Category Archives: EuroSun2008-5

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.

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.

Vertical fin ray trace analysis

The optical efficiency based on a surface reflectance measurements is 0.94, the gap between reflective surface and the absorber fin is 4 mm and the absorptance is 0.95. The first gap loss (green rays) is detected at an incident angle of 44 degrees which is depicted as a decrease in the optical efficiency seen in Fig. 6.

In Fig. 6 gap losses separate into roughly two ranges. A flat response occurs between 80 and 100 degrees and an abrupt efficiency drop occurs at 44 degrees. To show that the gap loss is the cause

of the optical efficiency drops, another simulation is run in which there is no gap loss. See Fig. 7. The graph depicts a rounded distribution with no abrupt jump in efficiency at any nominal angle.

To understand the nature of gap loss, the gap loss is plotted over the incident angles in Fig. 8. The gap losses are separated into roughly two levels.

image020

Fig. 12: Rays Striking the Horizontal Fin ICPC at a Nominal Angle of 150 Degrees.

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Fig. 13: Comparing Energy Efficiencies for Different Reflectances (Horizontal Fin ICPC).

Подпись: Fig. 11: Optical Efficiency (Horizontal Fin) from Incident Angles of 30 to 150. Подпись: Fig. 14: Comparing Efficiencies for Four Gaps of 0, 4, 6, and 10 mm (Horizontal Fin ICPC). Подпись: Fig.15: Laser and Sensor Assembly The reflectivity is now changed to 0.7 to achieve an optical efficiency curve that has a shape that is more of a dished appearance around the 90 degree incident angle. See Fig. 9.

Initial impacts of the implementation of the regulations on solar collectors

2.1. The education and training for project design professionals

Portuguese Civil Engineers are the legal responsible professionals for elaborate the water supply design projects. Until now they had to deal with simple equipments of hot water production but now they need to calculate a more complex and integrated system. After consulting a sufficient number of senior project design Civil Engineers they have responded that they are not very comfortable with this new technology and with all that is related with mechanical equipments. Traditional projects in the pass never forced them to know more about equipment subjects. Buildings are until now predominantly constructive but they are aware that intelligent and automatic buildings are a future inevitable reality. Despite there are some standard systems that function like a kit module, it is not sufficient to respond efficiently to the building design demand. In some cases, they are letting the design of solar collector system to be developed later by the installation firms with negative consequences on final building quality. Thus, it is absolutely essential that official institutions promote and stimulate even more training for these professionals. The process is been slow but some are making efforts to get training. Also, recent graduate Civil Engineers, who have just finishing theirs courses, faced almost the same problem mainly because many of the Institutions of Higher Education in Portugal still do not provide the required competencies on these subjects. In most of the courses there is still not an adequate and integrated group of curricular subjects that goes deep in these matters, providing the minimal competencies to accomplish sufficient professional practice in this area. Therefore, it would be necessary to make a great effort to introduce on the curricular course structures even more contents on those matters. And we can not forget other important aspect, as the firm’s know-how is still not very high there is always a tendency to charge more for the installation and maintenance operations and give not so qualified engineering consultant. Now, these firms must be prepared to correspond to a more informed attitude from design professionals.

Input-Output Controller (IOC)

The Input-Output Controller is a simulation based failure detection method available on the market since 2007. The first variant of the method monitors only the energy yields in the solar circuit. Furthermore, two temperature values in the storage are used as input for the simulation. A second approach also includes the buffer storage discharging. The IOC compares the daily measured and expected energy yields in the solar loop. The standard uncertainty (o) of the IOC-procedure, including measurements and yield calculation, is about 7 % (o). If the difference between measured and simulated yield is larger than 20 % (3 o) a fault is detected. This leads to a 99 % reliability for a correct fault prediction. Below a yield of 1.5 kWh/m2d the uncertainty margins are higher. There is a failure tree to establish if the fault occurred inside or outside the solar loop, and if it is for example the control or the solar station which causes the problems. The IOC is sold for 1190 € inclusive temperature and irradiance sensors, but without volume flow measurements. To be able to check the performance from home an extra data logger is necessary [9; 10].

