Category Archives: EuroSun2008-5

Double glass ETC with heat pipe

image066

In phases 2 and 3, the thermal performance of the ETC 6 was measured. It is can be seen from Fig. 4 that the performance ratio of ETC 6/ETC 4 decreases from summer to winter and increases from winter to summer. In summer the ETC 6 has a thermal performance maximum 28% higher than ETC 4 while in winter ETC 4 performs up to 8% better than ETC 6. This is due to the larger tube distance of ETC 6 and thus less shadow from neighbouring tubes. ETC 6 has a tube diameter to tube centre distance ratio of 0.69-0.73 which is a bit smaller than that of ETC 4, 0.75-0.81. It is reasonable that ETC 6 performs better than ETC 4 based on the thermal performance per m2 transparent area, while ETC 4 performs better than ETC 6 based on the thermal performance per m2 gross area, see Fig, 6, 7.

Ageing Performance of Collector Glazing Materials — Results from

20 Years of Outdoor Weathering

F. Ruesch*, S. Brunold

Institut fuer Solarenergie SPF, HSR University of Applied Sciences, Oberseestrasse 10, CH-8640

Rapperswil.

* Corresponding Author, florian. ruesch@solarenergv. ch
Abstract

The outdoor weathering performance of collector glazings was investigated over a time range of 20 years. A variety of 58 glazing types composed of glass and different polymeric materials were included. Five samples of each glazing type were exposed at two Swiss sites with differing climatic conditions. One sample from each type was collected, analyzed and stored following 40 days, 1, 3, 10 and 20 years of exposure.

The weathering properties of PMMA were in the range of glasses or even slightly better. Contrarily, material degradation was observed for PC, PET, PVC and UP. Soiling was strongly dependent on the exposure site and the glazing material. At the sub-urban site of Rapperswil (CH) a significant loss in solar transmittance in the range of 3-15% was measured. For the investigated fluoropolymers surprisingly high losses in transmittance (ETFE) or tendency for soil accumulation (FEP, PVF) was observed.

Keywords: outdoor weathering, ageing, collector glazing, polymeric glazing

1. Introduction

Solar collectors for hot water production provide a great potential for the exchange of fossil fuels with a renewable energy resource. Unfortunately, high investment costs still lower the economic success of this technology. The use of new polymeric collector materials offers a significant potential for cost savings in the installation and production process. In general the most important contribution to flat plate collector weight comes from the tempered glass glazing. A substitution with polymeric glazing could significantly reduce the weight to lower installation effort and cost. To justify the high investments for a solar thermal system, generally a long lifetime in the range of more than 20 years has to be ensured. However, up to now most producers of polymeric glazing materials do not present reliable data on the ageing performance of their products over such a long time period. This study provides data on the ageing and soiling properties of different polymeric materials and conventional solar glasses during 20 years of outdoor weathering. Major attention was turned to the change of the solar transmittance of the glazing materials as this measure is correlated very well to the change of the collector efficiency.

Frequency of severe hailstorms in Europe

Generally we talk about hail from a hailstone size larger than 0.5 cm. Smaller sizes are denoted as graupel or soft hail. However, the probability that hailstone sizes smaller than 2cm cause damages to solar thermal collectors or PV-modules can be assessed as marginal. On this account the following consideration about the trend of the frequency of severe hailstorms in Europe and their potential for economical losses will consider basically hailstones equal or larger than 2 cm. Primarily the following data presented here are based on observations of the ‘competence centre for local thunderstorms (Tordach)’ in Germany, Austria and Switzerland. This competence centre was founded in 1997 as a network of more than 30 scientists. They collect information about local thunderstorm in Europe and their associated climatologically secondary phenomena like hail in a period of 10 years. The main objective was to obtain reliable and complete climatologically records on these severe local storm phenomena in each of the three countries. The collected information was implemented into the European Severe Weather Database (ESWD) of the European Severe Weather Storm Laboratory (ESSL) which was under construction since 2002. Additionally, since 2006 the climatologically record of thunderstorms all over Europe takes place within the European Severe Weather Database ESWD. Fig. 2 shows the increase of severe hailstorms in Europe during the last 10 years. Each picture shows all in a time period of 4 years registered Hailstorms. Thereby each point represents one event. Additional shown is the geographical distribution of severe Hailstorms.

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Fig. 2. Accumulated hailstorm events during time periods of 4 years in Europe. Only events with hailstones

larger than 2 cm are shown. [1]

 

—————— 1————————————————- 1————————————————- 1——————- ►

01.01.1997-31.12.2000 01.01.2001 -31.12.2004 01.01.2005-02.08.2008

However this Fig. 2 also shows one problem in the approach of an overall European presentation of the development of the frequency of severe Hailstorms. The illustrated pictures were generated by the European thunderstorm database ESWD. As mentioned above, up to 2006 basically the data of the competence centre for local thunderstorms (Tordach), whose data acquisitions was limited on the countries Germany, Austria and Switzerland, are available for this database. Thus it must be assumed that the recognised unbalance of the frequency of hailstorms between South/Western respectively Eastern Europe and Central Europe displays not the real circumstances, but rather results in the not — continuous detection of severe hailstorms in South/Western — and Eastern Europe. This proposition is also backed up by the illustration of the period from the 01.01.2005 to 02.08.2008 which shows a more homogeneous distribution. The reason therefore is the climatologically registration of thunderstorms all over Europe within the ESWD which is performed since 2006. The development of the annual frequency of severe hailstorms during the last 10 years is given in Fig. 3. Apart from the given total number of hailstorms in Europe per year, the percentage of hailstorms registered in Germany, Austria and Switzerland based on the total number is given. This indicates two trends which are acting in opposite directions. On the one hand, the total number of hailstorms is dramatically increasing. On the other hand, the percentage of severe hailstorms in the countries Germany, Austria and Switzerland is decreasing in relation to the total number. Once more this fact makes clear the above mentioned major problem of the currently defective standardisation in the local systems of monitoring of the European countries and thus the different way of data collection. It also shows the importance and the

Подпись: Fig. 3. Annual frequency of severe Hailstorms in Europe (based on ESWD data).

mandatory provision of the further development of an overall European thunderstorm database to backup the assessment of the economical loss potential of severe hailstorms by an established database in the near future.

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.

Solar simulator with artificial cold sky

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Figure 4: Tracker under the solar simulator during indoor testing

The solar simulator’s lamp field consists of sixteen 1500 W metal halide lamps. In addition to the vertical and tilt movements, it is also possible to move the lamp field horizontally on the cantilever. This is necessary, as the tracker is mounted on tracks, which means that its distance from the lamp field cannot be varied. All position parameters are recorded electronically and shown on a screen. The actuators are driven via a wired remote control; positioning the lamp field over the collector is facilitated by a laser for cross projection and an ultrasonic distance sensor.

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