Category Archives: EuroSun2008-5

Evacuated Tube Reliability

After a year of operation several distinct patterns in the development of cracks in the evacuated tubes emerged. One of these involved the production sequence or, equivalently, the fin orientation and the other, the end of the tube where the crack occurred.

Подпись: the second half). Statistically, if one assumes that the entire production run is characterized by the overall fraction of cracked tubes of 0.05865 then the likelihood that the first half of the production run came from such a process is less than 0.3 percent. Moreover, after six years of operation only 3.6 percent of the vertically finned tubes had developed cracks, whereas the horizontally finned tubes continued to develop cracks at a much higher rate. Since the evacuated tubes were essentially hand built, this 3.6 percent failure rate is about what one would expect. The end caps of each end of the evacuated tubes were identical, each consisting of a dish shaped piece of glass and a metal cap bonded to the glass. At the top end a metal tubulation was brazed to the metal cap to provide flow of heated fluid. At the bottom end a metal tubulation was brazed to the metal cap to provide a means to evacuate the tube. Thus, only the top end was subject to both thermal stress (the 155C fluid) and mechanical stress (partial support of the fin and heat transport tube). One might expect the failure rates due to cracking to be higher at the top end of the tube than at the bottom. In fact the opposite occurred. Out of 19 cracked tubes after one year, 7 were cracked at their tops and 12 at their bottoms. Statistically, if one assumes that the true proportion of cracks at the top to be 60 percent, then there is only a 0.1 percent chance that one would observe seven or fewer cracks out of 19 at the top end. Optical Performance Modeling and Experimentation 2.1 Graphical Ray Tracing
Подпись: -80
Подпись: -100 1 1 1 1 1 1 1 1 1 1 -100 -80 -80 -40 -20 0 20 40 80 80 100
Подпись: Fig. 5: Rays Striking the Vertical fin ICPC at a Nominal Angle of 44 Degrees.
Подпись: Fig. 6: Optical Efficiency (Vertical Fin) from Nominal Angles of 15 to 165

Vertical and horizontal tube absorber orientations were produced in the first and second halves of the ICPC tube production run respectively. One year after installation 1.2 percent of the vertical fin orientation tubes and 9.8 percent of the horizontal tubes had developed cracks. This strongly suggests that there were distinct differences in the longevity of the vertically finned tubes versus that of the horizontally finned tubes (or, equivalently, of the first half of the production run versus

image018 image019

Fig. 4 and 5depict the results of an animated Fig. 10: Rays Striking the Horizontal Fin

graphical ray tracing simulation that has been icpc at a Nominal Angle of 30 Degrees.

designed to investigate the optical perperformance of the ICPC. See Duff, et al [7]. Factors

incorporated are the transmittance of the glass tube, the reflectivity of the reflective surface, the gap between the tube surface and the fin and the absorptance of the fin. The sun ray is simulated as discrete uniform rays over a range of incident angles from 15 degrees to 165 degrees. The rays are followed through the glass envelope, to the reflector and to the absorber fin. The number of rays absorbed is recorded. The collector efficiency graph of Fig. 6 shows the amount of energy absorbed during a typical daytime period.

Portuguese buildings market on solar collectors

In the 80s decade there was made an important investment in solar collector technology in Portugal but some problems related with an insufficient developed technology and inexperienced installer firms have been created a negative image on the public eye. On the year of 2000, some studies

estimated that Portugal had installed 239 500 m2 of total collector area but many were not operational and never performed up to expectations. This value was quite distant from Greece (2 815 000 m2) and Turkey (750 000 m2) but quite equal to Spain (399 922 m2) and Italy (344 000 m2) [5]. The studies that have been elaborated by the Solar Thermal Energy Observatory since 2003, demonstrate that collectors market is been growing. The more recent study, from 2006, reveals that the collector area installed was 28 300 m2, an increment of 49% relatively to the previous year, estimating that total area installed in Portugal was 253 000 of m2 [6]. Small domestic systems represented 65% of the total market and multi-residential buildings were residual. It is true that solar collectors market is growing but is still very far from the 150 000 m2 /year ambitioned initially. Noticed that new regulations were enacted in 2006 and before that it was not obligatory to install solar collectors. We predicted that solar collector market is going to have the so long expected increment. It is proof that new regulations usually take a transition period of one or two years to be completely adopted by all parts.

