Труды Всемирного конгресса Международного общества солнечной энергии

Подождите немного. Документ загружается.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

522

4.3 Size Exclusion Chromatography (SEC)

By SEC significant changes of the weight average

molecular weight values were determined for the

investigated amorphous grades PPE+PS and PC. For the

PPE+PS blend a significant decrease of average molecular

weight values of the PS component was detectable after

already 125 hours exposition in hot air. The decrease in

molecular weight can be attributed to chain scission of the

PS molecule, and is in agreement with the reduction in

oxidation temperature. In contrast, for the PPE component

of the blend an increase of the molecular weight values was

found after exposition in hot air. The increase can be

attributed to cross-linking of PPE. After aging in hot water

the SEC results showed a slight decrease in molecular

weight for both components, PS and PPE. For PC also a

slight decrease in SEC values was identified after

exposition to hot air at 140 °C. However, no significant

changes were recorded after exposition in hot water. For PP

no changes of the molecular weight values were obtained

for both exposition conditions, hot air and hot water.

4.4 Mechanical Investigations

In Figs. 3 and 4, the ultimate mechanical strain values

(strain-at-break) are plotted for the investigated plastics

grades as a function of ageing condition and time. After

aging in hot air, a strong decrease in ultimate strain, and

thus embrittlement, was obtained for PPE+PS already after

125 hours. After aging in hot water also a decrease of

ultimate strain values, however less pronounced, was

observable for the PPE+PS specimens. Although the

ultimate strain values of PC showed a significant scattering,

a significant embrittlement could be concluded from the

Fig. 3: Strain-to-break versus aging time in air at 140 °C.

Fig. 4: Strain-to-break versus aging time in water at 80 °C.

mechanical properties for both ageing conditions. In hot

water, a significant decrease of the ultimate strain values of

PC occurred after 4000 hours exposition time. In contrast to

investigated amorphous grades PPE+PS and PC, no

embrittlement was ascertainable for the semi-crystalline PP

grade, which exhibits a rather high ductility (high ultimate

strain values) in the reference and aged state.

5. CONCLUSIONS

The degradation behavior of two amorphous (PPE+PS and

PC) and one semi-crystalline (PP) polymer grades for

solar thermal applications was investigated by using

various characterization methods. It was shown, that

ATR-spectroscopy is an appropriate method to identify and

reveal chemical changes at the surface of samples. Thermal

analysis and SEC are meaningful methods to exhibit

changes within the bulk of the material. However, the

investigations emphasized that the most sensitive method

for the description of global ageing of polymeric materials

is tensile testing on unnotched samples and the evaluation

of ultimate mechanical properties (break values).

For the investigated materials the most significant changes

were obtained for the PPE+PS grade after exposition to hot

air at 140 °C. Ageing in hot water at 80 °C has a less

significant effect. For PPE+PS in hot air the ultimate strain,

molecular weight and oxidation temperature values

decreased significantly. For PC it was shown by tensile

testing that global ageing occurs. However, the global

ageing can be assumed to be less pronounced, because no

significant changes were reflected in the molecular weight

data. For PC also ageing in hot air at 140 °C is more severe

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

523

than exposition to hot water at 80 °C. The best long-term

stability was obtained for the investigated semi-crystalline

PP grade, showing neither changes in the ultimate

mechanical properties nor a decrease in molecular weight

or oxidation temperature. In the investigated temperature

and exposition time range, for PP only changes on the

specimen’s surface were detectable by ATR spectroscopy.

The results within this study clearly show, that an important

measure for a cost-efficient polymeric collector based on

the Solarnor concept is the reduction of the maximum

temperature in stagnation condition, in which the absorber

fluid is drained back.

