The impact of optical and thermal properties on the performance of flat plate solar collectors

Renewable Energy 28 (2003) 331–344 www.elsevier.com/locate/renene

The impact of optical and thermal properties on the performance of flat plate solar collectors B. Hellstrom a, M. Adsten a,b, P. Nostell a,b, B. Karlsson a, E. Wackelgard a,b,,∗ a

Energy and Building Design, Department of Construction and Architecture, Institute of Technology, Lund University, P.O. Box 118, S-221 00 Lund, Sweden b The Angstrom Laboratory, Department of Materials Science, Uppsala University, P.O. Box 534, S751 21 Uppsala, Sweden Received 25 January 2002; accepted 26 February 2002

Abstract The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 °C operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 °C. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1%. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 °C. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 °C. Including external booster reflectors increases the expected annual output at 50 °C to 19.9–29.4% depending on reflector material.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Solar thermal collector; Materials; Teflon; Antireflection treatment; Solar absorber; Booster reflector

∗

Corresponding author. Tel.: +46-18-471-0000; fax: +46-18-500-131. E-mail address: [email protected] (E. Wackelgard).

0960-1481/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 2 ) 0 0 0 4 0 - X

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Nomenclature F⬘ Collector efficiency factor (–) F⬘(ta)b Zero loss efficiency for the beam irradiation (at normal incidence) (–) F⬘(ta)d Zero loss efficiency for the diffuse irradiation (–) F⬘U1 First order effective heat loss coefficient (W/m2 K) F⬘U2 Temperature dependence of effective heat loss coefficient (W/m2 K2) G Global irradiance (W/m2) Beam irradiance (W/m2) Gb Diffuse irradiance (W/m2) Gd Coefficient for the heat transfer between the cover and the ambient hca (W/m2 K) Incidence angle modifier for beam irradiation (–) Ktab Incidence angle modifier for diffuse irradiation (–) Ktad (mC)e Effective thermal capacitance for the collector (J/m2 K) Collector delivered energy (W/m2) qu Specular solar reflectance (at 60° incidence) (–) Rs Total solar reflectance (at 60° incidence) (–) Rt Ambient air temperature (°C) ta Average fin temperature (°C) tf Mean fluid temperature in the collector (°C) tm U Collector heat loss coefficient (W/m2 K) Heat loss coefficient for the back and the sides of the collector Ub (W/m2 K) Length vs. collector height along the cover glass (–) Lr/H Reflector slope from horizontal (°) Br Effective specular reflectance of reflector for this application (–) Rs Total solar reflectance of the reflector (–) Rt apCPC Max Hsouth that the CPC accepts (°) B/H Width/height of collector aperture (–) dB/H dBrefl / Hcoll=reflector relative sideways over sizing (E–W direction) (–) Gap (Collector to reflector gap)/Hcoll (–) Slope of ground (°) Bg Greek letters a e q t

Absorptance (at normal incidence) (–) Emittance (hemispherical) (–) Incidence angle of the beam solar irradiation on the collector plane (°) Transmittance (at normal incidence) (–) or time (s)

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1. Introduction The impact of the material parameters on the energy output of solar collectors is not always well known, but with this knowledge it is possible to give priority to the most cost-effective improvements. The aim of this paper is to investigate the impact of material improvements of different collector components on the energy output. Starting with the energy output from a reference collector the impact on the energy gain due to different improvements are simulated. Two developments in the absorber are studied, an increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05. Several improvements in the collector glazing are evaluated, the addition of teflon honeycomb and single sheet and antireflection treatment of cover glass. A comparison between structured cover glass and unstructured glass is also made. The expression glazing is used for the whole transparent system with cover glass and transparent insulation. When only a single sheet is addressed the expression cover glass is used. The impact of adding external or internal booster reflectors was also studied. Including booster reflectors has the potential to make a cost-effective improvement since the reflector material is considerably cheaper than solar collectors are. A similar study involving some of the measures studied here has been made by Frei and Brunold [3]. This study involves absorber and glazing improvements and treats matters of economy, durability and future development. Annual output simulations are, however, only made for the absorber improvements and antireflective treatment of the cover glass. The calculations of the annually delivered heat have been performed using the collector array model in the MINSUN simulation program. The collector parameters used in the MINSUN program were mainly obtained from a recently developed calculation program that calculates the temperature-dependent heat loss coefficients as well as the incidence angle dependent optical efficiencies for the glazing. The program is mainly theoretically based, but has been verified through laboratory measurements [8].