3.3. Kassel University method (KU)

At Kassel University a failure detection method was developed, that combines a static algorithm based function control with dynamic simulation based failure detection [11]. The method consists of three steps. In the first step it is checked if too much data is missing due to data gaps and sensor defects. A minimum of 95 % of data points should be available to continue with the failure detection. In a second step a plausibility check is carried out, in which the correct operating of individual components is checked, similar to the approach used in [3]. The third step is a simulation based step in which the system is modelled with TRNSYS. Measured and simulated energy gains are compared at the heat exchanger for charging and or discharging the storage unit. If the difference is larger than the uncertainty margins on both sides an error is reported [12; 11].

Several failures were detected and partially identified. These were for example air in the collector field and a calcified heat exchanger. This approach is being further developed.

3.4. Guaranteed Solar Results (GRS)

In Guaranteed Result of Solar Thermal Systems the energy yield is guaranteed by the seller/builder of the system. Sophisticated measurement equipment is installed and monitors the system, costs for the measurement equipment and one year of operation are in the range of 10 k€. Daily averaged and monthly measured values are sent. Measured yearly energy yields are compared to simulations with f — chart, a simple simulation program, although also other simulation programs could be used. A comparison on a shorter basis is not possible, due to limitation of the simulation program. Large failures on a yearly basis can be detected; however failure analysis is not possible [13; 14].

Conclusion: Summary and Future Perspectives

A complete research and testing laboratory for scientific research in the field of solar energy utilization was built near Tripoli, Libya, for the CSES of the National Office for Research and Development by the general contractor, Bavaria Engineering GmbH, together with several project partners. TUV Rheinland Immissionsschutz und Energiesysteme GmbH, which recommended PSE AG as provider of the indoor/outdoor test stand for solar thermal collector testing, was project partner especially for the solar thermal test systems.

This paper describes the concept and realization of the test stand for solar thermal collectors. PSE AG and Fraunhofer ISE were able to use their years of experience with indoor and outdoor test stands and to develop the technology further.

PSE AG’s next project consists of the installation of a complete indoor test stand for solar thermal collectors at the French national institute for solar energy, INES, in the fall of 2008.

image174

Figure 5: Rendering of PSE s next generation of indoor test stands

Numerous innovations will also be realized in this test stand. Besides motor-driven lamp positioning, all test-stand-specific parameters will be centrally controlled and recorded so that tests can be easily documented and reproduced.

We are glad to be able to use our expertise for the realization of test stands for solar thermal collectors.

References

[1] Zahler, C.; Luginsland, F.; Haberle, A; Rommel, M.; Koschikowski, J.;

Fertigung und Installation eines Sonnensimulators fur das GREEN Labor an der Pontificia Universidade Catolica de Minas Gerais in Brasilien,

OTTI: 15. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2005 pp 462-467

[2] Haberle, A.; Berger, M.; Luginsland, F.; Zahler, C.; Rommel, M.; Baitsch, M.;Henning, H.-M.; Linear konzentrierender Fresnel-Kollektor fur Prozesswarmeanwendungen,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 185-190

[3] Haberle, A.; Luginsland, F.; Zahler, C.; Topor, A.; Reetz, C.; Apian-Bennewitz, P.;

Ein praziser undpreiswerter Sonnenstandssensor,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 144-149

Model description

1.1. Validated models

Validated models are used for the parameter analyses [4]. The validation was carried out in 2007 with measurement data from the four different evacuated tubular solar collectors tested in a test facility at the Technical University of Denmark. In Table 1 the values used and determined in the validation process are shown.

Table 1. Parameters for the four solar collectors.

Parameter

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Collector type

Total gross area of the collectors [m2]

2.07

2.07

3.53

3.53

Transparent area of the collectors [m2]

1.38

1.38

2.25

2.25

Number of tubes [-]

8

8

20

20

Glass tube radius [m]

0.05

0.05

0.035

0.035

Tube centre distance [m]

0.120

0.120

0.09026

0.09026

Collector panel tilt [°]

67

67

67

67

Collector azimuth [°]

0

0

0

0

Incidence angle modifier for the diffuse radiation [-]

0.9

0.9

0.9

0.9

Effective transmittance — absorptance product, [-]

0.83

0.83

0.83

0.83

Number of discretizations for the absorber [-]

41

41

33

15

Angle dependence of tau-alpha product based on tangent equation [-]

