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Development of 12.5 m2 solar collector panel for solar heating plants
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 84 (2004) 205–223
Development of 12.5 m2 solar collector panel for solar heating plants N.K. Vejen*, S. Furbo, L.J. Shah Department of Civil Engineering, Technical University of Denmark, Brovej-Building 118, Lyngby DK-2800 Kgs., Denmark Received 1 September 2001; accepted 23 January 2004
Abstract Theoretical and experimental investigations have elucidated how different changes in the design of the 12.5 m2 HT flat-plate solar collector from the Danish company ARCON Solvarme A/S influence the solar collector efficiency and the yearly thermal performance. The collector is designed for medium and large solar heating systems. Based on the theoretical findings a prototype of an improved HT solar collector was built and tested side-by-side with the original HT solar collector. The improved HT collector makes use of a changed insulation material, an absorber with improved absorptance and emittance, and a changed antireflection treated glass cover. Calculations based on the measured efficiencies showed that the yearly thermal performance is increased by 23–37% at operation temperatures between 40 C and 80 C when using the improved HT collector. The cost of the collector was however only increased with about 5%. r 2004 Elsevier B.V. All rights reserved. Keywords: Solar collector efficiency; Incidence angle modifier; Yearly thermal performance; Glass cover with antireflection surfaces; Absorber with improved absorptance and emittance
1. Introduction The aim of this work is to improve the efficiency of a solar collector from the Danish company ARCON Solvarme A/S with the trade name HT, and thereby improve the thermal performance of a solar heating plant. *Corresponding author. Tel.: +45-45-25-18-66; fax: +45-45-88-32-82. E-mail addresses: [email protected] (N.K. Vejen), [email protected] (S. Furbo), [email protected] (L.J. Shah). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.01.037
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Nomenclature Z Tm Ta G y ky
collector efficiency, dimensionless mean solar collector fluid temperature, C ambient air temperature, C solar irradiance on solar collector, W/m2 incidence angle, incidence angle modifier, dimensionless
Fig. 1. HT solar collector.
The HT solar collector is a high temperature solar collector suitable for medium and large solar heating systems and the collector has been used in almost 50% of all collector installations in large solar heating centrals built since 1985 up to now [1]. For instance, the HT solar collector is used in the world largest solar heating plant in Marstal, [2]. This solar heating plant runs at a typical operating temperature from 40 C to 80 C. The HT solar collector is a traditional flat plate solar collector with a transparent area of 12.53 m2. Fig. 1 shows the design of the HT solar collector. Data of the HT solar collector is given in Table 1.
2. Theoretical investigation Theoretical investigations of a number of design changes to an HT solar collector have been carried out with the program SOLEFF, developed by Rasmussen et al. [3]. The efficiency and the incidence angle modifier were calculated for a number of differently designed solar collectors. A 40% (weight%) propylene glycol/water mixture, which is typically used in collectors under Danish conditions, is used as the solar collector fluid. The mass flow rate through the solar collector used in the calculations is 25 kg/min corresponding to 2.0 kg/min/m2 solar collector. This flow
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Table 1 Data of HT solar collector Cover system Numbers: Material: Thickness: Absorber Type: No. of strips: Material: Surface: Channel system: Channel cross area: Thickness: Fluid volume: Strip dimensions (mm):
2 Glass+Teflon foil 4+0.025 mm
Sunstrip Niox 16 Copper tube/aluminum plate Selective nickel a ¼ 0:95; e ¼ 0:12 Horizontal parallel channels connected to two vertical manifold pipes at the right and left side of the collector 60 mm2 0.5 mm 8.5 kg
Frame Material: Dimensions:
Aluminum profiles 2.27 5.96 0.14 m
Insulation Material: Back side: Sides:
Glass wool 75 mm 30 mm
corresponds to a typical flow in a solar heating plant based on 10 serial connected HT collectors. A wind speed of 2 m/s is used in the calculations. Based on the theoretical investigations, costs and production issues, ARCON Solvarme A/S built an improved HT solar collector where the following components were changed: Insulation material, absorber, glass cover and antireflection treatment of the glass cover. Outdoor measurements were carried out on an HT solar collector and the improved HT solar collector situated side by side. The efficiency and the incidence angle modifier were measured for the two collectors. A program developed by Jensen et al. [4] was used to calculate the yearly thermal performance of a solar collector field, with HT solar collectors and the improved HT solar collector. In the program only the collector field is taken into account. Based on the solar collector efficiency equation, the incidence angle modifier and based on weather data on an hourly basis the thermal performance of the collector field is calculated during each hour of the year. Further it is assumed that the temperature
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distribution is uniform in the collector and constant throughout the year. The calculations were carried out for different temperature levels of the solar collector field. The yearly thermal performance is calculated on the basis of weather data from the Danish Design Reference Year, [5], with solar collectors facing south and with a collector tilt angle of 45 . Furthermore, the mean solar collector fluid temperature is kept constant during collector operation. 2.1. Insulation material The normally used Isover glass wool insulation as well as two Rockwool insulation materials are examined. The insulation thickness is 75 mm at the backside and 30 mm at the sides in all cases and the thermal conductivity is assumed constant. The independent Danish testing institute BVQ1 (http://www.vik.dk/index.html) supplied the following data for the thermal conductivity at 40 C for the three insulation materials: * * *
Glass wool: 0.044 W/mK. Rock wool x1: 0.040 W/mK. Rock wool x2: 0.036 W/mK.
Fig. 2 shows the solar collector efficiency for the HT solar collector with the three insulation materials for a solar irradiance of 800 W/m2 and an incidence angle of 0 . The efficiency is increased somewhat by changing the insulation material, especially at high temperature levels. Fig. 3 shows the yearly thermal performance as a function of the mean solar collector fluid temperature and the yearly percentage increase of the thermal performance by changing the insulation material. 0.8 0.7 0.6
Efficiency
0.5 0.4 Insulation material: 0.3 Glass wool 0.2
Rock wool #1 Rock wool #2
0.1 0 0
10
20
30
40
50
60
70
80
90
100
Tm -Ta , K
Fig. 2. Modelled collector efficiency of a standard HT collector with different insulation materials.