Spectral method (SPM)

The spectral method is based on analyzing the transient temperature changes in the collector circuit after the pump is started [5; 6]. Temperature signals on a secondly basis are transformed with a Fourier transformation in the spectral range. A failure free training phase results in a characteristic vector and an uncertainty boundary. A measured vector out of this range indicates a failure. Only one extra temperature sensor about a meter after the collector exit in the collector pipe is necessary. Several larger failures could be recognized, especially in high flow systems. These are e. g. a 40 % reduction of collector performance, a 20 % change in pump power and air in the heat exchanger. However, a failure free training phase of at least half a year is necessary and that may be difficult or even impossible.

3.2. Fault Detection with Artificial Neural Networks (ANN)

The development of a neural network-based fault diagnostic system for the solar circuit is still in a research phase. The method consists of three steps. In the prediction module, artificial neural networks are trained with fault-free system operating data obtained from a TRNSYS model. The model is trained so that 4 temperature values (collector in and output and storage in and output) can be predicted for different environmental conditions. The input consists of weather data (global and beam radiation, ambient temperature, incidence angle, wind speed, relative humidity, flow availability and inlet temperature), together with one of the other measured temperature values. In the second step residual values are calculated, which characterize e. g. the actual temperature increase in the collector compared to the predicted one. In the last step a diagnosis module is run. The failure detection was only successfully tested for introduced failures in TRNSYS [7; 8]. Since the network was trained with TRNSYS, and there are no measurement uncertainties it has to be seen how it compares to real system behaviour.

Performance investigations of differently designed heat-pipe. evacuated tubular collectors in the Artic climate

J. Dragsted1*, J. Fan1 & S. Furbo1

1 Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, 2800 Kgs. Lyngby,

Denmark

Janne Dragsted, iaa@byg. dtu. dk

Abstract

Evacuated tubular solar collectors have the advantages that they are designed to utilize the solar radiation from all directions, and that the heat loss from the collectors is low. This makes them ideal for Artic regions. This paper presents a theoretical investigation of four Sunda Technology evacuated tubular solar collectors’ thermal performance in the Arctic. Different design parameters for the collectors are investigated in terms of the thermal performance. The investigation shown that with improvements of different design parameters it is possible to reach an increase of the thermal performance of up to 9 %.

Keywords: Evacuated tubular collectors, heat pipe principle, thermal performance, TRNSYS, Arctic regions

1. Introduction

In this paper different designs of four evacuated tubular solar collectors are investigated in order to maximize the thermal performance of the solar collectors especially with the Arctic regions in mind. In the Arctic the reflection from the snow plays an important role for the total available energy from sun. Due to the midnight sun, where the sun stays on the sky all 24 hours of the day, there is a need for a solar collector that can utilize solar radiation from all directions. The collectors in the investigation are therefore placed in such a way that they can utilize solar radiation from all directions. No shadows from the surroundings are assumed. The heat loss from the collectors also plays an important role since the average ambient temperature during periods with the collectors in operation is low, around 0°C.

The evacuated tubular solar collectors have a low heat loss because of the vacuum inside the tubes, making them favourable for the Arctic regions.

The four different evacuated tubular solar collectors investigated are Seido 5-8, Seido 1-8, Seido 10-20 curved and Seido 10-20 flat from the Chinese company Sunda Technology Ltd see Fig 1. Seido 5-8 and Seido 1-8 are collectors with 8 glass tubes where the radius of the tubes is 5 cm. The absorber in Seido 5-8 is a curved absorber and the absorber in Seido 1-8 is a flat absorber. Seido 10-20 curved and Seido

image039image040image041image042

Fig 1. The four collectors from Sunda Technology Ltd.

10-20 flat are collectors with 20 tubes with a radius of 3.5 cm, where curved and flat refers to the design of the absorber. The locations used in the parametric analyse is Nuussuaq which is situated on the west coast of Greenland at latitude 70.4° and Sisimiut also situated on the west coast of Greenland at latitude 66.6°. For comparison reasons the location of Copenhagen, Denmark, is also used which is at latitude 55.3°. The thermal performances are calculated with different designs of the four solar collector types. The parameters which are investigated: The distance between the glass tubes, the design of the absorber, the radius of the glass tubes, and the transmittance-absorptance product. Further, the tilt and orientation of the collectors have been varied as well.