6. ACKNOWLEDGMENTS

The research work of this paper was performed at the

Polymer Competence Center Leoben GmbH within the

framework of the Kplus Program of the Austrian Ministry

of Traffic, Innovation and Technology with the contibutions

by the University of Leoben, Graz University of Technology,

Johannes Kepler University Linz, JOANNEUM RESEARCH

ForschungsgmbH and Upper Austrian Research GmbH.

The PCCL is funded by the Austrian Government and the

State Governments of Styria and Upper Austria.

7. REFERENCES

(1) W. Weiss, (2003). Solar Heating Systems for Houses,

Wiley Interscience, New York.

(2) M. G. Meir, J. B. Rekstad , The Development of a

Polymer Collector with Glazing, 1st Leobener

Symposium Polymeric Solar Materials, 2003,

November 6-7, Leoben, Austria, II-1.

(3) R. Hausner, C. Fink, Der Einfluss der Hydraulik auf die

thermische Belastung der Komponenten und der

Wärmeträgerflüssigkeit. Industry workshop, Solar

Combisystems Task26, 10th October 2001, Rapperswill,

CH, International Energy Agency, Austria, 48.

(4) P. Nitz, H. Hartwig, Solar Energy, 79 (2005) 573.

(5) A. Freeman, S. C. Mantell, J. H. Davidson, Solar

Energy, 79 (2005) 624.

(6) C. Wu, S. C. Mantell, J. H. Davidson, ASME J. of Solar

Energy Engineering 126 (2004) 581.

(7) R. W. Lang, A. Stern, G. Doerner, Angewandte

Makromolekulare Chemie 247 (1997) 131.

(8) S. Kahlen, G. M. Wallner, J. Fischer, M.G. Meir, J.

Rekstad, Basic Characterization of Polymeric Materials

for Solar Collector Absorbers, NorthSun 2005, May

25-27, Vilnius, Lithuania.

(9) DIN 53504 (1994). Testing of rubber; determination of

tensile strength at break, tensile stress at yield,

elongation at break and stress values in a tensile test.

(10) ISO 527-3 (2005). Plastics - Determination of tensile

properties - Part 3: Test conditions for films and

sheets.

(11) G. Dörner, R.W. Lang, Polymer Degradation and

Stability, 62, (1998) 431-440.

EXPERIMENTAL STUDY ON OPTICAL PROPERTIES OF THE COLLECTOR

Wang Juan

School of Science, Xi'an Jiaotong University

P.O. Box 2264

Xi 'an 710049, China

wangjuan_mm@163.com

Zhao Liang

State Key Laboratory of Multiphase Flow in Power

Engineering

Xi'an Jiaotong University

Xi 'an 710049, China

Li Huashan

State Key Laboratory of Multiphase Flow in Power Engineering

Xi'an Jiaotong University

Xi 'an 710049, China

li343@stu.xjtu.edu.cn

ABSTRACT

Solar collector is one of the most important parts of solar

chimney power plant. It plays an important role in

improving the efficiency and saving the cost of the whole

system. In this paper, several transparent materials which

could be used as the collector were chosen to study the

optical properties. The materials include polymethyl

methacrylate (PMMA), polycarbonate (PC), perspex (PS)

and ordinary glass. Firstly, an experiment device was

designed with the collector angles varying from 15

to 60

Secondly, different thickness of glass and different slope

angles of the collector of these materials mentioned above

on transmittance was compared. The effect of pollutions

caused by the natural environment such as dust, rain and

aging on these materials was considered in the experiment.

Based on the experimental results, a conclusion was made

that PMMA has the highest transmittance among the four

materials without considering other factors.

1. INTRODUCTION

The concept of solar chimney power plant was firstly

proposed by Schlaich in 1970s and later in 1980s studies

were carried out with a 50 kW power prototype in

Manzanares, Spain. Solar chimney power plant has three

essential elements—solar air collector, chimney and wind

turbines. Air is heated by solar radiation under a low

circular transparent collector open at the periphery. In the

center of the collector is a vertical chimney. As hot air rises

along the chimney and cold air comes in from the outer

perimeter. The energy contained in the updraft is converted

into mechanical energy by wind turbines at the base of the

tower, and into electrical energy by conventional

generators.