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700
8 7
2
Thermal performance, kWh/m year
800
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compaired to glass wool insulation, %
900
209
0 100
Meansolar collector fluid temperature, °C
Fig. 3. Calculated yearly thermal performance with different insulation materials.
The thermal performance of the collector can be increased somewhat by changing the insulation material. The higher the temperature level the higher the advantage by changing the insulation material. 2.2. Absorber It is investigated how the collector efficiency and the yearly thermal performance of the collector are influenced by the number of parallel channels in the absorber and by the absorptance and emittance of the absorber. 2.2.1. Number of parallel channels Calculations are carried out with 16 and 21 parallel channels in the absorber. The width of each sunstrip is decreased if 21 channels are used instead of 16 channels. Fig. 4 shows the collector efficiency at a solar irradiance of 800 W/m2 and an incidence angle of 0 and Fig. 5 shows the yearly thermal performance of the two collectors. The advantage by increasing the number of channels from 16 to 21 is most significant for low operating temperatures (approximately 2%-points); however, for higher operating temperatures the advantage decreases. The reason is that a change in the absorber design has the largest impact on the efficiency at low temperatures whereas for higher temperatures the heat loss is more dominant. 2.2.2. Strip type Calculations are carried out with the original absorber with an absorptance of 0.95 and emittance of 0.12 and with a new absorber with an absorptance of 0.96 and
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0.8
0.7
0.6
Efficiency
0.5
0.4
0.3
0.2
16 channels 21 channels
0.1
0 0
10
20
30
40
50
60
70
80
90
100
Tm -Ta , K
Fig. 4. Modelled solar collector efficiency with 16 and 21 channels.
800 Thermal performance, kWh/m2 year
9
16 channels 21 channels Improvement
8
700
7
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance going from 16 to 21 channels, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 5. Calculated yearly thermal performance with 16 and 21 channels.
emittance of 0.07. The absorbers have the same angular properties. The absorber has 16 parallel channels in both cases. Fig. 6 shows the efficiency at a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 7 shows the yearly thermal performance of the collector with the two absorbers. The efficiency for the collector with the new absorber is higher than the efficiency for the collector with the original absorber. This is especially the case for high temperature levels. The extra yearly thermal performance of the
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0.8 0.7
Efficiency
0.6 0.5 0.4 0.3 0.2
Original absorber New absorber
0.1 0 0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 tm-ta, K
Fig. 6. Modelled solar collector efficiency with the original and new absorber.
Original absorber New absorber Improvement
Thermal performance, kWh/m2 year
800
18 16
700
14
600
12
500
10
400
8
300
6
200
4
100
2
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original absorber, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 7. Calculated yearly thermal performance with the original and new absorber.
collector with the new absorber compared to the collector with the original absorber is increasing with increasing temperature level. 2.3. Glass cover Calculations are carried out with the original glass cover, a 4 mm glass with a pattern on the inner side of the glass and with a new 3.3 mm glass with a
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Table 2 Solar transmittance for two glass types for different incidence angles measured by the manufacturer Incidence angle ( )
Original glass
New glass
0 15 45 70
0.913 0.910 0.782 0.208
0.913 0.912 0.820 0.354
9
Original glass cover New glass cover Improvement
Thermal performance, kWh/m2 year
800
8
700
7
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original glass cover, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 8. Calculated yearly thermal performance with the original and the new glass cover.
less-structured pattern on the inner side of the glass. The transmittances for the two glass types are shown for different incidence angles in Table 2. The efficiency of the collector is the same for the two glass covers as long as the incidence angle is 0 . The incidence angle modifier is increased by replacing the original glass with the new glass cover. Fig. 8 shows the yearly thermal performance of the collector with the two glass covers. The yearly thermal performance is increased by 2–4% when the original glass cover is replaced with the new glass cover.
2.4. Glass cover with antireflection surfaces Calculations are carried out with the original glass cover and the original glass cover with commercial antireflection surfaces prepared by means of a liquid-phase etching by Sunarc Technology A/S. Fig. 9 shows the collector efficiency of the two
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0.8 0.7 0.6
Efficiency
0.5 0.4 0.3 0.2
Original glass cover Glass cover with antireflection surfaces
0.1 0 0
10
20
30
40
50
60
70
80
90
100
Tm-Ta, K
Fig. 9. Modelled solar collector efficiency with original and antireflection treated glass cover.
collectors for a solar irradiance of 800 W/m2 and an incidence angle of 0 . The original absorber and insulation are assumed. It is found that the collector start efficiency has improved from 0.75 to 0.79 by adding an antireflection surface to the glass cover. Based on the study of Vejen [6] and Furbo et al. [7] it is estimated that the incidence angle modifier is improved from ky ¼ 1 tan3:0 ðy=2Þ to ky ¼ 1 tan3:3 ðy=2Þ: Fig. 10 shows the yearly thermal performance of the two collectors. The extra yearly thermal performance of the collector with an antireflection treated glass cover compared to the collector with the original glass cover is increasing with increasing temperature level. The yearly thermal performance is increased by 12% at a mean solar collector temperature of 60 C. 2.5. Conclusion of theoretical investigations Based on the theoretical investigations, costs and production issues the following design changes to the HT solar collector were implemented: * * * *
Insulation material with lower thermal conductivity. Absorber with better optical properties. Glass cover with better optical properties. Glass cover with antireflection surfaces.
Fig. 11 shows the collector efficiencies for a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 12 shows the yearly thermal performances for the HT solar collector and for the HT solar collector where one design change is added at a time.
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Thermal performance, kWh/m2 year
900
27
800
24
700
21
600
18
500
15
400
12
300
9
200
6
Original glass cover Glass cover with antireflection surfaces Improvement
100
3
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original glass cover, %
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0 100
Mean solar collector fluid temperature, °C
Fig. 10. Yearly thermal performance with the original and the antireflection treated glass cover.
