An overview of methods for monitoring and failure detection has been presented in this paper. Several differences are highlighted by means of a partial multi-criteria analysis. The results are presented in a performance matrix, in which the methods are qualitatively evaluated with certain criteria. Quite a few methods are in an (advanced) stage of research and development, this complicates the analysis of the functioning of the different approaches. The Input/Output Controller, Guaranteed Solar Results and Manual monitoring can already be applied in commercially built solar thermal systems. Of those, the Input/Output Controller is the only one that analyses the measurement data automatically and provides an automatic failure indication.

However, none of the approaches include the auxiliary heating system. Several approaches, e. g. the method from Kassel University, are being developed further to increase the ability of detection and identification of failures. Furthermore practical experience has to be gained for a better evaluation of the performance of several approaches.

Acknowledgement

The authors gratefully acknowledge the financial support provided by the Marie Curie early stage Research Training Network ‘Advanced solar heating and cooling for buildings — SOLNET’ that is funded by the European Commission under contract MEST-CT-2005-020498 of the Sixth Framework Programme.

References

[1] Dodgson, J., Spackman, M., Pearman, A. D., Phillips, L. D., 2000. Multi-criteria Analysis: a Manual. Department of the Environment, Transport and Regions, London.

[2] Fink, C., Riva, R., Pertl, M., Wagner, W., 2006. OPTISOL — Messtechnisch begleitete Demonstrationsprojekte fur optimierte und standardisierte Solarsysteme im Mehrfamilienwohnbau. AEE — Institut fur Nachhaltige Technologien, Gleisdorf, Austria.

[3] Altgeld, H., Mahler, M., 1999. Funktionskontrolle bei kleinen thermischen Solaranlagen ohne Warmemengenmessung. Testzentrum Saarbrucken, Saarbrucken, Germany.

[4] Altgeld, H., 1999. Funktionskontrollen bei kleinen thermischen Solaranlagen ohne Warmemengenmessung. Hochschule fur Technik und Wirtschaft des Saarlandes, Saarbruecken, Germany.

[5] Grossenbacher, U., 2003. Qualitatssicherungssystem fur Solaranlagen; Methode zur permanenten Funktionskontrolle thermischer Solaranlagen. EnergieBuro Grossenbacher, Murten, Switzerland.

[6] Synetrum AG, 1998. Qualitatssicherung bei Solaranlagen: Permanente Funktionskontrolle. Synetrum AG, Murten, Switzerland.

[7] Kalogirou, S. A., Panteliou, S., Dentsoras, A., 1999. Modeling of solar domestic water heating systems using artificial neural networks. Solar Energy 65, pp. 335-342.

[8] Kalogirou, S., Lalot, S., Florides, G., Desmet, B., 2008. Development of a neural network-based fault diagnostic system for solar thermal applications. Solar Energy 82, pp. 164-172.

[9] Parisch, P., Vanoli, K., 2007. Quality assurance with the ISFH-Input/Output-Procedure 6-year-experience with 14 solar thermal systems. Proceedings of ESTEC 2007, Freiburg, Germany, pp. 315-320.

[10] Parisch, P., Vanoli, K., 2007. Wissenschaftlicher Schlussbericht Kapitel 1-6; Forschungsvorhaben: Wissenschaftlich-technische Untersuchung des ISFH-Input/Output-Verfahrens zur Ertragskontrolle solarthermischer Systeme sowie Entwicklung und Erprobung von Input/Output-Controllern. Institut fur Solarenergieforschung GmbH, Hameln/Emmerthal, Germany.

[11] Wiese, F., 2006. Langzeituberwachung grofler solarintegrierter Warmeversorgungsanlagen. Ph. D. Thesis, Kassel University, Kassel, Germany.

[12] Wiese, F., Vajen, K., Krause, M., Knoch, A., 2007. Automatic fault detection for big solar heating systems. Proceedings of ISES Solar World Congress, Beijing, China, pp. 759-763.

[13] Luboschik, U., Schalajda, P., Halagic, N., Heinzelmann, P. J., Backes, J., 1997. Garantierte Resultate von thermischen Solaranlagen; Ein Projekt zur Markteinfuhrung solarthermischer Anlagen. ASEW, Schluflbericht Projekt SE/475/93/DE/FR, EU project.

[14] Peuser, F., Remmers, K., Schnauss, M., 2002. Solar Thermal Systems. Solar Praxis AG, Germany.