In 2004, E. Bilgen designed the slope solar chimney power

plant. Slope solar chimney power plant is improved from

the tower chimney power plant. It is usually built on the

southern slope of a hill since it can receive longer sunlight.

The hill itself could be a part of the chimney, and this could

save the cost of chimney building. What’s more, the

ascending wind along the hill will enhance the entrance

wind speed.

As one of the primary parts of the slope solar chimney

power plant, the collector plays an important role in

increasing the efficiency and reducing the cost of the

system. In this paper, the transmittances of the materials

which could be used as the collector were studied through

the experiment devices designed by ourselves. The effect of

dust on the transmittance of the collector was observed in

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

525

the outside environment.

2. THEORETICAL ANALYSIS OF THE COLLECTOR

TRANSMITTANCE

2.1 Experiment Platform and Instruments

The picture of the experiment platform is as Fig. 1. There

are four platforms of 15°ˈ30°ˈ45°ˈ60° and each one has

four different transparent materials fixed onto it. The

materials were PMMA, polycarbonate, perspex and

ordinary glass. This experiment platform was aimed to

realize two functions: the highest transmittance among the

four materials just considering dust and aging outdoors; the

influence of different slope on the transmittance. YFJ-10

sunlight irradiatometer was used to measure the irradiation

and the transmittance of the transparent materials.

Fig. 1: Picture of the Experiment Platform.

2.2 The Relationship of the Incidence Angle and the

Slope

The experiment platform which faces south is based on the

advantage of south slope. As a result, the azimuth of the

collector is zero. In this case, the relationship of slope angle

β and the incidence angle θ at this slope is as follow:

cos cos sin sin cos cos cos

sin sin cos cos sin sin cos

cos( ( )) (1)

θβϕδβϕω

βϕδ ω βδ ϕ

βϕδ

+−

=−−

The sun hour angle ω is zero at noon and the local latitude

ϕ is

34.25

N in Xi’an, Shannxi province, China. The

declination of the sun

δ could be expressed by the

following expression:

[]23.45 sin 360(284 ) 365mδ =+ (2)

From the formula (2), we can see that the declination of the

sun is varying with the day number. Therefore, we choose a

certain day, May 13, 2007, to do the following research.

According to formula (1), the incidence angle at a slope of

β is:

15.7θβϕδβ=−+=−

(3)

The refraction angle could be expressed by refractive rate

and incidence angle as follows:

sin sinξ θθ= (4)

According to formula (3), (4), we could get the expression

of transmittance angle:

()()

arcsin sin 15.7βξθ −= (5)

2.3 The Transmittances at the Random Incidence Angles

When the solar ray irradiates at the surface of the collector,

the way it transmits through the collector was as follow in

Fig. 2:

Fig. 2: The solar ray transmits through the material.

According to Fig. 2, the expression of parameter

is:

cosL nl θ= (6)

If we just consider the function of reflection, as the

non-polarized light, the transmittance of solar ray is:

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

526

0.5[(1 ) (1 (2 1) )

(1 ) (1 (2 1) )]

τ ρρ

ρρ

=− +−

+− + −

(7)

sin ( ) sin ( )

tan ( ) tan ( )

ρθθ θθ

=− +

(8)

If the functions of the absorption and reflection were

considered at the same time, the total transmittance is:

0.5 [(1 ) (1 (2 1) )

(1 ) (1 (2 1) )]

τττ ρ ρ

ρρ

=⋅= ⋅− + −

+− + −

(9)

3. EXPERIMENTAL RESULT ANALYSIS OF THE

COLLECTOR TRANSMITTANCE

In this paper, one layer of the collector was studied.