1
0.9
0.8
0.7
Efficiency
0.6
0.5
0.4 HT collector 0.3 New insulation material 0.2
New insulation material, new absorber and new glass cover
0.1
New HT collector: New insulation material, new absorber, new glass cover with antireflection surface
0 0
10
20
30
40
50
60
70
80
Tm-Ta, K
Fig. 11. Modelled solar collector efficiency for different collector designs.
90
100
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1000 900
700
2
Thermal performance, kWh/m year
800
600 500 400
HT collector
300
New insulation material New insulation material and new absorber
200
New insulation material, new absorber and new glass cover 100 New HT collector: New insulation material, new absorber and new glass cover with antireflection surface 0 10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temperature, °C
Fig. 12. Calculated yearly thermal performance for different collector designs.
45
Percentage increase of yearly thermal performance compared to the HT collector, %
New insulation material 40
New insulation material and new absorber
35
New insulation material, new absorber and new glass cover
30
New HTcollector: New insulation material, new absorber and new glass cover with antireflection surfaces
25
20
15
10
5
0 0
10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temprature, °C
Fig. 13. Increase of yearly thermal performance compared to the HT for different collector designs.
Fig. 13 shows the percentage increase of yearly thermal performance compared to the HT collector for the different collector designs. It is expected that the yearly thermal performance of the new HT collector will be about 23% higher than the thermal performance of the original HT collector when
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the mean solar collector fluid temperature is 60 C. The percentage of extra yearly thermal performance of the new HT collector compared to the original HT collector is increasing with increasing temperature level. The two main reasons for the better performance are the new absorber and the antireflection surfaces of the glass cover. Based on the theoretical investigations, the following efficiencies at an incidence angle of 0 for the original HT collector and the new HT collector are found: *
Original HT: Z ¼ 0:75 3:00
*
Tm Ta ðTm Ta Þ2 0:0065 : G G
ð1Þ
Tm Ta ðTm Ta Þ2 0:0057 : G G
ð2Þ
New HT: Z ¼ 0:80 2:77
Furthermore, the incidence angle modifiers for the two collectors are: *
Original HT: ky ¼ 1 tan3:0 ðy=2Þ:
*
ð3Þ
New HT: ky ¼ 1 tan3:6 ðy=2Þ:
ð4Þ
3. Future improvements Further improvements of the thermal performance of solar heating plants could be achieved in different ways: *
*
*
Further improvements of the collector such as use of an even better absorber and decrease of heat loss caused by thermal bridges. Better design of the collector field by serial connection of collectors with different efficiencies, use of reflectors or tracking collectors. Use of water instead of propylene glycol/water mixture in summer periods without freeze risk.
For the improved HT collector it is investigated how the thermal performance is influenced if water is used as solar collector fluid instead of the normal 40% propylene glycol/water mixture. Water has a higher specific heat and thermal conductivity and a lower viscosity compared to the propylene glycol/water mixture. Further it is investigated how the thermal performance is influenced if the solar collector is tracking the sun.
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3.1. Water as solar collector fluid Fig. 14 shows the collector efficiencies for a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 15 shows the yearly thermal performances for the improved HT collector when water is used as solar collector fluid instead of a normal 40% propylene glycol/water mixture. The advantage by using water instead of a glycol/ water mixture is most significant for lower operating temperatures. The reason is that for high operating temperatures, especially the viscosity for the glycol/water mixture gets very close to the viscosity of water. Consequently, the difference between the convective heat transfer coefficients inside the pipes water and the glycol/water mixture is smaller for high operating temperatures and larger for low operating temperatures. The yearly thermal performance can be improved by 2–5% by using water instead of propylene glycol/water mixture. The improvement is due to the extra performance during the summer period as the solar radiation and thus the thermal performance during the winter period (November–February) is insignificant. 3.2. Tracking Fig. 16 shows the yearly thermal performances for the improved HT collector if it is in a fixed position and if it is tracking the sun so that the incidence angle will be 0 at all times. The yearly thermal performance can be improved by 35–73%. The percentage of extra yearly thermal performance of the tracking collector compared to the fixed collector is increasing with increasing temperature level. 0.9
0.8
0.7
Efficiency
0.6
0.5
0.4
0.3
Water 40% propylene glycol/water mixture
0.2
0.1
0 0
10
20
30
40
50
60
70
80
90
100
Tm-Ta, K
Fig. 14. Modelled solar collector efficiency when using water or propylene glycol/water mixture as solar collector fluid.
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900
8
Thermal performance, kWh/m2 year
800
Percentage increase of yearly thermal performance by changing from propylene glycol/water mixture to water, %
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7
700 6 600 5 500 4 400 3 300
Water
200
2
40% propylene glycol/water mixture 1
Improvement
100 0 0
10
20
30
40
50
60
70
80
0 100
90
Mean solar collector fluid temperature, °C
Fig. 15. Calculated yearly thermal performance using water or propylene glycol/water mixture as solar collector fluid.
80
1400
Thermal performance, kWh/m2 year
60 1000 50 800 40 600 30 400 20
Fixed Tracking
200
10
Improvement
0
0 0
10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temperature, °C
Fig. 16. Calculated yearly thermal performance with fixed and tracking solar collectors.
Percentage increase of yearly thermal performance by use of tracking collectors, %
70
1200
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4. Experimental investigations Based on the findings, a prototype of the new improved HT solar collector was built and tested side-by-side with the original HT solar collector. Fig. 17 shows the collectors side by side at the test site. The improved HT collector makes use of a changed insulation material, an absorber with increased absorptance and decreased emittance, and a changed glass cover with antireflection surfaces. The cost of the new collector was increased with about 5% compared to the old collector. Measurements have been carried out on the campus of DTU in the period from March 14 to May 12, 2002. The measurements are carried out according to ISO standard [8]. Wind speed was, however, not measured, but it is estimated that the wind speed was around 2 m/s. Based on the measurements, the efficiencies and incidence angle modifiers are found for the HT collector and the improved HT collector and based on these expressions, the expected yearly thermal performance of the two collectors is found. 4.1. Solar collector efficiency The two collectors are tested during clear weather at normal operating temperatures between ambient temperature and 95 C. The volume flow rate was 25 l/min per collector. 30 data points of the efficiency are obtained at different inlet temperatures. The efficiency curve is found by regression analyse of the measurements.