1.2. Testing limits for both test methodologies

For determination of thermal performance of solar collectors, measurement of the environmental and of working conditions relevant for the collector behaviour is needed. The test methodologies for this determination, steady state (SS) and quasi-dynamic (QD), considered in EN 12975 [1] have different requirements for test.

Table 1 summarizes the main requirements set out in the standard EN 12975-2 [1].

The less stringent limits on the validation of test periods for the QD test method mean that this method can be used to gather useful information during almost the whole daylight period throughout the year.

Considering a fix stand for installation of the collector to be tested, to perform a SS test, the period useful for test is reduced to a couple of hours on a day with clear sky conditions. This period of time to test according to SS test method can be larger if a tracking stand is used. But cloudy days can not be used for SS test method, independent of the type of stand used.

For QD test method, a clear day corresponds to almost 8 hours useful for test and cloudy days also give useful information for QD test method.

Calculations are carried out with different strip angles for the curved absorbers and different strip widths for the flat absorbers. The interval for the strip angle investigated is from 150° to 179°. The investigated strip width for Seido 1-8 is in the interval from 0.075 m to 0.095 m. For Seido 10-20 with

flat absorber the investigated interval is from 0.045 m to 0.065 m. The optimum strip angle and width of the absorbers are the values resulting in the largest absorber area. The optimum angles and widths for the solar collectors can be seen in Table 5.

A change of the strip angle and strip width from the values in Table 1 to 179° and 0.095m / 0.065 m will result in a change in the optimum tilt of the collectors. The largest change in the optimum tilt is again seen in Nuussuaq where the tilt is increased with 6° for Seido 5-8 and 1-8. For Seido 10-20 the increase in the tilt is
slightly less, around 2°. In Sisimiut and Copenhagen only a slight change is seen as a result of a change in the absorber area. The optimum orientation is also found for the optimum strip angle and width. Here the optimum orientation changes for Nuussuaq so that the solar collectors should be turned even more towards east so that they are turned between 55° and 60° from south towards east. For Sisimiut Seido 5-8 and 1-8 should be turned back towards south so that the solar collectors are only turned 6° and 4° degrees from south to west where they in the original optimum orientation were turned 12° and 10°. In Copenhagen the collectors should all be turned slightly more towards west, about 1°. The improvement of the thermal performance as a function of the mean collector fluid temperature is shown for Nuussuaq in Fig 4. The improvement is largest for Seido 10-20 with flat absorber where the improvement is about 17 % as an average for mean collector fluid temperatures from 20 °C to 100 °C. For Seido 5-8 Seido 1-8 the thermal performance decreases for high mean collector fluid temperatures.

This is caused by an increasing heat loss from the increase in absorber area. The same tendency is seen for Sisimiut and Copenhagen. The improvement in thermal performance for the four collectors at a mean collector fluid temperature of 60 °C is for Seido 5-8 about 1.5 %, for Seido 1-8 about 2 %, for Seido 10-20 with curved absorber about 12 % and for Seido 10­20 with flat absorber about 16%.

The requirements for performance testing of solar collectors and thermal solar systems are differ­ent. Therefore, it is common practice to use specifically designed test facilities for testing these two categories of solar thermal products. This results in at least two test facilities, each containing a large number of measuring equipment. Besides the fact that the set-up of the two test facilities re­quires a very substantial investment it also results in relatively high operational costs for the main­tenance of the two facilities and the calibration of all the different sensors.

In order to decrease the number of test facilities and thus to decrease the initial investment and the operational costs an all-in-one test facility was developed by SWT. A further requirement for this facility was some degree of mobility. It is possible to dismantle the whole facility within a couple of hours, load and ship it to any place and set it up again within a short time. This mobility also offers the advantage that the facility can be delivered as a ready to use turn-key product to the cus­tomer and put in operation within one day. Furthermore, the test facility is designed in such a way that it can be operated independent from a fresh water or cooling water net. Most importantly, the [9]

test facility conforms to the requirements of the standards ISO 9459-2 and ISO 9459-5 for system tests, and to the standards 12975-2 or ISO 9806 respectively for solar collectors. This offers sig­nificant advantages concerning the accreditation of the test facility.

For the testing of heat storages the ambient air temperature needs to be kept at a certain level. This is achieved by climatisation of the testing laboratory. By means of a constant ambient air temperature measuring errors are reduced.

Conclusion

Solar DHW-System Tests are carried out at Fraunhofer ISE since 1997. Until now it was possible to perform two tests of SDH systems at the same time. A completely new system testing facility was designed and engineered, which is set-up in September this year. With the new testing facility it is possible to perform four system tests at the same time. Furthermore, the boundary conditions during testing have been advanced (for example adjustable water temperature, climatisation of the testing laboratory, improved data acquisition system, etc.).

