Absorber; Polymer; Indentation test; Thermal degradation; Solar collector
1. Introduction Polymeric solar absorbers are considered as a promising alternative to conventional flat plate collectors for reducing the cost of solar thermal energy [1]. Unglazed solar collectors are widely used for heating of swimming pools [2]. The use of glazing improves the performance of polymeric absorbers and makes these suitable for space heating and domestic hot water preparation. The challenge is to find polymeric materials, which withstand the high operational and stagnation temperatures expected for glazed solar collectors. In particular the overheating periods during the service life will define the demands on a suitable polymeric material for solar collectors [3]. Stagnation conditions may change the structural and morphological parameters of polymeric materials and thus will have an impact on application relevant properties such as mechanical strength, dimensional stability or optical properties. Polymer degradation may deteriorate different properties of the solar absorber during its service life. Rather sensitive parameters for polymer degradation are the ultimate mechanical properties deformation at break and mechanical strength [4]. The most critical operational demand of a Corresponding author. Tel.: +47 2285 6461; fax: +47 2285 6422
E-mail address: [email protected] (A. Olivares). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.10.006
solar absorber is to avoid leaks of the heat carrier. Thus appropriate tools to evaluate these aspects are needed for a successful development of polymeric solar absorbers. Standard tests used for characterization of the polymeric materials before processing give limited information about the service life of the absorber because the processing from granules to the final product may change the properties of the polymeric material [5]. Hence, this work focused in the development of a test that can be applied directly on the absorber sheet. In this way the effect of the processing during the absorber production can be studied, however test on bulk material (polymer films) are currently carried out in collaboration with the Polymer Competence Center Leoben [4]. Using standard tests after the processing could be an option, but is hampered by the complex geometry of the absorber. Tensile tests require strong mechanical fixations, which are inappropriate for the geometry of the polymeric absorber. Hardness tests do not give information on the influence of the absorber geometry, which may play a major role. Impact tests (Charpy, IZOD) are principally designed for defined specimens according to ASTM/ISO and will be difficult to apply for the present absorber geometry. Hence a suitable test to evaluate the mechanical properties of the polymeric absorber requires that it can be applied directly on the absorber after the processing.
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It should give information on the strength and mechanical stability of the material as well as on the influence of the absorber geometry. The present work proposes the use of an indentation test as a tool for the evaluation of extruded polymeric absorbers. Such a test has not been reported previously, but the use of mechanical tests to characterize polymer degradation is a rather meaningful and sensitive approach [4,6,7]. 2. Absorber and functional demands The absorber is an extruded triple-wall sheet made of the material NORYLs Resin EN150SP [8]. The material is an amorphous polyphenylene/polystyrene (PPE/PS) blend, which was designed for the application in glazed solar collectors. NORYL blends have good temperature stability and low water absorption, 0.14% at 23 1C [9]. The absorber sheet’s outer dimensions are 10 mm 540 mm and the modules are available in different standard lengths (2, 3, 4 and 6 m). The absorber is designed and manufactured by Solarnor AS [10]. The absorber’s cross-section is shown in Fig. 1. The triple-wall sheet consists of three parallel plates connected by a series of inner walls that form two layers of intrinsic channels. The absorber is part of a drain-back collector system. The channels of the front layer transport the heat carrier (water) when the solar collector system is in operation, while the channels in the back layer remain filled with air and water vapour. During system standstill the heat carrier drains to the heat store and all channels are filled with air. Both ends of the absorber sheet are closed with end-caps with connections for the heat carrier’s forward- and return flow. The solar collector system is a vented system and the operational pressure in the solar loop is slightly below the atmospheric pressure. The hydrostatic pressure, which is caused by the water column inside the channels of the absorber sheet, is estimated: In the bottom of the absorber it can reach up to 50 kPa, which corresponds to an absorber of 5 m length filled with water and placed in vertical position. These conditions are illustrated by simulations with the software COMSOL Multiphysicss
Fig. 1. Cross-section of the polymeric extruded absorber showing the two layers of intrinsic channels and the bottom end-cap.
Fig. 2. Simulation of the stress distribution in a section of the absorber. The maximum stress in the extruded sheet is of the order of 1.6 MPa. The total absorber is assumed to be 5 m long, placed vertically and filled with water.
