Category Archives: EuroSun2008-5

Evaluation of Optical Properties

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

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

2. Results

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

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



Davos; losses in % (rel.)




Gain from cleaning in % (rel.)

Rapperswil; losses in % (rel.)




Gain from cleaning

Low Fe glass








Float glass








PMMA plate








PC plate




































PET, PVC and PC films








2.1. Glass

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

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

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

for the 20 year old glass samples from Rapperswil.





0 Uncleand x Soap О Ethanol




40 days 1 3 10 20 years


40 days 1 3 10 20 years




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


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

PMMA no declaration concerning UV-absorption




0.3 0.35 0.4 0.45 0.5

Wavelenght in micrometer


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

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

2.2. PC

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

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

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

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

4.4. Fluoropolymers

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

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

4.5. UP

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

Situation of European standards and quality assurance today

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

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

Application Area


Short description

Solar thermal energy


European standard:

efficiency and durability test of solar

thermal collectors

AS/NZS 2712:2007

Australian standard:

Efficiency and durability test of solar thermal collectors


IEC 61215: 2005-4

International standard: Efficiency and durability test of PV-modules

ASTM E 1038-05

International standard:

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

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

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

Horizontal fin ray trace analysis

Ray tracing analysis for the horizontal fin is shown

in Fig. 10. In Fig. 11 the optical efficiency plot for the horizontal fin ICPC shows an unbalanced

curve skewed to the right. The plot shows that the horizontal fin ICPC has greater energy collection efficiency in the evening than in the morning. As seen in Fig. 10, at an angle of 30 degrees, reflected ray striking the fin are both single and multiple reflections. As shown in Fig. 12, at an angle of 150 degrees, the only rays that are reflected to the fin are single reflection rays.

Fig. 13 shows efficiency effects of three different reflectivities of 1.00, 0.94, and 0.70. Efficiency is reduced for the smaller reflectivity values. The curve also skews more to the right for the smaller values of reflectivity.

Comparisons of different gaps between the absorber fin and the cover glass were analyzed by setting the reflectance to 1.00. As shown in Fig. 14, the efficiency is reduced as the gap between the absorber fin and the glass enclosure increases.

The building water supply design projects

The majority of the domestic water supply projects developed until now is based on very simply Domestic Hot Water (DHW) systems like electrically heated storage cylinder and gas or oil heating boilers. For Civil Engineers project designers the calculations were a quite simple task because they used to select the equipment without specific and demanding criteria. Nowadays, it is necessary to choose well the equipments to minimize its life-cycle cost because a minimum of three types of equipments must function together: the solar collector; the storage tank or cylinder

and the backup equipment (e. g. condensing oil or gas heating boilers). The system operation is also controlled by an automatic electronic unit. Not forgetting other system accessories, like pipes, pumps and valves [7,8]. The passive systems are commonly used on single residential buildings and they are more compact and easily mounted. But in multi residential buildings, there are several different consumers and it is necessary to think the most advantageous system installations option. Design professionals are dealing today with this dilemma because: they have little experience; they do not have much technical information to support their choices and there are not yet many multi­residential buildings with this technology in Portugal to learn with. Aware of this situation, a previous analysis for six system possible options (Table 1, Fig 1.) is presented next.

Table 1. Multi-residential buildings solar collector system options.



Water storage tank

Backup DWH System

Operation and maintenance management

System 1



Individual Boiler

All expenses in charge of each autonomous zone

System 2



storage tank internal burner

System 3 System 4





Individual boiler

water storage tank internal burner

A thermal energy meter for each autonomous zone or condominium management with a mensal rent

System 5



Collective boiler

A thermal energy meter for each autonomous zone

System 6



water storage tank internal burner

NOTE: Backup systems on electricity are not stimulated by regulations.

water storage tank


Fig 1. Multi-residential building solar collector system options.

Подпись: solar collectors solar collectors solar collectors

Each of these design options has, in a preliminary perspective, positive and negative points.