Fig. 17. The HT collector (left) and the improved HT collector (right) at the test site on the campus of DTU.
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The measurements showed that the efficiencies for the original and the improved HT solar collector for small incidence angles are: *
Original HT collector: Z ¼ 0:75 2:69
*
Tm Ta ðTm Ta Þ2 0:0050 : G G
ð5Þ
Improved HT collector: Z ¼ 0:82 2:44
Tm Ta ðTm Ta Þ2 0:0050 : G G
ð6Þ
The start efficiency is higher and the heat loss coefficient is lower for the improved HT collector compared to the HT collector. Fig. 18 shows the efficiency for the two collectors as a function of the difference between the mean solar collector fluid temperature and the ambient temperature at a solar irradiance of 800 W/m2 and at an incidence angle of 0 . To determine the incidence angle modifiers for the collectors, the collectors have been tested at incidence angles from 0 to 45 . Because of shadows from the surroundings in the morning, it has not been possible to make measurements at larger angles of incidence. Fig. 19 shows the measured incidence angle modifier at different incidence angles together with the tangent approximation of the incidence angle modifier for the two collectors. It is found that the tangent approximation of the incidence angle modifier is improved from ky ¼ 1 tan2:8 ðy=2Þ to ky ¼ 1 tan3:6 ðy=2Þ for the improved HT collector compared to the original HT collector. 90 80 70
Efficiency [%]
60 50 40 30
Measured HT collector HT collector, regression curve
20
Measured improved HT collector
10
Improved HT collector, regression curve
0 0
0.05
0.1
0.15
0.2
0.25
2
(Tm -T a)/G, [m ·K/W]
Fig. 18. Measured collector efficiency at an irradiance of approximately 800 W/m2.
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1.2 3.6
kθ = 1 - tan (θ/2)
Incidence angle modifier
1
0.8
0.6
2.8
kθ = 1 - tan (θ/2)
0.4
HT collector - Measurements HT collector - Curve fit Improved HT collector- Measurements Improved HTcollector - Curvefit
0.2
0 0
10
20
30
40
50
60
70
80
90
Incidence angle, °
Fig. 19. Incidence angle modifiers.
The measured efficiencies and incidence angle modifiers are in reasonable good agreement with the calculated efficiencies and incidence angle modifiers. 4.2. Yearly thermal performance The thermal performance of the HT collector and the improved HT collector is calculated on the basis of the efficiencies and incidence angle modifiers for the two collectors found from the measurements. Fig. 20 shows the yearly thermal performance for the HT collector and the improved HT collector together with the yearly percentage increase of the performance of the improved HT collector compared to the original HT collector. Solar collectors in solar heating plants are usually operating in the temperature range from 40 C to 80 C. In this temperature interval, the improved HT collector will have a 23–37% higher yearly thermal performance than the original HT collector. For an operating temperature of 60 C, the improvement of the yearly thermal performance is about 29%, which is a little bit higher than found in Fig. 13. The difference is mainly due to the fact that for the new HT collector, the measured collector efficiency is higher than the modelled collector efficiency.
5. Conclusions Theoretical investigations have elucidated how different changes in the design of the 12.5 m2 flat-plate solar collector panel HT from Arcon Solvarme A/S influence the efficiency and the yearly thermal performance of the collector.
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Yearly thermal performance, kWh/m2 year
1000
50
900
45
800
40
700
35
600
30
500
25
400
20
300
15
HT collector Improved HT collector Improvement
200 100
10 5
0 10
20
30
40 50 60 70 80 Mean solar collector fluid temperature, °C
90
Percentage increase of yearly thermal performance compared to the HT collector, %
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0 100
Fig. 20. Calculated yearly thermal performance of the HT collector and the improved HT collector based on the measured collector efficiencies.
The thermal performance of the collector can be improved by design changes. The largest increase in the thermal performance is achieved by using a glass cover with antireflection surfaces instead of a normal glass cover. Based on the theoretical investigations, costs and production issues the following design changes to the HT solar collector were implemented: *
*
*
*
Insulation material with lower thermal conductivity. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 3% at an operating temperature of 60 C. Absorber with better optical properties. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 6% at an operating temperature of 60 C. Glass cover with better optical properties. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 6% at an operating temperature of 60 C. Glass cover with antireflection surfaces. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 12% at an operating temperature of 60 C.
Further, measurements for the original and improved HT collectors were carried out and calculations based on the measured collector efficiencies showed that on a yearly basis the improved HT collector performs 23–37% better than the original HT
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collector in the temperature interval from 40 C to 80 C. The cost of the collector was however only increased with about 5%.
References [1] The European Large-Scale Solar Heating Network, http://main.hvac.chalmers.se/cshp//. [2] A. Heller, S. Furbo, First experience from the world largest fully commercial solar heating plant, Proceedings from ISES Solar World Congress, Taejon, Korea, 1997. [3] P.B. Rasmussen, S. Svendsen, SolEff Program til beregning af solfangeres effektivitet, Brugervejledning og generel programdokumentation, Thermal Insulation Laboratory, Technical University of Denmark, 1996. [4] K.L. Jensen, T. Nielsen, K.R. Andersen, Solfangerydelser i solvarmecentraler ved forskellige temperaturniveauer, Report from Department of Civil Engineering, Technical University of Denmark, 2001. [5] A. Skertveit, H. Lund, J.A. Oleth, Design Reference Year, Det Norske Institutt, Report No. 1194 Klima, 12/1994. [6] N.K. Vejen, Solfanger med antirefleksionsbehandlet glas, Department of Civil Engineering, Technical University of Denmark, SR-0032, 2000. [7] S. Furbo, L.J. Shah, Thermal advantages for solar heating Systems with a glass cover with antireflection surfaces, Proceedings of ISES Solar Word Congress, Adelaide, Australia, 2001. [8] EN 12975-2: Thermal solar systems and components, Solar collectors, Test Methods, 2001.