Therefore, the expression of the theoretical transmittance is:

cos

[( ( ]

0.5

1 )(1 ) 1 )(1 )

knl

τττ

ρρ ρρ

−

⋅+

=⋅=

−+ −+

(10)

3.1 The Influence of the Thickness on the Transmittance

of the Transparent Collector

The experiment platform of

slope angle was chosen to

study the effect of thickness. This experiment was

conducted in the same incidence and same material. The

date was 13

, May, 2007. The material was ordinary glass

with 1.52 refractive rate and

0.22cm

extinction

coefficient. According to formulas (3) and (5), we could

calculate the value of

14.0θ ≈

and

9.2θ ≈

respectively.

The relationship between total transmittance and the

thickness could be formulated as:

0.7337eτττ=⋅=

(11)

The experimental result with different thickness of the

same glass was given in Table 1.

TABLE 1: THE EXPERIMENTAL RUSULT OF THICK-

NESS ON TRANSMITTANCE

THICKNESS/mm TRANSMITTANCE

3mm 85.9

4mm 84.5

5mm 83.1

8mm 77.5

Fig. 3 showed the comparison of the theoretical relation-

ship and the experimental results.

Fig. 3: The influence of the thickness on transmittance.

It was obvious that the transmittance of the glass dropped

as the thickness increased both from the theoretical curve

and the experimental results in Fig. 3. At the range of 3mm

to 8mm, the experiment results accorded with theoretical

value very well and the error was less than 1%. The

transmittance differences among 3mm to 5mm were just

2.8%. As the area of the collector is very large in the real

system, the impact resistant performance must be

considered. Therefore, 5mm thickness was the best choice

referring to ordinary glass.

3.2 The Influence of the Slope Angle on the

Transmittance of the Transparent Collector

The transparent collector of the same material and

thickness was fixed to different slope experiment platforms.

The chosen material was still single layered ordinary glass

whose refractive rate was 1.52 and the extinction

coefficient was 0.22cm

-1

According to formula (9), (10), the theoretical expression

of the transmittance is:

cos

sin ( ) sin ( )

2sin( )sin( )

θθ θθ

−

⎡

⎛⎞

+− −

=⋅

⎢

⎜⎟

++ −

⎢

⎝⎠

⎣

tan ( ) tan ( )

()

tan ( ) tan ( )

θθ θθ

⎤

+− −

⎥

++ −

⎥

⎦

(12)

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

527

The experiment values of the transmittance with different

slopes were measured on the experiment platform and the

results were in Table 2.

TABLE 2: EXPERIMENTAL RESULTS OF TRANS-

MITTANCE WITH DIFFERENT SLOPE ANGLES

SLOPE ANGLES



TRANSMITTANCE 84.5 86.3 86.2 84.9

SLOPE ANGLES 30



TRANSMITTANCE 84.0 82.5 81.7 81.2

SLOPE ANGLES 60



TRANSMITTANCE 77.5 79.1 74.3 55.0

The comparison between the theoretical and experiment

results was shown in Fig. 4.

Fig. 4: Comparison between experimental and theoretical

results on different slopes.

From Fig. 4 showed that the results of experiment were

distributed evenly along the two sides of the theoretical

curve. When the slope angle was zero, the transmittance

was 84.5%, 0.64% higher than the theoretical value. At the

slopes of 20, 30 and 70 degrees, the results were almost the

same as the theoretical values. While the incidence ray

irradiated on the slope surface vertically, only scattering

light could transmit through the collector. Therefore, the

transmittance in this case was the lowest. At other slopes,

the differences between the theoretical and experimental

results were beyond 5.31%, which were all in the

anticipation. After 45 degrees, the transmittance dropped

sharply. The difference of the transmittance from zero to 45

degrees was beyond 5%.

3.3 The Influence of the Dust and Aging of the

Transparent Collector Surface on the Transmittance

The experiment was continued for about more than one

month to analyze the rule of the influence of dust and aging

on the transmittance. And the measurement was done every

other day. According to the data, the varying curves of the

influence with each slope were given in Fig. 5.