In addition, the new testing facility will not only allow the testing of solar heat storage tanks. Also detailed experimental investigations for the development and thermal characterization of more complex heat storage tanks with several heat sources will be possible.

References

[1] (2006). DIN EN 12976-1,2:2006 Thermische Solaranlagen und ihre Bauteile — Vorgefertigte Anlagen, Beuth Verlag Berlin

[2] (2008). prEN 12977-3 Thermal solar systems and components — Custom built systems — Part 3: Performance test methods for solar water heater stores

[3] (2006) prEN 15316-4.3:2006 Heating systems in buildings — Method for calculation of system energy requirements and system efficiencies — Part 2.2.3 Heat generation systems, thermal solar systems

With regard to the standard series EN 12975 for solar thermal collectors, a major change undertaken during this revision process was e. g. the presentation of the thermal performance by means of the so-called “power curve” instead of the efficiency curve previously used. An example of a power curve is shown in Figure 1.

The power curve describes the thermal power output per collector module or unit respectively as a function of the temperature difference between mean collector temperature tm [°C] and the ambient temperature ta [°C]. Since the power curve depends on the hemispherical solar irradiance G* used for its calculation the corresponding value of G* has to be mentioned together with the power curve. According to EN 12975-2:2006 a value of G* = 1000 W/m2 has to be used.

In comparison to the previously used collector efficiency curve the major advantage of the power curve is that the presentation of the collector output is not related to a certain area anymore. Hence the discussions about gross, aperture and absorber area become obsolete.

An other important modification made during the revision of the EN 12975 series is related to the mechanical load tests. The procedure was changed in such a way that the supporting frame is no longer subject of the standard. However the interface between supporting frame and collector is

used to fix the collector on the respective test rig. The load during the test is increased until the collector fails.

W. J. Platzer, A. Heimsath, C. Hildebrandt, A. Georg, G. Morin

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Corresponding Author, werner. platzer@fse. fraunhofer. de

Abstract

In order to accompany the demonstration of a full-size Linear Fresnel Collector (LFC) Fraunhofer ISE has developed characterization techniques to qualify optical and thermal key components within the projects »FRESQUALI« and »FRESNEL2« funded by the German Ministry of Environment, Nature Conservation and Nuclear Safety (BMU).

One key component for the LFC is the absorber tube with a Cermet coating, which has to be stable under atmospheric conditions for temperatures up to 450°C. Spectral emissivity and absorptivity changes have to be evaluated. Experiments show that coatings can improve due to heat treatment compared to freshly sputtered coatings.

Mirror optics using nearly flat primary glass mirrors and a secondary concentrator for the receiver construction are used to achieve a geometrical concentration in the order of 30-50 with respect to tube circumference. Deformations due to construction, internal stress of glass, wind load and gravity may lead to a distortion of the narrow focus line on the receiver. The acceptance angle of the secondary concentrator may deteriorate due to shape imperfections. Deterioration of specular reflectance due to dust and degradation also reduces performance. Several qualification techniques were used to support quality control for production and technical development.

Collector performance and possible improvements are discussed together with a comparison to parabolic trough collectors.

Keywords: Optical Testing, Thermal Testing, Durability, Optimization, Quality Control, Parabolic Trough, Linear Fresnel

1 Introduction

Having shown in previous feasibility studies [1,2] that the Linear Fresnel Collector (LFC) has a potential to generate direct steam sufficiently cheap to reduce the expected levelised electricity costs (LEC) of the solarthermal power plant by about 20% when compared to the parabolic through, the Fraunhofer Institute for Solar Energy Systems started developing key components for this new collector type with the support of the German Ministry of Environment, Nature Conservation and Nuclear Safety (BMU) within the project »FRESNEL2«. The aim was to produce an absorber coating being stable under atmospheric conditions up to temperatures of 450°C. Another point of concern was the bent mirror for the secondary concentrator which has to withstand temperatures between 250°C and 280°C. It is important for a economical viability of the LFC that the optical and thermal efficiency of the collector can be achieved in practice as simulated in the previous studies. Also the performance

should be stable over a long period of time, if not for the complete lifetime of a solar field. In our finished project mirror and absorber coatings with these specifications could be developed.