2. Parameter calculation and MINSUN simulation 2.1. Parameter calculations The parameters used in the collector model of the MINSUN program were derived using a recently developed calculation program further described in Ref. [8]. The program calculates the temperature-dependent heat loss coefficients as well as the incidence angle dependent optical efficiencies for glazings on a theoretical basis. The glazing can be equipped with honeycombs or with an arbitrary number of intermediate films between the absorber and the cover. The calculations are normally performed angle by angle and spectrally resolved, but integrated values can also be used. Optical data for the materials (such as plastic

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films, absorbers, etc.) are used as input to the program and can be obtained from spectrophotometric measurements. Heat loss calculations for collectors, single glazed or equipped with a honeycomb or a plastic film, have been compared with laboratory hot box measurements and has shown satisfying agreement [8]. For the optical calculations, the Fresnel formulas were considered sufficient for glazings with flat parallel sheets. For honeycomb glazings, imperfections such as corrugations in the material have made it necessary to use also angular dependent transmittance measurements on honeycomb structures to correct the model. No shadowing effects have been considered in the optical calculations. The program calculates the so-called total transmittance or g-value, which means that the fraction of the irradiation that is absorbed in the glazing, which is then gained by the collector through smaller net heat losses, is included in the solar transmittance. The parameters obtained with the calculation program were used in the MINSUN simulation program for performing calculations of annual energy outputs, using hourly steps. The collector model used in MINSUN is presented in Eq. (1) qu ⫽ F⬘(ta)bKtab(q)Gb ⫹ F⬘(ta)bKtadGd⫺F⬘U2(tm⫺ta)⫺F⬘U1(tm⫺ta)2

(1)

⫺(mC)edtm / dt The optical parameters were calculated from the incidence angle dependent loss-free efficiencies for (tm⫺ta) ⫽ 0ⴰC and the irradiance G ⫽ 500W / m2. The incident angle modifier for beam irradiance, Ktab, was given as values for every ten degrees. The heat loss coefficients, FU1 and FU2, were calculated from the obtained efficiencies at normal incidence for the temperature differences of (tm ⫺ ta) ⫽ 0, 30, and 50 °C. The heat capacitance parameter was for all the calculations was set to a fixed value, (mC)e ⫽ 7.5kJ / m2K. 2.2. Description of the MINSUN simulation program The MINSUN simulation program was originally developed to speed up simulations of large solar energy systems with seasonal storage. The program consists of two parts, the solar collector array model and the system model including storage, district-heating net, heat loads, and domestic hot water loads [1]. Since the first MINSUN version, the collector array model has been further developed with additional correction terms and functions using experience from solar collector testing [12,13]. In this study, only the collector array part was used. The collector array part of the MINSUN simulation program was chosen to perform the calculations presented here, because no knowledge about the system outside the collector array is needed, for example heat loads, tank sizes etc. Instead of detailed system information the program uses a fixed average operating (heat carrier fluid) temperature. Operating temperatures 30, 50 and 70 °C were simulated. The well-defined operating conditions make a comparison between different collectors more straightforward, since no system effects are included. If the operating