Implementing the mathematical model into a computer program

In order to reduce the time necessary to identify the best fitting surface and to ensure that the method is applied by all users in the same way the mathematical extrapolation procedure has been implemented into a Microsoft Excel based computer program named DHWScale.

For the solar domestic hot water system(s) of the product line that have/has been tested with the DST — method the following inputs have to be entered in the Excel sheet:

• For each tested system of the product line

— Collector area

— Storage tank volume

— Solar fraction fsol

• Number of systems tested from the product line

• Location (only Athens available up to now)

• Daily hot water consumption

With these data the program automatically computes the best fitting surface for the specific location and hot water consumption. When the corresponding surface is known the solar fraction can be computed for arbitrary sizes of collector area and storage tank volume.

System Families

It is possible within the Solar Keymark scheme rules to test and certify thermal solar collectors as families. This reduces the effort for the testing by far.

A similar procedure for SDHW systems is now in the stage of development among testing institutes in Europe. The “Centre Scientifique et Technique du Batiment” (CSTB) in France

developed the “Solen software” for the calculation of the efficiency of solar thermal heating systems in buildings according to prEN 15316-4.3:2006 [3].

The software has now been used to extrapolate between two tested forced-circulation systems. The systems were tested at Fraunhofer ISE using the Dynamic System Testing (DST) procedure according to EN 12976-2. The basic differences between the two systems are the size of the collector array and the size of the storage tank

System A: 2 collectors, Aa=4,72 m2; Storage tank, 295 l

System B: 3 collectors Aa=7,08 m2; Storage tank, 380 l

The following table shows the deviation between test results and simulations done using the “Solen software”. The simulations are adjusted with the test result of the other system in the “system family”. It is seen that for the location Davos, for instance, the deviation between test result and the simulation is smaller than for Athens. Looking at this particular system family the deviation for the locations Stockholm and Davos are very small. More tests and simulations have to be done to validate the procedure.

Table 1. The table shows the deviation between test results and simulations for the reference locations in Europe: Davos, Wurzburg, Athens and Stockholm.

Location: Davos

Energy demand Solar Contribution [kWh/a] [kWh/a]

Solar Fraction [-]

Deviation

[%]

DST-test-simulation, System A

5049

3024

0.60

Simulation, Solen software

5038

3030

0.60

0.41

DST-test-simulation, System B

5053

3936

0.78

Simulation, Solen software Location: WQrzburg

5038

3913

0.78

-0.29

DST-test-simulation, System A

4217

2016

0.48

Simulation, Solen software

4451

2032

0.46

-4.49

DST-test-simulation, System B

4212

2489

0.59

Simulation, Solen software

4451

2729

0.61

3.74

Location: Athens

DST-test-simulation, System A

3475

2332

0.67

Simulation, Solen software

3459

2229

0.64

-3.96

DST-test-simulation, System B

3470

2700

0.78

Simulation, Solen software Location: Stockholm

3459

2762

0.80

2.63

DST-test-simulation, System A

4647

2008

0.43

Simulation, Solen software DST-test-simulation, System B

4638

2016

0.44

0.61

4654

2560

0.55

Simulation, Solen software

4638

2541

0.55

-0.39

Increase of the loss potential caused by severe hailstorms

Подпись: Fig. 4. Annual growth rates of solar thermal collectors [2].