With the help of a second project »FRESQUALI«, also funded by the German Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), we developed further characterization and qualification methodologies which were used and tested using a local Fresnel process heat collector in Freiburg. They have been used also to assist the building of a LFC prototype on the Plataforma Solar de Almeria (see Figure 1). This paper describes the general approach and gives an overview of methodologies for quality assessment for the linear Fresnel collector and its subcomponents in laboratory or production phase. The design, construction, on-site quality control, commissioning and performance testing of the collector is presented elsewhere. [3,4]

S. Fischer* and H. Mdller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW), Pfaffenwaldring 6,

70550 Stuttgart, Germany

* Corresponding Author, fischer@itw. uni-stuttgart. de

Abstract

This paper presents the results of a CPC collector test according to EN 12975-2:2006 [1] using both accepted test procedures: the test method under steady state conditions and the quasi-dynamic test procedure. It is shown that only the test under quasi-dynamic conditions is suited to determine collector parameters that characterise the thermal performance of concentrating collectors such as CPC collectors to a desirable extent.

Keywords: Collector test, CPC collector, concentrating collectors, thermal performance

1. Introduction

CPC collectors (see figure 1) use a Compound Parabolic Concentrator as reflector in order to concentrate the solar irradiance on the absorber. Due to this concentration CPC collectors have, in comparison to the aperture area, a smaller absorber area which results in smaller heat losses. With reference to the operating temperature, CPC collectors therefore present a logical link between flat plate collectors and evacuated tube collectors.

CPC collectors available on the market usually have a concentration ratio in the range of C = 1 — 2. These concentration ratios, however, result in a smaller conversion of the diffuse irradiance compared to flat plate and evacuated tube collectors. During the determination of the thermal performance of CPC collectors this fact must be taken into account.

The European standard EN 12975 /1/ allows test methods to determine the thermal performance of solar thermal collectors with different test methods. At present the test method under steady state conditions as well as the method under quasi-dynamic conditions are used to determine the thermal performance of CPC collectors.

The present paper compares the results gained with the two different test methods. The comparison will show that the method under steady state conditions is not suitable to determine the thermal performance of CPC collectors correctly.

It is obvious that the diameter of hailstones is a good indicator for the damage potential of solar energy systems caused by hailstones. As shown in table 2, the standards for solar thermal collectors only define one value for the diameter. On the other hand, the standards for the quality assurance of PV- modules define in each case certain variations of the diameter. The major difficulty for the question on which ice ball diameter should be used to perform impact resistance tests is, that the kinetic energy of a hailstorm and the therewith associated damage potential, does not depend on this value alone. During a 16 years study in France (cf. 3), published in the Atmospheric Research, 3611 hailstorm events have been observed. The main topic of this study was to develop an understandable six class scale for public use to classify hailstorm events. But this is not of importance in this context. More interesting is that this study considered also the effective kinetic energy of severe thunderstorms with the result that the other most representative parameters, which directly influence the kinetic energy of hail events, are the total mass of hailstones per unit area, the total number of hailstones per unit area as well as the percentage of ground covered by hailstones [3]. Expectedly the diameter of hailstones is strongly correlated with the kinetic energy of hailstorms. But also the total mass of hailstones per unit area, which depends again on the total number of hailstones, is strongly correlated with the kinetic energy of a hailstorm. In consideration of all these parameter the resultant kinetic energy of a single hailstone against the grit size in cm is given in Fig. 7.

This shows that the values given in the standards are twice as high as those given in Fig. 7. From this point of view the normative requirement should be strong enough to ensure the impact resistance of solar energy systems. If we additionally take into account the frequency of hailstorms against the diameter of their hailstones, (cf. 3) it seems, that the usage of 25 mm grit size for testing procedures is

good to ensure the impact resistance of solar energy systems to severe hailstorms. This means, even though, that damages caused by larger hailstones, which are really seldom, have to be accepted.

The compliance of the requirements shown in Table 1 concerning the diameter as well as the mass of the ice balls are, verifiable in a easy way with the aid of the appropriate measurement equipment,. However, other requirements defining the quality of the ice balls such as “no cracks visible to the unaided eye” is subjected to the subjectiveness of the observer. Generally such a definition leads not to the same result concerning the quality of the ice balls. But the ice ball quality, independent from the listed parameters in Table 1, does have an essential influence of the result of the impact test, because a part of the energy of the impact of the ice ball is used for the destruction of the ice ball. The process of the production of ice balls therefore has to be defined in a way that the ice balls always show the same fracture mechanics. The production of the ice balls is therefore the most difficult and sensible procedure in respect to the performance of impact resistance tests. This fact results in the basic necessity to perform some representative studies to quantify the impact resistance of solar thermal collectors and photovoltaic-modules. For example, it is conceivable to define a material classification, with the objective to use other materials than ice, e. g. well defined polymer materials, which will cause the same result.