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temperature is varying within relatively small limits, this approximation is valid even for a system. The reflector model is based on traditional optical theory for diffuse and specular reflection in a planar reflector extended from the lower edge of the collector and sloping upwards in front of the collector. The main parameters are slope of the reflector, length and width and beam and total reflectance of the reflective surface material. Also incidence angle dependence of the reflectivity can be specified. By using limiting angles for solar altitude, also curved CPC reflectors can be modelled accurately. The model is carefully validated against long-term outdoor measurements ˚ ngstro¨ m laboratory. More details in combination with optical measurements at the A are found in [13]. The Hay and Davies model [2] was used for the diffuse radiation. The ground reflectance was set to zero in all cases, which is the standard value for collector fields. A collector tilt of 45° and azimuth 0°(south facing) was used. 2.3. Climate data Hourly average climate data for Stockholm, for a reference year based on SMHI (Swedish Meteorological and Hydrological Institute) measurements 1983–1992, was used in the MINSUN simulations. The International Energy Agency, Solar Heating and Cooling program, compiled this reference year. This year is characterised by a global and diffuse irradiation in the horizontal plane of 922 and 471 kWh/m2, respectively. Stockholm is situated on latitude 59° north and longitude 18° east.

3. Simulated cases 3.1. Reference solar collector The reference solar collector is a flat plate collector with a selective absorber and a single glass pane. For the absorber, e ⫽ 0.10 and a ⫽ 0.95 . was chosen. e is here the hemispherical, spectrally integrated emittance and a the absorptance integrated for the solar spectrum at normal incidence. F⬘ was modelled as F⬘ ⫽ 1⫺0.02∗U, where U is the collector heat loss coefficient. This equation was used for all the collectors, which explains why a change in the U-value also gives a change in the optical efficiency parameters, since they include F⬘. Compared to a more accurate model [2], this linear model is estimated to give a maximum error of 0.002 in F⬘ for the changes in U-value investigated here, corresponding to a relative error of ⬍ 1% in the changes of energy outputs. The glass pane has a normal solar transmittance of 0.90 and is placed 0.10 m above the absorber. The heat transfer coefficient between the glass cover and the ambient is set to a constant value, hca ⫽ 23W / m2K, which is recommended for windows in the International Standard ISO 9050. It can be discussed if it is appropriate to use the same value for a solar collector and a window since the surface temperature is different and the collector is facing another part of the sky compared to a window.

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The aim in this case is, however, to compare the different studied cases the choice is less critical. The convection in the gap above the absorber is calculated from experimentally derived correlations [6]. The back and sides of the collector has a heat loss coefficient of Ub ⫽ 1 ⫹ 0.0025∗(tf⫺ta)W / m2K No transparent insulation is included. The collectors described in the following sections are based on the reference collector, only the deviations from it are described. 3.2. Absorber As base case, a solar absorber of a ⫽ 0.95 and e ⫽ 0.10 was chosen since it is representative for commercial absorbers of today. Physical vapour deposition techniques for applying coatings on fin type absorbers has recently come in use in largescale industrial production. This has resulted in a lower thermal emittance for most types of commercial solar absorbers, ranging from about 0.05 to slightly more than 0.10 [9,14]. A similar significant improvement in solar absorptance has not been achieved since old techniques like (electro) chemical surface finishing gave very high solar absorptance, typically between 0.93 and 0.97 [5]. Two improvements compared to the base case have been considered in this study: an increase in solar absorptance to a ⫽ 0.97 and a decrease in emittance to e ⫽ 0.05 . The two cases a ⫽ 0.95 / e ⫽ 0.05 and a ⫽ 0.97 / e ⫽ 0.10 are realistic for commercial absorbers but the combined improvement, a ⫽ 0.97 / e ⫽ 0.05 has so far not been reported even for laboratory prepared samples. 3.3. Transparent insulation A 50 mm honeycomb structure, TIMax, with 8 × 8 mm cells, made from 25 µm teflon (FEP) plastic was investigated both through measurements and calculations. The U-values obtained from the measurements were approximately 0.1 W/m2 K (approximately 2%) higher than the calculated values, which were then corrected with this value. The difference may be a result from a small extent of inter-cell convection, since there was a problem to get the rather soft honeycomb structure attached to the cover glazing. The difference was not obtained with honeycombs of smaller cell width. The effects of installing a 25 µm teflon film t ⫽ (0.96) between the cover glazing and the absorber was also investigated. 3.4. Cover glass The glass used in the reference collector was an unstructured low-iron glass. Two alternatives to this glass were studied; an antireflective coated glass and a structured glass with the structure facing the absorber surface. The glass in both these alternatives were of low-iron type. Anti-reflective treatment increases the transmittance by the application of a thin film on both sides of the glass, [10,4]. With a homogeneous thin film on both sides