[11] for a section in the bottom of the absorber (see Fig. 2). The simulation was conducted with a simplified geometry of the extruded absorber to reduce the computational effort. A section of 10 mm length and 8 intrinsic channels in width were studied (instead of the 99 channels of the real extruded sheet); the rest of the section’s dimensions are equal to the extruded absorber shown in Fig. 1. The stress distribution is caused by 50 kPa of internal pressure. A maximum stress of 1.6 MPa is located at the round sides of the absorber. Such stress levels are usually lower than the internal stresses induced by processing via the melt and cooling the component. Hence, the calculated stress due to hydrostatic pressure is relatively low and not considered as a significant factor to cause a failure of the absorber sheet. Other impacts will also generate stress. The temperature of the absorber can under winter conditions be 20 1C and during summer at stagnation close to 150 1C. The temperature gradient along the sheet is an additional stress factor. If only a part of the absorber surface is irradiated due to shadow effect, the temperature gradient in the transition zone and also the stress can be significant. Manufacturing, transport and installation of the absorbers will also generate stress. Some of the stress situations occur only at room temperature, while other stress situations happen simultaneously with the presence of other stress factors. If the absorber is permanently filled with a heat carrier at a certain pressure, loads due to the hydraulic pressure and high temperatures appear simultaneously, in a drain-back concept the hydraulic pressure is present only at relatively low temperatures. Such design aspects are important to consider in relation to the choice of polymer material in the absorber sheet. A full analysis of the various stress situations that occurs during the service of the absorber in the drain-back concept investigated here gives a maximum stress of approx. 5 MPa.
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Additionally, thermal stagnation leads to high temperatures, which accelerate the degradation of the absorber. For typical collector installations under Norwegian climate (401 tilt angle, latitude 601) maximum temperatures up to 145 1C were measured [12]. As a result of this accelerated aging, the material looses ductility and becomes brittle, increasing the risk of having a fracture or leak during operation. How fast the polymeric material deteriorates its mechanical properties after the absorber extrusion is a major factor to determine the service life of the solar absorber. Hence the development of a method to study the mechanical properties of the extruded sheet is the first step to estimate its service life. 3. Instrumentation and experimental procedure The test instrumentation consists of an indenter (a piece of metal with a known geometry), which is forced into the test specimen by means of a hydraulic bench press. A S-type load cell from Tedea Huntleigh, Model 616 [13], measures the load during the indentation and a position transducer from Novotech, Series KL-1000 [14], measures the indenter’s penetration depth into the specimen. The indentation speed is controlled manually and is in
Fig. 4. Dimensions and geometries of the two indenters.
the range of 15 mm/min. The experimental set-up is shown in Fig. 3. The signals of the load cell and the position transducer are converted from analogue to digital by a data acquisition device (DAQ) from National Instruments, model PCI 6013 [15]. The software application LabVIEW (National Instruments) is used for programming the data logging procedures. LabView provides a graphical user interface, handles communication with the DAQ devices, file I/O, and displays measured data, etc. All data are saved in a computer. Two indenters are used to study the extruded absorber. Both indenters are made in brass and are referred to as prismatic and squared indenter, respectively (see Fig. 4). The indenters are used in different orientations, which are referred to as indenter modes (Fig. 5):
Fig. 3. The hydraulic bench press used for testing of the sheets.
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In mode-1, the squared indenter is forced into the extruded sheet with two faces parallel to the intrinsic channels. In mode-2 the prismatic indenter is forced into the extruded sheet with the contact edge parallel to and exactly in the middle between two walls of the intrinsic channels. In mode-3 the prismatic indenter is forced into the extruded sheet with the contact edge perpendicular to the intrinsic channels.
The specimens for the test are produced by cutting the absorber in segments of 87 mm 72 mm. The label indicates the zone of the absorber from where the specimen comes from. Fig. 6 shows the labelling nomenclature, numbers are related to the extrusion direction of the sheet and letters to the absorber’s cross-section. The results are plotted in a load–penetration depth curve as shown in Fig. 7. The limit of the curve at 2000 N is due to the nominal limit of the load cell. The uncertainties of the measured quantities caused by the instruments have been determined empirically using statistical methods [16]. The uncertainties of the measured quantities were determined to be 70.04 mm for the penetration depth and 75 N for the load. Fig. 7 shows typical test results for the three modes proposed for the indentation test. The initial deformation is characterized by a linear region of the
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Fig. 7. Typical load–penetration depth measurements of the structured absorber sheet for the three different indenter modes (see Fig. 5).
