Category Archives: EuroSun2008-5

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

Criteria

MM

FUKS

SP

IOC

ANN

GRS

KU

Automatic failure detection included?

++

++

++

++

++

Automatic failure identification included?

+

+-

+

Accuracy of failure detection

++

+-

?

+

?

+-

+

Accuracy of failure identification

++

+-

n. a.

+-

n. a.

n. a.

+-

Costs

(operational/hardware)

var

++ 100 €

+?

+

1190 €’

+?

10 k€2

+-

20-80€3

Monitored part of solar heating system (so far)4

var

sl

sl

sl, bs

sl

-aux

-aux

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

Abstract

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.

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Nuussuaq

75°

69°

76°

71°

Nuussuaq

-35°

-35°

-35°

-39°

Sisimiut

58°

58°

58°

55°

Sisimiut

12°

23°

10°

26°

Copenhagen

54°

51°

54°

52°

Copenhagen

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

 

image043

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.

 

image044

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.

 

image045

Distance between the center of the tubes [m]

 

image046image047

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

Abstract

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.

Experimental setup

The experiments are carried out in a domestic hot water tank and a space heating tank. Both tanks are of steel and have a volume of 400 litres with an internal height of 1.4 metres.

In the domestic hot water tank, fresh domestic water with high natural lime content, usual for Danish conditions, is led into the tank during cooling of the tank. During heating the water is circulated from the bottom of the tank through an external heat exchanger where it is heated and then led into the fabric pipe through the bottom of the tank. In this way domestic hot water is led into the fabric stratifier through the bottom of the tank. The inner surfaces of the domestic hot water tank are enamelled.

In the space heating tank, the same “dead” water is circulated. During heating the water is circulated from the bottom of the tank through an external heat exchanger where it is heated and then led into the fabric pipe through the bottom of the tank. In this way “dead” water is led into the fabric stratifier through the bottom of the tank. The inner surfaces of the space heating tank are not enamelled, no anode preventing corrosion is used and no additives preventing algae growth are added to the water.

image068

Fig. 1. The space heating tank (left) and the domestic hot water tank (right) used for the accelerated durability tests of the fabric inlet stratification pipes. In the front of the picture, a 27 kW heating element and two plate heat exchangers used for heating the water that circulates through the inlet stratifiers can be seen.

Подпись: Fig. 2. Fabric inlet stratifiers mounted in a circle at the cap in the bottom of the tank.

The fabric inlet stratifiers consist of two concentric fabric pipes, with diameters of 40 mm and 70 mm and the pipes are closed at the top. The pipes are mounted in a circle as shown in Fig. 2. The fabric inlet stratification pipes are secured at the cap in the bottom of the tanks, which is the only opening in the tank and spread over 93% of the tank height. Heating tests where cold tanks are heated with hot water through one fabric stratifier at the time, then cooled down again/heated again and so on are carried out during the test period. Tank temperatures, inlet and outlet temperatures and the volume flow rate through the inlet stratification pipes are measured during the whole test period.

2. Results

The results of the investigations show that several of the tested fabric pipes build up thermal stratification in exactly the same way after being in operation for a long time in the space heating tank. The investigations also show that this is not the case with any of the fabric pipes in the domestic hot water tank.

Table 2 shows the investigated fabric styles and the main results. Most likely there is a problem with deposits of lime attached to all pipes in the domestic hot water tank resulting in a strongly decreased ability to build up thermal stratification for all the fabric pipes tested.

For the space heating tank with “dead” water five of the tested fabric inlet stratifiers work at the end of the test period without decreased ability to build up thermal stratification, while two fabric inlet stratifiers have decreased ability to build up thermal stratification.

