Category Archives: EuroSun2008-5

Function Control without Heat Measurements (FUKS)

In the second half of the 90’s the FUKS-approach (function control without heat measurements) was developed in Germany as a cheap function control (<100€) [3]. This approach applies algorithms to measurement data and notices whether components function correctly. An overview of the developed algorithms is provided in table 3.2. The values in the algorithms could be adapted for other systems.

Several other algorithms were developed which can only be applied with additional sensors, e. g. resistance sensors for defect or inaccurate temperature sensors for the collector output or in the storage tank.

Table 3.2 Failure algorithms developed in the FUKS-approach [4]

Failure description

Result of failure

Algorithm

1. Collector circuit points interchanged

2. Collector T-sensor falsely positioned

Pump clocks

Pump running time < 10 s.

3. Leakage of heat exchanger

4. Power outage

System pressure too high

(Tkol = 20 °C) AND

(psystem psystem soll + 2 bar)

5. Incorrect controller software

6. False volume flow setting

7. Air in hydraulic circuit

8. dT setting is inappropriate

9. Defect input or output to the controller

dT too high

Pump on AND dT = dTsoll + 15 K

10. Gravity brake is open

11. Fouling of gravity brake

12. Time switch is programmed wrongly

Pump on at night

Pump on AND

time between 22:00 and 6:00

The method was for test purposes partially implemented in controllers of Esaa and Wagner. Several failures were recognized, but there were also false positives, detection of a failure while the system was functioning correctly. The approach stays cheap by using mainly sensors that are used for the control, however, with analysis and pressure measurements, the price may be slightly higher than mentioned. Although the method succeeds at detecting several failures in the lab, location of failures is limited, as can be seen in Table 3.2. Furthermore, there is no yield measurement, which could mean that large energy losses are not detected.

Two-Axis Tracker

PSE has been developing uniaxial and biaxial trackers for several years, for use in linear concentrating Fresnel collectors [2], heliostats, measurement of solar thermal collectors, etc. The mechanical design of this testing tracker is especially rugged; it has a maximum load allowance of 700 kg on the 5 x 2.5 m test surface. The guaranteed tracking precision is ±1°; however, the average precision during normal operations is much better. The tracking precision was verified using a “Heliosensor” [3], PSE’s sun position sensor.

Подпись: Figure 2: Data measurements taken with a heliosensor during calibration

The heliosensor is used to calibrate both the rotary encoder, which records the azimuth angle, and the inclination sensor, which records the zenith angle, during installation. After the sensors have been calibrated, the orientation of the testing surface is updated using an astronomical algorithm.

So as not to compromise the tracker’s precision, the track-laying and mounting of the two-axis tracker onto the moveable carriage must be carried out with especially high precision.

The tracker has three different operating modes: manual, automatic tracking, and user-defined tracking.

image172

Figure 3: Left: Two-axis tracker with a testing surface of 5 m x 2.5 m; Right: Screenshot of the corresponding software.

The tracker is operated using software installed on a local network PC, which communicates with the tracker’s controls, an embedded Linux system, via the network connection.

The testing surface is completed with four cross-flow fans, which ensure that the collector is ventilated according to current norms during measurements.

Determination of the Degree of Mixing

Before the start of test, mix the water in the storage tank by using a small pump to circulate the water from the top to the bottom of the store. The water in the store is assumed to be at a uniform temperature when the temperature of the water at the outlet of the store varies by less than 1 K for a period of 15 min. Draw off water from the store at a constant flow rate of 600 lit/hr. The cold water

entering the store shall be at a constant temperature of less than 30 °С, shall not fluctuate by more than

±0.25 K and shall not drift by more than ±0.2 K during the draw-off period. Measure the temperature

at least every 15 s and record an average value at least every time a tenth of the tank volume is drawn off. Fig. 8 shows that a mixing draw-off temperature and water flow rate trend and relationship change with draw-off time.

image038

Fig.8 Mixing raw-off temperature and flow rate graph 4.3. Determination of storage tank heat losses

Before the start of the test, mix the preconditioned water in the store by using a pump to circulate the water from the top to the bottom of the store. The water in the store is assumed to be at a uniform temperature when the temperature of the water at the outlet of the store varies by less than 1 K for a

period of 15 min. The average temperature over these 15 min is to be taken as the initial temperature of the tank. After cooling the tank for a period of between 12 h and 24 h, the water should be recirculated in the storage tank so that it reaches a uniform temperature. The temperature is assumed to be uniform when the temperature at the outlet of the tank varies by less than 1 K for a period of 15 min. The average temperature during this 15 min period shall be taken as the final temperature of the tank.