Fig. 5. Indenter modes: mode-1: squared indenter oriented with two faces parallel to the intrinsic channels; mode-2: prismatic indenter oriented with the contact edge parallel to the intrinsic channels; mode-3: prismatic indenter oriented with the contact edge perpendicular to the inner channels. Fig. 8. Simulation of mode-1: the stress distribution indicates that most of the deformation occurs in the internal walls direct under the squared indenter.
4. Simulation of the stress distribution for different indenter modes
Fig. 6. The complete absorber is cut in segments of 87 72 mm2, which are labelled as it is shown in the figure. The numbers are related to the extrusion direction and the letters to the absorber’s cross-section.
curves, which indicates the viscoelastic deformation of the specimen. The end of the viscoelastic region is visualized by a change in the curve’s linearity that indicates the viscoelastic limit. Beyond this point the specimen is under plastic deformation.
As mentioned in Section 2 the distribution of stress on the various parts of the absorber sheet can be studied by means of simulations. The indenter geometry and the orientation relative to the specimen produce different stress distributions on the internal structure of the structured sheet. The results may be relevant depending on which structure is considered critical for the application as absorber in a solar collector. In mode-1 the highest stress is located in the internal walls of the specimen, as illustrated in the computer simulation in Fig. 8. The thickness of the surface (1 mm) is small compared to the height of the internal walls (10 mm), this causes that most of the specimen’s deformation is
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Fig. 9. Simulation of mode-3: the stress distribution indicates that most of the deformation occurs in the internal walls in a zone of 8 mm directly under the prismatic indenter.
Fig. 10. Simulation of mode-2: the stress distribution indicates that the deformation happens in the top surface as much as in the internal walls closest to the prismatic indenter.
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Fig. 11. Cross-section during an indentation measurement in mode-2 (virgin absorber sheet).
penetration depth as for mode-1 and mode-3. This is evident from Fig. 7. Mode-2 also presents a larger plastic deformation of the specimen than the two previous cases. Fig. 11 shows the deformation of the specimen during testing. When the load is increased the internal walls of the specimen present buckling rather than compression, which accounts for parts of the changes of the slope in the plastic region of the curve (Fig. 7) (mode-2). Hence, we estimate that in mode-2 the deformations of the surface and the inner walls account for 40% and 60% of the total deformation, respectively (valid in the viscoelastic range of the curve). The surface deformation in mode-1 and mode-3 can be neglected. Indenter mode-2 is preferred to study the present polymeric absorber sheet because the surface strength is critical in preventing leaks during the operation of the absorber. 5. Accuracy of the experimental results
located on the internal walls. The computer simulation in Fig. 9 shows that mode-3 presents a similar situation as for mode-1, but for mode-3 the resistant area is reduced due to the sharp contact edge of the prismatic indenter. Most of the stress is located in a zone of 8 mm under the indenter. The resistant area in mode-3 is smaller than in mode-1, which accounts for the smaller slope in the elastic region of the load–penetration depth curve in Fig. 7. The analysis of mode-2 presents a more complex situation compared to the two previous cases. As Fig. 10 shows, the walls of the internal channels are in compression, and the specimen’s surface directly under the contact edge of the indenter bends. The total deformation is then the result of the compression of the internal walls and the deflection of the surface. The stress on the intrinsic walls and the surface is higher than for mode-1 and mode-3. Thus the slope of the load–penetration curves for mode-2 is not expected to reach as high load values for the same
Two conditions are necessary to secure reproducible results with the indentation test. The tested specimens should come from comparable zones of the extruded absorber and the indentation speed should not have large fluctuations. Specimens labelled as 1D, 2D (Fig. 6) are likely to show good agreement in the results because the extrusion direction of the absorber gives them similar properties. As Fig. 12 exhibits, the typical spread between specimens from comparable zones is small. Still the extrusion process introduces a variation of 75% relative to a homogenous absorber. As revealed in Fig. 13, specimens labelled as A, B, C are likely to show larger differences in the load–penetration depth curves. The absorber sheet is designed with internal channels in the centre with thinner walls than the internal channels at the sides. Hence, specimens labelled with
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Fig. 12. The typical variation between segments from similar zones of the extruded sheet is in the range of 75% (mode-2).