Systems 1 and 2 — All equipments are individual — Advantages: self management of the system by each family (they adapt the system to their consumptions needs); minimal problems and conflicts with neighbours; the solar panels could be mounted with different tilt angles for each apartment adapted to their consumptions needs. Disadvantages: great number of water pipes and accessories, more building interior space needed and more heat losses; more home interior space to install equipments; high maintenance expenses for each family; if one home produces solar energy

in excess it is not possible to redistribute it; in seasonal or not occupied spaces there is wasteful production of energy; more complexity on the system mounting. System 3 and 4 — Centralized solar collector — Advantages: less initial investment ; better optimization of the collector area and of the solar south-oriented roof area; optimization of the captured solar energy and a rational distribution for all consumers; management and maintenance in charge of the building condominium administration; the energy of seasonal occupied or not occupied homes can be used by others; each individual consumer must adapt their consumptions needs to a collective system. Disadvantages: more home interior space to install equipments; heat exchanger in inverted system; the householders must pay a service not total adapted to their consumptions needs; some homes can take more profit from the solar collector system then others, it depends of the consumption needs schedule and habits. System 5 and 6 — All equipments are sheared by all householders — Advantages: less pipes and accessories; more reduced initial investment; no individual system maintenance; householders pays a total service of hot water supply; better optimization of the collector area and of the solar south-oriented roof area. Disadvantages: initial part of the system with inverted supply system (water supply starts from the first floors); complex management of the return water of the circuit into the water storage tank; variations on hot water temperatures can occur; lack of preparation of the condominium administrations firms to manage this service; system regulated to a constant level of water temperatures not adapted to consume households schedule with less energy efficient management; requires an adapted collective interior space for a high capacity water storage tank and backup equipments; the building structure must resist to higher loads due to the heavy equipments installed; the building architecture must be prepared for maintenance, repair and substitution of big equipments; reduced preparation of the local authorities to deal with this collective systems (taxes to pay etc); big size systems are more vulnerable to operation problems and damages.

The first two systems require some initial investment and in some cases can lead to collector areas that are not effectively used. The second group of systems seems to have more vantages due to the fact that can result in a better flexibility in the system management, made by both condominium administration and individual consumers. The last systems also have some advantages but represent a more vulnerable operating system.

Results — Performance Matrix

The result of the comparison of the failure detection methods against the criteria described in §2.4 is presented in table 4.1. The performance matrix shows the performance of the failure detection method based on the different criteria. A few things need to be clarified before interpreting the table. First of all, it is assumed that without automatic failure detection there is a possibility for checking data with a manual analysis. This can be effective but is costly (dependent on the level of the analysis). This could be (and is partially) applied for all methods. Therefore, the criterion is limited to automatic failure detection. Because a lot of information is qualitative, it is hard to determine how much a method will cost in future application when it is still in the R&D phase. Also the effectiveness of a method may increase based on practical experience. Furthermore, the (literature) publications in general do not provide a very accurate description of effectiveness and accuracy of the method.

Table 4.1 Performance matrix









Automatic failure detection included?






Automatic failure identification included?




Accuracy of failure detection








Accuracy of failure identification



n. a.


n. a.

n. a.





++ 100 €



1190 €’


10 k€2



Monitored part of solar heating system (so far)4




sl, bs




Qualitative scale: ++ yes/very good/c

leap via +- = reasona

ble to — no/very bad/expensive

? = unclear

1 IOC: only hardware

2 costs for measurement equipment, including one year monitoring

3 Costs per month for 20 year monitoring and at least 30 monitoring systems sold. The main costs are

expected for maintenance and improvement of software (between € 15 and 50 per month) [11].

4 var = variable, sl = solar loop, — aux = whole system besides auxiliary heating system, bs = buffer

discharging loop (optional for IOC)

The IOC is the first method which could result into the implementation of a monitoring and failure detection method into general use of larger solar thermal systems. It has been tested and is commercially available against a reasonable price, but it does not apply to the whole solar system. Manual monitoring, though more costly, is much easier adapted to extensive variation in hydraulics and systems. The method developed at Kassel University is still in development, but could also provide an automatic monitoring solution for large systems. It includes more sensors and a larger part of the system than the IOC approach, and can therefore also analyse individual components. For very small systems the approach followed in FUKS detects several failures at reasonable additional costs.

However, so far none of the above described approaches takes the auxiliary heating system into account, which is also an important source of errors.

Comparison between Steady State and Quasi-dynamic test method. according to EN 12975 — application to flat plate collectors

Jose Afonso1, Nuno Mexa1, Maria Joao Carvalho ^

1 INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal
* Corresponding Author, mioaoa. carvalho@ineti. pt


Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) steady state test methodology according to EN 12975­2: section 6.1 and ii) quasi-dynamic test methodology according to EN 12975-2: section 6.3.

The most commonly used methodology is steady-state according to EN 12975-2: section

6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors.

But quasi-dynamic test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown by Rojas, D. et al. (2008).

As preparatory work for accreditation of this test methodology at the Solar Collector Testing Laboratory, tests according to the methodology were preformed.

The necessary tools for testing, such as data acquisition programme and a MLR (Multi Linear Regression) tool were developed. First results of the test of a flat plate collector are analysed. In the case of evacuated tubular collectors, test sequences were obtained and analysed showing the need for further development of the MLR tool.

