Category Archives: EuroSun2008-5

AN EXPERIMENTAL ANALYZE OF A SOLAR COLLECTOR

G. Zorer Gedik1* and A. Koyun2

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

Abstract

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

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

1. Introduction

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

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

image001
image002

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

Reflectivity Measurement

image025 image026

The device shown in Fig. 15 consists of a laser and detector mounted on a support structure that can be positioned to measure reflectance of mirror surface samples from the ICPC. Using this device, a map of reflector performance for the ICPC array has been generated.

Fig. 18. Third Level Reflectance Degradation. Fig. 19. Fourth Level Reflectance Degradation.

Four levels of reflectance degradation are identified for the Sacramento site. At level 1 degradation, shown in Fig. 16, the reflector still performs well with just a minor change in the reflector appearance. In level 2 degradation, shown in Fig. 17, there is some whitening of the reflector. In level 3 degradation, shown in Fig.18, there is a substantial amount of degradation of
the reflector. In level 4 degradation, shown in Fig. 19, most of reflector is gone and you can easily see through it.

At the site, all 336 tubes were categorized, one-by-one, by their reflectivity levels, existence of a glass crack, surface temperature, water leakage, and fin orientation. Each tube was divided into ten sections along its length. Degradation levels were identified and marked for each of the ten sections. Fig. 20 shows a color mapping of tube degradation information for a portion of the array.

Подпись:

Подпись: 3. Conclusions A detailed ray trace analysis for characterizing the optical performance of ICPC evacuated tubes has been described and its results illustrated. As a consequence of the ray tracing, it was found that reflectivity degradation will play a significant role in the reduction of array efficiency. The nature of reflectivity degradation depends on the type of failure, such as water leakage from the heat transport tube or cracks in the cover glass. Overall performance is also degraded by the loss of vacuum in the tube. An analysis of the performance consequences of reflector degradation and loss of vacuum degradation will be incorporated into the reliability study.

Reflector samples representative of the four different degradation levels were taken from the Sacramento site for measurements in the laser laboratory at Colorado State University, as in Fig. 21. The samples for the four levels of degradation and undegraded reflector samples were measured for their reflectivity by the laser and detector device. Using this device, a map of reflector performance for the ICPC array is being generated. The reflectance results are shown in Table 1 for each level of degradation.

References

[1] Garrison, J. D., Optimization of Fixed Solar Thermal Collectors, Solar Energy, v23, 1979

[2] Snail, J. J., O’Gallagher and R. Winston, A Stationary Evacuated Collector with Integrated Concentrator, Solar Energy, v33, 1983

[3] Подпись: Fig. 21: Laser and Sensor Assembly in the Colorado State University Laser Laboratory. Winston, R, O’Gallagher, J., Mahoney, A. R., Dudley, V. E. and Hoffman, R., “Initial Performance Measurements from a Low Concentration Version of an Integrated Compound Parabolic Concentrator (ICPC)”, Proceedings of the 1999 ASES Annual Conference, Albuquerque NM, June, 1998 [5] [6]

O’Gallagher, Tom Henkel and Jim Bergquam, “Performance of the Sacramento Demonstration ICPC Collector and Double Effect Chiller in 2000 and 2001”, Solar Energy, vol. 76, pages 175-180, January 2004.

[6] Duff, William, Jirachote Daosukho, Klaus Vanoli, Roland Winston, Joseph O’Gallagher, Tom Henkel and Jim Bergquam, “Comparisons of the Performance of Three Different Types of Evacuated Tubular Solar

Collectors”, American Solar Energy

Table 1: Measurement of Reflectivity S°ciety 2006 Denvei;

Colorado, July 2006.

[7] Подпись: Degradation Level Percent Reflectivity Good 93.48 1st 79.66 2nd 38.46 3rd 22.93 4th 1.24 Duff, William S. and Jirachote Daosukho, “A Performance and Reliability Study of a Novel ICPC Solar Collector Installation”, American Solar Energy Society 2007 Congress, Cleveland, Ohio, July 2007.