Solar Energy Materials & Solar Cells 84 (2004) 205–223
Development of 12.5 m2 solar collector panel for solar heating plants N.K. Vejen*, S. Furbo, L.J. Shah Department of Civil Engineering, Technical University of Denmark, Brovej-Building 118, Lyngby DK-2800 Kgs., Denmark Received 1 September 2001; accepted 23 January 2004
Abstract Theoretical and experimental investigations have elucidated how different changes in the design of the 12.5 m2 HT flat-plate solar collector from the Danish company ARCON Solvarme A/S influence the solar collector efficiency and the yearly thermal performance. The collector is designed for medium and large solar heating systems. Based on the theoretical findings a prototype of an improved HT solar collector was built and tested side-by-side with the original HT solar collector. The improved HT collector makes use of a changed insulation material, an absorber with improved absorptance and emittance, and a changed antireflection treated glass cover. Calculations based on the measured efficiencies showed that the yearly thermal performance is increased by 23–37% at operation temperatures between 40 C and 80 C when using the improved HT collector. The cost of the collector was however only increased with about 5%. r 2004 Elsevier B.V. All rights reserved. Keywords: Solar collector efficiency; Incidence angle modifier; Yearly thermal performance; Glass cover with antireflection surfaces; Absorber with improved absorptance and emittance
1. Introduction The aim of this work is to improve the efficiency of a solar collector from the Danish company ARCON Solvarme A/S with the trade name HT, and thereby improve the thermal performance of a solar heating plant. *Corresponding author. Tel.: +45-45-25-18-66; fax: +45-45-88-32-82. E-mail addresses: [email protected] (N.K. Vejen), [email protected] (S. Furbo), [email protected] (L.J. Shah). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.01.037
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Nomenclature Z Tm Ta G y ky
collector efficiency, dimensionless mean solar collector fluid temperature, C ambient air temperature, C solar irradiance on solar collector, W/m2 incidence angle, incidence angle modifier, dimensionless
Fig. 1. HT solar collector.
The HT solar collector is a high temperature solar collector suitable for medium and large solar heating systems and the collector has been used in almost 50% of all collector installations in large solar heating centrals built since 1985 up to now [1]. For instance, the HT solar collector is used in the world largest solar heating plant in Marstal, [2]. This solar heating plant runs at a typical operating temperature from 40 C to 80 C. The HT solar collector is a traditional flat plate solar collector with a transparent area of 12.53 m2. Fig. 1 shows the design of the HT solar collector. Data of the HT solar collector is given in Table 1.
2. Theoretical investigation Theoretical investigations of a number of design changes to an HT solar collector have been carried out with the program SOLEFF, developed by Rasmussen et al. [3]. The efficiency and the incidence angle modifier were calculated for a number of differently designed solar collectors. A 40% (weight%) propylene glycol/water mixture, which is typically used in collectors under Danish conditions, is used as the solar collector fluid. The mass flow rate through the solar collector used in the calculations is 25 kg/min corresponding to 2.0 kg/min/m2 solar collector. This flow
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Table 1 Data of HT solar collector Cover system Numbers: Material: Thickness: Absorber Type: No. of strips: Material: Surface: Channel system: Channel cross area: Thickness: Fluid volume: Strip dimensions (mm):
2 Glass+Teflon foil 4+0.025 mm
Sunstrip Niox 16 Copper tube/aluminum plate Selective nickel a ¼ 0:95; e ¼ 0:12 Horizontal parallel channels connected to two vertical manifold pipes at the right and left side of the collector 60 mm2 0.5 mm 8.5 kg
Frame Material: Dimensions:
Aluminum profiles 2.27 5.96 0.14 m
Insulation Material: Back side: Sides:
Glass wool 75 mm 30 mm
corresponds to a typical flow in a solar heating plant based on 10 serial connected HT collectors. A wind speed of 2 m/s is used in the calculations. Based on the theoretical investigations, costs and production issues, ARCON Solvarme A/S built an improved HT solar collector where the following components were changed: Insulation material, absorber, glass cover and antireflection treatment of the glass cover. Outdoor measurements were carried out on an HT solar collector and the improved HT solar collector situated side by side. The efficiency and the incidence angle modifier were measured for the two collectors. A program developed by Jensen et al. [4] was used to calculate the yearly thermal performance of a solar collector field, with HT solar collectors and the improved HT solar collector. In the program only the collector field is taken into account. Based on the solar collector efficiency equation, the incidence angle modifier and based on weather data on an hourly basis the thermal performance of the collector field is calculated during each hour of the year. Further it is assumed that the temperature
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distribution is uniform in the collector and constant throughout the year. The calculations were carried out for different temperature levels of the solar collector field. The yearly thermal performance is calculated on the basis of weather data from the Danish Design Reference Year, [5], with solar collectors facing south and with a collector tilt angle of 45 . Furthermore, the mean solar collector fluid temperature is kept constant during collector operation. 2.1. Insulation material The normally used Isover glass wool insulation as well as two Rockwool insulation materials are examined. The insulation thickness is 75 mm at the backside and 30 mm at the sides in all cases and the thermal conductivity is assumed constant. The independent Danish testing institute BVQ1 (http://www.vik.dk/index.html) supplied the following data for the thermal conductivity at 40 C for the three insulation materials: * * *
Glass wool: 0.044 W/mK. Rock wool x1: 0.040 W/mK. Rock wool x2: 0.036 W/mK.
Fig. 2 shows the solar collector efficiency for the HT solar collector with the three insulation materials for a solar irradiance of 800 W/m2 and an incidence angle of 0 . The efficiency is increased somewhat by changing the insulation material, especially at high temperature levels. Fig. 3 shows the yearly thermal performance as a function of the mean solar collector fluid temperature and the yearly percentage increase of the thermal performance by changing the insulation material. 0.8 0.7 0.6
Efficiency
0.5 0.4 Insulation material: 0.3 Glass wool 0.2
Rock wool #1 Rock wool #2
0.1 0 0
10
20
30
40
50
60
70
80
90
100
Tm -Ta , K
Fig. 2. Modelled collector efficiency of a standard HT collector with different insulation materials.
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700
8 7
2
Thermal performance, kWh/m year
800
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compaired to glass wool insulation, %
900
209
0 100
Meansolar collector fluid temperature, °C
Fig. 3. Calculated yearly thermal performance with different insulation materials.