Keywords: Solar thermal collectors, Test methods, Steady-state, Quasi-dynamic

1. Introduction

Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) Steady State (SS) test methodology according to EN 12975-2: section 6.1 [1] and ii) Quasi-Dynamic (QD) test methodology according to EN 12975-2: section

6.3 [1].

The most commonly used methodology is SS according to EN 12975-2: section 6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors. But QD test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown in reference [2].

The Solar Collector Testing Laboratory (LECS) of INETI is an accredited Laboratory for test of solar thermal collectors and the determination of thermal performance is made according to the SS test method (EN 12975-2:section 6.1 [1]). Two main reasons impose the need to develop the knowledge and the necessary practical conditions for use of the QD test methodology at LECS:

a) one is the improvement of use of time the collectors stay at the laboratory, allowing for a quicker response to the test requests [2];

b) the other is the fact that for some types of collectors, the QD test methodology allows for a better characterization of the collector behaviour [3, 4].

The SS test method requires steady conditions that significantly condition validation of the obtained data and therefore the time needed to conduct a test. The measured values for globlal irradiance, ambient temperature, fluid flow rate and fluid inlet temperature must not have significant variations (described in the standard [1]) for the set of data to be validated. When a steady state is not verified the set of obtained data is rejected.

The QD test method is much less restrictive. For example, the collector may continually be kept in the same position (facing south), regardless of the angle of incidence. There is also a greater variability of the measured values, which allows for more variable weather conditions. For this reason, use of the QD method to test solar collectors enables data to be obtained with less operator intervention. Because steady weather conditions are not required, solar collector tests may also be conducted over a greater annual period and in a more simplified manner. On the other hand, the calculation methodology, to determine the parameters that characterise the thermal performance of the collectors, is more complex.

In this work a short description of the test conditions for SS and QD test methods is presented in section 2. In this section, also the relevant equations and characteristic parameters for each test method are highlighted. Section 3. describes the different steps necessary or implementation of QD test method at the Laboratory. Section 4. presents and discusses the first test results obtained. In section 5. conclusions on the future development for implementation of QD test methodology, are presented.

Weather data

TRNSYS type 15-2 with weather data generated with Meteonorm 5.0 is used in the calculations. The weather data created with Meteonorm has been compared with reference years created based on a measurement period from 1992 to 2002. The amount of diffuse radiation is lower in the Meteonorm datafiles compared with the reference years. This is especially the case for Nuussuaq and Sisimiut. Further, the global radiation in the Meteonorm datafiles is higher than in the reference years. The direct radiation is therefore higher in the Meteonorm datafiles than the reference years.

2. Parameter variations

2.1. Collector tilt and orientation

Initial investigations are carried out with the validated models in order to determine the optimum tilt and orientation of the four solar collectors. The simulations are carried out with a constant mean solar collector fluid temperature of 60 °C during all operation hours of the year. The thermal performance of the collectors is investigated using the optimum tilt and orientation for each of the collectors. In Table 2 the values for the optimum tilt of the four solar collectors are given.

Table 2. Optimum tilt of the solar collectors. Table 3. Optimum orientation of the solar collectors.















































Previous investigations have shown that the optimum tilt roughly follows the latitude. In this case that is true for Nuussuaq and Copenhagen. For Sisimiut the optimum tilt is somewhat lower than expected, this is because the amount of beam radiation is high. In Table 3 the optimum orientations of the four solar collectors are shown. In Nuussuaq the collectors are best situated turned 35° or 39° from south towards

east. In Sisimiut the collectors with the curved absorbers should be turned 10° or 12° from south towards west. The collectors with flat absorbers should be turned 23° or 26° towards west. In Copenhagen the collectors should be turned slightly from south towards west. The following analyses of the different parameters will use the optimum tilt and orientation for each of the four solar collectors according to the location simulated. In Fig 2 the thermal performance of the



fluid temperature


Seido 5-8 —В— Seido 1-8

—9— Seido 10-20 with curved absorber —K— Seido 10-20 with flat absorber


Mean collector fluid temperature [ Cl


Fig 2. The thermal performance of the solar collectors in Nuussuaq using
optimum tilt and orientation.



increasing solar collector fluid temperature faster in Nuussuaq than in Copenhagen. This is caused by


the cold climate in Nuussuaq. The mean solar collector fluid temperature used in the following


analyses is 60 °C.