(5-a) (5-b)

(5-c) (5-d)

Legend:

Fig. 5: Comparison among different materials on

transmittance of different slope angles.

In Fig. 5, X axis denoted the number of days and Y axis

denoted the transmittance. The curves were fitted thrice

using least squares fit method. The slope angles were 15,

30, 45 and 60 degrees from the (5-a) to (5-d). It shows that

at each slope, the varying trend of the four different

materials were almost similar. At the first beginning, the

transmittances fell distinctly, then experienced a placid

period and at last dropped fast. It is evident that the

transmittance of PMMA was the highest among the four

materials and PS was the second highest one which was

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

528

about 5% lower than PMMA. PC was about 10%lower than

PMMA and ordinary glass was the lowest one which was

about 20% lower than the highest. From this point, PMMA

was the most suitable material to use as the collector. In

addition, from the fitted results, the changing rule of

PMMA was well-regulated at four slope angles.

4. CONCLUSIONS

The data from this investigation yield some results to the

performance of solar collectors. The transmittance of the

glass dropped while the thickness increased. Considering

the impact resistant performance, 5mm thickness was the

best choice referring to ordinary glass. The transmittance

was more than 80% before 45 degrees and it dropped

sharply after 45 degrees. What’s more, the difference of the

transmittance from zero to 45 degrees was beyond 5%, the

slope angle could be chosen no more than 45 degrees

considering the local latitude. Fig 5 shows that

transmittance of PMMA was similar at four slope angles

which was the highest. From this point, PMMA was the

more suitable to be used as the collector without

considering temperature, infrared reflection, heat transfer

coefficient and other factors, which would be studied in

future researches.

5. NOMENCLATURE

: slope

: number of glass cover

: day number

: extinction coefficient

ω : sunset hour angle

δ : declination of the sun

ϕ : local latitude ξ : refractive index

: the angle of incidence of beam radiation at slope of β

: the angle of refraction of beam radiation at slope of β

: the ray propagate distance through the glass

τ : transmittance considered only with absorbing function

τ : total transmittance

τ : transmittance considered only with reflecting function

: the reflectivity of directions’ linearly polarized

lights respectively

6. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial supports

from the National Natural Science Foundation of China

(No. 50506025 and 50521604).

7. REFERENCES

(1) E. Bilgen, “Solar Chimney Power Plants For High

Latitudes”, SOLAR ENERGY

March 2005, p. 449-

458.

(2) John A. Duffie, “Solar Energy Thermal Processes”, A

WILEY INTERSCIENCES PUBLICATION, 1974, p.

15.

(3) Wei Xuesong, “The Research of Thermal Property of

Greenhouse Covering Materials”, TIAN JIN UNIVER-

SITY

January 2005, p. 7-16

(4) Gorgon Tully, “solar heating systems—analyses and

design with the sun-pulse method”, 1981, p. 9-18.

(5) Wei Shengxian, Li Ming, Zhou Xizheng, “A theoretical

study on area compensation for non-directly- south-

facing solar collectors”, APPLIED THERMAL ENG-

INEERING

(6) K.S.Ong, “A Mathematical Model of A Solar Chimney”,

RENEWABLE ENERGY

, September 2002, p. 1054-

1056.

(7) Huang Guohua, Feasibility Analysis of Slope Solar

Chimney Power Plant. SOLAR ENERGY

. April 2005,

p. 46-47.

IMPACT OF AGEING ON THERMAL EFFICIENCY

OF SOLAR THERMAL COLLECTORS

Elke Streicher, Stephan Fischer, Harald Drück,

Hans-Müller Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

streicher@itw.uni-stuttgart.de

ABSTRACT

Today it is common practice to calculate the performance

of solar thermal systems or solar collectors based on the

results of a thermal performance test carried out with a new

solar collector. However, for an integral assessment of solar

thermal collectors and systems, it is necessary to consider

the change of performance with time due to degradation

effects. Degradation effects like a reduced transmittance

due to deposits on the transparent collector cover or a

reduction in the absorption coefficient of the absorber due

to high temperature oxidation mechanisms may result in

decreased collector efficiency. Up to now it is not possible

to take these degradation effects into account,

systematically.