The thermal performance of the collector can be increased somewhat by changing the insulation material. The higher the temperature level the higher the advantage by changing the insulation material. 2.2. Absorber It is investigated how the collector efficiency and the yearly thermal performance of the collector are influenced by the number of parallel channels in the absorber and by the absorptance and emittance of the absorber. 2.2.1. Number of parallel channels Calculations are carried out with 16 and 21 parallel channels in the absorber. The width of each sunstrip is decreased if 21 channels are used instead of 16 channels. Fig. 4 shows the collector efficiency at a solar irradiance of 800 W/m2 and an incidence angle of 0 and Fig. 5 shows the yearly thermal performance of the two collectors. The advantage by increasing the number of channels from 16 to 21 is most significant for low operating temperatures (approximately 2%-points); however, for higher operating temperatures the advantage decreases. The reason is that a change in the absorber design has the largest impact on the efficiency at low temperatures whereas for higher temperatures the heat loss is more dominant. 2.2.2. Strip type Calculations are carried out with the original absorber with an absorptance of 0.95 and emittance of 0.12 and with a new absorber with an absorptance of 0.96 and
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0.8
0.7
0.6
Efficiency
0.5
0.4
0.3
0.2
16 channels 21 channels
0.1
0 0
10
20
30
40
50
60
70
80
90
100
Tm -Ta , K
Fig. 4. Modelled solar collector efficiency with 16 and 21 channels.
800 Thermal performance, kWh/m2 year
9
16 channels 21 channels Improvement
8
700
7
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance going from 16 to 21 channels, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 5. Calculated yearly thermal performance with 16 and 21 channels.
emittance of 0.07. The absorbers have the same angular properties. The absorber has 16 parallel channels in both cases. Fig. 6 shows the efficiency at a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 7 shows the yearly thermal performance of the collector with the two absorbers. The efficiency for the collector with the new absorber is higher than the efficiency for the collector with the original absorber. This is especially the case for high temperature levels. The extra yearly thermal performance of the
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0.8 0.7
Efficiency
0.6 0.5 0.4 0.3 0.2
Original absorber New absorber
0.1 0 0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 tm-ta, K
Fig. 6. Modelled solar collector efficiency with the original and new absorber.
Original absorber New absorber Improvement
Thermal performance, kWh/m2 year
800
18 16
700
14
600
12
500
10
400
8
300
6
200
4
100
2
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original absorber, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 7. Calculated yearly thermal performance with the original and new absorber.
collector with the new absorber compared to the collector with the original absorber is increasing with increasing temperature level. 2.3. Glass cover Calculations are carried out with the original glass cover, a 4 mm glass with a pattern on the inner side of the glass and with a new 3.3 mm glass with a
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Table 2 Solar transmittance for two glass types for different incidence angles measured by the manufacturer Incidence angle ( )
Original glass
New glass
0 15 45 70
0.913 0.910 0.782 0.208
0.913 0.912 0.820 0.354
9
Original glass cover New glass cover Improvement
Thermal performance, kWh/m2 year
800
8
700
7
600
6
500
5
400
4
300
3
200
2
100
1
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original glass cover, %
900
0 100
Mean solar collector fluid temperature, °C
Fig. 8. Calculated yearly thermal performance with the original and the new glass cover.
less-structured pattern on the inner side of the glass. The transmittances for the two glass types are shown for different incidence angles in Table 2. The efficiency of the collector is the same for the two glass covers as long as the incidence angle is 0 . The incidence angle modifier is increased by replacing the original glass with the new glass cover. Fig. 8 shows the yearly thermal performance of the collector with the two glass covers. The yearly thermal performance is increased by 2–4% when the original glass cover is replaced with the new glass cover.
2.4. Glass cover with antireflection surfaces Calculations are carried out with the original glass cover and the original glass cover with commercial antireflection surfaces prepared by means of a liquid-phase etching by Sunarc Technology A/S. Fig. 9 shows the collector efficiency of the two
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0.8 0.7 0.6
Efficiency
0.5 0.4 0.3 0.2
Original glass cover Glass cover with antireflection surfaces
0.1 0 0
10
20
30
40
50
60
70
80
90
100
Tm-Ta, K
Fig. 9. Modelled solar collector efficiency with original and antireflection treated glass cover.
collectors for a solar irradiance of 800 W/m2 and an incidence angle of 0 . The original absorber and insulation are assumed. It is found that the collector start efficiency has improved from 0.75 to 0.79 by adding an antireflection surface to the glass cover. Based on the study of Vejen [6] and Furbo et al. [7] it is estimated that the incidence angle modifier is improved from ky ¼ 1 tan3:0 ðy=2Þ to ky ¼ 1 tan3:3 ðy=2Þ: Fig. 10 shows the yearly thermal performance of the two collectors. The extra yearly thermal performance of the collector with an antireflection treated glass cover compared to the collector with the original glass cover is increasing with increasing temperature level. The yearly thermal performance is increased by 12% at a mean solar collector temperature of 60 C. 2.5. Conclusion of theoretical investigations Based on the theoretical investigations, costs and production issues the following design changes to the HT solar collector were implemented: * * * *
Insulation material with lower thermal conductivity. Absorber with better optical properties. Glass cover with better optical properties. Glass cover with antireflection surfaces.
Fig. 11 shows the collector efficiencies for a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 12 shows the yearly thermal performances for the HT solar collector and for the HT solar collector where one design change is added at a time.
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900
27
800
24
700
21
600
18
500
15
400
12
300
9
200
6
Original glass cover Glass cover with antireflection surfaces Improvement
100
3
0 10
20
30
40
50
60
70
80
90
Percentage increase of yearly thermal performance compared to the original glass cover, %
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0 100
Mean solar collector fluid temperature, °C
Fig. 10. Yearly thermal performance with the original and the antireflection treated glass cover.