In order to eliminate small differences in the start temperature and inlet temperature from one experiment to the next in tests conducted with the same operation conditions, the normalized temperatures are used when comparing the thermal stratification profiles in the following figures. The normalized temperature is defined as:

(T — Ttank, start)/(Tinlet-Ttank, start ) (1)

In Fig. 3 — Fig. 5 normalized temperatures as functions of the height in the tanks obtained during heating periods are shown in the middle and at the end of the test period. For example, the heating curves for the fabric inlet stratification pipes style 769, 700-12 and 981 are shown for tests with a volume flow rate of 6 l/min, inlet temperatures of 44°C — 46°C over a time period of five months.

image070
image071

Curves after 10, 20, 30 and 40 minutes of heating are shown. From the figures it is clear that capability to build up thermal stratification is strongly decreased in the domestic hot water tank during the five-month period for the three shown fabric pipes. For the fabric pipes in the space heating tank, the capability of building up thermal stratification is slightly decreased for the fabric pipe style 981 while the capability of building up thermal stratification for the fabric pipes style 769 and 700-12 is unchanged during the five month-period.

Table 2. The tested fabric styles and main results.

Fabric Style #

Domestic hot water tank

Space heating tank

Style 361

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is decreased within the test period.

Style 769

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is unchanged within the test period.

Style 700-3

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is unchanged within the test period.

Style 700-12

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is unchanged within the test period.

Style 864

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is unchanged within the test period.

Style 867

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is unchanged within the test period.

Style 981

The fabric inlet stratifier fails to build up thermal stratification with time.

The performance of the fabric inlet stratifier is decreased within the test period.

-0.2 0 0.2 0.4 0.6 0.8 1 -0.2 0 0.2 0.4 0.6 0.8 1

(T_Ttank, start)/(TinleirTtank, start) [-] (T-T tank, start)/(TmlefTtank, start) [-]

Fig. 3. Temperature profiles for heating tests with the fabric inlet stratification pipe style 769 in the domestic
hot water tank (left) and the space heating tank (right). The volume flow rate is 6 l/min.

Fig. 6 shows the fresh fabric styles 700-12 (left) and 981 (right) before the tests. Fig. 7 and Fig. 8 show the fabric styles after the test period in the domestic hot water tank and in the space heating tank, respectively. The pictures are of the inside of the top of the inner fabrics. The fabric style 700-12 is a knit fabric with a dense structure while fabric style 981 is a woven fabric with relatively large holes in the structure. All the dark areas in the pictures are holes in the fabric structure. The pictures in Fig. 7 clearly show the lime deposits along the fabric fibres on both fabric styles. The pictures in Fig. 8 show smaller fractions of deposits, presumably from dirt and algae.

In the pictures, it can be clearly seen, that the fabrics are still permeable after the tests, especially the fabric style 981. However, deposits may result in stiffness of the fabric and hence prevent the fabric from contracting in order to equalize the pressure difference between the fabric pipe and the water in the tank in levels where the temperature in the pipe is higher than the temperature in the tank. This will result in the fabric starting to behave as a rigid pipe with holes where the pressure causes water to enter the fabric pipe at lower levels and thereby reducing the temperature in the pipe. Such a pipe is a mixing device rather than a stratification device [4].

Fig. 6. Pictures of two fabric styles before being tested. Left: Fabric style 700-12. Right: Fabric style 981.

 

image072

Fig. 7. Pictures of two fabric styles after being tested in the domestic hot water tank. Left: Fabric style 700­12. Right: Fabric style 981.

 

image073

Fig. 8. Pictures of two fabric styles after being tested in the space heating tank. Left: Fabric style 700-12.

Right: Fabric style 981.

 

image074

In order to quantify the mass of the deposits attached to the fabric pipes, the pipes were taken out of the tanks and weighted. Figure 9 shows the gained weight of each fabric pipe.

6

 

Style 700-12

■ Style 769

■ Style 700-3

■ Style 867 Style 361

■ Style 864 Style 981

 

Domestic hot water Domestic hot water Space heating tank — Space heating tank — tank — Inner pipe tank — Outer pipe Inner pipe Outer pipe

 

Fig. 9. Additional weight of the fabric inlet stratification pipes after the test period of one year.

For most fabrics, the deposit amounts are greater for the domestic hot water tank than for the space heating tank. However, for style 867 it is the other way around. For this fabric the ability to build up thermal stratification is not decreased in the space heating tank while the ability is strongly decreased in the domestic hot water tank in spite of the small deposited amounts. This indicates that it is the structure of the deposits that is important. Most likely the deposits result in rigid inflexible pipes while dirt and algae deposits at least for a long period of time do not destroy the flexibility of the fabric pipes.