In order to determine the extent of ageing and its impact on

the thermal performance of solar thermal collectors,

thermal efficiency tests have been carried out with 15

collectors, both in new and used state. The collectors were

exposed for a period of 3 years to normal weather

conditions. During that time the collectors were not filled

with a heat transfer fluid. Hence the conditions were similar

to the operating state “stagnation” where no usable heat is

removed from the collector and very high absorber

temperatures are reached. After the exposure the thermal

performance of the collectors was determined again. Based

on the collector efficiency parameters determined for the

new and the exposed (aged) collectors, the fractional

energy savings were calculated for a typical solar domestic

hot water system.

1. INTRODUCTION

Solar thermal collectors are one of the basic components of

solar thermal systems. However, the thermal performance

of solar collectors may be reduced over the years by ageing

effects. In order to quantify ageing and its impact on the

thermal performance of solar thermal collectors, thermal

efficiency tests have been carried out with 15 collectors in

new and used state. The selected collectors represent a

good cross-section of collector types available on the

German market in the years 2001 and 2002. The

investigation comprises 13 flat plate collectors with and

without antireflective coating of the glass cover, with

different insulation materials and different selective

coatings as well as two evacuated tube collectors working

according to the direct principle. One evacuated tube

collector is a so-called Sydney-Collector with a cylindrical

absorber with CPC reflector (CPC: compound parabolic

concentrator). The other one also works according to the

direct principle, with the collector loop heat transfer fluid

being transferred through the header with a so-called “wet

connection”. Each of the tubes of this type of evacuated

tube collector is equipped with a selective coated absorber

fin.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

530

The thermal efficiency of the collectors was previously

determined in 2001 or 2002 respectively in new state.

Hence the characteristic collector efficiency parameters for

the new collectors are known. Afterwards the collectors

have been exposed outdoors for a 3 year period to normal

weather conditions. During that time the collectors were

not filled with a heat transfer fluid. Therefore the

conditions were similar to the operating state “stagnation”

where no usable heat is removed from the collector and

hence very high absorber temperatures are reached. After

the exposure the thermal performance of the collectors was

determined again in 2005. Like the performance tests of the

collectors in new state, this was done by an outdoor test

according to European Standard EN 12975-2 using the

quasi-dynamic test method for the determination of the

characteristic collector efficiency parameters.

This paper presents a comparison of the characteristic

collector efficiency parameters (η

, a

) for the new and

the exposed (aged) collectors. Furthermore, it will be

shown how the changes in the collector efficiency

parameters due to ageing will influence the yearly energy

gain and the fractional energy savings of a typical solar

domestic hot water system.

2. IMPACT OF EXPOSURE ON CHARACTERISTIC

COLLECTOR EFFICIENCY PARAMETERS

In the following the changes of the collector efficiency

parameters due to the 3 years exposure will be presented

and discussed. The resulting changes of the conversion

factor η

and the effective heat transfer coefficients of the

15 solar thermal collectors are presented. For a better

assessment of the exposure impact on the heat transfer

coefficients a

and a

, the effective heat transfer coefficient

eff

(50K) at a temperature difference ΔT of 50 K (between

average fluid temperature and ambient temperature) was

determined according to equation 1.

(50 ) 50aKaaTwithTK=+⋅Δ Δ=

(1)

2.1 Conversion Factor

Figure 1 shows the conversion factors η

of the 15

collectors in new state and after the exposure. Nearly all of

the collectors show a slight decrease of the conversion

factor.