Distance between the center of the tubes [m]



one glass tube to the neighbouring glass tube is reduced. Seido 10-20 with curved and flat absorber decreases more rapidly with an increase in the distance than Seido 5-8 and Seido 1-8. This is due to the larger numbers of glass tubes in the Seido 10-20 solar collectors than the Seido 5-8 and Seido 1-8. The increased centre distance will therefore result in a strongly increased length of the manifold. The

Подпись:optimum distance for each of the collectors at the different locations is shown in Table

3. The optimum tilts of the solar collectors are affected by a change in the glass tube centre distance. A change in the distance from the values given in Table 1 to the optimum distance values for the solar collectors located in Nuussuaq will result in an increase in the optimum tilt with up to 10°. For Sisimiut and Copenhagen a change in the distance from the values form Table 1 to the optimum values will only result in a small change in the optimum tilt, about 1-2°. The optimum orientation is also found for the optimum distance between the glass tubes. For the solar collectors located in Nuussuaq the optimum orientation is turned even more from south towards east, from about 35° to 65°. In Sisimiut a change in the centre distance of the glass tubes with the curved absorbers will result in a decrease in the optimum orientation from 12° and 10° to 2° and 5° from south towards west. The collectors with the flat absorbers located in Sisimiut have an increase in the optimum orientation from 23° and 26° to 37° and 36° from south towards west. In Copenhagen the optimum orientation changes only slightly with a change in the centre tube distance from about 2° to 5° from south towards west. An improvement of the centre distance between the glass tubes will only result in slight improvement of the thermal performance of the collectors with the flat fins at a mean collector fluid temperature of 60 °C. For the collectors with the curved fin the improvement of the thermal performance is in the range of 8-10 %, the largest improvement seen in Nuussuaq and the lowest seen in Copenhagen. Again a decrease in performance is seen for all the collectors at all three locations with a mean collector fluid temperature is 100 °C

Development of Test Facilities for Solar Thermal Collectors and Systems

D. Bestenlehner[7], H. Widlroither[8], H. Drtick1, S. Fischer1, H. Mtiller-Steinhagen1

1Solar- und Warmetechnik Stuttgart (SWT)

Pfaffenwaldring 6, 70550 Stuttgart, Germany
Tel.: +49711 / 685-60155, Fax: +49711 / 685-63242
Email: bestenlehner@swt-technologie. de; drueck@swt-technologie. de
Internet: www. swt-technologie. de
2Fraunhofer-Institute for Industrial Engineering (IAO)
Nobelstrasse 12, 70569 Stuttgart, Germany
Email: harald. widlroither@iao. fraunhofer. de


As a consequence of the booming solar thermal business, a considerable number of new so­lar thermal products, such as solar thermal collectors or complete systems, are entering the market. For the determination of the thermal performance and to ensure the durability and reliability of these solar products, testing is an important aspect.

In order ensure comparable and representative results, tests of solar thermal products are car­ried out according to well established procedures. Such test procedures are specified e. g. in the European Standard EN 12975 or ISO 9806 for solar thermal collectors and in the stan­dard series ISO 9459 for solar thermal systems.

In order to perform the tests according to the above-mentioned standards, test laboratories and manufacturers require appropriate test facilities. Depending on the type of test, separate test facilities are used. These test facilities most often differ significantly, depending on the components to be tested. As a consequence for test laboratories and manufactures, the num­ber of test facilities increases if different products, such as solar thermal collectors and com­plete factory made solar domestic hot water systems, shall be tested. The growing number of individual test facilities results in relatively high investment costs and leads to significant operational costs e. g. for maintenance and calibration of sensors.

One possibility to overcome these problems and to reduce the number of different test facili­ties is to combine identical functions that are required for testing different products into only one multifunctional test facility.

For determination of the thermal performance of either a complete solar thermal system or only the solar collector, two different test facilities are typically used. Taking into account the two different ways to determine the thermal performance of a solar domestic hot water system (so-called CSTG1 -method and DST2 — method) even three different test facilities may

be required. To minimize the amount of hardware and the required investment capital as well as operational costs, a so-called three-in-one test facility was developed by SWT- Technologie, Germany.

This mobile, stand-alone solar thermal collector and system test facility is a complete test fa­cility for the determination of the thermal performance of solar thermal systems according to standards ISO 9459-2 (CSTG-method), ISO 9459-5 (DST-method) and the thermal perform­ance of solar collectors according to standards EN 12975 or ISO 9806 respectively. Due to this three-in-one approach, substantial investment costs for construction and operational costs for maintenance of three different solar test facilities can be saved by the test labora­tory or manufacturer respectively.