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it is possible to increase the integrated solar transmittance by as much as 6% for a film with optimum optical properties over the spectral range of the sun. Structured glass is not as good an option as plane glass since the transmittance decreases at a higher rate for angles of incidence above approximately 40°. The angular behaviour of the glass is of course dependent of the geometry of the surface structure. For the most frequently used structured glazing, AFG’s low iron glass, the pyramidal shape is too steep and this results in a poorer angular behaviour than for a plane glass. A more detailed study of the angular dependent optical properties of solar glazings is found in [7,11]. The transmittance at normal incidence was for the low iron glass 0.90 (reference collector cover glass), the same for the inward structured glass and 0.94 for the antireflection coated, low iron glass. For angles of incidence up to 40°, the solar transmittance is almost unchanged for all three samples. For the antireflection-coated glass, the incidence angle dependence is approximately the same as for the un-coated glass of the reference collector. 3.5. Optimised flat plate collector An ‘optimised’ collector was also simulated, using the material options that separately gave the largest gain compared to the reference collector. In this case the absorber has a ⫽ 0.97 and e ⫽ 0.05. The cover glass was antireflection treated and a teflon honeycomb was included. 3.6. External booster reflectors Booster reflectors between the collector rows significantly improve the performance of a solar collector in large systems at high latitudes. The increase in energy gain is cost-effective since the reflector material is considerably cheaper than the collector system. A sketch of the collector with reflector is shown in Fig. 1. In this paper, it was chosen to study three commercially available reflector materials, that are protected with different techniques: 1. PVF2 coated aluminium from Gasell AB in Sweden, Rt ⫽ 0.635,Rs ⫽ 0.572. 2. Anodised aluminium from Alanod in Germany (product number: 402 G/S), with a highly specular anodising layer of thickness 1.5 µm, Rt ⫽ 0.845, Rs ⫽ 0.803.

Fig. 1.

Sketch of two collector rows with an intermediate booster collector.

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3. Silver protected with glass from Erie Electroverre in Switzerland, Rt ⫽ Rs ⫽ 0.953. The simulation parameters are found in Table 1.

4. Results The results and the input parameters from the MINSUN simulations are presented in Table 2. Eight changes from the reference case are treated. The results that include external booster reflectors are treated separately. 1. 2. 3. 4. 5. 6. 7. 8.

e ⫽ 0.05; a ⫽ 0.97; e ⫽ 0.05 and a ⫽ 0.97, cases 1 and 2 combined; teflon film added; teflon honeycomb added; cover glass with structure facing the absorber; antireflection cover glass, t ⫽ 0.94; optimised collector, cases 3, 5, and 7 combined. The results in Table 2 are presented in the following ways:

1. The total annual energy output (kWh/m2). 2. The difference compared to the reference case (kWh/m2). 3. The relative difference to the reference case (%). At the end of Table 2, the MINSUN simulation parameters for the investigated cases are shown. The improvement in collector performance is moderate for an increase in the solar absorptance from 0.95 to 0.97 or a decrease in thermal emittance from 0.10 to 0.05. Separately they increase the collector energy output by less than 5% at 50 °C according to Table 2, but combining an enhancement of the solar absorptance to 0.97 with a decrease of the thermal emittance to 0.05 gives about 7% in relative increase of collector yield at this temperature. The combined effect is almost the sum of the Table 1 MINSUN simulation reflector parameters Material