[-]

Fig. 1: Conversion factors η

of the 15 tested collectors in

new state and after the exposure.

As the conversion factor is mainly dependent on the

transmittance-absorbance product, the decrease does not

necessarily result from a reduction in absorbance of the

selective coating, but also possibly from a reduction of

transmittance of the transparent collector cover. Former

investigations /1/ show that dust deposits on the cover are

often the reason for a lower transmittance and that this

problem can be solved with a thorough cleaning of the

collector cover. During the exposure no special cleaning

had taken place but deposits on the collector cover were

partly removed by natural rain. Prior to the thermal

efficiency tests the collectors were washed off. However, a

special cleaning with cleaning agents, as it was done in the

above mentioned investigation, was not carried out.

The comparison of the conversion factors in new state and

after the exposure results in an average reduction of the

conversion factor of -2.6 % (absolute). The entire range of

the absolute change comprises +0.5 to -4.5 %. From the

group of flat plate collectors, collector no. 7 shows the

smallest reduction of -1.8 % (absolute). The evacuated tube

collectors 14 and 15 show only very slight changes in the

conversion factors. Collector no. 15 with its absolute

reduction of 0.9 % is clearly under the average of -2.6 %.

Regarding collector no. 14 a slight increase of the

conversion factor of +0.5 % can be observed, which is

however within the limits of the measurement accuracy.

2.2 Effective Heat Transfer Coefficient

As can be seen in Figure 2, all collectors show only very

small changes in the effective heat transfer coefficients

eff

(50 K).

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

531

Fig. 2: Effective heat transfer coefficients a

eff

(50 K) in new

state and after the exposure.

Considering the power curves of the collectors in new state

and after the exposure, a parallel curve progression can be

observed in most cases, as can be seen for example in

Fig. 3 for collector No. 3. Hence no significant impact on

the effective heat transfer coefficients can be observed due

to the exposure.

The power curves are based on the determined collector

efficiency parameters and are plotted according to

EN 12975-2:2006, annex D, chapter 3 for a hemispherical

irradiance of G*=1000 W/m² (equation 2).

⎟

⎠

⎞

⎜

⎝

⎛

ϑ−ϑ

−

ϑ−ϑ

−⋅=

)(

aη*GAQ



(2)

Fig. 3: Power curves of collector No. 3, in new state and

after the exposure (for G*=1000 W/m²).

3. REDUCTION OF THERMAL PERFORMANCE DUE

TO AGEING

In order to be able to quantify the impact of ageing on the

energy gain of the solar thermal collectors, calculations

with the simulation program TRNSYS have been carried

out. This was done for using a typical German solar

domestic hot water heating system as reference. The

simulations were carried out in such a way, that only the

collector efficiency parameters for the collector were

changed.

Based on the collector efficiency parameters determined for

the new and the exposed (aged) collectors, the energy

output for each collector and the auxiliary heat demand

aux,net

required for a complete coverage of the hot water

demand were calculated.

The reference system for solar domestic hot water heating

used for that purpose is specified as follows:

collector: South orientation, inclination 45°, area: 5 m²

hot water

demand:

200 litres/day, tapping profile:

7 a.m. 80 litres, 12 a.m. 40 litres, 7 p.m. 80 litres

hot water demand temperature: 45 °C

cold water temperature: 10 ± 3 °C

yearly hot water demand: 2945 kWh

collector

circuit:

flow and return pipe 10 metres each, located

inside

store: total store volume: 300 litres

auxiliary volume: 150 litres

ambient temperature: 15 °C

auxiliary heating power: 15 kW,

domestic hot water set temperature: 52.5 °C

weather

data:

test reference year Würzburg (city located in the

middle of Germany)

annual irradiance in collector plane:

1231 kWh/m²

As a result of the simulations the fractional energy savings

sav

for the solar domestic hot water system were

determined according to equation 3.

Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2

Подождите немного. Документ загружается.