Fig. 14. Typical variations due to the speed of indentation (mode-3).
was found between the test performed with the hydraulic bench press (‘‘hand press’’) and the Instron machine (‘‘Instron 10 mm/min’’) (Fig. 14). The different slopes in the linear part of the curves are presumably due to specimens from different extrusions, which were investigated in Leoben and Oslo, thus the material of the extruded sheet was not exactly the same. The discontinuities in the curve of the hydraulic bench press (at approximately 2.1 mm penetration depth in Fig. 14) are due to the operator reaching the maximum range of the pump lever in the bench press, which causes a stop of the hydraulic flow for a few seconds until the start position of the lever is reached again. Considering the results shown in this section, it is fair to conclude that the hydraulic bench press is reliable enough to perform the test and reproducible results can be achieved with the proposed method if the conditions described above are taken into account. Fig. 13. Typical variation between segments from the different zones of the extruded sheet (mode-2).
different letters have larger variation of the ultimate properties and the indentation test results are difficult to compare. To study the effect of the variation in the indentation speed, the measurements with an Instron 4505 Universal Testing Machine were compared with the present results from the hydraulic bench press [17]. The indentation speed of the Instron testing machine can be accurately controlled over a wide range; testing speeds of 1, 10 and 100 mm/min were applied for comparison. The indentation speed of the hydraulic bench press is manually controlled by the operator and is in the range of 15 mm/min (see Fig. 3). Both tests were performed in mode-3. A good agreement
6. Thermal degradation The specimens are exposed to thermal degradation using an oven (type: TERMAKS Series TS8000) with stabilized airflow to control the exposure temperature. The specimens are exposed to constant temperature for different periods of times and then tested to study the reduction in their mechanical properties. Compared to the real condition of the polymeric absorber during operation, this procedure produces an accelerated aging in the specimens. Fig. 15 shows four specimens that have been exposed to 120 1C in the oven for different periods of time. The fifth specimen labelled Sunlab has been obtained from a solar collector exposed to outdoor weather conditions during a summer–winter period under thermal stagnation (208 days since April without water circulation). Measurements indicates that in stagnation conditions the polymeric
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Fig. 16. Typical failure in the plastic region of the load–penetration depth curve. Fig. 15. Different times of exposure to 120 1C compared with a specimen from a collector exposed to 208 days of outdoor weather condition (Oslo climate).
absorber can reach maximum temperatures of 145 1C [12], however in operative conditions stagnation is present in the absorber for short periods of time when the water flow of the system is interrupted. Several results as the ones shown in Fig. 15 indicate a significant effect of the temperature and the exposure time on the load–penetration curves. As a general conclusion, for a constant temperature, longer exposure times lead to a reduction of the maximum indentation load that the specimen can withstand during the test. A mathematical model for different temperature/time conditions may be used to predict how long time it will take to reduce the maximum indentation load to a level considered critical for the absorber operation. Using this model and the absorber temperature profile for one year of operation as a base for the temperature/time conditions during service life, it is possible to calculate after how may years the absorber will reach a level of degradation critical for its operation. Further details about the methods for estimation of the absorber service life will be discussed in a second paper [18]. Short exposure times at high temperatures mainly affect the plastic region of the curves. The typical failure observed in these cases is a permanent deflection on the surface and a fracture in the specimen’s internal walls, as seen in Fig. 16. This kind of failure is typically represented as a gradual reduction of the slope in the plastic region of the load–penetration curves (Fig. 15) (312 h). The results are consistent with the observation of severe reduction in the load at break after thermal exposure shown stress/strain measurements by Kahlen et al. [19]. If the exposure time is sufficiently long, the typical failure observed on the specimen is a break in the surface with almost no deformation on the specimen’s internal walls (Fig. 17). In the load–penetration curve, this is
Fig. 17. Typical failure in the viscoelastic region of the load–penetration depth curve.