3.8

3.8

3.8

3.8

Heat capacity of the fluid in the manifold tube [kJ/kgK]

3.8

3.8

3.8

3.8

Heat pipe length [m]

1.73

1.73

1.61

1.61

Absorber radius [m] / Absorber width [m]

0.041

0.085

0.027

0.056

Angle of strip [°]

164

164

Thickness of strip [mm]

0.47

0.47

0.60

0.60

Thermal conductivity of strip [W/mK]

238

238

238

238

Density of strip [kg/m3]

2700

2700

2700

2700

Heat capacity of strip [kJ/kgK]

0.896

0.896

0.896

0.896

Lowest evaporation temperature [°C]

15

15

15

15

Manifold heat exchange capacity rate [W/K]

10

10

10

10

Mass of the fluid inside the heat pipe [kg]

0.0038

0.0038

0.0038

0.0038

Effective heat capacity of the collector [kJ/K]

4.04

3.41

6.31

4.94

Validation of the mathematical model

For the validation of the developed mathematical model, the results of the DST tests of one thermosiphon system product line are used. Systems of this product line with different collector area and store volume were tested according to the DST method at the three laboratories CSTB, INETI and ITW. The results obtained from these tests were compared with the results that are obtained with the mathematical model. In Table 4, the test results based on the DST-test for the location of Athens and a hot water demand of 200 l/d are listed in the column fsol, DST.

In the same table, the results obtained with the mathematical model are depicted. The parameters of the different systems that have been entered in the program as “system tested” are defined in the line: “Input value fsol, DsT“. With the extrapolation tool DHWScale, the values fsol, calc are calculated for the remaining systems of the product line. In addition, the discrepancy between the calculated values of the solar fraction and the solar fractions obtained by the DST-method (Afsol = fSOl, DST — fsOi, caic ) and the relative error (srel) between both values is displayed. The relative error between the results obtained for fsol from the DST test and the calculation procedure is calculated by the following equation (5),

sol, DST

For the validation, three trials have been performed:

In the first trial (Trial 1) a thermosiphon system with Ac = 3.48 m2 and Vsto = 0.18 m3 (System no. 1) has been tested with the DST-method and a solar fraction of fsol, DST = 0.74 has been obtained. These values are now entered into the DHWScale program to extrapolate the results to systems of the same product line but different in size. For the systems 2 to 7 of the same product line the values of fsol can be found in the column “Trial 1, fsol, calc1”.

In a second trial it was assumed that a thermosiphon system with Ac = 3.96 m2 and Vst0 = 0.15 m3 (System no. 4) has been tested with the DST-method and a solar fraction of fsolDst = 0.70 has been obtained. Again, these values are entered in the DHWScale program. The results obtained for the other systems of the product line by means of extrapolation are listed in the column “Trial 2, fsol, calc1”.

For the third trial it is assumed that two thermosiphon systems (system 3 and system 7) have been tested with a DST-test. The parameters of both systems have been entered into the program as input data in order to obtain the solar fractions for the 5 remaining systems of the product line.

Table 4. Comparison of DST-results and the results from the mathematical model

DST

Trial 1

Trial 2

Trial 3

No

Ac

Vsto

fsol. DST

fsol, calc,1

A fsol / (ЄГЄі) [%]

fsol, calc,2

Afsol / (єГЄі) [%]

fsol, calc,3

Afsol / (єГЄі) [%]

System tested with DST

і

4

1 and 4

Input value fsol, DST

Ac = 3.48 Vsto = 0.18

fsol, DST = 0• 74

Ac = 3.96 Vsto = 0.15

fsol, DST = 0.70

Ac = 3.48; 3.96 Vsto = 0.18; 0.15

fsol, DST = 0. 74; 0.70

1

3.48

0.18

0.74

0.714

2.6 (3.5)

0.680

6.0 (8.1)

0.703

3.7 (5.0)

2

5.22

0.30

0.82

0.816

0.4 (0.5)

0.777

4.3 (5.2)

0.803

1.7 (2.1)

3

3.96

0.18

0.72

0.744

2.4 (3.3)

0.708

1.2 (1.7)

0.731

1.1 (1.5)

4

3.96

0.15

0.70

0.741

4.1( 5.9)

0.705

0.5 (0.7)

0.727

2.7 (3.9)