Lr / H(–)

Br (°) Rs (–)

Rt (–)

apCPC (°)

B/H (⫺)

dB/H (⫺)

Gap (⫺)

Bg (°)

PVF2 Al Anod Al Glass Ag

2.06 2.06 2.06

20 20 20

0.635 0.845 0.953

0.0 0.0 0.0

12 12 12

0.0 0.0 0.0

0.0 0.0 0.0

0.0 0.0 0.0

0.5715 0.8028 0.953

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Table 2 Annual energy gains (kWh/m2) from changes in the collector parameters for different operating temperatures and MINSUN parameters for each case Case

Annual collector energy output (kWh/m2)

Absolute difference collectorreference (kWh/m2)

Relative difference collectorreference (%)

1

2

3

tm (°C) Reference e ⫽ collector 0.05

a⫽ 0.97

e⫽ Teflon Teflon Struct. AR Optimised hc glazing glazing 0.05 / a film ⫽ 0.97

30

608

620

621

634

606

619

594

639

672

50 70

447 313

464 333

459 324

477 344

472 357

501 399

435 305

476 338

557 455

30

12

13

26

⫺2

11

⫺14

31

64

50 70

17 20

12 11

30 31

25 44

54 86

⫺12 ⫺8

29 25

110 142

30

2.0

2.1

4.3

⫺0.3

1.8

⫺2.3

5.1

10.5

50 70

3.8 6.4

2.7 3.5

6.7 9.9

5.6 14.1

12.1 27.5

⫺2.7 ⫺2.6

6.5 8.0

24.6 45.4

0.828

0.788

0.814

0.809

0.843

0.868

MINSUN F⬘(ta)b parameters (–), (W/m2 K2) (W/m2 K2) F⬘(ta)d F⬘U1 F⬘U2

4

5

6

7

8

0.809

0.813 0.824

0.721 3.42 0.0113

0.725 0.734 0.738 0.680 0.669 0.721 0.751 0.712 3.22 3.43 3.23 2.69 2.22 3.42 3.42 2.11 0.0106 0.0113 0.0106 0.0082 0.0062 0.0113 0.0113 0.0055

two individual measures. Fig. 2 illustrates the relative increase in collector energy output per 0.01 units in increased absorptance or decreased emittance for average operation temperatures between 30 and 70 °C. It can be seen in Fig. 2 that an increase of 0.01 units in absorptance is worth almost the double in energy gain at 50 °C compared to a corresponding decrease in emittance. However, the impact of lower thermal emittance becomes stronger at higher operation temperature. At an operating temperature of 50 °C, the increase in output for the teflon film collector is 6%, and the double, 12% for the teflon honeycomb collector. It should be noted that the improvements for the teflon film and honeycomb are to some extent

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Fig. 2. Relative change in collector energy output per 0.01 of absolute change in solar absorptance or thermal emittance as a function of operating temperature.

