Different SDHW systems (thermosiphon systems, integrated collector-storage systems and forced — circulation systems) have been tested at Fraunhofer ISE since 1997 The testing facilities for pre­assembled systems have now been expanded to satisfy the growing demand of these tests according to the European testing standard EN 12976. From now on it is possible to perform up to four tests of SDHW systems at the same time. Furthermore, it will be possible to carry out tests of heat storage tanks.

Elements of system testing

SDHW systems are tested for the Solar Keymark certification according EN 12976-1,2. This includes an efficiency test on the system, functional tests on the system and functional tests on the collector applied in the system. The tests will be briefly described in the following.

Efficiency test

The parameters of the systems are experimentally determined by an outdoor test with a number of testing days. The boundary conditions like daily irradiation, ambient temperature and cold water inlet temperature for a testing day to be valid are defined in the testing standard.

A simulation is carried out using the parameters which were determined in the efficiency test. The performance prediction simulations are carried out for different reference locations (Wurzburg, Stockholm, Davos and Athens are the standard locations but the calculations can be carried out also for other locations.).

Functional tests on the system

• Freeze resistance

The aim of the freeze resistance test is to check whether the necessary precautions have been taken to protect the system from temperatures below zero.

• Pressure resistance

Within the pressure resistance tests all circuits are checked on their ability to hold and withstand pressure.

Ability of the solar-plus-supplementary-system to cover the load

The intention of this test is to assess the ability of the system to provide the maximum daily load on days without contribution out of the solar loop.

• Over temperature protection

The test is done to make sure that the system is able to withstand a period of high solar radiation without hot water drawn from the tank. The intention is also to make sure that no dangerous situations occur for the user.

О

О D.

Fig. 3. Measurements of an over temperature test. The graph shows the irradiance, the power consumption of
the pump in the collector loop and the outlet temperature of the collector array (as mean values averaged
over five minutes). By means of the controller of the system the pump is operated intermittingly in order to
prevent the collector from overheating. The storage tank is cooled by the collector (it is a selective flat plate
collector in this case) until a certain threshold temperature is reached. The test showed that the risk of a
stagnation phase with evaporation of the fluid in the collector loop is reduced sufficiently..

Functional tests on the collector

Additional functional tests are performed on the collector of the system unless a complete test according EN 12975 has been performed already. The tests include:

• Exposure test

• High-temperature resistance test

• External thermal shock tests (twice)

• Internal thermal shock tests (twice)

• Rain penetration test

• Stagnation temperature

• Mechanical load test

• Final inspection

The kernel of the system is the neural networks. So, it is necessary to consider a first step to train the networks. This period should be as short as possible because during this period the system has

to be inspected regularly to be sure that nothing has happened (e. g. it is necessary to clean the collectors). It has been found that getting 30 values with a sampling period of 1 hour is a good compromise between sensitivity of the procedure and initial length.

At the end of the initialization period, three networks are available. They are used to estimate the temperatures. The latter are compared to the acquired temperatures through the computation of the RMSE. If the error is very small, the initial connection weights are stored in a matrix. If the error is higher than a threshold (0.05 for the collector array, 0.015 for the connecting pipes), the networks are re-trained. The new connection weights are stored in the connection weights matrix. If a given number of consecutive connection weights are different from the initial weights, an alarm is fired. It has been found that 5 consecutive values are sufficient, and do not lead to long delays between the fault and its detection.

2. Results

To generate the data, the typical meteorological year (TMY) files of Nicosia have been considered. Figure 4 shows the detection time for the F’ drift considering the two draw off profiles.

Although it seems that the detection is quite late, a plot of the temperatures about the detection time shows that it would be very difficult to detect the drift by the simple analysis of the temperatures (Fig. 5).

Figure 6 shows the detection time for the UL drift. On the one hand, it can be noted that a combined drift leads to an earlier detection. On the other hand, it has been checked that the defaults on the connecting pipes do not lead to any detection by the analysis of the collector array data.

3. Conclusions

An on-line fault diagnostic system has been presented. The main advantage of this system is that it does not need a long training period. It has been shown that the drifts are detected well before the increase of the auxiliary electrical power is higher than 7.5%. This means that the FDS is sensitive. As the neural networks are very simple, this should not be a problem to implement the FDS in real world applications.

Acknowledgements

The financial support of the French Ministry of Foreign Affairs (under contract EGIDE ZENON 11999SD) and of the Cyprus Research Foundation (under contract KY-rA/0305/02) is greatly acknowledged.

References

[1] S. Kalogirou, G. Florides, S. Lalot, B. Desmet, Development of a Neural Network-Based Fault Diagnostic System, World Renewable Energy Congress IX and Exhibition, on CD-ROM, 19-25 August 2006, Florence, Italy

[2] S. Kalogirou, S. Lalot, G. Florides, B. Desmet, Development of a Neural Network-Based Fault Diagnostic System for Solar Thermal Applications, Solar Energy, Vol. 82, No. 2, pp. 164-172, 2007.

[3] S. Lalot, O. P. Palsson, G. R. Jonsson, B. Desmet, 2007, Comparison of neural networks and Kalman filters performances for fouling detection in a heat exchanger, International Journal of Heat Exchangers

[4] S. Lalot, 2006, On-line detection of fouling in a water circulating temperature controller (WCTC) used in injection moulding, Part 1: principles, Applied Thermal Engineering, Volume 26, Issues 11-12, August 2006, Pages 1087-1094

[5] S. Lalot, 2006, On-line detection of fouling in a water circulating temperature controller (WCTC) used in injection moulding, Part 2: application, Applied Thermal Engineering, Volume 26, Issues 11-12, August 2006, Pages 1095-1105

[6] M. M. Prieto, J. M. Vallina, I. Suarez, and I. Martin, 2000, Application of a design code for estimating fouling on-line in a power plant condenser refrigerated by seawater, Proceedings of the ASME-ZSITS International Thermal Science Seminar, Bled (Slovenia), on CD-ROM

[7] U. Jordan, K. Vajen, Realistic Domestic Hot-Water Profiles in Different Time Scales, 2001, available at http://sel. me. wisc. edu/trnsys/trnlib/iea-shc-task26/iea-shc-task26-load-profiles-description-jordan. pdf

[8] I. Knight, N. Kreutzer, M. Manning, M. Swinton, H. Ribberink, 2007, European and Canadian non — HVAC Electric and DHW load profiles for use in simulating the performance of residual cogeneration systems. A report of subtask A of FC+COGEN-SIM the simulation of building integrated fuel cell and other cogeneration systems. Annex 42 of the International Energy Agency energy conservation in buildings and community systems programme

[9] S. Lalot, 2000, identification of the time parameters of solar collectors using artificial neural networks, EuroSun (ISES Europe Solar Congress), Copenhagen (Denmark), June 19-22

[10] M. Bosanac, S. Jensen, In-situ solar air collector array test, Solar Energy laboratory, Danish Technological Institute, December 1997

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.

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.

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.

—————— 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

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.

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.

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

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.

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.

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.

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.

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

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.