3. Conclusion

In this paper, automatic control system for domestic water heating test system has been studied and developed. Test procedure according to ISO 9459-2 is a series of complicated work and the whole process is a high cost test for water heating system. By the use of Labview a set of automatic control system can operate the test system, collect and record the corresponding data, and manipulate all sorts of equipments. In this case, it is easier and lower cost than has ever been before.

References

[1] ISO9459-1(1993). Solar heating—Domestic water heating systems—Part 1: Performance rating procedure using indoor test methods.

[2] ISO9459-2(1995). Solar heating—Domestic water heating systems—Part 2: Outdoor test methods performance characterization and yearly performance prediction of solar-only systems.

[3] ISO9459-3(1997). Solar heating—Domestic water heating systems—Part 3: Performance test for solar plus supplementary systems.

[4] ISO9459-4. Solar heating—Domestic water heating systems—Part 4: System performance characterization by means of components tests and computer simulation.

[5] ISO9459-5. Solar heating—Domestic water heating systems—Part 5: System performance characterization by means of whole system tests and computer simulation.

Selection of the best fitting surface

To be able to calculate the thermal performance of one specific thermosiphon DHW system, it is necessary to identify the surface that describes the thermal behaviour of the system in the best way. For the selection of the best fitting surface, the result of at least one DST test per product line is needed. In Table 2, an example for a product line of six different DHW systems is given. The solar fractions of system 1, 2 and 6 have been determined based on measurements according to the DST method and on a long term performance prediction. The solar fractions of system 3, 4 and 5 are not known a priori and have to be calculated by the extrapolation procedure. The best fitting surfaces is defined as the surface where the discrepancies between the values offsol obtained by the DST method and obtained with equation (2) are minimum. These values are compared with the fSol-values that have been derived directly for the DST measurements. The differences Afsol, between measured and predicted values is calculated with equation (3),

Подпись: (3)Подпись: (4)sol, i, j sol, measured sol, calc, i, j

with i = 1…3: system number, j = 1….12: surface number.

The total difference AFsol is defined as the summation over the differences Afsoi? i

AF, o =Z|A«,u

i

Table 2. Example for one typical product line

System no.

Collector aperture area [m2]

Storage tank volume [m3]

fsol [-]

1

2.0

0.3

0.572

2

4.0

0.3

0.725

3

6.0

0.3

?

4

2.0

0.5

?

5

4.0

0.5

?

6

6.0

0.5

0.830

It is assumed that the surface where the value of AFsol is minimal describes the system behaviour in the best way. This function will then be used to compute the solar fraction for the remaining systems 3, 4 and 5. Table 3 lists the results determined by the mathematical extrapolation procedure according to equation (3) for the solar fractions of systems 3, 4 and 5.

Table 3. Results determined by the extrapolation procedure for system 3, 4 and 5

System no.

Collector aperture

Storage tank

fsol [-]

area [m2]

volume [m3]

1st International Congress on Heating, Cooling, and Buildings^ 7th to 10th October, Lisbon — Portugal /

1

2.0

0.3

0.572

2

4.0

0.3

0.725

3

6.0

0.3

0.828

4

2.0

0.5

0.549

5

4.0

0.5

0.723

6

6.0

0.5

0.830

Development of new system testing facilities

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.).

image134

image135

Fig. 2.The parameters determined in the outdoor test are used for a prediction of the system output for reference locations in Europe.

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

Подпись:Подпись:Подпись:Подпись:Подпись:Подпись:Подпись: 20:00Подпись:Подпись: 1200Подпись:Подпись:Подпись:Подпись: 200Подпись:image150О D.

Подпись: 0 00

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 FDS procedure

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

Подпись: Fig. 4 Detection of the F' drift for two different draw off profiles

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).

image005

Fig. 5 Differential between temperatures when the system is stable all year long

and when F’ evolves

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.

Подпись: Figure 6: Detection of the UL drift

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

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.

image155

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.