depending on the absorber emittance. If the absorber had a higher emittance, the teflon film or honeycomb would have caused larger improvements, and vice versa. For the reference collector a plane glazing was used. Frequently, however, structured glazings are used in solar collectors due to aesthetic reasons. If the structure is facing the absorber surface, the transmittance decreases at higher angles of incidence, due to total internal reflection in the structure. This effect is enhanced for steeper structures. This choice of structure orientation decreases the energy output by approximately 2.5% for all considered operating temperatures. Commercially available antireflection treatment increases the normal solar transmittance of the glass by 4%. This increases the annual collector output with 6.5% at an operating temperature of 50 °C. With an ideal antireflection treatment it is possible to increase the solar transmittance further [10]. Cover glazings with ideal optical properties are available in laboratories, but the industrially produced antireflection coated glazings still need to be improved. For the optimum collector with a ⫽ 0.05, e ⫽ 0.97, a teflon honeycomb and an antireflection cover glazing, the gain compared to the reference collector is 25% at 50 °C operating temperature. Comparing the results of the optimised collector with the sum of the gains from each improvement shows that the effects are almost additive. An estimated annual energy balance for the reference and the optimised collector is shown in Fig. 3. The delivered heat from the reference, a ⫽ 0.97 / e ⫽ 0.05, and the optimised collector are compared in Fig. 4 at different operating temperatures. In Table 3 the results from MINSUN simulations for a reference collector with a flat booster reflector are shown. Three different commercially available booster reflectors were considered; PVF2 coated aluminium (PVF2), anodised aluminium (Anod. Al), and silver coated glass (Ag). For the operating temperature 50 °C the annual output increases by 19.9, 26.1, and 29.4% for each reflector, respectively. The relative improvement increases with increasing operating temperatures.

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Fig. 3.

341

Estimated annual energy balance in kWh/m2 for the reference and the optimised collectors.

Fig. 4. Annually delivered energy for the reference collector, a ⫽ 0.97 / e ⫽ 0.05 and the optimised collector as a function of operating temperature.

5. Conclusions The presented simulations show that several improvements can be achieved for solar thermal collectors. The largest efforts should be put on the improvements that lead to the largest increase in the collected annual heat per cost unit. Some of the presented improvements already exist for commercial collectors, such as teflon films and antireflection treated glass. Whereas others improvements, such as an absorber

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Table 3 Annual energy output for the reference collector with a flat booster reflector, absolute and relative difference due to addition of a booster reflector Op. temp. (°C)

PVF2

Anod. Al

Ag

Annual energy output (kWh/m2)

30 40 50 60 70

720 635 558 487 421

770 683 605 532 465

799 712 633 560 492

Absolute improvement with addition of reflector (kWh/m2)

30

112

162

191

40 50 60 70

112 111 110 108

160 158 155 152

189 186 183 179

Relative Improvement with addition of reflector (%)

30

15.6

21.0

23.9

40 50 60 70

17.6 19.9 22.6 25.7

23.4 26.1 29.1 32.7

26.5 29.4 32.7 36.4

simultaneously having an absorptance of a ⫽ 0.97 and a hemispherical emittance of e ⫽ 0.05, are difficult to achieve to a reasonable cost. By increasing the absorptance from 0.95 to 0.97 the annual energy output increases by 2.7% at an operating temperature of 50 °C. Decreasing the emittance from 0.10 to 0.05 leads to an increase of 3.8% for the same operating temperature. The improvements in either absorptance or emittance for the solar absorber are realistic to obtain; some of the new commercial absorbers already exhibit these characteristics. Improvements in absorptance and emittance are possible through optimisations in existing manufacturing process. The use of the versatile sputtering technique in the absorber manufacturing process increases the flexibility in comparison to previous deposition methods. It is therefore conceivable to alter the absorber coating performance without large investments. This means that a change in the absorber performance does not imply higher production costs. Including a teflon film leads to a performance increase of 5.6% and a teflon honeycomb to an increase of 12.1% at 50 °C operating temperature. Honeycombs made of other materials than teflon has also been investigated, but due to their problems with resisting high stagnation temperatures the teflon type was chosen. The major problem with using this polymer in honeycombs is its high material cost, since the amount of material required is about 12.5 times higher than for a flat single film. Including the single teflon sheet is, however, cost-effective, especially at high latitudes and high operating temperatures.