represented as a sharp break in the curve’s continuity in the viscoelastic region (Fig. 15) (1000 h). The failures shown in Figs. 16 and 17 suggest a change in the properties of the material as consequence of the thermal degradation. These results show that the reduction of the ductility of the polymer leads to a brittle sheet. The reduction of the material’s ductility also occurs under operational conditions as the Sunlab specimen in Fig. 15 reveals. However, due to the cycles of low and high temperature much longer exposition times are required to achieve the same level of degradation. 7. Results and conclusions The mechanical integrity of the polymeric absorber has been identified as a significant factor to determine its
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service life. Mechanical failure occurs when the absorber structure cannot withstand the stress related with the conditions of operation. Studies carried out on this particular absorber show that the stress levels during operation are relatively low (5 MPa). Experiments carried out during boiling of the heat carrier fluid inside the absorber have shown no significant increase in the stress levels, since the solar collector is implemented as a none pressurized system, however it may be a significant factor to be considered in pressurized systems. Cycles of temperature changes during day and night may also introduce a significant mechanical impact due to the cyclic stress associated with the thermal dilatation. In the specific case of NORYL the materials have been reported to stand a cyclic stress of 20 MPa during 500,000 cycles [8], thus this is not considered a main concern for this particular absorber. Further studies in the stress conditions during the absorber’s operation (including internal pressure test of the absorber and endcaps) have been carried out in collaboration with the Polymer Competence Center Leoben. Results of these investigations will be presented in the future by the corresponding authors. The criteria for the mechanical failure of the absorber has been defined using the load of indentation. Comparison of the stress levels during the absorber operation and stress levels in the specimens during the indentation test have been used with this purpose. For this polymeric absorber in particular, a limit of 160 N is proposed as a critical indentation load to assure that the absorber can withstand the stress conditions during operation without mechanical failure. Thermal aging has been identified as the main cause for deterioration of the polymeric absorber. Long exposure time to high temperatures produce a transition in the material’s properties from ductile to brittle. The phenomenon is critical for the absorber service life, since a brittle material means higher risk for leaks in the absorber due to the low ductility to absorb mechanical impacts. Hence a pressurized system is not recommended in this kind of absorber since higher stress levels may lead to earlier failures during the absorber’s service life. 8. Summary A simple indentation test has been developed to measure the mechanical properties of a structured polymeric sheet used as absorber in a glazed solar collector. The test results
are shown to be reproducible and characterize the stability and strength of the absorber sheet. Different indentation modes were demonstrated in order to determine a critical failure of the polymeric absorber. Mode-2 has been chosen as the best method, since it tests the stability and strength of the internal structure and the surface of the specimen. The thermal degradation impact in the absorber was characterized by the maximum load reached by the specimens during the indentation test. Higher temperatures and longer exposure times systematically lead to lower indentation loads in the test results. Hence characterizing the thermal degradation of the absorber with the indentation test gives the possibility for developing a model to estimate the absorber service life, a subject that will be discussed in a second paper [18]. References [1] P.T. Tsilingiris, Energy Convers. Manage. 40 (1999) 1237–1250. [2] W. Weiss, I. Bergmann, G. Faninger, Solar heat worldwide, IEA Solar Heating and Cooling Program, April 2007. [3] J.H. Davidson, S.C. Mantell, G.J. Jorgensen, Adv. Sol. Energy 15 (2003) 149. [4] G.M. Wallner, C. Weigl, R. Leitgeb, R.W. Lang, Polym. Degrad. Stability 85 (2004) 1065. [5] E.G. El’darov, F.V. Mamedov, V.M. Goldberg, G.E. Zaikov, Polym. Degrad. Stability 51 (1996) 271. [6] O. Guseva, A. Lichtblau, Polym. Test. 24 (2005) 718. [7] J. Kim, W.I. Lee, S.W. Tsai, Compos. Part B: Eng. 33 (2002) 531. [8] General Electric Company, GE Advanced Materials Plastics, NORYLs Resin EN150SP Data sheet, 2005. [9] General Electric Plastics, NORYLs profile-Modified PPOs Resins brochure, 1999. [10] Solarnor AS, Oslo, Norway /http://www.solarnor.comS. [11] COMSOL Inc., Burlington, US /http://www.comsol.comS. [12] J. Gjessing, Ventilering som metode for a˚ redusere stagnasjonstemperatur i solfangere, Master Thesis, Department of Physics, University of Oslo, 2006. [13] Vishay Americas, Shelton, US /http://www.vishay.comS. [14] Novotechnik, Ostfildern, Germany /http://www.novotechnik.deS. [15] National Instruments Corp., Austin, US /http://www.ni.comS. [16] S. Rabinovicˇ, Measurement errors and uncertainties, second ed., AIP Press, 1999. [17] S. Kahlen, G. Wallner, 2006. Aging and aging mechanisms of functional polymers for optical applications. Investigation on absorber materials, Interim Report, IR-S.9-01, University of Leoben, Austria. [18] A. Olivares, J. Rekstad, M. Meir, Modeling thermal degradation in extruded polymeric solar absorbers, Sol. Energy Mater. Sol. Cells, to be submitted. [19] S. Kahlen, G. Wallner, J. Fischer, M.G. Meir, J. Rekstad, Basic characterisation of Polymeric materials for solar collector absorbers, in: Proceedings of the NORTHSUN 2005 Conference, Vilnius, Lithuania, pp. 32–40.