The challenge of the estimation of the real existing loss potential at solar energy systems caused by severe hailstorms is given in the combination of high accounts on, at present, relative small areas, aggravated also by the high spatial concentration of such thunderstorms. This is also the reason why insurance and re-insurance companies accept such losses tacitly up to now and don’t itemize damages at solar energy systems in their loss statistics separately. Nevertheless, it is quite clear that the risk of damages on solar energy systems will enormously increase if we consider the rapid development of the solar thermal as soon as the PV-market in Europe in the last decades and if we also consider the aspiration of the EU to enhance the percentage of sustainable energy up to 20 % until 2020 and up to 50% until 2050 related to the overall energy demand, Fig. 4 and Fig. 5 show the annual rates of growth of solar thermal collectors and PV-modules which are registered up to now as well as predicted until 2020.

image158

Also the establishment of solar thermal as well as photovoltaic power stations for the industrial electricity generation and the increasing installation of large solar thermal systems to supply local heat grids or solar driven cooling systems as well as the furnishing of process heat for industrial processes results in a higher potential of economical losses. 88.8% of the present installed collector area of solar thermal systems in Germany are small systems up to 20 m2 Systems larger than 20 m2 are only 11.2%. Systems over 50 m2 even only 1.7%. For the compliance of the achieved objectives of the EU, the amount of large solar systems has to be increased appreciably. The 2007 published sustainability study of the Sarasin Bank predicted the annual growth rates in the field of large solar thermal systems as given in Fig. 6.

A further aspect which will influence the increase of the economical loss potential is given by the architectural integration of solar thermal collectors and PV-modules into the building shell. Solar energy systems will no longer be installed as several patchworks at the existing building shell but more and more as an integrated component of the building shell. Apart from the function just as an energy collecting device such integrated components have to fulfil other additional functions. Moreover, the efforts to exchange such integrated components in the case of some damages will be more expensive.

Update on European Standards for Thermal Solar Systems. and Components and on Solar Keymark Certification

H. Drtick*, H. Mtiller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, 70550 Stuttgart, Germany Tel.: +49 711 / 685-63553, Fax: +49 711 / 685-63503 Corresponding Author, email: drueck@itw. uni-stuttgart. de

Abstract

In the years 2000 and 2001 the first edition of the European standards for solar thermal systems and components was issued and started to replace all related national standards /1/. The three standard series are related to solar collectors as well as to factory made and custom built solar thermal systems. During the past four years the standards were revised and updated.

Based on the European standards, Solar Keymark certification was established in 2003. The Solar Keymark is the official CEN certification scheme for thermal solar collectors and factory made thermal solar systems /2/. Although the Solar Keymark is still relatively young, more than two thirds of all solar thermal collectors sold in Europe are already qualified with a Solar Keymark label. The specific Solar Keymark scheme rules forming the basis for Solar Keymark certification were revised and updated during the past two years. This was done to adopt the Solar Keymark certification process to present developments and to make Solar Keymark certification of factory made systems less expensive by introducing a so-called “flexible Solar Keymark certification” for system families or product lines respectively.

This paper describes the important changes and highlights resulting from the revision of the European solar standards. With regard to Solar Keymark certification, notable changes in the specific Solar Keymark scheme rules will be pointed out and the approach for flexible Solar Keymark certification will be discussed.

Keywords: European standards, Solar Keymark, testing, certification

1. Introduction

The solar thermal market is growing very dynamically. In order to ensure a certain amount of transparency and quality as a basis for a sustainable market development the existence of uniform standardised test procedures and product certification schemes are very important aspects.

With regard to the elaboration of European standards for solar thermal products the work started almost 15 years ago with the establishment of the European Standardisation Committee CEN TC 312 (CEN: Comite Europeen de Normalisation; TC: Technical Committee) in the year 1994. This activity was based on a proposal of the European manufacturer association ESIF (European Solar Industry Federation) which is today named ESTIF (European Solar Thermal Industry Federation). In the standardisation committee CEN TC 312 experts from industry as well as from research and testing institutions work, divided into several working groups, on aspect related to standardisation.

/

This first set of European standards for solar collectors, ‘factory made systems’ and ‘custom built systems’ was issued in 2000 and 2001 /1/. During the last two yeas these standard were revised and updated. The most important aspects resulting from this activity are described in chapter 3 to 5.

The Solar Keymark is the official CEN certification scheme for thermal solar collectors and factory made thermal solar systems. It requires that the products fulfil the requirements of the European Standard series EN 12975 and EN 12976 and that this is confirmed by an accredited testing laboratory. Furthermore, additional requirements such as yearly inspection of the production line and physical inspection of the product itself every second year, have to be fulfilled.