1
0.9
0.8
0.7
Efficiency
0.6
0.5
0.4 HT collector 0.3 New insulation material 0.2
New insulation material, new absorber and new glass cover
0.1
New HT collector: New insulation material, new absorber, new glass cover with antireflection surface
0 0
10
20
30
40
50
60
70
80
Tm-Ta, K
Fig. 11. Modelled solar collector efficiency for different collector designs.
90
100
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1000 900
700
2
Thermal performance, kWh/m year
800
600 500 400
HT collector
300
New insulation material New insulation material and new absorber
200
New insulation material, new absorber and new glass cover 100 New HT collector: New insulation material, new absorber and new glass cover with antireflection surface 0 10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temperature, °C
Fig. 12. Calculated yearly thermal performance for different collector designs.
45
Percentage increase of yearly thermal performance compared to the HT collector, %
New insulation material 40
New insulation material and new absorber
35
New insulation material, new absorber and new glass cover
30
New HTcollector: New insulation material, new absorber and new glass cover with antireflection surfaces
25
20
15
10
5
0 0
10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temprature, °C
Fig. 13. Increase of yearly thermal performance compared to the HT for different collector designs.
Fig. 13 shows the percentage increase of yearly thermal performance compared to the HT collector for the different collector designs. It is expected that the yearly thermal performance of the new HT collector will be about 23% higher than the thermal performance of the original HT collector when
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the mean solar collector fluid temperature is 60 C. The percentage of extra yearly thermal performance of the new HT collector compared to the original HT collector is increasing with increasing temperature level. The two main reasons for the better performance are the new absorber and the antireflection surfaces of the glass cover. Based on the theoretical investigations, the following efficiencies at an incidence angle of 0 for the original HT collector and the new HT collector are found: *
Original HT: Z ¼ 0:75 3:00
*
Tm Ta ðTm Ta Þ2 0:0065 : G G
ð1Þ
Tm Ta ðTm Ta Þ2 0:0057 : G G
ð2Þ
New HT: Z ¼ 0:80 2:77
Furthermore, the incidence angle modifiers for the two collectors are: *
Original HT: ky ¼ 1 tan3:0 ðy=2Þ:
*
ð3Þ
New HT: ky ¼ 1 tan3:6 ðy=2Þ:
ð4Þ
3. Future improvements Further improvements of the thermal performance of solar heating plants could be achieved in different ways: *
*
*
Further improvements of the collector such as use of an even better absorber and decrease of heat loss caused by thermal bridges. Better design of the collector field by serial connection of collectors with different efficiencies, use of reflectors or tracking collectors. Use of water instead of propylene glycol/water mixture in summer periods without freeze risk.
For the improved HT collector it is investigated how the thermal performance is influenced if water is used as solar collector fluid instead of the normal 40% propylene glycol/water mixture. Water has a higher specific heat and thermal conductivity and a lower viscosity compared to the propylene glycol/water mixture. Further it is investigated how the thermal performance is influenced if the solar collector is tracking the sun.
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3.1. Water as solar collector fluid Fig. 14 shows the collector efficiencies for a solar irradiance of 800 W/m2 and an incidence angle of 0 , and Fig. 15 shows the yearly thermal performances for the improved HT collector when water is used as solar collector fluid instead of a normal 40% propylene glycol/water mixture. The advantage by using water instead of a glycol/ water mixture is most significant for lower operating temperatures. The reason is that for high operating temperatures, especially the viscosity for the glycol/water mixture gets very close to the viscosity of water. Consequently, the difference between the convective heat transfer coefficients inside the pipes water and the glycol/water mixture is smaller for high operating temperatures and larger for low operating temperatures. The yearly thermal performance can be improved by 2–5% by using water instead of propylene glycol/water mixture. The improvement is due to the extra performance during the summer period as the solar radiation and thus the thermal performance during the winter period (November–February) is insignificant. 3.2. Tracking Fig. 16 shows the yearly thermal performances for the improved HT collector if it is in a fixed position and if it is tracking the sun so that the incidence angle will be 0 at all times. The yearly thermal performance can be improved by 35–73%. The percentage of extra yearly thermal performance of the tracking collector compared to the fixed collector is increasing with increasing temperature level. 0.9
0.8
0.7
Efficiency
0.6
0.5
0.4
0.3
Water 40% propylene glycol/water mixture
0.2
0.1
0 0
10
20
30
40
50
60
70
80
90
100
Tm-Ta, K
Fig. 14. Modelled solar collector efficiency when using water or propylene glycol/water mixture as solar collector fluid.
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900
8
Thermal performance, kWh/m2 year
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Percentage increase of yearly thermal performance by changing from propylene glycol/water mixture to water, %
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7
700 6 600 5 500 4 400 3 300
Water
200
2
40% propylene glycol/water mixture 1
Improvement
100 0 0
10
20
30
40
50
60
70
80
0 100
90
Mean solar collector fluid temperature, °C
Fig. 15. Calculated yearly thermal performance using water or propylene glycol/water mixture as solar collector fluid.
80
1400
Thermal performance, kWh/m2 year
60 1000 50 800 40 600 30 400 20
Fixed Tracking
200
10
Improvement
0
0 0
10
20
30
40
50
60
70
80
90
100
Mean solar collector fluid temperature, °C
Fig. 16. Calculated yearly thermal performance with fixed and tracking solar collectors.
Percentage increase of yearly thermal performance by use of tracking collectors, %
70
1200
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4. Experimental investigations Based on the findings, a prototype of the new improved HT solar collector was built and tested side-by-side with the original HT solar collector. Fig. 17 shows the collectors side by side at the test site. The improved HT collector makes use of a changed insulation material, an absorber with increased absorptance and decreased emittance, and a changed glass cover with antireflection surfaces. The cost of the new collector was increased with about 5% compared to the old collector. Measurements have been carried out on the campus of DTU in the period from March 14 to May 12, 2002. The measurements are carried out according to ISO standard [8]. Wind speed was, however, not measured, but it is estimated that the wind speed was around 2 m/s. Based on the measurements, the efficiencies and incidence angle modifiers are found for the HT collector and the improved HT collector and based on these expressions, the expected yearly thermal performance of the two collectors is found. 4.1. Solar collector efficiency The two collectors are tested during clear weather at normal operating temperatures between ambient temperature and 95 C. The volume flow rate was 25 l/min per collector. 30 data points of the efficiency are obtained at different inlet temperatures. The efficiency curve is found by regression analyse of the measurements.