5

3.96

0.30

0.75

0.750

0.0 (0.0)

0.713

3.7 (4.9)

0.742

0.8 (1.1)

6

1.86

0.30

0.61

0.582

2.8 (4.6)

0.550

6.0 (9.8)

0.583

2.7 (4.4)

7

3.72

0.30

0.77

0.735

3.5 (4.5)

0.688

8.2 (10.6)

0.727

4.3 (5.6)

8

5.58

0.30

0.82

0.830

1.0 (1.2)

0.791

2.9 (3.7)

0.816

0.4 (0.5)

As can be seen from Table 4, also for the system tested with DST, there is a discrepancy between the solar fraction obtained with the DST-method and the solar fraction obtained with the mathematical model. This results from the fact that there is no equation fsol = f (A, Vto) which exactly matches the default value from the DST-method.

The maximum relative error for Trial 1 is 5.9 % , 10.6 % for the second trial and 5.6 % for the third trial. The mean error, defined with

є1 +є2 +… + Є8
8

is 2.25 % (Trial 1), 5.9 % (Trial 2) and 3.0 % (Trial3). For the other hot water demands of 110 l/d and 300 l/d comparable results are obtained.

3. Conclusion

An extrapolation procedure including a mathematical model has been developed which can be used for determination of the solar fraction of systems which are part of a product line. By means of an extrapolation procedure, input values for only one or a few systems have to be determined by physical testing. The developed procedure has been implemented in an EXCEL based software tool named DHWScale.

As the model is based on second order polynomials, results can be obtained within seconds. The mathematical model has the advantage that only a few system tests are necessary to determine the solar fraction for a whole product line. This approach can offer the possibility to reduce the time and cost necessary to obtain Solar Keymark certification of factory made solar domestic hot water systems. The validation of the program with one thermosiphon product line tested by DST showed promising results. Depending on the desired accuracy already one system test may be sufficient (relative error of about 11 %). If two system tests are performed the relative error drops to 6 %. However, up to-date only DST-results for one product line are available. For a more profound assessment of the developed procedure more product lines have to be tested with the DST-method.

In principle it is possible to extend the DHWScale program towards other system designs such as forced circulated DHW systems with or without integrated auxiliary heating. First experiences have been gained with forced circulated DHW systems but a validation is necessary before any reliable statement about the accuracy of the results can be made. It is also expected that for other system types further parameters have to be taken into account such as the influence of the auxiliary heated volume. This will envoke a huge number of additional TRNSYS simulations.

The DHWscale program is a first approach for the determination of system test results by means of an extrapolation procedure. In principle the methodology of this approach can be extended to additional parameters (e. g. locations, loads) and other system concepts.

References

/1/ H. Druck, S. Fischer, H. Muller-Steinhagen,

Solar Keymark Testing of Solar Thermal Products; Proceedings of ISES 2007 Solar World Congress, September 18 to 21, 2007, Beijing, China, ISBN 978-7-302-16146-2, Tsinghua University Press, Beijing and Springer-Verlag GmbH Berlin Heidelberg, CD: ISBN 978-7-89486-623-3

/2/ DIN EN 12976, „Thermal solar systems and components — Factory made systems — Part 2: Test methods”

/3/ ISO 9459-5:1995, “Solar Heating — Domestic water heating systems — Part 2: Outdoor test methods for system performance characterisation and yearly performance prediction of solar system.

/4/ Research and experimental Validation on the DST Performance test Method for solar Domestic Water Heaters, Final Report, Contract No. SMT4-CT96-2067

Engineering and set-up of a new testing facility for solar thermal systems

At the Fraunhofer ISE a testing facility for solar domestic hot water systems (thermosiphon systems, systems with forced circulation, integral collector-storage systems) and heat storages is already in operation since 1997.

Now a new testing facility has been engineered to satisfy the growing demand for the tests on solar domestic hot water systems according EN 12976 [1]. This testing stand provides the possibility to perform tests of four different systems at the same time and independently.

The testing facility and its sensors fulfils very high testing and measuring requirements. The boundary conditions during testing can be handled in a far better way than in the previous test stand. This concerns, for example, the cold water inlet temperature during the tests and the ambient temperature of the storage tank.

The testing facility is designed for tests on complete solar heating systems and heat storages.