 

image075

5. Conclusion and outlook

The long time durability of seven different fabric inlet stratification pipes is investigated experimentally in a domestic hot water tank and in a space heating tank. The results show that the lime contained in the domestic water is deposited in the fabric pipes in the domestic hot water tank and that this destroys the capability of building up thermal stratification for the fabric pipe. The results also show that although dirt, algae etc. are deposited in the fabric pipe in the space heating tank, the capability of the fabric inlet stratifiers to build up thermal stratification is unchanged for five of the seven fabric pipes within an operation period corresponding to operation of a solar heating system with a solar collector area of 10 m2 with a volume flow rate of 0.2 l/min per m2 solar collector area in 2/3 year.

Most likely the deposits result in rigid inflexible pipes while dirt and algae deposits, at least for a long period, do not destroy the flexibility of the fabric pipes.

The investigations have also shown that there is a need for further research on:

• Durability tests for much longer period of time of fabric inlet stratification pipes in a space heating tank

• The influence on the amount of deposits of dirt, algae etc. in the fabric pipe by adding e. g. an anode for corrosion protection and additives to prevent algae growth in the space heating tank used for the durability tests

• The durability of fabric inlet stratification pipes at high temperatures (90-95°C)

• How the mounting of the fabric inlet stratification pipe influences the capability of building up thermal stratification

Nomenclature

T Measured temperature in the tank during the experiment [°C]

Ttank, start Measured temperature in the tank at the beginning of the experiment [°C]

Tinlet Measured inlet temperature [°C]

References

[1] E. Andersen, S. Furbo, J. Fan, (2007). Multilayer fabric stratification pipes for solar tanks, Solar Energy, Vol. 81, pp. 1219-1226.

[2] E. Andersen, S. Furbo, M. Hampel, W. Heidemann, H. Muller-Steinhagen, (2007). Investigations on stratification devices for hot water heat stores, International Journal of Energy Research, May 2007.

[3] E. Andersen, S. Furbo, (2006). Fabric inlet stratifiers for solar tanks with different volume flow rates, in proceedings of EuroSun 2006 Congress, Glasgow, Scotland

[4] L. J Shah, E. Andersen, S. Furbo, (2005). Theoretical and experimental investigations of inlet stratifiers for solar storage tanks. Applied Thermal Engineering 25, pp. 2086-2099.

PET and PVC

The PET samples became brittle and degenerated already after 10 years of exposure. In the case of PVC high losses in transmittance were observed, but mechanical destruction occurred only after 20 years. These samples showed a colour change over yellow and brown to a non-transparent black. This effect was accelerated when exposed under a protective glazing with high transmittance, indicating that temperature and not radiation dose is the driving parameter.

5. Conclusion

PMMA showed the lowest soiling of the investigated polymers and no significant degradation, which resulted in a better overall weathering performance than the tested glasses. Some PMMA — types showed a persistent UV-blocking over the whole 20 years of exposure. They can be recommended to serve as weather resistant UV-protection coating for other materials such as PC for example. The fluorinated polymer films suffered from surprisingly high total losses mainly caused by soiling (FEP and PVF). For the ETFE samples high losses remained even after cleaning with ethanol such that material degradation can’t be excluded. The tested PC types showed yellowing effects and material erosion already after some years of exposure. The UV-protection additives of the tested PC-types were not able to prevent material degradation at the surface. The tested PET, PVC and UP products are unsuitable for the use as collector glazing because of high losses in transmittance or fast mechanical destruction.

1

High losses in solar transmittance due to soiling were observed (especially in Rapperswil). Structured glass-surfaces for reflex reduction had no effect on soil accumulation. Soiling leads to elevated losses in the solar transmittance of the collector glazing which can be assigned to the collector efficiency within wide limits. For that reason a regular cleaning of solar collectors is recommended, especially for polluted sites or glazing composed of FEP or PVF. PMMA, FEP and PVF have a better cleaning capability than glasses.

Acknowledgements

The work presented has been supported by the Swiss Federal Office of Energy SFOE. Some results could have been contributed to the IEA SHC Task 39. Many thanks go to Ueli Frei who initiated this project and to Thomas Hauselmann for many hours of sample preparation and measurements.