Fig. 17. The HT collector (left) and the improved HT collector (right) at the test site on the campus of DTU.
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The measurements showed that the efficiencies for the original and the improved HT solar collector for small incidence angles are: *
Original HT collector: Z ¼ 0:75 2:69
*
Tm Ta ðTm Ta Þ2 0:0050 : G G
ð5Þ
Improved HT collector: Z ¼ 0:82 2:44
Tm Ta ðTm Ta Þ2 0:0050 : G G
ð6Þ
The start efficiency is higher and the heat loss coefficient is lower for the improved HT collector compared to the HT collector. Fig. 18 shows the efficiency for the two collectors as a function of the difference between the mean solar collector fluid temperature and the ambient temperature at a solar irradiance of 800 W/m2 and at an incidence angle of 0 . To determine the incidence angle modifiers for the collectors, the collectors have been tested at incidence angles from 0 to 45 . Because of shadows from the surroundings in the morning, it has not been possible to make measurements at larger angles of incidence. Fig. 19 shows the measured incidence angle modifier at different incidence angles together with the tangent approximation of the incidence angle modifier for the two collectors. It is found that the tangent approximation of the incidence angle modifier is improved from ky ¼ 1 tan2:8 ðy=2Þ to ky ¼ 1 tan3:6 ðy=2Þ for the improved HT collector compared to the original HT collector. 90 80 70
Efficiency [%]
60 50 40 30
Measured HT collector HT collector, regression curve
20
Measured improved HT collector
10
Improved HT collector, regression curve
0 0
0.05
0.1
0.15
0.2
0.25
2
(Tm -T a)/G, [m ·K/W]
Fig. 18. Measured collector efficiency at an irradiance of approximately 800 W/m2.
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1.2 3.6
kθ = 1 - tan (θ/2)
Incidence angle modifier
1
0.8
0.6
2.8
kθ = 1 - tan (θ/2)
0.4
HT collector - Measurements HT collector - Curve fit Improved HT collector- Measurements Improved HTcollector - Curvefit
0.2
0 0
10
20
30
40
50
60
70
80
90
Incidence angle, °
Fig. 19. Incidence angle modifiers.
The measured efficiencies and incidence angle modifiers are in reasonable good agreement with the calculated efficiencies and incidence angle modifiers. 4.2. Yearly thermal performance The thermal performance of the HT collector and the improved HT collector is calculated on the basis of the efficiencies and incidence angle modifiers for the two collectors found from the measurements. Fig. 20 shows the yearly thermal performance for the HT collector and the improved HT collector together with the yearly percentage increase of the performance of the improved HT collector compared to the original HT collector. Solar collectors in solar heating plants are usually operating in the temperature range from 40 C to 80 C. In this temperature interval, the improved HT collector will have a 23–37% higher yearly thermal performance than the original HT collector. For an operating temperature of 60 C, the improvement of the yearly thermal performance is about 29%, which is a little bit higher than found in Fig. 13. The difference is mainly due to the fact that for the new HT collector, the measured collector efficiency is higher than the modelled collector efficiency.
5. Conclusions Theoretical investigations have elucidated how different changes in the design of the 12.5 m2 flat-plate solar collector panel HT from Arcon Solvarme A/S influence the efficiency and the yearly thermal performance of the collector.
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50
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800
40
700
35
600
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25
400
20
300
15
HT collector Improved HT collector Improvement
200 100
10 5
0 10
20
30
40 50 60 70 80 Mean solar collector fluid temperature, °C
90
Percentage increase of yearly thermal performance compared to the HT collector, %
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0 100
Fig. 20. Calculated yearly thermal performance of the HT collector and the improved HT collector based on the measured collector efficiencies.
The thermal performance of the collector can be improved by design changes. The largest increase in the thermal performance is achieved by using a glass cover with antireflection surfaces instead of a normal glass cover. Based on the theoretical investigations, costs and production issues the following design changes to the HT solar collector were implemented: *
*
*
*
Insulation material with lower thermal conductivity. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 3% at an operating temperature of 60 C. Absorber with better optical properties. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 6% at an operating temperature of 60 C. Glass cover with better optical properties. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 6% at an operating temperature of 60 C. Glass cover with antireflection surfaces. The calculations showed that this improvement alone would give a yearly thermal performance increase of about 12% at an operating temperature of 60 C.
Further, measurements for the original and improved HT collectors were carried out and calculations based on the measured collector efficiencies showed that on a yearly basis the improved HT collector performs 23–37% better than the original HT
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collector in the temperature interval from 40 C to 80 C. The cost of the collector was however only increased with about 5%.
References [1] The European Large-Scale Solar Heating Network, http://main.hvac.chalmers.se/cshp//. [2] A. Heller, S. Furbo, First experience from the world largest fully commercial solar heating plant, Proceedings from ISES Solar World Congress, Taejon, Korea, 1997. [3] P.B. Rasmussen, S. Svendsen, SolEff Program til beregning af solfangeres effektivitet, Brugervejledning og generel programdokumentation, Thermal Insulation Laboratory, Technical University of Denmark, 1996. [4] K.L. Jensen, T. Nielsen, K.R. Andersen, Solfangerydelser i solvarmecentraler ved forskellige temperaturniveauer, Report from Department of Civil Engineering, Technical University of Denmark, 2001. [5] A. Skertveit, H. Lund, J.A. Oleth, Design Reference Year, Det Norske Institutt, Report No. 1194 Klima, 12/1994. [6] N.K. Vejen, Solfanger med antirefleksionsbehandlet glas, Department of Civil Engineering, Technical University of Denmark, SR-0032, 2000. [7] S. Furbo, L.J. Shah, Thermal advantages for solar heating Systems with a glass cover with antireflection surfaces, Proceedings of ISES Solar Word Congress, Adelaide, Australia, 2001. [8] EN 12975-2: Thermal solar systems and components, Solar collectors, Test Methods, 2001.