The building design project documents and organization

It would be important to ensure that the homeowner or the condominium could have an operating and instructions manual, identifying equipment schemes, safety procedures, conservations and maintenance information about the solar collector system. It is fundamental to implement some control and management measures during the life-cycle time of these equipments, especially in summer, to guarantee durability, efficiency and therefore turning this investment cost-effective. The obligatory for solar collector brings to discussion the implementation of a repair and maintenance design project for residential buildings, with a wider scope then solar collector’s subject. Residential building design is today facing fundamental changes and energy efficiency seems to be a start step to new design and property management approaches. The intelligent residential buildings concept, slightly inspired in a facility management concept in service buildings, is been slowly implemented in buildings. Monitoring and supervising devices as well as controlling and regulating systems in buildings appealed to the development of building automation. Other new building regulations are been enacted forward to get sustainable buildings, are examples: measures to better water supply and waste water management; more rigorous fire safety rules and security. A repair and maintenance design project turns to be essential and it is almost obvious that the obligatory use of solar collectors is going to accelerate this demand by

local government authorities. Also, the health and safety design project must be adapted to this new reality. A care should be taken to new risks, for example, the potential for excessive temperature and pressure in heat transfer fluids and stored water, the risk of Legionnaires’ disease, working at height and electrical safety.

Conclusion, discussion and outlook

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.

Steady State (SS) versus Quasi-dynamic (QD) test methodology

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.

Table 1: Test conditions and allowable deviations in relation to the mean values for a test period.

SS Method

QD Method

Value

Allowable deviation *

Value

Allowable deviation *

Global irradiance on collector aperture (G)

> 700 W. m-2

± 50 W. m-2

300 < G < 1100 W. m-2

Incidence angle (0)

< 20 °

Diffused fraction (Gd/G)

< 30 %

Air speed (u)

3 m. s-1 ± 1 m. s-1

1 < v < 4 m. s-1

Collector inlet temperature (te)

± 0,1 K

± 1 K

Ambient temperature (ta)

± 1,5 K

Mass flow rate (m )

± 1 %, 10 %*

± 0.02 kg. s-1.m-2

± 1 %; ± 10 %**

Regarding the mean values for a stable test period (Steady-State test) or for the total test period (Quasi-Dynamic test).

If there is no indication to the contrary, fluid flow rate should be approximately 0.2 kg. s-1 per square metre of collector reference area (A). Flow should be kept stable at ± 1% of the value determined during each test period, and should vary by no more than ± 10% compared to the other test periods._________________________________________________

Strip angle and strip width

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

Подпись: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.

Подпись: Fig 4. Improvement of the thermal performance as a result of a larger absorber area. 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 all-in-one test facility

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.

Ambient air temperature

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

Solar collectors — EN 12975 Part 1 and 2

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.

Table 3: Titles of European solar thermal standards at present

Number

Title “Thermal solar systems and components-…”

EN 12975-1:2006

Collectors — Part 1 — General requirements

EN 12975-2:2006

Collectors — Part 2 — Test methods

EN 12976-1:2006

Factory Made Systems — Part 1 — General requirements

EN 12976-2:2006

Factory Made Systems — Part 2 — Test methods

CEN/TS 12977-1:2009*

Custom Built Systems — Part 1 — General requirements for solar water heaters and combisystems

CEN/TS 12977-2:2008*

Custom Built Systems — Part 2 — Test methods for solar water heaters and combisystems

EN 12977-3:2008*

Custom Built Systems — Part 3 — Performance test methods for solar water heater stores.

CEN/TS 12977-4:2009*

Custom Built Systems — Part 4 — Performance test methods for solar combistores

CEN/TS 12977-5:2009*

Custom Built Systems — Part 5 — Performance test methods for control equipment

* expected year of publication.

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.

image007

Fig. 1. Example of a solar thermal collector “power curve”

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.

Quality Control of Concentrating Collector Components for the Optimization of Performance

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]

image076

Figure 1: View of the Fresnel demonstration collector by Solar Power Group and MAN Ferrostaal Power

Industry at the Plataforma Solar de Almeria.