References

[1] Gary Jorgensen, Adoption of General Methodology to Durability Assessment of Polymeric Glazing Materials, IEA SHC Task 27.

[2] J. Vitko, Jr, J. E. Shelby, Solarisation of Heliosat Glasses, Solar Energy Materials 3 (1980) 69-80.

[3] Colom X., Garcia T., Sunol J. J, Saurina J., Carrasco F., Properties of PMMA artificially aged, Journal of Non-Crystalline Solids 28 (2001) 308-312.

[4] G. F. Tjandraatmadja, L. S. Burn, M. C. Jollands, Evaluation of commercial polycarbonate optical properties after QUV-A radiation—the role of humidity in photodegradation, Polymer Degradation and Stability 78 (2002) 435-448.

[5] Ram A., Zilber O., Kenig S., Life expectation of polycarbonate. Polymer Engineering & Science. 25 (1985) 535-540.

[6] A. Davis, D. Sims, (1983). Weathering of Polymers, Applied Science Publishers LTD, Essex, England.

Normative requirements of „Impact Resistance Tests“

Table 2 shows a comparison of the present requirements of the methods of impact resistance tests of the transparent cover of solar thermal collectors and pv-modules which directly influence the energy transmitted during the impact and thus also the damage potential. Only requirements concerning tests with ice balls and no steal ball tests are listed.

Table 2. Normative requirements to the diameter, the mass and the velocity of the ice balls.

Durchmesser In mm

Masse In g

Geschwindigkeit in m/s

Kinetische Energie in J

EN12975-1,2:2006

25,0

7,53

23,0

1,99 ± 0,10

AS/NZS 2712:2007

25,0

7,50

23,0

1,98 ± 0,10

IEC61215: 2005-4

12,5

0,94

16,0

0,12 ± 0,01

15,0

1,63

17,8

0,26 ± 0,01

25,0

7,53

23,0

1,99 ± 0,10

35,0

20,70

27,2

7,66 ± 0,38

45,0

43,90

30,7

20,69 ± 1,04

55,0

80,20

33,9

46,08 ± 2,31

65,0

132,00

36,7

88,89 ± 4,45

75,0

203,00

39,5

158,37 ± 7,93

ASTM E 1038 — 5

15,0

1,60

17,2

0,24 ± 0,01

25,0

7,50

22,2

1,85 ± 0,09

35,0

20,60

26,3

7,11 ± 0,36

45,0

43,90

29,8

19,47 ± 0,98

55,0

80,10

32,9

43,42 ± 2,18

65,0

132,20

35,8

84,70 ± 4,24

75,0

203,00

38,5

150,07 ± 7,51

85,0

296,00

40,9

248,00 ± 12,41

The allowanced deviation of the diameter, the mass and the velocity of the ice balls, in all listed standards are ± 5% of the required values with exception of the Australian standard AS/NZS 2712:2007. The accepted deviations defined within the Australian standard are ± 1 mm in the diameter, ± 1 g in the mass of the ice ball and ± 1 m/s concerning the velocity of the ice ball. The given tolerances for the resulting kinetic energy are determined from these deviations according to the Gaussian error propagation. Unlike to the other listed standards the American standard ASTM E 1038 — 5 demands the calculation of the resultant velocity according to equation 1 and equation 2 against a given wind speed.

^ = vf + vl (1)

vt = 4.4^Jd (2)

where d is the diameter, vr is the resulting velocity, vt is the velocity of the ice ball and vw is the wind

speed (0 m/s, 15 m/s, 20 m/s or 30 m/s) chosen by the applicant of the test or the testing institution. However, for better comparison the wind speed is neglected in this consideration. Further requirements regarding the production, the handling and the quality of the ice balls as well as requirements concerning the test assembly, the framing conditions and the test procedures are quite similar in the discussed standards. The differences are:

• AS/NZS 2712:2007 defines a higher storage temperature of the ice balls than the other standards (0°C in comparison to -4°C ± 2°C).

• The maximum time between the removal of the ice ball from the storage container and the launching of the ice ball is not specified in AS/NZS 2712:2007 and ASTM E 1038 — 5. Both other standards specify a maximum time of 60 seconds.

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.