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Antireflection treated glass is commercially available for an additional cost of less than 9 USD/m2. The presented simulations indicate an increase of 6.5% when an antireflection treated cover glass, with 4% higher solar transmittance than an untreated one, is used in a collector operating at 50 °C. This means that this type of glass soon will be a standard component in flat plate collectors. Theoretically antireflection has the potential of increasing the annual heat delivery by 11% [10]. The study by Frei and Brunold [3] indicates a 5% increase in annual collector performance due to antireflection treatment of the cover glass. The investigation of the structured glass, with the structure facing the absorber, resulted in a deteriorated performance compared to the reference collector with a plane glass. In spite of this relatively poor performance, the structured glass AFGSolatech is frequently used in Swedish solar collectors. Switching from a structured glass to a plane glass would increase the output by approximately 2.7%. One reason for using structured glass is its ability to suppress the direct reflection of light from the slightly coloured absorber. In the case with the optimised collector where the absorber absorptance was increased, the emittance decreased, a teflon honeycomb included and the cover glass anti reflection treated an improvement of 24.6% was achieved. It should, however, be noted that the simultaneous improvement in absorber absorptance and emittance has so far not been reported. If the reference collector is combined with a flat booster reflector having an area twice the collector area, an increase of 19.9–29.4% at 50 °C depending on the reflector material being used is obtained. The contribution from the reflector is proportional to its specular reflectance. A second surface silver mirror is almost an ideal reflector material, with a 95% reflectance resulting in a 29.4% output increase. It is also relatively cheap, around 9 USD/m2. The sheet of glass is, however, very thin, it might therefore suffer from insufficient mechanical strength. Anodised aluminium is optically fairly good with a 26.1% increase in output, but the long-term stability in outdoor applications is limited to about 5 years, after which the specular reflectance decreases rapidly with time. The PVF2 coated sheet aluminium is a commercially produced roofing material. It has been used as corrugated reflectors in a number of large solar collector fields in Sweden and Denmark. It is long-term stable, has good mechanical strength, and is practical to install, but has a low specular reflectance. The performance increase with this reflector material is 19.9%.

Acknowledgements

This work was financed by the Foundation for Strategic Research (SSF), Vattenfall AB and the Swedish National Energy Administration. Jacob Jonsson is acknowledged for performing optical measurements and Bengt Perers and Arne Roos for fruitful discussions.

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References [1] Chant VG, Ha˚ kansson R. The MINSUN simulation and optimisation program. Application and users guide, IEA SH and C Task VII, Ottawa;1985. [2] Duffie JA, Beckman WA. Solar engineering of thermal processes., 2nd ed. New York: Wiley Interscience, 1991. [3] Frei U, Brunold S. Materials in high performance solar collectors. Proceedings of the World Renew Congress, vol. VI ;2000. p. 241–52. [4] Gombert A et al. Glazing with very high solar transmittance. Sol Energy 1998;62(3):177–88. [5] Granqvist CG. Materials science for solar energy conversion systems., 1st ed. Oxford: Pergamon Press, 1991. [6] Inaba H. Experimental study of natural convection in an inclined air layer. Int J Heat Mass Transr 1984;27(8):1127–39. [7] Helgesson A, Karlsson B, Nostell P. Angular dependent optical properties from outdoor measurements on solar glazings. In: Proceedings from Eurosun, Denmark; 2000. [8] Hellstro¨ m B, Karlsson B. Bengts bera¨ kningsprogram; 2001. [9] Lampert CM. International development and advances in solar selective absorbers. Proc Soc PhotoOpt Instrum Engr 1997;3138:134–45. [10] Nostell P, Roos A, Karlsson B. Anti-reflection of glazings for solar energy applications. Sol Energy Mat Sol Cells 1998;54:223–33. [11] Nostell P, Roos A, Karlsson B. Rev Sci Instr 2000;70(5):2481–94. [12] Perers B. Dynamic method for solar collector array testing and evaluation with standard database and simulation programs. Sol Energy 1993;50(6):517–26. [13] Perers B, Karlsson B. External reflectors for large solar collector arrays, simulation model and experimental results. Sol Energy 1993;51(5):327–37. [14] Wa¨ ckelga˚ rd E, Hultmark G. Industrially sputtered solar absorber surface. Sol Energy Mat Sol Cells 1998;54:165–70.