Since tests are specific to certain characteristics or properties of solar thermal systems and their components, it is obvious that not all possibly required tests can be performed with only a single test facility. Hence additional test facilities are required e. g. in order to perform durability and reliability tests of solar collectors.

1. Introduction

Testing of solar thermal collectors and systems is required in order to asses the thermal perform­ance and the quality of these products. This is especially necessary since solar thermal technology is a booming market and a wide range of solar collectors and systems are produced by numerous manufacturers all over the world.

Well established test procedures for solar collectors are specified in the European Standard EN 12975 or the international standard ISO 9806, and for solar thermal systems in ISO 9459-2 (CSTG-method) and ISO 9459-5 (DST-method).

In order to perform the tests specified in these standards, each test laboratory or manufacturer re­quires appropriate test facilities. Usually separate test facilities are used for collector and system testing. Typically, these test facilities are individually designed and installed at a specific location.

Emulated water supply mains

The energy balance of the storage tank is drawn a number of times within the course of the efficiency test of SDHW systems. This is achieved by means of highly accurate temperature sensors in the inlet and the outlet of the storage tank.

Doing this more than the entire volume of water of the storage tank is exchanged. This needs a large amount of fresh water.

The new testing laboratory is independent from the water supply mains. Those are emulated by a cold-water-buffer-tank in combination with a process thermostat having an adequate and sufficient cooling power.

This setup provides advantages to carry out the necessary measurements and experimental investigations.

It is possible to adjust the inlet temperature into the storage tank. That way, the cold water inlet temperature will be exactly the same between two tests. This enhances the reproducibility of the tests. Especially regarding the test of the “ability of the solar plus supplementary system to cover the load” mentioned above. This test is sensible to the cold water inlet temperature.

The adjustment of the cold water inlet temperature is possible with a very high accuracy (0.1 K). That is a basic requirement for the testing of heat storages according prEN 12977-3 [2]. Thus it is possible to not only perform the testing of SDHW systems but also of heat storages.

Measuring errors due to a fluctuating water inlet temperature are reduced. The public water main supply is not always at a constant temperature.

The closed circuit can b

e vented. Measuring errors due to air bubbles in the water mains supply are decreased.

Classification of thermal solar systems

As can be seen from the title of the standards, the thermal solar systems have been divided into two groups: “factory made systems” and “custom built systems”. This division was necessary in order to be able to include the whole spectrum of thermal solar systems in Europe, which ranges from small compact systems (thermo-siphon and integrated collector-storage-systems) to very large systems individually designed by engineers. The classification of a system “as factory” made or “custom built” is a choice of the final supplier in accordance to the following definitions:

Factory made solar heating systems are batch products with one trade name, sold as complete and ready to install kits, with fixed configuration. Systems of this class are considered as a single product and assessed as a whole. For the determination of its thermal performance, such a system is tested as one complete unit. If a factory made solar heating system is modified by changing its configuration

or by changing one or more of its components, the modified system is considered as a new system for which a new test report is necessary.

Custom built solar heating systems are either uniquely built, or assembled by choosing from an assortment of components. Systems of this category are regarded as a set of components. The components are separately tested and test results are integrated to an assessment of the whole system.

Custom built solar heating systems are subdivided into two categories:

Large custom built systems are uniquely designed for a specific situation. In general HVAC engineers, manufacturers or other experts design them (HVAC: heating, ventilation, air­conditioning).

Small custom built systems offered by a company are described in a so called assortment file, in which all components and possible system configurations, marketed by the company, are specified. Each possible combination of a system configuration with components from the assortment is considered as one custom built system.

Table 2 summarises the different types of thermal solar systems. As a consequence of this way of classification, forced circulation systems can be considered either as factory made or as custom built, depending on the market approach chosen by the final supplier. Hence it is essential that the performance of these systems is determined for the same set of reference conditions as specified in Annex B of EN 12976-2 and Annex A of ENV 12977-2.

Table 2: Division criteria for factory made and custom built thermal solar systems

Factory Made Solar Heating Systems

Custom Built Solar Heating Systems

Integral collector-storage systems for domestic hot water preparation

Forced-circulation systems for hot water preparation and/or space heating, assembled using components and configurations described in a documentation file (mostly small systems)

Thermosiphon systems for domestic hot water preparation

Forced-circulation systems as batch product with fixed configuration for domestic hot water preparation

Uniquely designed and assembled systems for hot water preparation and/or space heating (mostly large systems)

During the past four years the standards listed above were revised and updated. This procedure resulted in the publication of the following revised version or new parts of standards respectively (see Table 3). The most important changes and highlights resulting from the revision will be described separately for each of the three standard series in the following chapters.