Although the Solar Keymark is relatively young, as it was introduced to the market in 2003, up to now (summer 2008) approximately two thirds of all solar thermal collectors sold in Europe are already qualified with a Solar Keymark certificate. An overview on Solar Keymark certification is given in chapter 6.

Direct flow ETC

4. image067
Conclusion

Side-by-side tests of seven differently designed evacuated tubular collectors were carried out in an outdoor test facility. The observations from the measurements show that the direct flow ETC and the all-glass ETC have relatively high thermal performance m2 transparent area. The all-glass ETC with solar collector fluid in the tubes and the double-glass ETC with heat pipe perform relatively better in summer than in the rest of the year. This behaviour is insignificantly influenced by the mean collector fluid temperature. The heat pipe ETC with flat fin performs better than the ETC with curved fin in most of the test period and the superiority will increase in winter periods and in periods with high mean solar collector fluid temperature.

References

[1] Z. Q. Yin, “Development of Evacuated Tubular Collectors in China”, Proceedings of the Solar Thermal Industry Forum, Munich, April 22, 2008.

[2] W. B. Koldehoff, “The Solar Thermal Market-Today and Tomorrow”. Proceedings of the Solar Thermal Industry Forum, Munich, April 22, 2008.

[3] Z. He, H. Ge, F. Jiang, W. Li. A Comparison of Optical Performance between Evacuated Collector Tubes with Flat and Semicylindrical Absorbers. Solar Energy, 60 (2), 1997, PP. 109-117.

[4] J. Fan, J. Dragsted, S. Furbo. Side-by-side Tests of Differently Designed Evacuated Tubular Collectors. Proceedings of the 2007 Solar World Congress, pp. 634-637, Beijing, China, 2007.

Material selection and exposure

After a market analysis, which included all major distributors, a selection of 58 collector glazing types were chosen in the beginning of this long-term investigation (1984). An overview of tested samples is given in Table 1. These glazing types cover a variety of different material and plate types. Although the selection was made in 1984, the results still provide important information regarding the materials currently available on the market.

Table 1. Summary of the tested materials with the corresponding solar transmittance values.

Material

Number of Glazing Types

Solar Transmittance

Low Fe glass

8 (flat)

0.903-0.919

Fe containing float glass

8 (flat)

0.834-0.857

6 (flat)

0.832-0.843

PMMA Polymethylmetacrylat

6 (multi-skin)

0.685-0.803

5 (3 sinuous, 2 fiber reinforced)

0.831-0.869

5 (flat)

0.787-0.791

PC Polycarbonate

5 (multi-skin)

0.652-0.747

2 (films)

0.799-0.881

ETFE Ethylene-tetrafluoroethylene

3 (films)

0.921-0.932

FEP Fluorinated ethylene-propylene

2 (films)

0.956-0.957

PVF Polyvinylchlorid

1 (film)

0.881

PET Polyethylene teraphtalat

2 (films)

0.782-0.872

PVC Polyvinylchlorid

2 (films)

1 (special plate)

0.837-0.836

0.628

UP Unsaturated polyester

3 (fiber reinforced, 2 of them sinuous)

0.756-0.796

Two exposition sites with different climatic conditions were chosen (see Table 2.); Rapperswil representing a sub-urban location is home of the SPF institute. The alpine site of Davos is characterized by higher irradiation and lower temperature and air pollution.

Table 2. Main climatic parameters of the exposition sites.

City

 

CH-8640 Rapperswil

 

CH-7260 Davos Dorf

 

Подпись: 1556 AMSLПодпись: 1381 kWh/m2 per year 84.6 kWh/m2 per year 2.61 kWh/m2 per year 3.1 °C Rural/Forrestal Low pollution

Altitude

Total annual insolation Annual UVA insolation Annual UVB insolation Yearly mean temperature Site character Air pollution sources
417 AMSL

1093 kWh/m2 per year 60.7 kWh/m2 per year 2.08 kWh/m2 per year

9.3 °C Suburban

Train station and industries

Five samples of each glazing type were exposed at the two sites. Each sample covered a “mini collector” [1] which consists of a non-insulated box of solar selective coated stainless steel facing south at an inclination of 60°. One sample from each type was collected, analyzed and stored following 40 days, 1, 3, 10 and 20 years of exposure.