3. Conclusion

More attention should be given to the testing of solar collectors and PV-modules with respect to resistance against hailstorms. We have set up an apparatus for impact resistance tests at Fraunhofer ISE using ice balls as defined in the relevant European Standards. The present normative requirements are sufficient to ensure the impact resistance of solar thermal collectors and PV-modules to severe hailstorms. It is suggested to investigate if the test procedures can be simplified by using other projectiles than ice balls. Experimental studies have to be carried out with the objective to quantify the impact resistance of solar thermal systems.

References

[1] http://www. essl. org/ESWD/

[2] Dr. Matthias Fawer, Solarenergie 2007 — Der Hohenflug der Solarenergie halt an. Bank Sarasin & Cie AG, 2007

[3] J. Dessens, C. Berthet, J. L. Sanchez; A point hailfall classification based on hailpad measurements: The ANELFA scale, Atmospheric Research, Volume 83, Issues 2-4, February 2007, Pages 132-139, European Conference on Severe Storms 2004 — ECSS 2004, European Conference on Severe 2004

[4] CEI/IEC 61215: 2005-4, Crystalline silicon terrestrial photovoltaic (PV) modules — Desing qualification and approval, IEC 2005

[5] AS/NZS 2712:2007, Solar and heat pump water heaters — Design and construction, Jointly published by

Standards Australia, GPO Box 476, Sydney, NSW 2001 and Standards New Zealand, Private Bag 2439, Wellington 6020′

[6] EN12975-1,2:2006, Solar thermal systems and components — Part 2: Test methods; German Version, CEN 2006

[7] E 1038 — 5, Standard Test Method for Determining Resistance of Photovoltaic Modules to Hail by Impact with propelled Ice Balls, ASTM International

Xing Li1,2, Zhifeng Wang1 , Xiao Han2

1 Institute of Electrical Engineering, Chinese Academy of Sciences Beijing 100080, P. R.CHINA
2 Himin Solar Energy Group Co.,Ltd, Dezhou, Shangdong 253090, P. R.CHINA
Corresponding Author, lixing2002@126.com

Abstract

In the present paper, the automatic control system based on Labview for domestic solar water heating test system according ISO 9459-2 is investigated. It is a measurement and automation system composed of PXI (PCI extensions for instrumentation), control panel and all kinds of devices including temperature sensors, flow meters, magnetic valves, pumps and motors etc. Considering the outdoor test methods for system performance characterization of solar-only systems, in order to accurately measure the daily system performance, the coefficient of storage tank heat loss and the degree of mixing in the storage vessel during draw-off in sequence, it is essential to clarify the whole testing contents and design a set of logical control flow. In this study the program based on Labview is finished and adjusted successfully for selected configurations under the different measuring situations. The automation system is in good conditions and the accurate data is exactly attained according to the ISO standard. Besides, visualization of monitoring and evaluation of the test system is performed, giving insight into details of the measurement and control fields, which are valuable in the friendly interface between operator and test system.

Keywords: Automation, Labview, test system, solar water heating system

1. Introduction

As the environmental impacts of energy, solar thermal utilization attracted the extensive attention of the international community. China has become an annual output value of 6 billion industrial-scale solar water heater industries, and the possession of water heater stands on the leading position in the world. With the rapid development of solar thermal industry in China, 11 national standards with relevant to solar water heating systems have developed. But there is a big difference in the test procedures and test conditions between national standards and ISO standards. So test results of thermal performance according to the current GB standards can not be directly compare with the test results of ISO standards. Because the ISO standards are promulgated in the broad range of region and extensive applicability, our national standard system should be as far as possible with the convergence of the ISO standards in future, especially in the test conditions and test procedures. Otherwise no benchmark can be obeyed in the process of comparison between the test results of national standards and ISO
standards. In this paper, test method for domestic solar water heating system will be carried out according to ISO9459-2.

In order to improve the productivity and efficiency, it is necessary to develop a set of automatic control system to measure and auto-operate the test system. In the control system it will make use of PXI — based data collection system provided by National Instruments Company and design the control and data acquisition system based on Labview platform. Test data would be automatically measured and recorded; meanwhile control system could realize the corresponding control movements according to feedback of all kinds of sensors. In this paper, design principles, hardware structure, and software of the test system based on Labview will be presented and implemented.